Ameren (AEE) 10-K 2 Mar 15 2014 FY Annual report Financial data

Document_And_Entity_Informatio

Document And Entity Information (USD $)	12 Months Ended
	Dec. 31, 2014	Jan. 30, 2015	Jun. 30, 2014
Document Type	10-K
Amendment Flag	FALSE
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Trading Symbol	AEE
Entity Registrant Name	AMEREN CORP
Entity Central Index Key	1002910
Current Fiscal Year End Date	-19
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		242,634,798
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$9,918,910,542
Entity Well-known Seasoned Issuer	Yes
Union Electric Company
Document Type	10-K
Amendment Flag	FALSE
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Trading Symbol	AEE
Entity Registrant Name	UNION ELECTRIC CO
Entity Central Index Key	100826
Current Fiscal Year End Date	-19
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		102,123,834
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Well-known Seasoned Issuer	No
Ameren Illinois Company
Document Type	10-K
Amendment Flag	FALSE
Document Period End Date	31-Dec-14
Document Fiscal Year Focus	2014
Document Fiscal Period Focus	FY
Trading Symbol	AEE
Entity Registrant Name	Ameren Illinois Co
Entity Central Index Key	18654
Current Fiscal Year End Date	-19
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		25,452,373
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Well-known Seasoned Issuer	No

Consolidated_Statement_Of_Inco

Consolidated Statement Of Income (Loss) (USD $)	12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Operating Revenues:
Electric	$4,913		$4,832		$4,857
Gas	1,140		1,006		924
Total operating revenues	6,053		5,838		5,781
Operating Expenses:
Fuel	826		845		714
Purchased power	454		502		780
Gas purchased for resale	615		526		472
Other operations and maintenance	1,691		1,617		1,511
Depreciation and amortization	745		706		673
Taxes other than income taxes	468		458		443
Total operating expenses	4,799		4,654		4,593
Operating Income	1,254		1,184		1,188
Other Income and Expenses:
Miscellaneous income	79	[1]	69	[1]	70	[1]
Miscellaneous expense	22	[1]	26	[1]	37	[1]
Total other income (expense)	57		43		33
Interest charges	341		398		392
Income Before Income Taxes	970		829		829
Income taxes	377		311		307
Income from Continuing Operations	593		518		522
Income (Loss) from Discontinued Operations, Net of Tax (Note 16)	-1		-223		-1,496
Net Income (Loss)	592		295		-974
Less: Net Income (Loss) Attributable to Noncontrolling Interests:
Continuing Operations	6		6		6
Discontinued Operations					-6
Net Income (Loss):
Net income attributable to Ameren Corporation - continuing operations	587		512		516
Discontinued Operations	-1		-223		-1,490
Net income (loss) attributable to Ameren Corporation	586		289		-974
Pension and other postretirement activity, net of income taxes (benefit)	-12		30		-8
Comprehensive Income (Loss)	574		300		-932
Earnings (Loss) per Common Share – Basic:
Continuing Operations - Basic	$2.42		$2.11		$2.13
Discontinued Operations - Basic	$0		($0.92)		($6.14)
Earnings (Loss) per Common Share – Basic	$2.42		$1.19		($4.01)
Earnings (Loss) per Common Share – Diluted:
Continuing Operations - Diluted	$2.40		$2.10		$2.13
Discontinued Operations - Diluted	$0		($0.92)		($6.14)
Earnings (Loss) per Common Share – Diluted	$2.40		$1.18		($4.01)
Dividends per Common Share	$1.61		$1.60		$1.60
Average Common Shares Outstanding - Basic	242.6		242.6		242.6
Average Common Shares Outstanding - Diluted	244.4		244.5		243
Union Electric Company
Operating Revenues:
Electric	3,388		3,379		3,132
Gas	164		161		139
Other	1		1		1
Total operating revenues	3,553		3,541		3,272
Operating Expenses:
Fuel	826		845		714
Purchased power	119		127		78
Gas purchased for resale	82		78		64
Other operations and maintenance	946		915		827
Depreciation and amortization	473		454		440
Taxes other than income taxes	322		319		304
Total operating expenses	2,768		2,738		2,427
Operating Income	785		803		845
Other Income and Expenses:
Miscellaneous income	60		58		63
Miscellaneous expense	12		11		14
Total other income (expense)	48		47		49
Interest charges	211		210		223
Income Before Income Taxes	622		640		671
Income taxes	229		242		252
Net Income (Loss)	393		398		419
Net Income (Loss):
Net income (loss) attributable to Ameren Corporation	393		398		419
Other Comprehensive Income	0		0		0
Comprehensive Income (Loss)	393		398		419
Preferred Stock Dividends	3		3		3
Net Income Available to Common Stockholder	390		395		416
Ameren Illinois Company
Operating Revenues:
Electric	1,522		1,461		1,739
Gas	976		847		786
Other			3
Total operating revenues	2,498		2,311		2,525
Operating Expenses:
Purchased power	343		380		705
Gas purchased for resale	533		448		408
Other operations and maintenance	771		693		684
Depreciation and amortization	263		243		221
Taxes other than income taxes	138		132		130
Total operating expenses	2,048		1,896		2,148
Operating Income	450		415		377
Other Income and Expenses:
Miscellaneous income	17		10		7
Miscellaneous expense	8		9		17
Total other income (expense)	9		1		-10
Interest charges	112		143		129
Income Before Income Taxes	347		273		238
Income taxes	143		110		94
Net Income (Loss)	204		163		144
Net Income (Loss):
Net income (loss) attributable to Ameren Corporation	204		163		144
Pension and other postretirement activity, net of income taxes (benefit)	-3		-3		-3
Comprehensive Income (Loss)	201		160		141
Preferred Stock Dividends	3		3		3
Net Income Available to Common Stockholder	$201		$160		$141
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Consolidated_Statement_Of_Inco1

Consolidated Statement Of Income (Loss) (Parenthetical) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension and other postretirement activity, tax (benefit)	($7)	$16	($6)
Ameren Illinois Company
Pension and other postretirement activity, tax (benefit)	($2)	($2)	($2)

Consolidated_Statement_Of_Comp

Consolidated Statement Of Comprehensive Income (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income from Continuing Operations	$593	$518	$522
Pension and other postretirement activity, net of income taxes (benefit)	-12	30	-8
Comprehensive Income (Loss) from Continuing Operations	581	548	514
Less: Comprehensive Income from Continuing Operations Attributable to Noncontrolling Interests	6	6	6
Comprehensive Income from Continuing Operations Attributable to Ameren Corporation	575	542	508
Income (Loss) from Discontinued Operations, Net of Tax (Note 16)	-1	-223	-1,496
Comprehensive Income from Continuing Operations Attributable to Ameren Corporation		-18	58
Comprehensive Income (Loss) from Discontinued Operations	-1	-241	-1,438
Less: Comprehensive Income from Discontinuing Operations Attributable to Noncontrolling Interest		1	2
Comprehensive Income (Loss) from Discontinued Operations Attributable to Ameren Corporation	-1	-242	-1,440
Comprehensive Income (Loss)	574	300	-932
Ameren Illinois Company
Pension and other postretirement activity, net of income taxes (benefit)	-3	-3	-3
Comprehensive Income (Loss)	$201	$160	$141

Consolidated_Statement_Of_Comp1

Consolidated Statement Of Comprehensive Income (Parenthetical) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension and other postretirement activity, tax (benefit)	($7)	$16	($6)
Other Comprehensive Income (Loss) from Discontinued Operations Tax (Benefit)	$0	($10)	$40

Consolidated_Balance_Sheet

Consolidated Balance Sheet (USD $)	Dec. 31, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Current Assets:
Cash and cash equivalents	$5		$30
Accounts receivable - trade (less allowance for doubtful accounts)	423		404
Unbilled revenue	265		304
Miscellaneous accounts and notes receivable	81		196
Materials and supplies	524		526
Current regulatory assets	295		156
Current accumulated deferred income taxes, net	352		106
Other current assets	86		85
Assets of discontinued operations (Note 16)	15		165
Total current assets	2,046		1,972
Property, Plant and Equipment, Net	17,424	[1]	16,205	[1]
Investments and Other Assets:
Nuclear decommissioning trust fund	549		494
Goodwill	411		411
Regulatory assets	1,582		1,240
Other assets	664		720
Total investments and other assets	3,206		2,865
TOTAL ASSETS	22,676		21,042
Current Liabilities:
Current maturities of long-term debt	120		534
Short-term debt	714		368
Accounts and wages payable	711		806
Taxes accrued	46		55
Interest accrued	85		86
Current regulatory liabilities	106		216
Other current liabilities	434		351
Liabilities of discontinued operations (Note 16)	33		45
Total current liabilities	2,249		2,461
Long-term Debt, Net	6,120		5,504
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	3,923		3,250
Accumulated deferred investment tax credits	64		63
Regulatory liabilities	1,850		1,705
Asset retirement obligations	396		369
Pension and other postretirement benefits	705		466
Other deferred credits and liabilities	514		538
Total deferred credits and other liabilities	7,452		6,391
Commitments and Contingencies (Notes 2, 10, 14 and 15)
Stockholders' Equity:
Common stock	2		2
Other paid-in capital, principally premium on common stock	5,617		5,632
Preferred stock not subject to mandatory redemption	142		142
Retained earnings	1,103		907
Accumulated other comprehensive income (loss)	-9		3
Total stockholders' equity	6,713		6,544
Noncontrolling Interests	142		142
Total equity	6,855		6,686
TOTAL LIABILITIES AND EQUITY	22,676		21,042
Union Electric Company
Current Assets:
Cash and cash equivalents	1		1
Accounts receivable - trade (less allowance for doubtful accounts)	190		191
Accounts receivable - affiliates	65		1
Unbilled revenue	146		168
Miscellaneous accounts and notes receivable	35		57
Materials and supplies	347		352
Current regulatory assets	163		118
Current accumulated deferred income taxes, net	49		20
Other current assets	92		71
Total current assets	1,039		959
Property, Plant and Equipment, Net	10,867	[1]	10,452	[1]
Investments and Other Assets:
Nuclear decommissioning trust fund	549		494
Regulatory assets	695		534
Other assets	391		465
Total investments and other assets	1,635		1,493
TOTAL ASSETS	13,541		12,904
Current Liabilities:
Current maturities of long-term debt	120		109
Due to Related Parties, Current	0		105
Short-term debt	97		0
Accounts and wages payable	405		387
Accounts payable - affiliates	56		30
Taxes accrued	32		220
Interest accrued	58		57
Current regulatory liabilities	18		57
Other current liabilities	117		82
Total current liabilities	903		1,047
Long-term Debt, Net	3,879		3,648
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	2,806		2,524
Accumulated deferred investment tax credits	61		59
Regulatory liabilities	1,147		1,041
Asset retirement obligations	389		366
Pension and other postretirement benefits	274		189
Other deferred credits and liabilities	30		37
Total deferred credits and other liabilities	4,707		4,216
Commitments and Contingencies (Notes 2, 10, 14 and 15)
Stockholders' Equity:
Common stock	511		511
Other paid-in capital, principally premium on common stock	1,569		1,560
Preferred stock not subject to mandatory redemption	80		80
Retained earnings	1,892		1,842
Total stockholders' equity	4,052		3,993
TOTAL LIABILITIES AND EQUITY	13,541		12,904
Ameren Illinois Company
Current Assets:
Cash and cash equivalents	1		1
Accounts receivable - trade (less allowance for doubtful accounts)	212		201
Accounts receivable - affiliates	22		0
Unbilled revenue	119		135
Miscellaneous accounts and notes receivable	9		13
Materials and supplies	177		174
Current regulatory assets	129		38
Current accumulated deferred income taxes, net	160		45
Other current assets	15		26
Total current assets	844		633
Property, Plant and Equipment, Net	6,165		5,589
Investments and Other Assets:
Goodwill	411		411
Regulatory assets	883		701
Other assets	78		120
Total investments and other assets	1,372		1,232
TOTAL ASSETS	8,381		7,454
Current Liabilities:
Current maturities of long-term debt			0
Borrowings from money pool	15		56
Short-term debt	32
Accounts and wages payable	207		243
Accounts payable - affiliates	50		18
Taxes accrued	17		23
Customer deposits	77		79
Mark-to-market derivative liabilities	42		36
Environmental remediation	52		43
Current regulatory liabilities	84		159
Other current liabilities	124		114
Total current liabilities	700		771
Long-term Debt, Net	2,241		1,856
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,408		1,116
Accumulated deferred investment tax credits	3		4
Regulatory liabilities	703		664
Pension and other postretirement benefits	277		197
Accrued Environmental Loss Contingencies, Noncurrent	199		232
Other deferred credits and liabilities	189		166
Total deferred credits and other liabilities	2,779		2,379
Commitments and Contingencies (Notes 2, 10, 14 and 15)
Stockholders' Equity:
Common stock	0		0
Other paid-in capital, principally premium on common stock	1,980		1,965
Preferred stock not subject to mandatory redemption	62		62
Retained earnings	611		410
Accumulated other comprehensive income (loss)	8		11
Total stockholders' equity	2,661		2,448
TOTAL LIABILITIES AND EQUITY	$8,381		$7,454
[1]	(a)Amounts in Ameren and Ameren Missouri include two CTs under separate capital lease agreements. The gross cumulative asset value of those agreements was $233 million and $228 million at December 31, 2014 and 2013, respectively. The total accumulated depreciation associated with the two CTs was $66 million and $56 million at December 31, 2014 and 2013, respectively. In addition, Ameren Missouri has investments in debt securities, which were classified as held-to-maturity, related to the two CTs from the city of Bowling Green and Audrain County. As of December 31, 2014 and 2013, the carrying value of these debt securities was $294 million and $299 million, respectively.

Consolidated_Balance_Sheet_Par

Consolidated Balance Sheet (Parenthetical) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, except Share data, unless otherwise specified
Accounts receivable - trade, allowance for doubtful accounts	$21	$18
Common stock, par value	$0.01	$0.01
Common stock, shares authorized	400,000,000	400,000,000
Common stock, shares outstanding	242,600,000	242,600,000
Union Electric Company
Accounts receivable - trade, allowance for doubtful accounts	8	5
Common stock, par value	$5	$5
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares outstanding	102,100,000	102,100,000
Ameren Illinois Company
Accounts receivable - trade, allowance for doubtful accounts	$13	$13
Common Stock, No Par Value
Common stock, shares authorized	45,000,000	45,000,000
Common stock, shares outstanding	25,500,000	25,500,000

Consolidated_Statement_Of_Cash

Consolidated Statement Of Cash Flows (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Cash Flows From Operating Activities:
Net income (loss)	$592		$295		($974)
(Income) Loss from discontinued operations, net of tax	1		223		1,496
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	710		666		633
Amortization of nuclear fuel	81		71		83
Amortization of debt issuance costs and premium/discounts	22		24		20
Deferred income taxes and investment tax credits, net	451		410		257
Allowance for equity funds used during construction	-34	[1]	-37	[1]	-36	[1]
Share-based Compensation	25		27		29
Other	-24		23		-7
Changes in assets and liabilities:
Receivables	31		-60		30
Materials and supplies	3		60		-28
Accounts and wages payable	10		81		-34
Taxes accrued	-44		-195		-4
Regulatory assets and liabilities	-281		29		14
Assets, other	30		20		40
Liabilities, other	-28		-14		5
Pension and other postretirement benefits	-10		-28		-23
Counterparty collateral, net	22		41		41
Premiums paid on long-term debt repurchases					-138
Net cash provided by operating activities - continuing operations	1,557		1,636		1,404
Net cash provided by operating activities - discontinued operations	-6		57		286
Net cash provided by operating activities	1,551		1,693		1,690
Cash Flows From Investing Activities:
Capital expenditures	-1,785		-1,379		-1,063
Nuclear fuel expenditures	-74		-45		-91
Purchases of securities - nuclear decommissioning trust fund	-405		-214		-403
Sales and maturities of securities - nuclear decommissioning trust fund	391		196		384
Proceeds from Collection of Notes Receivable	95		6
Contributions to Note Receivable	-89		-5
Other	11		1		20
Net cash used in investing activities - continuing operations	-1,856		-1,440		-1,153
Net cash used in investing activities - discontinued operations	139		-283		-157
Net cash used in investing activities	-1,717		-1,723		-1,310
Cash Flows From Financing Activities:
Dividends on common stock	-390		-388		-382
Dividends paid to noncontrolling interest holders	-6		-6		-6
Short-term debt and credit facility repayments, net	346		368		-148
Redemptions, repurchases, and maturities:
Long-term debt	-697		-399		-760
Issuances:
Long-term debt	898		278		882
Capital issuance costs	-11		-2		-16
Other	1				4
Net cash provided by (used in) financing activities	141		-149		-426
Net change in cash and cash equivalents	-25		-179		-46
Cash and cash equivalents at beginning of year	30		209		255
Cash and cash equivalents at end of year	5		30		209
Less: Cash and cash equivalents of discontinued operations at end of year					25
Cash and cash equivalents at beginning of year	30		184
Cash and cash equivalents at end of year	5		30		184
Noncash financing activity – dividends on common stock					7
Cash Paid (Refunded) During the Year:
Interest net of capitalized	333		393		433
Income taxes, net	-27		8		1
Union Electric Company
Cash Flows From Operating Activities:
Net income (loss)	393		398		419
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	442		419		407
Amortization of nuclear fuel	81		71		83
FAC prudence review charge			26
Amortization of debt issuance costs and premium/discounts	7		7		6
Deferred income taxes and investment tax credits, net	245		65		287
Allowance for equity funds used during construction	-32		-31		-31
Other	3		1		8
Changes in assets and liabilities:
Receivables	-10		-59		27
Materials and supplies	8		45		-48
Accounts and wages payable	25		42		-27
Taxes accrued	-197		100		-46
Regulatory assets and liabilities	-68		68		-50
Assets, other	52		18		15
Liabilities, other			-29		14
Pension and other postretirement benefits	1		2		2
Premiums paid on long-term debt repurchases					-62
Net cash provided by operating activities	950		1,143		1,004
Cash Flows From Investing Activities:
Capital expenditures	-747		-648		-595
Nuclear fuel expenditures	-74		-45		-91
Purchases of securities - nuclear decommissioning trust fund	-405		-214		-403
Sales and maturities of securities - nuclear decommissioning trust fund	391		196		384
Money pool advances, net			-24		24
Tax grants received related to renewable energy properties					18
Other	-2				8
Net cash used in investing activities	-837		-687		-703
Cash Flows From Financing Activities:
Dividends on common stock	-340		-460		-400
Return of capital to parent	-215
Dividends on preferred stock	-3		-3		-3
Money pool borrowings, net	-105		105
Short-term debt and credit facility repayments, net	97
Redemptions, repurchases, and maturities:
Long-term debt	-109		-249		-427
Issuances:
Long-term debt	350				482
Capital issuance costs	-3				-7
Capital contribution from parent	215		4		1
Net cash provided by (used in) financing activities	-113		-603		-354
Net change in cash and cash equivalents			-147		-53
Cash and cash equivalents at beginning of year	1		148		201
Cash and cash equivalents at end of year	1		1		148
Noncash Or Part Noncash Capital Contribution From Parent	9
Cash Paid (Refunded) During the Year:
Interest net of capitalized	203		212		220
Income taxes, net	215		86		-3
Ameren Illinois Company
Cash Flows From Operating Activities:
Net income (loss)	204		163		144
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	259		238		214
Amortization of debt issuance costs and premium/discounts	13		15		11
Deferred income taxes and investment tax credits, net	196		104		104
Allowance for equity funds used during construction	-2		-6		-5
Other	-19		4		-11
Changes in assets and liabilities:
Receivables	-13		50		23
Materials and supplies	-4		15		20
Accounts and wages payable	7		19		-21
Taxes accrued	-7		28		3
Regulatory assets and liabilities	-215		-35		64
Assets, other	15		5		19
Liabilities, other	1		10		11
Pension and other postretirement benefits	-6		-8		-26
Counterparty collateral, net	14		43		40
Premiums paid on long-term debt repurchases					-76
Net cash provided by operating activities	445		651		519
Cash Flows From Investing Activities:
Capital expenditures	-835		-701		-442
Other	7		6		5
Net cash used in investing activities	-828		-695		-437
Cash Flows From Financing Activities:
Dividends on common stock			-110		-189
Dividends on preferred stock	-3		-3		-3
Money pool borrowings, net	-41		32		24
Short-term debt and credit facility repayments, net	32
Redemptions, repurchases, and maturities:
Long-term debt	-163		-150		-333
Issuances:
Long-term debt	548		278		400
Capital issuance costs	-6		-2		-6
Capital contribution from parent	15
Other	1				4
Net cash provided by (used in) financing activities	383		45		-103
Net change in cash and cash equivalents			1		-21
Cash and cash equivalents at beginning of year	1				21
Cash and cash equivalents at end of year	1		1
Cash Paid (Refunded) During the Year:
Interest net of capitalized	110		112		125
Income taxes, net	($44)		($23)		($22)
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Consolidated_Statement_Of_Cash1

Consolidated Statement Of Cash Flows (Parenthetical) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Capitalized interest	$18	$37	$30
Union Electric Company
Capitalized interest	16	16	15
Ameren Illinois Company
Capitalized interest	$2	$4	$2

Consolidated_Statement_Of_Stoc

Consolidated Statement Of Stockholders' Equity (USD $)	Total	Common Stock	Other Paid-In Capital	Retained Earnings	Derivative Financial Instruments	Deferred Retirement Benefit Costs	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interest	Total Ameren Corporation Stockholders' Equity	Union Electric Company	Union Electric Company	Union Electric Company	Union Electric Company	Union Electric Company	Ameren Illinois Company	Ameren Illinois Company	Ameren Illinois Company	Ameren Illinois Company	Ameren Illinois Company	Ameren Illinois Company
In Millions, unless otherwise specified											Common Stock	Other Paid-In Capital	Preferred Stock Not Subject To Mandatory Redemption	Retained Earnings		Other Paid-In Capital	Preferred Stock Not Subject To Mandatory Redemption	Retained Earnings	Deferred Retirement Benefit Costs	Accumulated Other Comprehensive Income (Loss)
Beginning of year at Dec. 31, 2011		$2	$5,598	$2,369	$7	($57)		$149				$1,555		$1,891		$1,965	$62	$408	$17
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	-974									419				419	144			144
Net income (loss) attributable to Ameren Corporation	-974			-974						419					144
Stock-based compensation activity			18
Capital contribution from parent										1		1				0
Common stock dividends				-389										-400				-189
Preferred stock dividends														-3				-3
Change in derivative financial instruments					18
Divestiture of derivative financial instruments					0
Change in deferred retirement benefit costs	8					24									3					-3
Divestiture of deferred retirement benefit costs						0
Net income attributable to noncontrolling interest holder								0
Dividends paid to noncontrolling interest holders								-6
Divestiture of noncontrolling interest								0
Other								8
Dividends	0
Stockholders' equity, end of year at Dec. 31, 2012									6,616	4,054					2,401
End of year at Dec. 31, 2012	6,767	2	5,616	1,006	25	-33	-8	151			511	1,556	80	1,907		1,965	62	360	14	14
End of year (shares) at Dec. 31, 2012	242.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	-143
Net income (loss) attributable to Ameren Corporation	-145									41					32
Stockholders' equity, end of year at Mar. 31, 2013
Beginning of year at Dec. 31, 2012	6,767	2	5,616	1,006	25	-33	-8	151			511	1,556	80	1,907		1,965	62	360	14	14
Beginning of year (shares) at Dec. 31, 2012	242.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	295									398				398	163			163
Net income (loss) attributable to Ameren Corporation	289			289						398					163
Stock-based compensation activity			16
Capital contribution from parent										4		4				0
Common stock dividends				-388										-460				-110
Preferred stock dividends														-3				-3
Change in derivative financial instruments					-21
Divestiture of derivative financial instruments					-4
Change in deferred retirement benefit costs	-30					29									3					-3
Divestiture of deferred retirement benefit costs						7
Net income attributable to noncontrolling interest holder								6
Dividends paid to noncontrolling interest holders								-6
Divestiture of noncontrolling interest								-9
Other								0
Dividends	0
Stockholders' equity, end of year at Dec. 31, 2013	6,544								6,544	3,993					2,448
End of year at Dec. 31, 2013	6,686	2	5,632	907	0	3	3	142			511	1,560	80	1,842		1,965	62	410	11	11
End of year (shares) at Dec. 31, 2013	242.6									102.1					25.5
Beginning of year at Sep. 30, 2013
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	38
Net income (loss) attributable to Ameren Corporation	37									33					22
Stockholders' equity, end of year at Dec. 31, 2013	6,544								6,544	3,993					2,448
End of year at Dec. 31, 2013	6,686	2					3				511		80				62		11
End of year (shares) at Dec. 31, 2013	242.6									102.1					25.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	98
Net income (loss) attributable to Ameren Corporation	96									48					54
Stockholders' equity, end of year at Mar. 31, 2014
Beginning of year at Dec. 31, 2013	6,686	2	5,632	907	0	3	3	142			511	1,560	80	1,842		1,965	62	410	11	11
Beginning of year (shares) at Dec. 31, 2013	242.6									102.1					25.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	592									393				393	204			204
Net income (loss) attributable to Ameren Corporation	586			586						393					204
Stock-based compensation activity			-15
Capital contribution from parent										215		224			15	15
Common stock dividends				-390										-340				0
Preferred stock dividends														-3				-3
Change in derivative financial instruments					0
Divestiture of derivative financial instruments					0
Change in deferred retirement benefit costs	12					-12									3					-3
Divestiture of deferred retirement benefit costs						0
Net income attributable to noncontrolling interest holder								6
Dividends paid to noncontrolling interest holders								-6
Divestiture of noncontrolling interest								0
Other								0
Return of capital to parent										215		-215
Stockholders' equity, end of year at Dec. 31, 2014	6,713								6,713	4,052					2,661
End of year at Dec. 31, 2014	6,855	2	5,617	1,103	0	-9	-9	142			511	1,569	80	1,892		1,980		611	8	8
End of year (shares) at Dec. 31, 2014	242.6									102.1					25.5
Beginning of year at Sep. 30, 2014
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	49
Net income (loss) attributable to Ameren Corporation	48									-5					46
Stockholders' equity, end of year at Dec. 31, 2014	6,713								6,713	4,052					2,661
End of year at Dec. 31, 2014	$6,855	$2					($9)				$511		$80						$8
End of year (shares) at Dec. 31, 2014	242.6									102.1					25.5

Summary_Of_Significant_Account

Summary Of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
	General
	Ameren, headquartered in St. Louis, Missouri, is a public utility holding company under PUHCA 2005, administered by the FERC. Ameren’s primary assets are its equity interests in its subsidiaries, including Ameren Missouri and Ameren Illinois. Ameren’s subsidiaries are separate, independent legal entities with separate businesses, assets, and liabilities. Dividends on Ameren’s common stock and the payment of other expenses by Ameren depend on distributions made to it by its subsidiaries. Ameren’s principal subsidiaries are listed below.
	•	Union Electric Company, doing business as Ameren Missouri, operates a rate-regulated electric generation, transmission, and distribution business and a rate-regulated natural gas transmission and distribution business in Missouri. Ameren Missouri was incorporated in Missouri in 1922 and is successor to a number of companies, the oldest of which was organized in 1881. It is the largest electric utility in the state of Missouri. It supplies electric and natural gas service to a 24,000-square-mile area in central and eastern Missouri. This area has an estimated population of 2.8 million and includes the Greater St. Louis area. Ameren Missouri supplies electric service to 1.2 million customers and natural gas service to 127,000 customers.
	•	Ameren Illinois Company, doing business as Ameren Illinois, operates rate-regulated electric and natural gas transmission and distribution businesses in Illinois. Ameren Illinois was created by the merger of CILCO and IP with and into CIPS in 2010. CIPS was incorporated in Illinois in 1923 and was the successor to a number of companies, the oldest of which was organized in 1902. Ameren Illinois supplies electric and natural gas utility service to portions of central and southern Illinois having an estimated population of 3.1 million in an area of 40,000 square miles. Ameren Illinois supplies electric service to 1.2 million customers and natural gas service to 813,000 customers.
	Ameren has various other subsidiaries responsible for activities such as the provision of shared services. Ameren also has a subsidiary, ATXI, that operates a FERC rate-regulated electric transmission business. ATXI is developing MISO-approved electric transmission projects, including the Illinois Rivers, Spoon River, and Mark Twain projects. Ameren is also pursuing reliability projects within Ameren Missouri's and Ameren Illinois' service territories as well as competitive electric transmission investment opportunities outside of these territories, including investments outside of MISO.
	In December 2013, Ameren completed the divestiture of New AER to IPH. In January 2014, Medina Valley completed its sale of the Elgin, Gibson City, and Grand Tower gas-fired energy centers to Rockland Capital. In addition, in 2013, Ameren abandoned the Meredosia and Hutsonville energy centers upon the completion of the divestiture of New AER to IPH. Ameren has begun to demolish the Hutsonville energy center and expects to demolish the Meredosia energy center thereafter. As a result of these events, Ameren segregated New AER’s and the Elgin, Gibson City, Grand Tower, Meredosia, and Hutsonville energy centers’ operating results, assets, and liabilities and presented them separately as discontinued operations for all periods presented in this report. Unless otherwise stated, these notes to the financial statements exclude discontinued operations for all periods presented. See Note 16 – Divestiture Transactions and Discontinued Operations for additional information regarding these transactions.
	The financial statements of Ameren are prepared on a consolidated basis, and therefore include the accounts of its majority-owned subsidiaries. Ameren Missouri and Ameren Illinois have no subsidiaries and therefore their financial statements are not prepared on a consolidated basis. All intercompany transactions have been eliminated. All tabular dollar amounts are in millions, unless otherwise indicated.
	Our accounting policies conform to GAAP. Our financial statements reflect all adjustments (which include normal, recurring adjustments) that are necessary, in our opinion, for a fair presentation of our results. The preparation of financial statements in conformity with GAAP requires management to make certain estimates and assumptions. Such estimates and assumptions affect reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the dates of financial statements, and the reported amounts of revenues and expenses during the reported periods. Actual results could differ from those estimates.
	Regulation
	We are regulated by the MoPSC, the ICC, and the FERC. We defer certain costs as assets pursuant to actions of rate regulators or because of expectations that we will be able to recover such costs in rates charged to customers. We also defer certain amounts as liabilities pursuant to actions of rate regulators or based on the expectation that such amounts will be returned to customers in future rates. Regulatory assets and liabilities are amortized consistent with the period of expected regulatory treatment. In addition to the cost recovery mechanisms discussed in the Purchased Gas, Power, and Fuel Rate-adjustment Mechanisms section below, Ameren Missouri and Ameren Illinois have approvals from regulators to use other cost recovery mechanisms. Ameren Missouri has a vegetation management and infrastructure inspection cost tracker, a pension and postretirement benefit cost tracker, an uncertain tax positions tracker, a renewable energy standards cost tracker, a solar rebate program tracker, a storm restoration cost tracker, and the MEEIA energy efficiency rider. Ameren Illinois' and ATXI's electric transmission rates are subject to formula ratemaking. Additionally, Ameren Illinois' electric distribution business participates in the performance-based formula ratemaking process established pursuant to the IEIMA. Ameren Illinois also has an environmental cost rider, an asbestos-related litigation rider, an energy efficiency rider, and a bad debt rider. See Note 2 – Rate and Regulatory Matters for additional information on regulatory assets and liabilities.
	Cash and Cash Equivalents
	Cash and cash equivalents include cash on hand and temporary investments purchased with an original maturity of three months or less.
	Allowance for Doubtful Accounts Receivable
	The allowance for doubtful accounts represents our estimate of existing accounts receivable that will ultimately be uncollectible. The allowance is calculated by applying estimated loss factors to various classes of outstanding receivables, including unbilled revenue. The loss factors used to estimate uncollectible accounts are based upon both historical collections experience and management’s estimate of future collections success given the existing and anticipated future collections environment. Ameren Illinois has a rate mechanism that adjusts rates for net write-offs of customer accounts receivable above or below those being collected in rates.
	Materials and Supplies
	Materials and supplies are recorded at the lower of cost or market. Cost is determined by the average-cost method. Materials and supplies are capitalized as inventory when purchased and then expensed or capitalized as plant assets when installed, as appropriate. The following table presents a breakdown of materials and supplies for each of the Ameren Companies at December 31, 2014 and 2013:
			Ameren Missouri				Ameren Illinois				Ameren
	2014
	Fuel(a)		$	134			$	—			$	134
	Gas stored underground		16				111				127
	Other materials and supplies		197				66				263
			$	347			$	177			$	524
	2013
	Fuel(a)		$	144			$	—			$	144
	Gas stored underground		17				110				127
	Other materials and supplies		191				64				255
			$	352			$	174			$	526
	(a)	Consists of coal, oil, and propane.
	Property and Plant, Net
	We capitalize the cost of additions to and betterments of units of property and plant. The cost includes labor, material, applicable taxes, and overhead. An allowance for funds used during construction, as discussed below, is also capitalized as a cost of our rate-regulated assets. Maintenance expenditures, including nuclear refueling and maintenance outages, are expensed as incurred. When units of depreciable property are retired, the original costs, less salvage values, are charged to accumulated depreciation. If environmental expenditures are related to assets currently in use, as in the case of the installation of pollution control equipment, the cost is capitalized and depreciated over the expected life of the asset. See Asset Retirement Obligations below and Note 3 – Property and Plant, Net, for additional information.
	Depreciation
	Depreciation is provided over the estimated lives of the various classes of depreciable property by applying composite rates on a straight-line basis to the cost basis of such property. The provision for depreciation for the Ameren Companies in 2014, 2013, and 2012 ranged from 3% to 4% of the average depreciable cost.
	Allowance for Funds Used During Construction
	We capitalize allowance for funds used during construction, or the cost of borrowed funds and the cost of equity funds (preferred and common stockholders’ equity) applicable to rate-regulated construction expenditures, in accordance with the utility industry's accounting practice. Allowance for funds used during construction does not represent a current source of cash funds. This accounting practice offsets the effect on earnings of the cost of financing during construction, and it treats such financing costs in the same manner as construction charges for labor and materials.
	Under accepted ratemaking practice, cash recovery of allowance for funds used during construction and other construction costs occurs when completed projects are placed in service and reflected in customer rates. The following table presents the annual allowance for funds used during construction rates that were used during 2014, 2013, and 2012:
		2014			2013			2012
	Ameren Missouri	7	%		8	%		8	%
	Ameren Illinois	2	%		8	%		9	%
	Goodwill
	Goodwill represents the excess of the purchase price of an acquisition over the fair value of the net assets acquired. Ameren and Ameren Illinois had goodwill of $411 million at December 31, 2014, and 2013.
	All of Ameren's and Ameren Illinois' goodwill at December 31, 2014 and 2013, was assigned to the Ameren Illinois reporting unit, which is also the Ameren Illinois reportable segment.
	We evaluate goodwill for impairment as of October 31 of each year, or more frequently if events and circumstances indicate that the asset might be impaired. Ameren and Ameren Illinois applied a qualitative goodwill evaluation model for their annual goodwill impairment test conducted as of October 31, 2014. Based on the results of Ameren’s and Ameren Illinois’ qualitative assessment, Ameren and Ameren Illinois believe it was more likely than not that the fair value of the Ameren Illinois reporting unit exceeded its carrying value as of October 31, 2014, indicating no impairment of Ameren’s or Ameren Illinois’ goodwill. The following factors, among others, were considered by Ameren and Ameren Illinois when assessing whether it was more likely than not that the fair value of the Ameren Illinois reporting unit exceeded its carrying value for the October 31, 2014, test:
	•	macroeconomic conditions, including those conditions within Ameren Illinois’ service territory;
	•	pending rate case outcomes and projections of future rate case outcomes;
	•	changes in laws and potential law changes;
	•	observable industry market multiples;
	•	achievement of IEIMA performance metrics and the yield of 30-year United States Treasury bonds;
	•	a potential reduction in the FERC-allowed return on equity related to transmission services; and
	•	actual and forecasted financial performance.
	The goodwill assigned to the Ameren Illinois reporting unit on the December 31, 2014 balance sheets of Ameren and Ameren Illinois had no accumulated goodwill impairment losses. Ameren and Ameren Illinois will continue to monitor the actual and forecasted operating results, cash flows, market capitalization, and observable industry market multiples of the Ameren Illinois reporting unit for signs of possible declines in estimated fair value and potential goodwill impairment.
	Impairment of Long-lived Assets
	We evaluate long-lived assets classified as held and used for impairment when events or changes in circumstances indicate that the carrying value of such assets may not be recoverable. Whether an impairment has occurred is determined by comparing the estimated undiscounted cash flows attributable to the assets to the carrying value of the assets. If the carrying value exceeds the undiscounted cash flows, we recognize an impairment charge equal to the amount by which the carrying value exceeds the estimated fair value of the assets. In the period in which we determine an asset meets held for sale criteria, we record an impairment charge to the extent the book value exceeds its estimated fair value less cost to sell.
	Investments
	Ameren and Ameren Missouri record investments held in Ameren Missouri's nuclear decommissioning trust fund at fair value. Losses on assets in the trust fund could result in higher funding requirements for decommissioning costs, which Ameren Missouri believes would be recovered in electric rates paid by its customers. Accordingly, Ameren and Ameren Missouri recognize a regulatory asset on their balance sheets for losses on investments held in the nuclear decommissioning trust fund. In addition, Ameren and Ameren Missouri recognize a regulatory liability on their balance sheets for gains on investments held in the nuclear decommissioning trust fund. As of December 31, 2014, the nuclear decommissioning trust fund had cumulative gains. See Note 9 – Nuclear Decommissioning Trust Fund Investments for additional information.
	Environmental Costs
	Liabilities for environmental costs are recorded on an undiscounted basis when it is probable that a liability has been incurred and the amount of the liability can be reasonably estimated. Costs are expensed or deferred as a regulatory asset when it is expected that the costs will be recovered from customers in future rates.
	Asset Retirement Obligations
	We are required to record the estimated fair value of legal obligations associated with the retirement of tangible long-lived assets in the period in which the liabilities are incurred and to capitalize a corresponding amount as part of the book value of the related long-lived asset. In subsequent periods, we are required to make adjustments to AROs based on changes in the estimated fair values of the obligations. Corresponding increases in asset book values are depreciated over the remaining useful life of the related asset. Uncertainties as to the probability, timing, or amount of cash expenditures associated with AROs affect our estimates of fair value. Ameren and Ameren Missouri have recorded AROs for retirement costs associated with Ameren Missouri’s Callaway energy center decommissioning costs, asbestos removal, CCR facilities, and river structures. Also, Ameren and Ameren Illinois have recorded AROs for retirement costs associated with asbestos removal. In addition, Ameren, Ameren Missouri, and Ameren Illinois have recorded AROs for the disposal of certain transformers. Ameren and Ameren Missouri are evaluating the potential effect of the EPA's new rule regarding the management and disposal of CCR on their AROs associated with ash ponds. See Note 15 – Commitments and Contingencies.
	Asset removal costs accrued by our rate-regulated operations that do not constitute legal obligations are classified as regulatory liabilities. See Note 2 – Rate and Regulatory Matters.
	The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the years ended December 31, 2014 and 2013:
		Ameren				Ameren				Ameren
		Missouri				Illinois
	Balance at December 31, 2012	$	346			$	3			$	349
Liabilities settled	(1		)		(a)				(1		)
Accretion in 2013(b)	19				(a)				19
Change in estimates(c)	2				(a)				2
Balance at December 31, 2013	$	366			$	3		(d)	$	369
Liabilities incurred	2				—				2
Liabilities settled	(2		)		(a)				(2		)
Accretion in 2014(b)	21				(a)				21
Change in estimates(c)(e)	2				4				6
Balance at December 31, 2014	$	389			$	7		(d)	$	396
(a)	Less than $1 million.
(b)	Accretion expense was recorded as an increase to regulatory assets at Ameren Missouri and Ameren Illinois.
(c)	Ameren Missouri changed its fair value estimates for asbestos removal in 2013 and 2014 and for certain CCR facilities in 2013.
(d)	Included in “Other deferred credits and liabilities” on the balance sheet.
(e)	Ameren Illinois changed its fair value estimate for asbestos removal in 2014.
Ameren and Ameren Missouri have nuclear decommissioning trust fund assets of $549 million and $494 million as of December 31, 2014 and 2013, respectively, which are for decommissioning of the Callaway energy center.
See Note 16 – Divestiture Transactions and Discontinued Operations for additional information on the AROs related to the abandoned Meredosia and Hutsonville energy centers, which are presented as discontinued operations and therefore not included in the table above.
Noncontrolling Interests
As of December 31, 2014 and 2013, Ameren’s noncontrolling interests included the preferred stock of Ameren Missouri and Ameren Illinois.
Operating Revenue
The Ameren Companies record operating revenue for electric or natural gas service when it is delivered to customers. We accrue an estimate of electric and natural gas revenues for service rendered but unbilled at the end of each accounting period.
Ameren Illinois participates in the performance-based formula ratemaking framework pursuant to the IEIMA. The IEIMA provides for an annual reconciliation of Ameren Illinois' electric delivery service revenue requirement. As of each balance sheet date, Ameren Illinois records its estimate of the electric delivery service revenue effect resulting from the reconciliation of the revenue requirement necessary to reflect the actual recoverable costs incurred for that year with the revenue requirement that was reflected in customer rates for that year. If the current year's revenue requirement is greater than the revenue requirement reflected in that year's customer rates, an increase to electric operating revenues with an offset to a regulatory asset is recorded to reflect the expected recovery of those additional costs from customers within the next two years. If the current year's revenue requirement is less than the revenue requirement reflected in that year's customer rates, a reduction to electric operating revenues with an offset to a regulatory liability is recorded to reflect the expected refund to customers within the next two years. See Note 2 – Rate and Regulatory Matters for information regarding Ameren Illinois' revenue requirement reconciliation pursuant to the IEIMA.
Similar to the IEIMA process described above, Ameren Illinois and ATXI record the impact of a revenue requirement reconciliation for each company's electric transmission jurisdiction, pursuant to FERC-approved rate treatment.
Accounting for MISO Transactions
MISO-related purchase and sale transactions are recorded by Ameren, Ameren Missouri, and Ameren Illinois using settlement information provided by MISO. Ameren Missouri records these purchase and sale transactions on a net hourly position. Ameren Missouri records net purchases in a single hour in “Operating Expenses – Purchased power” and net sales in a single hour in “Operating Revenues – Electric” in its statement of income. Ameren Illinois records net purchases in “Operating Expenses – Purchased power” in its statement of income to reflect all of its MISO transactions relating to the procurement of power for its customers. On occasion, Ameren Missouri's and Ameren Illinois' prior-period transactions will be resettled outside the routine settlement process because of a change in MISO’s tariff or a material interpretation thereof. In these cases, Ameren Missouri and Ameren Illinois recognize expenses associated with resettlements once the resettlement is probable and the resettlement amount can be estimated and recognize revenues once the resettlement amount is received.
Nuclear Fuel
Ameren Missouri’s cost of nuclear fuel is capitalized and then amortized to fuel expense on a unit-of-production basis. The cost is charged to "Operating Expenses – Fuel" in the statement of income.
Purchased Gas, Power and Fuel Rate-adjustment Mechanisms
Ameren Missouri and Ameren Illinois have various rate-adjustment mechanisms in place that provide for the recovery of purchased natural gas and electric fuel and purchased power costs without a traditional rate case proceeding. See Note 2 – Rate and Regulatory Matters for the regulatory assets and liabilities recorded at December 31, 2014 and 2013, related to the rate-adjustment mechanisms discussed below.
In Ameren Missouri’s and Ameren Illinois’ natural gas utility jurisdictions, changes in natural gas costs are reflected in billings to their natural gas utility customers through PGA clauses. The difference between actual natural gas costs and costs billed to customers in a given period is deferred as a regulatory asset or liability. The deferred amount is either billed or refunded to natural gas utility customers in a subsequent period.
In Ameren Illinois’ retail electric utility jurisdiction, changes in purchased power and transmission service costs are reflected in billings to its electric utility customers through pass-through rate-adjustment clauses. The difference between actual purchased power and transmission service costs and costs billed to customers in a given period is deferred as a regulatory asset or liability. The deferred amount is either billed or refunded to electric utility customers in a subsequent period.
Ameren Missouri has a FAC that allows an adjustment of electric rates three times per year for a pass-through to customers of 95% of changes in fuel and purchased power costs, including transportation charges and revenues, net of off-system sales, greater or less than the amount set in base rates, subject to MoPSC prudence review. The difference between the actual amounts incurred for these items and the amounts recovered from Ameren Missouri customers' base rates is deferred as a regulatory asset or liability. The deferred amounts are either billed or refunded to electric utility customers in a subsequent period.
Stock-based Compensation
Stock-based compensation cost is measured at the grant date based on the fair value of the award, net of an assumed forfeiture rate. Ameren recognizes as compensation expense the estimated fair value of stock-based compensation on a straight-line basis over the requisite service period. See Note 12 – Stock-based Compensation for additional information.
Excise Taxes
Ameren Missouri and Ameren Illinois collect certain excise taxes from customers that are levied on the sale or distribution of natural gas and electricity. Excise taxes are levied on Ameren Missouri's electric and natural gas businesses and on Ameren Illinois' natural gas business and are recorded gross in “Operating Revenues – Electric,” “Operating Revenues – Gas,” and “Operating Expenses – Taxes other than income taxes” on the statement of income or the statement of income and comprehensive income. Excise taxes for electric service in Illinois are levied on the customer and are therefore not included in Ameren Illinois' revenues and expenses. They are instead included in “Taxes accrued” on the balance sheet. The following table presents excise taxes recorded in “Operating Revenues – Electric,” “Operating Revenues – Gas,” and “Operating Expenses – Taxes other than income taxes” for the years ended December 31, 2014, 2013, and 2012:
	2014				2013				2012
Ameren Missouri	$	151			$	152			$	139
Ameren Illinois	64				61				54
Ameren	$	215			$	213			$	193
Unamortized Debt Discounts, Premiums, and Issuance Costs
Long-term debt discounts, premiums, and issuance costs are amortized over the lives of the related issuances. Credit facility fees are amortized over the credit facility term.
Income Taxes
Ameren uses an asset and liability approach for its financial accounting and reporting of income taxes, in accordance with authoritative accounting guidance. Deferred tax assets and liabilities are recognized for transactions that are treated differently for financial reporting and income tax return purposes. These deferred tax assets and liabilities are based on statutory tax rates.
We recognize that regulators will probably reduce future revenues for deferred tax liabilities that were initially recorded at rates in excess of the current statutory rate. Therefore, reductions in deferred tax liabilities that were recorded because of decreases in the statutory rate have been credited to a regulatory liability. A regulatory asset has been established to recognize the probable recovery through future customer rates of tax benefits related to the equity component of allowance for funds used during construction, as well as the effects of tax rate changes.
Investment tax credits used on tax returns for prior years have been deferred as a non-current liability. The credits are being amortized over the useful lives of the related investment. Deferred income taxes were recorded on the temporary difference represented by the deferred investment tax credits and a corresponding regulatory liability. This recognizes the expected reduction in rates for future lower income taxes associated with the amortization of the investment tax credits. See Note 13 – Income Taxes.
Ameren Missouri, Ameren Illinois, and all the other Ameren subsidiary companies are parties to a tax allocation agreement with Ameren (parent) that provides for the allocation of consolidated tax liabilities. The tax allocation agreement specifies that each party be allocated an amount of tax similar to that which would be owed or refunded had the party been separately subject to tax. Any net benefit attributable to the parent is reallocated to the other parties. This reallocation is treated as a capital contribution to the party receiving the benefit.
Earnings per Share
Basic earnings per share is computed by dividing net income attributable to Ameren Corporation common stockholders by the weighted-average number of common shares outstanding during the period. Diluted earnings per share is computed by dividing net income attributable to common stockholders by the diluted weighted-average number of common shares outstanding during the period. Diluted earnings per share reflects the potential dilution that would occur if certain stock-based performance share units were settled.
The following table presents Ameren’s basic and diluted earnings per share calculations and reconciles the weighted-average number of common shares outstanding to the diluted weighted-average number of common shares outstanding for the years ended December 31, 2014, 2013, and 2012:
	2014				2013				2012
Net income (loss) attributable to Ameren Corporation:
Continuing operations	$	587			$	512			$	516
Discontinued operations	(1		)		(223		)		(1,490		)
Net income (loss) attributable to Ameren Corporation	$	586			$	289			$	(974	)
Average common shares outstanding – basic	242.6				242.6				242.6
Assumed settlement of performance share units	1.8				1.9				0.4
Average common shares outstanding – diluted	244.4				244.5				243
Earnings (loss) per common share – basic:
Continuing operations	$	2.42			$	2.11			$	2.13
Discontinued operations	—				(0.92		)		(6.14		)
Earnings (loss) per common share – basic	$	2.42			$	1.19			$	(4.01	)
Earnings (loss) per common share – diluted:
Continuing operations	$	2.4			$	2.1			$	2.13
Discontinued operations	—				(0.92		)		(6.14		)
Earnings (loss) per common share – diluted	$	2.4			$	1.18			$	(4.01	)
There were no potentially dilutive securities excluded from the diluted earnings per share calculations for the years ended December 31, 2014, 2013, and 2012.
Capital Contributions and Return of Capital
In 2014, Ameren Missouri and Ameren Illinois received cash capital contributions of $215 million and $15 million, respectively, from Ameren (parent) as a result of the tax allocation agreement. Additionally, as of December 31, 2014, Ameren Missouri accrued a $9 million capital contribution related to the same agreement. In 2014, Ameren Missouri returned capital of $215 million to Ameren (parent).
Supplemental Cash Flow Information
The following table presents additional information regarding Ameren's consolidated statement of cash flows for the years ended December 31, 2014, 2013, and 2012:
	2014				2013				2012
Cash paid (refunded) during the year:
Interest
Continuing operations(a)	$	333			$	362			$	384
Discontinued operations(b)	—				31				49
	$	333			$	393			$	433
Income taxes, net
Continuing Operations	$	(41	)		$	116			$	10
Discontinued Operations	14				(108		)		(9		)
	$	(27	)		$	8			$	1
(a)	Net of $18 million, $20 million, and $17 million capitalized, respectively.
(b)	Net of $- million, $17 million, and $13 million capitalized, respectively.
See Note 3 – Property and Plant, Net, for information on accrued capital expenditures.
Accounting Changes and Other Matters
The following is a summary of recently adopted authoritative accounting guidance, as well as guidance issued but not yet adopted, that could affect the Ameren Companies.
Presentation of an Unrecognized Tax Benefit
In 2013, FASB issued additional authoritative accounting guidance, which became effective in 2014, to provide clarity for the financial statement presentation of an unrecognized tax benefit when a net operating loss carryforward, a similar tax loss, or a tax credit carryforward exists. The objective of this guidance is to eliminate diversity in practice related to the presentation of certain unrecognized tax benefits. It requires entities to present an unrecognized tax benefit as a reduction to a deferred tax asset for a net operating loss carryforward, a similar tax loss, or a tax credit carryforward. Previously, unrecognized tax benefits were recorded in “Other deferred credits and liabilities” on Ameren's, Ameren Missouri's, and Ameren Illinois' respective balance sheets. Beginning in 2014, unrecognized tax benefits are recorded as a reduction to the deferred tax assets for net operating losses and tax credit carryforwards within "Accumulated deferred income taxes, net" on our balance sheets. Unrecognized tax benefits that exceed these carryforwards are recorded in “Other deferred credits and liabilities” on the respective balance sheets. At December 31, 2014, unrecognized tax benefits of $52 million, $- million, and $- million were recorded in "Accumulated deferred income taxes, net" on Ameren's, Ameren Missouri's, and Ameren Illinois' balance sheets, respectively. At December 31, 2013, unrecognized tax benefits of $84 million, $15 million, and $- million previously recorded in "Other deferred credits and liabilities" on Ameren's, Ameren Missouri's, and Ameren Illinois' respective balance sheets were reclassified to "Accumulated deferred income taxes, net" for comparative purposes. The implementation of the additional authoritative accounting guidance did not affect the Ameren Companies' results of operations or liquidity, as this guidance is presentation-related only.
Reporting Discontinued Operations and Disclosures of Components of an Entity
In 2014, FASB issued authoritative accounting guidance that changes the criteria for reporting and qualifying for discontinued operations. Under the new guidance, a component of an entity, or a group of components of an entity, that either meets the criteria to be classified as held for sale or is disposed of by sale or otherwise is required to be reported in discontinued operations if the disposal represents a strategic shift that had, or will have, a major effect on an entity's operations and financial results. The guidance includes expanded disclosure requirements for discontinued operations and additional disclosures about a disposal of an individually significant component of an entity that does not qualify for discontinued operations presentation. The guidance is effective for the Ameren Companies in the first quarter of 2015 for components that are classified as held for sale or disposed of on or after January 1, 2015. Early adoption is permitted, but only for disposals or classifications as held for sale that have not been reported in financial statements previously issued. Therefore, Ameren's existing discontinued operations are not subject to the new disclosure requirements. The guidance will not affect the Ameren Companies' results of operations, financial position, or liquidity, as this guidance is presentation-related only.
Revenue from Contracts with Customers
In 2014, FASB issued authoritative accounting guidance to clarify the principles for recognizing revenue and to develop a common revenue standard for GAAP. The guidance requires an entity to recognize an amount of revenue for the transfer of promised goods or services to customers that reflects the consideration which the entity expects to be entitled to in exchange for those goods or services. The guidance also requires additional disclosures to enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. The guidance will be effective for the Ameren Companies in the first quarter of 2017. The guidance allows entities to choose one of two transition methods, either by applying the guidance retrospectively to each reporting period presented or by recording a cumulative effect adjustment to retained earnings in the period of initial adoption. The Ameren Companies are currently assessing the impacts of this guidance on their results of operations, financial position, and liquidity, as well as the transition method that they will use to adopt the guidance.

Rate_And_Regulatory_Matters

Rate And Regulatory Matters	12 Months Ended
	Dec. 31, 2014
Public Utilities, General Disclosures [Abstract]
RATE AND REGULATORY MATTERS	RATE AND REGULATORY MATTERS
	Below is a summary of significant regulatory proceedings and related lawsuits. We are unable to predict the ultimate outcome of these matters, the timing of the final decisions of the various agencies and courts, or the effect on our results of operations, financial position, or liquidity.
	Missouri
	2014 Electric Rate Case
	In July 2014, Ameren Missouri filed a request with the MoPSC seeking approval to increase its annual revenues for electric service. The request, as amended in February 2015, seeks an annual revenue increase of approximately $190 million. The amended rate request seeks recovery of increased net energy costs and rebates provided for customer-installed solar generation, as well as recovery of, and a return on, electric infrastructure investments. Approximately $100 million of the amended request relates to an increase in net energy costs above the levels included in base rates authorized by the MoPSC in its December 2012 electric rate order. Absent initiation of this general rate proceeding, 95% of those costs would have been reflected in rate adjustments implemented under Ameren Missouri’s existing FAC. The amended electric rate increase request is based on a 10.4% return on common equity, a capital structure composed of 51.8% common equity, an electric rate base of $7 billion, and a test year ended March 31, 2014, with certain pro forma adjustments through true-up dates of December 31, 2014 and January 1, 2015.
	Ameren Missouri's rate request also seeks continued use of the FAC and the regulatory tracking mechanisms for storm costs, vegetation management and infrastructure inspection costs, pension and postretirement benefits, and uncertain income tax positions that the MoPSC authorized in earlier electric rate orders.
	In October 2014, as part of this rate case proceeding, the MoOPC, the MIEC, and other parties filed a rate shift request that seeks to reduce Noranda’s electric rates with an offsetting increase in electric rates for Ameren Missouri’s other customers. Ameren Missouri supplies electricity to Noranda’s aluminum smelter in southeast Missouri under a 15-year agreement, that is subject to termination as early as 2020 upon at least five years notice by either party. Termination of the agreement by Ameren Missouri would require MoPSC approval.
	In February 2015, the MoPSC staff recommended an increase to Ameren Missouri's annual revenues of $89 million based on a return on equity of 9.25%. In addition, the MoPSC staff opposed the continued use of the regulatory tracking mechanisms for storm costs and vegetation management and infrastructure inspection costs. The MoPSC staff also opposed the recovery of $36 million in fixed costs not previously recovered associated with the accounting authority order discussed below.
	The MoPSC proceedings relating to the proposed electric service rate increase are ongoing and a decision by the MoPSC is expected by May 2015, with new rates effective by June 2015. Ameren Missouri cannot predict the level of any electric service rate change the MoPSC may approve or whether any rate increase that may eventually be approved will be sufficient for Ameren Missouri to recover its costs and to earn a reasonable return on its investments when the rate changes go into effect.
	FAC Prudence Review and Accounting Authority Order
	In July 2013, the MoPSC issued an order with respect to its review of Ameren Missouri’s FAC calculation for the period from October 1, 2009, to May 31, 2011. In this order, the MoPSC ruled that Ameren Missouri should have included in the FAC calculation all revenues and costs associated with certain long-term partial requirements sales that were made by Ameren Missouri because of the loss of Noranda's load caused by a severe ice storm in 2009. As a result of the order, in 2013 Ameren Missouri recorded a pretax charge to earnings of $26 million, including $1 million for interest, for its estimated obligation to refund to its electric customers the earnings associated with these sales previously recognized for the period from October 1, 2009, to May 31, 2011. Ameren Missouri recorded the charge to “Operating Revenues – Electric” and the related interest to “Interest Charges” with a corresponding offset to “Current regulatory liabilities.” No similar revenues were excluded from FAC calculations after May 2011.
	Separately, in July 2011, Ameren Missouri filed a request with the MoPSC for an accounting authority order that would allow Ameren Missouri to defer fixed costs totaling $36 million during the time period of March 1, 2009, to May 31, 2011, not previously recovered from Noranda as a result of the loss of load caused by the severe 2009 ice storm, for potential recovery in a future electric rate case. In November 2013, the MoPSC issued an accounting authority order that allowed Ameren Missouri to seek recovery of these fixed costs in an electric rate case. Ameren Missouri’s July 2014 electric rate case filing requested recovery of these fixed costs over five years. The MIEC and the MoOPC filed appeals of the MoPSC’s November 2013 accounting authority order with the Missouri Court of Appeals, Western District. In January 2015, the Missouri Court of Appeals, Western District upheld the MoPSC's order. Ameren Missouri has not recorded any potential revenue associated with this accounting authority order.
	MEEIA Filing
	In December 2014, Ameren Missouri filed an energy efficiency plan with the MoPSC under the MEEIA. This filing proposed a three-year plan that includes a portfolio of customer energy efficiency programs along with a cost recovery mechanism. If the plan is approved, beginning in January 2016, Ameren Missouri intends to invest $135 million over three years in the proposed customer energy efficiency programs. Ameren Missouri requested continued use of a MEEIA rider that allows it to collect from or refund to customers any difference in the actual amounts incurred and the amounts collected from customers for the MEEIA program costs and its lost revenues. In addition, Ameren Missouri requested incentives to earn additional revenues by achieving certain energy efficiency goals, including $25 million if 100% of its energy efficiency goals are achieved during the three-year period. Ameren Missouri must achieve at least 70% of its energy efficiency goals before it earns any incentive award.
	Illinois
	IEIMA
	Under the provisions of the IEIMA, Ameren Illinois’ electric delivery service rates are subject to an annual revenue requirement reconciliation to its actual costs. Throughout each year, Ameren Illinois records a regulatory asset or a regulatory liability and a corresponding increase or decrease to operating revenues for any differences between the revenue requirement reflected in customer rates for that year and its estimate of the probable increase or decrease in the revenue requirement expected to ultimately be approved by the ICC based on that year's actual costs incurred. As of December 31, 2014, Ameren Illinois had recorded regulatory assets of $101 million and $65 million to reflect its expected 2014 and 2013 revenue requirement reconciliation adjustments, respectively, with interest. As of December 31, 2013, Ameren Illinois had recorded a $65 million regulatory liability to reflect its 2012 revenue requirement reconciliation adjustment, which was refunded, with interest, to customers during 2014.
	In December 2014, the ICC issued an order in Ameren Illinois’ annual update filing approving a $204 million increase in Ameren Illinois’ electric delivery service revenue requirement beginning in January 2015. This update reflects an increase to the annual formula rate based on 2013 actual costs and expected net plant additions for 2014, an increase to include the 2013 revenue requirement reconciliation adjustment, which was recorded as a regulatory asset at December 31, 2014, and an increase resulting from the conclusion of the 2014 refund to customers for the 2012 revenue requirement reconciliation adjustment.
	In February 2014, Ameren Illinois filed an appeal of the ICC's December 2013 annual formula rate order to the Appellate Court of the Fourth District of Illinois regarding the rate treatment of accumulated deferred income taxes related to the transfer of former Ameren Missouri electric assets located in Illinois to Ameren Illinois. Ameren Illinois withdrew this appeal in February 2015.
	In the December 2013 order, the ICC disallowed, in part, the recovery from customers of the debt premium costs paid by Ameren Illinois for a tender offer in August 2012 to repurchase outstanding senior secured notes. As a result of the ICC order, in 2013, Ameren and Ameren Illinois each recorded a pretax charge to earnings of $15 million relating to the partial disallowance of the debt premium costs. In the December 2014 order discussed above, the ICC allowed partial recovery from customers of certain previously disallowed debt premium costs. Accordingly, in 2014, Ameren and Ameren Illinois each recorded a pretax increase to earnings of $11 million to reflect the partial recovery of the debt premium costs. Ameren and Ameren Illinois recorded the effects of both orders to “Interest charges” with a corresponding offset to “Regulatory assets.”
	2015 Natural Gas Delivery Service Rate Case
	In January 2015, Ameren Illinois filed a request with the ICC seeking approval to increase its annual revenues for natural gas delivery service by $53 million. The request was based on a 10.25% return on common equity, a capital structure composed of 50% common equity, and a rate base of $1.2 billion. In an attempt to reduce regulatory lag, Ameren Illinois used a 2016 future test year in this proceeding. Included in the request was a proposal to implement a decoupling rider mechanism for residential and small nonresidential customers. The decoupling rider would ensure that changes in natural gas sales volumes do not affect Ameren Illinois' annual natural gas revenues for these rate classes.
	A decision by the ICC in this proceeding is required by December 2015, with new rates expected to be effective in January 2016. Ameren Illinois cannot predict the level of any delivery service rate changes the ICC may approve, or whether the ICC will approve the decoupling rider, or whether any rate changes that may eventually be approved will be sufficient to enable Ameren Illinois to recover its costs and to earn a reasonable return on investments when the rate changes go into effect.
	2013 Natural Gas Delivery Service Rate Order
	In December 2013, the ICC issued a rate order that approved an increase in revenues for Ameren Illinois' natural gas delivery service of $32 million. The revenue increase was based on a 9.1% return on common equity, a capital structure composed of 51.7% common equity, and a rate base of $1.1 billion. The rate order was based on a 2014 future test year. The rate changes became effective January 1, 2014. In March 2014, Ameren Illinois filed with the Appellate Court of the Fourth District of Illinois an appeal of the allowed return on common equity included in the ICC's order and also appealed the rate treatment of accumulated deferred income taxes related to the transfer of former Ameren Missouri natural gas assets located in Illinois to Ameren Illinois. Ameren Illinois sought a 10.4% return on common equity in this rate case. In February 2015, Ameren Illinois withdrew its appeal solely as it related to the rate treatment of the accumulated deferred income taxes.
	ATXI Transmission Project
	ATXI's Spoon River project in northwest Illinois is a MISO-approved transmission line project with an expected cost of $150 million. In August 2014, ATXI made a filing with the ICC requesting a certificate of public convenience and necessity and project approval for the Spoon River project. A decision is expected from the ICC in 2015. A certificate of public convenience and necessity is required before ATXI can proceed with right-of-way acquisitions.
	Federal
	2011 Wholesale Distribution Rate Case
	In January 2011, Ameren Illinois filed a request with the FERC to increase its annual revenues for electric delivery service to its wholesale customers. These wholesale distribution revenues are treated as a deduction from Ameren Illinois’ revenue requirement in retail rate filings with the ICC, with no material effect on net income. In March 2011, the FERC issued an order authorizing the proposed rates to take effect, subject to refund when the final rates are determined. In September 2014, the FERC issued an order that finalized rates and resulted in refunds due to the wholesale customers. In October 2014, Ameren Illinois refunded $24 million, including interest, to the wholesale customers and requested a rehearing on certain aspects of the order.
	Ameren Illinois Electric Transmission Rate Refund
	In July 2012, the FERC issued an order concluding that Ameren Illinois improperly included acquisition premiums, including goodwill, in determining the common equity used in its electric transmission formula rate and thereby inappropriately recovered a higher amount from its electric transmission customers. The order required Ameren Illinois to make refunds to customers for such improperly included amounts. In August 2012, Ameren Illinois filed a request for a rehearing of this order.
	Ameren Illinois submitted a refund report in November 2012 and concluded that no refund was warranted. Several wholesale customers filed a protest with the FERC regarding that conclusion. In June 2013, the FERC issued an order that rejected Ameren Illinois' November 2012 refund report and provided guidance as to the filing of a new refund report. In July 2013, Ameren Illinois filed a revised refund report based on the guidance provided in the June 2013 order, as well as a request for a rehearing of that order. Ameren Illinois' July 2013 refund report also concluded that no refund was warranted.
	In June 2014, the FERC issued an order that denied Ameren Illinois’ rehearing requests of the July 2012 order and the June 2013 order. Separately, in June 2014, the FERC issued an order establishing settlement procedures and, if necessary, hearing procedures regarding Ameren Illinois’ July 2013 refund report. In July 2014, Ameren Illinois filed an appeal of the FERC order denying rehearing of the July 2012 and June 2013 orders with the United States Court of Appeals for the District of Columbia Circuit. Also in July 2014, Ameren Illinois filed a request for rehearing with the FERC of its June 2014 order regarding the July 2013 refund report. In November 2014, the United States Court of Appeals for the District of Columbia issued an order suspending the appeal until the related FERC proceedings have been completed.
	Ameren Illinois estimates the maximum pretax charge to earnings for this possible refund obligation through December 31, 2014, is $22 million. Ameren and Ameren Illinois recorded a current liability representing their estimate of the probable refund due to electric transmission customers based on the June 2014 order. If Ameren Illinois was to determine that a refund to its electric transmission customers in excess of the amount already recorded is probable, an additional charge to earnings would be recorded in the period in which that determination was made.
	FERC Complaint Cases
	In November 2013, a customer group filed a complaint case with the FERC seeking a reduction in the allowed base return on common equity for the FERC-regulated MISO transmission rate base under the MISO tariff to 9.15%. Currently, the FERC-allowed base return on common equity for MISO transmission owners is 12.38%. In October 2014, the FERC issued an order establishing settlement procedures and, if necessary, hearing procedures regarding the allowed base return on common equity. In January 2015, the settlement judge terminated settlement proceedings and the FERC scheduled the case for hearing proceedings, requiring an initial decision to be issued no later than November 30, 2015. As the original 15-month refund period for the November 2013 complaint case ended in February 2015, another customer complaint case was filed in February 2015. The February 2015 complaint case seeks a reduction in the allowed base return on common equity for the FERC-regulated MISO transmission rate base under the MISO tariff to 8.67%.
	In October 2014, the FERC issued an order in a proceeding, in which the Ameren Companies were not involved, reducing the allowed base return on common equity for New England transmission owners from 11.14% to 10.57%, with rate incentives allowed up to 11.74%. The FERC order in the New England transmission owners’ case applied observable market data from October 2012 to March 2013 to determine the allowed base return on common equity. The FERC expects the evidence and the calculation used in the New England transmission owners’ case to guide its decision in the MISO complaint case discussed above. The FERC calculation will establish the allowed base return on common equity, which specifies a unique time period for each complaint case, and will require multiple inputs based on observable market data specific to the utility industry and broader macroeconomic data. In January 2015, the settlement judge for the MISO complaint case ordered that July 13, 2015, should be the cut-off date for the observable market data to be used in the calculation of the allowed base return on common equity. Based on the information in these orders, Ameren and Ameren Illinois recorded current liabilities representing their estimate of the required refunds from the refund effective date of November 12, 2013, through December 31, 2014. Ameren Missouri did not record a liability as of December 31, 2014, and does not expect that a reduction in the FERC-allowed base return on common equity for MISO transmission owners would be material to its results of operations, financial position, or liquidity.
	In November 2014, we filed a request with the FERC to include an incentive adder of up to 50 basis points on the allowed base return on common equity for participation in an RTO, and we sought authorization to defer collection of it until after the issuance of the final order addressing the initial MISO complaint case discussed above. FERC approved the request to implement the incentive adder prospectively from January 6, 2015, and to defer collection of it until the issuance of the final order addressing the initial MISO complaint case.
	Ameren Missouri Power Purchase Agreement with Entergy
	Beginning in 2005, the FERC issued a series of orders addressing a complaint filed in 2001 by the Louisiana Public Service Commission against Entergy and certain of its affiliates. The complaint alleged unjust and unreasonable cost allocations. As a result of the FERC orders, Entergy began billing Ameren Missouri in 2007 for additional charges under a 165-megawatt power purchase agreement that expired August 31, 2009. In May 2012, the FERC issued an order stating that Entergy should not have included additional charges to Ameren Missouri under the power purchase agreement. Pursuant to the order, in June 2012, Entergy paid Ameren Missouri $31 million, with $24 million recorded as a reduction to “Operating Expenses – Purchased power” expense and $5 million for interest recorded as “Miscellaneous income” in the statement of income. The remaining $2 million was recorded as an offset to the FAC under-recovered regulatory asset for the amount refundable to customers. The amount of the Entergy refund recorded to the FAC regulatory asset related to the period when the FAC was effective; therefore, such costs were previously included in customer rates. In November 2013, Entergy filed an appeal of the FERC's May 2012 order with the United States Court of Appeals for the District of Columbia Circuit. Ameren is not able to predict when or how the court will rule on Entergy's appeal.
	The Louisiana Public Service Commission appealed the FERC’s orders regarding Louisiana Public Service Commission’s complaint against Entergy Services, Inc. to the United States Court of Appeals for the District of Columbia Circuit. That court ordered further FERC proceedings regarding Louisiana Public Service Commission’s complaint. Ameren Missouri estimates that it could incur an additional expense of up to $8 million if the FERC's May 2012 order is overturned on appeal. Ameren Missouri believes that the likelihood of incurring any expense is not probable, and therefore no liability has been recorded as of December 31, 2014.
	Combined Construction and Operating License
	In 2008, Ameren Missouri filed an application with the NRC for a COL for a new nuclear unit at Ameren Missouri's existing Callaway County, Missouri, energy center site. In 2009, Ameren Missouri suspended its efforts to build a new nuclear unit at the Callaway site, and the NRC suspended review of the COL application. The suspended status of the COL application currently extends through the end of 2015.
	Ameren Missouri estimates the total cost to obtain a COL for the Callaway site to be approximately $100 million. As of December 31, 2014, Ameren Missouri had capitalized investments of $69 million for the development of a new nuclear energy center. Ameren is currently evaluating all potential nuclear technologies in order to maintain an option for nuclear power in the future.
	All of Ameren Missouri's capitalized investments for the development of a new nuclear energy center will remain capitalized while management pursues options to maximize the value of its investment. If efforts to license additional nuclear generation are abandoned, the NRC does not extend the COL application suspended status, or if management concludes it is probable that the costs incurred will be disallowed in rates, a charge to earnings would be recognized in the period in which that determination is made.
	Regulatory Assets and Liabilities
	In accordance with authoritative accounting guidance regarding accounting for the effects of certain types of regulation, we defer certain costs as regulatory assets pursuant to actions of regulators or based on the expected ability to recover such costs in rates charged to customers. We may also defer certain amounts as regulatory liabilities because of actions of regulators or because of the expectation that such amounts will be returned to customers in future rates. The following table presents our regulatory assets and regulatory liabilities at December 31, 2014 and 2013:
			2014						2013
			Ameren		Ameren		Ameren			Ameren		Ameren		Ameren
			Missouri		Illinois					Missouri		Illinois
	Current regulatory assets:
	Under-recovered FAC(a)(b)		$	128	$	—	$	128		$	104	$	—	$	104
	Under-recovered Illinois electric power costs(c)		—		2		2			—		1		1
	Under-recovered PGA(c)		—		20		20			—		1		1
	MTM derivative losses(d)		32		42		74			14		36		50
	Energy efficiency riders(e)		3		—		3			—		—		—
	IEIMA revenue requirement reconciliation(a)(f)		—		65		65			—		—		—
	FERC revenue requirement reconciliation(a)(g)		—		—		3			—		—		—
	Total current regulatory assets		$	163	$	129	$	295		$	118	$	38	$	156
	Noncurrent regulatory assets:
	Pension and postretirement benefit costs(h)		$	148	$	275	$	423		$	44	$	140	$	184
	Income taxes(i)		253		3		256			230		7		237
	Asset retirement obligations(j)		—		5		5			—		5		5
Callaway costs(a)(k)		36		—		36			40		—		40
Unamortized loss on reacquired debt(a)(l)		72		80		152			77		74		151
Contaminated facilities costs(m)		—		251		251			—		271		271
MTM derivative losses(d)		14		144		158			8		118		126
Storm costs(n)		—		3		3			5		3		8
Demand-side costs before the MEEIA implementation(a)(o)		44		—		44			58		—		58
Workers’ compensation claims(p)		7		7		14			6		6		12
Credit facilities fees(q)		5		—		5			5		—		5
Common stock issuance costs(r)		2		—		2			4		—		4
Construction accounting for pollution control equipment(a)(s)		21		—		21			22		—		22
Solar rebate program(a)(t)		88		—		88			27		—		27
IEIMA revenue requirement reconciliation(a)(f)		—		101		101			—		65		65
FERC revenue requirement reconciliation(a)(g)		—		8		12			—		—		5
Other(u)		5		6		11			8		12		20
Total noncurrent regulatory assets		$	695	$	883	$	1,582		$	534	$	701	$	1,240
Current regulatory liabilities:
Over-recovered FAC(b)		$	—	$	—	$	—		$	26	$	—	$	26
Over-recovered Illinois electric power costs(c)		—		26		26			—		51		51
Over-recovered PGA(c)		2		25		27			5		29		34
MTM derivative gains(d)		16		1		17			26		1		27
Wholesale distribution refund(v)		—		—		—			—		13		13
IEIMA revenue requirement reconciliation(f)		—		—		—			—		65		65
FERC revenue requirement reconciliation(g)		—		11		11			—		—		—
Refund reserves for FERC orders and audit findings(w)		—		21		25			—		—		—
Total current regulatory liabilities		$	18	$	84	$	106		$	57	$	159	$	216
Noncurrent regulatory liabilities:
Income taxes(x)		$	41	$	14	$	55		$	37	$	3	$	40
Uncertain tax positions tracker(y)		7		—		7			1		—		1
Removal costs(z)		886		643		1,529			828		610		1,438
Asset retirement obligation(j)		182		—		182			146		—		146
Bad debt riders(aa)		—		7		7			—		8		8
Pension and postretirement benefit costs tracker(ab)		24		—		24			15		—		15
Energy efficiency riders(e)		—		39		39			3		33		36
FERC revenue requirement reconciliation(g)		—		—		—			—		10		10
Other(ac)		7		—		7			11		—		11
Total noncurrent regulatory liabilities		$	1,147	$	703	$	1,850		$	1,041	$	664	$	1,705
(a)	These assets earn a return.
(b)	Under-recovered or over-recovered fuel costs to be recovered through the FAC. Specific accumulation periods aggregate the under-recovered or over-recovered costs over four months, any related adjustments that occur over the following four months, and the recovery from customers that occurs over the next eight months.
(c)	Costs under- or over-recovered from utility customers. Amounts will be recovered from, or refunded to, customers within one year of the deferral.
(d)	Deferral of commodity-related derivative MTM losses or gains. See Note 7 – Derivative Financial Instruments for additional information.
(e)	The Ameren Missouri balance relates to the MEEIA. Beginning in January 2014, a MEEIA rider allowed Ameren Missouri to collect from or refund to customers any annual difference in the actual amounts incurred and the amounts collected from customers for the MEEIA program costs and its lost revenues. Under the MEEIA rider, collections from or refunds to customers occur one year after the program costs and lost revenues are incurred. The Ameren Illinois balance relates to a regulatory tracking mechanism to recover its electric and natural gas costs associated with developing, implementing, and evaluating customer energy efficiency and demand response programs. Any under-recovery or over-recovery will be collected from or refunded to customers over the 12 months following the plan year.
(f)	The difference between Ameren Illinois' annual revenue requirement calculated under the IEIMA's performance-based formula ratemaking framework and the revenue requirement included in customer rates for that year. Subject to ICC approval, these amounts will be collected from or refunded to customers within two years.
(g)	Ameren Illinois' and ATXI's annual revenue requirement reconciliation adjustments calculated pursuant to the FERC's electric transmission formula ratemaking framework. The under-recovery or over-recovery will be recovered from or refunded to customers within two years.
(h)	These costs are being amortized in proportion to the recognition of prior service costs (credits) and actuarial losses (gains) attributable to Ameren’s pension plan and postretirement benefit plans. See Note 11 – Retirement Benefits for additional information.
(i)	Offset to certain deferred tax liabilities for expected recovery of future income taxes when paid. This will be recovered over the expected life of the related assets.
(j)	Recoverable or refundable removal costs for AROs, including net realized and unrealized gains and losses related to the nuclear decommissioning trust fund investments. See Note 1 – Summary of Significant Accounting Policies – Asset Retirement Obligations.
(k)	Ameren Missouri’s Callaway energy center operations and maintenance expenses, property taxes, and carrying costs incurred between the plant in-service date and the date the plant was reflected in rates. These costs are being amortized over the remaining life of the energy center's current operating license, which expires in 2024.
(l)	Losses related to reacquired debt. These amounts are being amortized over the lives of the related new debt issuances or the original lives of the old debt issuances if no new debt was issued.
(m)	The recoverable portion of accrued environmental site liabilities that will be collected from electric and natural gas customers through ICC-approved cost recovery riders. The period of recovery will depend on the timing of remediation expenditures. See Note 15 – Commitments and Contingencies for additional information.
(n)	Ameren Missouri's actual storm costs that exceed the normalized storm costs for rate purposes. As approved by the December 2012 MoPSC electric rate order, the 2006, 2007, and 2008 storm costs were amortized through December 2014. The Ameren Illinois balance includes 2013 storm costs deferred in accordance with the IEIMA. These costs are being amortized over a five-year period beginning in 2013.
(o)	Demand-side costs incurred prior to implementation of the MEEIA in 2013, including the costs of developing, implementing and evaluating customer energy efficiency and demand response programs. Costs incurred from May 2008 through September 2008 are being amortized over a 10-year period that began in March 2009. Costs incurred from October 2008 through December 2009 are being amortized over a six-year period that began in July 2010. Costs incurred from January 2010 through February 2011 are being amortized over a six-year period that began in August 2011. Costs incurred from March 2011 through July 2012 are being amortized over a six-year period that began in January 2013.The amortization period for costs incurred from August 2012 through December 2012 will be determined in the July 2014 electric rate case.
(p)	The period of recovery will depend on the timing of actual expenditures.
(q)	Ameren Missouri’s costs incurred to enter into and maintain the 2012 Missouri Credit Agreement. Additional costs were incurred in December 2014 to amend and restate the 2012 Missouri Credit Agreement. These costs are being amortized over the life of the credit facility, ending in December 2019, to construction work in progress, which will be depreciated when assets are placed into service.
(r)	The MoPSC’s May 2010 electric rate order allowed Ameren Missouri to recover its portion of Ameren’s September 2009 common stock issuance costs. These costs are being amortized over five years, beginning in July 2010.
(s)	The MoPSC’s May 2010 electric rate order allowed Ameren Missouri to record an allowance for funds used during construction for pollution control equipment at its Sioux energy center until the cost of that equipment was included in customer rates. These costs will be amortized over the expected life of the Sioux energy center, which is currently through 2033.
(t)	Costs associated with Ameren Missouri's solar rebate program beginning in August 2012 to fulfill its renewable energy portfolio requirement. The amortization period for these costs will be three years, commencing with the effectiveness of Ameren Missouri's current July 2014 electric rate case.
(u)	The Ameren Illinois balance includes Ameren Illinois Merger integration and optimization costs, which are being amortized over four years, beginning in January 2012. The Ameren Illinois total also includes costs related to the 2013 natural gas delivery service rate case costs, which are being amortized over a two-year period that began in January 2014. At Ameren Missouri, the balance primarily includes the cost of renewable energy credits to fulfill its renewable energy portfolio requirement. Costs incurred from January 2010 through July 2012 are being amortized over three years, beginning in January 2013.
(v)	Estimated refund to wholesale electric customers as of December 31, 2013. See 2011 Wholesale Distribution Rate Case above.
(w)	Estimated refunds to transmission customers related to FERC orders and audit findings. In regards to the FERC orders, see Ameren Illinois Electric Transmission Rate Refund and FERC Complaint Cases above.
(x)	Unamortized portion of investment tax credits and federal excess deferred taxes. The unamortized portion of investment tax credits and the federal excess deferred taxes are being amortized over the expected life of the underlying assets.
(y)	The tracker is amortized over three years, beginning from the date the amounts are included in rates. See Note 13 - Income Taxes for additional information.
(z)	Estimated funds collected for the eventual dismantling and removal of plant from service, net of salvage value, upon retirement related to our rate-regulated operations.
(aa)	A regulatory tracking mechanism for the difference between the level of bad debt incurred by Ameren Illinois under GAAP and the level of such costs included in electric and natural gas rates. The over-recovery relating to 2012 was refunded to customers from June 2013 through May 2014. The over-recovery relating to 2013 is being refunded to customers from June 2014 through May 2015. The over-recovery relating to 2014 will be refunded to customers from June 2015 through May 2016.
(ab)	A regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs built into rates. For periods prior to August 2012, the MoPSC's December 2012 electric rate order directed the amortization to occur over five years, beginning in January 2013. For periods after August 2012, the amortization period will be determined in the July 2014 electric rate case.
(ac)	Balance includes the costs of renewable energy credits to fulfill Ameren Missouri's renewable energy portfolio requirement from August 2012 through December 2013, which were less than the amount included in rates. The balance also includes a regulatory tracking mechanism at Ameren Missouri for the difference between the level of storm costs incurred in a particular year and the level of such costs built into rates. The amortization periods for these over-recoveries will be determined in the July 2014 electric rate case.
Ameren, Ameren Missouri, and Ameren Illinois continually assess the recoverability of their regulatory assets. Under current accounting standards, regulatory assets are charged to earnings when it is no longer probable that such amounts will be recovered through future revenues. To the extent that payments of regulatory liabilities are no longer probable, the amounts are credited to earnings.

Property_And_Plant_Net

Property And Plant, Net	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
PROPERTY AND PLANT, NET	PROPERTY AND PLANT, NET
	The following table presents property and plant, net, for each of the Ameren Companies at December 31, 2014 and 2013:
			Ameren			Ameren			Other		Ameren(a)
			Missouri(a)			Illinois
	2014
	Property and plant, at original cost:
	Electric		$	17,052		$	6,517		$	344	$	23,913
	Natural gas		431			1,854			—		2,285
			17,483			8,371			344		26,198
	Less: Accumulated depreciation and amortization		7,086			2,422			251		9,759
			10,397			5,949			93		16,439
	Construction work in progress:
	Nuclear fuel in process		209			—			—		209
	Other		261			216			299		776
	Property and plant, net		$	10,867		$	6,165		$	392	$	17,424
	2013
	Property and plant, at original cost:
	Electric		$	15,964		$	5,426		$	336	$	21,726
	Natural gas		413			1,562			—		1,975
			16,377			6,988			336		23,701
	Less: Accumulated depreciation and amortization		6,766			1,627			251		8,644
			9,611			5,361			85		15,057
	Construction work in progress:
	Nuclear fuel in process		246			—			—		246
	Other		595			228			79		902
	Property and plant, net		$	10,452		$	5,589		$	164	$	16,205
	(a)	Amounts in Ameren and Ameren Missouri include two CTs under separate capital lease agreements. The gross cumulative asset value of those agreements was $233 million and $228 million at December 31, 2014 and 2013, respectively. The total accumulated depreciation associated with the two CTs was $66 million and $56 million at December 31, 2014 and 2013, respectively. In addition, Ameren Missouri has investments in debt securities, which were classified as held-to-maturity, related to the two CTs from the city of Bowling Green and Audrain County. As of December 31, 2014 and 2013, the carrying value of these debt securities was $294 million and $299 million, respectively.
	The following table provides accrued capital and nuclear fuel expenditures at December 31, 2014, 2013, and 2012, which represent noncash investing activity excluded from the accompanying statements of cash flows:
		Ameren(a)			Ameren			Ameren
					Missouri			Illinois
	Accrued capital expenditures:
	2014	$	181		$	72		$	59
	2013	175			74			86
	2012	107			63			37
	Accrued nuclear fuel expenditures:
	2014	13			13			(b)
	2013	8			8			(b)
	2012	8			8			(b)
	(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
	(b)	Not applicable.

ShortTerm_Debt_And_Liquidity

Short-Term Debt And Liquidity	12 Months Ended
	Dec. 31, 2014
Line of Credit Facility [Abstract]
SHORT-TERM DEBT AND LIQUIDITY	SHORT-TERM DEBT AND LIQUIDITY
	The liquidity needs of the Ameren Companies are typically supported through the use of available cash, short-term intercompany borrowings, drawings under committed bank credit agreements, or commercial paper issuances.
	2012 Credit Agreements
	On December 11, 2014, each of the 2012 Credit Agreements was amended and restated. The amended and restated agreements extended the maturity dates of the 2012 Credit Agreements from November 14, 2017, to December 11, 2019, resulting in $2.1 billion of credit provided through the extended maturity date. The facilities continue to include 24 international, national, and regional lenders, with no single lender providing more than $115 million of credit in aggregate.
	The obligations of each borrower under the respective 2012 Credit Agreements to which it is a party are several and not joint, and, except under limited circumstances relating to expenses and indemnities, the obligations of Ameren Missouri and Ameren Illinois under the respective 2012 Credit Agreements are not guaranteed by Ameren or any other subsidiary of Ameren. The maximum aggregate amount available to each borrower under each facility is shown in the following table (the amount being each borrower's "Borrowing Sublimit"):
			2012 Missouri Credit Agreement			2012 Illinois
						Credit Agreement
	Ameren		$	700		$	500
	Ameren Missouri		800			(a)
	Ameren Illinois		(a)			800
	(a)	Not applicable.
	Ameren has the option to seek additional commitments from existing or new lenders to increase the total facility size of the 2012 Credit Agreements up to a maximum amount of $1.2 billion for the 2012 Missouri Credit Agreement and $1.3 billion for the 2012 Illinois Credit Agreement. The 2012 Credit Agreements, as well as the Borrowing Sublimits of Ameren, Ameren Missouri, and Ameren Illinois, will mature and expire on December 11, 2019. The principal amount of each revolving loan owed by a borrower under any of the 2012 Credit Agreements to which it is a party will be due and payable no later than the maturity date of such 2012 Credit Agreement. The principal amount of each revolving loan owed by Ameren Missouri or Ameren Illinois under the applicable 2012 Credit Agreement will be due and payable no later than the earlier of the maturity date or 364 days after the date of such loan.
	The obligations of all borrowers under the 2012 Credit Agreements are unsecured. Loans are available on a revolving basis under each of the 2012 Credit Agreements. Funds borrowed may be repaid and, subject to satisfaction of the conditions to borrowing, reborrowed from time to time. At the election of each borrower, the interest rates on such loans will be the alternate base rate plus the margin applicable to the particular borrower and/or the eurodollar rate plus the margin applicable to the particular borrower. The applicable margins will be determined by the borrower's long-term unsecured credit ratings or, if no such ratings are then in effect, the borrower's corporate/issuer ratings then in effect. The 2012 Credit Agreements provide for the issuance of letters of credit for the account of the borrowers up to a maximum of 25% of the aggregate initial commitment under the applicable 2012 Credit Agreement. The borrowers have received commitments from the lenders to issue letters of credit up to $100 million under each of the 2012 Credit Agreements. In addition, the issuance of letters of credit is subject to the $2.1 billion overall combined facility borrowing limitations of the 2012 Credit Agreements.
	The borrowers will use the proceeds from any borrowings under the 2012 Credit Agreements for general corporate purposes, including working capital, commercial paper liquidity support, issuance of letters of credit, loan funding under the Ameren money pool arrangements, and other short-term intercompany loan arrangements, or for paying fees and expenses incurred in connection with the 2012 Credit Agreements. Both of the 2012 Credit Agreements are available to Ameren to support issuances under Ameren's commercial paper program, subject to borrowing sublimits. The 2012 Missouri Credit Agreement and the 2012 Illinois Credit Agreement are available to support issuances under Ameren Missouri's and Ameren Illinois' commercial paper programs, respectively. As of December 31, 2014, based on commercial paper outstanding and letters of credit issued under the 2012 Credit Agreements, the aggregate amount of credit capacity available to Ameren (parent), Ameren Missouri, and Ameren Illinois, collectively, at December 31, 2014, was $1.4 billion.
	Ameren, Ameren Missouri, and Ameren Illinois did not borrow under the 2012 Credit Agreements for the years ended December 31, 2014 and 2013.
	Commercial Paper
	The following table summarizes the borrowing activity and relevant interest rates under Ameren Missouri's and Ameren Illinois' commercial paper program, for the years ended December 31, 2014 and 2013:
			Ameren (parent)				Ameren Missouri			Ameren Illinois			Ameren Consolidated
	2014
	Average daily commercial paper outstanding		$	423			$	110		$	165		$	639
	Outstanding borrowings at period-end		585				97			32			714
	Weighted-average interest rate		0.36		%		0.38		%	0.32		%	0.36		%
	Peak commercial paper during period(a)		$	625			$	495		$	300		$	910
	Peak interest rate		0.75		%		0.7		%	0.6		%	0.75		%
	2013
	Average daily commercial paper outstanding		$	54			$	—		$	—		$	54
	Outstanding borrowings at period-end		368				—			—			368
	Weighted-average interest rate		0.56		%		—		%	—		%	0.56		%
	Peak commercial paper during period(a)		$	368			$	—		$	—		$	368
	Peak interest rate		0.85		%		—		%	—		%	0.85		%
	Indebtedness Provisions and Other Covenants
	The information below presents a summary of the Ameren Companies’ compliance with indebtedness provisions and other covenants.
	The 2012 Credit Agreements contain conditions for borrowings and issuances of letters of credit. These include the absence of default or unmatured default, material accuracy of representations and warranties (excluding any representation after the closing date as to the absence of material adverse change and material litigation, and the absence of any notice of violation, liability, or requirement under any environmental laws that could have a material adverse effect), and obtainment of required regulatory authorizations. In addition, it is a condition for any Ameren Illinois borrowing that, at the time of and after giving effect to such borrowing, Ameren Illinois not be in violation of any limitation on its ability to incur unsecured indebtedness contained in its articles of incorporation.
	The 2012 Credit Agreements also contain nonfinancial covenants, including restrictions on the ability to incur liens, to transact with affiliates, to dispose of assets, to make investments in or transfer assets to its affiliates, and to merge with other entities. The 2012 Credit Agreements require each of Ameren, Ameren Missouri, and Ameren Illinois to maintain consolidated indebtedness of not more than 65% of its consolidated total capitalization pursuant to a defined calculation set forth in the agreements. As of December 31, 2014, the ratios of consolidated indebtedness to total consolidated capitalization, calculated in accordance with the provisions of the 2012 Credit Agreements, were 50%, 49%, and 47%, for Ameren, Ameren Missouri, and Ameren Illinois, respectively. In addition, under the 2012 Illinois Credit Agreement and, by virtue of the cross-default provisions of the 2012 Missouri Credit Agreement, under the 2012 Missouri Credit Agreement, Ameren is required to maintain a ratio of consolidated funds from operations plus interest expense to consolidated interest expense of 2.0 to 1.0. However, the interest coverage requirement will only apply at such times as Ameren does not have a senior long-term unsecured credit rating of at least Baa3 from Moody's or BBB- from S&P. As of December 31, 2014, Ameren exceeded the rating requirements and the interest coverage requirement was not applicable. Failure of a borrower to satisfy a financial covenant constitutes an immediate default under the applicable 2012 Credit Agreement.
	The 2012 Credit Agreements contain default provisions that apply separately to each borrower; provided, however, that a default of Ameren Missouri or Ameren Illinois under the applicable 2012 Credit Agreement will also be deemed to constitute a default of Ameren under such agreement. Defaults include a cross-default to a default of such borrower under any other agreement covering outstanding indebtedness of such borrower and certain subsidiaries (other than project finance subsidiaries and nonmaterial subsidiaries) in excess of $75 million in the aggregate (including under the other 2012 Credit Agreement). However, under the default provisions of the 2012 Credit Agreements, any default of Ameren under any 2012 Credit Agreement that results solely from a default of Ameren Missouri or Ameren Illinois thereunder does not result in a cross-default of Ameren under the other 2012 Credit Agreement. Further, the 2012 Credit Agreement default provisions provide that an Ameren default under any of the 2012 Credit Agreements does not constitute a default by Ameren Missouri or Ameren Illinois.
	None of the Ameren Companies' credit agreements or financing agreements contain credit rating triggers that would cause a default or acceleration of repayment of outstanding balances. Management believes that the Ameren Companies were in compliance with the provisions and covenants of their credit agreements at December 31, 2014.
	Money Pools
	Ameren has money pool agreements with and among its subsidiaries to coordinate and provide for certain short-term cash and working capital requirements.
	Ameren Missouri, Ameren Illinois, and Ameren Services may participate in the utility money pool as both lenders and borrowers. Ameren may participate in the money pool only as a lender. Internal funds are surplus funds contributed to the money pool from participants. The primary sources of external funds for the money pool are the 2012 Credit Agreements and the commercial paper programs. The total amount available to the pool participants from the utility money pool at any given time is reduced by the amount of borrowings made by participants, but it is increased to the extent that the pool participants advance surplus funds to the utility money pool or remit funds from other external sources. The availability of funds is also determined by funding requirement limits established by regulatory authorizations. Participants receiving a loan under the money pool agreement must repay the principal amount of such loan, together with accrued interest. The rate of interest depends on the composition of internal and external funds in the utility money pool. The average interest rate for borrowing under the money pool for the year ended December 31, 2014, was 0.19% (2013 - 0.14%).
	See Note 14 – Related Party Transactions for the amount of interest income and expense from the money pool arrangements recorded by the Ameren Companies for the years ended December 31, 2014, 2013, and 2012.

LongTerm_Debt_And_Equity_Finan

Long-Term Debt And Equity Financings	12 Months Ended
	Dec. 31, 2014
Long-Term Debt And Equity Financings [Abstract]
LONG-TERM DEBT AND EQUITY FINANCINGS	LONG-TERM DEBT AND EQUITY FINANCINGS
	The following table presents long-term debt outstanding, including maturities due within one year, for the Ameren Companies as of December 31, 2014 and 2013:
		2014				2013
	Ameren (Parent):
	8.875% Senior unsecured notes due 2014	$	—			$	425
	Less: Maturities due within one year	—				(425		)
	Long-term debt, net	$	—			$	—
	Ameren Missouri:
	Senior secured notes:(a)
	5.50% Senior secured notes due 2014	—				104
	4.75% Senior secured notes due 2015	114				114
	5.40% Senior secured notes due 2016	260				260
	6.40% Senior secured notes due 2017	425				425
	6.00% Senior secured notes due 2018(b)	179				179
	5.10% Senior secured notes due 2018	199				199
	6.70% Senior secured notes due 2019(b)	329				329
	5.10% Senior secured notes due 2019	244				244
	5.00% Senior secured notes due 2020	85				85
	3.50% Senior secured notes due 2024	350				—
	5.50% Senior secured notes due 2034	184				184
	5.30% Senior secured notes due 2037	300				300
	8.45% Senior secured notes due 2039(b)	350				350
	3.90% Senior secured notes due 2042(b)	485				485
	Environmental improvement and pollution control revenue bonds:
	1992 Series due 2022(c)(d)	47				47
	1993 5.45% Series due 2028(e)	(e)				(e)
	1998 Series A due 2033(c)(d)	60				60
	1998 Series B due 2033(c)(d)	50				50
	1998 Series C due 2033(c)(d)	50				50
	Capital lease obligations:
	City of Bowling Green capital lease (Peno Creek CT) due 2022	54				59
	Audrain County capital lease (Audrain County CT) due 2023	240				240
	Total long-term debt, gross	4,005				3,764
	Less: Unamortized discount and premium	(6		)		(7		)
	Less: Maturities due within one year	(120		)		(109		)
	Long-term debt, net	$	3,879			$	3,648
		2014				2013
	Ameren Illinois:
	Senior secured notes:
	6.20% Senior secured notes due 2016(f)	$	54			$	54
	6.25% Senior secured notes due 2016(g)	75				75
	6.125% Senior secured notes due 2017(g)(h)	250				250
	6.25% Senior secured notes due 2018(g)(h)	144				144
	9.75% Senior secured notes due 2018(g)(h)	313				313
	2.70% Senior secured notes due 2022(g)(h)	400				400
	3.25% Senior secured notes due 2025(g)	300				—
	6.125% Senior secured notes due 2028(g)	60				60
	6.70% Senior secured notes due 2036(g)	61				61
	6.70% Senior secured notes due 2036(f)	42				42
	4.80% Senior secured notes due 2043(g)	280				280
	4.30% Senior secured notes due 2044(g)	250				—
	Environmental improvement and pollution control revenue bonds:
	5.90% Series 1993 due 2023(i)	(i)				32
	5.70% 1994A Series due 2024(j)	(j)				36
	5.95% 1993 Series C-1 due 2026(k)	—				35
	5.70% 1993 Series C-2 due 2026(k)	—				8
	1993 Series B-1 due 2028(d)(k)	17				17
	5.40% 1998A Series due 2028(j)	—				19
	5.40% 1998B Series due 2028(j)	—				33
	Fair-market value adjustments	—				4
	Total long-term debt, gross	2,246				1,863
	Less: Unamortized discount and premium	(5		)		(7		)
	Less: Maturities due within one year	—				—
	Long-term debt, net	$	2,241			$	1,856
Ameren consolidated long-term debt, net	$	6,120			$	5,504
(a)	These notes are collaterally secured by first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage indenture. The notes have a fall-away lien provision and will remain secured only as long as any first mortgage bonds issued under the Ameren Missouri mortgage indenture remain outstanding. Redemption, purchase, or maturity of all first mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the first mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the Ameren Missouri senior secured notes currently outstanding, we do not expect the first mortgage bond lien protection associated with these notes to fall away until 2042.
(b)	Ameren Missouri has agreed, during the life of these notes, not to optionally redeem, purchase or otherwise retire in full its first mortgage bonds. Ameren Missouri has also agreed to prevent a first mortgage bond release date from occurring as long as any of the 8.45% senior secured notes due 2039 and any of the 3.90% senior secured notes due 2042 remain outstanding.
(c)	These bonds are collaterally secured by first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage indenture and have a fall-away lien provision similar to that of Ameren Missouri's senior secured notes. The bonds are also backed by an insurance guarantee policy.
(d)	The interest rates, and the periods during which such rates apply, vary depending on our selection of defined rate modes. Maximum interest rates could reach 18% depending on the series of bonds. The average interest rates for 2014 and 2013 were as follows:
	2014		2013
Ameren Missouri 1992 Series due 2022	0.10%		0.17%
Ameren Missouri 1998 Series A due 2033	0.26%		0.34%
Ameren Missouri 1998 Series B due 2033	0.27%		0.33%
Ameren Missouri 1998 Series C due 2033	0.26%		0.34%
Ameren Illinois 1993 Series B-1 due 2028	0.21%		0.14%
(e)	These bonds are first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage bond indenture and are secured by substantially all Ameren Missouri property and franchises. The bonds are callable at 100% of par value. Less than $1 million principal amount of the bonds remain outstanding.
(f)	These notes are collaterally secured by first mortgage bonds issued by Ameren Illinois under the CILCO mortgage indenture. The notes have a fall-away lien provision, and Ameren Illinois could cause these notes to become unsecured at any time by redeeming the pollution control bonds 5.90% Series 1993 due 2023 (of which less than $1 million remains outstanding). Ameren Illinois may resecure these notes if it chooses.
(g)	These notes are collaterally secured by mortgage bonds issued by Ameren Illinois under the Ameren Illinois mortgage indenture. The notes have a fall-away lien provision and will remain secured only as long as any series of first mortgage bonds issued under the Ameren Illinois mortgage indenture remain outstanding. Redemption, purchase, or maturity of all mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the Ameren Illinois senior secured notes currently outstanding, we do not expect the mortgage bond lien protection associated with these notes to fall away until 2024.
(h)	Ameren Illinois has agreed, during the life of these notes, not to optionally redeem, purchase, or otherwise retire in full its Ameren Illinois mortgage bonds; therefore, an Ameren Illinois first mortgage bond release date will not occur as long as any of these notes are outstanding.
(i)	These bonds are first mortgage bonds issued by Ameren Illinois under the CILCO mortgage indenture and are secured by substantially all property of the former CILCO. The bonds are callable at 100% of par value. Less than $1 million principal amount of the bonds remain outstanding.
(j)	These bonds are mortgage bonds issued by Ameren Illinois under the Ameren Illinois mortgage indenture and are secured by substantially all property of the former IP and CIPS. The bonds are callable at 100% of par value. The bonds are also backed by an insurance guarantee policy. Less than $1 million principal amount of the bonds remain outstanding.
(k)	The bonds are callable at 100% of par value.
The following table presents the aggregate maturities of long-term debt, including current maturities, for the Ameren Companies at December 31, 2014:
		Ameren				Ameren				Ameren
		Missouri(a)				Illinois(a)				Consolidated
2015		$	120			$	—			$	120
2016		266				129				395
2017		431				250				681
2018		383				457				840
2019		581				—				581
Thereafter		2,224				1,410				3,634
Total		$	4,005			$	2,246			$	6,251
(a)	Excludes unamortized discount and premium of $6 million and $5 million at Ameren Missouri and Ameren Illinois, respectively.
All classes of Ameren Missouri’s and Ameren Illinois’ preferred stock are entitled to cumulative dividends, have voting rights, and are not subject to mandatory redemption. The preferred stock of Ameren's subsidiaries was included in "Noncontrolling Interests" on Ameren's consolidated balance sheet. The following table presents the outstanding preferred stock of Ameren Missouri and Ameren Illinois, which is redeemable, at the option of the issuer, at the prices shown below as of December 31, 2014 and 2013:
			Redemption Price(per share)				2014				2013
Ameren Missouri:
Without par value and stated value of $100 per share, 25 million shares authorized
$3.50 Series	130,000 shares		$	110			$	13			$	13
$3.70 Series	40,000 shares		104.75				4				4
$4.00 Series	150,000 shares		105.625				15				15
$4.30 Series	40,000 shares		105				4				4
$4.50 Series	213,595 shares		110			(a)	21				21
$4.56 Series	200,000 shares		102.47				20				20
$4.75 Series	20,000 shares		102.176				2				2
$5.50 Series A	14,000 shares		110				1				1
Total							$	80			$	80
Ameren Illinois:
With par value of $100 per share, 2 million shares authorized
4.00% Series	144,275 shares		$	101			$	14			$	14
4.08% Series	45,224 shares		103				5				5
4.20% Series	23,655 shares		104				2				2
4.25% Series	50,000 shares		102				5				5
4.26% Series	16,621 shares		103				2				2
4.42% Series	16,190 shares		103				2				2
4.70% Series	18,429 shares		103				2				2
4.90% Series	73,825 shares		102				7				7
4.92% Series	49,289 shares		103.5				5				5
5.16% Series	50,000 shares		102				5				5
6.625% Series	124,274 shares		100				12				12
7.75% Series	4,542 shares		100				1				1
Total							$	62			$	62
Total Ameren							$	142			$	142
(a)	In the event of voluntary liquidation, $105.50.
Ameren has 100 million shares of $0.01 par value preferred stock authorized, with no shares outstanding. Ameren Missouri has 7.5 million shares of $1 par value preference stock authorized, with no such preference stock outstanding. Ameren Illinois has 2.6 million shares of no par value preferred stock authorized, with no shares outstanding.
Ameren
In May 2014, Ameren (parent) repaid at maturity $425 million of its 8.875% senior unsecured notes, plus accrued interest. The notes were repaid with proceeds from commercial paper issuances.
Ameren filed a Form S-3 registration statement with the SEC in May 2014, authorizing the offering of 8.6 million additional shares of its common stock under DRPlus, which expires in May 2017. Shares of common stock sold under DRPlus are, at Ameren’s option, newly issued shares, treasury shares, or shares purchased in the open market or in privately negotiated transactions.
In October 2013, Ameren filed a Form S-8 registration statement with the SEC, authorizing the offering of 4 million additional shares of its common stock under its 401(k) plan. Shares of common stock sold under the 401(k) plan are, at Ameren’s option, newly issued shares, treasury shares, or shares purchased in the open market or in privately negotiated transactions.
In June 2012, Ameren, Ameren Missouri, and Ameren Illinois filed a Form S-3 shelf registration statement registering the issuance of an indeterminate amount of certain types of securities, which expires in June 2015.
From 2012 through 2014, Ameren shares for its DRPlus and its 401(k) plans were purchased in the open market.
Ameren Missouri
In April 2014, Ameren Missouri issued $350 million of 3.50% senior secured notes due April 15, 2024, with interest payable semiannually on April 15 and October 15 of each year, beginning October 15, 2014. Ameren Missouri received proceeds of $348 million, which were used to repay at maturity $104 million of its 5.50% senior secured notes due May 15, 2014 and to repay a portion of its short-term debt.
In October 2013, $44 million of Ameren Missouri’s 1993 5.45% Series tax-exempt first mortgage bonds were redeemed at par value plus accrued interest, and $200 million of Ameren Missouri’s 4.65% senior secured notes matured and were retired.
Ameren Illinois
In January 2014, Ameren Illinois redeemed the following environmental improvement and pollution control revenue bonds at par value plus accrued interest:
Senior Secured Notes	Principal Amount
5.90% Series 1993 due 2023(a)	$	32
5.70% 1994A Series due 2024(a)	36
1993 Series C-1 5.95% due 2026	35
1993 Series C-2 5.70% due 2026	8
5.40% 1998A Series due 2028	19
5.40% 1998B Series due 2028	33
Total amount redeemed	$	163
(a)	Less than $1 million principal amount of the bonds remain outstanding after redemption.
In June 2014, Ameren Illinois issued $250 million of 4.30% senior secured notes due July 1, 2044, with interest payable semiannually on January 1 and July 1, beginning January 1, 2015. Ameren Illinois received proceeds of $246 million, which were used to repay a portion of its short-term debt.
In December 2014, Ameren Illinois issued $300 million of 3.25% senior secured notes due March 1, 2025, with interest payable semiannually on March 1 and September 1, beginning March 1, 2015. Ameren Illinois received proceeds of $298 million, which were used to repay a portion of its short-term debt.
In December 2013, Ameren Illinois issued $280 million principal amount of 4.80% senior secured notes due December 15, 2043, with interest payable semiannually on June 15 and December 15, beginning June 15, 2014. Ameren Illinois received net proceeds of $276 million. The proceeds were used, together with other available cash, to repay at maturity $150 million of its 8.875% senior secured notes due December 15, 2013, and to repay its short-term debt.
Indenture Provisions and Other Covenants
Ameren Missouri’s and Ameren Illinois’ indentures and articles of incorporation include covenants and provisions related to issuances of first mortgage bonds and preferred stock. Ameren Missouri and Ameren Illinois are required to meet certain ratios to issue additional first mortgage bonds and preferred stock. A failure to achieve these ratios would not result in a default under these covenants and provisions but would restrict the companies’ ability to issue bonds or preferred stock. The following table summarizes the required and actual interest coverage ratios for interest charges, dividend coverage ratios, and bonds and preferred stock issuable as of December 31, 2014, at an assumed interest rate of 5% and dividend rate of 6%.
	Required Interest	Actual Interest		Bonds Issuable(b)				Required Dividend	Actual Dividend		Preferred Stock
	Coverage Ratio(a)	Coverage Ratio						Coverage Ratio(c)	Coverage Ratio		Issuable
Ameren Missouri	>2.0	4.3		$	3,605			>2.5	115.1		$	2,568
Ameren Illinois	>2.0	6.4		3,358			(d)	>1.5	2.7		208
(a)	Coverage required on the annual interest charges on first mortgage bonds outstanding and to be issued. Coverage is not required in certain cases when additional first mortgage bonds are issued on the basis of retired bonds.
(b)	Amount of bonds issuable based either on required coverage ratios or unfunded property additions, whichever is more restrictive. The amounts shown also include bonds issuable based on retired bond capacity of $832 million and $204 million at Ameren Missouri and Ameren Illinois, respectively.
(c)	Coverage required on the annual dividend on preferred stock outstanding and to be issued, as required in the respective company’s articles of incorporation.
(d)	Amount of bonds issuable by Ameren Illinois based on unfunded property additions and retired bonds solely under the former IP mortgage indenture.
Ameren Missouri and Ameren Illinois and certain other nonregistrant Ameren subsidiaries are subject to Section 305(a) of the Federal Power Act, which makes it unlawful for any officer or director of a public utility, as defined in the Federal Power Act, to participate in the making or paying of any dividend from any funds “properly included in capital account.” The FERC has consistently interpreted the provision to allow dividends to be paid as long as (1) the source of the dividends is clearly disclosed, (2) the dividends are not excessive, and (3) there is no self-dealing on the part of corporate officials. At a minimum, Ameren believes that dividends can be paid by its subsidiaries that are public utilities from net income and retained earnings. In addition, under Illinois law, Ameren Illinois may not pay any dividend on its stock, unless, among other things, its earnings and earned surplus are sufficient to declare and pay a dividend after provision is made for reasonable and proper reserves, or unless Ameren Illinois has specific authorization from the ICC.
Ameren Illinois’ articles of incorporation require dividend payments on its common stock to be based on ratios of common stock to total capitalization and other provisions related to certain operating expenses and accumulations of earned surplus. Ameren Illinois committed to the FERC to maintain a minimum 30% ratio of common stock equity to total capitalization. As of December 31, 2014, Ameren Illinois’ ratio of common stock equity to total capitalization was 53%.
In order for the Ameren Companies to issue securities in the future, they will have to comply with all applicable requirements in effect at the time of any such issuances.
Off-Balance-Sheet Arrangements
At December 31, 2014, none of the Ameren Companies had any off-balance-sheet financing arrangements, other than operating leases entered into in the ordinary course of business. None of the Ameren Companies expect to engage in any significant off-balance-sheet financing arrangements in the near future. See Note 16 – Divestiture Transactions and Discontinued Operations for Ameren (parent) guarantees and letters of credit issued to support New AER based on the transaction agreement with IPH.

Other_Income_And_Expenses

Other Income And Expenses	12 Months Ended
	Dec. 31, 2014
Other Nonoperating Income (Expense) [Abstract]
OTHER INCOME AND EXPENSES	OTHER INCOME AND EXPENSES
	The following table presents the components of "Other Income and Expenses" in the Ameren Companies’ statements of income (loss) for the years ended December 31, 2014, 2013, and 2012:
		2014			2013		2012
	Ameren:(a)
	Miscellaneous income:
	Allowance for equity funds used during construction	$	34		$	37	$	36
	Interest income on industrial development revenue bonds	27			27		28
	Interest income	10		(b)	3		4		(d)
	Other	8		(c)	2		2
	Total miscellaneous income	$	79		$	69	$	70
	Miscellaneous expense:
	Donations	$	10		$	12	$	24	(e)
	Other	12			14		13
	Total miscellaneous expense	$	22		$	26	$	37
	Ameren Missouri:
	Miscellaneous income:
	Allowance for equity funds used during construction	$	32		$	31	$	31
	Interest income on industrial development revenue bonds	27			27		28
	Interest income	1			—		4		(d)
	Total miscellaneous income	$	60		$	58	$	63
	Miscellaneous expense:
	Donations	$	6		$	4	$	9
	Other	6			7		5
	Total miscellaneous expense	$	12		$	11	$	14
	Ameren Illinois:
	Miscellaneous income:
	Allowance for equity funds used during construction	$	2		$	6	$	5
	Interest income	7		(b)	2		—
	Other	8		(c)	2		2
	Total miscellaneous income	$	17		$	10	$	7
	Miscellaneous expense:
	Donations	$	4		$	4	$	11	(e)
	Other	4			5		6
	Total miscellaneous expense	$	8		$	9	$	17
	(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.
	(b)	Includes Ameren Illinois' interest income received in 2014 relating to the 2013 and 2014 IEIMA revenue requirement reconciliation regulatory assets.
	(c)	Includes Ameren Illinois' income earned in 2014 from customer-requested construction.
	(d)	Includes Ameren Missouri's interest income relating to a refund of charges included in an expired power purchase agreement with Entergy. See Note 2 – Rate and Regulatory Matters for additional information.
	(e)	Includes Ameren Illinois' one-time $7.5 million contribution to the Illinois Science and Energy Innovation Trust pursuant to the IEIMA as a result of Ameren Illinois' participation in the electric delivery formula ratemaking process.

Derivative_Financial_Instrumen

Derivative Financial Instruments	12 Months Ended
	Dec. 31, 2014
Derivative Instrument Detail [Abstract]
DERIVATIVE FINANCIAL INSTRUMENTS	DERIVATIVE FINANCIAL INSTRUMENTS
	We use derivatives to manage the risk of changes in market prices for natural gas, power, and uranium, as well as the risk of changes in rail transportation surcharges through fuel oil hedges. Such price fluctuations may cause the following:
	•	an unrealized appreciation or depreciation of our contracted commitments to purchase or sell when purchase or sale prices under the commitments are compared with current commodity prices;
	•	market values of natural gas and uranium inventories that differ from the cost of those commodities in inventory; and
	•	actual cash outlays for the purchase of these commodities that differ from anticipated cash outlays.
	The derivatives that we use to hedge these risks are governed by our risk management policies for forward contracts, futures, options, and swaps. Our net positions are continually assessed within our structured hedging programs to determine whether new or offsetting transactions are required. The goal of the hedging program is generally to mitigate financial risks while ensuring that sufficient volumes are available to meet our requirements. Contracts we enter into as part of our risk management program may be settled financially, settled by physical delivery, or net settled with the counterparty.
	The following table presents open gross commodity contract volumes by commodity type for derivative assets and liabilities as of December 31, 2014 and 2013. As of December 31, 2014, these contracts ran through October 2017, October 2019, May 2032, and October 2016 for fuel oils, natural gas, power, and uranium, respectively.
		Quantity (in millions, except as indicated)
		2014			2013
	Commodity	Ameren Missouri	Ameren Illinois	Ameren	Ameren Missouri	Ameren Illinois	Ameren
	Fuel oils (in gallons)(a)	50	(b)	50	66	(b)	66
	Natural gas (in mmbtu)	28	108	136	28	108	136
	Power (in megawatthours)	1	11	12	3	11	14
	Uranium (pounds in thousands)	332	(b)	332	796	(b)	796
	(a)	Fuel oils consist of heating oil, ultra-low-sulfur diesel, and crude oil.
	(b)	Not applicable.
	Authoritative accounting guidance regarding derivative instruments requires that all contracts considered to be derivative instruments be recorded on the balance sheet at their fair values, unless the NPNS exception applies. See Note 8 – Fair Value Measurements for discussion of our methods of assessing the fair value of derivative instruments. Many of our physical contracts, such as our purchased power contracts, qualify for the NPNS exception to derivative accounting rules. The revenue or expense on NPNS contracts is recognized at the contract price upon physical delivery.
	If we determine that a contract meets the definition of a derivative and is not eligible for the NPNS exception, we review the contract to determine if it qualifies for hedge accounting. We also consider whether gains or losses resulting from such derivatives qualify for regulatory deferral. Derivative contracts that qualify for regulatory deferral are recorded at fair value, with changes in fair value recorded as regulatory assets or regulatory liabilities in the period in which the change occurs. We believe derivative losses and gains deferred as regulatory assets and regulatory liabilities are probable of recovery or refund through future rates charged to customers. Regulatory assets and regulatory liabilities are amortized to operating income as related losses and gains are reflected in rates charged to customers. Therefore, gains and losses on these derivatives have no effect on operating income. As of December 31, 2014 and 2013, all contracts that qualify for hedge accounting receive regulatory deferral.
	Authoritative accounting guidance permits companies to offset fair value amounts recognized for the right to reclaim cash collateral (a receivable) or the obligation to return cash collateral (a liability) against fair value amounts recognized for derivative instruments that are executed with the same counterparty under a master netting arrangement. The Ameren Companies did not elect to adopt this guidance for any eligible commodity contracts.
	The following table presents the carrying value and balance sheet location of all derivative commodity contracts, none of which were designated as hedging instruments, as of December 31, 2014 and 2013:
		Balance Sheet Location		Ameren			Ameren			Ameren
				Missouri			Illinois
	2014
	Fuel oils	Other current assets	$	2		$	—		$	2
	Natural gas	Other current assets		1			1			2
	Power	Other current assets		15			—			15
		Total assets	$	18		$	1		$	19
	Fuel oils	Other current liabilities	$	22		$	—		$	22
		Other deferred credits and liabilities		7			—			7
	Natural gas	MTM derivative liabilities		(a)			31			(a)
		Other current liabilities		6			—			37
		Other deferred credits and liabilities		6			13			19
	Power	MTM derivative liabilities		(a)			11			(a)
		Other current liabilities		3			—			14
		Other deferred credits and liabilities		—			131			131
	Uranium	Other current liabilities		2			—			2
		Total liabilities	$	46		$	186		$	232
	2013
	Fuel oils	Other current assets	$	6		$	—		$	6
		Other assets		3			—			3
	Natural gas	Other current assets		1			1			2
	Power	Other current assets		23			—			23
		Total assets	$	33		$	1		$	34
	Fuel oils	Other current liabilities	$	2		$	—		$	2
		Other deferred credits and liabilities		1			—			1
	Natural gas	MTM derivative liabilities		(a)			27			(a)
		Other current liabilities		5			—			32
		Other deferred credits and liabilities		6			19			25
	Power	MTM derivative liabilities		(a)			9			(a)
		Other current liabilities		4			—			13
		Other deferred credits and liabilities		—			99			99
	Uranium	Other current liabilities		5			—			5
		Other deferred credits and liabilities		1			—			1
		Total liabilities	$	24		$	154		$	178
	(a)	Balance sheet line item not applicable to registrant.
	The following table presents the cumulative amount of pretax net gains (losses) on all derivative instruments deferred in regulatory assets or regulatory liabilities as of December 31, 2014 and 2013:
			Ameren			Ameren			Ameren
			Missouri			Illinois
	2014
	Fuel oils derivative contracts(a)	$	(29	)	$	—		$	(29	)
	Natural gas derivative contracts(b)		(11	)		(43	)		(54	)
	Power derivative contracts(c)		12			(142	)		(130	)
	Uranium derivative contracts(d)		(2	)		—			(2	)
	2013
Fuel oils derivative contracts	$	2		$	—		$	2
Natural gas derivative contracts		(10	)		(45	)		(55	)
Power derivative contracts		19			(108	)		(89	)
Uranium derivative contracts		(6	)		—			(6	)
(a)	Represents net losses associated with fuel oils derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s rail transportation surcharges for coal through December 2017. Current losses deferred as regulatory assets include $21 million and $21 million at Ameren and Ameren Missouri, respectively.
(b)	Represents net losses associated with natural gas derivative contracts. These contracts are a partial hedge of natural gas requirements through October 2019 at Ameren and Ameren Missouri and through October 2018 at Ameren Illinois. Current gains deferred as regulatory liabilities include $2 million, $1 million, and $1 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively. Current losses deferred as regulatory assets include $37 million, $6 million, and $31 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively.
(c)	Represents net gains (losses) associated with power derivative contracts. These contracts are a partial hedge of power price requirements through May 2032 at Ameren and Ameren Illinois and through December 2015 at Ameren Missouri. Current gains deferred as regulatory liabilities include $15 million and $15 million at Ameren and Ameren Missouri, respectively. Current losses deferred as regulatory assets include $14 million, $3 million, and $11 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively.
(d)	Represents net losses associated with uranium derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri's uranium requirements through December 2016. Current losses deferred as regulatory assets include $2 million and $2 million at Ameren and Ameren Missouri, respectively.
Derivative instruments are subject to various credit-related losses in the event of nonperformance by counterparties to the transaction. Exchange-traded contracts are supported by the financial and credit quality of the clearing members of the respective exchanges and have nominal credit risk. In all other transactions, we are exposed to credit risk. Our credit risk management program involves establishing credit limits and collateral requirements for counterparties, using master netting arrangements, and reporting daily exposure to senior management.
We believe that entering into master netting arrangements mitigates the level of financial loss that could result from default by allowing net settlement of derivative assets and liabilities. We generally enter into the following master netting arrangements: (1) the International Swaps and Derivatives Association Agreement, a standardized financial natural gas and electric contract; (2) the Master Power Purchase and Sale Agreement, created by the Edison Electric Institute and the National Energy Marketers Association, a standardized contract for the purchase and sale of wholesale power; and (3) the North American Energy Standards Board Inc. Agreement, a standardized contract for the purchase and sale of natural gas. These master netting arrangements allow the counterparties to net settle sale and purchase transactions. Further, collateral requirements are calculated at the master netting arrangement level by counterparty.
The following table provides the recognized gross derivative balances and the net amounts of those derivatives subject to an enforceable master netting arrangement or similar agreement as of December 31, 2014 and 2013:
					Gross Amounts Not Offset on the Balance Sheet
Commodity Contracts Eligible to be Offset		Gross Amounts Recognized on the Balance Sheet			Derivative Instruments			Cash Collateral Received/Posted(a)			Net
											Amount
2014
Assets:
Ameren Missouri	$	18		$	5		$	—		$	13
Ameren Illinois		1			—			—			1
Ameren	$	19		$	5		$	—		$	14
Liabilities:
Ameren Missouri	$	46		$	5		$	5		$	36
Ameren Illinois		186			—			—			186
Ameren	$	232		$	5		$	5		$	222
2013
Assets:
Ameren Missouri	$	33		$	9		$	—		$	24
Ameren Illinois		1			1			—			—
Ameren	$	34		$	10		$	—		$	24
Liabilities:
Ameren Missouri	$	24		$	9		$	9		$	6
Ameren Illinois		154			1			15			138
Ameren	$	178		$	10		$	24		$	144
(a)	Cash collateral received reduces gross asset balances and is included in “Other current liabilities” and “Other deferred credits and liabilities” on the balance sheet. Cash collateral posted reduces gross liability balances and is included in “Other current assets” and “Other assets” on the balance sheet.
Concentrations of Credit Risk
In determining our concentrations of credit risk related to derivative instruments, we review our individual counterparties and categorize each counterparty into groupings according to the primary business in which each engages. We calculate maximum exposures based on the gross fair value of financial instruments, including NPNS and other accrual contracts. As of December 31, 2014, if counterparty groups were to fail completely to perform on contracts, Ameren, Ameren Missouri, and Ameren Illinois' maximum exposure was $5 million, $5 million, and $- million, respectively. The potential loss on counterparty exposures is reduced by the application of master netting arrangements and collateral held, to the extent of reducing the exposure to zero. As of December 31, 2014, the potential loss after consideration of the application of master netting arrangements and collateral held for Ameren, Ameren Missouri, and Ameren Illinois was $5 million, $5 million, and $- million, respectively.
Derivative Instruments with Credit Risk-Related Contingent Features
Our commodity contracts contain collateral provisions tied to the Ameren Companies’ credit ratings. If we were to experience an adverse change in our credit ratings, or if a counterparty with reasonable grounds for uncertainty regarding performance of an obligation requested adequate assurance of performance, additional collateral postings might be required. The following table presents, as of December 31, 2014, the aggregate fair value of all derivative instruments with credit risk-related contingent features in a gross liability position, the cash collateral posted, and the aggregate amount of additional collateral that could be required to be posted with counterparties. The additional collateral required is the net liability position allowed under the master netting arrangements assuming (1) the credit risk-related contingent features underlying these arrangements were triggered on December 31, 2014, and (2) those counterparties with rights to do so requested collateral.
	Aggregate Fair Value of				Cash				Potential Aggregate Amount of
	Derivative Liabilities(a)				Collateral Posted				Additional Collateral Required(b)
Ameren Missouri	$	96			$	4			$	88
Ameren Illinois	74				—				71
Ameren	$	170			$	4			$	159
(a)	Prior to consideration of master netting arrangements and including NPNS and other accrual contract exposures.
(b)	As collateral requirements with certain counterparties are based on master netting arrangements, the aggregate amount of additional collateral required to be posted is determined after consideration of the effects of such arrangements.

Fair_Value_Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	FAIR VALUE MEASUREMENTS
	Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. We use various methods to determine fair value, including market, income, and cost approaches. With these approaches, we adopt certain assumptions that market participants would use in pricing the asset or liability, including assumptions about market risk or the risks inherent in the inputs to the valuation. Inputs to valuation can be readily observable, market-corroborated, or unobservable. We use valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs. Authoritative accounting guidance established a fair value hierarchy that prioritizes the inputs used to measure fair value. All financial assets and liabilities carried at fair value are classified and disclosed in one of the following three hierarchy levels:
	Level 1: Inputs based on quoted prices in active markets for identical assets or liabilities. Level 1 assets and liabilities are primarily exchange-traded derivatives and assets, including cash and cash equivalents and listed equity securities, such as those held in Ameren Missouri’s nuclear decommissioning trust fund.
	The market approach is used to measure the fair value of equity securities held in Ameren Missouri's nuclear decommissioning trust fund. Equity securities in this fund are representative of the S&P 500 index, excluding securities of Ameren Corporation, owners and/or operators of nuclear power plants and the trustee and investment managers. The S&P 500 index comprises stocks of large-capitalization companies.
	Level 2: Market-based inputs corroborated by third-party brokers or exchanges based on transacted market data. Level 2 assets and liabilities include certain assets held in Ameren Missouri’s nuclear decommissioning trust fund, including corporate bonds and other fixed-income securities, United States Treasury and agency securities, and certain over-the-counter derivative instruments, including natural gas and financial power transactions.
	Fixed income securities are valued using prices from independent industry recognized data vendors who provide values that are either exchange-based or matrix-based. The fair value measurements of fixed income securities classified as Level 2 are based on inputs other than quoted prices that are observable for the asset or liability. Examples are matrix pricing, market corroborated pricing, and inputs such as yield curves and indices. Level 2 fixed income securities in the nuclear decommissioning trust fund are primarily corporate bonds, asset-backed securities and United States agency bonds.
	Derivative instruments classified as Level 2 are valued by corroborated observable inputs, such as pricing services or prices from similar instruments that trade in liquid markets. Our development and corroboration process entails obtaining multiple quotes or prices from outside sources. To derive our forward view to price our derivative instruments at fair value, we average the midpoints of the bid/ask spreads. To validate forward prices obtained from outside parties, we compare the pricing to recently settled market transactions. Additionally, a review of all sources is performed to identify any anomalies or potential errors. Further, we consider the volume of transactions on certain trading platforms in our reasonableness assessment of the averaged midpoint. Natural gas derivative contracts are valued based upon exchange closing prices without significant unobservable adjustments. Power derivatives contracts are valued based upon the use of multiple forward prices provided by third parties. The prices are averaged and shaped to a monthly profile when needed without significant unobservable adjustments.
	Level 3: Unobservable inputs that are not corroborated by market data. Level 3 assets and liabilities are valued by internally developed models and assumptions or methodologies that use significant unobservable inputs. Level 3 assets and liabilities include derivative instruments that trade in less liquid markets, where pricing is largely unobservable. We value Level 3 instruments by using pricing models with inputs that are often unobservable in the market, as well as certain internal assumptions. Our development and corroboration process entails obtaining multiple quotes or prices from outside sources. As a part of our reasonableness review, an evaluation of all sources is performed to identify any anomalies or potential errors.
	We perform an analysis each quarter to determine the appropriate hierarchy level of the assets and liabilities subject to fair value measurements. Financial assets and liabilities are classified in their entirety according to the lowest level of input that is significant to the fair value measurement. All assets and liabilities whose fair value measurement is based on significant unobservable inputs are classified as Level 3.
	The following table describes the valuation techniques and unobservable inputs for the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy for the period ended December 31, 2014:
			Fair Value										Weighted
			Assets			Liabilities				Valuation Technique(s)	Unobservable Input	Range	Average
	Level 3 Derivative asset and liability – commodity contracts(a):
	Ameren	Fuel oils	$	2		$	(8	)		Option model	Volatilities(%)(b)	Mar-39	32
										Discounted cash flow	Ameren Missouri credit risk(%)(b)(c)	0.43	(d)
											Escalation rate(%)(e)(f)	5	(d)
		Natural Gas	1			(2		)		Option model	Volatilities(%)(b)	31 - 144	63
											Nodal basis($/mmbtu)(e)	(0.40) - 0	-0.2
										Discounted cash flow	Nodal basis($/mmbtu)(e)	(0.40) - 0.10	-0.2
											Counterparty credit risk(%)(b)(c)	0.43 - 13	3
											Ameren Missouri and Ameren Illinois credit risk(%)(b)(c)	0.43	(d)
		Power(g)	11			(144		)		Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps($/MWh)(h)	27 - 50	32
											Estimated auction price for FTRs($/MW)(e)	(1,833) - 2,743	171
											Nodal basis($/MWh)(e)	(6) - 0	-2
											Counterparty credit risk(%)(b)(c)	0.26	(d)
											Ameren Missouri and Ameren Illinois credit risk(%)(b)(c)	0.43	(d)
										Fundamental energy production model	Estimated future gas prices($/mmbtu)(e)	5-Apr	4
											Escalation rate(%)(e)(i)	0 - 1	1
										Contract price allocation	Estimated renewable energy credit costs($/credit)(e)	7-May	6
		Uranium	—			(2		)		Discounted cash flow	Average forward uranium pricing($/pound)(e)	35 - 40	36
	Ameren Missouri	Fuel oils	$	2		$	(8	)		Option model	Volatilities(%)(b)	Mar-39	32
										Discounted cash flow	Ameren Missouri credit risk(%)(b)(c)	0.43	(d)
											Escalation rate(%)(e)(f)	5	(d)
		Natural Gas	—			(1		)		Option model	Volatilities(%)(b)	31 - 144	53
											Nodal basis($/mmbtu)(e)	(0.40) - 0	-0.3
										Discounted cash flow	Nodal basis($/mmbtu)(e)	-0.1	(d)
											Counterparty credit risk(%)(b)(c)	0.57 - 13	5
											Ameren Missouri credit risk(%)(b)(c)	0.43	(d)
		Power(g)	11			(2		)		Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps($/MWh)(b)	27 - 50	32
											Estimated auction price for FTRs($/MW)(e)	(1,833) - 2,743	171
											Counterparty credit risk(%)(b)(c)	0.26	(d)
											Ameren Missouri credit risk(%)(b)(c)	0.43	(d)
		Uranium	—			(2		)		Discounted cash flow	Average forward uranium pricing($/pound)(e)	35 - 40	36
	Ameren Illinois	Natural Gas	$	1		$	(1	)		Option model	Volatilities(%)(b)	50 - 144	94
											Nodal basis($/mmbtu)(e)	(0.10) - 0	-0.1
										Discounted cash flow	Nodal basis($/mmbtu)(e)	(0.40) - 0.10	-0.2
											Counterparty credit risk(%)(b)(c)	0.43 - 2	0.83
											Ameren Illinois credit risk(%)(b)(c)	0.43	(d)
			Fair Value										Weighted
			Assets			Liabilities				Valuation Technique(s)	Unobservable Input	Range	Average
		Power(g)	—			(142		)		Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps($/MWh)(e)	27 - 38	32
											Nodal basis($/MWh)(e)	(6) - 0	-2
											Ameren Illinois credit risk(%)(b)(c)	0.43	(d)
										Fundamental energy production model	Estimated future gas prices($/mmbtu)(e)	5-Apr	4
											Escalation rate(%)(e)(i)	0 - 1	1
										Contract price allocation	Estimated renewable energy credit costs($/credit)(e)	7-May	6
	(a)	The derivative asset and liability balances are presented net of counterparty credit considerations.
	(b)	Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.
	(c)	Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances.
	(d)	Not applicable.
	(e)	Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.
	(f)	Escalation rate applies to fuel oil prices 2017 and beyond.
	(g)	Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2018. Valuations beyond 2018 use fundamentally modeled pricing by month for peak and off-peak demand.
	(h)	The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes due to their opposing positions. As such, refer to the power sensitivity analysis for each company above.
(i)	Escalation rate applies to power prices 2026 and beyond.
The following table describes the valuation techniques and unobservable inputs for the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy as of December 31, 2013:
		Fair Value										Weighted
		Assets			Liabilities				Valuation Technique	Unobservable Input	Range	Average
Level 3 Derivative asset and liability – commodity contracts(a):
Ameren	Fuel oils	$	8		$	(3	)		Option model	Volatilities(%)(b)	Oct-35	16
									Discounted cash flow	Counterparty credit risk(%)(c)(d)	0.26 - 2	1
	Power(e)	21			(110		)		Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps($/MWh)(c)	25 - 51	32
										Estimated auction price for FTRs($/MW)(b)	(1,594) - 945	305
										Nodal basis($/MWh)(c)	(3) - (1)	-2
										Counterparty credit risk(%)(c)(d)	0.39 - 0.50	0.42
										Ameren Missouri and Ameren Illinois credit risk(%)(c)(d)	2	(f)
									Fundamental energy production model	Estimated future gas prices($/mmbtu)(b)	5-Apr	5
										Escalation rate(%)(b)(g)	4-Mar	4
									Contract price allocation	Estimated renewable energy credit costs($/credit)(b)	7-May	6
	Uranium	—			(6		)		Discounted cash flow	Average forward uranium pricing($/pound)(b)	34 - 41	36
Ameren Missouri	Fuel oils	$	8		$	(3	)		Option model	Volatilities(%)(b)	Oct-35	16
									Discounted cash flow	Counterparty credit risk(%)(c)(d)	0.26 - 2	1
	Power(e)	21			(2		)		Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps($/MWh)(c)	25 - 51	40
										Estimated auction price for FTRs($/MW)(b)	(1,594) - 945	305
										Nodal basis($/MWh)(c)	(3) - (1)	-2
										Counterparty credit risk(%)(c)(d)	0.39 - 0.50	0.42
										Ameren Missouri credit risk(%)(c)(d)	2	(f)
	Uranium	—			(6		)		Discounted cash flow	Average forward uranium pricing($/pound)(b)	34 - 41	36
Ameren Illinois	Power(e)	$	—		$	(108	)		Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps($/MWh)(b)	27 - 36	30
										Nodal basis($/MWh)(b)	(4) - 0	-2
										Ameren Illinois credit risk(%)(c)(d)	2	(f)
									Fundamental energy production model	Estimated future gas prices($/mmbtu)(b)	5-Apr	5
										Escalation rate(%)(b)(g)	4-Mar	4
									Contract price allocation	Estimated renewable energy credit costs($/credit)(b)	7-May	6
(a)	The derivative asset and liability balances are presented net of counterparty credit considerations.
(b)	Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.
(c)	Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.
(d)	Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances.
(e)	Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2017. Valuations beyond 2017 use fundamentally modeled pricing by month for peak and off-peak demand.
(f)	Not applicable.
(g)	Escalation rate applies to power prices 2026 and beyond.
In accordance with applicable authoritative accounting guidance, we consider nonperformance risk in our valuation of derivative instruments by analyzing the credit standing of our counterparties and considering any counterparty credit enhancements (e.g., collateral). The guidance also requires that the fair value measurement of liabilities reflect the nonperformance risk of the reporting entity, as applicable. Therefore, we have factored the impact of our credit standing, as well as any potential credit enhancements, into the fair value measurement of both derivative assets and derivative liabilities. Included in our valuation, and based on current market conditions, is a valuation adjustment for counterparty default derived from market data such as the price of credit default swaps, bond yields, and credit ratings. No gains or losses related to valuation adjustments for counterparty default risk were recorded at Ameren, Ameren Missouri, or Ameren Illinois in 2014, 2013 or 2012. At December 31, 2014, the counterparty default risk liability valuation adjustment related to derivative contracts totaled $1 million, less than $1 million, and $1 million, for Ameren, Ameren Missouri, and Ameren Illinois, respectively. At December 31, 2013, the counterparty default risk liability valuation adjustment related to derivative contracts totaled $3 million, less than $1 million, and $3 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively.
The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of December 31, 2014:
			Quoted Prices in				Significant Other				Significant Other				Total
			Active Markets for				Observable				Unobservable
			Identical Assets				Inputs				Inputs
			or Liabilities				(Level 2)				(Level 3)
			(Level 1)
Assets:
Ameren	Derivative assets – commodity contracts(a):
	Fuel oils		$	—			$	—			$	2			$	2
	Natural gas		—				1				1				2
	Power		—				4				11				15
	Total derivative assets – commodity contracts		$	—			$	5			$	14			$	19
	Nuclear decommissioning trust fund:
	Cash and cash equivalents		$	1			$	—			$	—			$	1
	Equity securities:
	U.S. large capitalization		364				—				—				364
	Debt securities:
	Corporate bonds		—				63				—				63
	Municipal bonds		—				2				—				2
	U.S. treasury and agency securities		—				102				—				102
	Asset-backed securities		—				10				—				10
	Other		—				5				—				5
	Total nuclear decommissioning trust fund		$	365			$	182			$	—			$	547	(b)
	Total Ameren		$	365			$	187			$	14			$	566
Ameren Missouri	Derivative assets – commodity contracts(a):
	Fuel oils		$	—			$	—			$	2			$	2
	Natural gas		—				1				—				1
	Power		—				4				11				15
	Total derivative assets – commodity contracts		$	—			$	5			$	13			$	18
			Quoted Prices in				Significant Other				Significant Other				Total
			Active Markets for				Observable				Unobservable
			Identical Assets				Inputs				Inputs
			or Liabilities				(Level 2)				(Level 3)
			(Level 1)
	Nuclear decommissioning trust fund:
	Cash and cash equivalents		$	1			$	—			$	—			$	1
	Equity securities:
	U.S. large capitalization		364				—				—				364
	Debt securities:
	Corporate bonds		—				63				—				63
	Municipal bonds		—				2				—				2
	U.S. treasury and agency securities		—				102				—				102
	Asset-backed securities		—				10				—				10
	Other		—				5				—				5
	Total nuclear decommissioning trust fund		$	365			$	182			$	—			$	547	(b)
	Total Ameren Missouri		$	365			$	187			$	13			$	565
Ameren Illinois	Derivative assets – commodity contracts(a):
	Natural gas		$	—			$	—			$	1			$	1
Liabilities:
Ameren	Derivative liabilities – commodity contracts(a):
	Fuel oils		$	21			$	—			$	8			$	29
	Natural gas		1				53				2				56
	Power		—				1				144				145
	Uranium		—				—				2				2
	Total Ameren		$	22			$	54			$	156			$	232
Ameren Missouri	Derivative liabilities – commodity contracts(a):
	Fuel oils		$	21			$	—			$	8			$	29
	Natural gas		1				10				1				12
	Power		—				1				2				3
	Uranium		—				—				2				2
	Total Ameren Missouri		$	22			$	11			$	13			$	46
Ameren Illinois	Derivative liabilities – commodity contracts(a):
	Natural gas		$	—			$	43			$	1			$	44
	Power		—				—				142				142
	Total Ameren Illinois		$	—			$	43			$	143			$	186
(a)	The derivative asset and liability balances are presented net of counterparty credit considerations.
(b)	Balance excludes $2 million of receivables, payables, and accrued income, net.
The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of December 31, 2013:
			Quoted Prices in				Significant Other				Significant				Total
			Active Markets for				Observable Inputs				Other
			Identical Assets				(Level 2)				Unobservable
			or Liabilities								Inputs
			(Level 1)								(Level 3)
Assets:
Ameren	Derivative assets – commodity contracts(a):
	Fuel oils		$	1			$	—			$	8			$	9
	Natural gas		—				2				—				2
	Power		—				2				21				23
	Total derivative assets – commodity contracts		$	1			$	4			$	29			$	34
			Quoted Prices in				Significant Other				Significant				Total
			Active Markets for				Observable Inputs				Other
			Identical Assets				(Level 2)				Unobservable
			or Liabilities								Inputs
			(Level 1)								(Level 3)
	Nuclear decommissioning trust fund:
	Cash and cash equivalents		$	3			$	—			$	—			$	3
	Equity securities:
	U.S. large capitalization		332				—				—				332
	Debt securities:
	Corporate bonds		—				52				—				52
	Municipal bonds		—				2				—				2
	U.S. treasury and agency securities		—				94				—				94
	Asset-backed securities		—				10				—				10
	Other		—				1				—				1
	Total nuclear decommissioning trust fund		$	335			$	159			$	—			$	494
	Total Ameren		$	336			$	163			$	29			$	528
Ameren Missouri	Derivative assets – commodity contracts(a):
	Fuel oils		$	1			$	—			$	8			$	9
	Natural gas		—				1				—				1
	Power		—				2				21				23
	Total derivative assets – commodity contracts		$	1			$	3			$	29			$	33
	Nuclear decommissioning trust fund:
	Cash and cash equivalents		$	3			$	—			$	—			$	3
	Equity securities:
	U.S. large capitalization		332				—				—				332
	Debt securities:
	Corporate bonds		—				52				—				52
	Municipal bonds		—				2				—				2
	U.S. treasury and agency securities		—				94				—				94
	Asset-backed securities		—				10				—				10
	Other		—				1				—				1
	Total nuclear decommissioning trust fund		$	335			$	159			$	—			$	494
	Total Ameren Missouri		$	336			$	162			$	29			$	527
Ameren Illinois	Derivative assets – commodity contracts(a):
	Natural gas		$	—			$	1			$	—			$	1
Liabilities:
Ameren	Derivative liabilities – commodity contracts(a):
	Fuel oils		$	—			$	—			$	3			$	3
	Natural gas		3				54				—				57
	Power		—				2				110				112
	Uranium		—				—				6				6
	Total Ameren		$	3			$	56			$	119			$	178
Ameren Missouri	Derivative liabilities – commodity contracts(a):
	Fuel oils		$	—			$	—			$	3			$	3
	Natural gas		3				8				—				11
	Power		—				2				2				4
	Uranium		—				—				6				6
	Total Ameren Missouri		$	3			$	10			$	11			$	24
Ameren Illinois	Derivative liabilities – commodity contracts(a):
	Natural gas		$	—			$	46			$	—			$	46
	Power		—				—				108				108
	Total Ameren Illinois		$	—			$	46			$	108			$	154
(a)	The derivative asset and liability balances are presented net of counterparty credit considerations.
The following table summarizes the changes in the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy for the year ended December 31, 2014:
		Net Derivative Commodity Contracts
		Ameren			Ameren			Ameren
		Missouri			Illinois
Fuel oils:
Beginning balance at January 1, 2014	$	5		$	(a)		$	5
Realized and unrealized gains (losses) included in regulatory assets/liabilities:		(9	)		(a)			(9	)
Settlements		(2	)		(a)			(2	)
Ending balance at December 31, 2014	$	(6	)	$	(a)		$	(6	)
Change in unrealized gains (losses) related to assets/liabilities held at December 31, 2014	$	(6	)	$	(a)		$	(6	)
Natural gas:
Beginning balance at January 1, 2014	$	—		$	—		$	—
Realized and unrealized gains (losses) included in regulatory assets/liabilities:		—			1			1
Purchases		—			(2	)		(2	)
Sales		(1	)		—			(1	)
Settlements		—			1			1
Ending balance at December 31, 2014	$	(1	)	$	—		$	(1	)
Change in unrealized gains (losses) related to assets/liabilities held at December 31, 2014	$	—		$	2		$	2
Power:
Beginning balance at January 1, 2014	$	19		$	(108	)	$	(89	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities:		(14	)		(39	)		(53	)
Purchases		34			—			34
Sales		(1	)		—			(1	)
Settlements		(29	)		5			(24	)
Ending balance at December 31, 2014	$	9		$	(142	)	$	(133	)
Change in unrealized gains (losses) related to assets/liabilities held at December 31, 2014	$	—		$	(43	)	$	(43	)
Uranium:
Beginning balance at January 1, 2014	$	(6	)	$	(a)		$	(6	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities:		(1	)		(a)			(1	)
Settlements		5			(a)			5
Ending balance at December 31, 2014	$	(2	)	$	(a)		$	(2	)
Change in unrealized gains (losses) related to assets/liabilities held at December 31, 2014	$	(1	)	$	(a)		$	(1	)
(a)	Not applicable.
The following table summarizes the changes in the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy for the year ended December 31, 2013:
		Net Derivative Commodity Contracts
		Ameren			Ameren			Ameren
		Missouri			Illinois
Fuel oils:
Beginning balance at January 1, 2013	$	5		$	(a)		$	5
Purchases		3			(a)			3
Sales		(1	)		(a)			(1	)
Settlements		(2	)		(a)			(2	)
Ending balance at December 31, 2013	$	5		$	(a)		$	5
Change in unrealized gains (losses) related to assets/liabilities held at December 31, 2013	$	—		$	(a)		$	—
Natural gas:
Beginning balance at January 1, 2013	$	—		$	—		$	—
Realized and unrealized gains (losses) included in regulatory assets/liabilities:		—			(1	)		(1	)
Purchases		—			1			1
Ending balance at December 31, 2013	$	—		$	—		$	—
Change in unrealized gains (losses) related to assets/liabilities held at December 31, 2013	$	—		$	—		$	—
Power:
Beginning balance at January 1, 2013	$	11		$	(111	)	$	(100	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities:		3			(18	)		(15	)
Purchases		40			—			40
Settlements		(36	)		21			(15	)
Transfers into Level 3		(3	)		—			(3	)
Transfers out of Level 3		4			—			4
Ending balance at December 31, 2013	$	19		$	(108	)	$	(89	)
Change in unrealized gains (losses) related to assets/liabilities held at December 31, 2013	$	(1	)	$	(24	)	$	(25	)
Uranium:
Beginning balance at January 1, 2013	$	(2	)	$	(a)		$	(2	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities:		(3	)		(a)			(3	)
Purchases		(2	)		(a)			(2	)
Settlements		1			(a)			1
Ending balance at December 31, 2013	$	(6	)	$	(a)		$	(6	)
Change in unrealized gains (losses) related to assets/liabilities held at December 31, 2013	$	(2	)	$	(a)		$	(2	)
(a)	Not applicable.
Transfers in or out of Level 3 represent either (1) existing assets and liabilities that were previously categorized as a higher level, but were recategorized to Level 3 because the inputs to the model became unobservable during the period or (2) existing assets and liabilities that were previously classified as Level 3, but were recategorized to a higher level because the lowest significant input became observable during the period. Transfers between Level 2 and Level 3 for power derivatives were primarily caused by changes in availability of financial trades observable on electronic exchanges between the periods. Any reclassifications are reported as transfers out of Level 3 at the fair value measurement reported at the beginning of the period in which the changes occur. For the years ended December 31, 2014 and 2013, there were no transfers between Level 1 and Level 2 related to derivative commodity contracts. For the year ended December 31, 2014, there were no transfers between Level 2 and Level 3 related to derivative commodity contracts. For the year ended December 31, 2013, there were $(3) million of transfers out of Level 2 into Level 3 and $4 million of transfers into Level 2 out of Level 3 related to power contracts at Ameren and Ameren Missouri.
See Note 11 – Retirement Benefits for the fair value hierarchy tables detailing Ameren’s pension and postretirement plan assets as of December 31, 2014, as well as a table summarizing the changes in Level 3 plan assets during 2014.
The Ameren Companies’ carrying amounts of cash and cash equivalents approximate fair value because of the short-term nature of these instruments. They are considered to be Level 1 in the fair value hierarchy. Ameren's and Ameren Missouri's carrying amounts of investments in debt securities related to the two CTs from the city of Bowling Green and Audrain County approximate fair value. These investments are classified as held-to-maturity. These investments are considered Level 2 in the fair value hierarchy, as they are valued based on similar market transactions. The Ameren Companies' short-term borrowings also approximate fair value because of their short-term nature. Short-term borrowings are considered to be Level 2 in the fair value hierarchy, as they are valued based on market rates for similar market transactions. The estimated fair value of long-term debt and preferred stock is based on the quoted market prices for same or similar issuances for companies with similar credit profiles or on the current rates offered to the Ameren Companies for similar financial instruments, which fair value measurement is considered Level 2 in the fair value hierarchy.
The following table presents the carrying amounts and estimated fair values of our long-term debt and preferred stock at December 31, 2014 and 2013:
	2014								2013
	Carrying Amount				Fair Value				Carrying Amount				Fair Value
Ameren:(a)
Long-term debt and capital lease obligations (including current portion)	$	6,240			$	7,135			$	6,038			$	6,584
Preferred stock	142				122				142				118
Ameren Missouri:
Long-term debt and capital lease obligations (including current portion)	$	3,999			$	4,518			$	3,757			$	4,124
Preferred stock	80				73				80				71
Ameren Illinois:
Long-term debt (including current portion)	$	2,241			$	2,517			$	1,856			$	2,028
Preferred stock	62				49				62				47
(a)	Preferred stock is recorded in "Noncontrolling Interests" on the consolidated balance sheet.

Nuclear_Decommissioning_Trust_

Nuclear Decommissioning Trust Fund Investments	12 Months Ended
	Dec. 31, 2014
Investments, Debt and Equity Securities [Abstract]
NUCLEAR DECOMMISSIONING TRUST FUND INVESTMENTS	NUCLEAR DECOMMISSIONING TRUST FUND INVESTMENTS
	Ameren Missouri has investments in debt and equity securities that are held in a trust fund for the purpose of funding the decommissioning of its Callaway energy center. We have classified these investments as available for sale, and we have recorded all such investments at their fair market value at December 31, 2014, and 2013. See Note 10 – Callaway Energy Center for additional information.
	Investments in the nuclear decommissioning trust fund have a target allocation of 60% to 70% in equity securities, with the balance invested in debt securities.
	The following table presents proceeds from the sale and maturities of investments in Ameren Missouri’s nuclear decommissioning trust fund and the gross realized gains and losses resulting from those sales for the years ended December 31, 2014, 2013, and 2012:
		2014			2013			2012
	Proceeds from sales and maturities	$	391		$	196		$	384
	Gross realized gains	7			7			6
	Gross realized losses	2			5			2
	Net realized and unrealized gains and losses are deferred and recorded as regulatory assets or regulatory liabilities on Ameren’s and Ameren Missouri’s balance sheets. This reporting is consistent with the method used to account for the decommissioning costs recovered in rates. Gains or losses associated with assets in the trust fund could result in lower or higher funding requirements for decommissioning costs, which are expected to be reflected in electric rates paid by Ameren Missouri’s customers. See Note 2 – Rate and Regulatory Matters.
	The following table presents the costs and fair values of investments in debt and equity securities in Ameren Missouri’s nuclear decommissioning trust fund at December 31, 2014 and 2013:
	Security Type	Cost			Gross Unrealized Gain			Gross Unrealized Loss		Fair Value
	2014
	Debt securities	$	175		$	7	$	(a)		$	182
	Equity securities	138			230			4		364
	Cash	1			—			—		1
	Other(b)	2			—			—		2
	Total	$	316		$	237	$	4		$	549
	2013
	Debt securities	$	157		$	4	$	2		$	159
	Equity securities	137			199			4		332
	Cash	3			—			—		3
	Other(b)	(a)			—			—		(a)
	Total	$	297		$	203	$	6		$	494
	(a)	Amount less than $1 million.
	(b)	Represents payables relating to pending security purchases, net of receivables related to pending security sales and interest receivables.
	The following table presents the costs and fair values of investments in debt securities in Ameren Missouri’s nuclear decommissioning trust fund according to their contractual maturities at December 31, 2014:
		Cost			Fair Value
	Less than 5 years	$	98		$	99
	5 years to 10 years	41			42
	Due after 10 years	36			41
	Total	$	175		$	182
	We have unrealized losses relating to certain available-for-sale investments included in our decommissioning trust fund, recorded as regulatory assets as discussed above. Decommissioning will not occur until the operating license for our nuclear energy center expires. Ameren Missouri submitted a license extension application to the NRC to extend the Callaway energy center’s operating license to 2044. The following table presents the fair value and the gross unrealized losses of the available-for-sale securities held in Ameren Missouri's nuclear decommissioning trust fund. They are aggregated by investment category and the length of time that individual securities have been in a continuous unrealized loss position at December 31, 2014:
		Less than 12 Months						12 Months or Greater				Total
		Fair Value			Gross			Fair Value			Gross	Fair Value			Gross
					Unrealized						Unrealized				Unrealized
					Losses						Losses				Losses
	Debt securities	$	28	$	(a)			$	8	$	(a)	$	36	$	(a)
	Equity securities	6			1			5			3	11			4
	Total	$	34	$	1			$	13	$	3	$	47	$	4
	(a)	Amount less than $1 million.

Callaway_Energy_Center

Callaway Energy Center	12 Months Ended
	Dec. 31, 2014
Nuclear Waste Matters [Abstract]
CALLAWAY ENERGY CENTER	CALLAWAY ENERGY CENTER
	Under the NWPA, the DOE is responsible for disposing of spent nuclear fuel from the Callaway energy center and other commercial nuclear energy centers. Under the NWPA, Ameren and other utilities that own and operate those energy centers are responsible for paying the disposal costs. The NWPA established the fee that these utilities pay the federal government for disposing of the spent nuclear fuel at one mill, or one-tenth of one cent, for each kilowatthour generated and sold by those plants. The NWPA also requires the DOE annually to review the nuclear waste fee against the cost of the nuclear waste disposal program and to propose to the United States Congress any fee adjustment necessary to offset the costs of the program. As required by the NWPA, Ameren Missouri and other utilities have entered into standard contracts with the federal government. The government, represented by the DOE, is responsible for implementing these provisions of the NWPA. Consistent with the NWPA and its standard contract, Ameren Missouri had historically collected one mill from its electric customers for each kilowatthour of electricity that it generates and sells from its Callaway energy center. However, as described below, Ameren Missouri has suspended collection of this fee.
	Although both the NWPA and the standard contract stated that the federal government would begin to dispose of spent nuclear fuel by 1998, the federal government is not meeting its disposal obligation. Ameren Missouri has sufficient installed capacity at the Callaway energy center to store its spent nuclear fuel generated through 2020 and it has the capability for additional storage capacity for spent nuclear fuel generated through the end of the energy center’s current licensed life. The DOE's delay in carrying out its obligation to dispose of spent nuclear fuel from the Callaway energy center is not expected to adversely affect the continued operations of the energy center.
	In January 2013, the DOE issued its plan for the management and disposal of spent nuclear fuel. The DOE's plan calls for a pilot interim storage facility to begin operation with an initial focus on accepting spent nuclear fuel from shutdown reactor sites by 2021. By 2025, a larger interim storage facility would be available, potentially co-located with the pilot facility on a geologic repository. The plan also proposes to begin operation of a permanent geological repository by 2048.
	Because the federal government is not meeting its disposal obligation, the Nuclear Energy Institute, a number of individual utilities, and the National Association of Regulatory Utility Commissioners sued the DOE in the United States Court of Appeals for the District of Columbia Circuit, seeking the suspension of the one mill nuclear waste fee. In November 2013, the court ordered the DOE to submit a proposal to the United States Congress to reduce the fee to zero. In January 2014, the DOE submitted that proposal, and it became effective in May 2014. Since the nuclear waste fee was previously included in Ameren Missouri’s FAC, the cost reduction will be passed on to electric utility customers with no material effect on Ameren’s or Ameren Missouri’s net income.
	As a result of the DOE's failure to begin to dispose of spent nuclear fuel from commercial nuclear energy centers and fulfill its contractual obligations, Ameren Missouri and other nuclear energy center owners have also sued the DOE to recover costs incurred for ongoing storage of their spent fuel. Ameren Missouri filed a breach of contract lawsuit to recover costs that it incurred through 2009. The lawsuit sought reimbursement for the cost of reracking the Callaway energy center’s spent fuel pool, for certain NRC fees, and for Missouri ad valorem taxes that Ameren Missouri would not have incurred had the DOE performed its contractual obligations. The parties entered into a settlement agreement that provides for annual recovery of additional spent fuel storage and related costs incurred from 2010 through 2016, with the ability to extend the recovery period as mutually agreed upon by the parties. Included in these reimbursements are costs related to a dry spent fuel storage facility that Ameren Missouri is constructing at its Callaway energy center. Ameren Missouri intends to begin transferring spent fuel assemblies to this facility in 2015. Ameren Missouri will continue to apply for reimbursement from the DOE for the cost to construct and operate the dry spent fuel storage facility along with related allowable costs.
	In December 2011, Ameren Missouri submitted a license extension application to the NRC to extend its Callaway energy center's operating license from 2024 to 2044. There is no deadline by which the NRC must act on this application. Among the rules upon which the NRC has historically relied in approving license extensions are rules dealing with the storage of spent nuclear fuel at the reactor site and with the NRC's confidence that permanent disposal of spent nuclear fuel will be available when needed. In a June 2012 decision, the United States Court of Appeals for the District of Columbia Circuit vacated these rules and remanded the case to the NRC, holding that the NRC's obligations under the National Environmental Policy Act required a more thorough environmental analysis in support of the NRC's waste confidence decision. As a result, the NRC stated that it would not issue licenses dependent on the vacated rules until it appropriately addressed the court's remand. In October 2014, after it completed the required environmental analysis, the NRC lifted its suspension on final licensing decisions. In February 2015, the staff of the NRC issued its recommendation that the NRC approve Ameren Missouri's application for a 20-year renewal of the Callaway energy center's operating license.
	Electric utility rates currently charged to customers provide for the recovery of the Callaway energy center's decommissioning costs, which include decontamination, dismantling, and site restoration costs, over an assumed 40-year life of the nuclear center, ending with the expiration of the energy center's current operating license in 2024. Amounts collected from customers are deposited into the external nuclear decommissioning trust fund to provide for the Callaway energy center’s decommissioning. It is assumed that the Callaway energy center site will be decommissioned through the immediate dismantlement method and removed from service. Ameren and Ameren Missouri have recorded an ARO for the Callaway energy center decommissioning costs at fair value, which represents the present value of estimated future cash outflows. Annual decommissioning costs of $7 million are included in the costs of service used to establish electric rates for Ameren Missouri's customers. Every three years, the MoPSC requires Ameren Missouri to file an updated cost study and funding analysis for decommissioning its Callaway energy center. Electric rates may be adjusted at such times to reflect changed estimates. The last cost study and funding analysis was filed with the MoPSC in September 2011. The MoPSC has authorized a delay of the 2014 cost study and funding analysis filing until 2015 pending the outcome of Ameren Missouri’s operating license extension application under review by the NRC. Following the NRC’s decision regarding Ameren Missouri’s operating license extension application, an updated cost study and a revised funding analysis will be filed. Rates charged to customers will be adjusted accordingly, as approved by the MoPSC, to reflect the operating license extension application decision, the updated cost study and the revised funding analysis. If the assumed return on trust assets is not earned, we believe that it is probable that any such earnings deficiency will be recovered in rates. The fair value of the trust fund for Ameren Missouri's Callaway energy center is reported as "Nuclear decommissioning trust fund" in Ameren's and Ameren Missouri's balance sheets. This amount is legally restricted and may be used only to fund the costs of nuclear decommissioning. Changes in the fair value of the trust fund are recorded as an increase or decrease to the nuclear decommissioning trust fund, with an offsetting adjustment to the related regulatory liability.
	See Note 2 – Rate and Regulatory Matters and Note 9 – Nuclear Decommissioning Trust Fund Investments for additional information related to the Callaway energy center.

Retirement_Benefits

Retirement Benefits	12 Months Ended
	Dec. 31, 2014
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]
RETIREMENT BENEFITS	RETIREMENT BENEFITS
	The primary objective of the Ameren pension and postretirement benefit plans is to provide eligible employees with pension and postretirement health care and life insurance benefits. Ameren offers defined benefit pension and postretirement benefit plans covering substantially all of its employees. Ameren uses a measurement date of December 31 for its pension and postretirement benefit plans. Ameren Missouri and Ameren Illinois each participate in Ameren’s single-employer pension and other postretirement plans. Ameren’s qualified pension plan is the Ameren Retirement Plan. Ameren also has an unfunded nonqualified pension plan, the Ameren Supplemental Retirement Plan, which is available for certain management employees and retirees to provide a supplemental benefit when their qualified pension plan benefits are capped to comply with Internal Revenue Code limitations. Ameren’s other postretirement plans are the Ameren Retiree Medical Plan and the Ameren Group Life Insurance Plan. Only Ameren subsidiaries participate in the plans listed above.
	In December 2013, Ameren completed the divestiture of New AER to IPH. In accordance with the transaction agreement, Ameren retained the pension obligations as of December 2, 2013, associated with the current and former employees of New AER and its subsidiaries who were included in the Ameren Retirement Plan and the Ameren Supplemental Retirement Plan. Ameren also retained the postretirement benefit obligations associated with the employees of New AER and its subsidiaries who were eligible to retire at December 2, 2013, and who were included in the Ameren Retiree Medical Plan and the Ameren Group Life Insurance Plan.
	Ameren’s unfunded obligation under its pension and other postretirement benefit plans was $710 million and $461 million as of December 31, 2014, and December 31, 2013, respectively. These net liabilities are recorded in "Other current liabilities," "Pension and other postretirement benefits," and "Other assets" on Ameren's consolidated balance sheet. The primary factor contributing to the increase in the unfunded obligation during 2014 was a 75 basis point decrease in the pension and other postretirement benefit plan discount rates used to determine the present value of the obligation. The offset to the increase in the unfunded obligation was primarily an increase to "Regulatory assets" on Ameren's, Ameren Missouri's, and Ameren Illinois' consolidated balance sheet.
	The following table presents the net benefit liability recorded on the balance sheets of each of the Ameren Companies as of December 31, 2014 and 2013:
		2014			2013
	Ameren(a)	$	710		$	461
	Ameren Missouri	277			191
	Ameren Illinois	278			159
	(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
	Ameren recognizes the underfunded status of its pension and postretirement plans as a liability on its consolidated balance sheet, with offsetting entries to accumulated OCI and regulatory assets, in accordance with authoritative accounting guidance. The following table presents the funded status of Ameren's pension and postretirement benefit plans as of December 31, 2014 and 2013. It also provides the amounts included in regulatory assets and accumulated OCI at December 31, 2014 and 2013, that have not been recognized in net periodic benefit costs.
		2014							2013
		Pension Benefits(a)				Postretirement			Pension Benefits(a)				Postretirement
						Benefits(a)							Benefits(a)
	Accumulated benefit obligation at end of year	$	4,176		$	(b)			$	3,698		$	(b)
	Change in benefit obligation:
	Net benefit obligation at beginning of year	$	3,900		$	1,096			$	4,051		$	1,157
	Service cost	79				19			91				22
	Interest cost	183				50			163				46
	Participant contributions	—				16			—				16
	Actuarial (gain) loss	462				84			(207		)		(76	)
	Curtailment gain(c)	—				—			—				(3	)
	Settlement(d)	—				—			—				(5	)
	Benefits paid	(214		)		(65	)		(198		)		(64	)
	Federal subsidy on benefits paid	(b)				3			(b)				3
	Net benefit obligation at end of year	4,410				1,203			3,900				1,096
	Change in plan assets:
	Fair value of plan assets at beginning of year	3,461				1,074			3,127				938
	Actual return on plan assets	448				75			376				156
	Employer contributions	99				6			156				25
	Federal subsidy on benefits paid	(b)				3			(b)				3
	Participant contributions	—				16			—				16
	Benefits paid	(214		)		(65	)		(198		)		(64	)
	Fair value of plan assets at end of year	3,794				1,109			3,461				1,074
	Funded status – deficiency	616				94			439				22
	Accrued benefit cost at December 31	$	616		$	94			$	439		$	22
	Amounts recognized in the balance sheet consist of:
	Noncurrent asset(e)	$	—		$	—			$	—		$	(9	)
	Current liability(f)	3				2			3				1
	Noncurrent liability	613				92			436				30
	Net liability recognized	$	616		$	94			$	439		$	22
	Amounts recognized in regulatory assets consist of:
	Net actuarial (gain) loss	$	452		$	(7	)		$	282		$	(71	)
	Prior service cost (credit)	(6		)		(16	)		(7		)		(20	)
	Amounts (pretax) recognized in accumulated OCI consist of:
	Net actuarial (gain) loss	29				(5	)		17				(12	)
	Prior service cost (credit)	—				(1	)		—				(1	)
	Total	$	475		$	(29	)		$	292		$	(104	)
	(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
	(b)	Not applicable.
	(c)	Effective with the divestiture of New AER on December 2, 2013, the liability for active management employees of New AER and its subsidiaries not eligible to retire were neither transferred to IPH nor retained by Ameren, which resulted in a curtailment gain. See Note 16 – Divestiture Transactions and Discontinued Operations for additional information on the divestiture.
	(d)	Effective with the divestiture of New AER on December 2, 2013, the liability for active union employees of New AER and its subsidiaries not eligible to retire was transferred to IPH based on the assumption of the collective bargaining agreements in place, which resulted in a settlement. See Note 16 – Divestiture Transactions and Discontinued Operations for additional information on the divestiture.
	(e)	Included in "Other assets" on Ameren's consolidated balance sheet.
	(f)	Included in "Other current liabilities" on Ameren's consolidated balance sheet.
	The following table presents the assumptions used to determine our benefit obligations at December 31, 2014 and 2013:
		Pension Benefits						Postretirement Benefits
		2014			2013			2014			2013
	Discount rate at measurement date	4	%		4.75	%		4	%		4.75	%
	Increase in future compensation	3.5			3.5			3.5			3.5
	Medical cost trend rate (initial)	(a)			(a)			5			5
	Medical cost trend rate (ultimate)	(a)			(a)			5			5
	Years to ultimate rate	(a)			(a)			—			—
	(a)	Not applicable
	Ameren determines discount rate assumptions by identifying a theoretical settlement portfolio of high-quality corporate bonds sufficient to provide for a plan's projected benefit payments. The settlement portfolio of bonds is selected from a pool of more than 700 high-quality corporate bonds. A single discount rate is then determined; that rate results in a discounted value of the plan's benefit payments that equates to the market value of the selected bonds. In addition, during 2014, Ameren adopted the Society of Actuaries 2014 Mortality Tables Report and Mortality Improvement Scale. The updated mortality tables assume increasing life expectancies for our employees and retirees, which resulted in an increase to our pension and other postretirement benefit obligations.
Funding
Pension benefits are based on the employees’ years of service and compensation. Ameren’s pension plans are funded in compliance with income tax regulations and federal funding or regulatory requirements. As a result, Ameren expects to fund its pension plan at a level equal to the greater of the pension expense or the legally required minimum contribution. Considering its assumptions at December 31, 2014, its investment performance in 2014, and its pension funding policy, Ameren expects to make annual contributions of $25 million to $115 million in each of the next five years, with aggregate estimated contributions of $290 million. We expect Ameren Missouri’s and Ameren Illinois’ portion of the future funding requirements to be 41% and 40%, respectively. These amounts are estimates. They may change based on actual investment performance, changes in interest rates, changes in our assumptions, changes in government regulations, and any voluntary contributions. Our funding policy for postretirement benefits is primarily to fund the Voluntary Employee Beneficiary Association (VEBA) trusts to match the annual postretirement expense.
The following table presents the cash contributions made to our defined benefit retirement plan and to our postretirement plans during 2014, 2013, and 2012:
	Pension Benefits												Postretirement Benefits
	2014				2013				2012				2014				2013		2012
Ameren Missouri	$	41			$	60			$	52			$	3			$	10	$	9
Ameren Illinois	39				50				46				2				11		35
Other	19				46				30				1				4		1
Ameren(a)	99				156				128				6				25		45
(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
Investment Strategy and Policies
Ameren manages plan assets in accordance with the “prudent investor” guidelines contained in ERISA. The investment committee, to the extent that authority is delegated to it by the finance committee of Ameren’s board of directors, implements investment strategy and asset allocation guidelines for the plan assets. The investment committee includes members of senior management. The investment committee’s goals are twofold: first, to ensure that sufficient funds are available to provide the benefits at the time they are payable; and second, to maximize total return on plan assets and to minimize expense volatility consistent with its tolerance for risk. Ameren delegates the task of investment management to specialists in each asset class. As appropriate, Ameren provides each investment manager with guidelines that specify allowable and prohibited investment types. The investment committee regularly monitors manager performance and compliance with investment guidelines.
The expected return on plan assets assumption is based on historical and projected rates of return for current and planned asset classes in the investment portfolio. Projected rates of return for each asset class were estimated after an analysis of historical experience, future expectations, and the volatility of the various asset classes. After considering the target asset allocation for each asset class, we adjusted the overall expected rate of return for the portfolio for historical and expected experience of active portfolio management results compared with benchmark returns and for the effect of expenses paid from plan assets. Ameren will use an expected return on plan assets for its pension plan assets and postretirement plan assets of 7.25% and 7.00%, respectively, in 2015. No plan assets are expected to be returned to Ameren during 2015.
Ameren’s investment committee strives to assemble a portfolio of diversified assets that does not create a significant concentration of risks. The investment committee develops asset allocation guidelines between asset classes, and it creates diversification through investments in assets that differ by type (equity, debt, real estate, private equity), duration, market capitalization, country, style (growth or value) and industry, among other factors. The diversification of assets is displayed in the target allocation table below. The investment committee also routinely rebalances the plan assets to adhere to the diversification goals. The investment committee’s strategy reduces the concentration of investment risk; however, Ameren is still subject to overall market risk. The following table presents our target allocations for 2015 and our pension and postretirement plans’ asset categories as of December 31, 2014 and 2013:
Asset	Target Allocation		Percentage of Plan Assets at December  31,
Category	2015		2014			2013
Pension Plan:
Cash and cash equivalents	0% - 5%		2	%		2	%
Equity securities:
U.S. large-capitalization	29% - 39%		34	%		36	%
U.S. small- and mid-capitalization	2% - 12%		7	%		8	%
International and emerging markets	9% - 19%		12	%		14	%
Total equity	50% - 60%		53	%		58	%
Debt securities	35% - 45%		41	%		36	%
Real estate	0% -   9%		4	%		4	%
Private equity	0% -   4%		(a)			(a)
Total			100	%		100	%
Postretirement Plans:
Cash and cash equivalents	0% - 10%		4	%		4	%
Equity securities:
U.S. large-capitalization	33% - 43%		40	%		41	%
U.S. small- and mid-capitalization	3% - 13%		7	%		8	%
International	10% - 20%		13	%		14	%
Total equity	55% - 65%		60	%		63	%
Debt securities	30% - 40%		36	%		33	%
Total			100	%		100	%
(a)	Less than 1% of plan assets.
In general, the United States large-capitalization equity investments are passively managed or indexed, whereas the international, emerging markets, United States small-capitalization, and United States mid-capitalization equity investments are actively managed by investment managers. Debt securities include a broad range of fixed income vehicles. Debt security investments in high-yield securities, emerging market securities, and non-United States dollar-denominated securities are owned by the plans, but in limited quantities to reduce risk. Most of the debt security investments are under active management by investment managers. Real estate investments include private real estate vehicles; however, Ameren does not, by policy, hold direct investments in real estate property. Ameren’s investment in private equity funds is spread among nine different limited partnerships, with invested capital ranging from $0.1 million to $5 million in each, which invest primarily in a diversified number of small United States-based companies. No further commitments may be made to private equity investments without approval by the finance committee of the board of directors. Additionally, Ameren’s investment committee allows investment managers to use derivatives, such as index futures, exchange traded funds, foreign exchange futures, and options, in certain situations, to increase or to reduce market exposure in an efficient and timely manner.
Fair Value Measurements of Plan Assets
Investments in the pension and postretirement benefit plans were stated at fair value as of December 31, 2014. The fair value of an asset is the amount that would be received upon its sale in an orderly transaction between market participants at the measurement date. Cash and cash equivalents have initial maturities of three months or less and are recorded at cost plus accrued interest. The carrying amounts of cash and cash equivalents approximate fair value because of the short-term nature of these instruments. Investments traded in active markets on national or international securities exchanges are valued at closing prices on the last business day on or before the measurement date. Securities traded in over-the-counter markets are valued based on quoted market prices, broker or dealer quotations, or alternative pricing sources with reasonable levels of price transparency. Derivative contracts are valued at fair value, as determined by the investment managers (or independent third parties on behalf of the investment managers), who use proprietary models and take into consideration exchange quotations on underlying instruments, dealer quotations, and other market information. The fair value of real estate is based on annual appraisal reports prepared by an independent real estate appraiser.
The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the pension plan assets measured at fair value as of December 31, 2014:
	Quoted Prices in				Significant Other				Significant Other				Total
	Active Markets for				Observable Inputs				Unobservable
	Identified Assets				(Level 2)				Inputs
	(Level 1)								(Level 3)
Cash and cash equivalents	$	—			$	38			$	—			$	38
Equity securities:
U.S. large-capitalization	—				1,331				—				1,331
U.S. small- and mid-capitalization	270				—				—				270
International and emerging markets	134				360				—				494
Debt securities:
Corporate bonds	—				1,026				—				1,026
Municipal bonds	—				175				—				175
U.S. treasury and agency securities	6				366				—				372
Other	—				31				—				31
Real estate	—				—				147				147
Private equity	—				—				13				13
Derivative assets	1				—				—				1
Total	$	411			$	3,327			$	160			$	3,898
Less: Medical benefit assets at December 31(a)													(125		)
Plus: Net receivables at December 31(b)													21
Fair value of pension plans assets at year end													$	3,794
(a)	Medical benefit (health and welfare) component for accounts maintained in accordance with Section 401(h) of the Internal Revenue Code to fund a portion of the postretirement obligation.
(b)	Receivables related to pending security sales, offset by payables related to pending security purchases.
The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the pension plan assets measured at fair value as of December 31, 2013:
	Quoted Prices in				Significant Other				Significant Other				Total
	Active Markets for				Observable Inputs				Unobservable
	Identified Assets or Liabilities				(Level 2)				Inputs
	(Level 1)								(Level 3)
Cash and cash equivalents	$	5			$	39			$	—			$	44
Equity securities:
U.S. large-capitalization	107				1,162				—				1,269
U.S. small- and mid-capitalization	273				—				—				273
International and emerging markets	143				372				—				515
Debt securities:
Corporate bonds	—				860				—				860
Municipal bonds	—				149				—				149
U.S. treasury and agency securities	—				256				—				256
Other	—				27				—				27
Real estate	—				—				131				131
Private equity	—				—				15				15
Derivative assets	1				—				—				1
Derivative liabilities	(1		)		—				—				(1		)
Total	$	528			$	2,865			$	146			$	3,539
Less: Medical benefit assets at December 31(a)													(112		)
Plus: Net receivables at December 31(b)													34
Fair value of pension plans assets at year end													$	3,461
(a)	Medical benefit (health and welfare) component for accounts maintained in accordance with Section 401(h) of the Internal Revenue Code to fund a portion of the postretirement obligation.
(b)	Receivables related to pending security sales, offset by payables related to pending security purchases.
The following table summarizes the changes in the fair value of the pension plan assets classified as Level 3 in the fair value hierarchy for each of the years ended December 31, 2014 and 2013:
	Beginning				Actual Return on				Actual Return on				Purchases,				Net		Ending Balance at
	Balance at				Plan Assets Related				Plan Assets Related				Sales, and				Transfers		December 31,
	January 1,				to Assets Still Held				to Assets Sold				Settlements, Net				into (out of)
					at the Reporting Date				During the Period								of Level 3
2014:00:00
Real estate	$	131			$	11			$	—			$	5			$	—	$	147
Private equity	15				(9		)		10				(3		)		—		13
2013:00:00
Real estate	$	118			$	9			$	—			$	4			$	—	$	131
Private equity	19				(9		)		11				(6		)		—		15
The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the postretirement benefit plans assets measured at fair value as of December 31, 2014:
	Quoted Prices in				Significant Other				Significant Other				Total
	Active Markets for				Observable Inputs				Unobservable
	Identified Assets				(Level 2)				Inputs
	(Level 1)								(Level 3)
Cash and cash equivalents	$	89			$	—			$	—			$	89
Equity securities:
U.S. large-capitalization	291				101				—				392
U.S. small- and mid-capitalization	70				—				—				70
International	37				94				—				131
Other	—				7								7
Debt securities:
Corporate bonds	—				105				—				105
Municipal bonds	—				111				—				111
U.S. treasury and agency securities	—				89				—				89
Other	—				44				—				44
Total	$	487			$	551			$	—			$	1,038
Plus: Medical benefit assets at December 31(a)													125
Less: Net payables at December 31(b)													(54		)
Fair value of postretirement benefit plans assets at year end													$	1,109
(a)	Medical benefit (health and welfare) component for 401(h) accounts to fund a portion of the postretirement obligation. These 401(h) assets are included in the pension plan assets shown above.
(b)	Payables related to pending security purchases, offset by interest receivables and receivables related to pending security sales.
The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the postretirement benefit plans assets measured at fair value as of December 31, 2013:
	Quoted Prices in				Significant Other				Significant Other				Total
	Active Markets for				Observable Inputs				Unobservable
	Identified Assets				(Level 2)				Inputs
	(Level 1)								(Level 3)
Cash and cash equivalents	$	77			$	—			$	—			$	77
Equity securities:
U.S. large-capitalization	297				101				—				398
U.S. small- and mid-capitalization	77				—				—				77
International	39				96				—				135
Other	—				2				—				2
Debt securities:
Corporate bonds	—				97				—				97
Municipal bonds	—				103				—				103
U.S. treasury and agency securities	—				72				—				72
Other	—				40				—				40
Total	$	490			$	511			$	—			$	1,001
Plus: Medical benefit assets at December 31(a)													112
Less: Net payables at December 31(b)													(39		)
Fair value of postretirement benefit plans assets at year end													$	1,074
(a)	Medical benefit (health and welfare) component for 401(h) accounts to fund a portion of the postretirement obligation. These 401(h) assets are included in the pension plan assets shown above.
(b)	Payables related to pending security purchases, offset by Medicare, interest receivables, and receivables related to pending security sales.
Net Periodic Benefit Cost
The following table presents the components of the net periodic benefit cost of our pension and postretirement benefit plans during 2014, 2013, and 2012:
	Pension Benefits				Postretirement Benefits
	Ameren(a)				Ameren(a)
2014
Service cost	$	79			$	19
Interest cost	183				50
Expected return on plan assets	(229		)		(65		)
Amortization of:
Prior service credit	(1		)		(5		)
Actuarial (gain) loss	49				(7		)
Net periodic benefit cost (benefit)	$	81			$	(8	)
2013
Service cost	$	91			$	22
Interest cost	163				46
Expected return on plan assets	(218		)		(62		)
Amortization of:
Prior service credit	(2		)		(6		)
Actuarial loss	87				8
Curtailment gain	(12		)		(7		)
Net periodic benefit cost(b)	$	109			$	1
2012
Service cost	$	81			$	22
Interest cost	166				47
Expected return on plan assets	(208		)		(56		)
Amortization of:
Transition obligation	—				2
Prior service credit	(3		)		(6		)
Actuarial loss	75				5
Net periodic benefit cost(c)	$	111			$	14
(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
(b)	The net periodic benefit cost includes a $6 million and a $7 million net gain for pension benefits and postretirement benefits, respectively, which was included in "Income (loss) from discontinued operations, net of taxes" on Ameren's consolidated statement of income (loss). This net gain includes the curtailment gain recognized in 2013 as a result of a significant reduction in employees as of the December 2, 2013 closing date of the New AER divestiture. See Note 16 – Divestiture Transactions and Discontinued Operations for additional information on the divestiture.
(c)	The net periodic benefit cost includes $9 million and $- million in total net costs for pension benefits and postretirement benefits, respectively, which were included in "Income (loss) from discontinued operations, net of taxes" on Ameren's consolidated statement of income (loss). See Note 16 – Divestiture Transactions and Discontinued Operations for additional information on the divestiture.
The current year expected return on plan assets is determined primarily by adjusting the prior year market-related asset value for current year contributions, disbursements, and expected return, plus 25% of the actual return in excess of (or less than) expected return for the four prior years.
The estimated amounts that will be amortized from regulatory assets and accumulated OCI into net periodic benefit cost in 2015 are as follows:
	Pension Benefits				Postretirement Benefits
	Ameren(a)				Ameren(a)
Regulatory assets:
Prior service credit	$	(1	)		$	(4	)
Net actuarial loss	86				15
Accumulated OCI:
Net actuarial (gain) loss	2				(2		)
Total	$	87			$	9
(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
Prior service cost is amortized on a straight-line basis over the average future service of active participants benefiting under the plan amendment. The net actuarial (gain) loss subject to amortization is amortized on a straight-line basis over 10 years.
The Ameren Companies are responsible for their share of the pension and postretirement benefit costs. The following table presents the pension costs and the postretirement benefit costs incurred and included in continuing operations for the years ended December 31, 2014, 2013, and 2012:
	Pension Costs												Postretirement Costs
	2014				2013				2012				2014				2013		2012
Ameren Missouri	$	50			$	69			$	63			$	3			$	8	$	10
Ameren Illinois	30				41				37				(9		)		—		4
Other	1				5				2				(2		)		—		—
Ameren(a)	81				115				102				(8		)		8		14
(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
The expected pension and postretirement benefit payments from qualified trust and company funds, which reflect expected future service, as of December 31, 2014, are as follows:
	Pension Benefits								Postretirement Benefits
	Paid from				Paid from				Paid from				Paid from
	Qualified				Company				Qualified				Company
	Trust				Funds				Trust				Funds
2015	$	253			$	3			$	58			$	2
2016	256				3				61				2
2017	257				4				64				2
2018	260				3				68				2
2019	260				3				70				2
2020 - 2024	1,273				11				388				11
The following table presents the assumptions used to determine net periodic benefit cost for our pension and postretirement benefit plans for the years ended December 31, 2014, 2013, and 2012:
	Pension Benefits									Postretirement Benefits
	2014			2013			2012			2014			2013			2012
Discount rate at measurement date	4.75	%		4	%		4.5	%		4.75	%		4	%		4.5	%
Expected return on plan assets	7.25			7.5			7.75			7			7.25			7.5
Increase in future compensation	3.5			3.5			3.5			3.5			3.5			3.5
Medical cost trend rate (initial)	(a)			(a)			(a)			5			5			5.5
Medical cost trend rate (ultimate)	(a)			(a)			(a)			5			5			5
Years to ultimate rate	(a)			(a)			(a)			—			—			1 year
(a)	Not applicable
The table below reflects the sensitivity of Ameren’s plans to potential changes in key assumptions:
	Pension Benefits								Postretirement Benefits
	Service Cost				Projected				Service Cost				Postretirement
	and Interest				Benefit				and Interest				Benefit
	Cost				Obligation				Cost				Obligation
0.25% decrease in discount rate	$	(1	)		$	138			$	1			$	39
0.25% increase in salary scale	2				13				—				—
1.00% increase in annual medical trend	—				—				3				36
1.00% decrease in annual medical trend	—				—				(2		)		(33		)
Other
Ameren sponsors a 401(k) plan for eligible employees. The Ameren 401(k) plan covered all eligible employees at December 31, 2014. The plan allowed employees to contribute a portion of their compensation in accordance with specific guidelines. Ameren matched a percentage of the employee contributions up to certain limits. The following table presents the portion of the matching contribution to the Ameren 401(k) plan attributable to the continuing operations for each of the Ameren Companies for the years ended December 31, 2014, 2013, and 2012:
	2014				2013				2012
Ameren Missouri	$	16			$	16			$	16
Ameren Illinois	11				10				9
Other	1				1				1
Ameren(a)	28				27				26
(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries.

StockBased_Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
STOCK-BASED COMPENSATION	STOCK-BASED COMPENSATION
	Ameren’s long-term incentive plan available for eligible employees and directors, the 2006 Incentive Plan, was replaced prospectively for new grants by the 2014 Incentive Plan effective in April 2014. The 2014 Incentive Plan provides for a maximum of 8 million common shares to be available for grant to eligible employees and directors. It retains many of the features of the 2006 Incentive Plan. To the extent that the issuance of a share that is subject to an outstanding award under the 2006 Incentive Plan would cause Ameren to exceed the maximum authorized shares under the 2006 Incentive Plan, the issuance of that share will take place under the 2014 Incentive Plan. This will reduce the maximum number of shares that may be granted under the 2014 Incentive Plan. The 2014 Incentive Plan awards may be stock options, stock appreciation rights, restricted stock, restricted stock units, performance shares, performance share units, cash-based awards, and other stock-based awards.
	A summary of nonvested shares at December 31, 2014, and changes during the year ended December 31, 2014, under the 2006 Incentive Plan and the 2014 Incentive Plan are presented below:
		Performance Share Units
		Share		Weighted-average
		Units		Fair Value per Share Unit
	Nonvested at January 1, 2014	1,218,544		$	33.23
	Granted(a)	688,323		38.9
	April Grants(b)	38,559		50.34
	Unearned or forfeited(c)	(97,432	)	34.42
	Earned and vested(d)	(685,617	)	36.12
	Nonvested at December 31, 2014	1,162,377		$	35.35
	(a)	Includes performance share units (share units) granted to certain executive and nonexecutive officers and other eligible employees in 2014 under the 2006 Incentive Plan and the 2014 Incentive Plan.
	(b)	In April 2014, certain executive officers were granted additional share units under the 2006 Incentive Plan and the 2014 Incentive Plan. The significant assumptions used to calculate fair value included a prorated three-year risk-free rate ranging from 0.76% to 0.79%, volatility of 12% to 18% for the peer group, and Ameren’s attainment of a three-year average earnings per share threshold during the performance period.
	(c)	Includes share units granted in 2012 that were not earned based on performance provisions of the award grants.
	(d)	Includes share units granted in 2012 that vested as of December 31, 2014, that were earned pursuant to the provisions of the award grants. Also includes share units that vested due to attainment of retirement eligibility by certain employees. Actual shares issued for retirement-eligible employees will vary depending on actual performance over the three-year measurement period.
	Ameren recorded compensation expense of $19 million, $20 million, and $22 million for the years ended December 31, 2014, 2013, and 2012, respectively, and a related tax benefit of $7 million, $8 million, and $8 million for the years ended December 31, 2014, 2013, and 2012, respectively. Ameren settled performance share units and restricted shares of $33 million, $11 million, and $11 million for the years ended December 31, 2014, 2013, and 2012. There were no significant compensation costs capitalized related to the performance share units during the years ended December 31, 2014, 2013, and 2012. As of December 31, 2014, total compensation cost of $18 million related to nonvested awards not yet recognized has expected to be recognized over a weighted-average period of 20 months.
	Performance Share Units
	Performance share units have been granted under the 2006 Incentive Plan and the 2014 Incentive Plan. A share unit vests and entitles an employee to receive shares of Ameren common stock (plus accumulated dividends) if, at the end of the three-year performance period, certain specified performance or market conditions have been met and if the individual remains employed by Ameren. The exact number of shares issued pursuant to a share unit varies from 0% to 200% of the target award, depending on actual company performance relative to the performance goals.
	The fair value of each share unit awarded in 2014, excluding the grants issued in April for certain executive officers, under the 2006 Incentive Plan and the 2014 Incentive Plan was determined to be $38.90. That amount was based on Ameren's closing common share price of $36.16 at December 31, 2013, and lattice simulations. Lattice simulations are used to estimate expected share payout based on Ameren's total shareholder return for a three-year performance period relative to the designated peer group beginning January 1, 2014. The simulations can produce a greater fair value for the share unit than the applicable closing common share price because they include the weighted payout scenarios in which an increase in the share price has occurred. The significant assumptions used to calculate fair value also included a three-year risk-free rate of 0.78%, volatility of 12% to 18% for the peer group, and Ameren's attainment of a three-year average earnings per share threshold during the performance period.
	The fair value of each share unit awarded in January 2013 under the 2006 Incentive Plan was determined to be $31.19. That amount was based on Ameren’s closing common share price of $30.72 at December 31, 2012, and lattice simulations. Lattice simulations are used to estimate expected share payout based on Ameren’s total shareholder return for a three-year performance period relative to the designated peer group beginning January 1, 2013. The simulations can produce a greater fair value for the share unit than the applicable closing common share price because they include the weighted payout scenarios in which an increase in the share price has occurred. The significant assumptions used to calculate fair value also included a three-year risk-free rate of 0.36%, volatility of 12% to 21% for the peer group, and Ameren’s attainment of a three-year average earnings per share threshold during the performance period.

Income_Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2014
Income Tax Disclosure [Abstract]
INCOME TAXES	INCOME TAXES
	The following table presents the principal reasons for the difference between the effective income tax rate and the statutory federal income tax rate for the years ended December 31, 2014, 2013, and 2012:
		Ameren Missouri			Ameren Illinois			Ameren
	2014
	Statutory federal income tax rate:	35	%		35	%		35	%
	Increases (decreases) from:
	Amortization of investment tax credit	(1	)		—			(1	)
	State tax	3			6			4
	Other permanent items	—			—			1
	Effective income tax rate	37	%		41	%		39	%
	2013
	Statutory federal income tax rate:	35	%		35	%		35	%
	Increases (decreases) from:
	Depreciation differences	—			(1	)		—
	Amortization of investment tax credit	(1	)		—			(1	)
	State tax	3			6			4
	Other permanent items	1			—			—
	Effective income tax rate	38	%		40	%		38	%
	2012
	Statutory federal income tax rate:	35	%		35	%		35	%
	Increases (decreases) from:
	Depreciation differences	(1	)		—			(1	)
	Amortization of investment tax credit	(1	)		(1	)		(1	)
	State tax	3			6			5
	Reserve for uncertain tax positions	1			—			—
	Other permanent items	—			—			(1	)
	Effective income tax rate	37	%		40	%		37	%
	The following table presents the components of income tax expense (benefit) for the years ended December 31, 2014, 2013, and 2012:
		Ameren Missouri				Ameren Illinois				Other				Ameren
	2014
	Current taxes:
	Federal	$	(13	)		$	(51	)		$	27			$	(37	)
	State	(3		)		(2		)		(32		)		(37		)
	Deferred taxes:
	Federal	222				159				(12		)		369
	State	28				38				22				88
	Deferred investment tax credits, amortization	(5		)		(1		)		—				(6		)
	Total income tax expense	$	229			$	143			$	5			$	377
	2013
	Current taxes:
	Federal	$	136			$	(15	)		$	(239	)	(a)	$	(118	)
	State	41				21				(43		)	(a)	19
	Deferred taxes:
	Federal	64				99				205			(a)	368
	State	6				6				36			(a)	48
	Deferred investment tax credits, amortization	(5		)		(1		)		—				(6		)
	Total income tax expense (benefit)	$	242			$	110			$	(41	)		$	311
	2012
	Current taxes:
	Federal	$	(25	)		$	(7	)		$	72			$	40
	State	(10		)		(3		)		23				10
	Deferred taxes:
	Federal	248				76				(120		)		204
	State	44				30				(14		)		60
	Deferred investment tax credits, amortization	(5		)		(2		)		—				(7		)
	Total income tax expense (benefit)	$	252			$	94			$	(39	)		$	307
	(a)	These amounts are substantially related to the reversal of unrecognized tax benefits as a result of IRS guidance related to the deductibility of expenditures to maintain, replace or improve steam or electric power generation property, along with casualty loss deductions for storm damage. The amounts also reflect the increase in deferred tax expense due to available net operating losses.
	The Illinois corporate income tax rate was increased to 9.5% from January 2011 through December 2014. The tax rate decreased to 7.75% on January 1, 2015 and is scheduled to decrease to 7.3% on January 1, 2025.
	The following table presents the deferred tax assets and deferred tax liabilities recorded as a result of temporary differences at December 31, 2014 and 2013:
		Ameren Missouri				Ameren Illinois				Other				Ameren
	2014
	Accumulated deferred income taxes, net liability (asset):
	Plant related	$	2,776			$	1,393			$	16			$	4,185
	Regulatory assets, net	82				(5		)		1				78
Deferred employee benefit costs	(80		)		(45		)		(95		)		(220		)
Revenue requirement reconciliation adjustments	—				66				3				69
Tax carryforwards	(107		)		(139		)		(429		)		(675		)
Other	86				(22		)		70				134
Total net accumulated deferred income tax liabilities (assets)(a)	$	2,757			$	1,248			$	(434	)		$	3,571
2013
Accumulated deferred income taxes, net liability (asset):
Plant related	$	2,513			$	1,243			$	13			$	3,769
Regulatory assets, net	74				2				—				76
Deferred employee benefit costs	(74		)		(85		)		(114		)		(273		)
Revenue requirement reconciliation adjustments	—				(4		)		2				(2		)
Tax carryforwards	(76		)		(95		)		(370		)		(541		)
Other	67				10				38				115
Total net accumulated deferred income tax liabilities (assets)(b)	$	2,504			$	1,071			$	(431	)		$	3,144
(a)	Includes $49 million recorded in "Other current assets" on Ameren Missouri's balance sheet as of December 31, 2014.
(b)	Includes $20 million recorded in "Other current assets" on Ameren Missouri's balance sheet as of December 31, 2013.
The following table presents the components of deferred tax assets relating to net operating loss carryforwards, tax credit carryforwards, and charitable contribution carryforwards at December 31, 2014:
	Ameren Missouri				Ameren Illinois				Other				Ameren
Net operating loss carryforwards:
Federal(a)	$	75			$	127			$	255			$	457
State(b)	11				10				53				74
Total net operating loss carryforwards	$	86			$	137			$	308			$	531
Tax credit carryforwards:
Federal(c)	$	21			$	1			$	77			$	99
State(d)	1				2				33				36
State valuation allowance(e)	(1		)		(1		)		(2		)		(4		)
Total tax credit carryforwards	$	21			$	2			$	108			$	131
Charitable contribution carryforwards(f)	$	—			$	—			$	19			$	19
Valuation allowance(g)	—				—				(6		)		(6		)
Total charitable contribution carryforwards	$	—			$	—			$	13			$	13
(a)	Will begin to expire in 2028.
(b)	Will begin to expire in 2020.
(c)	Will begin to expire in 2029.
(d)	Began to expire in 2013.
(e)	This balance increased by less than $1 million, $- million, and $- million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, during 2014.
(f)	These began to expire in 2013.
(g)	This balance increased by $3 million, $- million and $- million for Ameren, Ameren Missouri and Ameren Illinois, respectively, during 2014.
The following table presents the components of deferred tax assets relating to net operating loss carryforwards, tax credit carryforwards, and charitable contribution carryforwards at December 31, 2013:
	Ameren Missouri				Ameren Illinois				Other				Ameren
Net operating loss carryforwards:
Federal(a)	$	61			$	84			$	215			$	360
State(b)	3				11				34				48
Total net operating loss carryforwards	$	64			$	95			$	249			$	408
Tax credit carryforwards:
Federal(c)	$	12			$	—			$	76			$	88
State(d)	1				1				32				34
State valuation allowance(e)	(1		)		(1		)		(2		)		(4		)
Total tax credit carryforwards	$	12			$	—			$	106			$	118
Charitable contribution carryforwards(f)	$	—			$	—			$	18			$	18
Valuation allowance(g)	—				—				(3		)		(3		)
Total charitable contribution carryforwards	$	—			$	—			$	15			$	15
(a)	Will begin to expire in 2028
(b)	Will begin to expire in 2019.
(c)	Will begin to expire in 2029.
(d)	Began to expire in 2013.
(e)	Balance increased by $2 million, $- million, and $- million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, during 2013.
(f)	These began to expire in 2013.
(g)	This balance increased by $3 million, $- million, and $- million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, during 2013.
Uncertain Tax Positions
A reconciliation of the change in the unrecognized tax benefit balance during the years ended December 31, 2012, 2013, and 2014, is as follows:
	Ameren Missouri				Ameren Illinois				Other				Ameren
Unrecognized tax benefits – January 1, 2012	$	124			$	11			$	13			$	148
Increases based on tax positions prior to 2012	4				—				1				5
Decreases based on tax positions prior to 2012	(7		)		(1		)		(5		)		(13		)
Increases based on tax positions related to 2012	15				3				(1		)		17
Changes related to settlements with taxing authorities	—				—				—				—
Decreases related to the lapse of statute of limitations	—				—				(1		)		(1		)
Unrecognized tax benefits – December 31, 2012	$	136			$	13			$	7			$	156
Increases based on tax positions prior to 2013	—				2				5				7
Decreases based on tax positions prior to 2013	(122		)		(16		)		(5		)		(143		)
Increases (decreases) based on tax positions related to 2013	16				—				53			(a)	69
Changes related to settlements with taxing authorities	—				—				—				—
Decreases related to the lapse of statute of limitations	1				—				—				1
Unrecognized tax benefits – December 31, 2013	$	31			$	(1	)		$	60			$	90
Increases based on tax positions prior to 2014	1				1				4				6
Decreases based on tax positions prior to 2014	(32		)		(1		)		(9		)		(42		)
Increases based on tax positions related to 2014	—				—				—				—
Changes related to settlements with taxing authorities	—				—				—				—
Increases related to the lapse of statute of limitations	—				—				—				—
Unrecognized tax benefits (detriments) – December 31, 2014	$	—			$	(1	)		$	55			$	54
Total unrecognized tax benefits that, if recognized, would affect the effective tax rates as of December 31, 2012	$	3			$	(1	)		$	(1	)		$	1
Total unrecognized tax benefits (detriments) that, if recognized, would affect the effective tax rates as of December 31, 2013	$	3			$	—			$	51		(a)	$	54
Total unrecognized tax benefits that, if recognized, would affect the effective tax rates as of December 31, 2014	$	—			$	(1	)		$	53		(a)	$	52
(a)	Primarily due to tax positions relating to the New AER divestiture. The income statement impact of this unrecognized tax benefit was included in "Income (loss) from discontinued operations, net of taxes" on Ameren's consolidated statement of income (loss). See Note 16 – Divestiture Transactions and Discontinued Operations for additional information.
The Ameren Companies recognize interest charges (income) and penalties accrued on tax liabilities on a pretax basis as interest charges (income) or miscellaneous expense, respectively, in the statements of income.
A reconciliation of the change in the liability for interest on unrecognized tax benefits during the years ended December 31, 2012, 2013, and 2014, is as follows:
	Ameren Missouri				Ameren Illinois				Other				Ameren
Liability for interest – January 1, 2012	$	6			$	1			$	(2	)		$	5
Interest charges (income) for 2012	2				—				(1		)		1
Liability for interest – December 31, 2012	$	8			$	1			$	(3	)		$	6
Interest charges (income) for 2013	(8		)		(1		)		4				(5		)
Liability for interest – December 31, 2013	$	—			$	—			$	1			$	1
Interest charges (income) for 2014	—				—				(1		)		(1		)
Liability for interest – December 31, 2014	$	—			$	—			$	—			$	—
As of December 31, 2012, 2013, and 2014, the Ameren Companies have accrued no amount for penalties with respect to unrecognized tax benefits.
In 2014, final settlements for tax years 2007 through 2011 were reached with the IRS. These settlements, which resolved the uncertain tax positions associated with the timing of research tax deductions for these years, resulted in a decrease in Ameren’s and Ameren Missouri’s unrecognized tax benefits of $20 million, and $13 million, respectively. In addition, the settlement for tax years 2007 through 2011 provided certainty for the previously uncertain tax positions associated with the timing of research tax deductions for the remaining open tax years of 2012, 2013, and 2014. As a result, the certainty provided from the settlement resulted in an $18 million decrease in both Ameren’s and Ameren Missouri’s unrecognized tax benefits. The settlement also resulted in a $2 million increase to Ameren’s state unrecognized tax benefits. The net reduction in unrecognized tax benefits in 2014 did not materially affect income tax expense for the Ameren Companies.
In 2013, unrecognized tax benefits related to the deductibility of expenditures to maintain, replace, or improve steam or electric power generation property, along with casualty loss deductions for storm damage, were reduced by $103 million, $95 million, and $5 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively. This reduction in unrecognized tax benefits did not affect income tax expense for the Ameren Companies. However, the liability for interest related to these unrecognized tax benefits was released in 2013. In 2013, Ameren adopted an accounting method change as a result of guidance issued by the IRS, with respect to the amount and timing of the deductions to maintain, replace, or improve generation property.
It is reasonably possible that a settlement will be reached with the IRS in the next 12 months for the years 2012 and 2013. However, the Ameren Companies do not believe any settlements would have a material effect on its net income from continuing operations.
State income tax returns are generally subject to examination for a period of three years after filing. The state impact of any federal changes remains subject to examination by various states for a period of up to one year after formal notification to the states. The Ameren Companies currently do not have material state income tax issues under examination, administrative appeals, or litigation.
Ameren Missouri has an uncertain tax position tracker. Under Missouri's regulatory framework, uncertain tax positions do not reduce Ameren Missouri's electric rate base. When an uncertain income tax position liability is resolved, the MoPSC requires, through the uncertain tax position tracker, the creation of a regulatory asset or regulatory liability to reflect the time value, using the weighted-average cost of capital included in each of the electric rate orders in effect before the tax position was resolved, of the difference between the uncertain tax position liability that was excluded from rate base and the final tax liability. The resulting regulatory asset or liability will affect earnings in the year it is created and then will be amortized over three years beginning on the effective date of new rates established in the next electric rate case.

Related_Party_Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2014
Related Party Transactions [Abstract]
RELATED PARTY TRANSACTIONS	RELATED PARTY TRANSACTIONS
	The Ameren Companies have engaged in, and may in the future engage in, affiliate transactions in the normal course of business. These transactions primarily consist of natural gas and power purchases and sales, services received or rendered, and borrowings and lendings. Transactions between affiliates are reported as intercompany transactions on their financial statements, but are eliminated in consolidation for Ameren’s financial statements. Below are the material related party agreements.
	Electric Power Supply Agreements
	Capacity Supply Agreements
	Ameren Illinois must acquire capacity sufficient to meet its obligations to customers. Ameren Illinois uses periodic RFP processes that are administered by the IPA to contract capacity on behalf of its customers. Ameren Missouri participates in the RFP process and has been a winning supplier for certain periods. In 2010, Ameren Missouri contracted to supply a portion of Ameren Illinois’ capacity requirements for less than $1 million for the period from June 1, 2010, through May 31, 2013. In 2012, Ameren Missouri contracted to supply a portion of Ameren Illinois' capacity requirements for $1 million and $3 million for the 12 months ending May 31, 2014, and 2015, respectively.
	Energy Swaps and Energy Products
	Ameren Illinois must acquire energy sufficient to meet its obligations to customers.
	In 2011, Ameren Illinois used an RFP process, administered by the IPA, to procure energy products that settled physically from June 1, 2011, through May 31, 2014. Ameren Missouri was among the winning suppliers in the energy product RFP process. In 2011, Ameren Missouri and Ameren Illinois entered into energy product agreements by which Ameren Missouri agreed to sell and Ameren Illinois agreed to purchase approximately 16,800 megawatthours at approximately $37 per megawatthour during the 12 months ended May 31, 2012, approximately 40,800 megawatthours at approximately $29 per megawatthour during the 12 months ended May 31, 2013, and approximately 40,800 megawatthours at approximately $28 per megawatthour during the 12 months ended May 31, 2014. The energy product agreements between Ameren Missouri and Ameren Illinois for the periods ended May 31, 2012, and May 31, 2013, were for off-peak hours only.
	In 2014, Ameren Illinois used an RFP process, administered by the IPA, to procure energy products that will settle physically from December 1, 2014, through May 31, 2017. Ameren Missouri was among the winning suppliers in the energy product RFP process. As a result, Ameren Missouri and Ameren Illinois entered into energy product agreements by which Ameren Missouri agreed to sell and Ameren Illinois agreed to purchase approximately 168,400 megawatthours at approximately $51 per megawatthour during the period of January 1, 2015, through February 28, 2017.
	Interconnection and Transmission Agreements
	Ameren Missouri and Ameren Illinois are parties to an interconnection agreement for the use of their respective transmission lines and other facilities for the distribution of power. These agreements have no contractual expiration date, but may be terminated by either party with three years’ notice.
	Joint Ownership Agreement
	ATXI and Ameren Illinois have a joint ownership agreement to construct, own, operate, and maintain certain electric transmission assets in Illinois. Under the terms of this agreement, Ameren Illinois and ATXI are responsible for their applicable share of all costs related to the construction, operation, and maintenance of electric transmission systems. Currently, there are no construction projects or joint ownership of existing assets under this agreement.
	Support Services Agreements
	Ameren Services provides support services to its affiliates. The costs of support services, including wages, employee benefits, professional services, and other expenses, are based on, or are an allocation of, actual costs incurred. A shared services support agreement can be terminated at any time by the mutual agreement of Ameren Services and that affiliate or by either party with 60 days' notice before the end of a calendar year.
	In addition, Ameren Missouri and Ameren Illinois provide affiliates, primarily Ameren Services, with access to their facilities for administrative purposes. The cost of the rent and facility services are based on, or are an allocation of, actual costs incurred.
	Separately, Ameren Missouri and Ameren Illinois provide storm-related and miscellaneous support services to each other on an as-needed basis.
	Transmission Services
	Ameren Illinois takes transmission service from MISO for the retail load it serves in the AMIL pricing zone. ATXI is one of the transmission owners in the AMIL pricing zone. Accordingly ATXI receives transmission payments from Ameren Illinois through the MISO billing process.
	Money Pool
	See Note 4 – Short-term Debt and Liquidity and Note 5 – Long-term Debt and Equity Financings for a discussion of affiliate borrowing arrangements.
	Collateral Postings
	Under the terms of the Illinois power procurement agreements entered into through RFP processes administered by the IPA, suppliers must post collateral under certain market conditions to protect Ameren Illinois in the event of nonperformance. The collateral postings are unilateral, meaning that only the suppliers can be required to post collateral. Therefore, Ameren Missouri, as a winning supplier in the RFP process, may be required to post collateral. As of December 31, 2014 and 2013, there were no collateral postings required of Ameren Missouri related to the Illinois power procurement agreements.
	Tax Allocation Agreement
	See Note 1 – Summary of Significant Accounting Policies for a discussion of the tax allocation agreement. At December 31, 2014, Ameren Missouri and Ameren Illinois had an intercompany receivable balance with Ameren (parent) of $58 million and $15 million, respectively, related to the tax allocation agreement.
	The following table presents the impact on Ameren Missouri and Ameren Illinois of related party transactions for the years ended December 31, 2014, 2013, and 2012. It is based primarily on the agreements discussed above and the money pool arrangements discussed in Note 4 – Short-term Debt and Liquidity.
	Agreement	Income Statement Line Item			Ameren		Ameren
					Missouri		Illinois
	Ameren Missouri power supply agreements	Operating Revenues	2014	$	5	$	(a)
	with Ameren Illinois		2013		3		(a)
			2012		(b)		(a)
	Ameren Missouri and Ameren Illinois	Operating Revenues	2014		21		2
	rent and facility services		2013		21		1
			2012		19		1
	Ameren Missouri and Ameren Illinois	Operating Revenues	2014		1		(b)
	miscellaneous support services		2013		1		3
			2012		1		(b)
	Total Operating Revenues		2014	$	27	$	2
			2013		25		4
			2012		20		1
	Ameren Illinois power supply	Purchased Power	2014	$	(a)	$	5
	agreements with Ameren Missouri		2013		(a)		3
			2012		(a)		(b)
	Ameren Illinois transmission	Purchased Power	2014		(a)		2
	services with ATXI		2013		(a)		2
			2012		(a)		3
	Total Purchased Power		2014	$	(a)	$	7
			2013		(a)		5
			2012		(a)		3
	Ameren Services support services	Other Operations and	2014	$	124	$	109
	agreement	Maintenance	2013		116		93
			2012		106		88
	Insurance premiums(c)	Other Operations and	2014		(b)		(a)
		Maintenance	2013		(b)		(a)
			2012		(b)		(a)
	Total Other Operations and		2014	$	124	$	109
	Maintenance Expenses		2013		116		93
			2012		106		88
	Money pool borrowings (advances)	Interest (Charges)	2014	$	(b)	$	(b)
		Income	2013		(b)		(b)
			2012		(b)		(b)
	(a)	Not applicable.
	(b)	Amount less than $1 million.
	(c)	Represents insurance premiums paid to Missouri Energy Risk Assurance Company LLC, an affiliate, for replacement power.

Commitments_And_Contingencies

Commitments And Contingencies	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	COMMITMENTS AND CONTINGENCIES
	We are involved in legal, tax, and regulatory proceedings before various courts, regulatory commissions, authorities, and governmental agencies with respect to matters that arise in the ordinary course of business, some of which involve substantial amounts of money. We believe that the final disposition of these proceedings, except as otherwise disclosed in these notes to our financial statements, will not have a material adverse effect on our results of operations, financial position, or liquidity.
	See also Note 1 – Summary of Significant Accounting Policies, Note 2 – Rate and Regulatory Matters, Note 10 – Callaway Energy Center, Note 14 – Related Party Transactions, and Note 16 – Divestiture Transactions and Discontinued Operations in this report.
	Callaway Energy Center
	The following table presents insurance coverage at Ameren Missouri’s Callaway energy center at December 31, 2014. The property coverage and the nuclear liability coverage must be renewed on April 1 and January 1, respectively, of each year.
	Type and Source of Coverage	Maximum Coverages			Maximum Assessments
	Public liability and nuclear worker liability:
	American Nuclear Insurers	$	375		$	—
	Pool participation	13,241		(a)	128		(b)
		$	13,616	(c)	$	128
	Property damage:
	Nuclear Electric Insurance Limited	$	2,250	(d)	$	23	(e)
	European Mutual Association for Nuclear Insurance	500		(f)	—
		$	2,750		$	23
	Replacement power:
	Nuclear Electric Insurance Limited	$	490	(g)	$	9	(e)
	Missouri Energy Risk Assurance Company LLC	$	64	(h)	$	—
	(a)	Provided through mandatory participation in an industrywide retrospective premium assessment program.
	(b)	Retrospective premium under the Price-Anderson Act. This is subject to retrospective assessment with respect to a covered loss in excess of $375 million in the event of an incident at any licensed United States commercial reactor, payable at $19 million per year.
	(c)	Limit of liability for each incident under the Price-Anderson Act liability provisions of the Atomic Energy Act of 1954, as amended. A company could be assessed up to $128 million per incident for each licensed reactor it operates, with a maximum of $19 million per incident to be paid in a calendar year for each reactor. This limit is subject to change to account for the effects of inflation and changes in the number of licensed reactors.
	(d)	NEIL provides $2.25 billion in property damage, decontamination, and premature decommissioning insurance.
	(e)	All NEIL-insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL.
	(f)	European Mutual Association for Nuclear Insurance provides $500 million in excess of the $2.25 billion property coverage provided by NEIL.
	(g)	Provides replacement power cost insurance in the event of a prolonged accidental outage. Weekly indemnity is up to $4.5 million for 52 weeks, which commences after the first eight weeks of an outage, plus up to $3.6 million per week for a minimum of 71 weeks thereafter, for a total not exceeding the policy limit of $490 million. Nonradiation events are sub-limited to $328 million.
	(h)	Provides replacement power cost insurance in the event of a prolonged accidental outage. The coverage commences after the first 52 weeks of insurance coverage from NEIL concludes; it is a weekly indemnity of up to $0.9 million for 71 weeks in excess of the $3.6 million per week set forth above. Missouri Energy Risk Assurance Company LLC is an affiliate; it has reinsured this coverage with third-party insurance companies. See Note 14 – Related Party Transactions for more information on this affiliate transaction.
	The Price-Anderson Act is a federal law that limits the liability for claims from an incident involving any licensed United States commercial nuclear energy center. The limit is based on the number of licensed reactors. The limit of liability and the maximum potential annual payments are adjusted at least every five years for inflation to reflect changes in the Consumer Price Index. The most recent five-year inflationary adjustment became effective in September 2013. Owners of a nuclear reactor cover this exposure through a combination of private insurance and mandatory participation in a financial protection pool, as established by the Price-Anderson Act.
	Losses resulting from terrorist attacks on nuclear facilities are covered under NEIL’s policies, subject to an industrywide aggregate policy coverage limit of $3.24 billion within a 12-month period, or $1.83 billion for events not involving radiation contamination.
	If losses from a nuclear incident at the Callaway energy center exceed the limits of, or are not covered by, insurance or if coverage is unavailable, Ameren Missouri is at risk for any uninsured losses. If a serious nuclear incident were to occur, it could have a material adverse effect on Ameren’s and Ameren Missouri’s results of operations, financial position, or liquidity.
	Leases
	We lease various facilities, office equipment, plant equipment, and rail cars under capital and operating leases. The following table presents our lease obligations at December 31, 2014:
		2015			2016			2017		2018		2019		After 5 Years		Total
	Ameren:(a)
	Minimum capital lease payments(b)	$	33		$	33		$	33	$	32	$	32	$	360	$	523
	Less amount representing interest	27			27			27		26		25		97		229
	Present value of minimum capital lease payments	$	6		$	6		$	6	$	6	$	7	$	263	$	294
	Operating leases(c)	13			12			12		12		11		38		98
	Total lease obligations	$	19		$	18		$	18	$	18	$	18	$	301	$	392
	Ameren Missouri:
	Minimum capital lease payments(b)	$	33		$	33		$	33	$	32	$	32	$	360	$	523
	Less amount representing interest	27			27			27		26		25		97		229
	Present value of minimum capital lease payments	$	6		$	6		$	6	$	6	$	7	$	263	$	294
	Operating leases(c)	11			11			11		10		10		37		90
	Total lease obligations	$	17		$	17		$	17	$	16	$	17	$	300	$	384
	Ameren Illinois:
	Operating leases(c)	$	1		$	1		$	1	$	1	$	1	$	1	$	6
	(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.
	(b)	See Properties under Part I, Item 2, and Note 3 – Property and Plant, Net, of this report for additional information.
	(c)	Amounts related to certain land-related leases have indefinite payment periods. The annual obligations of $2 million, $1 million, and $1 million for Ameren, Ameren Missouri, and Ameren Illinois for these items are included in the 2015 through 2019 columns, respectively.
	The following table presents total rental expense, included in operating expenses, for the years ended December 31, 2014, 2013, and 2012:
		2014			2013			2012
	Ameren(a)	$	37		$	32		$	33
	Ameren Missouri	32			29			29
	Ameren Illinois	25			21			19
	(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.
	Other Obligations
	To supply a portion of the fuel requirements of our energy centers, we have entered into various long-term commitments for the procurement of coal, natural gas, nuclear fuel, and methane gas. We also have entered into various long-term commitments for purchased power and natural gas for distribution. The table below presents our estimated fuel, purchased power, and other commitments for fuel at December 31, 2014. Ameren’s and Ameren Missouri’s purchased power commitments include a 102-megawatt power purchase agreement with a wind farm operator, which expires in 2024. Ameren’s and Ameren Illinois’ purchased power commitments include the Ameren Illinois power purchase agreements entered into as part of the IPA-administered power procurement process. Included in the Other column are minimum purchase commitments under contracts for equipment, design and construction, and meter reading services at December 31, 2014. In addition, the Other column includes Ameren's and Ameren Missouri's obligations related to customer energy efficiency programs under the MEEIA as approved by the MoPSC's December 2012 electric rate order. Ameren Missouri expects to incur costs of $71 million in 2015 for these customer energy efficiency programs. See Note 2 – Rate and Regulatory Matters for additional information about the MEEIA.
		Coal			Natural			Nuclear		Purchased		Methane		Other		Total
					Gas(a)			Fuel		Power(b)		Gas
	Ameren:(c)
	2015	$	654		$	222		$	53	$	190	$	3	$	195	$	1,317
	2016	659			125			60		104		3		78		1,029
	2017	682			85			59		66		4		53		949
	2018	111			53			82		55		5		51		357
	2019	114			32			42		56		5		54		303
Thereafter	—			71			138		596		76		350		1,231
Total	$	2,220		$	588		$	434	$	1,067	$	96	$	781	$	5,186
Ameren Missouri:
2015	$	654		$	39		$	53	$	21	$	3	$	128	$	898
2016	659			22			60		21		3		39		804
2017	682			17			59		21		4		26		809
2018	111			11			82		21		5		27		257
2019	114			10			42		21		5		27		219
Thereafter	—			22			138		106		76		183		525
Total	$	2,220		$	121		$	434	$	211	$	96	$	430	$	3,512
Ameren Illinois:
2015	$	—		$	183		$	—	$	169	$	—	$	29	$	381
2016	—			103			—		83		—		24		210
2017	—			68			—		45		—		24		137
2018	—			42			—		34		—		24		100
2019	—			22			—		35		—		27		84
Thereafter	—			49			—		490		—		167		706
Total	$	—		$	467		$	—	$	856	$	—	$	295	$	1,618
(a)	Includes amounts for generation and for distribution.
(b)	The purchased power amounts for Ameren and Ameren Illinois include agreements through 2032 for renewable energy credits with various renewable energy suppliers. The agreements contain a provision that allows Ameren Illinois to reduce the quantity purchased in the event that Ameren Illinois would not be able to recover the costs associated with the renewable energy credits.
(c)	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
Environmental Matters
We are subject to various environmental laws and regulations enforced by federal, state, and local authorities. From the beginning phases of siting and development to the operation of existing or new electric generation, transmission and distribution facilities and natural gas storage, transmission and distribution facilities, our activities involve compliance with diverse environmental laws and regulations. These laws and regulations address emissions, discharges to water, water usage, impacts to air, land, and water, and chemical and waste handling. Complex and lengthy processes are required to obtain and renew approvals, permits, or licenses for new, existing or modified facilities. Additionally, the use and handling of various chemicals or hazardous materials require release prevention plans and emergency response procedures.
The EPA is developing and implementing environmental regulations that will have a significant impact on the electric utility industry. Over time, compliance with these regulations could be costly for certain companies, including Ameren Missouri, that operate coal-fired power plants. Significant new rules proposed or promulgated include the regulation of CO2 emissions from existing power plants through the proposed Clean Power Plan and from new power plants through the revised NSPS; revised national ambient air quality standards for ozone, fine particulates, SO2, and NOx emissions; the CSAPR, which requires further reductions of SO2 emissions and NOx emissions from power plants; a regulation governing management of CCR and CCR impoundments; the MATS, which require reduction of emissions of mercury, toxic metals, and acid gases from power plants; revised NSPS for particulate matter, SO2, and NOx emissions from new sources; new effluent standards applicable to waste water discharges from power plants and new regulations under the Clean Water Act that could require significant capital expenditures, such as modifications to water intake structures or new cooling towers at Ameren Missouri’s energy centers. Certain of these new and proposed regulations, if adopted, are likely to be challenged through litigation, so their ultimate implementation, as well as the timing of any such implementation, is uncertain. Although many details of the future regulations are unknown, the combined effects of the new and proposed environmental regulations could result in significant capital expenditures and increased operating costs for Ameren and Ameren Missouri. Compliance with these environmental laws and regulations could be prohibitively expensive, result in the closure or alteration of the operation of some of Ameren Missouri’s energy centers, or require capital investment. Ameren and Ameren Missouri expect these costs would be recoverable through rates, subject to MoPSC prudence review, but the nature and timing of costs, as well as the applicable regulatory framework, could result in regulatory lag.
As of December 31, 2014, Ameren and Ameren Missouri estimate capital expenditure investments of $350 million to $400 million through 2019 to comply with existing environmental regulations. Considerable uncertainty remains in this estimate. The actual amount of capital investments required to comply with existing environmental regulations may vary substantially from the above estimate due to uncertainty as to the precise compliance strategies that will be used and their ultimate cost, among other things. This estimate does not include the impacts of the proposed Clean Power Plan’s reduction in emissions of CO2, which is discussed below.
Ameren Missouri's current plan for compliance with existing environmental regulations for air emissions includes burning ultra-low-sulfur coal and installing new or optimizing existing pollution control equipment. Ameren Missouri has two scrubbers at its Sioux energy center, which are used to reduce SO2 emissions and other pollutants. Ameren Missouri's compliance plan assumes the installation of additional controls including mercury control technology at multiple energy centers within its coal-fired fleet through 2019. However, Ameren Missouri continues to evaluate its operations and options to determine how to comply with the CSAPR, the MATS, and other recently finalized or proposed EPA regulations. Ameren Missouri may be required to install additional pollution controls within the next six to 10 years. As the Clean Power Plan is still subject to revision by the EPA and implementation by the states, Ameren Missouri has not finalized a compliance plan for the proposed rule.
The following sections describe the more significant new or proposed environmental laws and rules and environmental enforcement and remediation matters that affect or could affect our operations.
Clean Air Act
Both federal and state laws require significant reductions in SO2 and NOx through either emission source reductions or the use and retirement of emission allowances. In 2005, the EPA issued regulations with respect to SO2 and NOx emissions (the CAIR). The CSAPR replaced the CAIR and became effective on January 1, 2015 for SO2 and annual NOx reductions, and on May 1, 2015, will become effective for ozone season NOx reductions. There will be further reductions in 2017 and in subsequent years. Ameren Missouri expects to have sufficiently reduced emissions and have sufficient allowances for 2015 to avoid making external purchases to comply with CSAPR. Ameren Missouri has already taken actions to prepare for the implementation of the CSAPR, including the installation of two scrubbers at its Sioux energy center and burning ultra-low sulfur coal. Ameren Missouri does not expect to make additional capital investments to comply with the CSAPR. However, Ameren Missouri will incur additional operations and maintenance costs to lower its emissions at one or more of its energy centers in compliance with the CSAPR. These higher operations and maintenance costs are expected to be collected from customers through the FAC or higher base rates.
In December 2011, the EPA issued the MATS under the Clean Air Act, which requires emission reductions for mercury and other hazardous air pollutants, such as acid gases, trace metals, and hydrogen chloride emissions. The MATS do not require a specific control technology to achieve the emission reductions. The MATS will apply to each unit at a coal-fired power plant. However, in certain cases, compliance can be achieved by averaging emissions from similar units at the same power plant. Compliance is required by April 2015 or, with a case-by-case extension, by April 2016. Ameren Missouri's Labadie and Meramec energy centers were granted extensions and expect to comply with the MATS by April 2016. Ameren Missouri expects to make additional capital investments to comply with the MATS. These capital expenditure investments are included in Ameren's and Ameren Missouri's estimate above. In addition, Ameren Missouri will incur additional operations and maintenance costs to lower its emissions at one or more of its energy centers in compliance with the MATS. These higher operations and maintenance costs are expected to be collected from customers through the FAC or higher base rates.
In December 2014, the EPA published its proposal to strengthen the 2008 national ambient air quality standard for ozone. A final standard is expected in October 2015, after which states that do not meet the standard must develop and implement plans to achieve compliance with the air quality standard. Ameren Missouri is currently evaluating the proposed standard and the possible effects on its operations.
Greenhouse Gas Regulation
Beginning in 2011, greenhouse gas emissions from stationary sources, such as power plants, became subject to regulation under the Clean Air Act. As a result of this action, Ameren Missouri is required to consider the emissions of greenhouse gases in any air permit application.
Recognizing the difficulties presented by regulating at once virtually all emitters of greenhouse gases, the EPA issued the “Tailoring Rule,” which established new higher emission thresholds for regulating greenhouse gas emissions from stationary sources, such as power plants, through operating permits and the NSR programs. The rule requires any source that already has an operating permit to have provisions relating to greenhouse gas emissions added to its permit upon renewal. Currently, all Ameren Missouri energy centers have operating permits that have been modified to address greenhouse gas emissions. In June 2014, the United States Supreme Court ruled that the EPA may regulate greenhouse gas emissions through operating permits and NSR programs at stationary sources that are already subject to those programs, but may not apply operating permits and NSR programs to non-stationary sources solely as a result of their greenhouse gas emissions. Ameren Missouri does not expect the decision to have a significant effect on its operations.
In January 2014, the EPA published proposed regulations that would set revised CO2 emissions standards for new power plants. The proposed standards would establish separate emissions limits for new natural-gas-fired plants and new coal-fired plants. In June 2014, the EPA proposed the Clean Power Plan, which sets forth CO2 emissions standards that would be applicable to existing power plants. The proposed Clean Power Plan would require each state to develop plans to achieve CO2 emission standards that the EPA calculated for each state. The EPA believes that the Clean Power Plan would achieve a 30% reduction in the nation's existing power plant CO2 emissions from 2005 levels by 2030. The proposed rule also has interim goals of aggressively reducing CO2 emissions by 2020. The EPA expects the proposed rule will be finalized in 2015. If the proposed rule is finalized, states would have one to three years to develop compliance plans. States would be allowed to develop independent plans or to join with other states to develop joint plans. Ameren Missouri is evaluating the proposed Clean Power Plan and the potential impact to its operations, including those related to electric system reliability. Significant uncertainty exists regarding the standard for existing power plants, as the finalized rule could be different from the proposed rule and will be subject to legal challenges, either of which could result in the amount and timing of CO2 emission standards being revised.
Preliminary studies suggest that if the proposed Clean Power Plan were to be finalized in its current form, Ameren Missouri may need to incur new or accelerated capital expenditures and increased fuel costs in order to achieve compliance. As proposed, the Clean Power Plan would require the states, including Missouri and Illinois, to submit compliance plans as early as 2016. The states’ compliance plans might require Ameren Missouri to construct natural gas-fired combined cycle generation and renewable generation, at a currently estimated cost of approximately $2 billion by 2020, that Ameren Missouri believes would otherwise not be necessary to meet the energy needs of its customers. Additionally, Missouri’s implementation of the proposed rules, if adopted, could result in the closure or alteration of the operation of some of Ameren Missouri’s coal and natural gas-fired energy centers, which could result in increased operating costs or impairment of assets. Ameren Missouri expects substantially all of these increased costs, which could begin in 2017, to be recoverable, subject to MoPSC prudence review, through substantially higher electric rates charged to its customers.
Future federal and state legislation or regulations that mandate limits on the emission of greenhouse gases may result in significant increases in capital expenditures and operating costs, which could lead to increased liquidity needs and higher financing costs. These compliance costs could be prohibitive at some of Ameren Missouri’s energy centers, which could result in the impairment of long-lived assets if costs are not recovered through rates. Mandatory limits on the emission of greenhouse gases could increase costs for its customers or have a material adverse effect on Ameren's and Ameren Missouri's results of operations, financial position, and liquidity if regulators delay or deny recovery in rates of these compliance costs. Ameren's and Ameren Missouri's earnings might benefit from increased investment to comply with greenhouse gas limitations to the extent that the investments are reflected and recovered timely in rates charged to customers.
NSR and Clean Air Litigation
In January 2011, the Department of Justice, on behalf of the EPA, filed a complaint against Ameren Missouri in the United States District Court for the Eastern District of Missouri. The EPA's complaint, as amended in October 2013, alleges that in performing projects at its Rush Island coal-fired energy center in 2007 and 2010, Ameren Missouri violated provisions of the Clean Air Act and Missouri law. In January 2012, the district court granted, in part, Ameren Missouri's motion to dismiss various aspects of the EPA's penalty claims. The EPA's claims for unspecified injunctive relief remain. Ameren Missouri believes its defenses are meritorious and is defending itself vigorously. However, there can be no assurances that it will be successful in its efforts.
The ultimate resolution of this matter could have a material adverse effect on the future results of operations, financial position, and liquidity of Ameren and Ameren Missouri. A resolution could result in increased capital expenditures for the installation of pollution control equipment, increased operations and maintenance expenses, and penalties. We are unable to predict the ultimate resolution of these matters or the costs that might be incurred.
Clean Water Act
In August 2014, the EPA published the final rule applicable to cooling water intake structures at existing power plants. The rule requires a case-by-case evaluation and plan for reducing the mortality of aquatic organisms impinged on the facility’s intake screens or entrained through the plant's cooling water system. Implementation of this rule will be administered through each power plant’s water discharge permitting process. All coal-fired and nuclear energy centers at Ameren Missouri are subject to this rule. The rule could have an adverse effect on Ameren’s and Ameren Missouri’s results of operations, financial position, and liquidity if its implementation requires the installation of cooling towers or extensive modifications to the cooling water systems at our energy centers and if those investments are not recovered timely in electric rates charged to our customers.
In April 2013, the EPA announced its proposal to revise the effluent limitation guidelines applicable to steam electric generating units under the Clean Water Act. Effluent limitation guidelines are national standards for wastewater discharges to surface water that are based on the effectiveness of available control technology. The EPA's proposed rule raised several compliance options that would prohibit effluent discharges of certain, but not all, waste streams and impose more stringent limitations on certain components in wastewater discharges from power plants. If the rule is enacted as proposed, Ameren Missouri would be subject to the revised limitations beginning as early as July 1, 2017, but no later than July 1, 2022. The EPA is expected to issue final guidelines in September 2015.
Ash Management
In December 2014, the EPA issued regulations regarding the management and disposal of CCR, which will affect future disposal and handling costs at Ameren Missouri's energy centers. The EPA regulations will be effective 180 days after publication in the Federal Register, which is anticipated in early 2015. The rule allows for the management of CCR as a solid waste, as well as for its continued beneficial uses, such as recycling, which could reduce the amount to be disposed. The rule established criteria regarding the structural integrity, location, and operation of CCR impoundments and landfills. It requires groundwater monitoring and closure of impoundments if the groundwater standards under the rule are not achieved. Ameren Missouri is currently evaluating the rule to determine its impact on current management of CCR and the potential costs associated with compliance. Ameren Missouri is also evaluating the potential effect the new rule will have on its AROs associated with ash ponds. Ameren Missouri's capital expenditure plan includes the cost of constructing landfills as part of its environmental compliance plan. Ameren Missouri expects certain of its ash ponds could be closed within the next five years.
The EPA's regulations issued in December 2014 regarding the management and disposal of CCR do not apply to inactive ash ponds at plants no longer in operation, such as the Meredosia and Hutsonville energy centers.
Remediation
We are involved in a number of remediation actions to clean up sites affected by hazardous substances, as required by federal and state law. Such laws require that responsible parties fund remediation actions regardless of their degree of fault, the legality of original disposal, or the ownership of a disposal site. Ameren Missouri and Ameren Illinois have each been identified by federal or state governments as a potentially responsible party at several contaminated sites.
As of December 31, 2014, Ameren Illinois owned or was otherwise responsible for 44 former MGP sites in Illinois. These sites are in various stages of investigation, evaluation, remediation, and closure. Ameren Illinois estimates it could substantially conclude remediation efforts at most of these sites by 2018. The ICC allows Ameren Illinois to recover remediation and litigation costs associated with its former MGP sites from its electric and natural gas utility customers through environmental adjustment rate riders. To be recoverable, such costs must be prudently incurred. Costs are subject to annual review by the ICC. As of December 31, 2014, Ameren Illinois estimated the obligation related to these former MGP sites at $250 million to $314 million. Ameren and Ameren Illinois recorded a liability of $250 million to represent their estimated minimum obligation for these sites, as no other amount within the range was a better estimate.
The scope and extent to which these former MGP sites are remediated may increase as remediation efforts continue. Considerable uncertainty remains in these estimates, as many factors can influence the ultimate actual costs, including site specific unanticipated underground structures, the degree to which groundwater is encountered, regulatory changes, local ordinances, and site accessibility. The actual costs may vary substantially from these estimates.
Ameren Illinois formerly used an off-site landfill, which Ameren Illinois did not own, in connection with the operation of a previously-owned energy center. Ameren Illinois could be required to perform certain maintenance activities at that landfill. As of December 31, 2014, Ameren Illinois estimated the obligation related to this site at $0.5 million to $6 million. Ameren Illinois recorded a liability of $0.5 million to represent its estimated minimum obligation for this site, as no other amount within the range was a better estimate. Ameren Illinois is also responsible for the cleanup of some underground storage tanks and a water treatment plant in Illinois. As of December 31, 2014, Ameren Illinois recorded a liability of $0.7 million to represent its best estimate of the obligation for these sites.
In 2008, the EPA issued an administrative order to Ameren Missouri pertaining to a former coal tar distillery operated by Koppers Company or its predecessor and successor companies. While Ameren Missouri is the current owner of the site, which is located in St. Louis, Missouri, it did not conduct any of the manufacturing operations involving coal tar or its byproducts. Ameren Missouri, along with two other potentially responsible parties, are performing a site investigation. As of December 31, 2014, Ameren Missouri estimated its obligation at $2 million to $5 million. Ameren Missouri recorded a liability of $2 million to represent its estimated minimum obligation, as no other amount within the range was a better estimate.
Ameren Missouri also participated in the investigation of various sites located in Sauget, Illinois. In 2000, the EPA notified Ameren Missouri and numerous other companies, including Solutia, Inc., that former landfills and lagoons at those sites may contain soil and groundwater contamination. These sites are known as Sauget Area 2. From about 1926 until 1976, Ameren Missouri operated an energy center adjacent to Sauget Area 2. Ameren Missouri currently owns a parcel of property at Sauget Area 2 that was once used as a landfill. Under the terms of an Administrative Order on Consent, Ameren Missouri joined with other potentially responsible parties to evaluate the extent of potential contamination with respect to Sauget Area 2.
In December 2013, the EPA issued its record of decision for Sauget Area 2 approving the investigation and the remediation alternatives recommended by the potentially responsible parties. Further negotiation among the potentially responsible parties will determine how to fund the implementation of the EPA-approved cleanup remedies. As of December 31, 2014, Ameren Missouri estimated its obligation related to Sauget Area 2 at $1 million to $2.5 million. Ameren Missouri recorded a liability of $1 million to represent its estimated minimum obligation, as no other amount within the range was a better estimate.
In 2012, Ameren Missouri signed an administrative order with the EPA and agreed to investigate soil and groundwater conditions at an Ameren Missouri-owned substation in St. Charles, Missouri. As of December 31, 2014, Ameren Missouri estimated the obligation related to this cleanup at $1.6 million to $4.5 million. Ameren Missouri recorded a liability of $1.6 million to represent its estimated minimum obligation for this site, as no other amount within the range was a better estimate.
Our operations or those of our predecessor companies involve the use of, disposal of, and in appropriate circumstances, the cleanup of substances regulated under environmental laws. We are unable to determine whether such practices will result in future environmental commitments or will affect our results of operations, financial position, or liquidity.
Pumped-storage Hydroelectric Facility Breach
In December 2005, there was a breach of the upper reservoir at Ameren Missouri's Taum Sauk pumped-storage hydroelectric energy center. This resulted in significant flooding in the local area, which damaged a state park. Ameren Missouri had liability insurance coverage for the Taum Sauk incident, subject to certain limits and deductibles.
In 2010, Ameren Missouri sued an insurance company that was providing Ameren Missouri with liability coverage on the date of the Taum Sauk incident. In the litigation, Ameren Missouri claims that the insurance company breached its duty to indemnify Ameren Missouri for losses resulting from the incident. In September 2014, the United States District Court for the Eastern District of Missouri ordered the case to be transferred to the United States District Court for the Southern District of New York for trial. The transfer order has been stayed pending resolution of Ameren Missouri’s October 2014 appeal of that order to the United States Court of Appeals for the Eighth Circuit.
In June 2014, Ameren Missouri reached a settlement with another group of insurers who provided Ameren Missouri with liability coverage on the date of the Taum Sauk incident. In accordance with that settlement, Ameren Missouri received a payment of $27 million.
As of December 31, 2014, Ameren Missouri had an insurance receivable of $41 million. It ultimately expects to collect this receivable from the remaining insurance company in the pending litigation described above. This receivable is included in “Other assets” on Ameren’s and Ameren Missouri’s balance sheets as of December 31, 2014. Ameren's and Ameren Missouri's results of operations, financial position, and liquidity could be adversely affected if Ameren Missouri's remaining liability insurance claim is not paid.
Asbestos-related Litigation
Ameren, Ameren Missouri, and Ameren Illinois have been named, along with numerous other parties, in a number of lawsuits filed by plaintiffs claiming varying degrees of injury from asbestos exposure at our present or former energy centers. Most have been filed in the Circuit Court of Madison County, Illinois. The total number of defendants named in each case varies, with 78 as the average number of parties as of December 31, 2014. Each lawsuit seeks unspecified damages that, if awarded at trial, typically would be shared among the various defendants.
The following table presents the pending asbestos-related lawsuits filed against the Ameren Companies as of December 31, 2014:
Ameren		Ameren	Ameren		Total(a)
		Missouri	Illinois
1		45	57		70
(a)	Total does not equal the sum of the subsidiary unit lawsuits because some of the lawsuits name multiple Ameren entities as defendants.
At December 31, 2014, Ameren, Ameren Missouri, and Ameren Illinois had liabilities of $12 million, $5 million, and $7 million, respectively, recorded to represent their best estimates of their obligations related to asbestos claims.
Ameren Illinois has a tariff rider to recover the costs of IP asbestos-related litigation claims, subject to the following terms: 90% of the cash expenditures in excess of the amount included in base electric rates is to be recovered from a trust fund that was established when Ameren acquired IP. At December 31, 2014, the trust fund balance was $22 million, including accumulated interest. If cash expenditures are less than the amount in base rates, Ameren Illinois will contribute 90% of the difference to the trust fund. Once the trust fund is depleted, 90% of allowed cash expenditures in excess of base rates will be recovered through charges assessed to customers under the tariff rider. The rider will permit recovery from electric customers within IP’s historical service territory.
Ameren Illinois Municipal Taxes
Ameren Illinois previously received tax liability notices from eight municipalities, including the City of O'Fallon, alleging that Ameren Illinois failed to collect prior-period taxes from certain customers in each municipality. In November 2014, Ameren Illinois reached a settlement agreement with the City of O'Fallon and paid $1 million for the prior-period taxes. With respect to the seven other communities, Ameren Illinois believes its defenses to the allegations are meritorious and its potential loss is immaterial.

Divestiture_Transactions_and_D

Divestiture Transactions and Discontinued Operations (Notes)	12 Months Ended
	Dec. 31, 2014
Discontinued Operations and Disposal Groups [Abstract]
DIVESTITURE TRANSACTIONS AND DISCONTINUED OPERATIONS	NOTE 16 – DIVESTITURE TRANSACTIONS AND DISCONTINUED OPERATIONS
	On December 2, 2013, Ameren completed the divestiture of New AER to IPH in accordance with the transaction agreement between Ameren and IPH dated March 14, 2013, as amended by a letter agreement dated December 2, 2013.
	Ameren retained certain pension and postretirement benefit obligations associated with current and former employees of AER, with the exception of the pension and postretirement benefit obligations associated with current and former employees of EEI, which were assumed by IPH. Ameren retained the Meredosia and Hutsonville energy centers, including their AROs. These energy centers were abandoned and had an immaterial property and plant asset balance as of December 31, 2014. The EPA's regulations issued in December 2014 regarding the management and disposal of CCR, as discussed in Note 15 – Commitments and Contingencies, do not apply to inactive ash ponds at plants no longer in operation, such as the Meredosia and Hutsonville energy centers. All other AROs associated with AER were assumed by New AER or by Rockland Capital, the third-party buyer of the Grand Tower energy center, as discussed below.
	The transaction agreement with IPH, as amended, provides that if the Elgin, Gibson City, and Grand Tower gas-fired energy centers are subsequently sold by Medina Valley and if Medina Valley receives additional proceeds from such sale, Medina Valley will pay Genco any proceeds from such sale, net of taxes and other expenses, in excess of the $137.5 million previously paid to Genco. On January 31, 2014, Medina Valley completed the sale of the Elgin, Gibson City, and Grand Tower gas-fired energy centers to Rockland Capital for a total purchase price of $168 million. The agreement with Rockland Capital requires $17 million of the purchase price to be held in escrow until January 31, 2016, to fund certain indemnity obligations, if any, of Medina Valley. The Rockland Capital escrow receivable balance and the corresponding payable due to Genco is reflected on Ameren's December 31, 2014, consolidated balance sheet in "Other assets" and in "Other deferred credits and liabilities," respectively. Medina Valley expects to pay Genco any remaining portion of the escrow balance on January 31, 2016. Ameren did not record a gain from its sale of the Elgin, Gibson City, and Grand Tower gas-fired energy centers.
	Discontinued Operations Presentation
	In March 2013, Ameren determined that New AER and the Elgin, Gibson City, and Grand Tower gas-fired energy centers qualified for discontinued operations presentation. In addition, in December 2013, coinciding with the completion of the divestiture of New AER to IPH, Ameren determined that the Meredosia and Hutsonville energy centers, which were both not operating, had been abandoned and also qualified for discontinued operations presentation. Ameren has begun to demolish the Hutsonville energy center and expects to demolish the Meredosia energy center thereafter. The disposal groups have been aggregated in the disclosures below. The following table presents the components of discontinued operations in Ameren's consolidated statement of income (loss) for the years ended December 31, 2014, 2013, and 2012:
		Year ended
		2014			2013				2012
	Operating revenues	$	1		$	1,037			$	1,047
	Operating expenses	(2		)	(1,207		)	(a)	(3,474		)	(b)
	Operating income (loss)	(1		)	(170		)		(2,427		)
	Other income (loss)	—			(1		)		—
	Interest charges	—			(39		)		(56		)
	Income (loss) before income taxes	(1		)	(210		)		(2,483		)
	Income tax (expense) benefit	—			(13		)		987
	Income (loss) from discontinued operations, net of taxes	$	(1	)	$	(223	)		$	(1,496	)
	(a)	Includes a $201 million pretax loss on disposal relating to the New AER divestiture.
	(b)	Includes a noncash pretax asset impairment charge of $2.58 billion to reduce the carrying value of AER's energy centers to their estimated fair value under held and used accounting guidance.
	Ameren’s results of operations for the year ended December 31, 2014, include adjustments for the New AER net working capital amount owed to IPH and for certain contingent liabilities associated with the New AER divestiture. In 2014, Ameren paid $13 million to IPH for the final working capital adjustment and a portion of the previously-recorded contingent liabilities. Additionally, Ameren recognized the operating revenues and operating expenses associated with the Elgin, Gibson City, and Grand Tower gas-fired energy centers prior to the completion of their sale to Rockland Capital on January 31, 2014. The final tax basis of the AER disposal group and the related tax benefit resulting from the transaction with IPH are dependent upon the resolution of tax matters under audit. It is reasonably possible in the next 12 months these tax audits will be completed. As a result, tax expense and benefits ultimately realized from the divestitures may differ materially from those recorded as of December 31, 2014, including the final resolution of Ameren's uncertain tax positions.
	Ameren recorded a pretax charge to earnings related to the New AER divestiture of $201 million for the year ended December 31, 2013. The loss was recorded in “Operating expenses” within the components of the discontinued operations statement of income (loss). Ameren did not receive any cash proceeds from IPH for the divestiture of New AER. In 2013, Ameren adjusted the accumulated deferred income taxes on its consolidated balance sheet to reflect the excess of tax basis over financial reporting basis of its stock investment in AER. This change in basis resulted in a discontinued operations deferred tax expense of $99 million, which was partially offset by the expected tax benefits of $86 million related to the pretax loss from discontinued operations, including the loss on disposal, during the year ended December 31, 2013.
	As discussed above, on January 31, 2014, Medina Valley completed the sale of the Elgin, Gibson City, and Grand Tower gas-fired energy centers to Rockland Capital for a total purchase price of $168 million. Ameren did not recognize a gain from the third-party sale to Rockland Capital for any value in excess of its $137.5 million carrying value for this disposal group because any excess amount that Medina Valley may receive, net of taxes and other expenses, over the carrying value, will ultimately be paid to Genco pursuant to the transaction agreement with IPH.
	New AER and the Elgin, Gibson City, and Grand Tower energy centers were impaired under held and used accounting guidance in 2012. In early 2012, the observable market price for power for delivery in that year and in future years in the Midwest sharply declined below 2011 levels. As a result of this sharp decline in the market price of power and the related impact on electric margins, Ameren evaluated, during the first quarter of 2012, whether the carrying values of Merchant Generation coal-fired energy centers were recoverable. AERG's Duck Creek energy center's carrying value exceeded its estimated undiscounted future cash flows. As a result, Ameren recorded a noncash pretax asset impairment charge of $628 million to reduce the carrying value of that energy center to its estimated fair value during the first quarter of 2012. In December 2012, Ameren determined that the estimated undiscounted cash flows during the period in which it expected to continue to own its Merchant Generation energy centers would be insufficient to recover the carrying value of those energy centers. Accordingly, Ameren recorded a noncash pretax impairment charge of $1.95 billion in the fourth quarter of 2012.
	The following table presents the carrying amounts of the components of assets and liabilities segregated on Ameren's consolidated balance sheets as discontinued operations at December 31, 2014 and 2013:
		31-Dec-14			31-Dec-13
	Assets of discontinued operations
	Accounts receivable and unbilled revenue	$	—		$	5
	Materials and supplies	—			5
	Property and plant, net	—			142
	Accumulated deferred income taxes, net(a)	15			13
	Total assets of discontinued operations	$	15		$	165
	Liabilities of discontinued operations
	Accounts payable and other current obligations	$	1		$	5
	Asset retirement obligations(b)	32			40
	Total liabilities of discontinued operations	$	33		$	45
	(a)	The December 31, 2014 balance primarily consists of deferred income tax assets related to the abandoned Meredosia and Hutsonville energy centers.
	(b)	Includes AROs associated with the abandoned Meredosia and Hutsonville energy centers of $32 million and $31 million at December 31, 2014 and 2013, respectively.
	Ameren has continuing transactions with New AER. Ameren Illinois has power supply agreements with Marketing Company, which are a result of the power procurement process in Illinois administered by the IPA, as required by the Illinois Public Utilities Act. Ameren Illinois continues to purchase power and to purchase trade receivables as required by Illinois law. Ameren Illinois and ATXI continue to sell transmission services to Marketing Company. Also, the transaction agreement requires Ameren (parent) to maintain certain guarantees discussed below. Immediately prior to the transaction agreement closing, the money pool borrowings through which Ameren provided cash collateral to Marketing Company were converted to a note payable to Ameren, with interest, on December 2, 2015, or sooner, as cash collateral requirements are reduced. Ameren has determined that the continuing cash flows generated by these arrangements are not significant and, accordingly, are not deemed to be direct cash flows of the divested business. Additionally, these arrangements do not provide Ameren with the ability to significantly influence the operating results of New AER. Ameren did not have significant continuing involvement with or material cash flows from the Elgin, Gibson City, or Grand Tower energy centers after their sale.
	Pursuant to the IPH transaction agreement, as amended, Ameren is obligated to pay up to $29 million for certain contingent liabilities as of December 31, 2014, which were included in "Other current liabilities" on Ameren's December 31, 2014 consolidated balance sheet.
	The note receivable from Marketing Company related to the cash collateral support provided to New AER was $12 million and $18 million at December 31, 2014 and 2013, respectively, and was reflected on Ameren's consolidated balance sheet in "Miscellaneous accounts and notes receivable" at December 31, 2014. This receivable is due to Ameren, with interest, on December 2, 2015, or sooner as cash collateral requirements are reduced. In addition, as of December 31, 2014, if Ameren’s credit ratings had been below investment grade, Ameren could have been required to post additional cash collateral in support of New AER in the amount of $26 million, which includes $11 million currently covered by Ameren guarantees. This cash collateral support is part of Ameren’s obligation to provide certain limited credit support to New AER until December 2, 2015, as discussed below.
	Ameren Guarantees and Letters of Credit
	The IPH transaction agreement, as amended, requires Ameren to maintain its financial obligations with respect to all credit support provided to New AER as of the December 2, 2013 closing date of the divestiture. Ameren must also provide such additional credit support as required by contracts entered into prior to the closing date, in each case until December 2, 2015. IPH shall indemnify Ameren for any payments Ameren makes pursuant to these credit support obligations if the counterparty does not return the posted collateral to Ameren. IPH's indemnification obligation is secured by certain AERG and Genco assets. In addition, Dynegy has provided a limited guarantee of $25 million to Ameren pursuant to which Dynegy will, among other things, guarantee IPH's indemnification obligations until December 2, 2015.
	In addition to the $29 million of contingent liabilities recorded on Ameren’s December 31, 2014 consolidated balance sheet, Ameren had a total of $114 million in guarantees outstanding for New AER that were not recorded on Ameren’s December 31, 2014 consolidated balance sheet, which included:
	•	$106 million related to guarantees supporting Marketing Company for physically and financially settled power transactions with its counterparties that were in place at the December 2, 2013 closing of the divestiture, as well as for Marketing Company's clearing broker and other service agreements. If Marketing Company did not fulfill its obligations to these counterparties who had active open positions as of December 31, 2014, Ameren would have been required under its guarantees to provide $11 million to the counterparties.
	•	$8 million related to requirements for lease agreements and potential environmental obligations. If New AER had not fulfilled its lease obligation as of December 31, 2014, Ameren would have been required to provide approximately $7 million to the leasing counterparty.
	Additionally, at December 31, 2014, Ameren had issued letters of credit totaling $9 million as credit support on behalf of New AER.
	Ameren has not recorded a reserve for these contingent obligations because it does not believe a payment with respect to any of these guarantees or letters of credit was probable as of December 31, 2014.

Segment_Information

Segment Information	12 Months Ended
	Dec. 31, 2014
Segment Reporting Information, Profit (Loss) [Abstract]
SEGMENT INFORMATION	SEGMENT INFORMATION
	Ameren has two reportable segments: Ameren Missouri and Ameren Illinois. Ameren Missouri and Ameren Illinois each have one reportable segment. The Ameren Missouri segment for both Ameren and Ameren Missouri includes all the operations of Ameren Missouri as described in Note 1 – Summary of Significant Accounting Policies. The Ameren Illinois segment for both Ameren and Ameren Illinois consists of all of the operations of Ameren Illinois as described in Note 1 – Summary of Significant Accounting Policies. The category called Other primarily includes Ameren parent company activities, Ameren Services, and ATXI. The Other category also includes certain corporate activities previously included in the Merchant Generation segment. See Note 16 – Divestiture Transactions and Discontinued Operations for additional information.
	The following table presents information about the reported revenues and specified items reflected in Ameren’s net income attributable to Ameren Corporation and capital expenditures from continuing operations for the years ended December 31, 2014, 2013, and 2012, and total assets in continuing operations as of December 31, 2014, 2013, and 2012:
		Ameren		Ameren		Other				Intersegment			Consolidated
		Missouri		Illinois						Eliminations
	2014
	External revenues	$	3,526	$	2,496	$	31			$	—		$	6,053
	Intersegment revenues	27		2		2				(31		)	—
	Depreciation and amortization	473		263		9				—			745
	Interest and dividend income	28		7		2				—			37
	Interest charges	211		112		18				—			341
	Income taxes	229		143		5				—			377
	Net income (loss) attributable to Ameren Corporation from continuing operations	390		201		(4		)		—			587
	Capital expenditures	747		835		203			(a)	—			1,785
	Total assets	13,541		8,381		942				(203		)	22,661		(b)
	2013
	External revenues	$	3,516	$	2,307	$	15			$	—		$	5,838
	Intersegment revenues	25		4		2				(31		)	—
	Depreciation and amortization	454		243		9				—			706
	Interest and dividend income	27		2		1				—			30
	Interest charges	210		143		45				—			398
	Income taxes (benefit)	242		110		(41		)		—			311
	Net income (loss) attributable to Ameren Corporation from continuing operations	395		160		(43		)		—			512
	Capital expenditures	648		701		30			(a)	—			1,379
	Total assets	12,904		7,454		752				(233		)	20,877		(b)
	2012
	External revenues	$	3,252	$	2,524	$	5			$	—		$	5,781
	Intersegment revenues	20		1		3				(24		)	—
	Depreciation and amortization	440		221		12				—			673
	Interest and dividend income	32		—		—				—			32
	Interest charges	223		129		40				—			392
	Income taxes (benefit)	252		94		(39		)		—			307
	Net income (loss) attributable to Ameren Corporation from continuing operations	416		141		(41		)		—			516
	Capital expenditures	595		442		26			(a)	—			1,063
	Total assets	13,043		7,282		1,228				(934		)	20,619		(b)
	.
	(a)	Includes the elimination of intercompany transfers.
	(b)	Excludes total assets from discontinued operations of $15 million, $165 million, and $1,611 million as of December 31, 2014, 2013, and 2012, respectively.

Selected_Quarterly_Information

Selected Quarterly Information	12 Months Ended
	Dec. 31, 2014
Selected Quarterly Financial Information [Abstract]
SELECTED QUARTERLY INFORMATION	SELECTED QUARTERLY INFORMATION (Unaudited) (In millions, except per share amounts)
	Ameren	2014														2013
	Quarter ended (a)	31-Mar			30-Jun			30-Sep				31-Dec				31-Mar			30-Jun			30-Sep			31-Dec
	Operating revenues	$	1,594		$	1,419		$	1,670			$	1,370			$	1,475		$	1,403		$	1,638		$	1,322
	Operating income	246			322			561				125				185			261			567			171
	Net income (loss)	98			150			295				49				(143		)	96			304			38
	Net income attributable to Ameren Corporation – continuing operations	$	97		$	150		$	294			$	46			$	54		$	105		$	305		$	48
	Net income (loss) attributable to Ameren Corporation – discontinued operations	(1		)	(1		)	(1		)		2				(199		)	(10		)	(3		)	(11		)
	Net income (loss) attributable to Ameren Corporation	$	96		$	149		$	293			$	48			$	(145	)	$	95		$	302		$	37
	Earnings per common share – basic – continuing operations	$	0.4		$	0.62		$	1.21			$	0.19			$	0.22		$	0.44		$	1.26		$	0.19
	Earnings (loss) per common share – basic – discontinued operations	—			(0.01		)	—				0.01				(0.82		)	(0.05		)	(0.01		)	(0.04		)
	Earnings (loss) per common share – basic	$	0.4		$	0.61		$	1.21			$	0.2			$	(0.60	)	$	0.39		$	1.25		$	0.15
	Earnings per common share – diluted – continuing operations	$	0.4		$	0.62		$	1.2			$	0.19			$	0.22		$	0.44		$	1.25		$	0.19
	Earnings (loss) per common share – diluted – discontinued operations	—			(0.01		)	—				0.01				(0.82		)	(0.05		)	(0.01		)	(0.04		)
	Earnings (loss) per common share – diluted	$	0.4		$	0.61		$	1.2			$	0.2			$	(0.60	)	$	0.39		$	1.24		$	0.15
	(a)	The sum of quarterly amounts, including per share amounts, may not equal amounts reported for year-to-date periods. This is due to the effects of rounding and to changes in the number of weighted-average shares outstanding each period.
	Ameren Missouri Quarter ended		Operating			Operating			Net Income				Net Income (Loss)
			Revenues			Income			(Loss)				Available
													to Common
													Stockholder
	March 31, 2014		$	817		$	119		$	48			$	47
	March 31, 2013		796			111			41				40
	June 30, 2014		900			243			127				126
	June 30, 2013		889			179			85				84
	September 30, 2014		1,097			394			223				222
	September 30, 2013		1,093			417			239				238
	December 31, 2014		739			29			(5		)		(5		)
	December 31, 2013		763			96			33				33
	Ameren Illinois Quarter ended		Operating			Operating			Net Income				Net Income
			Revenues			Income							Available
													to Common
													Stockholder
	March 31, 2014		$	774		$	120		$	54			$	53
	March 31, 2013		684			85			32				31
	June 30, 2014		519			75			29				28
	June 30, 2013		516			87			32				31
	September 30, 2014		572			158			75				75
	September 30, 2013		547			158			77				77
	December 31, 2014		633			97			46				45
	December 31, 2013		564			85			22				21

Schedule_I_Condensed_Financial

Schedule I - Condensed Financial Information Of Parent	12 Months Ended
	Dec. 31, 2014
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Condensed Financial Information Of Parent
	SCHEDULE I – CONDENSED FINANCIAL INFORMATION OF PARENT
	AMEREN CORPORATION
	CONDENSED STATEMENT OF INCOME (LOSS) AND COMPREHENSIVE INCOME (LOSS)
	For the Years Ended December 31, 2014, 2013, and 2012
	(In millions)	2014			2013			2012
	Operating revenues	$	—		$	—		$	—
	Operating expenses	11			26			17
	Operating loss	(11		)	(26		)	(17		)
	Equity in earnings of subsidiaries	607			546			546
	Interest income from affiliates	3			3			3
	Total other income (expense), net	2			(5		)	(4		)
	Interest charges	16			42			39
	Income tax (benefit)	(2		)	(36		)	(27		)
	Net Income Attributable to Ameren Corporation – Continuing Operations	587			512			516
	Net Loss Attributable to Ameren Corporation – Discontinued Operations	(1		)	(223		)	(1,490		)
	Net Income (Loss) Attributable to Ameren Corporation	$	586		$	289		$	(974	)
	Net Income Attributable to Ameren Corporation – Continuing Operations	$	587		$	512		$	516
	Other Comprehensive Income (Loss), Net of Taxes:
	Pension and other postretirement benefit plan activity, net of income taxes (benefit) of $(7), $16, and $(6), respectively	(12		)	30			(8		)
	Comprehensive Income from Continuing Operations Attributable to Ameren Corporation	575			542			508
	Net Loss Attributable to Ameren Corporation – Discontinued Operations	(1		)	(223		)	(1,490		)
	Other Comprehensive Income (Loss) from Discontinued Operations, Net of Income Taxes	—			(19		)	50
	Comprehensive Loss from Discontinued Operations Attributable to Ameren Corporation	(1		)	(242		)	(1,440		)
	Comprehensive Income (Loss) Attributable to Ameren Corporation	$	574		$	300		$	(932	)
	SCHEDULE I – CONDENSED FINANCIAL INFORMATION OF PARENT
	AMEREN CORPORATION
	CONDENSED BALANCE SHEET
	(In millions)	31-Dec-14			31-Dec-13
	Assets:
	Cash and cash equivalents	$	1		$	11
	Advances to money pool	55			334
	Accounts receivable – affiliates	28			18
	Notes receivable – affiliates	94			9
	Miscellaneous accounts and notes receivable	39			125
	Current accumulated deferred income taxes, net	143			41
	Other current assets	14			1
	Total current assets	374			539
	Investments in subsidiaries – continuing operations	6,680			6,336
	Investments in subsidiaries – discontinued operations	(4		)	(5		)
	Note receivable – ATXI	100			51
	Accumulated deferred income taxes, net	264			570
	Other assets	152			141
	Total assets	$	7,566		$	7,632
	Liabilities and Stockholders’ Equity:
	Current maturities of long-term debt	$	—		$	425
	Short-term debt	585			368
	Accounts payable	—			119
	Accounts payable – affiliates	88			4
	Other current liabilities	52			20
	Total current liabilities	725			936
	Other deferred credits and liabilities	128			152
	Total liabilities	853			1,088
	Commitments and Contingencies (Notes 4 and 5)
	Stockholders’ Equity:
	Common stock, $.01 par value, 400.0 shares authorized – shares outstanding of 242.6	2			2
	Other paid-in capital, principally premium on common stock	5,617			5,632
	Retained earnings	1,103			907
	Accumulated other comprehensive income (loss)	(9		)	3
	Total stockholders’ equity	6,713			6,544
	Total liabilities and stockholders’ equity	$	7,566		$	7,632
	SCHEDULE I – CONDENSED FINANCIAL INFORMATION OF PARENT
	AMEREN CORPORATION
	CONDENSED STATEMENT OF CASH FLOWS
For the Years Ended December 31, 2014, 2013, and 2012
(In millions)	2014			2013			2012
Net cash flows provided by operating activities	$	514		$	453		$	532
Cash flows from investing activities:
Money pool advances, net	279			(371		)	24
Notes receivable – affiliates, net	(134		)	(23		)	(20		)
Investments in subsidiaries	(280		)	(50		)	(2		)
Distributions from subsidiaries	215			1			21
Proceeds from note receivable – Marketing Company	95			6			—
Contributions to note receivable – Marketing Company	(89		)	(5		)	—
Other	(12		)	(3		)	(5		)
Net cash flows provided by (used in) investing activities	74			(445		)	18
Cash flows from financing activities:
Dividends on common stock	(390		)	(388		)	(382		)
Short-term debt, net	217			368			(148		)
Maturities of long-term debt	(425		)	—			—
Net cash flows used in financing activities	(598		)	(20		)	(530		)
Net change in cash and cash equivalents	$	(10	)	$	(12	)	$	20
Cash and cash equivalents at beginning of year	11			23			3
Cash and cash equivalents at end of year	$	1		$	11		$	23
Cash dividends received from consolidated subsidiaries	$	340		$	570		$	610
Noncash investing activity – divestiture	$	—		$	494		$	—
Noncash investing activity – investments in subsidiaries	(19		)	—			—
Noncash financing activity – dividends on common stock	—			—			(7		)
AMEREN CORPORATION (parent company only)
NOTES TO CONDENSED FINANCIAL STATEMENTS
December 31, 2014
NOTE 1 – BASIS OF PRESENTATION
Ameren Corporation (parent company only) is a public utility holding company that conducts substantially all of its business operations through its subsidiaries. In accordance with authoritative accounting guidance, Ameren Corporation (parent company only) has accounted for wholly owned subsidiaries using the equity method. These financial statements are presented on a condensed basis.
Beginning in 2014, unrecognized tax benefits are recorded as a reduction to the deferred tax assets for net operating losses and tax credit carryforwards within "Accumulated deferred income taxes, net" on Ameren Corporation's (parent company only) balance sheets. At December 31, 2014, unrecognized tax benefits of $53 million were recorded in "Accumulated deferred income taxes, net" on Ameren Corporation's (parent company only) balance sheet. At December 31, 2013, unrecognized tax benefits of $53 million previously recorded in "Other deferred credits and liabilities" on Ameren Corporation's (parent company only) balance sheet were reclassified to "Accumulated deferred income taxes, net" for comparative purposes. See Note 1 – Summary of Significant Accounting Policies under Part II, Item 8, of this report for additional information.
Additional disclosures relating to the parent company financial statements are included within the combined notes under Part II, Item 8, of this report. See Note 1 – Summary of Significant Accounting Policies and Note 14 – Related Party Transactions under Part II, Item 8, of this report for information on the tax allocation agreement between Ameren (parent) and its subsidiaries.
NOTE 2 – SHORT-TERM DEBT AND LIQUIDITY
See Note 4 – Short-term Debt and Liquidity under Part II, Item 8, of this report for a description and details of short-term debt and liquidity needs of Ameren Corporation (parent company only).
NOTE 3 – LONG-TERM OBLIGATIONS
In May 2014, Ameren (parent) repaid at maturity $425 million of its 8.875% senior unsecured notes, plus accrued interest. The notes were repaid with proceeds from commercial paper issuances.
NOTE 4 – COMMITMENTS AND CONTINGENCIES
See Note 15 – Commitments and Contingencies and Note 16 – Divestiture Transactions and Discontinued Operations under Part II, Item 8, of this report for a description of all material contingencies, guarantees, and letters of credit outstanding of Ameren Corporation (parent company only).
NOTE 5 – NEW AER DIVESTITURE AND DISCONTINUED OPERATIONS
In December 2012, Ameren determined that it intended to, and it was probable that it would, exit its Merchant Generation business before the end of the previously estimated useful lives of that business's long-lived assets. As a result of the 2012 determination, Ameren Corporation (parent company only) recorded a pretax impairment charge of $1.88 billion to reduce its investment in certain of the Merchant Generation segment's coal and natural-gas-fired energy centers to their estimated fair values. In December 2013, Ameren completed a divestiture that included a significant portion of that business. As a result of the divestiture in 2013, Ameren Corporation (parent company only) recorded a pretax loss on disposal of $201 million. These charges were included within "Net Loss Attributable to Ameren Corporation – Discontinued Operations" in the Ameren Corporation (parent company only) Condensed Statement of Income (Loss) and Comprehensive Income (Loss) for the years ended December 31, 2013 and 2012.
The "Miscellaneous accounts and notes receivable" on the December 31, 2013 Ameren Corporation (parent company only) Condensed Balance Sheet included a receivable from Dynegy related to the non-state-regulated subsidiary money pool borrowing balance as of the divestiture date of certain New AER subsidiaries. Additionally, a payable to Dynegy of the estimated working capital adjustment required under the terms of the agreement with IPH was reflected in "Accounts payable" on the December 31, 2013 Ameren Corporation (parent company only) Condensed Balance Sheet. In 2014, the receivable and payable were finalized and settled, along with certain contingent liabilities associated with the New AER divestiture, resulting in a net $13 million payment to IPH.
See Note 16 – Divestiture Transactions and Discontinued Operations under Part II, Item 8, of this report for additional information on the impairment charges recognized in 2013 and 2012 as well as the divestiture.

Schedule_II_Valuation_And_Qual

Schedule II - Valuation And Qualifying Accounts	12 Months Ended
	Dec. 31, 2014
Valuation and Qualifying Accounts [Abstract]
Valuation And Qualifying Accounts
	SCHEDULE II – VALUATION AND QUALIFYING ACCOUNTS
	FOR THE YEARS ENDED DECEMBER 31, 2014, 2013, AND 2012
	(in millions)
	Column A	Column B		Column C				Column D		Column E
	Description	Balance at		-1		-2		Deductions(b)		Balance at End
		Beginning		Charged to Costs		Charged to Other				of Period
		of Period		and Expenses		Accounts(a)
	Ameren:
	Deducted from assets – allowance for doubtful accounts:
	2014	$	18	$	36	$	4	$	37	$	21
	2013	17		35		4		38		18
	2012	20		30		2		35		17
	Deferred tax valuation allowance:
	2014	$	7	$	3	$	—	$	—	$	10
	2013	2		5		—		—		7
	2012	1		1		—		—		2
	Ameren Missouri:
	Deducted from assets – allowance for doubtful accounts:
	2014	$	5	$	16	$	—	$	13	$	8
	2013	5		16		—		16		5
	2012	7		11		—		13		5
	Deferred tax valuation allowance:
	2014	$	1	$	—	$	—	$	—	$	1
	2013	1		—		—		—		1
	2012	1		—		—		—		1
	Ameren Illinois:
	Deducted from assets – allowance for doubtful accounts:
	2014	$	13	$	20	$	4	$	24	$	13
	2013	12		19		4		22		13
	2012	13		19		2		22		12
	Deferred tax valuation allowance:
	2014	$	1	$	—	$	—	$	—	$	1
	2013	1		—		—		—		1
	2012	—		1		—		—		1
	(a)	Uncollectible account reserve associated with receivables purchased by Ameren Illinois from alternative retail electric suppliers, as required by the Illinois Public Utilities Act.
	(b)	Uncollectible accounts charged off, less recoveries.

Summary_Of_Significant_Account1

Summary Of Significant Accounting Policies (Policy)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
General	Ameren, headquartered in St. Louis, Missouri, is a public utility holding company under PUHCA 2005, administered by the FERC. Ameren’s primary assets are its equity interests in its subsidiaries, including Ameren Missouri and Ameren Illinois. Ameren’s subsidiaries are separate, independent legal entities with separate businesses, assets, and liabilities. Dividends on Ameren’s common stock and the payment of other expenses by Ameren depend on distributions made to it by its subsidiaries. Ameren’s principal subsidiaries are listed below.
	•	Union Electric Company, doing business as Ameren Missouri, operates a rate-regulated electric generation, transmission, and distribution business and a rate-regulated natural gas transmission and distribution business in Missouri. Ameren Missouri was incorporated in Missouri in 1922 and is successor to a number of companies, the oldest of which was organized in 1881. It is the largest electric utility in the state of Missouri. It supplies electric and natural gas service to a 24,000-square-mile area in central and eastern Missouri. This area has an estimated population of 2.8 million and includes the Greater St. Louis area. Ameren Missouri supplies electric service to 1.2 million customers and natural gas service to 127,000 customers.
	•	Ameren Illinois Company, doing business as Ameren Illinois, operates rate-regulated electric and natural gas transmission and distribution businesses in Illinois. Ameren Illinois was created by the merger of CILCO and IP with and into CIPS in 2010. CIPS was incorporated in Illinois in 1923 and was the successor to a number of companies, the oldest of which was organized in 1902. Ameren Illinois supplies electric and natural gas utility service to portions of central and southern Illinois having an estimated population of 3.1 million in an area of 40,000 square miles. Ameren Illinois supplies electric service to 1.2 million customers and natural gas service to 813,000 customers.
	Ameren has various other subsidiaries responsible for activities such as the provision of shared services. Ameren also has a subsidiary, ATXI, that operates a FERC rate-regulated electric transmission business. ATXI is developing MISO-approved electric transmission projects, including the Illinois Rivers, Spoon River, and Mark Twain projects. Ameren is also pursuing reliability projects within Ameren Missouri's and Ameren Illinois' service territories as well as competitive electric transmission investment opportunities outside of these territories, including investments outside of MISO.
Consolidation	The financial statements of Ameren are prepared on a consolidated basis, and therefore include the accounts of its majority-owned subsidiaries. Ameren Missouri and Ameren Illinois have no subsidiaries and therefore their financial statements are not prepared on a consolidated basis. All intercompany transactions have been eliminated. All tabular dollar amounts are in millions, unless otherwise indicated.
	Our accounting policies conform to GAAP. Our financial statements reflect all adjustments (which include normal, recurring adjustments) that are necessary, in our opinion, for a fair presentation of our results. The preparation of financial statements in conformity with GAAP requires management to make certain estimates and assumptions. Such estimates and assumptions affect reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the dates of financial statements, and the reported amounts of revenues and expenses during the reported periods. Actual results could differ from those estimates.
Public Utilities	Purchased Gas, Power and Fuel Rate-adjustment Mechanisms
	Ameren Missouri and Ameren Illinois have various rate-adjustment mechanisms in place that provide for the recovery of purchased natural gas and electric fuel and purchased power costs without a traditional rate case proceeding. See Note 2 – Rate and Regulatory Matters for the regulatory assets and liabilities recorded at December 31, 2014 and 2013, related to the rate-adjustment mechanisms discussed below.
	In Ameren Missouri’s and Ameren Illinois’ natural gas utility jurisdictions, changes in natural gas costs are reflected in billings to their natural gas utility customers through PGA clauses. The difference between actual natural gas costs and costs billed to customers in a given period is deferred as a regulatory asset or liability. The deferred amount is either billed or refunded to natural gas utility customers in a subsequent period.
	In Ameren Illinois’ retail electric utility jurisdiction, changes in purchased power and transmission service costs are reflected in billings to its electric utility customers through pass-through rate-adjustment clauses. The difference between actual purchased power and transmission service costs and costs billed to customers in a given period is deferred as a regulatory asset or liability. The deferred amount is either billed or refunded to electric utility customers in a subsequent period.
	Ameren Missouri has a FAC that allows an adjustment of electric rates three times per year for a pass-through to customers of 95% of changes in fuel and purchased power costs, including transportation charges and revenues, net of off-system sales, greater or less than the amount set in base rates, subject to MoPSC prudence review. The difference between the actual amounts incurred for these items and the amounts recovered from Ameren Missouri customers' base rates is deferred as a regulatory asset or liability. The deferred amounts are either billed or refunded to electric utility customers in a subsequent period.
	Environmental Costs
	Liabilities for environmental costs are recorded on an undiscounted basis when it is probable that a liability has been incurred and the amount of the liability can be reasonably estimated. Costs are expensed or deferred as a regulatory asset when it is expected that the costs will be recovered from customers in future rates.
	Investments
	Ameren and Ameren Missouri record investments held in Ameren Missouri's nuclear decommissioning trust fund at fair value. Losses on assets in the trust fund could result in higher funding requirements for decommissioning costs, which Ameren Missouri believes would be recovered in electric rates paid by its customers. Accordingly, Ameren and Ameren Missouri recognize a regulatory asset on their balance sheets for losses on investments held in the nuclear decommissioning trust fund. In addition, Ameren and Ameren Missouri recognize a regulatory liability on their balance sheets for gains on investments held in the nuclear decommissioning trust fund. As of December 31, 2014, the nuclear decommissioning trust fund had cumulative gains.
	Regulation
	We are regulated by the MoPSC, the ICC, and the FERC. We defer certain costs as assets pursuant to actions of rate regulators or because of expectations that we will be able to recover such costs in rates charged to customers. We also defer certain amounts as liabilities pursuant to actions of rate regulators or based on the expectation that such amounts will be returned to customers in future rates. Regulatory assets and liabilities are amortized consistent with the period of expected regulatory treatment. In addition to the cost recovery mechanisms discussed in the Purchased Gas, Power, and Fuel Rate-adjustment Mechanisms section below, Ameren Missouri and Ameren Illinois have approvals from regulators to use other cost recovery mechanisms. Ameren Missouri has a vegetation management and infrastructure inspection cost tracker, a pension and postretirement benefit cost tracker, an uncertain tax positions tracker, a renewable energy standards cost tracker, a solar rebate program tracker, a storm restoration cost tracker, and the MEEIA energy efficiency rider. Ameren Illinois' and ATXI's electric transmission rates are subject to formula ratemaking. Additionally, Ameren Illinois' electric distribution business participates in the performance-based formula ratemaking process established pursuant to the IEIMA. Ameren Illinois also has an environmental cost rider, an asbestos-related litigation rider, an energy efficiency rider, and a bad debt rider. See Note 2 – Rate and Regulatory Matters for additional information on regulatory assets and liabilities.
Cash and Cash Equivalents	Cash and Cash Equivalents
	Cash and cash equivalents include cash on hand and temporary investments purchased with an original maturity of three months or less.
Allowance for Doubtful Accounts Receivable	Allowance for Doubtful Accounts Receivable
	The allowance for doubtful accounts represents our estimate of existing accounts receivable that will ultimately be uncollectible. The allowance is calculated by applying estimated loss factors to various classes of outstanding receivables, including unbilled revenue. The loss factors used to estimate uncollectible accounts are based upon both historical collections experience and management’s estimate of future collections success given the existing and anticipated future collections environment. Ameren Illinois has a rate mechanism that adjusts rates for net write-offs of customer accounts receivable above or below those being collected in rates.
Materials and Supplies	Materials and supplies are recorded at the lower of cost or market. Cost is determined by the average-cost method. Materials and supplies are capitalized as inventory when purchased and then expensed or capitalized as plant assets when installed, as appropriate.
Property and Plant	Property and Plant, Net
	We capitalize the cost of additions to and betterments of units of property and plant. The cost includes labor, material, applicable taxes, and overhead. An allowance for funds used during construction, as discussed below, is also capitalized as a cost of our rate-regulated assets. Maintenance expenditures, including nuclear refueling and maintenance outages, are expensed as incurred. When units of depreciable property are retired, the original costs, less salvage values, are charged to accumulated depreciation. If environmental expenditures are related to assets currently in use, as in the case of the installation of pollution control equipment, the cost is capitalized and depreciated over the expected life of the asset. See Asset Retirement Obligations below and Note 3 – Property and Plant, Net, for additional information.
	Depreciation
	Depreciation is provided over the estimated lives of the various classes of depreciable property by applying composite rates on a straight-line basis to the cost basis of such property. The provision for depreciation for the Ameren Companies in 2014, 2013, and 2012 ranged from 3% to 4% of the average depreciable cost.
Allowance for Funds Used During Construction	Allowance for Funds Used During Construction
	We capitalize allowance for funds used during construction, or the cost of borrowed funds and the cost of equity funds (preferred and common stockholders’ equity) applicable to rate-regulated construction expenditures, in accordance with the utility industry's accounting practice. Allowance for funds used during construction does not represent a current source of cash funds. This accounting practice offsets the effect on earnings of the cost of financing during construction, and it treats such financing costs in the same manner as construction charges for labor and materials.
	Under accepted ratemaking practice, cash recovery of allowance for funds used during construction and other construction costs occurs when completed projects are placed in service and reflected in customer rates.
Goodwill	Goodwill represents the excess of the purchase price of an acquisition over the fair value of the net assets acquired. Ameren and Ameren Illinois had goodwill of $411 million at December 31, 2014, and 2013.
	All of Ameren's and Ameren Illinois' goodwill at December 31, 2014 and 2013, was assigned to the Ameren Illinois reporting unit, which is also the Ameren Illinois reportable segment.
	We evaluate goodwill for impairment as of October 31 of each year, or more frequently if events and circumstances indicate that the asset might be impaired. Ameren and Ameren Illinois applied a qualitative goodwill evaluation model for their annual goodwill impairment test conducted as of October 31, 2014. Based on the results of Ameren’s and Ameren Illinois’ qualitative assessment, Ameren and Ameren Illinois believe it was more likely than not that the fair value of the Ameren Illinois reporting unit exceeded its carrying value as of October 31, 2014, indicating no impairment of Ameren’s or Ameren Illinois’ goodwill. The following factors, among others, were considered by Ameren and Ameren Illinois when assessing whether it was more likely than not that the fair value of the Ameren Illinois reporting unit exceeded its carrying value for the October 31, 2014, test:
	•	macroeconomic conditions, including those conditions within Ameren Illinois’ service territory;
	•	pending rate case outcomes and projections of future rate case outcomes;
	•	changes in laws and potential law changes;
	•	observable industry market multiples;
	•	achievement of IEIMA performance metrics and the yield of 30-year United States Treasury bonds;
	•	a potential reduction in the FERC-allowed return on equity related to transmission services; and
	•	actual and forecasted financial performance.
	The goodwill assigned to the Ameren Illinois reporting unit on the December 31, 2014 balance sheets of Ameren and Ameren Illinois had no accumulated goodwill impairment losses. Ameren and Ameren Illinois will continue to monitor the actual and forecasted operating results, cash flows, market capitalization, and observable industry market multiples of the Ameren Illinois reporting unit for signs of possible declines in estimated fair value and potential goodwill impairment.
Impairment of Long-lived Assets	We evaluate long-lived assets classified as held and used for impairment when events or changes in circumstances indicate that the carrying value of such assets may not be recoverable. Whether an impairment has occurred is determined by comparing the estimated undiscounted cash flows attributable to the assets to the carrying value of the assets. If the carrying value exceeds the undiscounted cash flows, we recognize an impairment charge equal to the amount by which the carrying value exceeds the estimated fair value of the assets. In the period in which we determine an asset meets held for sale criteria, we record an impairment charge to the extent the book value exceeds its estimated fair value less cost to sell.
Asset Retirement Obligations	Asset Retirement Obligations
	We are required to record the estimated fair value of legal obligations associated with the retirement of tangible long-lived assets in the period in which the liabilities are incurred and to capitalize a corresponding amount as part of the book value of the related long-lived asset. In subsequent periods, we are required to make adjustments to AROs based on changes in the estimated fair values of the obligations. Corresponding increases in asset book values are depreciated over the remaining useful life of the related asset. Uncertainties as to the probability, timing, or amount of cash expenditures associated with AROs affect our estimates of fair value. Ameren and Ameren Missouri have recorded AROs for retirement costs associated with Ameren Missouri’s Callaway energy center decommissioning costs, asbestos removal, CCR facilities, and river structures. Also, Ameren and Ameren Illinois have recorded AROs for retirement costs associated with asbestos removal. In addition, Ameren, Ameren Missouri, and Ameren Illinois have recorded AROs for the disposal of certain transformers. Ameren and Ameren Missouri are evaluating the potential effect of the EPA's new rule regarding the management and disposal of CCR on their AROs associated with ash ponds. See Note 15 – Commitments and Contingencies.
	Asset removal costs accrued by our rate-regulated operations that do not constitute legal obligations are classified as regulatory liabilities.
Noncontrolling Interest	Noncontrolling Interests
	As of December 31, 2014 and 2013, Ameren’s noncontrolling interests included the preferred stock of Ameren Missouri and Ameren Illinois.
Revenue	Operating Revenue
	The Ameren Companies record operating revenue for electric or natural gas service when it is delivered to customers. We accrue an estimate of electric and natural gas revenues for service rendered but unbilled at the end of each accounting period.
	Ameren Illinois participates in the performance-based formula ratemaking framework pursuant to the IEIMA. The IEIMA provides for an annual reconciliation of Ameren Illinois' electric delivery service revenue requirement. As of each balance sheet date, Ameren Illinois records its estimate of the electric delivery service revenue effect resulting from the reconciliation of the revenue requirement necessary to reflect the actual recoverable costs incurred for that year with the revenue requirement that was reflected in customer rates for that year. If the current year's revenue requirement is greater than the revenue requirement reflected in that year's customer rates, an increase to electric operating revenues with an offset to a regulatory asset is recorded to reflect the expected recovery of those additional costs from customers within the next two years. If the current year's revenue requirement is less than the revenue requirement reflected in that year's customer rates, a reduction to electric operating revenues with an offset to a regulatory liability is recorded to reflect the expected refund to customers within the next two years. See Note 2 – Rate and Regulatory Matters for information regarding Ameren Illinois' revenue requirement reconciliation pursuant to the IEIMA.
	Similar to the IEIMA process described above, Ameren Illinois and ATXI record the impact of a revenue requirement reconciliation for each company's electric transmission jurisdiction, pursuant to FERC-approved rate treatment.
Cost Of Sales	Accounting for MISO Transactions
	MISO-related purchase and sale transactions are recorded by Ameren, Ameren Missouri, and Ameren Illinois using settlement information provided by MISO. Ameren Missouri records these purchase and sale transactions on a net hourly position. Ameren Missouri records net purchases in a single hour in “Operating Expenses – Purchased power” and net sales in a single hour in “Operating Revenues – Electric” in its statement of income. Ameren Illinois records net purchases in “Operating Expenses – Purchased power” in its statement of income to reflect all of its MISO transactions relating to the procurement of power for its customers. On occasion, Ameren Missouri's and Ameren Illinois' prior-period transactions will be resettled outside the routine settlement process because of a change in MISO’s tariff or a material interpretation thereof. In these cases, Ameren Missouri and Ameren Illinois recognize expenses associated with resettlements once the resettlement is probable and the resettlement amount can be estimated and recognize revenues once the resettlement amount is received.
	Nuclear Fuel
	Ameren Missouri’s cost of nuclear fuel is capitalized and then amortized to fuel expense on a unit-of-production basis. The cost is charged to "Operating Expenses – Fuel" in the statement of income.
Stock-Based Compensation	Stock-based Compensation
	Stock-based compensation cost is measured at the grant date based on the fair value of the award, net of an assumed forfeiture rate. Ameren recognizes as compensation expense the estimated fair value of stock-based compensation on a straight-line basis over the requisite service period.
Excise Taxes	Excise Taxes
	Ameren Missouri and Ameren Illinois collect certain excise taxes from customers that are levied on the sale or distribution of natural gas and electricity. Excise taxes are levied on Ameren Missouri's electric and natural gas businesses and on Ameren Illinois' natural gas business and are recorded gross in “Operating Revenues – Electric,” “Operating Revenues – Gas,” and “Operating Expenses – Taxes other than income taxes” on the statement of income or the statement of income and comprehensive income. Excise taxes for electric service in Illinois are levied on the customer and are therefore not included in Ameren Illinois' revenues and expenses. They are instead included in “Taxes accrued” on the balance sheet.
Unamortized Debt Discount, Premium, And Expense	Unamortized Debt Discounts, Premiums, and Issuance Costs
	Long-term debt discounts, premiums, and issuance costs are amortized over the lives of the related issuances. Credit facility fees are amortized over the credit facility term.
Income Taxes	Income Taxes
	Ameren uses an asset and liability approach for its financial accounting and reporting of income taxes, in accordance with authoritative accounting guidance. Deferred tax assets and liabilities are recognized for transactions that are treated differently for financial reporting and income tax return purposes. These deferred tax assets and liabilities are based on statutory tax rates.
	We recognize that regulators will probably reduce future revenues for deferred tax liabilities that were initially recorded at rates in excess of the current statutory rate. Therefore, reductions in deferred tax liabilities that were recorded because of decreases in the statutory rate have been credited to a regulatory liability. A regulatory asset has been established to recognize the probable recovery through future customer rates of tax benefits related to the equity component of allowance for funds used during construction, as well as the effects of tax rate changes.
	Investment tax credits used on tax returns for prior years have been deferred as a non-current liability. The credits are being amortized over the useful lives of the related investment. Deferred income taxes were recorded on the temporary difference represented by the deferred investment tax credits and a corresponding regulatory liability. This recognizes the expected reduction in rates for future lower income taxes associated with the amortization of the investment tax credits. See Note 13 – Income Taxes.
	Ameren Missouri, Ameren Illinois, and all the other Ameren subsidiary companies are parties to a tax allocation agreement with Ameren (parent) that provides for the allocation of consolidated tax liabilities. The tax allocation agreement specifies that each party be allocated an amount of tax similar to that which would be owed or refunded had the party been separately subject to tax. Any net benefit attributable to the parent is reallocated to the other parties. This reallocation is treated as a capital contribution to the party receiving the benefit.
Earnings Per Share	Earnings per Share
	Basic earnings per share is computed by dividing net income attributable to Ameren Corporation common stockholders by the weighted-average number of common shares outstanding during the period. Diluted earnings per share is computed by dividing net income attributable to common stockholders by the diluted weighted-average number of common shares outstanding during the period. Diluted earnings per share reflects the potential dilution that would occur if certain stock-based performance share units were settled.
Accounting Changes and Other Matters	Accounting Changes and Other Matters
	The following is a summary of recently adopted authoritative accounting guidance, as well as guidance issued but not yet adopted, that could affect the Ameren Companies.
	Presentation of an Unrecognized Tax Benefit
	In 2013, FASB issued additional authoritative accounting guidance, which became effective in 2014, to provide clarity for the financial statement presentation of an unrecognized tax benefit when a net operating loss carryforward, a similar tax loss, or a tax credit carryforward exists. The objective of this guidance is to eliminate diversity in practice related to the presentation of certain unrecognized tax benefits. It requires entities to present an unrecognized tax benefit as a reduction to a deferred tax asset for a net operating loss carryforward, a similar tax loss, or a tax credit carryforward. Previously, unrecognized tax benefits were recorded in “Other deferred credits and liabilities” on Ameren's, Ameren Missouri's, and Ameren Illinois' respective balance sheets. Beginning in 2014, unrecognized tax benefits are recorded as a reduction to the deferred tax assets for net operating losses and tax credit carryforwards within "Accumulated deferred income taxes, net" on our balance sheets. Unrecognized tax benefits that exceed these carryforwards are recorded in “Other deferred credits and liabilities” on the respective balance sheets. At December 31, 2014, unrecognized tax benefits of $52 million, $- million, and $- million were recorded in "Accumulated deferred income taxes, net" on Ameren's, Ameren Missouri's, and Ameren Illinois' balance sheets, respectively. At December 31, 2013, unrecognized tax benefits of $84 million, $15 million, and $- million previously recorded in "Other deferred credits and liabilities" on Ameren's, Ameren Missouri's, and Ameren Illinois' respective balance sheets were reclassified to "Accumulated deferred income taxes, net" for comparative purposes. The implementation of the additional authoritative accounting guidance did not affect the Ameren Companies' results of operations or liquidity, as this guidance is presentation-related only.
	Reporting Discontinued Operations and Disclosures of Components of an Entity
	In 2014, FASB issued authoritative accounting guidance that changes the criteria for reporting and qualifying for discontinued operations. Under the new guidance, a component of an entity, or a group of components of an entity, that either meets the criteria to be classified as held for sale or is disposed of by sale or otherwise is required to be reported in discontinued operations if the disposal represents a strategic shift that had, or will have, a major effect on an entity's operations and financial results. The guidance includes expanded disclosure requirements for discontinued operations and additional disclosures about a disposal of an individually significant component of an entity that does not qualify for discontinued operations presentation. The guidance is effective for the Ameren Companies in the first quarter of 2015 for components that are classified as held for sale or disposed of on or after January 1, 2015. Early adoption is permitted, but only for disposals or classifications as held for sale that have not been reported in financial statements previously issued. Therefore, Ameren's existing discontinued operations are not subject to the new disclosure requirements. The guidance will not affect the Ameren Companies' results of operations, financial position, or liquidity, as this guidance is presentation-related only.
	Revenue from Contracts with Customers
	In 2014, FASB issued authoritative accounting guidance to clarify the principles for recognizing revenue and to develop a common revenue standard for GAAP. The guidance requires an entity to recognize an amount of revenue for the transfer of promised goods or services to customers that reflects the consideration which the entity expects to be entitled to in exchange for those goods or services. The guidance also requires additional disclosures to enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. The guidance will be effective for the Ameren Companies in the first quarter of 2017. The guidance allows entities to choose one of two transition methods, either by applying the guidance retrospectively to each reporting period presented or by recording a cumulative effect adjustment to retained earnings in the period of initial adoption. The Ameren Companies are currently assessing the impacts of this guidance on their results of operations, financial position, and liquidity, as well as the transition method that they will use to adopt the guidance.

Summary_Of_Significant_Account2

Summary Of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2014
Accounting Policies [Abstract]
Schedule Of Materials And Supplies	The following table presents a breakdown of materials and supplies for each of the Ameren Companies at December 31, 2014 and 2013:
			Ameren Missouri				Ameren Illinois				Ameren
	2014
	Fuel(a)		$	134			$	—			$	134
	Gas stored underground		16				111				127
	Other materials and supplies		197				66				263
			$	347			$	177			$	524
	2013
	Fuel(a)		$	144			$	—			$	144
	Gas stored underground		17				110				127
	Other materials and supplies		191				64				255
			$	352			$	174			$	526
	(a)	Consists of coal, oil, and propane.
Schedule Of Rates Used For Allowance For Funds Used During Construction	The following table presents the annual allowance for funds used during construction rates that were used during 2014, 2013, and 2012:
		2014			2013			2012
	Ameren Missouri	7	%		8	%		8	%
	Ameren Illinois	2	%		8	%		9	%
Schedule Of Asset Retirement Obligations	The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the years ended December 31, 2014 and 2013:
		Ameren				Ameren				Ameren
		Missouri				Illinois
	Balance at December 31, 2012	$	346			$	3			$	349
	Liabilities settled	(1		)		(a)				(1		)
	Accretion in 2013(b)	19				(a)				19
	Change in estimates(c)	2				(a)				2
	Balance at December 31, 2013	$	366			$	3		(d)	$	369
	Liabilities incurred	2				—				2
	Liabilities settled	(2		)		(a)				(2		)
	Accretion in 2014(b)	21				(a)				21
	Change in estimates(c)(e)	2				4				6
	Balance at December 31, 2014	$	389			$	7		(d)	$	396
	(a)	Less than $1 million.
	(b)	Accretion expense was recorded as an increase to regulatory assets at Ameren Missouri and Ameren Illinois.
	(c)	Ameren Missouri changed its fair value estimates for asbestos removal in 2013 and 2014 and for certain CCR facilities in 2013.
	(d)	Included in “Other deferred credits and liabilities” on the balance sheet.
	(e)	Ameren Illinois changed its fair value estimate for asbestos removal in 2014.
Schedule Of Excise Taxes	The following table presents excise taxes recorded in “Operating Revenues – Electric,” “Operating Revenues – Gas,” and “Operating Expenses – Taxes other than income taxes” for the years ended December 31, 2014, 2013, and 2012:
		2014				2013				2012
	Ameren Missouri	$	151			$	152			$	139
	Ameren Illinois	64				61				54
	Ameren	$	215			$	213			$	193
Schedule of Earnings Per Share, Basic and Diluted	The following table presents Ameren’s basic and diluted earnings per share calculations and reconciles the weighted-average number of common shares outstanding to the diluted weighted-average number of common shares outstanding for the years ended December 31, 2014, 2013, and 2012:
		2014				2013				2012
	Net income (loss) attributable to Ameren Corporation:
	Continuing operations	$	587			$	512			$	516
	Discontinued operations	(1		)		(223		)		(1,490		)
	Net income (loss) attributable to Ameren Corporation	$	586			$	289			$	(974	)
	Average common shares outstanding – basic	242.6				242.6				242.6
	Assumed settlement of performance share units	1.8				1.9				0.4
	Average common shares outstanding – diluted	244.4				244.5				243
	Earnings (loss) per common share – basic:
	Continuing operations	$	2.42			$	2.11			$	2.13
	Discontinued operations	—				(0.92		)		(6.14		)
	Earnings (loss) per common share – basic	$	2.42			$	1.19			$	(4.01	)
	Earnings (loss) per common share – diluted:
	Continuing operations	$	2.4			$	2.1			$	2.13
	Discontinued operations	—				(0.92		)		(6.14		)
	Earnings (loss) per common share – diluted	$	2.4			$	1.18			$	(4.01	)
Schedule of Cash Flow, Supplemental Disclosures [Table Text Block]	Supplemental Cash Flow Information
	The following table presents additional information regarding Ameren's consolidated statement of cash flows for the years ended December 31, 2014, 2013, and 2012:
		2014				2013				2012
	Cash paid (refunded) during the year:
	Interest
	Continuing operations(a)	$	333			$	362			$	384
	Discontinued operations(b)	—				31				49
		$	333			$	393			$	433
	Income taxes, net
	Continuing Operations	$	(41	)		$	116			$	10
	Discontinued Operations	14				(108		)		(9		)
		$	(27	)		$	8			$	1
	(a)	Net of $18 million, $20 million, and $17 million capitalized, respectively.
	(b)	Net of $- million, $17 million, and $13 million capitalized, respectively.

Rate_And_Regulatory_Matters_Ta

Rate And Regulatory Matters (Tables)	12 Months Ended
	Dec. 31, 2014
Public Utilities, General Disclosures [Abstract]
Schedule Of Regulatory Assets And Liabilities	The following table presents our regulatory assets and regulatory liabilities at December 31, 2014 and 2013:
			2014						2013
			Ameren		Ameren		Ameren			Ameren		Ameren		Ameren
			Missouri		Illinois					Missouri		Illinois
	Current regulatory assets:
	Under-recovered FAC(a)(b)		$	128	$	—	$	128		$	104	$	—	$	104
	Under-recovered Illinois electric power costs(c)		—		2		2			—		1		1
	Under-recovered PGA(c)		—		20		20			—		1		1
	MTM derivative losses(d)		32		42		74			14		36		50
	Energy efficiency riders(e)		3		—		3			—		—		—
	IEIMA revenue requirement reconciliation(a)(f)		—		65		65			—		—		—
	FERC revenue requirement reconciliation(a)(g)		—		—		3			—		—		—
	Total current regulatory assets		$	163	$	129	$	295		$	118	$	38	$	156
	Noncurrent regulatory assets:
	Pension and postretirement benefit costs(h)		$	148	$	275	$	423		$	44	$	140	$	184
	Income taxes(i)		253		3		256			230		7		237
	Asset retirement obligations(j)		—		5		5			—		5		5
	Callaway costs(a)(k)		36		—		36			40		—		40
	Unamortized loss on reacquired debt(a)(l)		72		80		152			77		74		151
	Contaminated facilities costs(m)		—		251		251			—		271		271
	MTM derivative losses(d)		14		144		158			8		118		126
	Storm costs(n)		—		3		3			5		3		8
	Demand-side costs before the MEEIA implementation(a)(o)		44		—		44			58		—		58
	Workers’ compensation claims(p)		7		7		14			6		6		12
	Credit facilities fees(q)		5		—		5			5		—		5
	Common stock issuance costs(r)		2		—		2			4		—		4
	Construction accounting for pollution control equipment(a)(s)		21		—		21			22		—		22
	Solar rebate program(a)(t)		88		—		88			27		—		27
	IEIMA revenue requirement reconciliation(a)(f)		—		101		101			—		65		65
	FERC revenue requirement reconciliation(a)(g)		—		8		12			—		—		5
	Other(u)		5		6		11			8		12		20
	Total noncurrent regulatory assets		$	695	$	883	$	1,582		$	534	$	701	$	1,240
	Current regulatory liabilities:
	Over-recovered FAC(b)		$	—	$	—	$	—		$	26	$	—	$	26
	Over-recovered Illinois electric power costs(c)		—		26		26			—		51		51
	Over-recovered PGA(c)		2		25		27			5		29		34
	MTM derivative gains(d)		16		1		17			26		1		27
	Wholesale distribution refund(v)		—		—		—			—		13		13
	IEIMA revenue requirement reconciliation(f)		—		—		—			—		65		65
	FERC revenue requirement reconciliation(g)		—		11		11			—		—		—
	Refund reserves for FERC orders and audit findings(w)		—		21		25			—		—		—
	Total current regulatory liabilities		$	18	$	84	$	106		$	57	$	159	$	216
	Noncurrent regulatory liabilities:
	Income taxes(x)		$	41	$	14	$	55		$	37	$	3	$	40
	Uncertain tax positions tracker(y)		7		—		7			1		—		1
	Removal costs(z)		886		643		1,529			828		610		1,438
	Asset retirement obligation(j)		182		—		182			146		—		146
	Bad debt riders(aa)		—		7		7			—		8		8
	Pension and postretirement benefit costs tracker(ab)		24		—		24			15		—		15
	Energy efficiency riders(e)		—		39		39			3		33		36
	FERC revenue requirement reconciliation(g)		—		—		—			—		10		10
	Other(ac)		7		—		7			11		—		11
	Total noncurrent regulatory liabilities		$	1,147	$	703	$	1,850		$	1,041	$	664	$	1,705
	(a)	These assets earn a return.
	(b)	Under-recovered or over-recovered fuel costs to be recovered through the FAC. Specific accumulation periods aggregate the under-recovered or over-recovered costs over four months, any related adjustments that occur over the following four months, and the recovery from customers that occurs over the next eight months.
	(c)	Costs under- or over-recovered from utility customers. Amounts will be recovered from, or refunded to, customers within one year of the deferral.
	(d)	Deferral of commodity-related derivative MTM losses or gains. See Note 7 – Derivative Financial Instruments for additional information.
	(e)	The Ameren Missouri balance relates to the MEEIA. Beginning in January 2014, a MEEIA rider allowed Ameren Missouri to collect from or refund to customers any annual difference in the actual amounts incurred and the amounts collected from customers for the MEEIA program costs and its lost revenues. Under the MEEIA rider, collections from or refunds to customers occur one year after the program costs and lost revenues are incurred. The Ameren Illinois balance relates to a regulatory tracking mechanism to recover its electric and natural gas costs associated with developing, implementing, and evaluating customer energy efficiency and demand response programs. Any under-recovery or over-recovery will be collected from or refunded to customers over the 12 months following the plan year.
	(f)	The difference between Ameren Illinois' annual revenue requirement calculated under the IEIMA's performance-based formula ratemaking framework and the revenue requirement included in customer rates for that year. Subject to ICC approval, these amounts will be collected from or refunded to customers within two years.
	(g)	Ameren Illinois' and ATXI's annual revenue requirement reconciliation adjustments calculated pursuant to the FERC's electric transmission formula ratemaking framework. The under-recovery or over-recovery will be recovered from or refunded to customers within two years.
	(h)	These costs are being amortized in proportion to the recognition of prior service costs (credits) and actuarial losses (gains) attributable to Ameren’s pension plan and postretirement benefit plans. See Note 11 – Retirement Benefits for additional information.
	(i)	Offset to certain deferred tax liabilities for expected recovery of future income taxes when paid. This will be recovered over the expected life of the related assets.
	(j)	Recoverable or refundable removal costs for AROs, including net realized and unrealized gains and losses related to the nuclear decommissioning trust fund investments. See Note 1 – Summary of Significant Accounting Policies – Asset Retirement Obligations.
	(k)	Ameren Missouri’s Callaway energy center operations and maintenance expenses, property taxes, and carrying costs incurred between the plant in-service date and the date the plant was reflected in rates. These costs are being amortized over the remaining life of the energy center's current operating license, which expires in 2024.
(l)	Losses related to reacquired debt. These amounts are being amortized over the lives of the related new debt issuances or the original lives of the old debt issuances if no new debt was issued.
(m)	The recoverable portion of accrued environmental site liabilities that will be collected from electric and natural gas customers through ICC-approved cost recovery riders. The period of recovery will depend on the timing of remediation expenditures. See Note 15 – Commitments and Contingencies for additional information.
(n)	Ameren Missouri's actual storm costs that exceed the normalized storm costs for rate purposes. As approved by the December 2012 MoPSC electric rate order, the 2006, 2007, and 2008 storm costs were amortized through December 2014. The Ameren Illinois balance includes 2013 storm costs deferred in accordance with the IEIMA. These costs are being amortized over a five-year period beginning in 2013.
(o)	Demand-side costs incurred prior to implementation of the MEEIA in 2013, including the costs of developing, implementing and evaluating customer energy efficiency and demand response programs. Costs incurred from May 2008 through September 2008 are being amortized over a 10-year period that began in March 2009. Costs incurred from October 2008 through December 2009 are being amortized over a six-year period that began in July 2010. Costs incurred from January 2010 through February 2011 are being amortized over a six-year period that began in August 2011. Costs incurred from March 2011 through July 2012 are being amortized over a six-year period that began in January 2013.The amortization period for costs incurred from August 2012 through December 2012 will be determined in the July 2014 electric rate case.
(p)	The period of recovery will depend on the timing of actual expenditures.
(q)	Ameren Missouri’s costs incurred to enter into and maintain the 2012 Missouri Credit Agreement. Additional costs were incurred in December 2014 to amend and restate the 2012 Missouri Credit Agreement. These costs are being amortized over the life of the credit facility, ending in December 2019, to construction work in progress, which will be depreciated when assets are placed into service.
(r)	The MoPSC’s May 2010 electric rate order allowed Ameren Missouri to recover its portion of Ameren’s September 2009 common stock issuance costs. These costs are being amortized over five years, beginning in July 2010.
(s)	The MoPSC’s May 2010 electric rate order allowed Ameren Missouri to record an allowance for funds used during construction for pollution control equipment at its Sioux energy center until the cost of that equipment was included in customer rates. These costs will be amortized over the expected life of the Sioux energy center, which is currently through 2033.
(t)	Costs associated with Ameren Missouri's solar rebate program beginning in August 2012 to fulfill its renewable energy portfolio requirement. The amortization period for these costs will be three years, commencing with the effectiveness of Ameren Missouri's current July 2014 electric rate case.
(u)	The Ameren Illinois balance includes Ameren Illinois Merger integration and optimization costs, which are being amortized over four years, beginning in January 2012. The Ameren Illinois total also includes costs related to the 2013 natural gas delivery service rate case costs, which are being amortized over a two-year period that began in January 2014. At Ameren Missouri, the balance primarily includes the cost of renewable energy credits to fulfill its renewable energy portfolio requirement. Costs incurred from January 2010 through July 2012 are being amortized over three years, beginning in January 2013.
(v)	Estimated refund to wholesale electric customers as of December 31, 2013. See 2011 Wholesale Distribution Rate Case above.
(w)	Estimated refunds to transmission customers related to FERC orders and audit findings. In regards to the FERC orders, see Ameren Illinois Electric Transmission Rate Refund and FERC Complaint Cases above.
(x)	Unamortized portion of investment tax credits and federal excess deferred taxes. The unamortized portion of investment tax credits and the federal excess deferred taxes are being amortized over the expected life of the underlying assets.
(y)	The tracker is amortized over three years, beginning from the date the amounts are included in rates. See Note 13 - Income Taxes for additional information.
(z)	Estimated funds collected for the eventual dismantling and removal of plant from service, net of salvage value, upon retirement related to our rate-regulated operations.
(aa)	A regulatory tracking mechanism for the difference between the level of bad debt incurred by Ameren Illinois under GAAP and the level of such costs included in electric and natural gas rates. The over-recovery relating to 2012 was refunded to customers from June 2013 through May 2014. The over-recovery relating to 2013 is being refunded to customers from June 2014 through May 2015. The over-recovery relating to 2014 will be refunded to customers from June 2015 through May 2016.
(ab)	A regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs built into rates. For periods prior to August 2012, the MoPSC's December 2012 electric rate order directed the amortization to occur over five years, beginning in January 2013. For periods after August 2012, the amortization period will be determined in the July 2014 electric rate case.
(ac)	Balance includes the costs of renewable energy credits to fulfill Ameren Missouri's renewable energy portfolio requirement from August 2012 through December 2013, which were less than the amount included in rates. The balance also includes a regulatory tracking mechanism at Ameren Missouri for the difference between the level of storm costs incurred in a particular year and the level of such costs built into rates. The amortization periods for these over-recoveries will be determined in the July 2014 electric rate case.

Property_And_Plant_Net_Tables

Property And Plant, Net (Tables)	12 Months Ended
	Dec. 31, 2014
Property, Plant and Equipment [Abstract]
Schedule Of Property And Plant, Net	The following table presents property and plant, net, for each of the Ameren Companies at December 31, 2014 and 2013:
			Ameren			Ameren			Other		Ameren(a)
			Missouri(a)			Illinois
	2014
	Property and plant, at original cost:
	Electric		$	17,052		$	6,517		$	344	$	23,913
	Natural gas		431			1,854			—		2,285
			17,483			8,371			344		26,198
	Less: Accumulated depreciation and amortization		7,086			2,422			251		9,759
			10,397			5,949			93		16,439
	Construction work in progress:
	Nuclear fuel in process		209			—			—		209
	Other		261			216			299		776
	Property and plant, net		$	10,867		$	6,165		$	392	$	17,424
	2013
	Property and plant, at original cost:
	Electric		$	15,964		$	5,426		$	336	$	21,726
	Natural gas		413			1,562			—		1,975
			16,377			6,988			336		23,701
	Less: Accumulated depreciation and amortization		6,766			1,627			251		8,644
			9,611			5,361			85		15,057
	Construction work in progress:
	Nuclear fuel in process		246			—			—		246
	Other		595			228			79		902
	Property and plant, net		$	10,452		$	5,589		$	164	$	16,205
	(a)	Amounts in Ameren and Ameren Missouri include two CTs under separate capital lease agreements. The gross cumulative asset value of those agreements was $233 million and $228 million at December 31, 2014 and 2013, respectively. The total accumulated depreciation associated with the two CTs was $66 million and $56 million at December 31, 2014 and 2013, respectively. In addition, Ameren Missouri has investments in debt securities, which were classified as held-to-maturity, related to the two CTs from the city of Bowling Green and Audrain County. As of December 31, 2014 and 2013, the carrying value of these debt securities was $294 million and $299 million, respectively.
Accrued Capital Expenditures	The following table provides accrued capital and nuclear fuel expenditures at December 31, 2014, 2013, and 2012, which represent noncash investing activity excluded from the accompanying statements of cash flows:
		Ameren(a)			Ameren			Ameren
					Missouri			Illinois
	Accrued capital expenditures:
	2014	$	181		$	72		$	59
	2013	175			74			86
	2012	107			63			37
	Accrued nuclear fuel expenditures:
	2014	13			13			(b)
	2013	8			8			(b)
	2012	8			8			(b)
	(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
	(b)	Not applicable.

ShortTerm_Debt_And_Liquidity_T

Short-Term Debt And Liquidity (Tables)	12 Months Ended
	Dec. 31, 2014
Line of Credit Facility [Abstract]
Schedule Of Maximum Aggregate Amount Available On Credit Agreements	The maximum aggregate amount available to each borrower under each facility is shown in the following table (the amount being each borrower's "Borrowing Sublimit"):
			2012 Missouri Credit Agreement			2012 Illinois
						Credit Agreement
	Ameren		$	700		$	500
	Ameren Missouri		800			(a)
	Ameren Illinois		(a)			800
	(a)	Not applicable.
Schedule of Commercial Paper	The following table summarizes the borrowing activity and relevant interest rates under Ameren Missouri's and Ameren Illinois' commercial paper program, for the years ended December 31, 2014 and 2013:
			Ameren (parent)				Ameren Missouri			Ameren Illinois			Ameren Consolidated
	2014
	Average daily commercial paper outstanding		$	423			$	110		$	165		$	639
	Outstanding borrowings at period-end		585				97			32			714
	Weighted-average interest rate		0.36		%		0.38		%	0.32		%	0.36		%
	Peak commercial paper during period(a)		$	625			$	495		$	300		$	910
	Peak interest rate		0.75		%		0.7		%	0.6		%	0.75		%
	2013
	Average daily commercial paper outstanding		$	54			$	—		$	—		$	54
	Outstanding borrowings at period-end		368				—			—			368
	Weighted-average interest rate		0.56		%		—		%	—		%	0.56		%
	Peak commercial paper during period(a)		$	368			$	—		$	—		$	368
	Peak interest rate		0.85		%		—		%	—		%	0.85		%

LongTerm_Debt_And_Equity_Finan1

Long-Term Debt And Equity Financings (Tables)	12 Months Ended
	Dec. 31, 2014
Debt Instrument [Line Items]
Schedule of Long-term Debt Instruments	The following table presents long-term debt outstanding, including maturities due within one year, for the Ameren Companies as of December 31, 2014 and 2013:
		2014				2013
	Ameren (Parent):
	8.875% Senior unsecured notes due 2014	$	—			$	425
	Less: Maturities due within one year	—				(425		)
	Long-term debt, net	$	—			$	—
	Ameren Missouri:
	Senior secured notes:(a)
	5.50% Senior secured notes due 2014	—				104
	4.75% Senior secured notes due 2015	114				114
	5.40% Senior secured notes due 2016	260				260
	6.40% Senior secured notes due 2017	425				425
	6.00% Senior secured notes due 2018(b)	179				179
	5.10% Senior secured notes due 2018	199				199
	6.70% Senior secured notes due 2019(b)	329				329
	5.10% Senior secured notes due 2019	244				244
	5.00% Senior secured notes due 2020	85				85
	3.50% Senior secured notes due 2024	350				—
	5.50% Senior secured notes due 2034	184				184
	5.30% Senior secured notes due 2037	300				300
	8.45% Senior secured notes due 2039(b)	350				350
	3.90% Senior secured notes due 2042(b)	485				485
	Environmental improvement and pollution control revenue bonds:
	1992 Series due 2022(c)(d)	47				47
	1993 5.45% Series due 2028(e)	(e)				(e)
	1998 Series A due 2033(c)(d)	60				60
	1998 Series B due 2033(c)(d)	50				50
	1998 Series C due 2033(c)(d)	50				50
	Capital lease obligations:
	City of Bowling Green capital lease (Peno Creek CT) due 2022	54				59
	Audrain County capital lease (Audrain County CT) due 2023	240				240
	Total long-term debt, gross	4,005				3,764
	Less: Unamortized discount and premium	(6		)		(7		)
	Less: Maturities due within one year	(120		)		(109		)
	Long-term debt, net	$	3,879			$	3,648
		2014				2013
	Ameren Illinois:
	Senior secured notes:
	6.20% Senior secured notes due 2016(f)	$	54			$	54
	6.25% Senior secured notes due 2016(g)	75				75
	6.125% Senior secured notes due 2017(g)(h)	250				250
	6.25% Senior secured notes due 2018(g)(h)	144				144
	9.75% Senior secured notes due 2018(g)(h)	313				313
	2.70% Senior secured notes due 2022(g)(h)	400				400
	3.25% Senior secured notes due 2025(g)	300				—
	6.125% Senior secured notes due 2028(g)	60				60
	6.70% Senior secured notes due 2036(g)	61				61
	6.70% Senior secured notes due 2036(f)	42				42
	4.80% Senior secured notes due 2043(g)	280				280
	4.30% Senior secured notes due 2044(g)	250				—
	Environmental improvement and pollution control revenue bonds:
	5.90% Series 1993 due 2023(i)	(i)				32
	5.70% 1994A Series due 2024(j)	(j)				36
	5.95% 1993 Series C-1 due 2026(k)	—				35
	5.70% 1993 Series C-2 due 2026(k)	—				8
	1993 Series B-1 due 2028(d)(k)	17				17
	5.40% 1998A Series due 2028(j)	—				19
	5.40% 1998B Series due 2028(j)	—				33
	Fair-market value adjustments	—				4
	Total long-term debt, gross	2,246				1,863
	Less: Unamortized discount and premium	(5		)		(7		)
	Less: Maturities due within one year	—				—
	Long-term debt, net	$	2,241			$	1,856
	Ameren consolidated long-term debt, net	$	6,120			$	5,504
(a)	These notes are collaterally secured by first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage indenture. The notes have a fall-away lien provision and will remain secured only as long as any first mortgage bonds issued under the Ameren Missouri mortgage indenture remain outstanding. Redemption, purchase, or maturity of all first mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the first mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the Ameren Missouri senior secured notes currently outstanding, we do not expect the first mortgage bond lien protection associated with these notes to fall away until 2042.
(b)	Ameren Missouri has agreed, during the life of these notes, not to optionally redeem, purchase or otherwise retire in full its first mortgage bonds. Ameren Missouri has also agreed to prevent a first mortgage bond release date from occurring as long as any of the 8.45% senior secured notes due 2039 and any of the 3.90% senior secured notes due 2042 remain outstanding.
(c)	These bonds are collaterally secured by first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage indenture and have a fall-away lien provision similar to that of Ameren Missouri's senior secured notes. The bonds are also backed by an insurance guarantee policy.
(d)	The interest rates, and the periods during which such rates apply, vary depending on our selection of defined rate modes. Maximum interest rates could reach 18% depending on the series of bonds. The average interest rates for 2014 and 2013 were as follows:
	2014		2013
Ameren Missouri 1992 Series due 2022	0.10%		0.17%
Ameren Missouri 1998 Series A due 2033	0.26%		0.34%
Ameren Missouri 1998 Series B due 2033	0.27%		0.33%
Ameren Missouri 1998 Series C due 2033	0.26%		0.34%
Ameren Illinois 1993 Series B-1 due 2028	0.21%		0.14%
(e)	These bonds are first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage bond indenture and are secured by substantially all Ameren Missouri property and franchises. The bonds are callable at 100% of par value. Less than $1 million principal amount of the bonds remain outstanding.
(f)	These notes are collaterally secured by first mortgage bonds issued by Ameren Illinois under the CILCO mortgage indenture. The notes have a fall-away lien provision, and Ameren Illinois could cause these notes to become unsecured at any time by redeeming the pollution control bonds 5.90% Series 1993 due 2023 (of which less than $1 million remains outstanding). Ameren Illinois may resecure these notes if it chooses.
(g)	These notes are collaterally secured by mortgage bonds issued by Ameren Illinois under the Ameren Illinois mortgage indenture. The notes have a fall-away lien provision and will remain secured only as long as any series of first mortgage bonds issued under the Ameren Illinois mortgage indenture remain outstanding. Redemption, purchase, or maturity of all mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the Ameren Illinois senior secured notes currently outstanding, we do not expect the mortgage bond lien protection associated with these notes to fall away until 2024.
(h)	Ameren Illinois has agreed, during the life of these notes, not to optionally redeem, purchase, or otherwise retire in full its Ameren Illinois mortgage bonds; therefore, an Ameren Illinois first mortgage bond release date will not occur as long as any of these notes are outstanding.
(i)	These bonds are first mortgage bonds issued by Ameren Illinois under the CILCO mortgage indenture and are secured by substantially all property of the former CILCO. The bonds are callable at 100% of par value. Less than $1 million principal amount of the bonds remain outstanding.
(j)	These bonds are mortgage bonds issued by Ameren Illinois under the Ameren Illinois mortgage indenture and are secured by substantially all property of the former IP and CIPS. The bonds are callable at 100% of par value. The bonds are also backed by an insurance guarantee policy. Less than $1 million principal amount of the bonds remain outstanding.
(k)	The bonds are callable at 100% of par value.
Schedule Of Maturities Of Long-Term Debt	The following table presents the aggregate maturities of long-term debt, including current maturities, for the Ameren Companies at December 31, 2014:
			Ameren				Ameren				Ameren
			Missouri(a)				Illinois(a)				Consolidated
	2015		$	120			$	—			$	120
	2016		266				129				395
	2017		431				250				681
	2018		383				457				840
	2019		581				—				581
	Thereafter		2,224				1,410				3,634
	Total		$	4,005			$	2,246			$	6,251
	(a)	Excludes unamortized discount and premium of $6 million and $5 million at Ameren Missouri and Ameren Illinois, respectively.
Schedule Of Outstanding Preferred Stock
				Redemption Price(per share)				2014				2013
	Ameren Missouri:
	Without par value and stated value of $100 per share, 25 million shares authorized
	$3.50 Series	130,000 shares		$	110			$	13			$	13
	$3.70 Series	40,000 shares		104.75				4				4
	$4.00 Series	150,000 shares		105.625				15				15
	$4.30 Series	40,000 shares		105				4				4
	$4.50 Series	213,595 shares		110			(a)	21				21
	$4.56 Series	200,000 shares		102.47				20				20
	$4.75 Series	20,000 shares		102.176				2				2
	$5.50 Series A	14,000 shares		110				1				1
	Total							$	80			$	80
	Ameren Illinois:
	With par value of $100 per share, 2 million shares authorized
	4.00% Series	144,275 shares		$	101			$	14			$	14
	4.08% Series	45,224 shares		103				5				5
	4.20% Series	23,655 shares		104				2				2
	4.25% Series	50,000 shares		102				5				5
	4.26% Series	16,621 shares		103				2				2
	4.42% Series	16,190 shares		103				2				2
	4.70% Series	18,429 shares		103				2				2
	4.90% Series	73,825 shares		102				7				7
	4.92% Series	49,289 shares		103.5				5				5
	5.16% Series	50,000 shares		102				5				5
	6.625% Series	124,274 shares		100				12				12
	7.75% Series	4,542 shares		100				1				1
	Total							$	62			$	62
	Total Ameren							$	142			$	142
	(a)	In the event of voluntary liquidation, $105.50.
Schedule of Required and Actual Debt Ratios	The following table summarizes the required and actual interest coverage ratios for interest charges, dividend coverage ratios, and bonds and preferred stock issuable as of December 31, 2014, at an assumed interest rate of 5% and dividend rate of 6%.
		Required Interest	Actual Interest		Bonds Issuable(b)				Required Dividend	Actual Dividend		Preferred Stock
		Coverage Ratio(a)	Coverage Ratio						Coverage Ratio(c)	Coverage Ratio		Issuable
	Ameren Missouri	>2.0	4.3		$	3,605			>2.5	115.1		$	2,568
	Ameren Illinois	>2.0	6.4		3,358			(d)	>1.5	2.7		208
	(a)	Coverage required on the annual interest charges on first mortgage bonds outstanding and to be issued. Coverage is not required in certain cases when additional first mortgage bonds are issued on the basis of retired bonds.
	(b)	Amount of bonds issuable based either on required coverage ratios or unfunded property additions, whichever is more restrictive. The amounts shown also include bonds issuable based on retired bond capacity of $832 million and $204 million at Ameren Missouri and Ameren Illinois, respectively.
	(c)	Coverage required on the annual dividend on preferred stock outstanding and to be issued, as required in the respective company’s articles of incorporation.
	(d)	Amount of bonds issuable by Ameren Illinois based on unfunded property additions and retired bonds solely under the former IP mortgage indenture.
Ameren Illinois Company
Debt Instrument [Line Items]
Debt Redemption [Table Text Block]	In January 2014, Ameren Illinois redeemed the following environmental improvement and pollution control revenue bonds at par value plus accrued interest:
	Senior Secured Notes	Principal Amount
	5.90% Series 1993 due 2023(a)	$	32
	5.70% 1994A Series due 2024(a)	36
	1993 Series C-1 5.95% due 2026	35
	1993 Series C-2 5.70% due 2026	8
	5.40% 1998A Series due 2028	19
	5.40% 1998B Series due 2028	33
	Total amount redeemed	$	163
	(a)	Less than $1 million principal amount of the bonds remain outstanding

Other_Income_And_Expenses_Tabl

Other Income And Expenses (Tables)	12 Months Ended
	Dec. 31, 2014
Other Nonoperating Income (Expense) [Abstract]
Other Income And Expenses	The following table presents the components of "Other Income and Expenses" in the Ameren Companies’ statements of income (loss) for the years ended December 31, 2014, 2013, and 2012:
		2014			2013		2012
	Ameren:(a)
	Miscellaneous income:
	Allowance for equity funds used during construction	$	34		$	37	$	36
	Interest income on industrial development revenue bonds	27			27		28
	Interest income	10		(b)	3		4		(d)
	Other	8		(c)	2		2
	Total miscellaneous income	$	79		$	69	$	70
	Miscellaneous expense:
	Donations	$	10		$	12	$	24	(e)
	Other	12			14		13
	Total miscellaneous expense	$	22		$	26	$	37
	Ameren Missouri:
	Miscellaneous income:
	Allowance for equity funds used during construction	$	32		$	31	$	31
	Interest income on industrial development revenue bonds	27			27		28
	Interest income	1			—		4		(d)
	Total miscellaneous income	$	60		$	58	$	63
	Miscellaneous expense:
	Donations	$	6		$	4	$	9
	Other	6			7		5
	Total miscellaneous expense	$	12		$	11	$	14
	Ameren Illinois:
	Miscellaneous income:
	Allowance for equity funds used during construction	$	2		$	6	$	5
	Interest income	7		(b)	2		—
	Other	8		(c)	2		2
	Total miscellaneous income	$	17		$	10	$	7
	Miscellaneous expense:
	Donations	$	4		$	4	$	11	(e)
	Other	4			5		6
	Total miscellaneous expense	$	8		$	9	$	17
	(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.
	(b)	Includes Ameren Illinois' interest income received in 2014 relating to the 2013 and 2014 IEIMA revenue requirement reconciliation regulatory assets.
	(c)	Includes Ameren Illinois' income earned in 2014 from customer-requested construction.
	(d)	Includes Ameren Missouri's interest income relating to a refund of charges included in an expired power purchase agreement with Entergy. See Note 2 – Rate and Regulatory Matters for additional information.
	(e)	Includes Ameren Illinois' one-time $7.5 million contribution to the Illinois Science and Energy Innovation Trust pursuant to the IEIMA as a result of Ameren Illinois' participation in the electric delivery formula ratemaking process.

Derivative_Financial_Instrumen1

Derivative Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2014
Derivative Instrument Detail [Abstract]
Open Gross Derivative Volumes By Commodity Type	The following table presents open gross commodity contract volumes by commodity type for derivative assets and liabilities as of December 31, 2014 and 2013. As of December 31, 2014, these contracts ran through October 2017, October 2019, May 2032, and October 2016 for fuel oils, natural gas, power, and uranium, respectively.
		Quantity (in millions, except as indicated)
		2014			2013
	Commodity	Ameren Missouri	Ameren Illinois	Ameren	Ameren Missouri	Ameren Illinois	Ameren
	Fuel oils (in gallons)(a)	50	(b)	50	66	(b)	66
	Natural gas (in mmbtu)	28	108	136	28	108	136
	Power (in megawatthours)	1	11	12	3	11	14
	Uranium (pounds in thousands)	332	(b)	332	796	(b)	796
	(a)	Fuel oils consist of heating oil, ultra-low-sulfur diesel, and crude oil.
	(b)	Not applicable.
Derivative Instruments Carrying Value	The following table presents the carrying value and balance sheet location of all derivative commodity contracts, none of which were designated as hedging instruments, as of December 31, 2014 and 2013:
		Balance Sheet Location		Ameren			Ameren			Ameren
				Missouri			Illinois
	2014
	Fuel oils	Other current assets	$	2		$	—		$	2
	Natural gas	Other current assets		1			1			2
	Power	Other current assets		15			—			15
		Total assets	$	18		$	1		$	19
	Fuel oils	Other current liabilities	$	22		$	—		$	22
		Other deferred credits and liabilities		7			—			7
	Natural gas	MTM derivative liabilities		(a)			31			(a)
		Other current liabilities		6			—			37
		Other deferred credits and liabilities		6			13			19
	Power	MTM derivative liabilities		(a)			11			(a)
		Other current liabilities		3			—			14
		Other deferred credits and liabilities		—			131			131
	Uranium	Other current liabilities		2			—			2
		Total liabilities	$	46		$	186		$	232
	2013
	Fuel oils	Other current assets	$	6		$	—		$	6
		Other assets		3			—			3
	Natural gas	Other current assets		1			1			2
	Power	Other current assets		23			—			23
		Total assets	$	33		$	1		$	34
	Fuel oils	Other current liabilities	$	2		$	—		$	2
		Other deferred credits and liabilities		1			—			1
	Natural gas	MTM derivative liabilities		(a)			27			(a)
		Other current liabilities		5			—			32
		Other deferred credits and liabilities		6			19			25
	Power	MTM derivative liabilities		(a)			9			(a)
		Other current liabilities		4			—			13
		Other deferred credits and liabilities		—			99			99
	Uranium	Other current liabilities		5			—			5
		Other deferred credits and liabilities		1			—			1
		Total liabilities	$	24		$	154		$	178
	(a)	Balance sheet line item not applicable to registrant.
Cumulative Pretax Net Gains (Losses) On All Derivative Instruments In OCI	The following table presents the cumulative amount of pretax net gains (losses) on all derivative instruments deferred in regulatory assets or regulatory liabilities as of December 31, 2014 and 2013:
			Ameren			Ameren			Ameren
			Missouri			Illinois
	2014
	Fuel oils derivative contracts(a)	$	(29	)	$	—		$	(29	)
	Natural gas derivative contracts(b)		(11	)		(43	)		(54	)
	Power derivative contracts(c)		12			(142	)		(130	)
	Uranium derivative contracts(d)		(2	)		—			(2	)
	2013
	Fuel oils derivative contracts	$	2		$	—		$	2
	Natural gas derivative contracts		(10	)		(45	)		(55	)
	Power derivative contracts		19			(108	)		(89	)
	Uranium derivative contracts		(6	)		—			(6	)
	(a)	Represents net losses associated with fuel oils derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s rail transportation surcharges for coal through December 2017. Current losses deferred as regulatory assets include $21 million and $21 million at Ameren and Ameren Missouri, respectively.
	(b)	Represents net losses associated with natural gas derivative contracts. These contracts are a partial hedge of natural gas requirements through October 2019 at Ameren and Ameren Missouri and through October 2018 at Ameren Illinois. Current gains deferred as regulatory liabilities include $2 million, $1 million, and $1 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively. Current losses deferred as regulatory assets include $37 million, $6 million, and $31 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively.
	(c)	Represents net gains (losses) associated with power derivative contracts. These contracts are a partial hedge of power price requirements through May 2032 at Ameren and Ameren Illinois and through December 2015 at Ameren Missouri. Current gains deferred as regulatory liabilities include $15 million and $15 million at Ameren and Ameren Missouri, respectively. Current losses deferred as regulatory assets include $14 million, $3 million, and $11 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively.
	(d)	Represents net losses associated with uranium derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri's uranium requirements through December 2016. Current losses deferred as regulatory assets include $2 million and $2 million at Ameren and Ameren Missouri, respectively.
Derivative Instruments With Credit Risk-Related Contingent Features	The following table presents, as of December 31, 2014, the aggregate fair value of all derivative instruments with credit risk-related contingent features in a gross liability position, the cash collateral posted, and the aggregate amount of additional collateral that could be required to be posted with counterparties. The additional collateral required is the net liability position allowed under the master netting arrangements assuming (1) the credit risk-related contingent features underlying these arrangements were triggered on December 31, 2014, and (2) those counterparties with rights to do so requested collateral.
		Aggregate Fair Value of				Cash				Potential Aggregate Amount of
		Derivative Liabilities(a)				Collateral Posted				Additional Collateral Required(b)
	Ameren Missouri	$	96			$	4			$	88
	Ameren Illinois	74				—				71
	Ameren	$	170			$	4			$	159
	(a)	Prior to consideration of master netting arrangements and including NPNS and other accrual contract exposures.
	(b)	As collateral requirements with certain counterparties are based on master netting arrangements, the aggregate amount of additional collateral required to be posted is determined after consideration of the effects of such arrangements.
Offsetting Derivative Assets and Liabilities [Table Text Block]	The following table provides the recognized gross derivative balances and the net amounts of those derivatives subject to an enforceable master netting arrangement or similar agreement as of December 31, 2014 and 2013:
						Gross Amounts Not Offset on the Balance Sheet
	Commodity Contracts Eligible to be Offset		Gross Amounts Recognized on the Balance Sheet			Derivative Instruments			Cash Collateral Received/Posted(a)			Net
												Amount
	2014
	Assets:
	Ameren Missouri	$	18		$	5		$	—		$	13
	Ameren Illinois		1			—			—			1
	Ameren	$	19		$	5		$	—		$	14
	Liabilities:
	Ameren Missouri	$	46		$	5		$	5		$	36
	Ameren Illinois		186			—			—			186
	Ameren	$	232		$	5		$	5		$	222
	2013
	Assets:
	Ameren Missouri	$	33		$	9		$	—		$	24
	Ameren Illinois		1			1			—			—
	Ameren	$	34		$	10		$	—		$	24
	Liabilities:
	Ameren Missouri	$	24		$	9		$	9		$	6
	Ameren Illinois		154			1			15			138
	Ameren	$	178		$	10		$	24		$	144
	(a)	Cash collateral received reduces gross asset balances and is included in “Other current liabilities” and “Other deferred credits and liabilities” on the balance sheet. Cash collateral posted reduces gross liability balances and is included in “Other current assets” and “Other assets” on the balance sheet.

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2014
Fair Value Disclosures [Abstract]
Fair Value Inputs, Assets and Liabilities, Quantitative Information	The following table describes the valuation techniques and unobservable inputs for the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy for the period ended December 31, 2014:
			Fair Value										Weighted
			Assets			Liabilities				Valuation Technique(s)	Unobservable Input	Range	Average
	Level 3 Derivative asset and liability – commodity contracts(a):
	Ameren	Fuel oils	$	2		$	(8	)		Option model	Volatilities(%)(b)	Mar-39	32
										Discounted cash flow	Ameren Missouri credit risk(%)(b)(c)	0.43	(d)
											Escalation rate(%)(e)(f)	5	(d)
		Natural Gas	1			(2		)		Option model	Volatilities(%)(b)	31 - 144	63
											Nodal basis($/mmbtu)(e)	(0.40) - 0	-0.2
										Discounted cash flow	Nodal basis($/mmbtu)(e)	(0.40) - 0.10	-0.2
											Counterparty credit risk(%)(b)(c)	0.43 - 13	3
											Ameren Missouri and Ameren Illinois credit risk(%)(b)(c)	0.43	(d)
		Power(g)	11			(144		)		Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps($/MWh)(h)	27 - 50	32
											Estimated auction price for FTRs($/MW)(e)	(1,833) - 2,743	171
											Nodal basis($/MWh)(e)	(6) - 0	-2
											Counterparty credit risk(%)(b)(c)	0.26	(d)
											Ameren Missouri and Ameren Illinois credit risk(%)(b)(c)	0.43	(d)
										Fundamental energy production model	Estimated future gas prices($/mmbtu)(e)	5-Apr	4
											Escalation rate(%)(e)(i)	0 - 1	1
										Contract price allocation	Estimated renewable energy credit costs($/credit)(e)	7-May	6
		Uranium	—			(2		)		Discounted cash flow	Average forward uranium pricing($/pound)(e)	35 - 40	36
	Ameren Missouri	Fuel oils	$	2		$	(8	)		Option model	Volatilities(%)(b)	Mar-39	32
										Discounted cash flow	Ameren Missouri credit risk(%)(b)(c)	0.43	(d)
											Escalation rate(%)(e)(f)	5	(d)
		Natural Gas	—			(1		)		Option model	Volatilities(%)(b)	31 - 144	53
											Nodal basis($/mmbtu)(e)	(0.40) - 0	-0.3
										Discounted cash flow	Nodal basis($/mmbtu)(e)	-0.1	(d)
											Counterparty credit risk(%)(b)(c)	0.57 - 13	5
											Ameren Missouri credit risk(%)(b)(c)	0.43	(d)
		Power(g)	11			(2		)		Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps($/MWh)(b)	27 - 50	32
											Estimated auction price for FTRs($/MW)(e)	(1,833) - 2,743	171
											Counterparty credit risk(%)(b)(c)	0.26	(d)
											Ameren Missouri credit risk(%)(b)(c)	0.43	(d)
		Uranium	—			(2		)		Discounted cash flow	Average forward uranium pricing($/pound)(e)	35 - 40	36
	Ameren Illinois	Natural Gas	$	1		$	(1	)		Option model	Volatilities(%)(b)	50 - 144	94
											Nodal basis($/mmbtu)(e)	(0.10) - 0	-0.1
										Discounted cash flow	Nodal basis($/mmbtu)(e)	(0.40) - 0.10	-0.2
											Counterparty credit risk(%)(b)(c)	0.43 - 2	0.83
											Ameren Illinois credit risk(%)(b)(c)	0.43	(d)
			Fair Value										Weighted
			Assets			Liabilities				Valuation Technique(s)	Unobservable Input	Range	Average
		Power(g)	—			(142		)		Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps($/MWh)(e)	27 - 38	32
											Nodal basis($/MWh)(e)	(6) - 0	-2
											Ameren Illinois credit risk(%)(b)(c)	0.43	(d)
										Fundamental energy production model	Estimated future gas prices($/mmbtu)(e)	5-Apr	4
											Escalation rate(%)(e)(i)	0 - 1	1
										Contract price allocation	Estimated renewable energy credit costs($/credit)(e)	7-May	6
	(a)	The derivative asset and liability balances are presented net of counterparty credit considerations.
	(b)	Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.
	(c)	Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances.
	(d)	Not applicable.
	(e)	Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.
	(f)	Escalation rate applies to fuel oil prices 2017 and beyond.
	(g)	Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2018. Valuations beyond 2018 use fundamentally modeled pricing by month for peak and off-peak demand.
	(h)	The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes due to their opposing positions. As such, refer to the power sensitivity analysis for each company above.
	(i)	Escalation rate applies to power prices 2026 and beyond.
	The following table describes the valuation techniques and unobservable inputs for the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy as of December 31, 2013:
			Fair Value										Weighted
			Assets			Liabilities				Valuation Technique	Unobservable Input	Range	Average
	Level 3 Derivative asset and liability – commodity contracts(a):
	Ameren	Fuel oils	$	8		$	(3	)		Option model	Volatilities(%)(b)	Oct-35	16
										Discounted cash flow	Counterparty credit risk(%)(c)(d)	0.26 - 2	1
		Power(e)	21			(110		)		Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps($/MWh)(c)	25 - 51	32
											Estimated auction price for FTRs($/MW)(b)	(1,594) - 945	305
										Nodal basis($/MWh)(c)	(3) - (1)	-2
										Counterparty credit risk(%)(c)(d)	0.39 - 0.50	0.42
										Ameren Missouri and Ameren Illinois credit risk(%)(c)(d)	2	(f)
									Fundamental energy production model	Estimated future gas prices($/mmbtu)(b)	5-Apr	5
										Escalation rate(%)(b)(g)	4-Mar	4
									Contract price allocation	Estimated renewable energy credit costs($/credit)(b)	7-May	6
	Uranium	—			(6		)		Discounted cash flow	Average forward uranium pricing($/pound)(b)	34 - 41	36
Ameren Missouri	Fuel oils	$	8		$	(3	)		Option model	Volatilities(%)(b)	Oct-35	16
									Discounted cash flow	Counterparty credit risk(%)(c)(d)	0.26 - 2	1
	Power(e)	21			(2		)		Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps($/MWh)(c)	25 - 51	40
										Estimated auction price for FTRs($/MW)(b)	(1,594) - 945	305
										Nodal basis($/MWh)(c)	(3) - (1)	-2
										Counterparty credit risk(%)(c)(d)	0.39 - 0.50	0.42
										Ameren Missouri credit risk(%)(c)(d)	2	(f)
	Uranium	—			(6		)		Discounted cash flow	Average forward uranium pricing($/pound)(b)	34 - 41	36
Ameren Illinois	Power(e)	$	—		$	(108	)		Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps($/MWh)(b)	27 - 36	30
										Nodal basis($/MWh)(b)	(4) - 0	-2
										Ameren Illinois credit risk(%)(c)(d)	2	(f)
									Fundamental energy production model	Estimated future gas prices($/mmbtu)(b)	5-Apr	5
										Escalation rate(%)(b)(g)	4-Mar	4
									Contract price allocation	Estimated renewable energy credit costs($/credit)(b)	7-May	6
(a)	The derivative asset and liability balances are presented net of counterparty credit considerations.
(b)	Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.
(c)	Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.
(d)	Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances.
(e)	Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2017. Valuations beyond 2017 use fundamentally modeled pricing by month for peak and off-peak demand.
(f)	Not applicable.
(g)	Escalation rate applies to power prices 2026 and beyond.
Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis	The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of December 31, 2014:
				Quoted Prices in				Significant Other				Significant Other				Total
				Active Markets for				Observable				Unobservable
				Identical Assets				Inputs				Inputs
				or Liabilities				(Level 2)				(Level 3)
				(Level 1)
	Assets:
	Ameren	Derivative assets – commodity contracts(a):
		Fuel oils		$	—			$	—			$	2			$	2
		Natural gas		—				1				1				2
		Power		—				4				11				15
		Total derivative assets – commodity contracts		$	—			$	5			$	14			$	19
		Nuclear decommissioning trust fund:
		Cash and cash equivalents		$	1			$	—			$	—			$	1
		Equity securities:
		U.S. large capitalization		364				—				—				364
		Debt securities:
		Corporate bonds		—				63				—				63
		Municipal bonds		—				2				—				2
		U.S. treasury and agency securities		—				102				—				102
		Asset-backed securities		—				10				—				10
		Other		—				5				—				5
		Total nuclear decommissioning trust fund		$	365			$	182			$	—			$	547	(b)
		Total Ameren		$	365			$	187			$	14			$	566
	Ameren Missouri	Derivative assets – commodity contracts(a):
		Fuel oils		$	—			$	—			$	2			$	2
		Natural gas		—				1				—				1
		Power		—				4				11				15
		Total derivative assets – commodity contracts		$	—			$	5			$	13			$	18
				Quoted Prices in				Significant Other				Significant Other				Total
				Active Markets for				Observable				Unobservable
				Identical Assets				Inputs				Inputs
				or Liabilities				(Level 2)				(Level 3)
				(Level 1)
		Nuclear decommissioning trust fund:
		Cash and cash equivalents		$	1			$	—			$	—			$	1
		Equity securities:
		U.S. large capitalization		364				—				—				364
		Debt securities:
		Corporate bonds		—				63				—				63
		Municipal bonds		—				2				—				2
		U.S. treasury and agency securities		—				102				—				102
		Asset-backed securities		—				10				—				10
		Other		—				5				—				5
		Total nuclear decommissioning trust fund		$	365			$	182			$	—			$	547	(b)
		Total Ameren Missouri		$	365			$	187			$	13			$	565
	Ameren Illinois	Derivative assets – commodity contracts(a):
		Natural gas		$	—			$	—			$	1			$	1
	Liabilities:
	Ameren	Derivative liabilities – commodity contracts(a):
		Fuel oils		$	21			$	—			$	8			$	29
		Natural gas		1				53				2				56
		Power		—				1				144				145
		Uranium		—				—				2				2
		Total Ameren		$	22			$	54			$	156			$	232
	Ameren Missouri	Derivative liabilities – commodity contracts(a):
		Fuel oils		$	21			$	—			$	8			$	29
		Natural gas		1				10				1				12
		Power		—				1				2				3
		Uranium		—				—				2				2
		Total Ameren Missouri		$	22			$	11			$	13			$	46
	Ameren Illinois	Derivative liabilities – commodity contracts(a):
		Natural gas		$	—			$	43			$	1			$	44
		Power		—				—				142				142
	Total Ameren Illinois		$	—			$	43			$	143			$	186
(a)	The derivative asset and liability balances are presented net of counterparty credit considerations.
(b)	Balance excludes $2 million of receivables, payables, and accrued income, net.
The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of December 31, 2013:
			Quoted Prices in				Significant Other				Significant				Total
			Active Markets for				Observable Inputs				Other
			Identical Assets				(Level 2)				Unobservable
			or Liabilities								Inputs
			(Level 1)								(Level 3)
Assets:
Ameren	Derivative assets – commodity contracts(a):
	Fuel oils		$	1			$	—			$	8			$	9
	Natural gas		—				2				—				2
	Power		—				2				21				23
	Total derivative assets – commodity contracts		$	1			$	4			$	29			$	34
			Quoted Prices in				Significant Other				Significant				Total
			Active Markets for				Observable Inputs				Other
			Identical Assets				(Level 2)				Unobservable
			or Liabilities								Inputs
			(Level 1)								(Level 3)
	Nuclear decommissioning trust fund:
	Cash and cash equivalents		$	3			$	—			$	—			$	3
	Equity securities:
	U.S. large capitalization		332				—				—				332
	Debt securities:
	Corporate bonds		—				52				—				52
	Municipal bonds		—				2				—				2
	U.S. treasury and agency securities		—				94				—				94
	Asset-backed securities		—				10				—				10
	Other		—				1				—				1
	Total nuclear decommissioning trust fund		$	335			$	159			$	—			$	494
	Total Ameren		$	336			$	163			$	29			$	528
Ameren Missouri	Derivative assets – commodity contracts(a):
	Fuel oils		$	1			$	—			$	8			$	9
	Natural gas		—				1				—				1
	Power		—				2				21				23
	Total derivative assets – commodity contracts		$	1			$	3			$	29			$	33
	Nuclear decommissioning trust fund:
	Cash and cash equivalents		$	3			$	—			$	—			$	3
	Equity securities:
	U.S. large capitalization		332				—				—				332
	Debt securities:
	Corporate bonds		—				52				—				52
	Municipal bonds		—				2				—				2
	U.S. treasury and agency securities		—				94				—				94
	Asset-backed securities		—				10				—				10
	Other		—				1				—				1
	Total nuclear decommissioning trust fund		$	335			$	159			$	—			$	494
	Total Ameren Missouri		$	336			$	162			$	29			$	527
Ameren Illinois	Derivative assets – commodity contracts(a):
	Natural gas		$	—			$	1			$	—			$	1
Liabilities:
Ameren	Derivative liabilities – commodity contracts(a):
	Fuel oils		$	—			$	—			$	3			$	3
	Natural gas		3				54				—				57
	Power		—				2				110				112
	Uranium		—				—				6				6
	Total Ameren		$	3			$	56			$	119			$	178
Ameren Missouri	Derivative liabilities – commodity contracts(a):
	Fuel oils		$	—			$	—			$	3			$	3
	Natural gas		3				8				—				11
	Power		—				2				2				4
	Uranium		—				—				6				6
	Total Ameren Missouri		$	3			$	10			$	11			$	24
Ameren Illinois	Derivative liabilities – commodity contracts(a):
	Natural gas		$	—			$	46			$	—			$	46
	Power		—				—				108				108
	Total Ameren Illinois		$	—			$	46			$	108			$	154
(a)	The derivative asset and liability balances are presented net of counterparty credit considerations.
Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level 3 In The Fair Value Hierarchy	The following table summarizes the changes in the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy for the year ended December 31, 2014:
			Net Derivative Commodity Contracts
			Ameren			Ameren			Ameren
			Missouri			Illinois
	Fuel oils:
	Beginning balance at January 1, 2014	$	5		$	(a)		$	5
	Realized and unrealized gains (losses) included in regulatory assets/liabilities:		(9	)		(a)			(9	)
	Settlements		(2	)		(a)			(2	)
	Ending balance at December 31, 2014	$	(6	)	$	(a)		$	(6	)
	Change in unrealized gains (losses) related to assets/liabilities held at December 31, 2014	$	(6	)	$	(a)		$	(6	)
	Natural gas:
	Beginning balance at January 1, 2014	$	—		$	—		$	—
	Realized and unrealized gains (losses) included in regulatory assets/liabilities:		—			1			1
	Purchases		—			(2	)		(2	)
	Sales		(1	)		—			(1	)
	Settlements		—			1			1
	Ending balance at December 31, 2014	$	(1	)	$	—		$	(1	)
	Change in unrealized gains (losses) related to assets/liabilities held at December 31, 2014	$	—		$	2		$	2
	Power:
	Beginning balance at January 1, 2014	$	19		$	(108	)	$	(89	)
	Realized and unrealized gains (losses) included in regulatory assets/liabilities:		(14	)		(39	)		(53	)
	Purchases		34			—			34
	Sales		(1	)		—			(1	)
	Settlements		(29	)		5			(24	)
	Ending balance at December 31, 2014	$	9		$	(142	)	$	(133	)
	Change in unrealized gains (losses) related to assets/liabilities held at December 31, 2014	$	—		$	(43	)	$	(43	)
	Uranium:
	Beginning balance at January 1, 2014	$	(6	)	$	(a)		$	(6	)
	Realized and unrealized gains (losses) included in regulatory assets/liabilities:		(1	)		(a)			(1	)
	Settlements		5			(a)			5
	Ending balance at December 31, 2014	$	(2	)	$	(a)		$	(2	)
	Change in unrealized gains (losses) related to assets/liabilities held at December 31, 2014	$	(1	)	$	(a)		$	(1	)
	(a)	Not applicable.
	The following table summarizes the changes in the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy for the year ended December 31, 2013:
			Net Derivative Commodity Contracts
			Ameren			Ameren			Ameren
			Missouri			Illinois
	Fuel oils:
	Beginning balance at January 1, 2013	$	5		$	(a)		$	5
	Purchases		3			(a)			3
	Sales		(1	)		(a)			(1	)
	Settlements		(2	)		(a)			(2	)
	Ending balance at December 31, 2013	$	5		$	(a)		$	5
	Change in unrealized gains (losses) related to assets/liabilities held at December 31, 2013	$	—		$	(a)		$	—
	Natural gas:
	Beginning balance at January 1, 2013	$	—		$	—		$	—
	Realized and unrealized gains (losses) included in regulatory assets/liabilities:		—			(1	)		(1	)
	Purchases		—			1			1
	Ending balance at December 31, 2013	$	—		$	—		$	—
	Change in unrealized gains (losses) related to assets/liabilities held at December 31, 2013	$	—		$	—		$	—
	Power:
	Beginning balance at January 1, 2013	$	11		$	(111	)	$	(100	)
	Realized and unrealized gains (losses) included in regulatory assets/liabilities:		3			(18	)		(15	)
	Purchases		40			—			40
	Settlements		(36	)		21			(15	)
	Transfers into Level 3		(3	)		—			(3	)
	Transfers out of Level 3		4			—			4
	Ending balance at December 31, 2013	$	19		$	(108	)	$	(89	)
	Change in unrealized gains (losses) related to assets/liabilities held at December 31, 2013	$	(1	)	$	(24	)	$	(25	)
	Uranium:
	Beginning balance at January 1, 2013	$	(2	)	$	(a)		$	(2	)
	Realized and unrealized gains (losses) included in regulatory assets/liabilities:		(3	)		(a)			(3	)
	Purchases		(2	)		(a)			(2	)
	Settlements		1			(a)			1
Ending balance at December 31, 2013	$	(6	)	$	(a)		$	(6	)
Change in unrealized gains (losses) related to assets/liabilities held at December 31, 2013	$	(2	)	$	(a)		$	(2	)
(a)	Not applicable.
Schedule Of Carrying Amounts And Estimated Fair Values Of Long-Term Debt And Preferred Stock	following table presents the carrying amounts and estimated fair values of our long-term debt and preferred stock at December 31, 2014 and 2013:
		2014								2013
		Carrying Amount				Fair Value				Carrying Amount				Fair Value
	Ameren:(a)
	Long-term debt and capital lease obligations (including current portion)	$	6,240			$	7,135			$	6,038			$	6,584
	Preferred stock	142				122				142				118
	Ameren Missouri:
	Long-term debt and capital lease obligations (including current portion)	$	3,999			$	4,518			$	3,757			$	4,124
	Preferred stock	80				73				80				71
	Ameren Illinois:
	Long-term debt (including current portion)	$	2,241			$	2,517			$	1,856			$	2,028
	Preferred stock	62				49				62				47
	(a)	Preferred stock is recorded in "Noncontrolling Interests" on the consolidated balance sheet.

Nuclear_Decommissioning_Trust_1

Nuclear Decommissioning Trust Fund Investments (Tables)	12 Months Ended
	Dec. 31, 2014
Nuclear Decommissioning Trust Fund Investments [Line Items]
Fair Value Of Securities In Nuclear Decommissioning Trust Fund	The following table presents the costs and fair values of investments in debt and equity securities in Ameren Missouri’s nuclear decommissioning trust fund at December 31, 2014 and 2013:
	Security Type	Cost			Gross Unrealized Gain			Gross Unrealized Loss		Fair Value
	2014
	Debt securities	$	175		$	7	$	(a)		$	182
	Equity securities	138			230			4		364
	Cash	1			—			—		1
	Other(b)	2			—			—		2
	Total	$	316		$	237	$	4		$	549
	2013
	Debt securities	$	157		$	4	$	2		$	159
	Equity securities	137			199			4		332
	Cash	3			—			—		3
	Other(b)	(a)			—			—		(a)
	Total	$	297		$	203	$	6		$	494
	(a)	Amount less than $1 million.
	(b)	Represents payables relating to pending security purchases, net of receivables related to pending security sales and interest receivables.
Schedule Of Unrealized Losses On Available-For-Sale Investments Included In Nuclear Decommissioning Trust Fund	They are aggregated by investment category and the length of time that individual securities have been in a continuous unrealized loss position at December 31, 2014:
		Less than 12 Months						12 Months or Greater				Total
		Fair Value			Gross			Fair Value			Gross	Fair Value			Gross
					Unrealized						Unrealized				Unrealized
					Losses						Losses				Losses
	Debt securities	$	28	$	(a)			$	8	$	(a)	$	36	$	(a)
	Equity securities	6			1			5			3	11			4
	Total	$	34	$	1			$	13	$	3	$	47	$	4
	(a)	Amount less than $1 million.
Regulatory Asset
Nuclear Decommissioning Trust Fund Investments [Line Items]
Proceeds From Sale Of Investments In Nuclear Decommissioning Trust Fund And Gross Realized Gains And Losses	The following table presents proceeds from the sale and maturities of investments in Ameren Missouri’s nuclear decommissioning trust fund and the gross realized gains and losses resulting from those sales for the years ended December 31, 2014, 2013, and 2012:
		2014			2013			2012
	Proceeds from sales and maturities	$	391		$	196		$	384
	Gross realized gains	7			7			6
	Gross realized losses	2			5			2
Fair Value Of Securities In Nuclear Decommissioning Trust Fund According To Their Contractual Maturities	The following table presents the costs and fair values of investments in debt securities in Ameren Missouri’s nuclear decommissioning trust fund according to their contractual maturities at December 31, 2014:
		Cost			Fair Value
	Less than 5 years	$	98		$	99
	5 years to 10 years	41			42
	Due after 10 years	36			41
	Total	$	175		$	182

Retirement_Benefits_Tables

Retirement Benefits (Tables)	12 Months Ended
	Dec. 31, 2014
Summary Of Benefit Liability Recorded	The following table presents the net benefit liability recorded on the balance sheets of each of the Ameren Companies as of December 31, 2014 and 2013:
		2014			2013
	Ameren(a)	$	710		$	461
	Ameren Missouri	277			191
	Ameren Illinois	278			159
	(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
Funded Status Of Benefit Plans And Amounts Included In Regulatory Assets And OCI	The following table presents the funded status of Ameren's pension and postretirement benefit plans as of December 31, 2014 and 2013. It also provides the amounts included in regulatory assets and accumulated OCI at December 31, 2014 and 2013, that have not been recognized in net periodic benefit costs.
		2014							2013
		Pension Benefits(a)				Postretirement			Pension Benefits(a)				Postretirement
						Benefits(a)							Benefits(a)
	Accumulated benefit obligation at end of year	$	4,176		$	(b)			$	3,698		$	(b)
	Change in benefit obligation:
	Net benefit obligation at beginning of year	$	3,900		$	1,096			$	4,051		$	1,157
	Service cost	79				19			91				22
	Interest cost	183				50			163				46
	Participant contributions	—				16			—				16
	Actuarial (gain) loss	462				84			(207		)		(76	)
	Curtailment gain(c)	—				—			—				(3	)
	Settlement(d)	—				—			—				(5	)
	Benefits paid	(214		)		(65	)		(198		)		(64	)
	Federal subsidy on benefits paid	(b)				3			(b)				3
	Net benefit obligation at end of year	4,410				1,203			3,900				1,096
	Change in plan assets:
	Fair value of plan assets at beginning of year	3,461				1,074			3,127				938
	Actual return on plan assets	448				75			376				156
	Employer contributions	99				6			156				25
	Federal subsidy on benefits paid	(b)				3			(b)				3
	Participant contributions	—				16			—				16
	Benefits paid	(214		)		(65	)		(198		)		(64	)
	Fair value of plan assets at end of year	3,794				1,109			3,461				1,074
	Funded status – deficiency	616				94			439				22
	Accrued benefit cost at December 31	$	616		$	94			$	439		$	22
	Amounts recognized in the balance sheet consist of:
	Noncurrent asset(e)	$	—		$	—			$	—		$	(9	)
	Current liability(f)	3				2			3				1
	Noncurrent liability	613				92			436				30
	Net liability recognized	$	616		$	94			$	439		$	22
	Amounts recognized in regulatory assets consist of:
	Net actuarial (gain) loss	$	452		$	(7	)		$	282		$	(71	)
	Prior service cost (credit)	(6		)		(16	)		(7		)		(20	)
	Amounts (pretax) recognized in accumulated OCI consist of:
	Net actuarial (gain) loss	29				(5	)		17				(12	)
	Prior service cost (credit)	—				(1	)		—				(1	)
	Total	$	475		$	(29	)		$	292		$	(104	)
	(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
	(b)	Not applicable.
Assumptions Used To Determine Benefit Obligations	The following table presents the assumptions used to determine our benefit obligations at December 31, 2014 and 2013:
		Pension Benefits						Postretirement Benefits
		2014			2013			2014			2013
	Discount rate at measurement date	4	%		4.75	%		4	%		4.75	%
	Increase in future compensation	3.5			3.5			3.5			3.5
	Medical cost trend rate (initial)	(a)			(a)			5			5
	Medical cost trend rate (ultimate)	(a)			(a)			5			5
	Years to ultimate rate	(a)			(a)			—			—
Schedule Of Cash Contributions Made To Benefit Plans	The following table presents the cash contributions made to our defined benefit retirement plan and to our postretirement plans during 2014, 2013, and 2012:
		Pension Benefits												Postretirement Benefits
		2014				2013				2012				2014				2013		2012
	Ameren Missouri	$	41			$	60			$	52			$	3			$	10	$	9
	Ameren Illinois	39				50				46				2				11		35
	Other	19				46				30				1				4		1
	Ameren(a)	99				156				128				6				25		45
	(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
Target Allocation Of The Plans' Asset Categories	The following table presents our target allocations for 2015 and our pension and postretirement plans’ asset categories as of December 31, 2014 and 2013:
	Asset	Target Allocation		Percentage of Plan Assets at December  31,
	Category	2015		2014			2013
	Pension Plan:
	Cash and cash equivalents	0% - 5%		2	%		2	%
	Equity securities:
	U.S. large-capitalization	29% - 39%		34	%		36	%
	U.S. small- and mid-capitalization	2% - 12%		7	%		8	%
	International and emerging markets	9% - 19%		12	%		14	%
	Total equity	50% - 60%		53	%		58	%
	Debt securities	35% - 45%		41	%		36	%
	Real estate	0% -   9%		4	%		4	%
	Private equity	0% -   4%		(a)			(a)
	Total			100	%		100	%
	Postretirement Plans:
	Cash and cash equivalents	0% - 10%		4	%		4	%
	Equity securities:
	U.S. large-capitalization	33% - 43%		40	%		41	%
	U.S. small- and mid-capitalization	3% - 13%		7	%		8	%
	International	10% - 20%		13	%		14	%
	Total equity	55% - 65%		60	%		63	%
	Debt securities	30% - 40%		36	%		33	%
	Total			100	%		100	%
Changes In The Fair Value Of Plan Assets Classified As Level 3	The following table summarizes the changes in the fair value of the pension plan assets classified as Level 3 in the fair value hierarchy for each of the years ended December 31, 2014 and 2013:
		Beginning				Actual Return on				Actual Return on				Purchases,				Net		Ending Balance at
		Balance at				Plan Assets Related				Plan Assets Related				Sales, and				Transfers		December 31,
		January 1,				to Assets Still Held				to Assets Sold				Settlements, Net				into (out of)
						at the Reporting Date				During the Period								of Level 3
	2014:00:00
	Real estate	$	131			$	11			$	—			$	5			$	—	$	147
	Private equity	15				(9		)		10				(3		)		—		13
	2013:00:00
	Real estate	$	118			$	9			$	—			$	4			$	—	$	131
	Private equity	19				(9		)		11				(6		)		—		15
Components Of Net Periodic Benefit Cost	The following table presents the components of the net periodic benefit cost of our pension and postretirement benefit plans during 2014, 2013, and 2012:
		Pension Benefits				Postretirement Benefits
		Ameren(a)				Ameren(a)
	2014
	Service cost	$	79			$	19
	Interest cost	183				50
	Expected return on plan assets	(229		)		(65		)
	Amortization of:
	Prior service credit	(1		)		(5		)
	Actuarial (gain) loss	49				(7		)
	Net periodic benefit cost (benefit)	$	81			$	(8	)
	2013
	Service cost	$	91			$	22
	Interest cost	163				46
	Expected return on plan assets	(218		)		(62		)
	Amortization of:
	Prior service credit	(2		)		(6		)
	Actuarial loss	87				8
	Curtailment gain	(12		)		(7		)
	Net periodic benefit cost(b)	$	109			$	1
	2012
	Service cost	$	81			$	22
	Interest cost	166				47
	Expected return on plan assets	(208		)		(56		)
	Amortization of:
	Transition obligation	—				2
	Prior service credit	(3		)		(6		)
	Actuarial loss	75				5
	Net periodic benefit cost(c)	$	111			$	14
	(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
Summary Of Estimated Amortizable Amounts From Regulatory Assets and Accumulated OCI Into Net Periodic Benefit Cost	The estimated amounts that will be amortized from regulatory assets and accumulated OCI into net periodic benefit cost in 2015 are as follows:
		Pension Benefits				Postretirement Benefits
		Ameren(a)				Ameren(a)
	Regulatory assets:
	Prior service credit	$	(1	)		$	(4	)
	Net actuarial loss	86				15
	Accumulated OCI:
	Net actuarial (gain) loss	2				(2		)
	Total	$	87			$	9
	(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
Summary Of Benefit Plan Costs Incurred	The Ameren Companies are responsible for their share of the pension and postretirement benefit costs. The following table presents the pension costs and the postretirement benefit costs incurred and included in continuing operations for the years ended December 31, 2014, 2013, and 2012:
		Pension Costs												Postretirement Costs
		2014				2013				2012				2014				2013		2012
	Ameren Missouri	$	50			$	69			$	63			$	3			$	8	$	10
	Ameren Illinois	30				41				37				(9		)		—		4
	Other	1				5				2				(2		)		—		—
	Ameren(a)	81				115				102				(8		)		8		14
	(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
Schedule Of Expected Payments From Qualified Trust And Company Funds	The expected pension and postretirement benefit payments from qualified trust and company funds, which reflect expected future service, as of December 31, 2014, are as follows:
		Pension Benefits								Postretirement Benefits
		Paid from				Paid from				Paid from				Paid from
		Qualified				Company				Qualified				Company
		Trust				Funds				Trust				Funds
	2015	$	253			$	3			$	58			$	2
	2016	256				3				61				2
	2017	257				4				64				2
	2018	260				3				68				2
	2019	260				3				70				2
	2020 - 2024	1,273				11				388				11
Assumptions Used To Determine Net Periodic Benefit Cost	The following table presents the assumptions used to determine net periodic benefit cost for our pension and postretirement benefit plans for the years ended December 31, 2014, 2013, and 2012:
		Pension Benefits									Postretirement Benefits
		2014			2013			2012			2014			2013			2012
	Discount rate at measurement date	4.75	%		4	%		4.5	%		4.75	%		4	%		4.5	%
	Expected return on plan assets	7.25			7.5			7.75			7			7.25			7.5
	Increase in future compensation	3.5			3.5			3.5			3.5			3.5			3.5
	Medical cost trend rate (initial)	(a)			(a)			(a)			5			5			5.5
	Medical cost trend rate (ultimate)	(a)			(a)			(a)			5			5			5
	Years to ultimate rate	(a)			(a)			(a)			—			—			1 year
Schedule Of Potential Changes In Key Assumptions	The table below reflects the sensitivity of Ameren’s plans to potential changes in key assumptions:
		Pension Benefits								Postretirement Benefits
		Service Cost				Projected				Service Cost				Postretirement
		and Interest				Benefit				and Interest				Benefit
		Cost				Obligation				Cost				Obligation
	0.25% decrease in discount rate	$	(1	)		$	138			$	1			$	39
	0.25% increase in salary scale	2				13				—				—
	1.00% increase in annual medical trend	—				—				3				36
	1.00% decrease in annual medical trend	—				—				(2		)		(33		)
Schedule Of Matching Contributions	The following table presents the portion of the matching contribution to the Ameren 401(k) plan attributable to the continuing operations for each of the Ameren Companies for the years ended December 31, 2014, 2013, and 2012:
		2014				2013				2012
	Ameren Missouri	$	16			$	16			$	16
	Ameren Illinois	11				10				9
	Other	1				1				1
	Ameren(a)	28				27				26
	(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
Pension Benefits
Target Allocation Of The Plans' Asset Categories	The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the pension plan assets measured at fair value as of December 31, 2014:
		Quoted Prices in				Significant Other				Significant Other				Total
		Active Markets for				Observable Inputs				Unobservable
		Identified Assets				(Level 2)				Inputs
		(Level 1)								(Level 3)
	Cash and cash equivalents	$	—			$	38			$	—			$	38
	Equity securities:
	U.S. large-capitalization	—				1,331				—				1,331
	U.S. small- and mid-capitalization	270				—				—				270
	International and emerging markets	134				360				—				494
	Debt securities:
	Corporate bonds	—				1,026				—				1,026
	Municipal bonds	—				175				—				175
	U.S. treasury and agency securities	6				366				—				372
	Other	—				31				—				31
	Real estate	—				—				147				147
	Private equity	—				—				13				13
	Derivative assets	1				—				—				1
	Total	$	411			$	3,327			$	160			$	3,898
	Less: Medical benefit assets at December 31(a)													(125		)
	Plus: Net receivables at December 31(b)													21
	Fair value of pension plans assets at year end													$	3,794
	(a)	Medical benefit (health and welfare) component for accounts maintained in accordance with Section 401(h) of the Internal Revenue Code to fund a portion of the postretirement obligation.
	(b)	Receivables related to pending security sales, offset by payables related to pending security purchases.
	The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the pension plan assets measured at fair value as of December 31, 2013:
		Quoted Prices in				Significant Other				Significant Other				Total
		Active Markets for				Observable Inputs				Unobservable
		Identified Assets or Liabilities				(Level 2)				Inputs
		(Level 1)								(Level 3)
	Cash and cash equivalents	$	5			$	39			$	—			$	44
	Equity securities:
	U.S. large-capitalization	107				1,162				—				1,269
	U.S. small- and mid-capitalization	273				—				—				273
	International and emerging markets	143				372				—				515
	Debt securities:
	Corporate bonds	—				860				—				860
	Municipal bonds	—				149				—				149
	U.S. treasury and agency securities	—				256				—				256
	Other	—				27				—				27
	Real estate	—				—				131				131
	Private equity	—				—				15				15
	Derivative assets	1				—				—				1
	Derivative liabilities	(1		)		—				—				(1		)
	Total	$	528			$	2,865			$	146			$	3,539
	Less: Medical benefit assets at December 31(a)													(112		)
	Plus: Net receivables at December 31(b)													34
	Fair value of pension plans assets at year end													$	3,461
	(a)	Medical benefit (health and welfare) component for accounts maintained in accordance with Section 401(h) of the Internal Revenue Code to fund a portion of the postretirement obligation.
	(b)	Receivables related to pending security sales, offset by payables related to pending security purchases.
Postretirement Benefits
Target Allocation Of The Plans' Asset Categories	The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the postretirement benefit plans assets measured at fair value as of December 31, 2014:
		Quoted Prices in				Significant Other				Significant Other				Total
		Active Markets for				Observable Inputs				Unobservable
		Identified Assets				(Level 2)				Inputs
		(Level 1)								(Level 3)
	Cash and cash equivalents	$	89			$	—			$	—			$	89
	Equity securities:
	U.S. large-capitalization	291				101				—				392
	U.S. small- and mid-capitalization	70				—				—				70
	International	37				94				—				131
	Other	—				7								7
	Debt securities:
	Corporate bonds	—				105				—				105
	Municipal bonds	—				111				—				111
	U.S. treasury and agency securities	—				89				—				89
	Other	—				44				—				44
	Total	$	487			$	551			$	—			$	1,038
	Plus: Medical benefit assets at December 31(a)													125
	Less: Net payables at December 31(b)													(54		)
	Fair value of postretirement benefit plans assets at year end													$	1,109
	(a)	Medical benefit (health and welfare) component for 401(h) accounts to fund a portion of the postretirement obligation. These 401(h) assets are included in the pension plan assets shown above.
	(b)	Payables related to pending security purchases, offset by interest receivables and receivables related to pending security sales.
	The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the postretirement benefit plans assets measured at fair value as of December 31, 2013:
		Quoted Prices in				Significant Other				Significant Other				Total
		Active Markets for				Observable Inputs				Unobservable
		Identified Assets				(Level 2)				Inputs
		(Level 1)								(Level 3)
	Cash and cash equivalents	$	77			$	—			$	—			$	77
	Equity securities:
	U.S. large-capitalization	297				101				—				398
	U.S. small- and mid-capitalization	77				—				—				77
	International	39				96				—				135
	Other	—				2				—				2
	Debt securities:
	Corporate bonds	—				97				—				97
	Municipal bonds	—				103				—				103
	U.S. treasury and agency securities	—				72				—				72
	Other	—				40				—				40
	Total	$	490			$	511			$	—			$	1,001
	Plus: Medical benefit assets at December 31(a)													112
	Less: Net payables at December 31(b)													(39		)
	Fair value of postretirement benefit plans assets at year end													$	1,074
	(a)	Medical benefit (health and welfare) component for 401(h) accounts to fund a portion of the postretirement obligation. These 401(h) assets are included in the pension plan assets shown above.
	(b)	Payables related to pending security purchases, offset by Medicare, interest receivables, and receivables related to pending security sales.

StockBased_Compensation_Tables

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary Of Nonvested Shares Related To Long-Term Incentive Plan	A summary of nonvested shares at December 31, 2014, and changes during the year ended December 31, 2014, under the 2006 Incentive Plan and the 2014 Incentive Plan are presented below:
		Performance Share Units
		Share		Weighted-average
		Units		Fair Value per Share Unit
	Nonvested at January 1, 2014	1,218,544		$	33.23
	Granted(a)	688,323		38.9
	April Grants(b)	38,559		50.34
	Unearned or forfeited(c)	(97,432	)	34.42
	Earned and vested(d)	(685,617	)	36.12
	Nonvested at December 31, 2014	1,162,377		$	35.35
	(a)	Includes performance share units (share units) granted to certain executive and nonexecutive officers and other eligible employees in 2014 under the 2006 Incentive Plan and the 2014 Incentive Plan.
	(b)	In April 2014, certain executive officers were granted additional share units under the 2006 Incentive Plan and the 2014 Incentive Plan. The significant assumptions used to calculate fair value included a prorated three-year risk-free rate ranging from 0.76% to 0.79%, volatility of 12% to 18% for the peer group, and Ameren’s attainment of a three-year average earnings per share threshold during the performance period.
	(c)	Includes share units granted in 2012 that were not earned based on performance provisions of the award grants.
	(d)	Includes share units granted in 2012 that vested as of December 31, 2014, that were earned pursuant to the provisions of the award grants.

Income_Taxes_Tables

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2014																Dec. 31, 2013
Income Tax Disclosure [Abstract]
Schedule Of Effective Income Tax Rate Reconciliation	The following table presents the principal reasons for the difference between the effective income tax rate and the statutory federal income tax rate for the years ended December 31, 2014, 2013, and 2012:
		Ameren Missouri			Ameren Illinois			Ameren
	2014
	Statutory federal income tax rate:	35	%		35	%		35	%
	Increases (decreases) from:
	Amortization of investment tax credit	(1	)		—			(1	)
	State tax	3			6			4
	Other permanent items	—			—			1
	Effective income tax rate	37	%		41	%		39	%
	2013
	Statutory federal income tax rate:	35	%		35	%		35	%
	Increases (decreases) from:
	Depreciation differences	—			(1	)		—
	Amortization of investment tax credit	(1	)		—			(1	)
	State tax	3			6			4
	Other permanent items	1			—			—
	Effective income tax rate	38	%		40	%		38	%
	2012
	Statutory federal income tax rate:	35	%		35	%		35	%
	Increases (decreases) from:
	Depreciation differences	(1	)		—			(1	)
	Amortization of investment tax credit	(1	)		(1	)		(1	)
	State tax	3			6			5
	Reserve for uncertain tax positions	1			—			—
	Other permanent items	—			—			(1	)
	Effective income tax rate	37	%		40	%		37	%
Schedule Of Components Of Income Tax Expense (Benefit)	The following table presents the components of income tax expense (benefit) for the years ended December 31, 2014, 2013, and 2012:
		Ameren Missouri				Ameren Illinois				Other				Ameren
	2014
	Current taxes:
	Federal	$	(13	)		$	(51	)		$	27			$	(37	)
	State	(3		)		(2		)		(32		)		(37		)
	Deferred taxes:
	Federal	222				159				(12		)		369
	State	28				38				22				88
	Deferred investment tax credits, amortization	(5		)		(1		)		—				(6		)
	Total income tax expense	$	229			$	143			$	5			$	377
	2013
	Current taxes:
	Federal	$	136			$	(15	)		$	(239	)	(a)	$	(118	)
	State	41				21				(43		)	(a)	19
	Deferred taxes:
	Federal	64				99				205			(a)	368
	State	6				6				36			(a)	48
	Deferred investment tax credits, amortization	(5		)		(1		)		—				(6		)
	Total income tax expense (benefit)	$	242			$	110			$	(41	)		$	311
	2012
	Current taxes:
	Federal	$	(25	)		$	(7	)		$	72			$	40
	State	(10		)		(3		)		23				10
	Deferred taxes:
	Federal	248				76				(120		)		204
	State	44				30				(14		)		60
	Deferred investment tax credits, amortization	(5		)		(2		)		—				(7		)
	Total income tax expense (benefit)	$	252			$	94			$	(39	)		$	307
	(a)	These amounts are substantially related to the reversal of unrecognized tax benefits as a result of IRS guidance related to the deductibility of expenditures to maintain, replace or improve steam or electric power generation property, along with casualty loss deductions for storm damage. The amounts also reflect the increase in deferred tax expense due to available net operating losses.
Schedule Of Deferred Tax Assets And Liabilities Resulting From Temporary Differences	The following table presents the deferred tax assets and deferred tax liabilities recorded as a result of temporary differences at December 31, 2014 and 2013:
		Ameren Missouri				Ameren Illinois				Other				Ameren
	2014
	Accumulated deferred income taxes, net liability (asset):
	Plant related	$	2,776			$	1,393			$	16			$	4,185
	Regulatory assets, net	82				(5		)		1				78
	Deferred employee benefit costs	(80		)		(45		)		(95		)		(220		)
	Revenue requirement reconciliation adjustments	—				66				3				69
	Tax carryforwards	(107		)		(139		)		(429		)		(675		)
	Other	86				(22		)		70				134
	Total net accumulated deferred income tax liabilities (assets)(a)	$	2,757			$	1,248			$	(434	)		$	3,571
	2013
	Accumulated deferred income taxes, net liability (asset):
	Plant related	$	2,513			$	1,243			$	13			$	3,769
	Regulatory assets, net	74				2				—				76
	Deferred employee benefit costs	(74		)		(85		)		(114		)		(273		)
	Revenue requirement reconciliation adjustments	—				(4		)		2				(2		)
	Tax carryforwards	(76		)		(95		)		(370		)		(541		)
	Other	67				10				38				115
	Total net accumulated deferred income tax liabilities (assets)(b)	$	2,504			$	1,071			$	(431	)		$	3,144
	(a)	Includes $49 million recorded in "Other current assets" on Ameren Missouri's balance sheet as of December 31, 2014.
	(b)	Includes $20 million recorded in "Other current assets" on Ameren Missouri's balance sheet as of December 31, 2013.
Schedule Of Net Operating Loss Carryforwards And Tax Credit Carryforwards	2013																The following table presents the components of deferred tax assets relating to net operating loss carryforwards, tax credit carryforwards, and charitable contribution carryforwards at December 31, 2013:
	The following table presents the components of deferred tax assets relating to net operating loss carryforwards, tax credit carryforwards, and charitable contribution carryforwards at December 31, 2014:
																		Ameren Missouri			Ameren Illinois			Other			Ameren
		Ameren Missouri				Ameren Illinois				Other				Ameren			Net operating loss carryforwards:
	Net operating loss carryforwards:																Federal(a)	$	61		$	84		$	215		$	360
	Federal(a)	$	75			$	127			$	255			$	457
																	State(b)	3			11			34			48
	State(b)	11				10				53				74
																	Total net operating loss carryforwards	$	64		$	95		$	249		$	408
	Total net operating loss carryforwards	$	86			$	137			$	308			$	531
																	Tax credit carryforwards:
	Tax credit carryforwards:																Federal(c)	$	12		$	—		$	76		$	88
	Federal(c)	$	21			$	1			$	77			$	99
																	State(d)	1			1			32			34
	State(d)	1				2				33				36
																	State valuation allowance(e)	(1		)	(1		)	(2		)	(4		)
	State valuation allowance(e)	(1		)		(1		)		(2		)		(4		)	Total tax credit carryforwards	$	12		$	—		$	106		$	118
	Total tax credit carryforwards	$	21			$	2			$	108			$	131
																	Charitable contribution carryforwards(f)	$	—		$	—		$	18		$	18
	Charitable contribution carryforwards(f)	$	—			$	—			$	19			$	19
																	Valuation allowance(g)	—			—			(3		)	(3		)
	Valuation allowance(g)	—				—				(6		)		(6		)
																	Total charitable contribution carryforwards	$	—		$	—		$	15		$	15
	Total charitable contribution carryforwards	$	—			$	—			$	13			$	13
																	(a)	Will begin to expire in 2028
	(a)	Will begin to expire in 2028.
																	(b)	Will begin to expire in 2019.
	(b)	Will begin to expire in 2020.
																	(c)	Will begin to expire in 2029.
	(c)	Will begin to expire in 2029.
																	(d)	Began to expire in 2013.
	(d)	Began to expire in 2013.
																	(e)	Balance increased by $2 million, $- million, and $- million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, during 2013.
	(e)	This balance increased by less than $1 million, $- million, and $- million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, during 2014.
																	(f)	These began to expire in 2013.
	(f)	These began to expire in 2013.
																	(g)	This balance increased by $3 million, $- million, and $- million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, during 2013.
	(g)	This balance increased by $3 million, $- million and $- million for Ameren, Ameren Missouri and Ameren Illinois, respectively, during 2014.
Schedule Of Changes To Unrecognized Tax Benefits And Related Interest	A reconciliation of the change in the unrecognized tax benefit balance during the years ended December 31, 2012, 2013, and 2014, is as follows:
		Ameren Missouri				Ameren Illinois				Other				Ameren
	Unrecognized tax benefits – January 1, 2012	$	124			$	11			$	13			$	148
	Increases based on tax positions prior to 2012	4				—				1				5
	Decreases based on tax positions prior to 2012	(7		)		(1		)		(5		)		(13		)
	Increases based on tax positions related to 2012	15				3				(1		)		17
	Changes related to settlements with taxing authorities	—				—				—				—
	Decreases related to the lapse of statute of limitations	—				—				(1		)		(1		)
	Unrecognized tax benefits – December 31, 2012	$	136			$	13			$	7			$	156
	Increases based on tax positions prior to 2013	—				2				5				7
	Decreases based on tax positions prior to 2013	(122		)		(16		)		(5		)		(143		)
	Increases (decreases) based on tax positions related to 2013	16				—				53			(a)	69
	Changes related to settlements with taxing authorities	—				—				—				—
	Decreases related to the lapse of statute of limitations	1				—				—				1
	Unrecognized tax benefits – December 31, 2013	$	31			$	(1	)		$	60			$	90
	Increases based on tax positions prior to 2014	1				1				4				6
	Decreases based on tax positions prior to 2014	(32		)		(1		)		(9		)		(42		)
	Increases based on tax positions related to 2014	—				—				—				—
	Changes related to settlements with taxing authorities	—				—				—				—
	Increases related to the lapse of statute of limitations	—				—				—				—
	Unrecognized tax benefits (detriments) – December 31, 2014	$	—			$	(1	)		$	55			$	54
	Total unrecognized tax benefits that, if recognized, would affect the effective tax rates as of December 31, 2012	$	3			$	(1	)		$	(1	)		$	1
	Total unrecognized tax benefits (detriments) that, if recognized, would affect the effective tax rates as of December 31, 2013	$	3			$	—			$	51		(a)	$	54
	Total unrecognized tax benefits that, if recognized, would affect the effective tax rates as of December 31, 2014	$	—			$	(1	)		$	53		(a)	$	52
	(a)	Primarily due to tax positions relating to the New AER divestiture. The income statement impact of this unrecognized tax benefit was included in "Income (loss) from discontinued operations, net of taxes" on Ameren's consolidated statement of income (loss). See Note 16 – Divestiture Transactions and Discontinued Operations for additional information.
Reconciliation Of Changes In Liability For Interest On Unrecognized Tax Benefits	A reconciliation of the change in the liability for interest on unrecognized tax benefits during the years ended December 31, 2012, 2013, and 2014, is as follows:
		Ameren Missouri				Ameren Illinois				Other				Ameren
	Liability for interest – January 1, 2012	$	6			$	1			$	(2	)		$	5
	Interest charges (income) for 2012	2				—				(1		)		1
	Liability for interest – December 31, 2012	$	8			$	1			$	(3	)		$	6
	Interest charges (income) for 2013	(8		)		(1		)		4				(5		)
	Liability for interest – December 31, 2013	$	—			$	—			$	1			$	1
	Interest charges (income) for 2014	—				—				(1		)		(1		)
	Liability for interest – December 31, 2014	$	—			$	—			$	—			$	—

Related_Party_Transactions_Tab

Related Party Transactions (Tables)	12 Months Ended
	Dec. 31, 2014
Related Party Transactions [Abstract]
Schedule of Related Party Transactions	The following table presents the impact on Ameren Missouri and Ameren Illinois of related party transactions for the years ended December 31, 2014, 2013, and 2012. It is based primarily on the agreements discussed above and the money pool arrangements discussed in Note 4 – Short-term Debt and Liquidity.
	Agreement	Income Statement Line Item			Ameren		Ameren
					Missouri		Illinois
	Ameren Missouri power supply agreements	Operating Revenues	2014	$	5	$	(a)
	with Ameren Illinois		2013		3		(a)
			2012		(b)		(a)
	Ameren Missouri and Ameren Illinois	Operating Revenues	2014		21		2
	rent and facility services		2013		21		1
			2012		19		1
	Ameren Missouri and Ameren Illinois	Operating Revenues	2014		1		(b)
	miscellaneous support services		2013		1		3
			2012		1		(b)
	Total Operating Revenues		2014	$	27	$	2
			2013		25		4
			2012		20		1
	Ameren Illinois power supply	Purchased Power	2014	$	(a)	$	5
	agreements with Ameren Missouri		2013		(a)		3
			2012		(a)		(b)
	Ameren Illinois transmission	Purchased Power	2014		(a)		2
	services with ATXI		2013		(a)		2
			2012		(a)		3
	Total Purchased Power		2014	$	(a)	$	7
			2013		(a)		5
			2012		(a)		3
	Ameren Services support services	Other Operations and	2014	$	124	$	109
	agreement	Maintenance	2013		116		93
			2012		106		88
	Insurance premiums(c)	Other Operations and	2014		(b)		(a)
		Maintenance	2013		(b)		(a)
			2012		(b)		(a)
	Total Other Operations and		2014	$	124	$	109
	Maintenance Expenses		2013		116		93
			2012		106		88
	Money pool borrowings (advances)	Interest (Charges)	2014	$	(b)	$	(b)
		Income	2013		(b)		(b)
			2012		(b)		(b)
	(a)	Not applicable.
	(b)	Amount less than $1 million.
	(c)	Represents insurance premiums paid to Missouri Energy Risk Assurance Company LLC, an affiliate, for replacement power.

Commitments_And_Contingencies_

Commitments And Contingencies (Tables)	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies Disclosure [Abstract]
Schedule Of Insurance Coverage At Callaway Energy Center	The following table presents insurance coverage at Ameren Missouri’s Callaway energy center at December 31, 2014. The property coverage and the nuclear liability coverage must be renewed on April 1 and January 1, respectively, of each year.
	Type and Source of Coverage	Maximum Coverages			Maximum Assessments
	Public liability and nuclear worker liability:
	American Nuclear Insurers	$	375		$	—
	Pool participation	13,241		(a)	128		(b)
		$	13,616	(c)	$	128
	Property damage:
	Nuclear Electric Insurance Limited	$	2,250	(d)	$	23	(e)
	European Mutual Association for Nuclear Insurance	500		(f)	—
		$	2,750		$	23
	Replacement power:
	Nuclear Electric Insurance Limited	$	490	(g)	$	9	(e)
	Missouri Energy Risk Assurance Company LLC	$	64	(h)	$	—
	(a)	Provided through mandatory participation in an industrywide retrospective premium assessment program.
	(b)	Retrospective premium under the Price-Anderson Act. This is subject to retrospective assessment with respect to a covered loss in excess of $375 million in the event of an incident at any licensed United States commercial reactor, payable at $19 million per year.
	(c)	Limit of liability for each incident under the Price-Anderson Act liability provisions of the Atomic Energy Act of 1954, as amended. A company could be assessed up to $128 million per incident for each licensed reactor it operates, with a maximum of $19 million per incident to be paid in a calendar year for each reactor. This limit is subject to change to account for the effects of inflation and changes in the number of licensed reactors.
	(d)	NEIL provides $2.25 billion in property damage, decontamination, and premature decommissioning insurance.
	(e)	All NEIL-insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL.
	(f)	European Mutual Association for Nuclear Insurance provides $500 million in excess of the $2.25 billion property coverage provided by NEIL.
	(g)	Provides replacement power cost insurance in the event of a prolonged accidental outage. Weekly indemnity is up to $4.5 million for 52 weeks, which commences after the first eight weeks of an outage, plus up to $3.6 million per week for a minimum of 71 weeks thereafter, for a total not exceeding the policy limit of $490 million. Nonradiation events are sub-limited to $328 million.
	(h)	Provides replacement power cost insurance in the event of a prolonged accidental outage. The coverage commences after the first 52 weeks of insurance coverage from NEIL concludes; it is a weekly indemnity of up to $0.9 million for 71 weeks in excess of the $3.6 million per week set forth above. Missouri Energy Risk Assurance Company LLC is an affiliate; it has reinsured this coverage with third-party insurance companies. See Note 14 – Related Party Transactions for more information on this affiliate transaction.
Schedule Of Lease Obligations	We lease various facilities, office equipment, plant equipment, and rail cars under capital and operating leases. The following table presents our lease obligations at December 31, 2014:
		2015			2016			2017		2018		2019		After 5 Years		Total
	Ameren:(a)
	Minimum capital lease payments(b)	$	33		$	33		$	33	$	32	$	32	$	360	$	523
	Less amount representing interest	27			27			27		26		25		97		229
	Present value of minimum capital lease payments	$	6		$	6		$	6	$	6	$	7	$	263	$	294
	Operating leases(c)	13			12			12		12		11		38		98
	Total lease obligations	$	19		$	18		$	18	$	18	$	18	$	301	$	392
	Ameren Missouri:
	Minimum capital lease payments(b)	$	33		$	33		$	33	$	32	$	32	$	360	$	523
	Less amount representing interest	27			27			27		26		25		97		229
	Present value of minimum capital lease payments	$	6		$	6		$	6	$	6	$	7	$	263	$	294
	Operating leases(c)	11			11			11		10		10		37		90
	Total lease obligations	$	17		$	17		$	17	$	16	$	17	$	300	$	384
	Ameren Illinois:
	Operating leases(c)	$	1		$	1		$	1	$	1	$	1	$	1	$	6
	(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.
	(b)	See Properties under Part I, Item 2, and Note 3 – Property and Plant, Net, of this report for additional information.
	(c)	Amounts related to certain land-related leases have indefinite payment periods. The annual obligations of $2 million, $1 million, and $1 million for Ameren, Ameren Missouri, and Ameren Illinois for these items are included in the 2015 through 2019 columns, respectively.
Schedule Of Rental Expense	The following table presents total rental expense, included in operating expenses, for the years ended December 31, 2014, 2013, and 2012:
		2014			2013			2012
	Ameren(a)	$	37		$	32		$	33
	Ameren Missouri	32			29			29
	Ameren Illinois	25			21			19
	(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.
Schedule Of Estimated Purchased Commitments	December 31, 2014. Ameren’s and Ameren Missouri’s purchased power commitments include a 102-megawatt power purchase agreement with a wind farm operator, which expires in 2024. Ameren’s and Ameren Illinois’ purchased power commitments include the Ameren Illinois power purchase agreements entered into as part of the IPA-administered power procurement process. Included in the Other column are minimum purchase commitments under contracts for equipment, design and construction, and meter reading services at December 31, 2014. In addition, the Other column includes Ameren's and Ameren Missouri's obligations related to customer energy efficiency programs under the MEEIA as approved by the MoPSC's December 2012 electric rate order. Ameren Missouri expects to incur costs of $71 million in 2015 for these customer energy efficiency programs. See Note 2 – Rate and Regulatory Matters for additional information about the MEEIA.
		Coal			Natural			Nuclear		Purchased		Methane		Other		Total
					Gas(a)			Fuel		Power(b)		Gas
	Ameren:(c)
	2015	$	654		$	222		$	53	$	190	$	3	$	195	$	1,317
	2016	659			125			60		104		3		78		1,029
	2017	682			85			59		66		4		53		949
	2018	111			53			82		55		5		51		357
	2019	114			32			42		56		5		54		303
	Thereafter	—			71			138		596		76		350		1,231
	Total	$	2,220		$	588		$	434	$	1,067	$	96	$	781	$	5,186
	Ameren Missouri:
	2015	$	654		$	39		$	53	$	21	$	3	$	128	$	898
	2016	659			22			60		21		3		39		804
	2017	682			17			59		21		4		26		809
	2018	111			11			82		21		5		27		257
	2019	114			10			42		21		5		27		219
	Thereafter	—			22			138		106		76		183		525
	Total	$	2,220		$	121		$	434	$	211	$	96	$	430	$	3,512
	Ameren Illinois:
	2015	$	—		$	183		$	—	$	169	$	—	$	29	$	381
	2016	—			103			—		83		—		24		210
	2017	—			68			—		45		—		24		137
	2018	—			42			—		34		—		24		100
	2019	—			22			—		35		—		27		84
	Thereafter	—			49			—		490		—		167		706
	Total	$	—		$	467		$	—	$	856	$	—	$	295	$	1,618
	(a)	Includes amounts for generation and for distribution.
	(b)	The purchased power amounts for Ameren and Ameren Illinois include agreements through 2032 for renewable energy credits with various renewable energy suppliers. The agreements contain a provision that allows Ameren Illinois to reduce the quantity purchased in the event that Ameren Illinois would not be able to recover the costs associated with the renewable energy credits.
	(c)	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
Schedule Of Asbestos-Related Litigation Pending Lawsuits	The following table presents the pending asbestos-related lawsuits filed against the Ameren Companies as of December 31, 2014:
	Ameren		Ameren	Ameren		Total(a)
			Missouri	Illinois
	1		45	57		70
	(a)	Total does not equal the sum of the subsidiary unit lawsuits because some of the lawsuits name multiple Ameren entities as defendants.

Divestiture_Transactions_and_D1

Divestiture Transactions and Discontinued Operations (Tables)	12 Months Ended
	Dec. 31, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Schedule of Disposal Groups, Including Discontinued Operations, Income Statement, Balance Sheet and Additional Disclosures	The following table presents the components of discontinued operations in Ameren's consolidated statement of income (loss) for the years ended December 31, 2014, 2013, and 2012:
		Year ended
		2014			2013				2012
	Operating revenues	$	1		$	1,037			$	1,047
	Operating expenses	(2		)	(1,207		)	(a)	(3,474		)	(b)
	Operating income (loss)	(1		)	(170		)		(2,427		)
	Other income (loss)	—			(1		)		—
	Interest charges	—			(39		)		(56		)
	Income (loss) before income taxes	(1		)	(210		)		(2,483		)
	Income tax (expense) benefit	—			(13		)		987
	Income (loss) from discontinued operations, net of taxes	$	(1	)	$	(223	)		$	(1,496	)
	(a)	Includes a $201 million pretax loss on disposal relating to the New AER divestiture.
	(b)	Includes a noncash pretax asset impairment charge of $2.58 billion to reduce the carrying value of AER's energy centers to their estimated fair value under held and used accounting guidance.
	The following table presents the carrying amounts of the components of assets and liabilities segregated on Ameren's consolidated balance sheets as discontinued operations at December 31, 2014 and 2013:
		31-Dec-14			31-Dec-13
	Assets of discontinued operations
	Accounts receivable and unbilled revenue	$	—		$	5
	Materials and supplies	—			5
	Property and plant, net	—			142
	Accumulated deferred income taxes, net(a)	15			13
	Total assets of discontinued operations	$	15		$	165
	Liabilities of discontinued operations
	Accounts payable and other current obligations	$	1		$	5
	Asset retirement obligations(b)	32			40
	Total liabilities of discontinued operations	$	33		$	45
	(a)	The December 31, 2014 balance primarily consists of deferred income tax assets related to the abandoned Meredosia and Hutsonville energy centers.
	(b)	Includes AROs associated with the abandoned Meredosia and Hutsonville energy centers of $32 million and $31 million at December 31, 2014 and 2013, respectively.

Segment_Information_Tables

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2014
Segment Reporting Information, Profit (Loss) [Abstract]
Schedule Of Segment Reporting Information, By Segment	The following table presents information about the reported revenues and specified items reflected in Ameren’s net income attributable to Ameren Corporation and capital expenditures from continuing operations for the years ended December 31, 2014, 2013, and 2012, and total assets in continuing operations as of December 31, 2014, 2013, and 2012:
		Ameren		Ameren		Other				Intersegment			Consolidated
		Missouri		Illinois						Eliminations
	2014
	External revenues	$	3,526	$	2,496	$	31			$	—		$	6,053
	Intersegment revenues	27		2		2				(31		)	—
	Depreciation and amortization	473		263		9				—			745
	Interest and dividend income	28		7		2				—			37
	Interest charges	211		112		18				—			341
	Income taxes	229		143		5				—			377
	Net income (loss) attributable to Ameren Corporation from continuing operations	390		201		(4		)		—			587
	Capital expenditures	747		835		203			(a)	—			1,785
	Total assets	13,541		8,381		942				(203		)	22,661		(b)
	2013
	External revenues	$	3,516	$	2,307	$	15			$	—		$	5,838
	Intersegment revenues	25		4		2				(31		)	—
	Depreciation and amortization	454		243		9				—			706
	Interest and dividend income	27		2		1				—			30
	Interest charges	210		143		45				—			398
	Income taxes (benefit)	242		110		(41		)		—			311
	Net income (loss) attributable to Ameren Corporation from continuing operations	395		160		(43		)		—			512
	Capital expenditures	648		701		30			(a)	—			1,379
	Total assets	12,904		7,454		752				(233		)	20,877		(b)
	2012
	External revenues	$	3,252	$	2,524	$	5			$	—		$	5,781
	Intersegment revenues	20		1		3				(24		)	—
	Depreciation and amortization	440		221		12				—			673
	Interest and dividend income	32		—		—				—			32
	Interest charges	223		129		40				—			392
	Income taxes (benefit)	252		94		(39		)		—			307
	Net income (loss) attributable to Ameren Corporation from continuing operations	416		141		(41		)		—			516
	Capital expenditures	595		442		26			(a)	—			1,063
	Total assets	13,043		7,282		1,228				(934		)	20,619		(b)
	.
	(a)	Includes the elimination of intercompany transfers.

Selected_Quarterly_Information1

Selected Quarterly Information (Tables)	12 Months Ended
	Dec. 31, 2014
Selected Quarterly Financial Information [Abstract]
Summary Of Selected Quarterly Information	SELECTED QUARTERLY INFORMATION (Unaudited) (In millions, except per share amounts)
	Ameren	2014														2013
	Quarter ended (a)	31-Mar			30-Jun			30-Sep				31-Dec				31-Mar			30-Jun			30-Sep			31-Dec
	Operating revenues	$	1,594		$	1,419		$	1,670			$	1,370			$	1,475		$	1,403		$	1,638		$	1,322
	Operating income	246			322			561				125				185			261			567			171
	Net income (loss)	98			150			295				49				(143		)	96			304			38
	Net income attributable to Ameren Corporation – continuing operations	$	97		$	150		$	294			$	46			$	54		$	105		$	305		$	48
	Net income (loss) attributable to Ameren Corporation – discontinued operations	(1		)	(1		)	(1		)		2				(199		)	(10		)	(3		)	(11		)
	Net income (loss) attributable to Ameren Corporation	$	96		$	149		$	293			$	48			$	(145	)	$	95		$	302		$	37
	Earnings per common share – basic – continuing operations	$	0.4		$	0.62		$	1.21			$	0.19			$	0.22		$	0.44		$	1.26		$	0.19
	Earnings (loss) per common share – basic – discontinued operations	—			(0.01		)	—				0.01				(0.82		)	(0.05		)	(0.01		)	(0.04		)
	Earnings (loss) per common share – basic	$	0.4		$	0.61		$	1.21			$	0.2			$	(0.60	)	$	0.39		$	1.25		$	0.15
	Earnings per common share – diluted – continuing operations	$	0.4		$	0.62		$	1.2			$	0.19			$	0.22		$	0.44		$	1.25		$	0.19
	Earnings (loss) per common share – diluted – discontinued operations	—			(0.01		)	—				0.01				(0.82		)	(0.05		)	(0.01		)	(0.04		)
	Earnings (loss) per common share – diluted	$	0.4		$	0.61		$	1.2			$	0.2			$	(0.60	)	$	0.39		$	1.24		$	0.15
	(a)	The sum of quarterly amounts, including per share amounts, may not equal amounts reported for year-to-date periods. This is due to the effects of rounding and to changes in the number of weighted-average shares outstanding each period.
	Ameren Missouri Quarter ended		Operating			Operating			Net Income				Net Income (Loss)
			Revenues			Income			(Loss)				Available
													to Common
													Stockholder
	March 31, 2014		$	817		$	119		$	48			$	47
	March 31, 2013		796			111			41				40
	June 30, 2014		900			243			127				126
	June 30, 2013		889			179			85				84
	September 30, 2014		1,097			394			223				222
	September 30, 2013		1,093			417			239				238
	December 31, 2014		739			29			(5		)		(5		)
	December 31, 2013		763			96			33				33
	Ameren Illinois Quarter ended		Operating			Operating			Net Income				Net Income
			Revenues			Income							Available
													to Common
													Stockholder
	March 31, 2014		$	774		$	120		$	54			$	53
	March 31, 2013		684			85			32				31
	June 30, 2014		519			75			29				28
	June 30, 2013		516			87			32				31
	September 30, 2014		572			158			75				75
	September 30, 2013		547			158			77				77
	December 31, 2014		633			97			46				45
	December 31, 2013		564			85			22				21

Summary_Of_Significant_Account3

Summary Of Significant Accounting Policies (Narrative) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Accounting Policies [Line Items]
Nuclear decommissioning trust fund	$549	$494
Average performance share units excluded from calculation	0	0	0
Unrecognized Tax Benefit Recorded In Other Deferred Credits And Liabilities	52	84
Unrecognized Tax Benefits	54	90	156	148
Goodwill	411	411
Ameren Illinois Company
Accounting Policies [Line Items]
Capital contribution from parent	15
Unrecognized Tax Benefit Recorded In Other Deferred Credits And Liabilities	0	0
Unrecognized Tax Benefits	-1	-1	13	11
Goodwill	411	411
Public Utilities, Area Serviced	40,000
Public Utilities, Estimated Population of Service Territory	3,100,000
Union Electric Company
Accounting Policies [Line Items]
Capital contribution from parent	215	4	1
Noncash Or Part Noncash Capital Contribution From Parent	9
Nuclear decommissioning trust fund	549	494
Unrecognized Tax Benefit Recorded In Other Deferred Credits And Liabilities	0	15
Unrecognized Tax Benefits	0	31	136	124
Tax grants received related to renewable energy properties			18
Public Utilities, Area Serviced	24,000
Public Utilities, Estimated Population of Service Territory	2,800,000
Return of capital to parent	($215)
Minimum
Accounting Policies [Line Items]
Percent of average depreciable cost	3.00%	3.00%	3.00%
Maximum
Accounting Policies [Line Items]
Percent of average depreciable cost	4.00%	4.00%	4.00%
Power | Ameren Illinois Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers	1,200,000
Power | Union Electric Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers	1,200,000
Natural Gas | Ameren Illinois Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers	813,000
Natural Gas | Union Electric Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers	127,000
FAC | Union Electric Company
Accounting Policies [Line Items]
Sharing Level For Fac	95.00%

Summary_Of_Significant_Account4

Summary Of Significant Accounting Policies (Schedule Of Material And Supplies) (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Accounting Policies [Line Items]
Fuel	$134	[1]	$144	[1]
Gas stored underground	127		127
Other materials and supplies	263		255
Total materials and supplies	524		526
Union Electric Company
Accounting Policies [Line Items]
Fuel	134	[1]	144	[1]
Gas stored underground	16		17
Other materials and supplies	197		191
Total materials and supplies	347		352
Ameren Illinois Company
Accounting Policies [Line Items]
Gas stored underground	111		110
Other materials and supplies	66		64
Total materials and supplies	$177		$174
[1]	(a)Consists of coal, oil, and propane.

Summary_Of_Significant_Account5

Summary Of Significant Accounting Policies (Schedule Of Rates Used For Allowance For Funds Used During Construction) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Union Electric Company
Accounting Policies [Line Items]
Allowance for funds used during construction, rate	7.00%	8.00%	8.00%
Ameren Illinois Company
Accounting Policies [Line Items]
Allowance for funds used during construction, rate	2.00%	8.00%	9.00%

Summary_Of_Significant_Account6

Summary Of Significant Accounting Policies (Schedule Of Asset Retirement Obligations) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance	$369,000,000		$349,000,000
Liabilities incurred	2,000,000
Liabilities settled	-2,000,000		-1,000,000
Accretion in period	21,000,000	[1]	19,000,000	[1]
Change in estimates	6,000,000	[2],[3]	2,000,000	[3]
Balance	396,000,000		369,000,000
Nuclear decommissioning trust fund	549,000,000		494,000,000
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance	366,000,000		346,000,000
Liabilities incurred	2,000,000
Liabilities settled	-2,000,000		-1,000,000
Accretion in period	21,000,000	[1]	19,000,000	[1]
Change in estimates	2,000,000	[3]	2,000,000	[3]
Balance	389,000,000		366,000,000
Nuclear decommissioning trust fund	549,000,000		494,000,000
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance	3,000,000	[4]	3,000,000
Liabilities settled	-1,000,000	[5]	-1,000,000	[5]
Accretion in period	1,000,000	[1],[5]	1,000,000	[1],[5]
Change in estimates	4,000,000	[1]	1,000,000	[5]
Balance	$7,000,000	[4]	$3,000,000	[4]
[1]	(b)Accretion expense was recorded as an increase to regulatory assets at Ameren Missouri and Ameren Illinois.
[2]	(e)Ameren Illinois changed its fair value estimate for asbestos removal in 2014.
[3]	c)Ameren Missouri changed its fair value estimates for asbestos removal in 2013 and 2014 and for certain CCR facilities in 2013.
[4]	(d)Included in “Other deferred credits and liabilities” on the balance sheet.
[5]	(a)Less than $1 million.

Summary_Of_Significant_Account7

Summary Of Significant Accounting Policies (Schedule Of Excise Taxes) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accounting Policies [Line Items]
Excise tax expense	$215	$213	$193
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	151	152	139
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$64	$61	$54

Summary_Of_Significant_Account8

Summary Of Significant Accounting Policies (Basic and Diluted Earnings Per Share Calculations) (Details) (USD $)	3 Months Ended								12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Accounting Policies [Abstract]
Continuing Operations	$46	$294	$150	$97	$48	$305	$105	$54	$587	$512	$516
Discontinued Operations	2	-1	-1	-1	-11	-3	-10	-199	-1	-223	-1,490
Net income (loss) attributable to Ameren Corporation	$48	$293	$149	$96	$37	$302	$95	($145)	$586	$289	($974)
Average Common Shares Outstanding - Basic									242.6	242.6	242.6
Assumed Settlement of Performance Share Units									1.8	1.9	0.4
Average Common Shares Outstanding - Diluted									244.4	244.5	243
Continuing Operations - Basic	$0.19	$1.21	$0.62	$0.40	$0.19	$1.26	$0.44	$0.22	$2.42	$2.11	$2.13
Discontinued Operations - Basic	$0.01		($0.01)	$0	($0.04)	($0.01)	($0.05)	($0.82)	$0	($0.92)	($6.14)
Earnings (Loss) per Common Share – Basic	$0.20	$1.21	$0.61	$0.40	$0.15	$1.25	$0.39	($0.60)	$2.42	$1.19	($4.01)
Continuing Operations - Diluted	$0.19	$1.20	$0.62	$0.40	$0.19	$1.25	$0.44	$0.22	$2.40	$2.10	$2.13
Discontinued Operations - Diluted	$0.01	$0	($0.01)	$0	($0.04)	($0.01)	($0.05)	($0.82)	$0	($0.92)	($6.14)
Earnings (Loss) per Common Share – Diluted	$0.20	$1.20	$0.61	$0.40	$0.15	$1.24	$0.39	($0.60)	$2.40	$1.18	($4.01)
Average performance share units excluded from calculation									0	0	0

Summary_Of_Significant_Account9

Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Supplemental Cash Flow Information) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Supplemental Cash Flow Information [Line Items]
Interest Paid, Capitalized	$18		$37		$30
Interest Paid, Net	333		393		433
Income Taxes Paid, Net	-27		8		1
Continuing Operations
Supplemental Cash Flow Information [Line Items]
Interest Paid, Capitalized	18		20		17
Interest Paid, Net	333	[1]	362	[1]	384	[1]
Income Taxes Paid, Net	-41		116		10
Discontinued Operations
Supplemental Cash Flow Information [Line Items]
Interest Paid, Capitalized	0		17		13
Interest Paid, Net			31	[2]	49	[2]
Income Taxes Paid, Net	$14		($108)		($9)
[1]	(a)Net of $18 million, $20 million, and $17 million capitalized, respectively.
[2]	(b)Net of $- million, $17 million, and $13 million capitalized, respectively.

Rate_And_Regulatory_Matters_Ra

Rate And Regulatory Matters Rate and Regulatory Matters (Narrative-Missouri) (Details) (USD $)	12 Months Ended			1 Months Ended	3 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jul. 31, 2014	Dec. 31, 2014
Rate And Regulatory Matters [Line Items]
Regulatory assets	$1,582,000,000	$1,240,000,000			$1,582,000,000
Interest Expense	341,000,000	398,000,000	392,000,000
Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	695,000,000	534,000,000			695,000,000
Interest Expense	211,000,000	210,000,000	223,000,000
Electric Distribution | Pending Rate Case [Member] | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount				190,000,000
Requested Rate Increase Related to Net Energy Costs				100,000,000
Sharing Level For Fac				95.00%
Public Utilities, Requested Return on Equity, Percentage				10.40%
Public Utilities, Requested Equity Capital Structure, Percentage				51.80%
Rate Base				7,000,000,000
Electric Distribution | Accounting Authority Order Request | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	36,000,000				36,000,000
MEEIA | Electric Distribution | Union Electric Company
Rate And Regulatory Matters [Line Items]
Energy Efficiency Program Spending					135,000,000
Incentive Award if Energy Efficiency Goals Are Achieved					25,000,000
Achieved Percentage of Energy Efficiency Earnings For Incentive Award	100.00%				100.00%
Incentive Award if Energy Efficiency Goals Are Achieved, Period					3 years
Minimum Percentage of Energy Efficiency Goal Achievement For Company To Be Eligible For Incentive Award					70.00%
MOPSC [Member] | Electric Distribution | Pending Rate Case [Member] | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount					89,000,000
Public Utilities, Requested Return on Equity, Percentage					9.25%
Fac Prudence Review | Subsequent Periods After September 30, 2009 | Union Electric Company
Rate And Regulatory Matters [Line Items]
Customer Refund Liability, Current	26,000,000				26,000,000
Interest Expense	$1,000,000

Rate_And_Regulatory_Matters_Na

Rate And Regulatory Matters (Narrative-Illinois) (Details) (USD $)	12 Months Ended	3 Months Ended	12 Months Ended	1 Months Ended
	Dec. 31, 2013	Dec. 31, 2014	Dec. 31, 2014	Jan. 31, 2015	Dec. 09, 2013
Rate And Regulatory Matters [Line Items]
Regulatory assets	$1,240,000,000	$1,582,000,000	$1,582,000,000
Current regulatory assets	156,000,000	295,000,000	295,000,000
Current regulatory liabilities	216,000,000	106,000,000	106,000,000
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	701,000,000	883,000,000	883,000,000
Current regulatory assets	38,000,000	129,000,000	129,000,000
Current regulatory liabilities	159,000,000	84,000,000	84,000,000
Ameren Illinois Company | Final Rate Order | Gas Distribution
Rate And Regulatory Matters [Line Items]
Authorized increase in revenue from utility service	32,000,000
Rate base	1,100,000,000
Public Utilities, Approved Return on Equity, Percentage	9.10%
Percent of capital structure composed of equity	51.70%
Ameren Illinois Company | Final Rate Order | Electric Distribution
Rate And Regulatory Matters [Line Items]
Disallowed costs associated with debt redemption					15,000,000
Allowed Costs Associated with Debt Redemption		11,000,000
Ameren Illinois Company | IEMA Revenue Requirement Reconciliation | IEIMA | Electric Distribution
Rate And Regulatory Matters [Line Items]
Regulatory assets		101,000,000	101,000,000
Current regulatory assets		65,000,000	65,000,000
Current regulatory liabilities	65,000,000
Authorized increase in revenue from utility service		204,000,000
Ameren Illinois Company | Rate order appeal [Member] | Gas Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Return on Equity, Percentage	10.40%
ATXI
Rate And Regulatory Matters [Line Items]
Projected Transmission Project Costs			150,000,000
Subsequent Event | Ameren Illinois Company | Pending Rate Case [Member] | Gas Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount				53,000,000
Public Utilities, Requested Return on Equity, Percentage				10.25%
Public Utilities, Requested Equity Capital Structure, Percentage				50.00%
Rate base				$1,200,000,000

Rate_And_Regulatory_Matters_Ra1

Rate And Regulatory Matters Rate and Regulatory Matters (Narrative-Federal) (Details) (USD $)	12 Months Ended						1 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		Nov. 30, 2014	Feb. 28, 2015	Sep. 30, 2014
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	$106		$216
Purchased power	454		502		780
Miscellaneous income	79	[1]	69	[1]	70	[1]
Midwest Independent Transmission System Operator, Inc [Member] | Pending Ferc Case [Member]
Rate And Regulatory Matters [Line Items]
Customer Requested Rate on Equity	9.15%
Rate of return on common equity	12.38%
Incentive adder to FERC allowed base return on common equity							50
New England Transmission Owners [Member] | Pending Ferc Case [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Return on Equity, Percentage	11.14%
Wholesale Distribution Rate Case
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities									24
Subsequent Event | Midwest Independent Transmission System Operator, Inc [Member] | Pending Ferc Case [Member]
Rate And Regulatory Matters [Line Items]
Customer Requested Rate on Equity								8.67%
Minimum | New England Transmission Owners [Member] | Pending Ferc Case [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Return on Equity, Percentage	10.57%
Maximum | New England Transmission Owners [Member] | Pending Ferc Case [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Return on Equity, Percentage	11.74%
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	84		159
Purchased power	343		380		705
Miscellaneous income	17		10		7
Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	18		57
Purchased power	119		127		78
Miscellaneous income	60		58		63
Union Electric Company | Entergy Refund
Rate And Regulatory Matters [Line Items]
Proceeds from Legal Settlements			31
Purchased power					24
Miscellaneous income					5
Reduction To Under-recovered Asset					2
New Nuclear Energy Center COL [Member] | Union Electric Company
Rate And Regulatory Matters [Line Items]
Investments in Power and Distribution Projects	69
New Nuclear Energy Center COL [Member] | Union Electric Company | Maximum
Rate And Regulatory Matters [Line Items]
Investments in Power and Distribution Projects	100
Pending Ferc Case [Member] | Ameren Illinois Company | Maximum
Rate And Regulatory Matters [Line Items]
Loss Contingency, Estimate of Possible Loss	22
Pending Ferc Case [Member] | Union Electric Company | Power Purchase Agreement With Entergy Arkansas
Rate And Regulatory Matters [Line Items]
Purchased power	$8
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Rate_And_Regulatory_Matters_Sc

Rate And Regulatory Matters (Schedule Of Regulatory Assets And Liabilities) (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Rate And Regulatory Matters [Line Items]
Current regulatory assets	$295		$156
Regulatory assets	1,582		1,240
Current regulatory liabilities	106		216
Regulatory liabilities	1,850		1,705
Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	163		118
Regulatory assets	695		534
Current regulatory liabilities	18		57
Regulatory liabilities	1,147		1,041
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	129		38
Regulatory assets	883		701
Current regulatory liabilities	84		159
Regulatory liabilities	703		664
Under-Recovered FAC
Rate And Regulatory Matters [Line Items]
Current regulatory assets	128	[1],[2]	104	[1],[2]
Under-Recovered FAC | Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	128	[1],[2]	104	[1],[2]
Under-Recovered FAC | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	0	[1],[2]
Under-Recovered Illinois Electric Power Costs
Rate And Regulatory Matters [Line Items]
Current regulatory assets	2	[3]	1	[3]
Under-Recovered Illinois Electric Power Costs | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	2	[3]	1	[3]
Under-Recovered PGA
Rate And Regulatory Matters [Line Items]
Current regulatory assets	20	[3]	1	[3]
Under-Recovered PGA | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	20	[3]	1	[3]
MTM Derivative Losses
Rate And Regulatory Matters [Line Items]
Current regulatory assets	74	[4]	50	[4]
Regulatory assets	158	[4]	126	[4]
MTM Derivative Losses | Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	32	[4]	14	[4]
Regulatory assets	14	[4]	8	[4]
MTM Derivative Losses | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	42	[4]	36	[4]
Regulatory assets	144	[4]	118	[4]
Energy Efficiency Rider
Rate And Regulatory Matters [Line Items]
Current regulatory assets	3	[5]
Regulatory liabilities	39	[5]	36	[5]
Energy Efficiency Rider | Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	3	[5]
Regulatory liabilities			3	[5]
Energy Efficiency Rider | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	39	[5]	33	[5]
IEMA Revenue Requirement Reconciliation
Rate And Regulatory Matters [Line Items]
Current regulatory assets	65	[1],[6]
Regulatory assets	101	[1],[6]	65	[1],[6]
Current regulatory liabilities			65	[6]
IEMA Revenue Requirement Reconciliation | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory assets	65	[1],[6]
Regulatory assets	101	[1],[6]	65	[1],[6]
Current regulatory liabilities			65	[6]
FERC Revenue Requirement Reconciliation
Rate And Regulatory Matters [Line Items]
Current regulatory assets	3	[1],[7]
Regulatory assets	12	[1],[7]	5	[1],[7]
Current regulatory liabilities	11	[7]
Regulatory liabilities			10	[7]
FERC Revenue Requirement Reconciliation | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	8	[1],[7]
Current regulatory liabilities	11	[7]
Regulatory liabilities			10	[7]
Pension And Postretirement Benefit Costs
Rate And Regulatory Matters [Line Items]
Regulatory assets	423	[8]	184	[8]
Pension And Postretirement Benefit Costs | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	148	[8]	44	[8]
Pension And Postretirement Benefit Costs | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	275	[8]	140	[8]
Income Taxes
Rate And Regulatory Matters [Line Items]
Regulatory assets	256	[9]	237	[9]
Regulatory liabilities	55	[10]	40	[10]
Income Taxes | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	253	[9]	230	[9]
Regulatory liabilities	41	[10]	37	[10]
Income Taxes | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	3	[9]	7	[9]
Regulatory liabilities	14	[10]	3	[10]
Asset Retirement Obligation
Rate And Regulatory Matters [Line Items]
Regulatory assets	5	[11]	5	[11]
Regulatory liabilities	182	[11]	146	[11]
Asset Retirement Obligation | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	182	[11]	146	[11]
Asset Retirement Obligation | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	5	[11]	5	[11]
Callaway Costs
Rate And Regulatory Matters [Line Items]
Regulatory assets	36	[1],[12]	40	[1],[12]
Callaway Costs | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	36	[1],[12]	40	[1],[12]
Unamortized Loss On Reacquired Debt
Rate And Regulatory Matters [Line Items]
Regulatory assets	152	[1],[13]	151	[1],[13]
Unamortized Loss On Reacquired Debt | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	72	[1],[13]	77	[1],[13]
Unamortized Loss On Reacquired Debt | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	80	[1],[13]	74	[1],[13]
Recoverable Costs Contaminated Facilities
Rate And Regulatory Matters [Line Items]
Regulatory assets	251	[14]	271	[14]
Recoverable Costs Contaminated Facilities | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	251	[14]	271	[14]
Storm Costs
Rate And Regulatory Matters [Line Items]
Regulatory assets	3	[15]	8	[15]
Storm Costs | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets			5	[15]
Storm Costs | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	3	[15]	3	[15]
Demand-Side Costs
Rate And Regulatory Matters [Line Items]
Regulatory assets	44	[1],[16]	58	[1],[16]
Demand-Side Costs | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	44	[1],[16]	58	[1],[16]
Reserve For Workers' Compensation Liabilities
Rate And Regulatory Matters [Line Items]
Regulatory assets	14	[17]	12	[17]
Reserve For Workers' Compensation Liabilities | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	7	[17]	6	[17]
Reserve For Workers' Compensation Liabilities | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	7	[17]	6	[17]
Credit Facilities Fees
Rate And Regulatory Matters [Line Items]
Regulatory assets	5	[18]	5	[18]
Credit Facilities Fees | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	5	[18]	5	[18]
Common Stock Issuance Costs
Rate And Regulatory Matters [Line Items]
Regulatory assets	2	[19]	4	[19]
Common Stock Issuance Costs | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	2	[19]	4	[19]
Construction Accounting For Pollution Control Equipment
Rate And Regulatory Matters [Line Items]
Regulatory assets	21	[1],[20]	22	[1],[20]
Construction Accounting For Pollution Control Equipment | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	21	[1],[20]	22	[1],[20]
Solar Rebates
Rate And Regulatory Matters [Line Items]
Regulatory assets	88	[1],[21]	27	[1],[21]
Solar Rebates | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	88	[1],[21]	27	[1],[21]
Other
Rate And Regulatory Matters [Line Items]
Regulatory assets	11	[22]	20	[22]
Regulatory liabilities	7	[23]	11	[23]
Other | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	5	[22]	8	[22]
Regulatory liabilities	7	[23]	11	[23]
Other | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	6	[22]	12	[22]
Over-Recovered FAC
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	0	[2]	26	[2]
Over-Recovered FAC | Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	0	[2]	26	[2]
Over-Recovered FAC | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	0	[2]	0	[2]
Over-Recovered Illinois Electric Power Costs
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	26	[3]	51	[3]
Over-Recovered Illinois Electric Power Costs | Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	0	[3]	0	[3]
Over-Recovered Illinois Electric Power Costs | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	26	[3]	51	[3]
Over-Recovered PGA
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	27	[3]	34	[3]
Over-Recovered PGA | Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	2	[3]	5	[3]
Over-Recovered PGA | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	25	[3]	29	[3]
MTM Derivative Gains
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	17	[4]	27	[4]
MTM Derivative Gains | Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	16	[4]	26	[4]
MTM Derivative Gains | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	1	[4]	1	[4]
Wholesale Distribution Refund
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities			13	[24]
Wholesale Distribution Refund | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities			13	[24]
Refund Reserves for FERC Orders and Audit Findings [Member]
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	25	[25]
Refund Reserves for FERC Orders and Audit Findings [Member] | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	21	[25]
Uncertain tax positions tracker [Member]
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	7	[26]	1	[26]
Uncertain tax positions tracker [Member] | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	7	[26]	1	[26]
Removal Costs
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	1,529	[27]	1,438	[27]
Removal Costs | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	886	[27]	828	[27]
Removal Costs | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	643	[27]	610	[27]
Bad Debt Rider
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	7	[28]	8	[28]
Bad Debt Rider | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	7	[28]	8	[28]
Pension And Postretirement Benefit Costs Tracker
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	24	[29]	15	[29]
Pension And Postretirement Benefit Costs Tracker | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	$24	[29]	$15	[29]
[1]	(a)These assets earn a return.
[2]	(b)Under-recovered or over-recovered fuel costs to be recovered through the FAC. Specific accumulation periods aggregate the under-recovered or over-recovered costs over four months, any related adjustments that occur over the following four months, and the recovery from customers that occurs over the next eight months.
[3]	(c)Costs under- or over-recovered from utility customers. Amounts will be recovered from, or refunded to, customers within one year of the deferral.
[4]	(d)Deferral of commodity-related derivative MTM losses or gains. See Note 7 – Derivative Financial Instruments for additional information.
[5]	(e)The Ameren Missouri balance relates to the MEEIA. Beginning in January 2014, a MEEIA rider allowed Ameren Missouri to collect from or refund to customers any annual difference in the actual amounts incurred and the amounts collected from customers for the MEEIA program costs and its lost revenues. Under the MEEIA rider, collections from or refunds to customers occur one year after the program costs and lost revenues are incurred. The Ameren Illinois balance relates to a regulatory tracking mechanism to recover its electric and natural gas costs associated with developing, implementing, and evaluating customer energy efficiency and demand response programs. Any under-recovery or over-recovery will be collected from or refunded to customers over the 12 months following the plan year.
[6]	(f)The difference between Ameren Illinois' annual revenue requirement calculated under the IEIMA's performance-based formula ratemaking framework and the revenue requirement included in customer rates for that year. Subject to ICC approval, these amounts will be collected from or refunded to customers within two years.
[7]	(g)Ameren Illinois' and ATXI's annual revenue requirement reconciliation adjustments calculated pursuant to the FERC's electric transmission formula ratemaking framework. The under-recovery or over-recovery will be recovered from or refunded to customers within two years.
[8]	(h)These costs are being amortized in proportion to the recognition of prior service costs (credits) and actuarial losses (gains) attributable to Ameren’s pension plan and postretirement benefit plans. See Note 11 – Retirement Benefits for additional information.
[9]	(i)Offset to certain deferred tax liabilities for expected recovery of future income taxes when paid. This will be recovered over the expected life of the related assets.
[10]	(x)Unamortized portion of investment tax credits and federal excess deferred taxes. The unamortized portion of investment tax credits and the federal excess deferred taxes are being amortized over the expected life of the underlying assets.
[11]	(j)Recoverable or refundable removal costs for AROs, including net realized and unrealized gains and losses related to the nuclear decommissioning trust fund investments. See Note 1 – Summary of Significant Accounting Policies – Asset Retirement Obligations.
[12]	(k)Ameren Missouri’s Callaway energy center operations and maintenance expenses, property taxes, and carrying costs incurred between the plant in-service date and the date the plant was reflected in rates. These costs are being amortized over the remaining life of the energy center's current operating license, which expires in 2024.
[13]	(l)Losses related to reacquired debt. These amounts are being amortized over the lives of the related new debt issuances or the original lives of the old debt issuances if no new debt was issued.
[14]	(m)The recoverable portion of accrued environmental site liabilities that will be collected from electric and natural gas customers through ICC-approved cost recovery riders. The period of recovery will depend on the timing of remediation expenditures. See Note 15 – Commitments and Contingencies for additional information.
[15]	(n)Ameren Missouri's actual storm costs that exceed the normalized storm costs for rate purposes. As approved by the December 2012 MoPSC electric rate order, the 2006, 2007, and 2008 storm costs were amortized through December 2014. The Ameren Illinois balance includes 2013 storm costs deferred in accordance with the IEIMA. These costs are being amortized over a five-year period beginning in 2013.
[16]	(o)Demand-side costs incurred prior to implementation of the MEEIA in 2013, including the costs of developing, implementing and evaluating customer energy efficiency and demand response programs. Costs incurred from May 2008 through September 2008 are being amortized over a 10-year period that began in March 2009. Costs incurred from October 2008 through December 2009 are being amortized over a six-year period that began in July 2010. Costs incurred from January 2010 through February 2011 are being amortized over a six-year period that began in August 2011. Costs incurred from March 2011 through July 2012 are being amortized over a six-year period that began in January 2013.The amortization period for costs incurred from August 2012 through December 2012 will be determined in the July 2014 electric rate case.
[17]	(p)The period of recovery will depend on the timing of actual expenditures.
[18]	(q)Ameren Missouri’s costs incurred to enter into and maintain the 2012 Missouri Credit Agreement. Additional costs were incurred in December 2014 to amend and restate the 2012 Missouri Credit Agreement. These costs are being amortized over the life of the credit facility, ending in December 2019, to construction work in progress, which will be depreciated when assets are placed into service.
[19]	(r)The MoPSC’s May 2010 electric rate order allowed Ameren Missouri to recover its portion of Ameren’s September 2009 common stock issuance costs. These costs are being amortized over five years, beginning in July 2010.
[20]	(s)The MoPSC’s May 2010 electric rate order allowed Ameren Missouri to record an allowance for funds used during construction for pollution control equipment at its Sioux energy center until the cost of that equipment was included in customer rates. These costs will be amortized over the expected life of the Sioux energy center, which is currently through 2033.
[21]	(t)Costs associated with Ameren Missouri's solar rebate program beginning in August 2012 to fulfill its renewable energy portfolio requirement. The amortization period for these costs will be three years, commencing with the effectiveness of Ameren Missouri's current July 2014 electric rate case.
[22]	(u)The Ameren Illinois balance includes Ameren Illinois Merger integration and optimization costs, which are being amortized over four years, beginning in January 2012. The Ameren Illinois total also includes costs related to the 2013 natural gas delivery service rate case costs, which are being amortized over a two-year period that began in January 2014. At Ameren Missouri, the balance primarily includes the cost of renewable energy credits to fulfill its renewable energy portfolio requirement. Costs incurred from January 2010 through July 2012 are being amortized over three years, beginning in January 2013.
[23]	(ac)Balance includes the costs of renewable energy credits to fulfill Ameren Missouri's renewable energy portfolio requirement from August 2012 through December 2013, which were less than the amount included in rates. The balance also includes a regulatory tracking mechanism at Ameren Missouri for the difference between the level of storm costs incurred in a particular year and the level of such costs built into rates. The amortization periods for these over-recoveries will be determined in the July 2014 electric rate case.
[24]	(v)Estimated refund to wholesale electric customers as of December 31, 2013. See 2011 Wholesale Distribution Rate Case above.
[25]	(w)Estimated refunds to transmission customers related to FERC orders and audit findings. In regards to the FERC orders, see Ameren Illinois Electric Transmission Rate Refund and FERC Complaint Cases above.
[26]	(y)The tracker is amortized over three years, beginning from the date the amounts are included in rates. See Note 13 - Income Taxes for additional information.
[27]	(z)Estimated funds collected for the eventual dismantling and removal of plant from service, net of salvage value, upon retirement related to our rate-regulated operations.
[28]	(aa)A regulatory tracking mechanism for the difference between the level of bad debt incurred by Ameren Illinois under GAAP and the level of such costs included in electric and natural gas rates. The over-recovery relating to 2012 was refunded to customers from June 2013 through May 2014. The over-recovery relating to 2013 is being refunded to customers from June 2014 through May 2015. The over-recovery relating to 2014 will be refunded to customers from June 2015 through May 2016.
[29]	(ab)A regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs built into rates. For periods prior to August 2012, the MoPSC's December 2012 electric rate order directed the amortization to occur over five years, beginning in January 2013. For periods after August 2012, the amortization period will be determined in the July 2014 electric rate case.

Property_And_Plant_Net_Schedul

Property And Plant, Net (Schedule Of Property And Plant, Net) (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
In Millions, unless otherwise specified	equipment
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	26,198	[1]	$23,701	[1]
Accumulated depreciation and amortization	9,759	[1]	8,644	[1]
Property and plant, before construction work in progress	16,439	[1]	15,057	[1]
Property, Plant and Equipment, Net	17,424	[1]	16,205	[1]
Number of combustion turbine electric generation equipment under capital lease agreements	2
Number of capital lease agreements	2
Capital lease agreements, gross asset value	233		228
Total accumulated depreciation, capital lease agreements	66		56
Held-to-maturity Securities	294		299
Electric
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	23,913	[1]	21,726	[1]
Gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	2,285	[1]	1,975	[1]
Nuclear Fuel
Property, Plant and Equipment [Line Items]
Construction work in progress	209	[1]	246	[1]
Other Energy
Property, Plant and Equipment [Line Items]
Construction work in progress	776	[1]	902	[1]
Union Electric Company
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	17,483	[1]	16,377	[1]
Accumulated depreciation and amortization	7,086	[1]	6,766	[1]
Property and plant, before construction work in progress	10,397	[1]	9,611	[1]
Property, Plant and Equipment, Net	10,867	[1]	10,452	[1]
Union Electric Company | Electric
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	17,052	[1]	15,964	[1]
Union Electric Company | Gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	431	[1]	413	[1]
Union Electric Company | Nuclear Fuel
Property, Plant and Equipment [Line Items]
Construction work in progress	209	[1]	246	[1]
Union Electric Company | Other Energy
Property, Plant and Equipment [Line Items]
Construction work in progress	261	[1]	595	[1]
Ameren Illinois Company
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	8,371		6,988
Accumulated depreciation and amortization	2,422		1,627
Property and plant, before construction work in progress	5,949		5,361
Property, Plant and Equipment, Net	6,165		5,589
Ameren Illinois Company | Electric
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	6,517		5,426
Ameren Illinois Company | Gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	1,854		1,562
Ameren Illinois Company | Nuclear Fuel
Property, Plant and Equipment [Line Items]
Construction work in progress	0		0
Ameren Illinois Company | Other Energy
Property, Plant and Equipment [Line Items]
Construction work in progress	216		228
Other
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	344		336
Accumulated depreciation and amortization	251		251
Property and plant, before construction work in progress	93		85
Property, Plant and Equipment, Net	392		164
Other | Electric
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	344		336
Other | Gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	0		0
Other | Nuclear Fuel
Property, Plant and Equipment [Line Items]
Construction work in progress	0		0
Other | Other Energy
Property, Plant and Equipment [Line Items]
Construction work in progress	299		$79
[1]	(a)Amounts in Ameren and Ameren Missouri include two CTs under separate capital lease agreements. The gross cumulative asset value of those agreements was $233 million and $228 million at December 31, 2014 and 2013, respectively. The total accumulated depreciation associated with the two CTs was $66 million and $56 million at December 31, 2014 and 2013, respectively. In addition, Ameren Missouri has investments in debt securities, which were classified as held-to-maturity, related to the two CTs from the city of Bowling Green and Audrain County. As of December 31, 2014 and 2013, the carrying value of these debt securities was $294 million and $299 million, respectively.

Property_And_Plant_Net_Accrued

Property And Plant, Net (Accrued Capital Expenditures) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	$181	[1]	$175	[1]	$107	[1]
Union Electric Company
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	72		74		63
Ameren Illinois Company
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	59		86		37
Nuclear Fuel
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	13	[1]	8	[1]	8	[1]
Nuclear Fuel | Union Electric Company
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	$13		$8		$8
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries.

ShortTerm_Debt_And_Liquidity_N

Short-Term Debt And Liquidity (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Illinois Credit Agreement 2012 | Maximum
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	1,300,000,000
Illinois Credit Agreement 2012 | Ameren Illinois Company
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	800,000,000
Actual debt-to-capital ratio	0.47
Missouri Credit Agreement 2012 | Maximum
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	1,200,000,000
Missouri Credit Agreement 2012 | Union Electric Company
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	800,000,000
Actual debt-to-capital ratio	0.49
Credit Agreements 2012
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	1,400,000,000
Covenant terms, default provisions, maximum indebtedness	75,000,000
Multiyear Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	2,100,000,000
Number of lenders	24
Line of credit facility, maximum borrowing capacity, per lender	115,000,000
Actual debt-to-capital ratio	0.5
Minimum ratio of consolidated funds from operations plus interest expense to consolidated interest expense as of balance sheet date	2.0 to 1.0
Letters of credit portion of aggregate commitment	25.00%
Line of Credit Facility, Commitment Fee Amount	100,000,000
Multiyear Credit Facility | Maximum
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.65
Utilities
Line of Credit Facility [Line Items]
Short Term Debt, Weighted Average Interest Rate During Period	0.19%	0.14%
New Ameren Energy Resources Company, LLC
Line of Credit Facility [Line Items]
Line of Credit Facility, Commitment Fee Amount	9,000,000

ShortTerm_Debt_And_Liquidity_S

Short-Term Debt And Liquidity (Schedule Of Maximum Aggregate Amount Available On Credit Agreements) (Details) (USD $)	Dec. 31, 2014
Illinois Credit Agreement 2012 | Parent Company
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$500,000,000
Illinois Credit Agreement 2012 | Ameren Illinois Company
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	800,000,000
Missouri Credit Agreement 2012 | Parent Company
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	700,000,000
Missouri Credit Agreement 2012 | Union Electric Company
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$800,000,000

ShortTerm_Debt_And_Liquidity_S1

Short-Term Debt And Liquidity Short-Term Debt And Liquidity (Commercial Paper) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Commercial Paper
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$639	$54
Commercial paper outstanding	714	368
Weighted average interest rate	0.36%	0.56%
Peak short-term borrowings	910	368
Peak short-term borrowings interest rate	0.75%	0.85%
Union Electric Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	110	0
Weighted average interest rate	0.38%	0.00%
Peak short-term borrowings	495	0
Peak short-term borrowings interest rate	0.70%	0.00%
Ameren Illinois Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	165	0
Weighted average interest rate	0.32%	0.00%
Peak short-term borrowings	300	0
Peak short-term borrowings interest rate	0.60%	0.00%
Ameren Illinois Company | Commercial Paper
Line of Credit Facility [Line Items]
Commercial paper outstanding	32	0
Ameren Missouri [Member] | Commercial Paper
Line of Credit Facility [Line Items]
Commercial paper outstanding	97	0
Ameren (parent) [Domain]
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	423	54
Weighted average interest rate	0.36%	0.56%
Peak short-term borrowings	625	368
Peak short-term borrowings interest rate	0.75%	0.85%
Ameren (parent) [Domain] | Commercial Paper
Line of Credit Facility [Line Items]
Commercial paper outstanding	$585	$368

LongTerm_Debt_And_Equity_Finan2

Long-Term Debt And Equity Financings (Narrative) (Details) (USD $)	12 Months Ended					1 Months Ended				3 Months Ended		1 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012	Jan. 31, 2014		31-May-14	Apr. 30, 2014	Dec. 31, 2013		Jun. 30, 2014	Dec. 31, 2014		Oct. 31, 2013	15-May-14
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized	100,000,000												100,000,000
Preferred stock, par value	$0.01												$0.01
Preferred stock, shares outstanding	0												0
Common stock, shares authorized	400,000,000		400,000,000							400,000,000			400,000,000
Repayments of Other Long-term Debt	$697,000,000		$399,000,000		$760,000,000
Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized	7,500,000												7,500,000
Preferred stock, par value	$1												$1
Common stock, shares authorized	150,000,000		150,000,000							150,000,000			150,000,000
Repayments of Other Long-term Debt	109,000,000		249,000,000		427,000,000
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized	2,600,000												2,600,000
Common stock, shares authorized	45,000,000		45,000,000							45,000,000			45,000,000
Repayments of Other Long-term Debt	163,000,000		150,000,000		333,000,000
Debt instrument face amount						1,000,000	[1]
Common stock equity to capitalization ratio	53.00%												53.00%
Ameren Illinois Company | Environmental Improvement And Pollution Control Revenue Bonds
Long-Term Debt And Equity Financings [Line Items]
Redemptions of long-term debt						163,000,000
Parent Company
Long-Term Debt And Equity Financings [Line Items]
Common stock, shares authorized								8,600,000							4,000,000
Repayments of Other Long-term Debt	425,000,000		0		0
Ameren Missouri and Ameren Illinois
Long-Term Debt And Equity Financings [Line Items]
Bonds interest rate assumption	5.00%												5.00%
Dividend rate on preferred shares, percentage	6.00%
Series1993570 Due2024 [Member] | Ameren Illinois Company | Environmental Improvement And Pollution Control Revenue Bonds
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	1,000,000	[2]											1,000,000	[2]
Senior Unsecured Notes8875 Due2014 [Member] | Parent Company
Long-Term Debt And Equity Financings [Line Items]
Repayments of Other Long-term Debt								425,000,000
Debt instrument face amount			425,000,000							425,000,000
Long-term debt interest rate	8.88%							8.88%					8.88%
Senior Secured Notes, 3.50%, Due 2024 [Member] | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount									350,000,000
Long-term debt interest rate									3.50%
Proceeds from issuance of secured debt									348,000,000
5.50% Senior secured notes due 2014 | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	0	[3]	104,000,000	[3]						104,000,000	[3]		0	[3]
Long-term debt interest rate	5.50%												5.50%			5.50%
Redemptions of long-term debt								104,000,000
Senior Secured Notes 4.80% Due 2043 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	280,000,000	[4]	280,000,000	[4]						280,000,000	[4]		280,000,000	[4]
Long-term debt interest rate	4.80%		4.80%							4.80%			4.80%
Proceeds from issuance of secured debt										276,000,000
Senior Secured Notes 8.875% Due 2013 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate			8.88%							8.88%
Redemptions of long-term debt										150,000,000
1993 5.45% Series due 2028 | Union Electric Company | Environmental Improvement And Pollution Control Revenue Bonds
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	1,000,000	[2]											1,000,000	[2]
Long-term debt interest rate	5.45%												5.45%		5.45%
Redemptions of long-term debt										44,000,000
4.65% Senior secured notes due 2013 | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate															4.65%
Redemptions of long-term debt										200,000,000
Senior Secured Notes, 4.30%, Due 2044 [Member] [Member] | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount												250,000,000
Long-term debt interest rate												4.30%
Proceeds from issuance of secured debt												246,000,000
Senior Secured Notes, 3 Point 25, Due 2025 [Member] | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	300,000,000	[4],[5]											300,000,000	[4],[5]
Long-term debt interest rate	3.25%												3.25%
Proceeds from issuance of secured debt													298,000,000
Senior Secured Notes, 2.70%, Due 2022 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	400,000,000	[4],[5]	400,000,000	[4],[5]						400,000,000	[4],[5]		400,000,000	[4],[5]
Long-term debt interest rate	2.70%												2.70%
3.90% Senior secured notes due 2042 | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	485,000,000	[3],[6]	485,000,000	[3],[6]						485,000,000	[3],[6]		485,000,000	[3],[6]
Long-term debt interest rate	3.90%												3.90%
Series 1993 5.90% Due 2023 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	1,000,000	[2]											1,000,000	[2]
Long-term debt interest rate	5.90%												5.90%
Series 1993 5.90% Due 2023 | Ameren Illinois Company | Environmental Improvement And Pollution Control Revenue Bonds
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	1,000,000	[2],[7]	32,000,000	[7]						32,000,000	[7]		1,000,000	[2],[7]
Long-term debt interest rate	5.90%												5.90%
Redemptions of long-term debt						$32,000,000	[8]
Minimum | Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Common stock equity to capitalization ratio	30.00%												30.00%
[1]	These bonds are mortgage bonds issued by Ameren Illinois under the Ameren Illinois mortgage indenture and are secured by substantially all property of the former IP and CIPS. The bonds are callable at 100% of par value. The bonds are also backed by an insurance guarantee policy. Less than $1 million principal amount of the bonds remain outstanding.
[2]	These bonds are first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage bond indenture and are secured by substantially all Ameren Missouri property and franchises. The bonds are callable at 100% of par value. Less than $1 million principal amount of the bonds remain outstanding.
[3]	These notes are collaterally secured by first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage indenture. The notes have a fall-away lien provision and will remain secured only as long as any first mortgage bonds issued under the Ameren Missouri mortgage indenture remain outstanding. Redemption, purchase, or maturity of all first mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the first mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the Ameren Missouri senior secured notes currently outstanding, we do not expect the first mortgage bond lien protection associated with these notes to fall away until 2042.
[4]	These notes are collaterally secured by mortgage bonds issued by Ameren Illinois under the Ameren Illinois mortgage indenture. The notes have a fall-away lien provision and will remain secured only as long as any series of first mortgage bonds issued under the Ameren Illinois mortgage indenture remain outstanding. Redemption, purchase, or maturity of all mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the Ameren Illinois senior secured notes currently outstanding, we do not expect the mortgage bond lien protection associated with these notes to fall away until 2024.
[5]	Ameren Illinois has agreed, during the life of these notes, not to optionally redeem, purchase, or otherwise retire in full its Ameren Illinois mortgage bonds; therefore, an Ameren Illinois first mortgage bond release date will not occur as long as any of these notes are outstanding.
[6]	Ameren Missouri has agreed, during the life of these notes, not to optionally redeem, purchase or otherwise retire in full its first mortgage bonds. Ameren Missouri has also agreed to prevent a first mortgage bond release date from occurring as long as any of the 8.45% senior secured notes due 2039 and any of the 3.90% senior secured notes due 2042 remain outstanding.
[7]	These bonds are first mortgage bonds issued by Ameren Illinois under the CILCO mortgage indenture and are secured by substantially all property of the former CILCO. The bonds are callable at 100% of par value. Less than $1 million principal amount of the bonds remain outstanding.
[8]	Less than $1 million principal amount of the bonds remain outstanding after redemption.

LongTerm_Debt_And_Equity_Finan3

Long-Term Debt And Equity Financings (Schedule Of Long-Term Debt Outstanding) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		31-May-14	Jan. 31, 2014		Oct. 31, 2013	15-May-14
Debt Instrument [Line Items]
Less: Maturities due within one year	($120,000,000)		($534,000,000)
Long-term Debt, Net	6,120,000,000		5,504,000,000
Parent Company
Debt Instrument [Line Items]
Less: Maturities due within one year	0		-425,000,000
Parent Company | 8.875% Senior unsecured notes due 2014
Debt Instrument [Line Items]
Debt instrument face amount			425,000,000
Long-term debt interest rate	8.88%				8.88%
Long-term debt maturity date	2014
Union Electric Company
Debt Instrument [Line Items]
Long-term debt, gross	4,005,000,000		3,764,000,000
Less: Unamortized discount and premium	6,000,000		7,000,000
Less: Maturities due within one year	-120,000,000		-109,000,000
Long-term Debt, Net	3,879,000,000		3,648,000,000
Union Electric Company | City Of Bowling Green Capital Lease Peno Creek Ct
Debt Instrument [Line Items]
Capital lease obligations	54,000,000		59,000,000
Long-term debt maturity date	2022
Union Electric Company | Audrain County Capital Lease Audrain County Ct
Debt Instrument [Line Items]
Capital lease obligations	240,000,000		240,000,000
Long-term debt maturity date	2023
Ameren Illinois Company
Debt Instrument [Line Items]
Debt instrument face amount						1,000,000	[1]
Fair-market value adjustments			4,000,000
Long-term debt, gross	2,246,000,000		1,863,000,000
Less: Unamortized discount and premium	5,000,000		7,000,000
Less: Maturities due within one year			0
Long-term Debt, Net	2,241,000,000		1,856,000,000
Secured Debt | Union Electric Company | 4.65% Senior secured notes due 2013
Debt Instrument [Line Items]
Long-term debt interest rate								4.65%
Secured Debt | Union Electric Company | 5.50% Senior secured notes due 2014
Debt Instrument [Line Items]
Debt instrument face amount	0	[2]	104,000,000	[2]
Long-term debt interest rate	5.50%								5.50%
Long-term debt maturity date	2014
Secured Debt | Union Electric Company | 4.75% Senior secured notes due 2015
Debt Instrument [Line Items]
Debt instrument face amount	114,000,000	[2]	114,000,000	[2]
Long-term debt interest rate	4.75%
Long-term debt maturity date	2015
Secured Debt | Union Electric Company | 5.40% Senior secured notes due 2016
Debt Instrument [Line Items]
Debt instrument face amount	260,000,000	[2]	260,000,000	[2]
Long-term debt interest rate	5.40%
Long-term debt maturity date	2016
Secured Debt | Union Electric Company | 6.40% Senior secured notes due 2017
Debt Instrument [Line Items]
Debt instrument face amount	425,000,000	[2]	425,000,000	[2]
Long-term debt interest rate	6.40%
Long-term debt maturity date	2017
Secured Debt | Union Electric Company | 6.00% Senior secured notes due 2018
Debt Instrument [Line Items]
Debt instrument face amount	179,000,000	[2],[3]	179,000,000	[2],[3]
Long-term debt interest rate	6.00%
Long-term debt maturity date	2018
Secured Debt | Union Electric Company | 5.10% Senior secured notes due 2018
Debt Instrument [Line Items]
Debt instrument face amount	199,000,000	[2]	199,000,000	[2]
Long-term debt interest rate	5.10%
Long-term debt maturity date	2018
Secured Debt | Union Electric Company | 6.70% Senior secured notes due 2019
Debt Instrument [Line Items]
Debt instrument face amount	329,000,000	[2],[3]	329,000,000	[2],[3]
Long-term debt interest rate	6.70%
Long-term debt maturity date	2019
Secured Debt | Union Electric Company | 5.10% Senior secured notes due 2019
Debt Instrument [Line Items]
Debt instrument face amount	244,000,000	[2]	244,000,000	[2]
Long-term debt interest rate	5.10%
Long-term debt maturity date	2019
Secured Debt | Union Electric Company | 5.00% Senior secured notes due 2020
Debt Instrument [Line Items]
Debt instrument face amount	85,000,000	[2]	85,000,000	[2]
Long-term debt interest rate	5.00%
Long-term debt maturity date	2020
Secured Debt | Union Electric Company | Senior Secured Notes350 Due2024 [Member]
Debt Instrument [Line Items]
Debt instrument face amount	350,000,000	[2]
Long-term debt interest rate	3.50%
Long-term debt maturity date	2024
Secured Debt | Union Electric Company | 5.50% Senior secured notes due 2034
Debt Instrument [Line Items]
Debt instrument face amount	184,000,000	[2]	184,000,000	[2]
Long-term debt interest rate	5.50%
Long-term debt maturity date	2034
Secured Debt | Union Electric Company | 5.30% Senior secured notes due 2037
Debt Instrument [Line Items]
Debt instrument face amount	300,000,000	[2]	300,000,000	[2]
Long-term debt interest rate	5.30%
Long-term debt maturity date	2037
Secured Debt | Union Electric Company | 8.45% Senior secured notes due 2039
Debt Instrument [Line Items]
Debt instrument face amount	350,000,000	[2],[3]	350,000,000	[2],[3]
Long-term debt interest rate	8.45%
Long-term debt maturity date	2039
Secured Debt | Union Electric Company | 3.90% Senior secured notes due 2042
Debt Instrument [Line Items]
Debt instrument face amount	485,000,000	[2],[3]	485,000,000	[2],[3]
Long-term debt interest rate	3.90%
Long-term debt maturity date	2042
Secured Debt | Ameren Illinois Company | Senior Secured Notes 8.875% Due 2013
Debt Instrument [Line Items]
Long-term debt interest rate			8.88%
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.20% Due 2016
Debt Instrument [Line Items]
Debt instrument face amount	54,000,000	[4]	54,000,000	[4]
Long-term debt interest rate	6.20%
Long-term debt maturity date	2016
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.25% Due 2016
Debt Instrument [Line Items]
Debt instrument face amount	75,000,000	[5]	75,000,000	[5]
Long-term debt interest rate	6.25%
Long-term debt maturity date	2016
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.125% Due 2017
Debt Instrument [Line Items]
Debt instrument face amount	250,000,000	[5],[6]	250,000,000	[5],[6]
Long-term debt interest rate	6.13%
Long-term debt maturity date	2017
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.25% Due 2018
Debt Instrument [Line Items]
Debt instrument face amount	144,000,000	[5],[6]	144,000,000	[5],[6]
Long-term debt interest rate	6.25%
Long-term debt maturity date	2018
Secured Debt | Ameren Illinois Company | Senior Secured Notes 9.75% Due 2018
Debt Instrument [Line Items]
Debt instrument face amount	313,000,000	[5],[6]	313,000,000	[5],[6]
Long-term debt interest rate	9.75%
Long-term debt maturity date	2018
Secured Debt | Ameren Illinois Company | Senior Secured Notes, 2.70%, Due 2022
Debt Instrument [Line Items]
Debt instrument face amount	400,000,000	[5],[6]	400,000,000	[5],[6]
Long-term debt interest rate	2.70%
Long-term debt maturity date	2022
Secured Debt | Ameren Illinois Company | Senior Secured Notes, 3 Point 25, Due 2025 [Member]
Debt Instrument [Line Items]
Debt instrument face amount	300,000,000	[5],[6]
Long-term debt interest rate	3.25%
Long-term debt maturity date	2025
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.125% Due 2028
Debt Instrument [Line Items]
Debt instrument face amount	60,000,000	[5]	60,000,000	[5]
Long-term debt interest rate	6.13%
Long-term debt maturity date	2028
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.70% Due 2036
Debt Instrument [Line Items]
Debt instrument face amount	61,000,000	[5]	61,000,000	[5]
Long-term debt interest rate	6.70%
Long-term debt maturity date	2036
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.70% Due 2036
Debt Instrument [Line Items]
Debt instrument face amount	42,000,000	[4]	42,000,000	[4]
Long-term debt interest rate	6.70%
Long-term debt maturity date	2036
Secured Debt | Ameren Illinois Company | Senior Secured Notes 4.80% Due 2043
Debt Instrument [Line Items]
Debt instrument face amount	280,000,000	[5]	280,000,000	[5]
Long-term debt interest rate	4.80%		4.80%
Long-term debt maturity date	2043
Secured Debt | Ameren Illinois Company | Senior Secured Notes430 Due2044 [Member]
Debt Instrument [Line Items]
Debt instrument face amount	250,000,000	[5]
Long-term debt interest rate	4.30%
Long-term debt maturity date	2044
Secured Debt | Ameren Illinois Company | Series 1993 5.90% Due 2023
Debt Instrument [Line Items]
Debt instrument face amount	1,000,000	[7]
Long-term debt interest rate	5.90%
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1992 Series due 2022
Debt Instrument [Line Items]
Debt instrument face amount	47,000,000	[8],[9]	47,000,000	[8],[9]
Long-term debt maturity date	2022
Debt instrument, interest rate, maximum	18.00%
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1993 5.45% Series due 2028
Debt Instrument [Line Items]
Debt instrument face amount	1,000,000	[7]
Long-term debt interest rate	5.45%							5.45%
Long-term debt maturity date	2028
Redemption price, percentage	100.00%
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1998 Series A due 2033
Debt Instrument [Line Items]
Debt instrument face amount	60,000,000	[8],[9]	60,000,000	[8],[9]
Long-term debt maturity date	2033
Debt instrument, interest rate, maximum	18.00%
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1998 Series B due 2033
Debt Instrument [Line Items]
Debt instrument face amount	50,000,000	[8],[9]	50,000,000	[8],[9]
Long-term debt maturity date	2033
Debt instrument, interest rate, maximum	18.00%
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1998 Series C due 2033
Debt Instrument [Line Items]
Debt instrument face amount	50,000,000	[8],[9]	50,000,000	[8],[9]
Long-term debt maturity date	2033
Debt instrument, interest rate, maximum	18.00%
Environmental Improvement And Pollution Control Revenue Bonds | Ameren Illinois Company | Series 1993 5.90% Due 2023
Debt Instrument [Line Items]
Debt instrument face amount	1,000,000	[10],[7]	32,000,000	[10]
Long-term debt interest rate	5.90%
Long-term debt maturity date	2023
Redemption price, percentage	100.00%
Environmental Improvement And Pollution Control Revenue Bonds | Ameren Illinois Company | Series 1994 A 5.70% Due 2024
Debt Instrument [Line Items]
Debt instrument face amount	1,000,000	[1]	36,000,000	[1]
Long-term debt interest rate	5.70%
Long-term debt maturity date	2024
Redemption price, percentage	100.00%
Environmental Improvement And Pollution Control Revenue Bonds | Ameren Illinois Company | Series C-1 1993 5.95% Due 2026
Debt Instrument [Line Items]
Debt instrument face amount			35,000,000	[11]
Long-term debt interest rate	5.95%
Long-term debt maturity date	2026
Redemption price, percentage	100.00%
Environmental Improvement And Pollution Control Revenue Bonds | Ameren Illinois Company | Series C-2 1993 5.70% Due 2026
Debt Instrument [Line Items]
Debt instrument face amount			8,000,000	[11]
Long-term debt interest rate	5.70%
Long-term debt maturity date	2026
Redemption price, percentage	100.00%
Environmental Improvement And Pollution Control Revenue Bonds | Ameren Illinois Company | Series B-1 1993 Due 2028
Debt Instrument [Line Items]
Debt instrument face amount	17,000,000	[11],[8]	17,000,000	[11],[8]
Long-term debt maturity date	2028
Redemption price, percentage	100.00%
Environmental Improvement And Pollution Control Revenue Bonds | Ameren Illinois Company | Series1998A 5.40% Due 2028
Debt Instrument [Line Items]
Debt instrument face amount			19,000,000	[1]
Long-term debt interest rate	5.40%
Long-term debt maturity date	2028
Redemption price, percentage	100.00%
Environmental Improvement And Pollution Control Revenue Bonds | Ameren Illinois Company | Series1998B 5.40% Due 2028
Debt Instrument [Line Items]
Debt instrument face amount			$33,000,000	[1]
Long-term debt interest rate	5.40%
Long-term debt maturity date	2028
Redemption price, percentage	100.00%
[1]	These bonds are mortgage bonds issued by Ameren Illinois under the Ameren Illinois mortgage indenture and are secured by substantially all property of the former IP and CIPS. The bonds are callable at 100% of par value. The bonds are also backed by an insurance guarantee policy. Less than $1 million principal amount of the bonds remain outstanding.
[2]	These notes are collaterally secured by first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage indenture. The notes have a fall-away lien provision and will remain secured only as long as any first mortgage bonds issued under the Ameren Missouri mortgage indenture remain outstanding. Redemption, purchase, or maturity of all first mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the first mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the Ameren Missouri senior secured notes currently outstanding, we do not expect the first mortgage bond lien protection associated with these notes to fall away until 2042.
[3]	Ameren Missouri has agreed, during the life of these notes, not to optionally redeem, purchase or otherwise retire in full its first mortgage bonds. Ameren Missouri has also agreed to prevent a first mortgage bond release date from occurring as long as any of the 8.45% senior secured notes due 2039 and any of the 3.90% senior secured notes due 2042 remain outstanding.
[4]	hese notes are collaterally secured by first mortgage bonds issued by Ameren Illinois under the CILCO mortgage indenture. The notes have a fall-away lien provision, and Ameren Illinois could cause these notes to become unsecured at any time by redeeming the pollution control bonds 5.90% Series 1993 due 2023 (of which less than $1 million remains outstanding). Ameren Illinois may resecure these notes if it chooses.
[5]	These notes are collaterally secured by mortgage bonds issued by Ameren Illinois under the Ameren Illinois mortgage indenture. The notes have a fall-away lien provision and will remain secured only as long as any series of first mortgage bonds issued under the Ameren Illinois mortgage indenture remain outstanding. Redemption, purchase, or maturity of all mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the Ameren Illinois senior secured notes currently outstanding, we do not expect the mortgage bond lien protection associated with these notes to fall away until 2024.
[6]	Ameren Illinois has agreed, during the life of these notes, not to optionally redeem, purchase, or otherwise retire in full its Ameren Illinois mortgage bonds; therefore, an Ameren Illinois first mortgage bond release date will not occur as long as any of these notes are outstanding.
[7]	These bonds are first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage bond indenture and are secured by substantially all Ameren Missouri property and franchises. The bonds are callable at 100% of par value. Less than $1 million principal amount of the bonds remain outstanding.
[8]	The interest rates, and the periods during which such rates apply, vary depending on our selection of defined rate modes. Maximum interest rates could reach 18% depending on the series of bonds. The average interest rates for 2014 and 2013 were as follows: 2014 2013Ameren Missouri 1992 Series due 20220.10% 0.17%Ameren Missouri 1998 Series A due 20330.26% 0.34%Ameren Missouri 1998 Series B due 20330.27% 0.33%Ameren Missouri 1998 Series C due 20330.26% 0.34%Ameren Illinois 1993 Series B-1 due 20280.21% 0.14%
[9]	These bonds are collaterally secured by first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage indenture and have a fall-away lien provision similar to that of Ameren Missouri's senior secured notes. The bonds are also backed by an insurance guarantee policy.
[10]	These bonds are first mortgage bonds issued by Ameren Illinois under the CILCO mortgage indenture and are secured by substantially all property of the former CILCO. The bonds are callable at 100% of par value. Less than $1 million principal amount of the bonds remain outstanding.
[11]	The bonds are callable at 100% of par value.

LongTerm_Debt_And_Equity_Finan4

Long-Term Debt And Equity Financings (Schedule Of Average Interest Rates) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
1992 Series due 2022 | Union Electric Company
Debt Instrument [Line Items]
Debt instrument, interest rate during period	0.10%	0.17%
1998 Series A due 2033 | Union Electric Company
Debt Instrument [Line Items]
Debt instrument, interest rate during period	0.26%	0.34%
1998 Series B due 2033 | Union Electric Company
Debt Instrument [Line Items]
Debt instrument, interest rate during period	0.27%	0.33%
1998 Series C due 2033 | Union Electric Company
Debt Instrument [Line Items]
Debt instrument, interest rate during period	0.26%	0.34%
Series B-1 1993 Due 2028 | Ameren Illinois Company
Debt Instrument [Line Items]
Debt instrument, interest rate during period	0.21%	0.14%
Secured Debt | 3.90% Senior secured notes due 2042 | Union Electric Company
Debt Instrument [Line Items]
Long-term debt interest rate	3.90%

LongTerm_Debt_And_Equity_Finan5

Long-Term Debt And Equity Financings (Schedule Of Maturities Of Long-Term Debt) (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Debt Instrument [Line Items]
2015	$120
2016	395
2017	681
2018	840
2019	581
Thereafter	3,634
Total	6,251
Union Electric Company
Debt Instrument [Line Items]
2015	120	[1]
2016	266	[1]
2017	431	[1]
2018	383	[1]
2019	581	[1]
Thereafter	2,224	[1]
Total	4,005	[1]
Unamortized discount and premium	6		7
Ameren Illinois Company
Debt Instrument [Line Items]
2016	129	[1]
2017	250	[1]
2018	457	[1]
Thereafter	1,410	[1]
Total	2,246	[1]
Unamortized discount and premium	$5		$7
[1]	Excludes unamortized discount and premium of $6 million and $5 million at Ameren Missouri and Ameren Illinois, respectively.

LongTerm_Debt_And_Equity_Finan6

Long-Term Debt And Equity Financings (Schedule Of Outstanding Preferred Stock) (Details) (USD $)	12 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, par value	$0.01
Preferred stock, authorized	100,000,000
Preferred stock, shares outstanding	0
Preferred stock, issued	$142		$142
Preferred stock, voluntary liquidation	$106
Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, par value	$1
Preferred stock, authorized	7,500,000
Preferred stock, issued	80		80
Union Electric Company | Par Value $100 [Member]
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, par value	$100
Preferred stock, authorized	25,000,000
Union Electric Company | $3.50 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount	$3.50
Preferred stock, shares outstanding	130,000
Preferred stock, redemption price per share	$110
Preferred stock, issued	13		13
Union Electric Company | $3.70 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount	$3.70
Preferred stock, shares outstanding	40,000
Preferred stock, redemption price per share	$105
Preferred stock, issued	4		4
Union Electric Company | $4.00 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount	$4
Preferred stock, shares outstanding	150,000
Preferred stock, redemption price per share	$106
Preferred stock, issued	15		15
Union Electric Company | $4.30 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount	$4.30
Preferred stock, shares outstanding	40,000
Preferred stock, redemption price per share	$105
Preferred stock, issued	4		4
Union Electric Company | $4.50 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount	$4.50
Preferred stock, shares outstanding	213,595
Preferred stock, redemption price per share	$110	[1]
Preferred stock, issued	21		21
Union Electric Company | $4.56 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount	$4.56
Preferred stock, shares outstanding	200,000
Preferred stock, redemption price per share	$102
Preferred stock, issued	20		20
Union Electric Company | $4.75 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount	$4.75
Preferred stock, shares outstanding	20,000
Preferred stock, redemption price per share	$102
Preferred stock, issued	2		2
Union Electric Company | $5.50 Series A
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount	$5.50
Preferred stock, shares outstanding	14,000
Preferred stock, redemption price per share	$110
Preferred stock, issued	1		1
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized	2,600,000
Preferred stock, issued	62		62
Ameren Illinois Company | Par Value $100 [Member]
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, par value	$100
Preferred stock, authorized	2,000,000
Ameren Illinois Company | 4.00% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage	4.00%
Preferred stock, shares outstanding	144,275
Preferred stock, redemption price per share	$101
Preferred stock, issued	14		14
Ameren Illinois Company | 4.08% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage	4.08%
Preferred stock, shares outstanding	45,224
Preferred stock, redemption price per share	$103
Preferred stock, issued	5		5
Ameren Illinois Company | 4.20% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage	4.20%
Preferred stock, shares outstanding	23,655
Preferred stock, redemption price per share	$104
Preferred stock, issued	2		2
Ameren Illinois Company | 4.25% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage	4.25%
Preferred stock, shares outstanding	50,000
Preferred stock, redemption price per share	$102
Preferred stock, issued	5		5
Ameren Illinois Company | 4.26% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage	4.26%
Preferred stock, shares outstanding	16,621
Preferred stock, redemption price per share	$103
Preferred stock, issued	2		2
Ameren Illinois Company | 4.42% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage	4.42%
Preferred stock, shares outstanding	16,190
Preferred stock, redemption price per share	$103
Preferred stock, issued	2		2
Ameren Illinois Company | 4.70% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage	4.70%
Preferred stock, shares outstanding	18,429
Preferred stock, redemption price per share	$103
Preferred stock, issued	2		2
Ameren Illinois Company | 4.90% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage	4.90%
Preferred stock, shares outstanding	73,825
Preferred stock, redemption price per share	$102
Preferred stock, issued	7		7
Ameren Illinois Company | 4.92% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage	4.92%
Preferred stock, shares outstanding	49,289
Preferred stock, redemption price per share	$104
Preferred stock, issued	5		5
Ameren Illinois Company | 5.16% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage	5.16%
Preferred stock, shares outstanding	50,000
Preferred stock, redemption price per share	$102
Preferred stock, issued	5		5
Ameren Illinois Company | 6.625% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage	6.63%
Preferred stock, shares outstanding	124,274
Preferred stock, redemption price per share	$100
Preferred stock, issued	12		12
Ameren Illinois Company | 7.75% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage	7.75%
Preferred stock, shares outstanding	4,542
Preferred stock, redemption price per share	$100
Preferred stock, issued	$1		$1
[1]	In the event of voluntary liquidation, $105.50

Recovered_Sheet1

Long-Term Debt and Equity Financings (Schedule of Debt Redemptions) (Details) (Ameren Illinois Company, USD $)	1 Months Ended
	Jan. 31, 2014		Dec. 31, 2014		Dec. 31, 2013
Debt Instrument [Line Items]
Debt instrument face amount	$1,000,000	[1]
Secured Debt | Senior Secured Notes 9.75% Due 2018
Debt Instrument [Line Items]
Debt instrument face amount			313,000,000	[2],[3]	313,000,000	[2],[3]
Secured Debt | Series 1993 5.90% Due 2023
Debt Instrument [Line Items]
Debt instrument face amount			1,000,000	[4]
Secured Debt | Senior Secured Notes 6.25% Due 2018
Debt Instrument [Line Items]
Debt instrument face amount			144,000,000	[2],[3]	144,000,000	[2],[3]
Environmental Improvement And Pollution Control Revenue Bonds
Debt Instrument [Line Items]
Redemptions of long-term debt	163,000,000
Environmental Improvement And Pollution Control Revenue Bonds | Series 1993 5.90% Due 2023
Debt Instrument [Line Items]
Redemptions of long-term debt	32,000,000	[5]
Debt instrument face amount			1,000,000	[4],[6]	32,000,000	[6]
Environmental Improvement And Pollution Control Revenue Bonds | Series 1994 A 5.70% Due 2024
Debt Instrument [Line Items]
Redemptions of long-term debt	36,000,000	[5]
Debt instrument face amount			1,000,000	[1]	36,000,000	[1]
Environmental Improvement And Pollution Control Revenue Bonds | Series C-1 1993 5.95% Due 2026
Debt Instrument [Line Items]
Redemptions of long-term debt	35,000,000
Debt instrument face amount					35,000,000	[7]
Environmental Improvement And Pollution Control Revenue Bonds | Series C-2 1993 5.70% Due 2026
Debt Instrument [Line Items]
Redemptions of long-term debt	8,000,000
Debt instrument face amount					8,000,000	[7]
Environmental Improvement And Pollution Control Revenue Bonds | Series1998A 5.40% Due 2028
Debt Instrument [Line Items]
Redemptions of long-term debt	19,000,000
Debt instrument face amount					19,000,000	[1]
Environmental Improvement And Pollution Control Revenue Bonds | Series1998B 5.40% Due 2028
Debt Instrument [Line Items]
Redemptions of long-term debt	33,000,000
Debt instrument face amount					$33,000,000	[1]
[1]	These bonds are mortgage bonds issued by Ameren Illinois under the Ameren Illinois mortgage indenture and are secured by substantially all property of the former IP and CIPS. The bonds are callable at 100% of par value. The bonds are also backed by an insurance guarantee policy. Less than $1 million principal amount of the bonds remain outstanding.
[2]	These notes are collaterally secured by mortgage bonds issued by Ameren Illinois under the Ameren Illinois mortgage indenture. The notes have a fall-away lien provision and will remain secured only as long as any series of first mortgage bonds issued under the Ameren Illinois mortgage indenture remain outstanding. Redemption, purchase, or maturity of all mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the Ameren Illinois senior secured notes currently outstanding, we do not expect the mortgage bond lien protection associated with these notes to fall away until 2024.
[3]	Ameren Illinois has agreed, during the life of these notes, not to optionally redeem, purchase, or otherwise retire in full its Ameren Illinois mortgage bonds; therefore, an Ameren Illinois first mortgage bond release date will not occur as long as any of these notes are outstanding.
[4]	These bonds are first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage bond indenture and are secured by substantially all Ameren Missouri property and franchises. The bonds are callable at 100% of par value. Less than $1 million principal amount of the bonds remain outstanding.
[5]	Less than $1 million principal amount of the bonds remain outstanding after redemption.
[6]	These bonds are first mortgage bonds issued by Ameren Illinois under the CILCO mortgage indenture and are secured by substantially all property of the former CILCO. The bonds are callable at 100% of par value. Less than $1 million principal amount of the bonds remain outstanding.
[7]	The bonds are callable at 100% of par value.

Recovered_Sheet2

Long-Term Debt and Equity Financings (Schedule of Required and Actual Debt Ratios) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014
Union Electric Company
Debt Instrument [Line Items]
Bonds Issuable Based On Coverage Ratio	3,605	[1]
Preferred Stock Issuable Based On Coverage Ratio	2,568
Retired Bond Capacity	832
Union Electric Company | Actual Interest Coverage Ratio
Debt Instrument [Line Items]
Interest Coverage Ratio	4.3
Dividend Coverage Ratio	115.1
Ameren Illinois Company
Debt Instrument [Line Items]
Bonds Issuable Based On Coverage Ratio	3,358	[1],[2]
Preferred Stock Issuable Based On Coverage Ratio	208
Retired Bond Capacity	204
Ameren Illinois Company | Actual Interest Coverage Ratio
Debt Instrument [Line Items]
Interest Coverage Ratio	6.4
Dividend Coverage Ratio	2.7
Minimum | Union Electric Company | Required Dividend Coverage Ratio
Debt Instrument [Line Items]
Interest Coverage Ratio	2	[3]
Dividend Coverage Ratio	2.5	[4]
Minimum | Ameren Illinois Company | Required Dividend Coverage Ratio
Debt Instrument [Line Items]
Interest Coverage Ratio	2	[3]
Dividend Coverage Ratio	1.5	[4]
[1]	Amount of bonds issuable based either on required coverage ratios or unfunded property additions, whichever is more restrictive. The amounts shown also include bonds issuable based on retired bond capacity of $832 million and $204 million at Ameren Missouri and Ameren Illinois, respectively.
[2]	Amount of bonds issuable by Ameren Illinois based on unfunded property additions and retired bonds solely under the former IP mortgage indenture.
[3]	Coverage required on the annual interest charges on first mortgage bonds outstanding and to be issued. Coverage is not required in certain cases when additional first mortgage bonds are issued on the basis of retired bonds.
[4]	Coverage required on the annual dividend on preferred stock outstanding and to be issued, as required in the respective company’s articles of incorporation.

Other_Income_And_Expenses_Othe

Other Income And Expenses (Other Income And Expenses) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$34	[1]	$37	[1]	$36	[1]
Interest income on industrial development revenue bonds	27	[1]	27	[1]	28	[1]
Interest and dividend income	10	[1],[2]	3	[1]	4	[1],[3]
Other	8	[1],[4]	2	[1]	2	[1]
Total miscellaneous income	79	[1]	69	[1]	70	[1]
Donations	10	[1]	12	[1]	24	[1],[5]
Other	12	[1]	14	[1]	13	[1]
Total miscellaneous expense	22	[1]	26	[1]	37	[1]
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	32		31		31
Interest income on industrial development revenue bonds	27		27		28
Interest and dividend income	1				4	[3]
Total miscellaneous income	60		58		63
Donations	6		4		9
Other	6		7		5
Total miscellaneous expense	12		11		14
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	2		6		5
Interest and dividend income	7	[2]	2
Other	8	[4]	2		2
Total miscellaneous income	17		10		7
Donations	4		4		11	[5]
Other	4		5		6
Total miscellaneous expense	8		9		17
Illinois Science and Energy Innovation Trust | Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
One-time Donation					$7.50
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.
[2]	Includes Ameren Illinois' interest income received in 2014 relating to the 2013 and 2014 IEIMA revenue requirement reconciliation regulatory assets.
[3]	Includes Ameren Missouri's interest income relating to a refund of charges included in an expired power purchase agreement with Entergy. See Note 2 – Rate and Regulatory Matters for additional information.
[4]	Includes Ameren Illinois' income earned in 2014 from customer-requested construction.
[5]	Includes Ameren Illinois' one-time $7.5 million contribution to the Illinois Science and Energy Innovation Trust pursuant to the IEIMA as a result of Ameren Illinois' participation in the electric delivery formula ratemaking process.

Derivative_Financial_Instrumen2

Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Details)	Dec. 31, 2014		Dec. 31, 2013
	gal		gal
Fuel Oils
Derivative [Line Items]
Quantity	50,000,000	[1]	66,000,000	[1]
Natural Gas
Derivative [Line Items]
Quantity	136,000,000		136,000,000
Power
Derivative [Line Items]
Quantity	12,000,000		14,000,000
Uranium
Derivative [Line Items]
Quantity	332,000		796,000
Union Electric Company | Fuel Oils
Derivative [Line Items]
Quantity	50,000,000	[1]	66,000,000	[1]
Union Electric Company | Natural Gas
Derivative [Line Items]
Quantity	28,000,000		28,000,000
Union Electric Company | Power
Derivative [Line Items]
Quantity	1,000,000		3,000,000
Union Electric Company | Uranium
Derivative [Line Items]
Quantity	332,000		796,000
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Quantity	108,000,000		108,000,000
Ameren Illinois Company | Power
Derivative [Line Items]
Quantity	11,000,000		11,000,000
[1]	Fuel oils consist of heating oil, ultra-low-sulfur diesel, and crude oil.

Derivative_Financial_Instrumen3

Derivative Financial Instruments (Derivative Instruments Carrying Value) (Details) (Not Designated As Hedging Instrument, USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Derivative [Line Items]
Derivative assets	$19	$34
Derivative liabilities	232	178
Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets	2	6
Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets		3
Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities	22	2
Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	7	1
Natural Gas | Other current assets
Derivative [Line Items]
Derivative assets	2	2
Natural Gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	37	32
Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	19	25
Power | Other current assets
Derivative [Line Items]
Derivative assets	15	23
Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	14	13
Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	131	99
Uranium | Other current liabilities
Derivative [Line Items]
Derivative liabilities	2	5
Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities		1
Union Electric Company
Derivative [Line Items]
Derivative assets	18	33
Derivative liabilities	46	24
Union Electric Company | Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets	2	6
Union Electric Company | Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets		3
Union Electric Company | Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities	22	2
Union Electric Company | Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	7	1
Union Electric Company | Natural Gas | Other current assets
Derivative [Line Items]
Derivative assets	1	1
Union Electric Company | Natural Gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	6	5
Union Electric Company | Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	6	6
Union Electric Company | Power | Other current assets
Derivative [Line Items]
Derivative assets	15	23
Union Electric Company | Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	3	4
Union Electric Company | Uranium | Other current liabilities
Derivative [Line Items]
Derivative liabilities	2	5
Union Electric Company | Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities		1
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	1	1
Derivative liabilities	186	154
Ameren Illinois Company | Natural Gas | Other current assets
Derivative [Line Items]
Derivative assets	1	1
Ameren Illinois Company | Natural Gas | MTM derivative liabilities
Derivative [Line Items]
Derivative liabilities	31	27
Ameren Illinois Company | Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	13	19
Ameren Illinois Company | Power | MTM derivative liabilities
Derivative [Line Items]
Derivative liabilities	11	9
Ameren Illinois Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	$131	$99

Derivative_Financial_Instrumen4

Derivative Financial Instruments (Cumulative Amount Of Pretax Net Gains (Losses) On All Derivative Instruments In OCI) (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Derivative [Line Items]
Current gains deferred as regulatory liabilities	$106		$216
Current regulatory assets	295		156
Power
Derivative [Line Items]
Current gains deferred as regulatory liabilities	15
Current regulatory assets	14
Fuel Oils
Derivative [Line Items]
Current gains deferred as regulatory liabilities	21
Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	2
Current regulatory assets	37
Uranium
Derivative [Line Items]
Current regulatory assets	2
Regulatory Liabilities Or Assets | Power
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	-130	[1]	-89
Regulatory Liabilities Or Assets | Fuel Oils
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	-29	[2]	2
Regulatory Liabilities Or Assets | Natural Gas
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	-54	[3]	-55
Regulatory Liabilities Or Assets | Uranium
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	-2	[4]	-6
Union Electric Company
Derivative [Line Items]
Current gains deferred as regulatory liabilities	18		57
Current regulatory assets	163		118
Union Electric Company | Power
Derivative [Line Items]
Current gains deferred as regulatory liabilities	15
Current regulatory assets	3
Union Electric Company | Fuel Oils
Derivative [Line Items]
Current gains deferred as regulatory liabilities	21
Union Electric Company | Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current regulatory assets	6
Union Electric Company | Uranium
Derivative [Line Items]
Current regulatory assets	2
Union Electric Company | Regulatory Liabilities Or Assets | Power
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	12	[1]	19
Union Electric Company | Regulatory Liabilities Or Assets | Fuel Oils
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	-29	[2]	2
Union Electric Company | Regulatory Liabilities Or Assets | Natural Gas
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	-11	[3]	-10
Union Electric Company | Regulatory Liabilities Or Assets | Uranium
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	-2	[4]	-6
Ameren Illinois Company
Derivative [Line Items]
Current gains deferred as regulatory liabilities	84		159
Current regulatory assets	129		38
Ameren Illinois Company | Power
Derivative [Line Items]
Current regulatory assets	11
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current regulatory assets	31
Ameren Illinois Company | Regulatory Liabilities Or Assets | Power
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	-142	[1]	-108
Ameren Illinois Company | Regulatory Liabilities Or Assets | Natural Gas
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	($43)	[3]	($45)
[1]	Represents net gains (losses) associated with power derivative contracts. These contracts are a partial hedge of power price requirements through May 2032 at Ameren and Ameren Illinois and through December 2015 at Ameren Missouri. Current gains deferred as regulatory liabilities include $15 million and $15 million at Ameren and Ameren Missouri, respectively. Current losses deferred as regulatory assets include $14 million, $3 million, and $11 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively.
[2]	Represents net losses associated with fuel oils derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s rail transportation surcharges for coal through December 2017. Current losses deferred as regulatory assets include $21 million and $21 million at Ameren and Ameren Missouri, respectively.
[3]	Represents net losses associated with natural gas derivative contracts. These contracts are a partial hedge of natural gas requirements through October 2019 at Ameren and Ameren Missouri and through October 2018 at Ameren Illinois. Current gains deferred as regulatory liabilities include $2 million, $1 million, and $1 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively. Current losses deferred as regulatory assets include $37 million, $6 million, and $31 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively.
[4]	Represents net losses associated with uranium derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri's uranium requirements through December 2016. Current losses deferred as regulatory assets include $2 million and $2 million at Ameren and Ameren Missouri, respectively.

Derivative_Financial_Instrumen5

Derivative Financial Instruments Derivative Financial Instruments (Offsetting Derivative Assets and Liabilities) (Details) (Commodity Contract, USD $)	Dec. 31, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	$19	[1]	$34	[1]
Derivative Instruments	5		10
Net Amount	14		24
Gross Amounts Recognized in the Balance Sheet	232	[1]	178	[1]
Derivative Instruments	5		10
Cash Collateral Received/Posted	5	[2]	24	[2]
Net Amount	222		144
Union Electric Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	18	[1]	33	[1]
Derivative Instruments	5		9
Net Amount	13		24
Gross Amounts Recognized in the Balance Sheet	46	[1]	24	[1]
Derivative Instruments	5		9
Cash Collateral Received/Posted	5	[2]	9	[2]
Net Amount	36		6
Ameren Illinois Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	1		1
Derivative Instruments			1
Net Amount	1
Gross Amounts Recognized in the Balance Sheet	186	[1]	154	[1]
Derivative Instruments			1
Cash Collateral Received/Posted			15	[2]
Net Amount	$186		$138
[1]	The derivative asset and liability balances are presented net of counterparty credit considerations.
[2]	Cash collateral received reduces gross asset balances and is included in “Other current liabilities” and “Other deferred credits and liabilities” on the balance sheet. Cash collateral posted reduces gross liability balances and is included in “Other current assets” and “Other assets” on the balance sheet.

Derivative_Financial_Instrumen6

Derivative Financial Instruments (Maximum Exposure If Counterparties Fail To Perform On Contracts) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	$5
Union Electric Company
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	$5

Derivative_Financial_Instrumen7

Derivative Financial Instruments (Potential Loss On Counterparty Exposures) (Details) (USD $)	Dec. 31, 2014
In Millions, unless otherwise specified
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$5
Union Electric Company
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$5

Derivative_Financial_Instrumen8

Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Details) (USD $)	Dec. 31, 2014
In Millions, unless otherwise specified
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	$170	[1]
Cash Collateral Posted	4
Potential Aggregate Amount of Additional Collateral Required	159	[2]
Union Electric Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	96	[1]
Cash Collateral Posted	4
Potential Aggregate Amount of Additional Collateral Required	88	[2]
Ameren Illinois Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	74	[1]
Cash Collateral Posted	0
Potential Aggregate Amount of Additional Collateral Required	$71	[2]
[1]	Prior to consideration of master netting arrangements and including NPNS and other accrual contract exposures.
[2]	As collateral requirements with certain counterparties are based on master netting arrangements, the aggregate amount of additional collateral required to be posted is determined after consideration of the effects of such arrangements.

Derivative_Financial_Instrumen9

Derivative Financial Instruments (Cash Flow Hedges) (Details) (USD $)	3 Months Ended								12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative [Line Items]
Operating revenues	$1,370	$1,670	$1,419	$1,594	$1,322	$1,638	$1,403	$1,475	$6,053	$5,838	$5,781

Fair_Value_Measurements_Schedu

Fair Value Measurements (Schedule of Valuation Process and Unobservable Inputs) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Discounted Cash Flow | Minimum | Fuel Oils
Fair Value Inputs [Abstract]
Escalation rate	5.00%	[1],[2]
Counterparty credit risk			0.26%	[3],[4]
Credit risk	0.43%	[3],[4]
Discounted Cash Flow | Minimum | Natural Gas
Fair Value Inputs [Abstract]
Counterparty credit risk	0.43%	[3],[4]
Credit risk	0.43%	[3],[4]
Nodal basis	-0.4	[1]
Discounted Cash Flow | Minimum | Power
Fair Value Inputs [Abstract]
Counterparty credit risk	0.26%	[3],[4]	0.39%	[3],[4]
Credit risk	0.43%	[3],[4]	2.00%	[3],[4]
Average bid/ask consensus peak and off-peak pricing	27	[5]	25	[4]
Estimated auction price for FTRs	-1,833	[1]	-1,594	[1]
Nodal basis	-6	[1]	-3	[4]
Discounted Cash Flow | Minimum | Uranium
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	35	[1]	34	[1]
Discounted Cash Flow | Minimum | Union Electric Company | Fuel Oils
Fair Value Inputs [Abstract]
Escalation rate	5.00%	[1],[2]
Counterparty credit risk			0.26%	[3],[4]
Credit risk	0.43%	[3],[4]
Discounted Cash Flow | Minimum | Union Electric Company | Natural Gas
Fair Value Inputs [Abstract]
Counterparty credit risk	0.57%	[3],[4]
Credit risk	0.43%	[3],[4]
Nodal basis	-0.1	[1]
Discounted Cash Flow | Minimum | Union Electric Company | Power
Fair Value Inputs [Abstract]
Counterparty credit risk	0.26%	[3],[4]	0.39%	[3],[4]
Credit risk	0.43%	[3],[4]	2.00%	[3],[4]
Average bid/ask consensus peak and off-peak pricing	27	[4]	25	[4]
Estimated auction price for FTRs	-1,833	[1]	-1,594	[1]
Nodal basis			-3	[4]
Discounted Cash Flow | Minimum | Union Electric Company | Uranium
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	35	[1]	34	[1]
Discounted Cash Flow | Minimum | Ameren Illinois Company | Natural Gas
Fair Value Inputs [Abstract]
Counterparty credit risk	0.43%	[3],[4]
Credit risk	0.43%	[3],[4]
Nodal basis	-0.4	[1]
Discounted Cash Flow | Minimum | Ameren Illinois Company | Power
Fair Value Inputs [Abstract]
Credit risk	0.43%	[3],[4]	2.00%	[3],[4]
Nodal basis	-6	[1]	-4	[1]
Average bid/ask consensus pricing	27	[1]	27	[1]
Discounted Cash Flow | Maximum | Fuel Oils
Fair Value Inputs [Abstract]
Escalation rate	5.00%	[1],[2]
Counterparty credit risk			2.00%	[3],[4]
Credit risk	0.43%	[3],[4]
Discounted Cash Flow | Maximum | Natural Gas
Fair Value Inputs [Abstract]
Counterparty credit risk	13.00%	[3],[4]
Credit risk	0.43%	[3],[4]
Nodal basis	0.1	[1]
Discounted Cash Flow | Maximum | Power
Fair Value Inputs [Abstract]
Counterparty credit risk	0.26%	[3],[4]	0.50%	[3],[4]
Credit risk	0.43%	[3],[4]	2.00%	[3],[4]
Average bid/ask consensus peak and off-peak pricing	50	[5]	51	[4]
Estimated auction price for FTRs	2,743	[1]	945	[1]
Nodal basis	0	[1]	-1	[4]
Discounted Cash Flow | Maximum | Uranium
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	40	[1]	41	[1]
Discounted Cash Flow | Maximum | Union Electric Company | Fuel Oils
Fair Value Inputs [Abstract]
Escalation rate	5.00%	[1],[2]
Counterparty credit risk			2.00%	[3],[4]
Credit risk	0.43%	[3],[4]
Discounted Cash Flow | Maximum | Union Electric Company | Natural Gas
Fair Value Inputs [Abstract]
Counterparty credit risk	13.00%	[3],[4]
Credit risk	0.43%	[3],[4]
Nodal basis	-0.1	[1]
Discounted Cash Flow | Maximum | Union Electric Company | Power
Fair Value Inputs [Abstract]
Counterparty credit risk	0.26%	[3],[4]	0.50%	[3],[4]
Credit risk	0.43%	[3],[4]	2.00%	[3],[4]
Average bid/ask consensus peak and off-peak pricing	50	[4]	51	[4]
Estimated auction price for FTRs	2,743	[1]	945	[1]
Nodal basis			-1	[4]
Discounted Cash Flow | Maximum | Union Electric Company | Uranium
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	40	[1]	41	[1]
Discounted Cash Flow | Maximum | Ameren Illinois Company | Natural Gas
Fair Value Inputs [Abstract]
Counterparty credit risk	2.00%	[3],[4]
Credit risk	0.43%	[3],[4]
Nodal basis	0.1	[1]
Discounted Cash Flow | Maximum | Ameren Illinois Company | Power
Fair Value Inputs [Abstract]
Credit risk	0.43%	[3],[4]	2.00%	[3],[4]
Nodal basis	0	[1]	0	[1]
Average bid/ask consensus pricing	38	[1]	36	[1]
Discounted Cash Flow | Weighted Average | Fuel Oils
Fair Value Inputs [Abstract]
Escalation rate	5.00%	[1],[2]
Counterparty credit risk			1.00%	[3],[4]
Credit risk	0.43%	[3],[4]
Discounted Cash Flow | Weighted Average | Natural Gas
Fair Value Inputs [Abstract]
Counterparty credit risk	3.00%	[3],[4]
Credit risk	0.43%	[3],[4]
Nodal basis	-0.2	[1]
Discounted Cash Flow | Weighted Average | Power
Fair Value Inputs [Abstract]
Counterparty credit risk	0.26%	[3],[4]	0.42%	[3],[4]
Credit risk	0.43%	[3],[4]	2.00%	[3],[4]
Average bid/ask consensus peak and off-peak pricing	32	[5]	32	[4]
Estimated auction price for FTRs	171	[1]	305	[1]
Nodal basis	-2	[1]	-2	[4]
Discounted Cash Flow | Weighted Average | Uranium
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	36	[1]	36	[1]
Discounted Cash Flow | Weighted Average | Union Electric Company | Fuel Oils
Fair Value Inputs [Abstract]
Escalation rate	5.00%	[1],[2]
Counterparty credit risk			1.00%	[3],[4]
Credit risk	0.43%	[3],[4]
Discounted Cash Flow | Weighted Average | Union Electric Company | Natural Gas
Fair Value Inputs [Abstract]
Counterparty credit risk	5.00%	[3],[4]
Credit risk	0.43%	[3],[4]
Nodal basis	-0.1	[1]
Discounted Cash Flow | Weighted Average | Union Electric Company | Power
Fair Value Inputs [Abstract]
Counterparty credit risk	0.26%	[3],[4]	0.42%	[3],[4]
Credit risk	0.43%	[3],[4]	2.00%	[3],[4]
Average bid/ask consensus peak and off-peak pricing	32	[4]	40	[4]
Estimated auction price for FTRs	171	[1]	305	[1]
Nodal basis			-2	[4]
Discounted Cash Flow | Weighted Average | Union Electric Company | Uranium
Fair Value Inputs [Abstract]
Average bid/ask consensus pricing	36	[1]	36	[1]
Discounted Cash Flow | Weighted Average | Ameren Illinois Company | Natural Gas
Fair Value Inputs [Abstract]
Counterparty credit risk	0.83%	[3],[4]
Credit risk	0.43%	[3],[4]
Nodal basis	-0.2	[1]
Discounted Cash Flow | Weighted Average | Ameren Illinois Company | Power
Fair Value Inputs [Abstract]
Credit risk	0.43%	[3],[4]	2.00%	[3],[4]
Nodal basis	-2	[1]	-2	[1]
Average bid/ask consensus pricing	32	[1]	30	[1]
Option Model | Minimum | Fuel Oils
Fair Value Inputs [Abstract]
Volatilities	3.00%	[4]	10.00%	[1]
Option Model | Minimum | Natural Gas
Fair Value Inputs [Abstract]
Volatilities	31.00%	[4]
Nodal basis	-0.4	[1]
Option Model | Minimum | Union Electric Company | Fuel Oils
Fair Value Inputs [Abstract]
Volatilities	3.00%	[4]	10.00%	[1]
Option Model | Minimum | Union Electric Company | Natural Gas
Fair Value Inputs [Abstract]
Volatilities	31.00%	[4]
Nodal basis	-0.4	[1]
Option Model | Minimum | Ameren Illinois Company | Natural Gas
Fair Value Inputs [Abstract]
Volatilities	50.00%	[4]
Nodal basis	-0.1	[1]
Option Model | Maximum | Fuel Oils
Fair Value Inputs [Abstract]
Volatilities	39.00%	[4]	35.00%	[1]
Option Model | Maximum | Natural Gas
Fair Value Inputs [Abstract]
Volatilities	144.00%	[4]
Nodal basis	0	[1]
Option Model | Maximum | Union Electric Company | Fuel Oils
Fair Value Inputs [Abstract]
Volatilities	39.00%	[4]	35.00%	[1]
Option Model | Maximum | Union Electric Company | Natural Gas
Fair Value Inputs [Abstract]
Volatilities	144.00%	[4]
Nodal basis	0	[1]
Option Model | Maximum | Ameren Illinois Company | Natural Gas
Fair Value Inputs [Abstract]
Volatilities	144.00%	[4]
Nodal basis	0	[1]
Option Model | Weighted Average | Fuel Oils
Fair Value Inputs [Abstract]
Volatilities	32.00%	[4]	16.00%	[1]
Option Model | Weighted Average | Natural Gas
Fair Value Inputs [Abstract]
Volatilities	63.00%	[4]
Nodal basis	-0.2	[1]
Option Model | Weighted Average | Union Electric Company | Fuel Oils
Fair Value Inputs [Abstract]
Volatilities	32.00%	[4]	16.00%	[1]
Option Model | Weighted Average | Union Electric Company | Natural Gas
Fair Value Inputs [Abstract]
Volatilities	53.00%	[4]
Nodal basis	-0.3	[1]
Option Model | Weighted Average | Ameren Illinois Company | Natural Gas
Fair Value Inputs [Abstract]
Volatilities	94.00%	[4]
Nodal basis	-0.1	[1]
Fundamental Energy Production Model | Minimum | Power
Fair Value Inputs [Abstract]
Escalation rate	0.00%	[6]	3.00%	[1],[7]
Estimated future gas prices	4	[1]	4	[1]
Fundamental Energy Production Model | Minimum | Ameren Illinois Company | Power
Fair Value Inputs [Abstract]
Escalation rate	0.00%	[1],[7]	3.00%	[1],[7]
Estimated future gas prices	4	[1]	4	[1]
Fundamental Energy Production Model | Maximum | Power
Fair Value Inputs [Abstract]
Escalation rate	1.00%	[6]	4.00%	[1],[7]
Estimated future gas prices	5	[1]	5	[1]
Fundamental Energy Production Model | Maximum | Ameren Illinois Company | Power
Fair Value Inputs [Abstract]
Escalation rate	1.00%	[1],[7]	4.00%	[1],[7]
Estimated future gas prices	5	[1]	5	[1]
Fundamental Energy Production Model | Weighted Average | Power
Fair Value Inputs [Abstract]
Escalation rate	1.00%	[1],[7]	4.00%	[1],[7]
Estimated future gas prices	4	[1]	5	[1]
Fundamental Energy Production Model | Weighted Average | Ameren Illinois Company | Power
Fair Value Inputs [Abstract]
Escalation rate	1.00%	[1],[7]	4.00%	[1],[7]
Estimated future gas prices	4	[1]	5	[1]
Contract Price Allocation | Minimum | Power
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	5	[1]	5	[1]
Contract Price Allocation | Minimum | Ameren Illinois Company | Power
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	5	[1]	5	[1]
Contract Price Allocation | Maximum | Power
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	7	[1]	7	[1]
Contract Price Allocation | Maximum | Ameren Illinois Company | Power
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	7	[1]	7	[1]
Contract Price Allocation | Weighted Average | Power
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	6	[1]	6	[1]
Contract Price Allocation | Weighted Average | Ameren Illinois Company | Power
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs	6	[1]	6	[1]
Derivative liabilities | Fuel Oils
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	-8	[6]	-3	[6]
Derivative liabilities | Natural Gas
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	2	[6]
Derivative liabilities | Power
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	-144	[6],[8]	-110	[6],[9]
Derivative liabilities | Uranium
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	-2	[6]	-6	[6]
Derivative liabilities | Union Electric Company | Fuel Oils
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	-8	[6]	-3	[6]
Derivative liabilities | Union Electric Company | Natural Gas
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	-1	[6]
Derivative liabilities | Union Electric Company | Power
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	-2	[6],[8]	-2	[6],[9]
Derivative liabilities | Union Electric Company | Uranium
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	-2	[6]	-6	[6]
Derivative liabilities | Ameren Illinois Company | Natural Gas
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	-1	[6]
Derivative liabilities | Ameren Illinois Company | Power
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	-142	[6],[8]	-108	[6],[9]
Derivative assets | Fuel Oils
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	2	[6]	8	[6]
Derivative assets | Natural Gas
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	1	[6]
Derivative assets | Power
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	11	[6],[8]	21	[6],[9]
Derivative assets | Union Electric Company | Fuel Oils
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	2	[6]	8	[6]
Derivative assets | Union Electric Company | Power
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	11	[6],[8]	21	[6],[9]
Derivative assets | Ameren Illinois Company | Natural Gas
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	1	[6]
[1]	Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.
[2]	Escalation rate applies to fuel oil prices 2017 and beyond.
[3]	Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances.
[4]	Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.
[5]	The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes due to their opposing positions. As such, refer to the power sensitivity analysis for each company above.
[6]	The derivative asset and liability balances are presented net of counterparty credit considerations.
[7]	Escalation rate applies to power prices 2026 and beyond.
[8]	Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2018. Valuations beyond 2018 use fundamentally modeled pricing by month for peak and off-peak demand.
[9]	Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2017. Valuations beyond 2017 use fundamentally modeled pricing by month for peak and off-peak demand.

Fair_Value_Measurements_Narrat

Fair Value Measurements (Narrative) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Counterparty default risk liability valuation adjustment related to derivative contracts	$1	$3
Union Electric Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Counterparty default risk liability valuation adjustment related to derivative contracts	1	1
Ameren Illinois Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Counterparty default risk liability valuation adjustment related to derivative contracts	$1	$3

Fair_Value_Measurements_Schedu1

Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	$547	[1]	$494
Assets	566	[2]	528	[2]
Excluded receivables, payables, and accrued income, net	2
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	365		335
Assets	365	[2]	336	[2]
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	182		159
Assets	187	[2]	163	[2]
Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	14	[2]	29	[2]
Cash And Cash Equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	1		3
Cash And Cash Equivalents | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	1		3
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	19	[2]	34	[2]
Derivative liabilities	232	[2]	178	[2]
Commodity Contract | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets			1	[2]
Derivative liabilities	22	[2]	3	[2]
Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	5	[2]	4	[2]
Derivative liabilities	54	[2]	56	[2]
Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	14	[2]	29	[2]
Derivative liabilities	156	[2]	119	[2]
Commodity Contract | Fuel Oils
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	[2]	9	[2]
Derivative liabilities	29	[2]	3	[2]
Commodity Contract | Fuel Oils | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets			1	[2]
Derivative liabilities	21	[2]
Commodity Contract | Fuel Oils | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	[2]	8	[2]
Derivative liabilities	8	[2]	3	[2]
Commodity Contract | Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	[2]	2	[2]
Derivative liabilities	56	[2]	57	[2]
Commodity Contract | Natural Gas | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	1	[2]	3	[2]
Commodity Contract | Natural Gas | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	[2]	2	[2]
Derivative liabilities	53	[2]	54	[2]
Commodity Contract | Natural Gas | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	[2]
Derivative liabilities	2	[2]
Commodity Contract | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	15	[2]	23	[2]
Derivative liabilities	145	[2]	112	[2]
Commodity Contract | Power | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	4	[2]	2	[2]
Derivative liabilities	1	[2]	2	[2]
Commodity Contract | Power | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	11	[2]	21	[2]
Derivative liabilities	144	[2]	110	[2]
Commodity Contract | Uranium
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	2	[2]	6	[2]
Commodity Contract | Uranium | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	2	[2]	6	[2]
Equity Securities | U.S. Large Capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	364		332
Equity Securities | U.S. Large Capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	364		332
Debt Securities | Corporate Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	63		52
Debt Securities | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	63		52
Debt Securities | Municipal Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	2		2
Debt Securities | Municipal Bonds | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	2		2
Debt Securities | U.S. treasury and agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	102		94
Debt Securities | U.S. treasury and agency securities | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	102		94
Debt Securities | Asset-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	10		10
Debt Securities | Asset-Backed Securities | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	10		10
Debt Securities | Other debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	5		1
Debt Securities | Other debt securities | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	5		1
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	547	[1]	494
Assets	565	[2]	527	[2]
Union Electric Company | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	365		335
Assets	365	[2]	336	[2]
Union Electric Company | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	182		159
Assets	187	[2]	162	[2]
Union Electric Company | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets	13	[2]	29	[2]
Union Electric Company | Cash And Cash Equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	1		3
Union Electric Company | Cash And Cash Equivalents | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	1		3
Union Electric Company | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	18	[2]	33	[2]
Derivative liabilities	46	[2]	24	[2]
Union Electric Company | Commodity Contract | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets			1	[2]
Derivative liabilities	22	[2]	3	[2]
Union Electric Company | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	5	[2]	3	[2]
Derivative liabilities	11	[2]	10	[2]
Union Electric Company | Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	13	[2]	29	[2]
Derivative liabilities	13	[2]	11	[2]
Union Electric Company | Commodity Contract | Fuel Oils
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	[2]	9	[2]
Derivative liabilities	29	[2]	3	[2]
Union Electric Company | Commodity Contract | Fuel Oils | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets			1	[2]
Derivative liabilities	21	[2]
Union Electric Company | Commodity Contract | Fuel Oils | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	[2]	8	[2]
Derivative liabilities	8	[2]	3	[2]
Union Electric Company | Commodity Contract | Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	[2]	1	[2]
Derivative liabilities	12	[2]	11	[2]
Union Electric Company | Commodity Contract | Natural Gas | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	1	[2]	3	[2]
Union Electric Company | Commodity Contract | Natural Gas | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	[2]	1	[2]
Derivative liabilities	10	[2]	8	[2]
Union Electric Company | Commodity Contract | Natural Gas | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	1	[2]
Union Electric Company | Commodity Contract | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	15	[2]	23	[2]
Derivative liabilities	3	[2]	4	[2]
Union Electric Company | Commodity Contract | Power | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	4	[2]	2	[2]
Derivative liabilities	1	[2]	2	[2]
Union Electric Company | Commodity Contract | Power | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	11	[2]	21	[2]
Derivative liabilities	2	[2]	2	[2]
Union Electric Company | Commodity Contract | Uranium
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	2	[2]	6	[2]
Union Electric Company | Commodity Contract | Uranium | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	2	[2]	6	[2]
Union Electric Company | Equity Securities | U.S. Large Capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	364		332
Union Electric Company | Equity Securities | U.S. Large Capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	364		332
Union Electric Company | Debt Securities | Corporate Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	63		52
Union Electric Company | Debt Securities | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	63		52
Union Electric Company | Debt Securities | Municipal Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	2		2
Union Electric Company | Debt Securities | Municipal Bonds | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	2		2
Union Electric Company | Debt Securities | U.S. treasury and agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	102		94
Union Electric Company | Debt Securities | U.S. treasury and agency securities | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	102		94
Union Electric Company | Debt Securities | Asset-Backed Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	10		10
Union Electric Company | Debt Securities | Asset-Backed Securities | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	10		10
Union Electric Company | Debt Securities | Other debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	5		1
Union Electric Company | Debt Securities | Other debt securities | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	5		1
Ameren Illinois Company | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1		1
Derivative liabilities	186	[2]	154	[2]
Ameren Illinois Company | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	43	[2]	46	[2]
Ameren Illinois Company | Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	143	[2]	108	[2]
Ameren Illinois Company | Commodity Contract | Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	[2]	1	[1]
Derivative liabilities	44	[2]	46	[2]
Ameren Illinois Company | Commodity Contract | Natural Gas | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets			1	[1]
Derivative liabilities	43	[2]	46	[2]
Ameren Illinois Company | Commodity Contract | Natural Gas | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	[2]
Derivative liabilities	1	[2]
Ameren Illinois Company | Commodity Contract | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	142	[2]	108	[2]
Ameren Illinois Company | Commodity Contract | Power | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	$142	[2]	$108	[2]
[1]	Balance excludes $2 million of receivables, payables, and accrued income, net.
[2]	The derivative asset and liability balances are presented net of counterparty credit considerations.

Fair_Value_Measurements_Schedu2

Fair Value Measurements (Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level 3 In The Fair Value Hierarchy) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fuel Oils
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$5	$5
Included in regulatory assets/liabilities	-9
Purchases		3
Sales		-1
Settlements	-2	-2
Ending balance	-6	5
Change in unrealized gains (losses) related to assets/liabilities still held	-6
Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	0
Included in regulatory assets/liabilities	1	-1
Purchases	-2	1
Sales	-1
Settlements	1
Ending balance	-1	0
Change in unrealized gains (losses) related to assets/liabilities still held	2
Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	-89	-100
Included in regulatory assets/liabilities	-53	-15
Purchases	34	40
Sales	-1
Settlements	-24	-15
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		-3
Transfers out of Level 3		4
Ending balance	-133	-89
Change in unrealized gains (losses) related to assets/liabilities still held	-43	-25
Uranium
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	-6	-2
Included in regulatory assets/liabilities	-1	-3
Purchases		-2
Settlements	5	1
Ending balance	-2	-6
Change in unrealized gains (losses) related to assets/liabilities still held	-1	-2
Union Electric Company | Fuel Oils
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	5	5
Included in regulatory assets/liabilities	-9
Purchases		3
Sales		-1
Settlements	-2	-2
Ending balance	-6	5
Change in unrealized gains (losses) related to assets/liabilities still held	-6
Union Electric Company | Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	0
Purchases		0
Sales	-1
Ending balance	-1	0
Union Electric Company | Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	19	11
Included in regulatory assets/liabilities	-14	3
Purchases	34	40
Sales	-1
Settlements	-29	-36
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		-3
Transfers out of Level 3		4
Ending balance	9	19
Change in unrealized gains (losses) related to assets/liabilities still held		-1
Union Electric Company | Uranium
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	-6	-2
Included in regulatory assets/liabilities	-1	-3
Purchases		-2
Settlements	5	1
Ending balance	-2	-6
Change in unrealized gains (losses) related to assets/liabilities still held	-1	-2
Ameren Illinois Company | Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	0
Included in regulatory assets/liabilities	1	-1
Purchases	-2	1
Settlements	1
Ending balance	0	0
Change in unrealized gains (losses) related to assets/liabilities still held	2
Ameren Illinois Company | Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	-108	-111
Included in regulatory assets/liabilities	-39	-18
Settlements	5	21
Ending balance	-142	-108
Change in unrealized gains (losses) related to assets/liabilities still held	($43)	($24)

Fair_Value_Measurements_Schedu3

Fair Value Measurements (Schedule Of Transfers Between Fair Value Hierarchy Levels) (Details) (Power, USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfer into Level 3/Transfers out of Level 2	($3)
Transfers out of Level 3/Transfers into Level 2	4
Union Electric Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Transfer into Level 3/Transfers out of Level 2	-3
Transfers out of Level 3/Transfers into Level 2	$4

Fair_Value_Measurements_Schedu4

Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Long-Term Debt And Preferred Stock) (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt and Capital Lease Obligations	$6,240	[1]	$6,038	[1]
Preferred stock	142	[1]	142	[1]
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt and capital lease obligations (including current portion)	7,135	[1]	6,584	[1]
Preferred stock	122	[1]	118	[1]
Union Electric Company | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt and Capital Lease Obligations	3,999		3,757
Preferred stock	80		80
Union Electric Company | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt and capital lease obligations (including current portion)	4,518		4,124
Preferred stock	73		71
Ameren Illinois Company | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt and Capital Lease Obligations	2,241		1,856
Preferred stock	62		62
Ameren Illinois Company | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt and capital lease obligations (including current portion)	2,517		2,028
Preferred stock	$49		$47
[1]	Preferred stock is recorded in "Noncontrolling Interests" on the consolidated balance sheet.

Nuclear_Decommissioning_Trust_2

Nuclear Decommissioning Trust Fund Investments (Proceeds From The Sale Of Investments And Related Gross Realized Gains And Losses In Nuclear Decommissioning Trust Fund) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Nuclear Decommissioning Trust Fund Investments [Line Items]
Sales and maturities of securities - nuclear decommissioning trust fund	$391	$196	$384
Gross realized gains	7	7	6
Gross realized losses	2	5	2
Minimum | Nuclear Decommissioning Trust Fund
Nuclear Decommissioning Trust Fund Investments [Line Items]
Trust fund investments, target allocation percentage	60.00%
Maximum | Nuclear Decommissioning Trust Fund
Nuclear Decommissioning Trust Fund Investments [Line Items]
Trust fund investments, target allocation percentage	70.00%
Union Electric Company
Nuclear Decommissioning Trust Fund Investments [Line Items]
Sales and maturities of securities - nuclear decommissioning trust fund	$391	$196	$384

Nuclear_Decommissioning_Trust_3

Nuclear Decommissioning Trust Fund Investments (Fair Values Of Investments In Debt And Equity Securities In Nuclear Decommissioning Trust Fund) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Nuclear Decommissioning Trust Fund Investments [Line Items]
Cost	$316	$297
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	237	203
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	4	6
Fair Value	549	494
Debt Securities
Nuclear Decommissioning Trust Fund Investments [Line Items]
Cost	175	157
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	7	4
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	1	2
Fair Value	182	159
Equity Securities
Nuclear Decommissioning Trust Fund Investments [Line Items]
Cost	138	137
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	230	199
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax	4	4
Fair Value	364	332
Cash
Nuclear Decommissioning Trust Fund Investments [Line Items]
Cost	1	3
Fair Value	1	3
Other Debt And Equity Securities
Nuclear Decommissioning Trust Fund Investments [Line Items]
Cost	2	1
Fair Value	$2	$1

Nuclear_Decommissioning_Trust_4

Nuclear Decommissioning Trust Fund Investments (Costs And Fair Values Of Investments In Debt Securities In Nuclear Decommissioning Trust Fund According To Contractual Maturities) (Details) (USD $)	Dec. 31, 2014
In Millions, unless otherwise specified
Investments, Debt and Equity Securities [Abstract]
Cost, Less than 5 years	$98
Cost, 5 years to 10 years	41
Cost, Due after 10 years	36
Cost, Total	175
Fair Value, Less than 5 years	99
Fair Value, 5 years to 10 years	42
Fair Value, Due after 10 years	41
Fair Value, Total	$182

Nuclear_Decommissioning_Trust_5

Nuclear Decommissioning Trust Fund Investments (Fair Value And The Gross Unrealized Losses Of The Available-For-Sale Securities Held In Nuclear Decommissioning Trust Fund) (Details) (USD $)	Dec. 31, 2014
Nuclear Decommissioning Trust Fund Investments [Line Items]
Less than 12 months, fair value	$34,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	3,000,000
12 months or greater, fair value	13,000,000
Total, fair value	47,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	4,000,000
Debt Securities
Nuclear Decommissioning Trust Fund Investments [Line Items]
Less than 12 months, fair value	28,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	1
12 months or greater, fair value	8,000,000
Total, fair value	36,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	1,000,000
Equity Securities
Nuclear Decommissioning Trust Fund Investments [Line Items]
Less than 12 months, fair value	6,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	1,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	3,000,000
12 months or greater, fair value	5,000,000
Total, fair value	11,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$4,000,000

Callaway_Energy_Center_Details

Callaway Energy Center (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
	mill
Loss Contingencies [Line Items]
Nwf Fee Number Of Mills	1
Annual decommissioning costs included in costs of service		$7	$7
Nuclear Plant
Loss Contingencies [Line Items]
Useful life	40 years
Annual decommissioning costs included in costs of service	7

Retirement_Benefits_Narrative_

Retirement Benefits (Narrative) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
	bond
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded Status of Plan	$710	[1]	$461	[1]
Defined Benefit Plan Assumptions Used Calculating Benefit Obligation, Change in Discount Rate	0.75%
Number of high-quality corporate bonds	700
Defined benefit plan, estimated future employer contributions during the next five years	290
Actual return in excess of (or less than) expected return, percentage	25.00%
Expected return on plan assets, period	4 years
Amortization basis, straight line, in years	10 years
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded Status of Plan	-616	[1]	-439	[1]
Assumptions used calculating net periodic benefit cost, expected long-term return on assets in future years	7.25%
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded Status of Plan	-94	[1]	-22	[1]
Assumptions used calculating net periodic benefit cost, expected long-term return on assets in future years	7.00%
Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, estimated future employer contributions during the next five years	25
Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, estimated future employer contributions during the next five years	115
Private equity
Defined Benefit Plan Disclosure [Line Items]
Number of limited partnerships in private equity funds	9
Minimum invested capital within limited partnership investments	0.1
Maximum invested capital within limited partnership investments	$5
Union Electric Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Future funding requirement, percentage	41.00%
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Future funding requirement, percentage	40.00%
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries.

Retirement_Benefits_Summary_Of

Retirement Benefits (Summary Of Benefit Liability Recorded) (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	$710	[1]	$461	[1]
Union Electric Company
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	277		191
Ameren Illinois Company
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	$278		$159
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries.

Retirement_Benefits_Funded_Sta

Retirement Benefits (Funded Status Of Benefit Plans And Amounts Included In Regulatory Assets And OCI) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Change in plan assets:
Funded status – deficiency	($710)	[1]	($461)	[1]
Amounts recognized in the balance sheet consist of:
Noncurrent liability	705		466
Net liability recognized	710	[1]	461	[1]
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation at end of year	4,176	[1]	3,698	[1]
Change in benefit obligation:
Net benefit obligation at beginning of year	3,900	[1]	4,051	[1]
Service cost	79	[1],[2]	91	[1],[2]	81	[2]
Interest cost	183	[1],[2]	163	[1],[2]	166	[2]
Actuarial (gain) loss	462	[1]	-207	[1]
Benefits paid	-214	[1]	-198	[1]
Net benefit obligation at end of year	4,410	[1]	3,900	[1]	4,051	[1]
Change in plan assets:
Fair value of plan assets at beginning of year	3,461	[1]	3,127	[1]
Actual return on plan assets	448	[1]	376	[1]
Employer contributions	99	[1]	156	[1]	128	[1]
Benefits paid	-214	[1]	-198	[1]
Fair value of plan assets at end of year	3,794	[1]	3,461	[1]	3,127	[1]
Funded status – deficiency	616	[1]	439	[1]
Accrued benefit cost at December 31	616	[1]	439	[1]
Amounts recognized in the balance sheet consist of:
Current liability(f)	3	[1],[3]	3	[1],[3]
Noncurrent liability	613	[1]	436	[1]
Net liability recognized	616	[1]	439	[1]
Amounts recognized in regulatory assets consist of:
Net actuarial (gain) loss	452	[1]	282	[1]
Prior service cost (credit)	-6	[1]	-7	[1]
Amounts (pretax) recognized in accumulated OCI consist of:
Net actuarial (gain) loss	29	[1]	17	[1]
Prior service cost (credit)			0	[1]
Total	475	[1]	292	[1]
Postretirement Benefits
Change in benefit obligation:
Net benefit obligation at beginning of year	1,096	[1]	1,157	[1]
Service cost	19	[1],[2]	22	[1],[2]	22	[2]
Interest cost	50	[1],[2]	46	[1],[2]	47	[2]
Participant contributions	16	[1]	16	[1]
Actuarial (gain) loss	84	[1]	-76	[1]
Curtailment gain(c)			-3	[2],[4]
Settlement			5	[5]
Benefits paid	-65	[1]	-64	[1]
Federal subsidy on benefits paid	3	[1]	3	[1]
Net benefit obligation at end of year	1,203	[1]	1,096	[1]	1,157	[1]
Change in plan assets:
Fair value of plan assets at beginning of year	1,074	[1]	938	[1]
Actual return on plan assets	75	[1]	156	[1]
Employer contributions	6	[1]	25	[1]	45	[1]
Federal subsidy on benefits paid	3	[1]	3	[1]
Participant contributions	16	[1]	16	[1]
Benefits paid	-65	[1]	-64	[1]
Fair value of plan assets at end of year	1,109	[1]	1,074	[1]	938	[1]
Funded status – deficiency	94	[1]	22	[1]
Accrued benefit cost at December 31	94	[1]	22	[1]
Amounts recognized in the balance sheet consist of:
Noncurrent asset	0	[1],[6]	9
Current liability(f)	2	[1],[3]	1	[1],[3]
Noncurrent liability	92	[1]	30	[1]
Net liability recognized	94	[1]	22	[1]
Amounts recognized in regulatory assets consist of:
Net actuarial (gain) loss	-7	[1]	-71	[1]
Prior service cost (credit)	-16	[1]	-20	[1]
Amounts (pretax) recognized in accumulated OCI consist of:
Net actuarial (gain) loss	-5	[1]	-12	[1]
Prior service cost (credit)	-1	[1]	-1	[1]
Total	($29)	[1]	($104)	[1]
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
[2]	Includes amounts for Ameren registrant and nonregistrant subsidiaries
[3]	Included in "Other current liabilities" on Ameren's consolidated balance sheet.
[4]	Effective with the divestiture of New AER on December 2, 2013, the liability for active management employees of New AER and its subsidiaries not eligible to retire were neither transferred to IPH nor retained by Ameren, which resulted in a curtailment gain. See Note 16 – Divestiture Transactions and Discontinued Operations for additional information on the divestiture.
[5]	Not applicable.
[6]	Included in "Other assets" on Ameren's consolidated balance sheet.

Retirement_Benefits_Assumption

Retirement Benefits (Assumptions Used To Determine Benefit Obligations) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate at measurement date	4.00%	4.75%
Increase in future compensation	3.50%	3.50%
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate at measurement date	4.00%	4.75%
Increase in future compensation	3.50%	3.50%
Medical cost trend rate (initial)	5.00%	5.00%
Medical cost trend rate (ultimate)	5.00%	5.00%

Retirement_Benefits_Cash_Contr

Retirement Benefits (Cash Contributions Made To Benefit Plans) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	$99	[1]	$156	[1]	$128	[1]
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	6	[1]	25	[1]	45	[1]
Union Electric Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	41		60		52
Union Electric Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	3		10		9
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	39		50		46
Ameren Illinois Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	2		11		35
Other | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	19		46		30
Other | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	$1		$4		$1
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries.

Retirement_Benefits_Target_All

Retirement Benefits (Target Allocation Of The Plans' Asset Categories) (Details)	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Actual Plan Asset Allocations	100.00%		100.00%
Pension Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation	0.00%
Maximum Target Allocation	5.00%
Defined Benefit Plan, Actual Plan Asset Allocations	2.00%		2.00%
Pension Benefits | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation	50.00%
Maximum Target Allocation	60.00%
Defined Benefit Plan, Actual Plan Asset Allocations	53.00%		58.00%
Pension Benefits | U.S. large capitalization
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation	29.00%
Maximum Target Allocation	39.00%
Defined Benefit Plan, Actual Plan Asset Allocations	34.00%		36.00%
Pension Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation	2.00%
Maximum Target Allocation	12.00%
Defined Benefit Plan, Actual Plan Asset Allocations	7.00%		8.00%
Pension Benefits | International and emerging markets
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation	9.00%
Maximum Target Allocation	19.00%
Defined Benefit Plan, Actual Plan Asset Allocations	12.00%		14.00%
Pension Benefits | Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation	35.00%
Maximum Target Allocation	45.00%
Defined Benefit Plan, Actual Plan Asset Allocations	41.00%		36.00%
Pension Benefits | Real estate
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation	0.00%
Maximum Target Allocation	9.00%
Defined Benefit Plan, Actual Plan Asset Allocations	4.00%		4.00%
Pension Benefits | Private equity
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation	0.00%
Maximum Target Allocation	4.00%
Defined Benefit Plan, Actual Plan Asset Allocations	1.00%	[1]	1.00%	[1]
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Actual Plan Asset Allocations	100.00%		100.00%
Postretirement Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation	0.00%
Maximum Target Allocation	10.00%
Defined Benefit Plan, Actual Plan Asset Allocations	4.00%		4.00%
Postretirement Benefits | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation	55.00%
Maximum Target Allocation	65.00%
Defined Benefit Plan, Actual Plan Asset Allocations	60.00%		63.00%
Postretirement Benefits | U.S. large capitalization
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation	33.00%
Maximum Target Allocation	43.00%
Defined Benefit Plan, Actual Plan Asset Allocations	40.00%		41.00%
Postretirement Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation	3.00%
Maximum Target Allocation	13.00%
Defined Benefit Plan, Actual Plan Asset Allocations	7.00%		8.00%
Postretirement Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation	10.00%
Maximum Target Allocation	20.00%
Defined Benefit Plan, Actual Plan Asset Allocations	13.00%		14.00%
Postretirement Benefits | Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation	30.00%
Maximum Target Allocation	40.00%
Defined Benefit Plan, Actual Plan Asset Allocations	36.00%		33.00%
[1]	(a)Less than 1% of plan assets.

Retirement_Benefits_Fair_Value

Retirement Benefits (Fair Value Of Plan Assets Utilizing Fair Value Hierarchy) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Real estate
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Purchases, Sales, and Settlements	$5		$4
Fair value of plan assets	147		131		118
Private equity
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Purchases, Sales, and Settlements	-3		-6
Fair value of plan assets	13		15		19
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,794	[1]	3,461	[1]	3,127	[1]
Pension Benefits | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	411		528
Pension Benefits | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,327		2,865
Pension Benefits | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	160		146
Pension Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	38		44
Pension Benefits | Cash And Cash Equivalents | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0		5
Pension Benefits | Cash And Cash Equivalents | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	38		39
Pension Benefits | U.S. large capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,331		1,269
Pension Benefits | U.S. large capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0		107
Pension Benefits | U.S. large capitalization | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,331		1,162
Pension Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	270		273
Pension Benefits | U.S. small and mid-capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	270		273
Pension Benefits | U.S. small and mid-capitalization | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0		0
Pension Benefits | International and emerging markets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	494		515
Pension Benefits | International and emerging markets | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	134		143
Pension Benefits | International and emerging markets | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	360		372
Pension Benefits | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,026		860
Pension Benefits | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,026		860
Pension Benefits | Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	175		149
Pension Benefits | Municipal Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	175		149
Pension Benefits | U.S. treasury and agency securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	372		256
Pension Benefits | U.S. treasury and agency securities | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6
Pension Benefits | U.S. treasury and agency securities | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	366		256
Pension Benefits | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	31		27
Pension Benefits | Other | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	31		27
Pension Benefits | Real estate
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	147		131
Pension Benefits | Real estate | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	147		131
Pension Benefits | Private equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13		15
Pension Benefits | Private equity | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13		15
Pension Benefits | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1		1
Pension Benefits | Derivative assets | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1		1
Pension Benefits | Derivative liabilities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets			-1
Pension Benefits | Derivative liabilities | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets			-1
Pension Benefits | Medical benefit assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	-125	[2]	-112	[2]
Pension Benefits | Net receivables
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	21	[3]	34	[3]
Pension Benefits | Includes Medical Benefit Component Under Section401 H And Excludes Receivables Related To Pending Security Sales [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,898		3,539
Pension Benefits | Excludes Medical Benefit Component Under Section401 H And Includes Receivables Related To Pending Security Sales [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,794		3,461
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,109	[1]	1,074	[1]	938	[1]
Postretirement Benefits | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	487		490
Postretirement Benefits | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	551		511
Postretirement Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	89		77
Postretirement Benefits | Cash And Cash Equivalents | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	89		77
Postretirement Benefits | Cash And Cash Equivalents | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0		0
Postretirement Benefits | U.S. large capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	392		398
Postretirement Benefits | U.S. large capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	291		297
Postretirement Benefits | U.S. large capitalization | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	101		101
Postretirement Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	70		77
Postretirement Benefits | U.S. small and mid-capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	70		77
Postretirement Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	131		135
Postretirement Benefits | International | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	37		39
Postretirement Benefits | International | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	94		96
Postretirement Benefits | Other Equity | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7		2
Postretirement Benefits | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	105		97
Postretirement Benefits | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	105		97
Postretirement Benefits | Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	111		103
Postretirement Benefits | Municipal Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	111		103
Postretirement Benefits | U.S. treasury and agency securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	89		72
Postretirement Benefits | U.S. treasury and agency securities | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	89		72
Postretirement Benefits | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	44		40
Postretirement Benefits | Other | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	44		40
Postretirement Benefits | Medical benefit assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	125	[2]	112	[2]
Postretirement Benefits | Net payables
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	-54	[4]	-39	[4]
Postretirement Benefits | Excludes Medical Benefit Component Under Section401 H And Excludes Payables Related To Pending Security Sales [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,038		1,001
Postretirement Benefits | Includes Medical Benefit Component Under Section401 H And Excludes Payables Related To Pending Security Sales [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$1,109		$1,074
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
[2]	Medical benefit (health and welfare) component for accounts maintained in accordance with Section 401(h) of the Internal Revenue Code to fund a portion of the postretirement obligation.
[3]	Receivables related to pending security sales, offset by payables related to pending security purchases.
[4]	Payables related to pending security purchases, offset by interest receivables and receivables related to pending security sales.

Retirement_Benefits_Changes_In

Retirement Benefits (Changes In The Fair Value Of Plan Assets Classified As Level 3) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Real estate
Change in plan assets:
Fair value of plan assets at beginning of year	$131	$118
Actual Return on Plan Assets Related to Assets Still Held at the Reporting Date	11	9
Purchases, Sales and Settlements, net	5	4
Fair value of plan assets at end of year	147	131
Private equity
Change in plan assets:
Fair value of plan assets at beginning of year	15	19
Actual Return on Plan Assets Related to Assets Still Held at the Reporting Date	-9	-9
Actual Return on Plan Assets Related to Assets Sold During the Period	10	11
Purchases, Sales and Settlements, net	-3	-6
Fair value of plan assets at end of year	$13	$15

Retirement_Benefits_Components

Retirement Benefits (Components Of Net Periodic Benefit Cost) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$79	[1],[2]	$91	[1],[2]	$81	[1]
Interest cost	183	[1],[2]	163	[1],[2]	166	[1]
Expected return on plan assets	-229	[1]	-218	[1]	-208	[1]
Prior service credit	-1	[1]	-2	[1]	-3	[1]
Actuarial (gain) loss	49	[1]	87	[1]	75	[1]
Curtailment			-12	[1]
Net periodic benefit cost (benefit)	81	[1]	109	[1],[3]	111	[1],[4]
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	19	[1],[2]	22	[1],[2]	22	[1]
Interest cost	50	[1],[2]	46	[1],[2]	47	[1]
Expected return on plan assets	-65	[1]	-62	[1]	-56	[1]
Transition obligation					2	[1]
Prior service credit	-5	[1]	-6	[1]	-6	[1]
Actuarial (gain) loss	-7	[1]	8	[1]	5	[1]
Curtailment			-7	[1]
Net periodic benefit cost (benefit)	-8	[1]	1	[1],[3]	14	[1],[4]
New Ameren Energy Resources Company, LLC | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (benefit)	6	[1],[3]	9	[1],[3]
New Ameren Energy Resources Company, LLC | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (benefit)	$7	[1],[3]
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries
[2]	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
[3]	(b)The net periodic benefit cost includes a $6 million and a $7 million net gain for pension benefits and postretirement benefits, respectively, which was included in "Income (loss) from discontinued operations, net of taxes" on Ameren's consolidated statement of income (loss). This net gain includes the curtailment gain recognized in 2013 as a result of a significant reduction in employees as of the December 2, 2013 closing date of the New AER divestiture. See Note 16 – Divestiture Transactions and Discontinued Operations for additional information on the divestiture.
[4]	(c)The net periodic benefit cost includes $9 million and $- million in total net costs for pension benefits and postretirement benefits, respectively, which were included in "Income (loss) from discontinued operations, net of taxes" on Ameren's consolidated statement of income (loss). See Note 16 – Divestiture Transactions and Discontinued Operations for additional information on the divestiture.

Retirement_Benefits_Summary_Of1

Retirement Benefits (Summary Of Estimated Amortizable Amounts From Regulatory Assets and Accumulated OCI Into Net Periodic Benefit Cost) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	($1)	[1]
Net actuarial loss	86	[1]
Net actuarial (gain) loss	2	[1]
Net periodic benefit cost	87	[1]
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	-4	[1]
Net actuarial loss	15	[1]
Net actuarial (gain) loss	-2	[1]
Net periodic benefit cost	$9	[1]
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries.

Retirement_Benefits_Summary_Of2

Retirement Benefits (Summary Of Benefit Plan Costs Incurred) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$81	[1]	$109	[1],[2]	$111	[1],[3]
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	-8	[1]	1	[1],[2]	14	[1],[3]
Union Electric Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	50		69		63
Union Electric Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	3		8		10
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	30		41		37
Ameren Illinois Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	-9		0		4
Other | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	1		5		2
Other | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	-2				0
Parent Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	81	[4]	115	[4]	102	[4]
Parent Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	($8)	[4]	$8	[4]	$14	[4]
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries
[2]	(b)The net periodic benefit cost includes a $6 million and a $7 million net gain for pension benefits and postretirement benefits, respectively, which was included in "Income (loss) from discontinued operations, net of taxes" on Ameren's consolidated statement of income (loss). This net gain includes the curtailment gain recognized in 2013 as a result of a significant reduction in employees as of the December 2, 2013 closing date of the New AER divestiture. See Note 16 – Divestiture Transactions and Discontinued Operations for additional information on the divestiture.
[3]	(c)The net periodic benefit cost includes $9 million and $- million in total net costs for pension benefits and postretirement benefits, respectively, which were included in "Income (loss) from discontinued operations, net of taxes" on Ameren's consolidated statement of income (loss). See Note 16 – Divestiture Transactions and Discontinued Operations for additional information on the divestiture.
[4]	(a)Includes amounts for Ameren registrant and nonregistrant subsidiaries

Retirement_Benefits_Schedule_O

Retirement Benefits (Schedule Of Expected Payments From Qualified Trust And Company Funds) (Details) (USD $)	Dec. 31, 2014
In Millions, unless otherwise specified
Pension Benefits | Paid From Qualified Trust
Defined Benefit Plan Disclosure [Line Items]
2015	$253
2016	256
2017	257
2018	260
2019	260
2020 - 2024	1,273
Pension Benefits | Paid From Company Funds
Defined Benefit Plan Disclosure [Line Items]
2015	3
2016	3
2017	4
2018	3
2019	3
2020 - 2024	11
Postretirement Benefits | Paid From Qualified Trust
Defined Benefit Plan Disclosure [Line Items]
2015	58
2016	61
2017	64
2018	68
2019	70
2020 - 2024	388
Postretirement Benefits | Paid From Company Funds
Defined Benefit Plan Disclosure [Line Items]
2015	2
2016	2
2017	2
2018	2
2019	2
2020 - 2024	$11

Retirement_Benefits_Assumption1

Retirement Benefits (Assumptions Used To Determine Net Periodic Benefit Cost) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate at measurement date	4.75%	4.00%	4.50%
Expected return on plan assets	7.25%	7.50%	7.75%
Increase in future compensation	3.50%	3.50%	3.50%
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate at measurement date	4.75%	4.00%	4.50%
Expected return on plan assets	7.00%	7.25%	7.50%
Increase in future compensation	3.50%	3.50%	3.50%
Medical cost trend rate (initial)	5.00%	5.00%	5.50%
Medical cost trend rate (ultimate)	5.00%	5.00%	5.00%
Years to ultimate rate			1 year

Retirement_Benefits_Schedule_O1

Retirement Benefits (Schedule Of Potential Changes In Key Assumptions) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service Cost and Interest Cost, .25% decrease in discount rate	($1)
Benefit Obligation, .25% decrease in discount rate	138
Service Cost and Interest Cost, .25% increase in salary rate	2
Benefit Obligation, .25% increase in salary rate	13
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service Cost and Interest Cost, .25% decrease in discount rate	1
Benefit Obligation, .25% decrease in discount rate	39
Service Cost and Interest Cost, 1.00% increase in annual medical trend	3
Benefit Obligation, 1.00% increase in annual medical trend	36
Service Cost and Interest Cost, 1.00% decrease in annual medical trend	-2
Benefit Obligation, 1.00% decrease in annual medical trend	($33)

Retirement_Benefits_Schedule_O2

Retirement Benefits (Schedule Of Matching Contributions) (Details) (401 (K), USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$28	[1]	$27	[1]	$26	[1]
Union Electric Company
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	16		16		16
Ameren Illinois Company
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	11		10		9
Other
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$1		$1		$1
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries.

StockBased_Compensation_Narrat

Stock-Based Compensation (Narrative) (Details) (USD $)	12 Months Ended				1 Months Ended
In Millions, except Share data, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013	Dec. 31, 2012	Apr. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum shares available for grants	8,000,000
Share-based compensation expense	$19		$20	$22
Employee service share-based compensation, tax benefit from compensation expense	7		8	8
Amounts paid to settle share units	33		11	11
Compensation cost not yet recognized	$18
Expected weighted average recognition period for share-based compensation expense, in months	20 months
Performance Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance period	3 years
Percentage of shares issued per share unit, minimum	0.00%
Percentage of shares issued per share unit, maximum	200.00%
Fair value of each share unit, per share	$38.90	[1]	$31.19		$50.34	[2]
Closing common share price			$36.16	$30.72
Three-year risk-free rate	0.78%		0.36%
Volatility rate, minimum	12.00%		12.00%		12.00%
Volatility rate, maximum	18.00%		21.00%		18.00%
[1]	Includes performance share units (share units) granted to certain executive and nonexecutive officers and other eligible employees in 2014 under the 2006 Incentive Plan and the 2014 Incentive Plan.
[2]	In April 2014, certain executive officers were granted additional share units under the 2006 Incentive Plan and the 2014 Incentive Plan. The significant assumptions used to calculate fair value included a prorated three-year risk-free rate ranging from 0.76% to 0.79%, volatility of 12% to 18% for the peer group, and Ameren’s attainment of a three-year average earnings per share threshold during the performance period.

StockBased_Compensation_Summar

Stock-Based Compensation (Summary Of Nonvested Shares) (Details) (Performance Share Units, USD $)	1 Months Ended		12 Months Ended
	Apr. 30, 2014		Dec. 31, 2014		Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Share Units, Nonvested at beginning of year			1,218,544
Share Units, Granted	38,559	[1]	688,323	[2]
Share Units, Unearned or forfeited			-97,432	[3]
Share Units, Earned and vested			-685,617	[4]
Share Units, Nonvested at end of year			1,162,377		1,218,544
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Weighted-average Fair Value per Unit, Nonvested at beginning of year			$33.23
Weighted-averageFair Value per Unit, granted	$50.34	[1]	$38.90	[2]	$31.19
Weighted-average Fair Value per Unit, Unearned or forfeited			$34.42	[3]
Weighted-average Fair Value per Unit, Earned and vested			$36.12	[4]
Weighted-average Fair Value per Unit, Nonvested at end of year			$35.35		$33.23
Three-year risk-free rate			0.78%		0.36%
Volatility rate, minimum	12.00%		12.00%		12.00%
Volatility rate, maximum	18.00%		18.00%		21.00%
Maximum
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Three-year risk-free rate	0.79%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Three-year risk-free rate	0.76%
[1]	In April 2014, certain executive officers were granted additional share units under the 2006 Incentive Plan and the 2014 Incentive Plan. The significant assumptions used to calculate fair value included a prorated three-year risk-free rate ranging from 0.76% to 0.79%, volatility of 12% to 18% for the peer group, and Ameren’s attainment of a three-year average earnings per share threshold during the performance period.
[2]	Includes performance share units (share units) granted to certain executive and nonexecutive officers and other eligible employees in 2014 under the 2006 Incentive Plan and the 2014 Incentive Plan.
[3]	Includes share units granted in 2012 that were not earned based on performance provisions of the award grants.
[4]	Includes share units granted in 2012 that vested as of December 31, 2014, that were earned pursuant to the provisions of the award grants. Also includes share units that vested due to attainment of retirement eligibility by certain employees. Actual shares issued for retirement-eligible employees will vary depending on actual performance over the three-year measurement period.

Income_Taxes_Schedule_Of_Effec

Income Taxes (Schedule Of Effective Income Tax Rate Reconciliation) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Taxes [Line Items]
Statutory federal income tax rate:	35.00%	35.00%	35.00%
Depreciation differences			-1.00%
Amortization of investment tax credit	-1.00%	-1.00%	-1.00%
State tax	4.00%	4.00%	5.00%
Other permanent items	1.00%	0.00%	-1.00%
Effective income tax rate	39.00%	38.00%	37.00%
Union Electric Company
Income Taxes [Line Items]
Statutory federal income tax rate:	35.00%	35.00%	35.00%
Depreciation differences		0.00%	-1.00%
Amortization of investment tax credit	-1.00%	-1.00%	-1.00%
State tax	3.00%	3.00%	3.00%
Other permanent items(a)			1.00%
Other permanent items	0.00%	1.00%	0.00%
Effective income tax rate	37.00%	38.00%	37.00%
Ameren Illinois Company
Income Taxes [Line Items]
Statutory federal income tax rate:	35.00%	35.00%	35.00%
Depreciation differences		-1.00%
Amortization of investment tax credit	0.00%	0.00%	-1.00%
State tax	6.00%	6.00%	6.00%
Effective income tax rate	41.00%	40.00%	40.00%

Income_Taxes_Schedule_Of_Compo

Income Taxes (Schedule Of Components Of Income Tax Expense (Benefit)) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Income Taxes [Line Items]
Current Federal taxes	($37)		($118)		$40
Current State taxes	-37		19		10
Deferred Federal taxes	369		368		204
Deferred State taxes	88		48		60
Deferred investment tax credits, amortization	-6		-6		-7
Total income tax expense	377		311		307
Union Electric Company
Income Taxes [Line Items]
Current Federal taxes	-13		136		-25
Current State taxes	-3		41		-10
Deferred Federal taxes	222		64		248
Deferred State taxes	28		6		44
Deferred investment tax credits, amortization	-5		-5		-5
Total income tax expense	229		242		252
Ameren Illinois Company
Income Taxes [Line Items]
Current Federal taxes	-51		-15		-7
Current State taxes	-2		21		-3
Deferred Federal taxes	159		99		76
Deferred State taxes	38		6		30
Deferred investment tax credits, amortization	-1		-1		-2
Total income tax expense	143		110		94
Other
Income Taxes [Line Items]
Current Federal taxes	27	[1]	-239	[1]	72
Current State taxes	-32	[1]	-43	[1]	23
Deferred Federal taxes	-12	[1]	205	[1]	-120
Deferred State taxes	22	[1]	36	[1]	-14
Deferred investment tax credits, amortization	0		0		0
Total income tax expense	$5		($41)		($39)
[1]	These amounts are substantially related to the reversal of unrecognized tax benefits as a result of IRS guidance related to the deductibility of expenditures to maintain, replace or improve steam or electric power generation property, along with casualty loss deductions for storm damage. The amounts also reflect the increase in deferred tax expense due to available net operating losses.

Income_Taxes_Schedule_Of_Defer

Income Taxes (Schedule Of Deferred Tax Assets And Liabilities Resulting From Temporary Differences) (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Income Taxes [Line Items]
Plant related	$4,185		$3,769
Regulatory assets, net	78		76
Deferred benefit costs	-220		-273
Revenue Requirement Reconciliation Adjustment	69
Revenue requirement reconciliation adjustments			-2
ARO	-675		-541
Other	134		115
Total net accumulated deferred income tax liabilities	3,571	[1]	3,144	[2]
Current assets	352		106
Union Electric Company
Income Taxes [Line Items]
Plant related	2,776		2,513
Regulatory assets, net	82		74
Deferred benefit costs	-80		-74
ARO	-107		-76
Other	86		67
Total net accumulated deferred income tax liabilities	2,757	[1]	2,504	[2]
Current assets	49		20
Ameren Illinois Company
Income Taxes [Line Items]
Plant related	1,393		1,243
Regulatory assets, net			2
Regulatory assets, net	-5
Deferred benefit costs	-45		-85
Revenue Requirement Reconciliation Adjustment	66
Revenue requirement reconciliation adjustments			-4
ARO	-139		-95
Other			10
Deferred Tax Assets, Other	-22
Total net accumulated deferred income tax liabilities	1,248	[1]	1,071	[2]
Current assets	160		45
Other
Income Taxes [Line Items]
Plant related	16		13
Regulatory assets, net	1
Deferred benefit costs	-95		-114
Revenue Requirement Reconciliation Adjustment	3		2
ARO	-429		-370
Other	70		38
Total net accumulated deferred income tax liabilities	($434)	[1]	($431)	[2]
[1]	Includes $49 million recorded in "Other current assets" on Ameren Missouri's balance sheet as of December 31, 2014.
[2]	Includes $20 million recorded in "Other current assets" on Ameren Missouri's balance sheet as of December 31, 2013

Income_Taxes_Schedule_Of_Net_O

Income Taxes (Schedule Of Net Operating Loss Carryforwards And Tax Credit Carryforwards) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$531		$408
Tax credit carryforwards	131		118
Tax credit carryforward, expiration period start	1-Jan-13		1-Jan-13
Change in valuation allowance	3		3
Deferred Tax Assets, Charitable Contribution Carryforwards	19	[1]	18	[1]
Deferred Tax Assets, Valuation Allowance, Noncurrent	6	[2]	-3	[3]
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	13		15
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	457	[4]	360	[5]
Tax credit carryforwards	99	[6]	88	[6]
Net operating loss carryforward, expiration period start	1-Jan-28		1-Jan-28
Tax credit carryforward, expiration period start	1-Jan-29		1-Jan-29
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	74	[7]	48	[8]
Tax credit carryforwards	36	[9]	34	[9]
Tax Credit Carryforward, Valuation Allowance	-4	[10]	4	[11]
Net operating loss carryforward, expiration period start	1-Jan-20		1-Jan-19
Tax credit carryforward, expiration period start	1-Jan-13		1-Jan-13
Change in valuation allowance	1		2
Union Electric Company
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	86		64
Tax credit carryforwards	21		12
Union Electric Company | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	75	[4]	61	[5]
Tax credit carryforwards	21	[6]	12	[6]
Union Electric Company | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	11	[7]	3	[8]
Tax credit carryforwards	1	[9]	1	[9]
Tax Credit Carryforward, Valuation Allowance	-1	[10]	-1	[11]
Ameren Illinois Company
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	137		95
Tax credit carryforwards	2
Ameren Illinois Company | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	127	[4]	84	[5]
Tax credit carryforwards	1	[6]	0	[6]
Ameren Illinois Company | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	10	[7]	11	[8]
Tax credit carryforwards	2	[9]	1	[9]
Tax Credit Carryforward, Valuation Allowance	-1	[10]	-1	[11]
Other
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	308		249
Tax credit carryforwards	108		106
Deferred Tax Assets, Charitable Contribution Carryforwards	19	[1]	18	[1]
Deferred Tax Assets, Valuation Allowance, Noncurrent	6	[2]	-3	[3]
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	13		15
Other | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	255	[4]	215	[5]
Tax credit carryforwards	77	[6]	76	[6]
Other | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	53	[7]	34	[8]
Tax credit carryforwards	33	[9]	32	[9]
Tax Credit Carryforward, Valuation Allowance	($2)	[10]	($2)	[11]
[1]	(f)These began to expire in 2013.
[2]	(g)This balance increased by $3 million, $- million and $- million for Ameren, Ameren Missouri and Ameren Illinois, respectively, during 2014.
[3]	(g)This balance increased by $3 million, $- million, and $- million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, during 2013.
[4]	Will begin to expire in 2028.
[5]	Will begin to expire in 2028
[6]	Will begin to expire in 2029
[7]	Will begin to expire in 2020
[8]	Will begin to expire in 2019
[9]	Began to expire in 2013
[10]	This balance increased by less than $1 million, $- million, and $- million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, during 2014.
[11]	Balance increased by $2 million, $- million, and $- million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, during 2013

Income_Taxes_Schedule_Of_Chang

Income Taxes (Schedule Of Changes To Unrecognized Tax Benefits And Related Interest) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits	$90		$156		$148
Increases based on tax positions	6		7		5
Decreases based on tax positions	-42		-143		-13
Increases based on tax positions related to current period	0		69		17
Changes related to settlements with taxing authorities	-20		0
Decreases related to the lapse of statute of limitations			1		-1
Unrecognized tax benefits	54		90		156
Total unrecognized tax benefits (detriments) that, if recognized, would impact the effective tax rates	52		54		1
Union Electric Company
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits	31		136		124
Increases based on tax positions	1		0		4
Decreases based on tax positions	-32		-122		-7
Increases based on tax positions related to current period	0		16		15
Changes related to settlements with taxing authorities	-18		0
Decreases related to the lapse of statute of limitations			1
Unrecognized tax benefits	0		31		136
Total unrecognized tax benefits (detriments) that, if recognized, would impact the effective tax rates	0		3		3
Ameren Illinois Company
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits	-1		13		11
Increases based on tax positions	1		2		0
Decreases based on tax positions	-1		-16		-1
Increases based on tax positions related to current period	0				3
Changes related to settlements with taxing authorities			0
Unrecognized tax benefits	-1		-1		13
Total unrecognized tax benefits (detriments) that, if recognized, would impact the effective tax rates	-1				-1
Other
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Unrecognized tax benefits	60		7		13
Increases based on tax positions	4		5		1
Decreases based on tax positions	-9		-5		-5
Increases based on tax positions related to current period	0		53	[1]	-1
Changes related to settlements with taxing authorities			0
Decreases related to the lapse of statute of limitations					-1
Unrecognized tax benefits	55		60		7
Total unrecognized tax benefits (detriments) that, if recognized, would impact the effective tax rates	$53	[1]	$51	[1]	($1)
[1]	Primarily due to tax positions relating to the New AER divestiture. The income statement impact of this unrecognized tax benefit was included in "Income (loss) from discontinued operations, net of taxes" on Ameren's consolidated statement of income (loss). See Note 16 – Divestiture Transactions and Discontinued Operations for additional information.

Income_Taxes_Reconciliation_Of

Income Taxes (Reconciliation Of The Change In The Liability For Interest On Unrecognized Tax Benefits) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Contingency [Line Items]
Liability for interest	$1	$6	$5
Interest charges (income)	-1	-5	1
Liability for interest	0	1	6
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities	20	0
Union Electric Company
Income Tax Contingency [Line Items]
Liability for interest	0	8	6
Interest charges (income)	0	-8	2
Liability for interest	0	0	8
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities	18	0
Ameren Illinois Company
Income Tax Contingency [Line Items]
Liability for interest		1	1
Interest charges (income)		-1	0
Liability for interest	0	0	1
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities		0
Other
Income Tax Contingency [Line Items]
Liability for interest	1	-3	-2
Interest charges (income)	-1	4	-1
Liability for interest	0	1	-3
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities		0
2007 through 2011 tax year | Union Electric Company
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities	$13

Income_Taxes_Narrative_Details

Income Taxes (Narrative) (Details) (USD $)	12 Months Ended			1 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 31, 2011
Income Taxes [Line Items]
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities	$20	$0
State corporate income tax rate	4.00%	4.00%	5.00%
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions	103
Maximum | Illinois Corporate Income Tax
Income Taxes [Line Items]
State corporate income tax rate				9.50%
Union Electric Company
Income Taxes [Line Items]
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities	18	0
State corporate income tax rate	3.00%	3.00%	3.00%
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions	95
Ameren Illinois Company
Income Taxes [Line Items]
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities		0
State corporate income tax rate	6.00%	6.00%	6.00%
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions	5
Income Tax Rate in 2015 | Illinois Corporate Income Tax
Income Taxes [Line Items]
State corporate income tax rate	7.75%
Income Tax Rate in 2025 | Illinois Corporate Income Tax
Income Taxes [Line Items]
State corporate income tax rate	7.30%
State
Income Taxes [Line Items]
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities	$2

Related_Party_Transactions_Det

Related Party Transactions (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	$27		$25		$20
Operating Expenses	124		116		106
Union Electric Company | Ameren Missouri Power Supply Agreements with Ameren Illinois
Related Party Transaction [Line Items]
Operating Revenues	5		3		1	[1]
Union Electric Company | Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	21		21		19
Union Electric Company | Ameren Missouri and Ameren Illinois miscellaneous support services
Related Party Transaction [Line Items]
Operating Revenues	1		1		1
Union Electric Company | Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	124		116		106
Union Electric Company | Insurance Premiums
Related Party Transaction [Line Items]
Operating Expenses	1	[1],[2]	1	[1],[2]	1	[1],[2]
Union Electric Company | Money Pool
Related Party Transaction [Line Items]
Interest (Charges) Income	1	[1]	1	[1]	1	[1]
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	2		4		1
Operating Expenses	109		93		88
Ameren Illinois Company | Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	2		1		1
Ameren Illinois Company | Ameren Missouri and Ameren Illinois miscellaneous support services
Related Party Transaction [Line Items]
Operating Revenues	1		3		1	[1]
Ameren Illinois Company | Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Operating Expenses	5		1	[1]
Ameren Illinois Company | Ameren Illinois transmission agreements with ATXI
Related Party Transaction [Line Items]
Operating Expenses	2		2		3
Ameren Illinois Company | Purchased Power
Related Party Transaction [Line Items]
Operating Expenses	7		5		3
Ameren Illinois Company | Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	109		93		88
Ameren Illinois Company | Money Pool
Related Party Transaction [Line Items]
Interest (Charges) Income	$1	[1]	$1	[1]	$0	[1]
[1]	Amount less than $1 million.
[2]	Represents insurance premiums paid to Missouri Energy Risk Assurance Company LLC, an affiliate, for replacement power.

Related_Party_Transactions_Nar

Related Party Transactions (Narrative) (Details) (USD $)	1 Months Ended			12 Months Ended
In Millions, unless otherwise specified	Apr. 30, 2012	31-May-11	Apr. 30, 2010	Dec. 31, 2014
		MWh		MWh
Period Five | Union Electric Company
Related Party Transaction [Line Items]
Energy Supply Agreements Amount	$1
Period Five | Ameren Illinois Company | Union Electric Company
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power		40,800
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate		28
Period Six | Union Electric Company
Related Party Transaction [Line Items]
Energy Supply Agreements Amount	3
Period Three | Ameren Illinois Company | Union Electric Company
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power		16,800
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate		37
Period Four | Union Electric Company
Related Party Transaction [Line Items]
Energy Supply Agreements Amount			$1
Period Four | Ameren Illinois Company | Union Electric Company
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power		40,800
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate		29
Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power				168,400
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate				51

Commitments_And_Contingencies_1

Commitments And Contingencies (Callaway Nuclear Energy Center) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014
Commitments and Contingencies [Line Items]
Threshold Amount For Retrospective Insurance Assessment For Covered Loss Under Public Liability And Nuclear Worker Liability Insurance Policy	$375,000,000
Insurance aggregate maximum coverage	13,616,000,000	[1]
Insurance maximum coverage per incident	128,000,000
Annual payment in the event of an incident at any licensed commercial reactor	19,000,000
Aggregate maximum assessment per incident under Price-Anderson liability provisions of Atomic Energy Act	128,000,000
Maximum annual payment to be paid in a calendar year per reactor incident under liability provisions of Atomic Energy Act	19,000,000
Amount of coverage in excess of primary property liability coverage	2,250,000,000
Amount of weekly indemnity coverage commencing eight weeks after power outage	4,500,000
Number of weeks of coverage after the first eight weeks of an outage	1 year
Amount of additional weekly indemnity coverage commencing after initial indemnity coverage	3,600,000
Number of additional weeks after initial indemnity coverage for power outage, minimum	1 year 4 months 10 days
Amount of weekly indemnity coverage thereafter not exceeding policy limit	490,000,000
Amount of secondary weekly indemnity coverage for prolonged nuclear plant outage in excess of primary indemnity coverage	900,000
Aggregate nuclear power industry insurance policy limit for losses from terrorist attacks within twelve month period	3,240,000,000
Public Liability And Nuclear Worker Liability - American Nuclear Insurers
Commitments and Contingencies [Line Items]
Insurance aggregate maximum coverage	375,000,000
Insurance maximum coverage per incident	0
Public Liability And Nuclear Worker Liability - Pool Participation
Commitments and Contingencies [Line Items]
Insurance aggregate maximum coverage	13,241,000,000	[2]
Insurance maximum coverage per incident	128,000,000	[3]
Property Damage - Nuclear Electric Insurance Ltd
Commitments and Contingencies [Line Items]
Insurance aggregate maximum coverage	2,250,000,000	[4]
Insurance maximum coverage per incident	23,000,000	[5]
Replacement Power - Nuclear Electric Insurance Ltd
Commitments and Contingencies [Line Items]
Insurance aggregate maximum coverage	490,000,000	[6]
Insurance maximum coverage per incident	9,000,000	[5]
Replacement Power - Energy Risk Assurance Company
Commitments and Contingencies [Line Items]
Insurance aggregate maximum coverage	64,000,000	[7]
Insurance maximum coverage per incident	0
Property Damage European Mutual Association for Nuclear Insurance
Commitments and Contingencies [Line Items]
Insurance aggregate maximum coverage	500,000,000	[8]
Amount of primary property liability coverage	500,000,000
Property Damage
Commitments and Contingencies [Line Items]
Insurance aggregate maximum coverage	2,750,000,000
Insurance maximum coverage per incident	23,000,000
Non-radiation event [Member]
Commitments and Contingencies [Line Items]
Aggregate nuclear power industry insurance policy limit for losses from terrorist attacks within twelve month period	$1,830,000,000
[1]	Limit of liability for each incident under the Price-Anderson Act liability provisions of the Atomic Energy Act of 1954, as amended. A company could be assessed up to $128 million per incident for each licensed reactor it operates, with a maximum of $19 million per incident to be paid in a calendar year for each reactor. This limit is subject to change to account for the effects of inflation and changes in the number of licensed reactors.
[2]	Provided through mandatory participation in an industrywide retrospective premium assessment program.
[3]	Retrospective premium under the Price-Anderson Act. This is subject to retrospective assessment with respect to a covered loss in excess of $375 million in the event of an incident at any licensed United States commercial reactor, payable at $19 million per year.
[4]	NEIL provides $2.25 billion in property damage, decontamination, and premature decommissioning insurance.
[5]	NEIL-insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL.
[6]	Provides replacement power cost insurance in the event of a prolonged accidental outage. Weekly indemnity is up to $4.5 million for 52 weeks, which commences after the first eight weeks of an outage, plus up to $3.6 million per week for a minimum of 71 weeks thereafter, for a total not exceeding the policy limit of $490 million. Nonradiation events are sub-limited to $328 million.
[7]	Provides replacement power cost insurance in the event of a prolonged accidental outage. The coverage commences after the first 52 weeks of insurance coverage from NEIL concludes; it is a weekly indemnity of up to $0.9 million for 71 weeks in excess of the $3.6 million per week set forth above. Missouri Energy Risk Assurance Company LLC is an affiliate; it has reinsured this coverage with third-party insurance companies. See Note 14 – Related Party Transactions for more information on this affiliate transaction.
[8]	European Mutual Association for Nuclear Insurance provides $500 million in excess of the $2.25 billion property coverage provided by NEIL.

Commitments_And_Contingencies_2

Commitments And Contingencies (Schedule Of Lease Obligations) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Commitments and Contingencies [Line Items]
Capital lease payments, due in one year	$33	[1],[2]
Capital lease payments, due in two years	33	[1],[2]
Capital lease payments, due in three years	33	[1],[2]
Capital lease payments, due in four years	32	[1],[2]
Capital lease payments, due in five years	32	[1],[2]
Capital lease payments, After 5 Years	360	[1],[2]
Capital lease payments, Total	523	[1],[2]
Less Amount representing interest, due in one year	27	[1]
Less Amount representing interest, due in two years	27	[1]
Less Amount representing interest, due in three years	27	[1]
Less Amount representing interest, due in four years	26	[1]
Less Amount representing interest, due in 5 years	25	[1]
Less Amount representing interest, After 5 Years	97	[1]
Less Amount representing interest, Total	229	[1]
Present value of minimum capital lease payments, due in one year	6	[1]
Present value of minimum capital lease payments, due in two years	6	[1]
Present value of minimum capital lease payments, due in three years	6	[1]
Present value of minimum capital lease payments, due in four years	6	[1]
Present value of minimum capital lease payments, due in five years	7	[1]
Present value of minimum capital lease payments, After 5 Years	263	[1]
Present value of minimum capital lease payments, Total	294	[1]
Operating leases, due in one year	13	[1],[3]
Operating leases, due in two years	12	[1],[3]
Operating leases, due in three years	12	[1],[3]
Operating leases, due in four years	12	[1],[3]
Operating leases, due in five years	11	[1],[3]
Operating leases, After 5 Years	38	[1],[3]
Operating leases, Total	98	[1],[3]
Total lease obligations, due in one year	19	[1]
Total lease obligations, due in two years	18	[1]
Total lease obligations, due in three years	18	[1]
Total lease obligations, due in four years	18	[1]
Total lease obligations, due in five years	18	[1]
Total lease obligations, After 5 Years	301	[1]
Total lease obligations, Total	392	[1]
Annual obligation for real estate leases and railroad licenses	2
Total rental expense	37	[4]	32	[4]	33	[4]
Union Electric Company
Commitments and Contingencies [Line Items]
Capital lease payments, due in one year	33	[2]
Capital lease payments, due in two years	33	[2]
Capital lease payments, due in three years	33	[2]
Capital lease payments, due in four years	32	[2]
Capital lease payments, due in five years	32	[2]
Capital lease payments, After 5 Years	360	[2]
Capital lease payments, Total	523	[2]
Less Amount representing interest, due in one year	27
Less Amount representing interest, due in two years	27
Less Amount representing interest, due in three years	27
Less Amount representing interest, due in four years	26
Less Amount representing interest, due in 5 years	25
Less Amount representing interest, After 5 Years	97
Less Amount representing interest, Total	229
Present value of minimum capital lease payments, due in one year	6
Present value of minimum capital lease payments, due in two years	6
Present value of minimum capital lease payments, due in three years	6
Present value of minimum capital lease payments, due in four years	6
Present value of minimum capital lease payments, due in five years	7
Present value of minimum capital lease payments, After 5 Years	263
Present value of minimum capital lease payments, Total	294
Operating leases, due in one year	11	[3]
Operating leases, due in two years	11	[3]
Operating leases, due in three years	11	[3]
Operating leases, due in four years	10	[3]
Operating leases, due in five years	10	[3]
Operating leases, After 5 Years	37	[3]
Operating leases, Total	90	[3]
Total lease obligations, due in one year	17
Total lease obligations, due in two years	17
Total lease obligations, due in three years	17
Total lease obligations, due in four years	16
Total lease obligations, due in five years	17
Total lease obligations, After 5 Years	300
Total lease obligations, Total	384
Annual obligation for real estate leases and railroad licenses	1
Total rental expense	32		29		29
Ameren Illinois Company
Commitments and Contingencies [Line Items]
Operating leases, due in one year	1	[3]
Operating leases, due in two years	1	[3]
Operating leases, due in three years	1	[3]
Operating leases, due in four years	1	[3]
Operating leases, due in five years	1	[3]
Operating leases, After 5 Years	1	[3]
Operating leases, Total	6	[3]
Annual obligation for real estate leases and railroad licenses	1
Total rental expense	$25		$21		$19
[1]	ncludes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.
[2]	See Properties under Part I, Item 2, and Note 3 – Property and Plant, Net, of this report for additional information.
[3]	Amounts related to certain land-related leases have indefinite payment periods. The annual obligations of $2 million, $1 million, and $1 million for Ameren, Ameren Missouri, and Ameren Illinois for these items are included in the 2015 through 2019 columns, respectively.
[4]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Commitments_And_Contingencies_3

Commitments And Contingencies (Schedule Of Estimated Purchased Power Commitments) (Details) (USD $)	Dec. 31, 2014
In Millions, unless otherwise specified
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2014	$1,317	[1]
2015	1,029	[1]
2016	949	[1]
2017	357	[1]
2018	303	[1]
Thereafter	1,231	[1]
Total	5,186	[1]
Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2014	654	[1]
2015	659	[1]
2016	682	[1]
2017	111	[1]
2018	114	[1]
Thereafter	0	[1]
Total	2,220	[1]
Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2014	222	[1],[2]
2015	125	[1],[2]
2016	85	[1],[2]
2017	53	[1],[2]
2018	32	[1],[2]
Thereafter	71	[1],[2]
Total	588	[1],[2]
Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2014	53	[1]
2015	60	[1]
2016	59	[1]
2017	82	[1]
2018	42	[1]
Thereafter	138	[1]
Total	434	[1]
Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2014	190	[1],[3]
2015	104	[1],[3]
2016	66	[1],[3]
2017	55	[1],[3]
2018	56	[1],[3]
Thereafter	596	[1],[3]
Total	1,067	[1],[3]
Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2014	3	[1]
2015	3	[1]
2016	4	[1]
2017	5	[1]
2018	5	[1]
Thereafter	76	[1]
Total	96	[1]
Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2014	195	[1]
2015	78	[1]
2016	53	[1]
2017	51	[1]
2018	54	[1]
Thereafter	350	[1]
Total	781	[1]
Union Electric Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2014	898
2015	804
2016	809
2017	257
2018	219
Thereafter	525
Total	3,512
Union Electric Company | Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2014	654
2015	659
2016	682
2017	111
2018	114
Thereafter	0
Total	2,220
Union Electric Company | Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2014	39	[2]
2015	22	[2]
2016	17	[2]
2017	11	[2]
2018	10	[2]
Thereafter	22	[2]
Total	121	[2]
Union Electric Company | Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2014	53
2015	60
2016	59
2017	82
2018	42
Thereafter	138
Total	434
Union Electric Company | Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2014	21
2015	21
2016	21
2017	21
2018	21
Thereafter	106
Total	211
Union Electric Company | Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2014	3
2015	3
2016	4
2017	5
2018	5
Thereafter	76
Total	96
Union Electric Company | Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2014	128
2015	39
2016	26
2017	27
2018	27
Thereafter	183
Total	430
Ameren Illinois Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2014	381
2015	210
2016	137
2017	100
2018	84
Thereafter	706
Total	1,618
Ameren Illinois Company | Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2014	183	[2]
2015	103	[2]
2016	68	[2]
2017	42	[2]
2018	22	[2]
Thereafter	49	[2]
Total	467	[2]
Ameren Illinois Company | Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2014	169	[3]
2015	83	[3]
2016	45	[3]
2017	34	[3]
2018	35	[3]
Thereafter	490	[3]
Total	856	[3]
Ameren Illinois Company | Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2014	29
2015	24
2016	24
2017	24
2018	27
Thereafter	167
Total	295
Investment in Energy Efficiency Programs | Union Electric Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2015	$71
[1]	(c)Includes amounts for Ameren registrant and nonregistrant subsidiaries.
[2]	(a)Includes amounts for generation and for distribution.
[3]	(b)The purchased power amounts for Ameren and Ameren Illinois include agreements through 2032 for renewable energy credits with various renewable energy suppliers. The agreements contain a provision that allows Ameren Illinois to reduce the quantity purchased in the event that Ameren Illinois would not be able to recover the costs associated with the renewable energy credits.

Commitments_And_Contingencies_4

Commitments And Contingencies (Environmental Matters) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014
EPA Clean Power Plan CO2 Proposed Reductions	30.00%
Estimated Environmental Expenditures to Comply with Clean Power Plan	$2,000,000,000
Manufactured Gas Plant
Accrual for environmental loss contingencies	250,000,000
Manufactured Gas Plant | Ameren Illinois Company
Number of remediation sites	44
Loss contingency range of possible loss, minimum	250,000,000
Loss contingency range of possible loss, maximum	314,000,000
Accrual for environmental loss contingencies	250,000,000
Former Coal Ash Landfill | Ameren Illinois Company
Loss contingency range of possible loss, minimum	500,000
Loss contingency range of possible loss, maximum	6,000,000
Accrual for environmental loss contingencies	500,000
Other Environmental | Ameren Illinois Company
Accrual for environmental loss contingencies	700,000
Former Coal Tar Distillery | Union Electric Company
Loss contingency range of possible loss, minimum	2,000,000
Loss contingency range of possible loss, maximum	5,000,000
Accrual for environmental loss contingencies	2,000,000
Sauget Area 2 | Union Electric Company
Loss contingency range of possible loss, minimum	1,000,000
Loss contingency range of possible loss, maximum	2,500,000
Accrual for environmental loss contingencies	1,000,000
Substation in St Charles, Missouri | Union Electric Company
Loss contingency range of possible loss, minimum	1,600,000
Loss contingency range of possible loss, maximum	4,500,000
Accrual for environmental loss contingencies	1,600,000
Minimum
Estimated capital costs to comply with existing and known federal and state air emissions regulations	350,000,000
Maximum
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$400,000,000

Commitments_And_Contingencies_5

Commitments And Contingencies (Pumped-Storage Hydroelectric Facility Breach) (Details) (Union Electric Company, USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014
Union Electric Company
Commitments and Contingencies [Line Items]
Litigation Settlement, Amount	$27
Insurance Settlements Receivable	$41

Commitments_And_Contingencies_6

Commitments And Contingencies (Asbestos-Related Litigation And Tax Exemptions And Credits) (Details) (USD $)	12 Months Ended		3 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2014
	defendant
Asbestos-Related
Loss Contingencies [Line Items]
Number of defendants	78
Number of pending asbestos lawsuits as of the balance sheet date	70	[1]	70	[1]
Loss contingency range of possible loss, minimum	$12		$12
Percent of allowed cash expenditures in excess of base rates to be recovered through charges assessed to customers	90.00%		90.00%
Asbestos trust fund balance	22
Percent of difference to be contributed to the asbestos trust fund if cash expenditures are less than amount included in base electric rates.	90.00%		90.00%
Parent Company | Asbestos-Related
Loss Contingencies [Line Items]
Number of pending asbestos lawsuits as of the balance sheet date	1		1
Union Electric Company | Asbestos-Related
Loss Contingencies [Line Items]
Number of pending asbestos lawsuits as of the balance sheet date	45		45
Loss contingency range of possible loss, minimum	5		5
Ameren Illinois Company | Asbestos-Related
Loss Contingencies [Line Items]
Number of pending asbestos lawsuits as of the balance sheet date	57		57
Loss contingency range of possible loss, minimum	7		7
State | Ameren Illinois Company
Loss Contingencies [Line Items]
Loss Contingency, Damages Awarded, Value			$1
[1]	(a)Total does not equal the sum of the subsidiary unit lawsuits because some of the lawsuits name multiple Ameren entities as defendants.

Divestiture_Transactions_and_D2

Divestiture Transactions and Discontinued Operations (Narrative) (Details) (USD $)	9 Months Ended	12 Months Ended	1 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2012	Jan. 31, 2014	Dec. 31, 2013	Dec. 31, 2013	Mar. 31, 2012	Dec. 31, 2014	Mar. 14, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Asset retirement obligations				$369	$369		$396
Shutdown Of Meredosia And Hutsonville Energy Centers [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Asset retirement obligations				31	31		32
New Ameren Energy Resources Company, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment of assets to be disposed of		2,580
Working Capital and Contingent Liability Payment	13
Loss contingency range of possible loss, maximum							29
Collateral to be posted if credit ratings are below investment grade							26
Buyer's indemnification guarantee obligation								25
Loss Contingency Accrual							29
Guarantees Outstanding							114
Letters of Credit Outstanding, Amount							9
Notes Payable, Other Payables | New Ameren Energy Resources Company, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Notes, Loans and Financing Receivable, Net, Noncurrent				18	18		12
Elgin, Gibson City and Grand Tower Energy Centers
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Sales of Business, Affiliate and Productive Assets			168	137.5
Proceeds from Asset Sale Held in Escrow			17
Disposal group, fair value				137.5	137.5
Merchant Generation
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment of assets to be disposed of		1,950			201
Discontinued operations deferred tax expense					99
Discontinued operations deferred tax benefit					86
Merchant Generation | Duck Creek Energy Center
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment of assets to be disposed of						628
Ameren Energy Marketing Company
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Guarantees, Maximum Exposure							11
Guarantees Outstanding							106
Guarantee Type, Other [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Guarantees, Maximum Exposure							7
Guarantees Outstanding							$8

Divestiture_Transactions_and_D3

Divestiture Transactions and Discontinued Operations (Components of Discontinued Operations in Consolidated Statement of Income) (Details) (USD $)	12 Months Ended					3 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013		Dec. 31, 2012		Mar. 31, 2012
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Operating revenues	$1	$1,037		$1,047
Operating expenses	-2	-1,207	[1]	-3,474	[2]
Operating income (loss)	-1	-170		-2,427
Other income (loss)	0	-1		0
Interest charges	0	-39		-56
Income (loss) before income taxes	-1	-210		-2,483
Income tax (expense) benefit	0	-13		987
Income (Loss) from Discontinued Operations, Net of Tax (Note 16)	-1	-223		-1,496
New Ameren Energy Resources Company, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment of assets to be disposed of				2,580
Merchant Generation
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment of assets to be disposed of		201		1,950
Merchant Generation | Duck Creek Energy Center
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Impairment of assets to be disposed of						$628
[1]	(a)Includes a $201 million pretax loss on disposal relating to the New AER divestiture.
[2]	(b)Includes a noncash pretax asset impairment charge of $2.58 billion to reduce the carrying value of AER's energy centers to their estimated fair value under held and used accounting guidance.

Divestiture_Transactions_and_D4

Divestiture Transactions and Discontinued Operations (Components of Assets and Liabilities on Consolidated Balance Sheet) (Details) (USD $)	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
In Millions, unless otherwise specified
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Asset retirement obligations	$396		$369
Assets of discontinued operations
Accounts receivable and unbilled revenue	0		5
Materials and supplies	0		5
Property and plant, net	0		142
Accumulated deferred income taxes, net	15	[1]	13	[1]
Total assets of discontinued operations	15		165		1,611
Liabilities of discontinued operations
Accounts payable and other current obligations	1		5
Asset retirement obligations	32	[2]	40	[2]
Total liabilities of discontinued operations	33		45
Shutdown Of Meredosia And Hutsonville Energy Centers [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Asset retirement obligations	$32		$31
[1]	(a)The December 31, 2014 balance primarily consists of deferred income tax assets related to the abandoned Meredosia and Hutsonville energy centers.
[2]	(b)Includes AROs associated with the abandoned Meredosia and Hutsonville energy centers of $32 million and $31 million at December 31, 2014 and 2013, respectively.

Segment_Information_Details

Segment Information (Details) (USD $)	3 Months Ended										12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013		Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
											segment
Segment Reporting Information [Line Items]
Number of reportable segments											2
External revenues	$1,370		$1,670	$1,419	$1,594	$1,322		$1,638	$1,403	$1,475	$6,053		$5,838		$5,781
Depreciation and amortization											745		706		673
Interest Expense											341		398		392
Income taxes (benefit)											377		311		307
Net income (loss) attributable to Ameren Corporation	48		293	149	96	37		302	95	-145	586		289		-974
Capital expenditures											1,785		1,379		1,063
Total assets	22,676					21,042					22,676		21,042
Assets of discontinued operations (Note 16)	15					165					15		165		1,611
Income (Loss) from Continuing Operations Attributable to Parent	46		294	150	97	48		305	105	54	587		512		516
Union Electric Company
Segment Reporting Information [Line Items]
External revenues											3,526		3,516		3,252
Revenue from Related Parties											27		25		20
Depreciation and amortization											473		454		440
Interest and dividend income											28		27		32
Interest Expense											211		210		223
Income taxes (benefit)											229		242		252
Net income (loss) attributable to Ameren Corporation											390		395		416
Capital expenditures											747		648		595
Total assets	13,541					12,904					13,541		12,904		13,043
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues											2,496		2,307		2,524
Revenue from Related Parties											2		4		1
Depreciation and amortization											263		243		221
Interest and dividend income											7		2		0
Interest Expense											112		143		129
Income taxes (benefit)											143		110		94
Net income (loss) attributable to Ameren Corporation											201		160		141
Capital expenditures											835		701		442
Total assets	8,381					7,454					8,381		7,454		7,282
Other Segment
Segment Reporting Information [Line Items]
External revenues											31		15		5
Revenue from Related Parties											2		2		3
Depreciation and amortization											9		9		12
Interest and dividend income											2		1
Interest Expense											18		45		40
Income taxes (benefit)											5		-41		-39
Net income (loss) attributable to Ameren Corporation											-4		-43		-41
Capital expenditures											203	[1]	30	[1]	26	[1]
Total assets	942					752					942		752		1,228
Intersegment Elimination
Segment Reporting Information [Line Items]
Revenue from Related Parties											-31		-31		-24
Total assets	-203					-233					-203		-233		-934
Segment, Continuing Operations
Segment Reporting Information [Line Items]
External revenues											6,053		5,838		5,781
Depreciation and amortization											745		706		673
Interest and dividend income											37		30		32
Interest Expense											341		398		392
Income taxes (benefit)											377		311		307
Capital expenditures											1,785		1,379		1,063
Total assets	22,661	[2]				20,877	[2]				22,661	[2]	20,877	[2]	20,619	[2]
Union Electric Company
Segment Reporting Information [Line Items]
External revenues	739		1,097	900	817	763		1,093	889	796	3,553		3,541		3,272
Revenue from Related Parties											27		25		20
Depreciation and amortization											473		454		440
Interest Expense											211		210		223
Income taxes (benefit)											229		242		252
Net income (loss) attributable to Ameren Corporation	-5		223	127	48	33		239	85	41	393		398		419
Capital expenditures											747		648		595
Total assets	13,541					12,904					13,541		12,904
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	633		572	519	774	564		547	516	684	2,498		2,311		2,525
Revenue from Related Parties											2		4		1
Depreciation and amortization											263		243		221
Interest Expense											112		143		129
Income taxes (benefit)											143		110		94
Net income (loss) attributable to Ameren Corporation	46		75	29	54	22		77	32	32	204		163		144
Capital expenditures											835		701		442
Total assets	$8,381					$7,454					$8,381		$7,454
[1]	Includes the elimination of intercompany transfers.
[2]	Excludes total assets from discontinued operations of $15 million, $165 million, and $1,611 million as of December 31, 2014, 2013, and 2012, respectively.

Selected_Quarterly_Information2

Selected Quarterly Information (Summary Of Selected Quarterly Information) (Details) (USD $)	3 Months Ended								12 Months Ended
In Millions, except Per Share data, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Selected Quarterly Financial Information [Line Items]
Operating revenues	$1,370	$1,670	$1,419	$1,594	$1,322	$1,638	$1,403	$1,475	$6,053	$5,838	$5,781
Operating Income	125	561	322	246	171	567	261	185	1,254	1,184	1,188
Net income (loss)	49	295	150	98	38	304	96	-143	592	295	-974
Net income attributable to Ameren Corporation - continuing operations	46	294	150	97	48	305	105	54	587	512	516
Net Income (Loss) Attributable to Ameren Corporation - Discontinued Operations	2	-1	-1	-1	-11	-3	-10	-199	-1	-223	-1,490
Net income (loss) attributable to Ameren Corporation	48	293	149	96	37	302	95	-145	586	289	-974
Earnings per common share - basic - continuing operations	$0.19	$1.21	$0.62	$0.40	$0.19	$1.26	$0.44	$0.22	$2.42	$2.11	$2.13
Earnings (loss) per common share - basic - discontinued operations	$0.01		($0.01)	$0	($0.04)	($0.01)	($0.05)	($0.82)	$0	($0.92)	($6.14)
Earnings (loss) per common share - basic	$0.20	$1.21	$0.61	$0.40	$0.15	$1.25	$0.39	($0.60)	$2.42	$1.19	($4.01)
Earnings per common share - diluted - continuing operations	$0.19	$1.20	$0.62	$0.40	$0.19	$1.25	$0.44	$0.22	$2.40	$2.10	$2.13
Earnings (loss) per common share - diluted - discontinued operations	$0.01	$0	($0.01)	$0	($0.04)	($0.01)	($0.05)	($0.82)	$0	($0.92)	($6.14)
Earnings (loss) per common share - diluted	$0.20	$1.20	$0.61	$0.40	$0.15	$1.24	$0.39	($0.60)	$2.40	$1.18	($4.01)
Union Electric Company
Selected Quarterly Financial Information [Line Items]
Operating revenues	739	1,097	900	817	763	1,093	889	796	3,553	3,541	3,272
Operating Income	29	394	243	119	96	417	179	111	785	803	845
Net income (loss)									393	398	419
Net income (loss) attributable to Ameren Corporation	-5	223	127	48	33	239	85	41	393	398	419
Net Income Available to Common Stockholder	-5	222	126	47	33	238	84	40	390	395	416
Ameren Illinois Company
Selected Quarterly Financial Information [Line Items]
Operating revenues	633	572	519	774	564	547	516	684	2,498	2,311	2,525
Operating Income	97	158	75	120	85	158	87	85	450	415	377
Net income (loss)									204	163	144
Net income (loss) attributable to Ameren Corporation	46	75	29	54	22	77	32	32	204	163	144
Net Income Available to Common Stockholder	$45	$75	$28	$53	$21	$77	$31	$31	$201	$160	$141

Schedule_I_Condensed_Financial1

Schedule I - Condensed Financial Information Of Parent (Statement of Income) (Details) (USD $)	3 Months Ended								12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Operating revenues	$1,370	$1,670	$1,419	$1,594	$1,322	$1,638	$1,403	$1,475	$6,053		$5,838		$5,781
Operating expenses									4,799		4,654		4,593
Operating Income	125	561	322	246	171	567	261	185	1,254		1,184		1,188
Interest income from affiliates									10	[1],[2]	3	[1]	4	[1],[3]
Other Nonoperating Income (Expense)									57		43		33
Interest charges									341		398		392
Income taxes (benefit)									377		311		307
Net income attributable to Ameren Corporation - continuing operations	46	294	150	97	48	305	105	54	587		512		516
Net Income (Loss) Attributable to Ameren Corporation - Discontinued Operations	2	-1	-1	-1	-11	-3	-10	-199	-1		-223		-1,490
Net income (loss) attributable to Ameren Corporation	48	293	149	96	37	302	95	-145	586		289		-974
Comprehensive Income from Continuing Operations
Pension and other postretirement activity, net of income taxes (benefit)									-12		30		-8
Comprehensive Income (Loss) from Continuing Operations, Net of Tax, Portion Attributable to Parent									575		542		508
Other Comprehensive Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest											-18		58
Comprehensive Income (Loss) from Discontinued Operations									-1		-242		-1,440
Comprehensive Income (Loss) Attributable to Ameren Corporation									574		300		-932
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement activity, tax (benefit)									-7		16		-6
Parent Company
Condensed Financial Statements, Captions [Line Items]
Operating expenses									11		26		17
Operating Income									-11		-26		-17
Equity in earnings of subsidiaries									607		546		546
Interest income from affiliates									3		3		3
Other Nonoperating Income (Expense)									2		-5		-4
Interest charges									16		42		39
Income taxes (benefit)									-2		-36		-27
Net income attributable to Ameren Corporation - continuing operations									587		512		516
Net Income (Loss) Attributable to Ameren Corporation - Discontinued Operations									-1		-223		-1,490
Net income (loss) attributable to Ameren Corporation									586		289		-974
Comprehensive Income from Continuing Operations
Pension and other postretirement activity, net of income taxes (benefit)									-12		30		-8
Comprehensive Income (Loss) from Continuing Operations, Net of Tax, Portion Attributable to Parent									575		542		508
Other Comprehensive Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest									0		-19		50
Comprehensive Income (Loss) from Discontinued Operations									-1		-242		-1,440
Comprehensive Income (Loss) Attributable to Ameren Corporation									574		300		-932
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement activity, tax (benefit)									($7)		$16		($6)
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.
[2]	Includes Ameren Illinois' interest income received in 2014 relating to the 2013 and 2014 IEIMA revenue requirement reconciliation regulatory assets.
[3]	Includes Ameren Missouri's interest income relating to a refund of charges included in an expired power purchase agreement with Entergy. See Note 2 – Rate and Regulatory Matters for additional information.

Schedule_I_Condensed_Financial2

Schedule I - Condensed Financial Information Of Parent (Balance Sheet) (Details) (USD $)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
In Millions, unless otherwise specified
ASSETS
Cash and cash equivalents	$5	$30	$184
Miscellaneous accounts and notes receivable	81	196
Current accumulated deferred income taxes, net	352	106
Other current assets	86	85
Total current assets	2,046	1,972
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	13	15
Other non-current assets	664	720
TOTAL ASSETS	22,676	21,042
LIABILITIES AND EQUITY
Current maturities of long-term debt	120	534
Short-term debt	714	368
Accounts Payable and Accrued Liabilities, Current	711	806
Other current liabilities	434	351
Total current liabilities	2,249	2,461
Other deferred credits and liabilities	514	538
Commitments and Contingencies
Retained earnings	1,103	907
Accumulated other comprehensive income (loss)	-9	3
Total equity	6,855	6,686	6,767
TOTAL LIABILITIES AND EQUITY	22,676	21,042
Parent Company
ASSETS
Cash and cash equivalents	1	11	23	3
Advances to money pool	55	334
Accounts receivable - affiliates	28	18
Notes Receivable, Related Parties, Current	94	9
Miscellaneous accounts and notes receivable	39	125
Current accumulated deferred income taxes, net	143	41
Other current assets	14	1
Total current assets	374	539
Investments in subsidiaries - continuing operations	6,680	6,336
Investments in subsidiaries - discontinued operations	-4	-5
Note receivable - affiliates	100	51
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	264	570
Other non-current assets	152	141
TOTAL ASSETS	7,566	7,632
LIABILITIES AND EQUITY
Current maturities of long-term debt	0	425
Short-term debt	585	368
Accounts Payable and Accrued Liabilities, Current	0	119
Accounts payable – affiliates	88	4
Other current liabilities	52	20
Total current liabilities	725	936
Other deferred credits and liabilities	128	152
Total liabilities	853	1,088
Common stock, $.01 par value, 400.0 shares authorized – shares outstanding of 242.6	2	2
Other paid-in capital, principally premium on common stock	5,617	5,632
Retained earnings	1,103	907
Accumulated other comprehensive income (loss)	-9	3
Total equity	6,713	6,544
TOTAL LIABILITIES AND EQUITY	$7,566	$7,632

Schedule_I_Condensed_Financial3

Schedule I - Condensed Financial Information Of Parent (Statement of Cash Flows) (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	$1,551	$1,693	$1,690
Cash Flows From Investing Activities:
Proceeds from Collection of Notes Receivable	95	6
Contributions to Note Receivable	-89	-5
Other	11	1	20
Net Cash Provided by (Used in) Investing Activities	-1,717	-1,723	-1,310
Cash flows from financing activities:
Dividends on common stock	-390	-388	-382
Short-term debt and credit facility borrowings, net	346	368	-148
Repayments of Other Long-term Debt	-697	-399	-760
Net cash provided by (used in) financing activities	141	-149	-426
Net change in cash and cash equivalents	-25	-179	-46
Cash and cash equivalents at beginning of year	30	184
Cash and cash equivalents at end of year	5	30	184
Noncash financing activity – dividends on common stock			7
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	514	453	532
Cash Flows From Investing Activities:
Money pool advances, net	279	-371	24
Intercompany notes receivable, net	-134	-23	-20
Investments in subsidiaries	-280	-50	-2
Return of investments	215	1	21
Proceeds from Collection of Notes Receivable	95	6	0
Contributions to Note Receivable	-89	-5	0
Other	-12	-3	-5
Net Cash Provided by (Used in) Investing Activities	74	-445	18
Cash flows from financing activities:
Dividends on common stock	-390	-388	-382
Short-term debt and credit facility borrowings, net	217	368	-148
Repayments of Other Long-term Debt	-425	0	0
Net cash provided by (used in) financing activities	-598	-20	-530
Net change in cash and cash equivalents	-10	-12	20
Cash and cash equivalents at beginning of year	11	23	3
Cash and cash equivalents at end of year	1	11	23
Cash dividends received from consolidated subsidiaries	340	570	610
Noncash investing activity - divestiture		494	0
Noncash Investing activity- investment in subsidiaries	-19	0	0
Noncash financing activity – dividends on common stock			($7)

Schedule_I_Condensed_Financial4

Schedule I - Condensed Financial Information Of Parent (Impairment and Other Charges) (Details) (USD $)	12 Months Ended			9 Months Ended	1 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2014	31-May-14
Impairment and Other Charges [Line Items]
Unrecognized Tax Benefit Recorded In Other Deferred Credits And Liabilities	$52	$84
Repayments of Other Long-term Debt	697	399	760
Parent Company
Impairment and Other Charges [Line Items]
Unrecognized Tax Benefit Recorded In Other Deferred Credits And Liabilities	53	53
Repayments of Other Long-term Debt	425	0	0
Parent Company | Merchant Generation
Impairment and Other Charges [Line Items]
Impairment charge on long-lived assets and related charges		201	1,880
New Ameren Energy Resources Company, LLC
Impairment and Other Charges [Line Items]
Working Capital and Contingent Liability Payment				13
Senior Unsecured Notes8875 Due2014 [Member] | Parent Company
Impairment and Other Charges [Line Items]
Repayments of Other Long-term Debt					$425
Long-term debt interest rate	8.88%				8.88%

Schedule_II_Valuation_And_Qual1

Schedule II - Valuation And Qualifying Accounts (Details) (USD $)	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Allowance For Doubtful Accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	$18		$17		$20
Charged to Costs and Expenses	36		35		30
Charged to Other Accounts	4	[1]	4	[1]	2	[1]
Deductions	37	[2]	38	[2]	35	[2]
Balance at End of Period	21		18		17
Valuation Allowance of Deferred Tax Assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	7		2		1
Charged to Costs and Expenses	3		5		1
Balance at End of Period	10		7		2
Union Electric Company | Allowance For Doubtful Accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	5		5		7
Charged to Costs and Expenses	16		16		11
Deductions	13	[2]	16	[2]	13	[2]
Balance at End of Period	8		5		5
Union Electric Company | Valuation Allowance of Deferred Tax Assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period							1
Balance at End of Period	1		1		1		1
Ameren Illinois Company | Allowance For Doubtful Accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	13		12		13
Charged to Costs and Expenses	20		19		19
Charged to Other Accounts	4	[1]	4	[1]	2	[1]
Deductions	24	[2]	22	[2]	22	[2]
Balance at End of Period	13		13		12
Ameren Illinois Company | Valuation Allowance of Deferred Tax Assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Charged to Costs and Expenses					1
Balance at End of Period	$1		$1		$1
[1]	Uncollectible account reserve associated with receivables purchased by Ameren Illinois from alternative retail electric suppliers, as required by the Illinois Public Utilities Act.
[2]	Uncollectible accounts charged off, less recoveries.