Ameren (AEE) 10-Q 10 Nov 14 2014 Q3 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	9 Months Ended
	Sep. 30, 2014
Entity Information [Line Items]	'
Document Type	'10-Q
Amendment Flag	'false
Document Period End Date	30-Sep-14
Document Fiscal Year Focus	'2014
Document Fiscal Period Focus	'Q3
Trading Symbol	'AEE
Entity Registrant Name	'AMEREN CORP
Entity Central Index Key	'0001002910
Current Fiscal Year End Date	'--12-31
Entity Filer Category	'Large Accelerated Filer
Entity Common Stock, Shares Outstanding	242,634,798
Union Electric Company	'
Entity Information [Line Items]	'
Document Type	'10-Q
Amendment Flag	'false
Document Period End Date	30-Sep-14
Document Fiscal Year Focus	'2014
Document Fiscal Period Focus	'Q3
Entity Registrant Name	'UNION ELECTRIC CO
Entity Central Index Key	'0000100826
Current Fiscal Year End Date	'--12-31
Entity Filer Category	'Non-accelerated Filer
Entity Common Stock, Shares Outstanding	102,123,834
Ameren Illinois Company	'
Entity Information [Line Items]	'
Document Type	'10-Q
Amendment Flag	'false
Document Period End Date	30-Sep-14
Document Fiscal Year Focus	'2014
Document Fiscal Period Focus	'Q3
Entity Registrant Name	'AMEREN ILLINOIS CO
Entity Central Index Key	'0000018654
Current Fiscal Year End Date	'--12-31
Entity Filer Category	'Non-accelerated Filer
Entity Common Stock, Shares Outstanding	25,452,373

Consolidated_Statement_of_Inco

Consolidated Statement of Income (Loss) (USD $)	3 Months Ended				9 Months Ended
In Millions, except Per Share data, unless otherwise specified	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Operating Revenues:	'		'		'		'
Electric	$1,523		$1,507		$3,864		$3,823
Gas	147		131		819		693
Total operating revenues	1,670		1,638		4,683		4,516
Operating Expenses:	'		'		'		'
Fuel	236		222		638		648
Purchased power	112		128		335		400
Gas purchased for resale	49		42		432		344
Other operations and maintenance	404		383		1,236		1,229
Depreciation and amortization	187		175		551		528
Taxes other than income taxes	121		121		362		354
Total operating expenses	1,109		1,071		3,554		3,503
Operating Income	561		567		1,129		1,013
Other Income and Expenses:	'		'		'		'
Miscellaneous income	21	[1]	20	[1]	60	[1]	51	[1]
Miscellaneous expense	7	[1]	5	[1]	20	[1]	18	[1]
Total other income (expense)	14		15		40		33
Interest Charges	85		88		266		289
Income Before Income Taxes	490		494		903		757
Income Taxes	194		187		357		288
Income from Continuing Operations	296		307		546		469
Loss from Discontinued Operations, Net of Taxes (Note 12)	-1		-3		-3		-212
Net Income (Loss)	295		304		543		257
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	'		-5		3		5
Comprehensive Income	293		292		541		241
Less: Net Income (Loss) Attributable to Noncontrolling Interests:	'		'		'		'
Net Income from Continuing Operations Attributable to Noncontrolling Interests	2		2		5		5
Net Income (Loss):	'		'		'		'
Continuing Operations	294		305		541		464
Discontinued Operations	-1		-3		-3		-212
Net Income (Loss)	293		302		538		252
Earnings Per Share, Basic [Abstract]	'		'		'		'
Continuing Operations	$1.21		$1.26		$2.23		$1.92
Discontinued Operations	'		($0.01)		($0.01)		($0.88)
Earnings Per Share, Basic	$1.21		$1.25		$2.22		$1.04
Earnings Per Share, Diluted [Abstract]	'		'		'		'
Continuing Operations	$1.20		$1.25		$2.21		$1.91
Discontinued Operations	'		($0.01)		($0.01)		($0.88)
Earnings Per Share, Diluted	$1.20		$1.24		$2.20		$1.03
Dividends per Common Share	$0.40		$0.40		$1.20		$1.20
Average Common Shares Outstanding - Basic	242.6		242.6		242.6		242.6
Average Common Shares Outstanding - Diluted	244.3		245.1		244.3		244.4
Union Electric Company	'		'		'		'
Operating Revenues:	'		'		'		'
Electric	1,076		1,075		2,696		2,667
Gas	21		17		117		110
Other	'		1		1		1
Total operating revenues	1,097		1,093		2,814		2,778
Operating Expenses:	'		'		'		'
Fuel	236		222		638		648
Purchased power	25		33		86		100
Gas purchased for resale	7		4		58		52
Other operations and maintenance	228		212		677		686
Depreciation and amortization	118		114		351		338
Taxes other than income taxes	89		91		248		247
Total operating expenses	703		676		2,058		2,071
Operating Income	394		417		756		707
Other Income and Expenses:	'		'		'		'
Miscellaneous income	15		16		45		44
Miscellaneous expense	4		2		10		10
Total other income (expense)	11		14		35		34
Interest Charges	53		43		159		159
Income Before Income Taxes	352		388		632		582
Income Taxes	129		149		234		217
Net Income (Loss)	223		239		398		365
Other Comprehensive Income	0		0		0		0
Comprehensive Income	223		239		398		365
Net Income (Loss):	'		'		'		'
Net Income (Loss)	223		239		398		365
Earnings Per Share, Diluted [Abstract]	'		'		'		'
Preferred Stock Dividends	1		1		3		3
Net Income Available to Common Stockholder	222		238		395		362
Ameren Illinois Company	'		'		'		'
Operating Revenues:	'		'		'		'
Electric	445		432		1,162		1,160
Gas	127		115		703		585
Other	'		'		'		2
Total operating revenues	572		547		1,865		1,747
Operating Expenses:	'		'		'		'
Purchased power	89		96		256		303
Gas purchased for resale	43		38		374		292
Other operations and maintenance	185		166		580		538
Depreciation and amortization	66		59		193		182
Taxes other than income taxes	31		30		109		102
Total operating expenses	414		389		1,512		1,417
Operating Income	158		158		353		330
Other Income and Expenses:	'		'		'		'
Miscellaneous income	4		4		12		7
Miscellaneous expense	2		3		7		7
Total other income (expense)	2		1		5		0
Interest Charges	31		31		90		96
Income Before Income Taxes	129		128		268		234
Income Taxes	54		51		110		93
Net Income (Loss)	75		77		158		141
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	'		'		-2		-2
Comprehensive Income	75		77		156		139
Net Income (Loss):	'		'		'		'
Net Income (Loss)	75		77		158		141
Earnings Per Share, Diluted [Abstract]	'		'		'		'
Preferred Stock Dividends	'		'		2		2
Net Income Available to Common Stockholder	$75		$77		$156		$139
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Consolidated_Statement_of_Inco1

Consolidated Statement of Income (Loss) (Parenthetical) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Pension and other postretirement benefit plan activity, tax expense (benefit)	'	($5)	$3	$3
Ameren Illinois Company	'	'	'	'
Pension and other postretirement benefit plan activity, tax expense (benefit)	($1)	($1)	($2)	($2)

Consolidated_Statement_of_Comp

Consolidated Statement of Comprehensive Income (Loss) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Income from Continuing Operations	$296	$307	$546	$469
Other Comprehensive Income (Loss), Net of Taxes	'	'	'	'
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	'	-5	3	5
Comprehensive Income from Continuing Operations	296	302	549	474
Less: Comprehensive Income from Continuing Operations Attributable to Noncontrolling Interests	2	2	5	5
Comprehensive Income from Continuing Operations Attributable to Ameren Corporation	294	300	544	469
Loss from Discontinued Operations, Net of Taxes	-1	-3	-3	-212
Other Comprehensive Loss from Discontinued Operations, Net of Taxes	'	-5	'	-16
Comprehensive Loss from Discontinued Operations Attributable to Ameren Corporation	-1	-8	-3	-228
Comprehensive Income	$293	$292	$541	$241

Consolidated_Statement_of_Comp1

Consolidated Statement of Comprehensive Income (Loss) Consolidated Statement of Income (Parenthetical) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Statement of Comprehensive Income [Abstract]	'	'	'	'
Pension and other postretirement benefit plan activity, tax expense (benefit)	'	($5)	$3	$3

Consolidated_Balance_Sheet

Consolidated Balance Sheet (USD $)	Sep. 30, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Current Assets:	'		'
Cash and cash equivalents	$13		$30
Accounts receivable - trade (less allowance for doubtful accounts)	467		404
Unbilled revenue	203		304
Miscellaneous accounts and notes receivable	117		196
Materials and supplies	561		526
Current regulatory assets	199		156
Current accumulated deferred income taxes, net	301		106
Other current assets	66		85
Assets of discontinued operations (Note 12)	15		165
Total current assets	1,942		1,972
Property and Plant, Net	16,991		16,205
Investments and Other Assets:	'		'
Nuclear decommissioning trust fund	529		494
Goodwill	411		411
Intangible assets	20		22
Regulatory assets	1,259		1,240
Other assets	724		698
Total investments and other assets	2,943		2,865
TOTAL ASSETS	21,876		21,042
Current Liabilities:	'		'
Current maturities of long-term debt	119		534
Short-term Debt	753		368
Accounts and wages payable	466		806
Taxes accrued	161		55
Interest accrued	105		86
Current regulatory liabilities	132		216
Other current liabilities	350		351
Liabilities of discontinued operations (Note 12)	33		45
Total current liabilities	2,119		2,461
Long-term Debt, Net	5,825		5,504
Deferred Credits and Other Liabilities:	'		'
Accumulated deferred income taxes, net	3,845		3,250
Accumulated deferred investment tax credits	59		63
Regulatory liabilities	1,805		1,705
Asset retirement obligations	385		369
Pension and other postretirement benefits	400		466
Other deferred credits and liabilities	522		538
Total deferred credits and other liabilities	7,016		6,391
Commitments and Contingencies	'		'
Stockholders' Equity:	'		'
Common Stock	2		2
Other paid-in capital	5,612		5,632
Retained earnings	1,154		907
Accumulated other comprehensive income (loss)	6		3
Stockholder's equity	6,774		6,544
Noncontrolling Interest	142	[1]	142
Total equity	6,916		6,686
TOTAL LIABILITIES AND EQUITY	21,876		21,042
Union Electric Company	'		'
Current Assets:	'		'
Cash and cash equivalents	1		1
Accounts receivable - trade (less allowance for doubtful accounts)	261		191
Accounts receivable - affiliates	12		1
Unbilled revenue	134		168
Miscellaneous accounts and notes receivable	86		57
Materials and supplies	350		352
Current regulatory assets	137		118
Other current assets	40		71
Total current assets	1,021		959
Property and Plant, Net	10,660		10,452
Investments and Other Assets:	'		'
Nuclear decommissioning trust fund	529		494
Intangible assets	20		22
Regulatory assets	539		534
Other assets	410		443
Total investments and other assets	1,498		1,493
TOTAL ASSETS	13,179		12,904
Current Liabilities:	'		'
Current maturities of long-term debt	119		109
Borrowings from money pool	'		105
Short-term Debt	65		'
Accounts and wages payable	189		387
Accounts payable - affiliates	32		30
Taxes accrued	200		220
Interest accrued	66		57
Current regulatory liabilities	11		57
Other current liabilities	99		82
Total current liabilities	781		1,047
Long-term Debt, Net	3,885		3,648
Deferred Credits and Other Liabilities:	'		'
Accumulated deferred income taxes, net	2,656		2,524
Accumulated deferred investment tax credits	55		59
Regulatory liabilities	1,107		1,041
Asset retirement obligations	383		366
Pension and other postretirement benefits	147		189
Other deferred credits and liabilities	44		37
Total deferred credits and other liabilities	4,392		4,216
Commitments and Contingencies	'		'
Stockholders' Equity:	'		'
Common Stock	511		511
Other paid-in capital	1,560		1,560
Preferred stock not subject to mandatory redemption	80		80
Retained earnings	1,970		1,842
Stockholder's equity	4,121		3,993
TOTAL LIABILITIES AND EQUITY	13,179		12,904
Ameren Illinois Company	'		'
Current Assets:	'		'
Cash and cash equivalents	1		1
Accounts receivable - trade (less allowance for doubtful accounts)	192		201
Accounts receivable - affiliates	2		'
Unbilled revenue	69		135
Miscellaneous accounts and notes receivable	6		13
Materials and supplies	211		174
Current regulatory assets	62		38
Current accumulated deferred income taxes, net	125		45
Other current assets	17		26
Total current assets	685		633
Property and Plant, Net	6,030		5,589
Investments and Other Assets:	'		'
Goodwill	411		411
Regulatory assets	712		701
Other assets	145		120
Total investments and other assets	1,268		1,232
TOTAL ASSETS	7,983		7,454
Current Liabilities:	'		'
Borrowings from money pool	16		56
Short-term Debt	189		'
Accounts and wages payable	212		243
Accounts payable - affiliates	28		18
Taxes accrued	16		23
Customer deposits	71		79
Current environmental remediation	53		43
Current regulatory liabilities	121		159
Other current liabilities	148		150
Total current liabilities	854		771
Long-term Debt, Net	1,940		1,856
Deferred Credits and Other Liabilities:	'		'
Accumulated deferred income taxes, net	1,330		1,116
Accumulated deferred investment tax credits	3		4
Regulatory liabilities	698		664
Pension and other postretirement benefits	189		197
Environmental remediation	202		232
Other deferred credits and liabilities	165		166
Total deferred credits and other liabilities	2,587		2,379
Commitments and Contingencies	'		'
Stockholders' Equity:	'		'
Common Stock	0		0
Other paid-in capital	1,965		1,965
Preferred stock not subject to mandatory redemption	62		62
Retained earnings	566		410
Accumulated other comprehensive income (loss)	9		11
Stockholder's equity	2,602		2,448
TOTAL LIABILITIES AND EQUITY	$7,983		$7,454
[1]	Included the 20% EEI ownership interest not owned by Ameren prior to the divestiture of New AER to IPH. Prior to the divestiture of New AER, the assets and liabilities of EEI were consolidated in Ameren’s balance sheet at a 100% ownership level and were included in “Assets of discontinued operations” and “Liabilities of discontinued operations,” respectively. The divestiture of New AER, which included EEI, was completed in the fourth quarter of 2013. See Note 12 - Divestiture Transactions and Discontinued Operations for additional information.

Consolidated_Balance_Sheet_Par

Consolidated Balance Sheet (Parenthetical) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Millions, except Per 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	400
Common stock, shares outstanding	242.6	242.6
Union Electric Company	'	'
Accounts receivable - trade allowance for doubtful accounts	7	5
Common stock, par value	$5	$5
Common stock, shares authorized	150	150
Common stock, shares outstanding	102.1	102.1
Ameren Illinois Company	'	'
Accounts receivable - trade allowance for doubtful accounts	$14	$13
Common stock, no par value	'	'
Common stock, shares authorized	45	45
Common stock, shares outstanding	25.5	25.5

Consolidated_Statement_of_Cash

Consolidated Statement of Cash Flows (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014		Sep. 30, 2013
Cash Flows From Operating Activities:	'		'
Net income (loss)	$543		$257
Loss from discontinued operations, net of taxes	3		212
Adjustments to reconcile net income (loss) to net cash provided by operating activities:	'		'
Depreciation and amortization	526		500
Amortization of nuclear fuel	70		46
Amortization of debt issuance costs and premium/discounts	16		18
Deferred income taxes and investment tax credits, net	370		258
Allowance for equity funds used during construction	-26	[1]	-26	[1]
Stock-based compensation costs	20		19
Other	-9		14
Changes in assets and liabilities:	'		'
Receivables	16		-88
Materials and supplies	-34		7
Accounts and wages payable	-187		-102
Taxes accrued	100		104
Assets, other	-123		20
Liabilities, other	-70		-24
Pension and other postretirement benefits	-27		-34
Counterparty collateral, net	20		34
Net cash provided by operating activities – continuing operations	1,208		1,215
Net cash provided by (used in) operating activities – discontinued operations	-5		99
Net cash provided by operating activities	1,203		1,314
Cash Flows From Investing Activities:	'		'
Capital expenditures	-1,310		-943
Nuclear fuel expenditures	-28		-34
Purchases of securities - nuclear decommissioning trust fund	-365		-147
Sales and maturities of securities - nuclear decommissioning trust fund	354		134
Proceeds from Notes Receivable	79		'
Contributions to Note Receivable	-84		'
Other	3		-1
Net cash used in investing activities – continuing operations	-1,351		-991
Net cash provided by (used in) investing activities – discontinued operations	139		-42
Net cash used in investing activities	-1,212		-1,033
Cash Flows From Financing Activities:	'		'
Dividends on common stock	-291		-291
Dividends paid to noncontrolling interest holders	-5		-5
Short-term debt, net	385		'
Repayments of Other Long-term Debt	-692		'
Proceeds from Issuance of Long-term Debt	598		'
Capital Issuance Costs	-4		'
Other	1		'
Net cash provided by (used in) financing activities – continuing operations	-8		-296
Net cash used in financing activities – discontinued operations	'		'
Net cash used in financing activities	-8		-296
Net change in cash and cash equivalents	-17		-15
Cash and cash equivalents at beginning of period	30		209
Cash and cash equivalents at end of period	13		194
Less cash and cash equivalents at end of period, discontinued operations	'		25
Cash and cash equivalents at beginning of period	30		'
Cash and cash equivalents at end of period	13		169
Union Electric Company	'		'
Cash Flows From Operating Activities:	'		'
Net income (loss)	398		365
Adjustments to reconcile net income (loss) to net cash provided by operating activities:	'		'
Depreciation and amortization	329		313
Amortization of nuclear fuel	70		46
FAC prudence review charge	'		26
Amortization of debt issuance costs and premium/discounts	5		6
Deferred income taxes and investment tax credits, net	139		62
Allowance for equity funds used during construction	-24		-22
Other	1		1
Changes in assets and liabilities:	'		'
Receivables	-76		-148
Materials and supplies	3		27
Accounts and wages payable	-151		-124
Taxes accrued	-22		260
Assets, other	-10		59
Liabilities, other	6		-78
Pension and other postretirement benefits	-8		-12
Net cash provided by operating activities	660		781
Cash Flows From Investing Activities:	'		'
Capital expenditures	-548		-480
Nuclear fuel expenditures	-28		-34
Money pool advances, net	'		24
Purchases of securities - nuclear decommissioning trust fund	-365		-147
Sales and maturities of securities - nuclear decommissioning trust fund	354		134
Other	-6		-3
Net cash used in investing activities	-593		-506
Cash Flows From Financing Activities:	'		'
Dividends on common stock	-268		-320
Dividends on preferred stock	-3		-3
Money pool borrowings, net	-105		'
Redemptions of Long-term Debt	-104		'
Short-term debt, net	65		'
Proceeds from Issuance of Long-term Debt	350		'
Capital Issuance Costs	-2		'
Net cash used in financing activities	-67		-323
Net change in cash and cash equivalents	'		-48
Cash and cash equivalents at beginning of period	1		148
Cash and cash equivalents at end of period	1		100
Ameren Illinois Company	'		'
Cash Flows From Operating Activities:	'		'
Net income (loss)	158		141
Adjustments to reconcile net income (loss) to net cash provided by operating activities:	'		'
Depreciation and amortization	190		178
Amortization of debt issuance costs and premium/discounts	10		11
Deferred income taxes and investment tax credits, net	136		120
Other	-6		-7
Changes in assets and liabilities:	'		'
Receivables	80		66
Materials and supplies	-37		-20
Accounts and wages payable	1		31
Taxes accrued	-5		-2
Assets, other	-102		-33
Liabilities, other	-31		1
Pension and other postretirement benefits	-12		-13
Counterparty collateral, net	14		34
Net cash provided by operating activities	396		507
Cash Flows From Investing Activities:	'		'
Capital expenditures	-633		-462
Other	6		6
Net cash used in investing activities	-627		-456
Cash Flows From Financing Activities:	'		'
Dividends on common stock	'		-45
Dividends on preferred stock	-2		-2
Money pool borrowings, net	-40		-3
Redemptions of Long-term Debt	-163		'
Short-term debt, net	189		'
Proceeds from Issuance of Long-term Debt	248		'
Capital Issuance Costs	-2		'
Other	1		'
Net cash used in financing activities	231		-50
Net change in cash and cash equivalents	'		1
Cash and cash equivalents at beginning of period	1		'
Cash and cash equivalents at end of period	$1		$1
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Summary_Of_Significant_Account

Summary Of Significant Accounting Policies	9 Months Ended
	Sep. 30, 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 FERC. Ameren’s primary assets are its equity interests in its subsidiaries. Ameren’s subsidiaries are separate, independent legal entities with separate businesses, assets, and liabilities. Dividends on Ameren’s common stock and the payment of parent company expenses by Ameren depend on distributions made to it by its subsidiaries. Ameren’s principal subsidiaries are listed below. Also see the Glossary of Terms and Abbreviations at the front of this report and in the Form 10-K.
	•	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 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 a rate-regulated electric and natural gas transmission and distribution business in Illinois. Ameren Illinois supplies electric service to 1.2 million customers and natural gas service to 807,000 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 and is constructing the Illinois Rivers project.
	The operating results, assets, and liabilities for New AER and the Elgin, Gibson City, Grand Tower, Meredosia, and Hutsonville energy centers have been presented separately as discontinued operations for all periods presented in this report. Unless otherwise stated, these notes to Ameren’s financial statements exclude discontinued operations for all periods presented. On January 31, 2014, Medina Valley completed its sale of the Elgin, Gibson City, and Grand Tower gas-fired energy centers to Rockland Capital. See Note 12 - Divestiture Transactions and Discontinued Operations in this report for additional information regarding the discontinued operations presentation and Note 16 - Divestiture Transactions and Discontinued Operations under Part II, Item 8, of the Form 10-K for additional information regarding Ameren’s divestiture of New AER in December 2013.
	The financial statements of Ameren are prepared on a consolidated basis, and therefore include the accounts of its majority-owned subsidiaries. All significant intercompany transactions have been eliminated. Ameren Missouri and Ameren Illinois have no subsidiaries, and therefore their financial statements are not prepared on a consolidated basis. 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. The results of operations of an interim period may not give a true indication of results that may be expected for a full year. These financial statements should be read in conjunction with the financial statements and the notes thereto included in the Form 10-K.
	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 three and nine months ended September 30, 2014, and 2013:
		Three Months								Nine Months
		2014				2013				2014			2013
	Net income (loss) attributable to Ameren Corporation:
	Continuing operations	$	294			$	305			$	541		$	464
	Discontinued operations	(1		)		(3		)		(3		)	(212		)
	Net income attributable to Ameren Corporation	$	293			$	302			$	538		$	252
	Average common shares outstanding - basic	242.6				242.6				242.6			242.6
	Assumed settlement of performance share units	1.7				2.5				1.7			1.8
	Average common shares outstanding - diluted	244.3				245.1				244.3			244.4
	Earnings (loss) per common share – basic:
	Continuing operations	$	1.21			$	1.26			$	2.23		$	1.92
	Discontinued operations	—				(0.01		)		(0.01		)	(0.88		)
	Earnings per common share – basic	$	1.21			$	1.25			$	2.22		$	1.04
	Earnings (loss) per common share – diluted:
	Continuing operations	$	1.2			$	1.25			$	2.21		$	1.91
	Discontinued operations	—				(0.01		)		(0.01		)	(0.88		)
	Earnings per common share – diluted	$	1.2			$	1.24			$	2.2		$	1.03
	There were no potentially dilutive securities excluded from the diluted earnings per share calculations for the three and nine months ended September 30, 2014, and 2013.
	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 April 24, 2014. The 2014 Incentive Plan provides for a maximum of 8 million common shares to be available for grant to eligible employees and directors, and 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, as of April 24, 2014, 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 and will therefore 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 performance share units at September 30, 2014, and changes during the nine months ended September 30, 2014, under the 2006 Incentive Plan and the 2014 Incentive Plan are presented below:
		Performance Share Units
		Share Units		Weighted-average Fair Value Per Share Unit at Grant Date
	Nonvested at January 1, 2014	1,218,544		$	33.23
	Granted(a)	685,026		38.9
	April Grants(b)	38,559		50.34
	Forfeitures	(65,847	)	33.82
	Vested(c)	(123,295	)	38.64
	Nonvested at September 30, 2014	1,752,987		$	35.42
	(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)	Share units vested due to the 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.
	The fair value of each share unit awarded in 2014, excluding the April Grants, 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 stockholder 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.
	Intangible Assets
	Ameren and Ameren Missouri classify renewable energy credits and emission allowances as intangible assets. Ameren Illinois consumes renewable energy credits as they are purchased through the IPA procurement process and expenses them immediately. Ameren Missouri’s emission allowances are allocated by the EPA and therefore are recorded at zero cost. We evaluate intangible assets for impairment if events or changes in circumstances indicate that their carrying amount might be impaired.
	At September 30, 2014, Ameren’s and Ameren Missouri’s intangible assets consisted of renewable energy credits obtained through wind and solar power purchase agreements. The book values of both Ameren’s and Ameren Missouri’s renewable energy credits were $20 million and $22 million at September 30, 2014 and December 31, 2013, respectively.
	Ameren Missouri’s and Ameren Illinois’ renewable energy credits and Ameren Missouri’s emission allowances are charged to “Purchased power” expense and “Fuel” expense, respectively, as they are used in operations. The following table presents amortization expense based on usage of renewable energy credits and emission allowances, net of gains from sales, for Ameren, Ameren Missouri and Ameren Illinois, during the three and nine months ended September 30, 2014, and 2013:
			Three Months							Nine Months
		2014				2013				2014			2013
	Ameren Missouri	$	1			$	—			$	7		$	(a)
	Ameren Illinois		1				2				7			9
	Ameren	$	2			$	2			$	14		$	9
	(a)	Less than $1 million.
	Excise Taxes
	Excise taxes levied on us are reflected on Ameren Missouri electric customer bills and on Ameren Missouri and Ameren Illinois natural gas customer bills. They 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 reflected on Ameren Illinois electric customer bills are imposed on the customer and are therefore not included in revenues and expenses. They are 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 three and nine months ended September 30, 2014, and 2013:
		Three Months								Nine Months
		2014				2013				2014			2013
	Ameren Missouri	$	47			$	49			$	120		$	120
	Ameren Illinois	9				10				46			43
	Ameren	$	56			$	59			$	166		$	163
	Uncertain Tax Positions
With the adoption of new accounting guidance in the first quarter of 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 our balance sheets. At September 30, 2014, unrecognized tax benefits of $89 million, $13 million, and $2 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. For additional information see the Accounting and Reporting Developments section below.
The following table presents the total amount of reserves for unrecognized tax benefits (detriments) related to uncertain tax positions as of September 30, 2014, and December 31, 2013:
	30-Sep-14				December 31,
					2013
Ameren	$	97			$	90
Ameren Missouri	35				31
Ameren Illinois	1				(1		)
The following table presents the amount of reserves for unrecognized tax benefits, included in the table above, related to uncertain tax positions that would impact results of operations, if recognized, as of September 30, 2014, and December 31, 2013:
	30-Sep-14				December 31,
					2013
Ameren	$	55			$	54
Ameren Missouri	3				3
Ameren Illinois	—				—
In October 2014, a settlement was reached with the Appeals Office of the IRS for the years 2007 through 2010. During the fourth quarter of 2014, this settlement, which is primarily related to uncertain tax positions associated with the timing of research tax deductions, will result in a decrease in Ameren’s uncertain tax benefits of $16 million, of which $9 million is related to Ameren Missouri. This settlement will not have a material impact on Ameren’s or Ameren Missouri’s results of operations or liquidity.
Ameren’s federal income tax returns for the years 2011 and 2012 are before the Appeals Office of the IRS. It is reasonably possible that a settlement will be reached with the Appeals Office of the IRS in the next 12 months for the years 2011 and 2012. The potential settlement, which would primarily relate to uncertain tax positions associated with the timing of research tax deductions, is expected to result in a decrease in Ameren’s uncertain tax benefits of $6 million, all of which relates to Ameren Missouri and none of which will have a material impact on their respective results of operations or liquidity.
Ameren’s federal income tax return for the year 2013 is currently under examination by the IRS and it is reasonably possible that a settlement will be reached with the IRS examination team in the next 12 months for that year. The potential settlement, which would relate to the timing of research tax deductions and the tax basis of certain leases related to the divestiture of the merchant generation business, is expected to result in a decrease in Ameren’s uncertain tax benefits of $73 million, of which $17 million relates to Ameren Missouri and $1 million relates to Ameren Illinois. Although we are unable to estimate the impact of any potential settlement at this time, up to $55 million of the Ameren total could increase net income from Ameren’s discontinued operations. Settlement of the remaining $18 million of uncertain tax positions at Ameren, as well as those positions at Ameren Missouri and Ameren Illinois, are associated with the timing of deductions and will not have a material impact on our results of operations.
In addition, it is reasonably possible that other events will occur during the next 12 months that would cause the total amount of our unrecognized tax benefits to fluctuate. However, other than as described above, we do not believe any such fluctuations would be material to our results of operations, financial position, or liquidity.
State income tax returns are generally subject to examination for a period of three years after filing of the return. We do not currently have material state income tax issues under examination, administrative appeals, or litigation. 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.
Asset Retirement Obligations
AROs at Ameren, Ameren Missouri and Ameren Illinois increased at September 30, 2014, compared to December 31, 2013, to reflect the accretion of obligations to their fair value and an additional ARO at Ameren and Ameren Missouri of $2 million related to the retirement costs for a CCR storage facility, partially offset by immaterial settlements.
Noncontrolling Interests
As of September 30, 2014, Ameren's noncontrolling interests were composed of the preferred stock not subject to mandatory redemption of Ameren Missouri and Ameren Illinois. All noncontrolling interests are classified as a component of equity separate from Ameren's equity on its consolidated balance sheet. A reconciliation of the equity changes attributable to the noncontrolling interests at Ameren for the three and nine months ended September 30, 2014, and 2013, are shown below:
	Three Months								Nine Months
	2014				2013				2014			2013
Noncontrolling interests, beginning of period	$	142			$	151		(a)	$	142		$	151		(a)
Net income from continuing operations attributable to noncontrolling interests	2				2				5			5
Dividends paid to noncontrolling interest holders	(2		)		(2		)		(5		)	(5		)
Noncontrolling interests, end of period	$	142			$	151		(a)	$	142		$	151		(a)
(a)	Included the 20% EEI ownership interest not owned by Ameren prior to the divestiture of New AER to IPH. Prior to the divestiture of New AER, the assets and liabilities of EEI were consolidated in Ameren’s balance sheet at a 100% ownership level and were included in “Assets of discontinued operations” and “Liabilities of discontinued operations,” respectively. The divestiture of New AER, which included EEI, was completed in the fourth quarter of 2013. See Note 12 - Divestiture Transactions and Discontinued Operations for additional information.
Accounting and Reporting Developments
The following is a summary of recently adopted or issued authoritative accounting guidance relevant to the Ameren Companies.
Presentation of an Unrecognized Tax Benefit
In July 2013, FASB issued additional authoritative accounting guidance 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 to the extent a net operating loss carryforward, a similar tax loss, or a tax credit carryforward is available under the tax law. This guidance was effective for the Ameren Companies beginning in the first quarter of 2014. 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 the first quarter 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. For comparative purposes, the Ameren Companies reclassified the December 31, 2013 balances in accordance with the new guidance as discussed in the Uncertain Tax Positions section above. 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 Disposals of Components of an Entity
In April 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 has 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 will be 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 would not be 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 May 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 to which the entity expects to be entitled 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 Ameren Companies are currently assessing the impacts of this guidance.

Rate_And_Regulatory_Matters

Rate And Regulatory Matters	9 Months Ended
	Sep. 30, 2014
Public Utilities, General Disclosures [Abstract]	'
RATE AND REGULATORY MATTERS	'
	RATE AND REGULATORY MATTERS
	Below is a summary of updates to significant regulatory proceedings and related lawsuits. See also Note 2 - Rate and Regulatory Matters under Part II, Item 8, of the Form 10-K. 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 impact on our results of operations, financial position, or liquidity.
	Missouri
	Rate Shift Complaint and Earnings Complaint Cases
	In February 2014, Noranda and 37 residential customers filed a rate shift complaint case and an earnings complaint case with the MoPSC.
	On August 20, 2014, the MoPSC issued an order that rejected Noranda’s and the residential customers’ request in the rate shift complaint case. On September 12, 2014, Noranda, the MoOPC, the MIEC, and other parties filed a rehearing request, which was subsequently denied by the MoPSC.
	In the earnings complaint case, Noranda and the residential customers asserted that Ameren Missouri’s electric service business is earning more than the 9.8% return on common equity authorized in the MoPSC's December 2012 electric rate order. The MoOPC, the MIEC, and other parties, participated in the earnings complaint case. On October 1, 2014, the MoPSC issued an order that rejected Noranda’s and the residential customers’ request in the earnings complaint case. On October 30, 2014, Noranda, the MoOPC, the MIEC, and other parties filed a rehearing request, which was subsequently denied by the MoPSC.
	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 by $264 million. The 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, additional electric infrastructure investments made for the benefit of Ameren Missouri’s customers. Plant additions to rate base since the last electric rate order are expected to total approximately $1.4 billion through the true-up date in this rate case and include electric infrastructure investments for upgrades to the electrostatic precipitators at the coal-fired Labadie energy center, the replacement of the nuclear reactor vessel head at the Callaway energy center, two new substations in St. Louis, and the O’Fallon solar energy center, among other additions. Approximately $127 million of the request relates to an increase in net energy costs above the current levels included in base rates previously authorized by the MoPSC in its December 2012 electric rate order, 95% of which, absent initiation of this general rate proceeding, would have been reflected in rate adjustments implemented under Ameren Missouri’s existing FAC. The electric rate increase request is based on a 10.4% return on common equity, a capital structure composed of 51.6% common equity, an electric rate base for Ameren Missouri of $7.3 billion, and a test year ended March 31, 2014, with certain pro-forma adjustments expected through the true-up date of December 31, 2014.
	As a part of its filing, Ameren Missouri also requested continued use of the FAC and the regulatory tracking mechanisms for storm costs, vegetation management/ infrastructure inspection costs, pension and postretirement benefits, and uncertain income tax positions that the MoPSC previously authorized in earlier electric rate orders.
	The MoPSC proceeding relating to the proposed electric service rate increase will take place over a period of up to 11 months and a decision by the MoPSC in such proceeding is expected by May 2015, with rates effective by June 2015. Ameren Missouri cannot predict the level of any electric service rate change the MoPSC may approve, when any rate change may go into effect, or whether any rate increase that may eventually be approved will be sufficient for Ameren Missouri to recover its costs and earn a reasonable return on its investments when the increase goes into effect.
	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 given by either party. Termination of the agreement by Ameren Missouri would require MoPSC approval.
	Accounting Authority Order
	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 that were 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. In February 2014, the MIEC and the MoOPC filed appeals of the MoPSC’s November 2013 accounting authority order with the Missouri Court of Appeals, Western District. Ameren Missouri has not recorded any potential revenue associated with this accounting authority order.
	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 in effect for customer billings 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 September 30, 2014, Ameren Illinois had recorded regulatory assets of $76 million and $64 million, respectively, to reflect its expected 2014 and 2013 revenue requirement reconciliation adjustments, with interest. As of September 30, 2014, Ameren Illinois had recorded a regulatory liability of $13 million to reflect its 2012 revenue requirement reconciliation adjustment, with interest, which is being refunded to customers during 2014.
	In December 2013, the ICC issued an order in Ameren Illinois' annual formula rate update filing, which was based on 2012 recoverable costs and expected net plant additions for 2013. The ICC order established rates for 2014. In February 2014, Ameren Illinois filed an appeal 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.
	In April 2014, Ameren Illinois filed with the ICC its annual electric delivery service formula rate update to establish the revenue requirement used to set rates for 2015. Pending ICC approval, Ameren Illinois’ update filing, as revised in July 2014, will result in a $205 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 annual reconciliation of the revenue requirement in effect for 2013 to the actual costs incurred in that year, and an increase resulting from the conclusion of a refund to customers in 2014 for the 2012 revenue requirement reconciliation. In August 2014, the ICC staff submitted its revised calculation of the revenue requirement. The ICC staff recommended a $205 million increase in Ameren Illinois’ electric delivery service revenue requirement. Other intervenors requested an electric delivery service revenue requirement up to $7 million lower than the revenue requirement recommended by the ICC staff. In October 2014, the administrative law judges issued a proposed order that reflected an increase to Ameren Illinois’ electric delivery service revenue requirement of $204 million. A final ICC decision on this April 2014 filing is expected by December 2014.
	2013 Natural Gas Delivery Service Rate Case
	In December 2013, the ICC issued a rate order that approved an increase in revenues for 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 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 with the Appellate Court of the Fourth District of Illinois. Ameren Illinois sought a 10.4% return on common equity in this rate case.
	ATXI Transmission Project
	The Spoon River project in northwest Illinois is a MISO-approved transmission line project. 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 acquisition.
	Federal
	2011 Wholesale Distribution Rate Case
	In January 2011, Ameren Illinois filed a request with FERC to increase its annual revenues for electric delivery service for 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 impact on net income. In March 2011, FERC issued an order authorizing the proposed rates to take effect, subject to refund when the final rates are determined. In November 2012, a FERC administrative law judge issued an initial decision finding that refunds were due to the wholesale customers. In September 2014, FERC issued an order affirming certain findings in the initial decision. Ameren and Ameren Illinois recognized in “Current regulatory liabilities” an estimate of $24 million and $13 million as of September 30, 2014, and December 31, 2013, respectively, for the refund due to the wholesale customers relating to billings since March 2011. 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, 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 FERC regarding Ameren Illinois’ conclusion that no refund was warranted. In June 2013, 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, FERC issued an order that denied Ameren Illinois’ rehearing requests of the July 2012 order and the June 2013 order. Separately, in June 2014, FERC issued an order that established hearing and settlement procedures for Ameren Illinois’ July 2013 refund report. In July 2014, Ameren Illinois filed an appeal of FERC’s orders 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 separately filed a request for rehearing with FERC of its June 2014 order regarding the July 2013 refund report.
	Ameren Illinois estimates the maximum pretax charge to earnings for this possible refund obligation through December 31, 2014, would be $19 million, before interest charges. For the nine months ended September 30, 2014, Ameren and Ameren Illinois recorded a $4 million reduction to “Operating Revenues - Electric” with a corresponding increase to “Current regulatory liabilities” for its estimate of the refund due to electric transmission customers based on the June 2014 order. If Ameren Illinois were 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 is made.
	FERC Complaint Case
	In November 2013, a customer group filed a complaint case with FERC seeking a reduction in the allowed base return on common equity for FERC-regulated MISO transmission rate base to 9.15%, as well as a limit on the common equity ratio, under the MISO tariff. Currently, the FERC-allowed base return on common equity for MISO transmission owners is 12.38%. In October 2014, FERC issued an order establishing settlement procedures and, if necessary, hearing procedures regarding the allowed base return on common equity and denied all other aspects of the MISO complaint case. This complaint case could result in a reduction to Ameren Illinois' and ATXI's allowed base return on common equity, which would result in a refund for transmission service revenues earned from the refund effective date of November 12, 2013.
	In October 2014, FERC issued an order, which confirmed its June 2014 order, 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 orders 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. FERC expects the evidence and the calculation used in the New England transmission owners’ case to guide its decision in the MISO complaint case. The calculation FERC will use to establish the allowed base return on common equity, which is based on a unique time period for each complaint case, will require multiple inputs based on observable market data specific to the utility industry and broader macroeconomic data. The unique time period of observable market data for the MISO complaint case has not been established by FERC. Due to the wide range of potential outcomes and significant uncertainty regarding the value of inputs required in FERC’s calculation, the Ameren Companies cannot reasonably estimate the impact, if any, that a resolution in the MISO complaint case could have on their allowed base return on common equity.
	On November 6, 2014, we filed a request with FERC to include an incentive adder of up to 50 basis points for participation in an RTO on the allowed base return on common equity. The filing requests a November 7, 2014 effective date and seeks authorization to defer collection of the incentive adder until after the issuance of the final order addressing the pending MISO complaint case discussed above. FERC is required to issue an order within 60 days of our filing.
	If MISO’s allowed base return on common equity was lowered to 10.57%, as established in the New England transmission owners’ case, with no additional rate incentives, the required refund for Ameren and Ameren Illinois would be $14 million and $11 million, respectively, from the refund effective date of November 12, 2013 through September 30, 2014. The estimated annual reduction in revenues if the MISO return on common equity was 10.57% for Ameren and Ameren Illinois would be $16 million and $12 million, respectively. Ameren Missouri would 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. If Ameren and Ameren Illinois were to determine that a refund to their electric transmission customers could be reasonably estimated, a charge to earnings would be recorded for the refund in the period in which that determination is made.
	Ameren Missouri Power Purchase Agreement with Entergy
	Beginning in 2005, 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, which expired August 31, 2009. In May 2012, 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. In July 2012, Entergy filed an appeal of FERC's May 2012 order to the United States Court of Appeals for the District of Columbia Circuit, which was subsequently dismissed on a procedural issue. In November 2013, Entergy refiled the appeal of 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 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. In April 2008, that court ordered further FERC proceedings regarding Louisiana Public Service Commission’s complaint. The court ordered FERC to explain its previous denial of retroactive refunds and the implementation of prospective charges. Ameren Missouri is unable to predict when or how FERC will respond to the court’s decisions. Ameren Missouri estimates that it could incur an additional expense of up to $25 million if FERC orders retroactive application for the years 2001 to 2005. Ameren Missouri believes that the likelihood of incurring an expense is not probable, and therefore no liability has been recorded as of September 30, 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 its existing Missouri nuclear energy center site, and the NRC suspended review of the COL application.
	Ameren Missouri estimated the total cost required to obtain a small modular reactor COL to be $80 million to $120 million. As of September 30, 2014, Ameren Missouri had capitalized investments of $69 million for the development of a new nuclear energy center. Ameren Missouri 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 or 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.

ShortTerm_Debt_And_Liquidity

Short-Term Debt And Liquidity	9 Months Ended
	Sep. 30, 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, drawings under committed credit agreements, commercial paper issuances, or, in the case of Ameren Missouri and Ameren Illinois, short-term intercompany borrowings.
	The 2012 Missouri Credit Agreement and the 2012 Illinois Credit Agreement, both of which expire on November 14, 2017, were not utilized for direct borrowings during the nine months ended September 30, 2014, but they were used to support commercial paper issuances and to issue letters of credit. Based on letters of credit issued under the 2012 Credit Agreements, as well as commercial paper outstanding, the aggregate amount of credit capacity available to Ameren (parent), Ameren Missouri and Ameren Illinois, collectively, at September 30, 2014, was $1.3 billion.
	Commercial Paper
	The following table presents commercial paper outstanding at Ameren (parent), Ameren Missouri and Ameren Illinois as of September 30, 2014, and December 31, 2013. Ameren Illinois established a commercial paper program in May 2014.
		30-Sep-14				31-Dec-13
	Ameren (parent)	$	499			$	368
	Ameren Missouri	65				—
	Ameren Illinois	189				—
	Ameren Consolidated	$	753			$	368
	The following table summarizes the commercial paper activity and relevant interest rates under Ameren’s (parent), Ameren Missouri’s and Ameren Illinois’ commercial paper programs for the nine months ended September 30, 2014, and 2013:
			Ameren (parent)				Ameren Missouri			Ameren Illinois			Ameren Consolidated
	2014
	Average daily commercial paper outstanding		$	386			$	141		$	157		$	609
	Weighted-average interest rate		0.36		%		0.38		%	0.31		%	0.35		%
	Peak commercial paper during period(a)		$	531			$	495		$	300		$	907
	Peak interest rate		0.75		%		0.7		%	0.34		%	0.75		%
	2013
	Average daily commercial paper outstanding		$	26			$	—		$	—		$	26
	Weighted-average interest rate		0.52		%		—		%	—		%	0.52		%
	Peak commercial paper during period(a)		$	92			$	—		$	—		$	92
	Peak interest rate		0.85		%		—		%	—		%	0.85		%
	(a)	The timing of peak commercial paper issuances varies by company, and therefore the peak amounts presented by company might not equal the Ameren Consolidated peak commercial paper issuances for the period.
	Indebtedness Provisions and Other Covenants
	The information below is a summary of the Ameren Companies’ compliance with indebtedness provisions and other covenants within the 2012 Credit Agreements. See Note 4 – Short-term Debt and Liquidity under Part II, Item 8, in the Form 10-K for a detailed description of these provisions.
	The 2012 Credit Agreements 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 September 30, 2014, the ratios of consolidated indebtedness to total consolidated capitalization, calculated in accordance with the provisions of the 2012 Credit Agreements, were 49%, 48% and 46%, 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, Ameren is required to maintain a ratio of consolidated funds from operations plus interest expense to consolidated interest expense of at least 2.0 to 1.0, to be calculated quarterly, as of the end of the most recent four fiscal quarters then ending, in accordance with the 2012 Illinois Credit Agreement. Ameren’s ratio as of September 30, 2014, was 6.1 to 1.0. Failure of a borrower to satisfy a financial covenant constitutes an immediate default under the applicable 2012 Credit Agreement. The calculation of Ameren’s ratios discussed above includes both continuing and discontinued operations.
	None of the Ameren Companies' credit agreements or financing arrangements contain credit rating triggers that would cause a default or acceleration of repayment of outstanding balances. The Ameren Companies were in compliance with the provisions and covenants of their credit agreements at September 30, 2014.
	Money Pools
	Ameren (parent) has money pool agreements with and among its subsidiaries to coordinate and provide for certain short-term cash and working capital requirements. Ameren Services is responsible for the operation and administration of the money pool agreements.
	Ameren Missouri, Ameren Illinois and Ameren Services may participate in the utility money pool as both lenders and borrowers. Ameren (parent) may participate in the utility money pool only as a lender. Surplus internal funds are contributed to the utility money pool from participants. The primary sources of external funds for the utility 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 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 utility 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 utility money pool for the three and nine months ended September 30, 2014, was 0.10% and 0.23%, respectively (2013 - 0.05% and 0.08%, respectively).
	See Note 8 - Related Party Transactions for the amount of interest income and expense from the money pool arrangements recorded by the Ameren Companies for the three and nine months ended September 30, 2014, and 2013.

LongTerm_Debt_And_Equity_Finan

Long-Term Debt And Equity Financings

9 Months Ended

Sep. 30, 2014

Long-Term Debt And Equity Financings [Abstract]

'

LONG-TERM DEBT AND EQUITY FINANCINGS

'

LONG-TERM DEBT

Ameren (parent)

In May 2014, Ameren (parent) repaid at maturity $425 million of its 8.875% senior unsecured notes due May 15, 2014, plus accrued interest. The notes were repaid with proceeds from commercial paper issuances.

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.

Ameren Illinois

In January 2014, Ameren Illinois redeemed the following environmental improvement and pollution control revenue bonds at par value plus accrued interest:

Environmental improvement and pollution control revenue bonds

Principal Amount

5.90% Series 1993 due 2023(a)

$

32

5.70% 1994A Series due 2024(a)

36

5.95% 1993 Series C-1 due 2026

35

5.70% 1993 Series C-2 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 of each year, beginning January 1, 2015. Ameren Illinois received proceeds of $246 million, which were used to repay a portion of 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 and dividend coverage ratios and bonds and preferred stock issuable as of September 30, 2014, at an assumed annual 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

4.6

$

3,304

≥2.5

126.3

$

2,823

Ameren Illinois

≥2.0

6.7

3,636

(d)

≥1.5

2.4

203

(e)

(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 $833 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.

(e)

Preferred stock issuable is restricted by the amount of preferred stock that is currently authorized by Ameren Illinois’ articles of incorporation.

Ameren Missouri and Ameren Illinois and certain other 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.” 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 FERC to maintain a minimum 30% equity capital structure. As of September 30, 2014, Ameren Illinois had a 54% equity capital structure.

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 September 30, 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 12 - 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	9 Months Ended
	Sep. 30, 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 for the three and nine months ended September 30, 2014, and 2013:
		Three Months				Nine Months
		2014		2013		2014		2013
	Ameren:(a)
	Miscellaneous income:
	Allowance for equity funds used during construction	$	10	$	10	$	26	$	26
	Interest income on industrial development revenue bonds	6		7		20		21
	Interest income	3		2		8		3
	Other	2		1		6		1
	Total miscellaneous income	$	21	$	20	$	60	$	51
	Miscellaneous expense:
	Donations	$	3	$	2	$	9	$	7
	Other	4		3		11		11
	Total miscellaneous expense	$	7	$	5	$	20	$	18
	Ameren Missouri:
	Miscellaneous income:
	Allowance for equity funds used during construction	$	9	$	8	$	24	$	22
	Interest income on industrial development revenue bonds	6		7		20		21
	Interest income	—		1		1		1
	Total miscellaneous income	$	15	$	16	$	45	$	44
	Miscellaneous expense:
	Donations	$	2	$	—	$	5	$	3
	Other	2		2		5		7
	Total miscellaneous expense	$	4	$	2	$	10	$	10
	Ameren Illinois:
	Miscellaneous income:
	Allowance for equity funds used during construction	$	1	$	2	$	2	$	4
	Interest income	2		1		5		2
	Other	1		1		5		1
	Total miscellaneous income	$	4	$	4	$	12	$	7
	Miscellaneous expense:
	Donations	$	—	$	—	$	3	$	3
	Other	2		3		4		4
	Total miscellaneous expense	$	2	$	3	$	7	$	7
	(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Derivative_Financial_Instrumen

Derivative Financial Instruments	9 Months Ended
	Sep. 30, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]	'
DERIVATIVE FINANCIAL INSTRUMENTS	'
	DERIVATIVE FINANCIAL INSTRUMENTS
	We use derivatives to manage the risk of changes in market prices for natural gas, diesel, power, and uranium. 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 September 30, 2014, and December 31, 2013. As of September 30, 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)	52		(b)		52		66		(b)		66
	Natural gas (in mmbtu)	23		102		125		28		108		136
	Power (in megawatthours)	1		11		12		3		11		14
	Uranium (pounds in thousands)	557		(b)		557		796		(b)		796
	(a)	Fuel oils consist of ultra-low-sulfur diesel, on-highway 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 7 - Fair Value Measurements for a 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 September 30, 2014, and December 31, 2013, all contracts that qualify for hedge accounting received 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 the same 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 September 30, 2014, and December 31, 2013:
		Balance Sheet Location		Ameren  Missouri				Ameren  Illinois				Ameren
	2014
	Fuel oils	Other current assets		$	3			$	—			$	3
	Natural gas	Other current assets		—				1				1
		Other assets		—				1				1
	Power	Other current assets		10				—				10
		Other assets		1				—				1
		Total assets		$	14			$	2			$	16
	Fuel oils	Other current liabilities		$	5			$	—			$	5
		Other deferred credits and liabilities		1				—				1
	Natural gas	Other current liabilities		3				16				19
		Other deferred credits and liabilities		3				6				9
	Power	Other current liabilities		6				8				14
		Other deferred credits and liabilities		—				116				116
	Uranium	Other current liabilities		2				—				2
		Other deferred credits and liabilities		1				—				1
		Total liabilities		$	21			$	146			$	167
	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	Other current liabilities		5				27				32
		Other deferred credits and liabilities		6				19				25
	Power	Other current liabilities		4				9				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
	The following table presents the cumulative amount of pretax net gains (losses) on all derivative instruments deferred as regulatory assets or regulatory liabilities as of September 30, 2014, and December 31, 2013:
		Ameren				Ameren				Ameren
		Missouri				Illinois
	2014
	Fuel oils derivative contracts(a)	$	(5	)		$	—			$	(5	)
	Natural gas derivative contracts(b)	(6		)		(20		)		(26		)
	Power derivative contracts(c)	5				(124		)		(119		)
	Uranium derivative contracts(d)	(3		)		—				(3		)
	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 oil derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s transportation costs for coal through December 2017. Current losses deferred as regulatory assets include $4 million and $4 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 $1 million and $1 million at Ameren and Ameren Illinois, respectively. Current losses deferred as regulatory assets include $19 million, $3 million, and $16 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 $10 million and $10 million at Ameren and Ameren Missouri. Current losses deferred as regulatory assets include $14 million, $6 million, and $8 million at Ameren, Ameren Missouri and Ameren Illinois, respectively.
(d)	Represents net losses on 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 trading and netting agreements, and reporting daily exposure to senior management.
We believe that entering into master trading and netting agreements 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 trading and netting agreements: (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 trading and netting agreements allow the counterparties to net settle sale and purchase transactions. Further, collateral requirements are calculated at the master trading and netting agreement 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 September 30, 2014, and December 31, 2013:
						Gross Amounts Not Offset in the Balance Sheet
Commodity Contracts Eligible to be Offset		Gross Amounts Recognized in the Balance Sheet				Derivative Instruments				Cash Collateral Received/Posted(a)			Net
													Amount
2014
Assets:
Ameren Missouri		$	14			$	7			$	—		$	7
Ameren Illinois		2				1				—			1
Ameren		$	16			$	8			$	—		$	8
Liabilities:
Ameren Missouri		$	21			$	7			$	5		$	9
Ameren Illinois		146				1				—			145
Ameren		$	167			$	8			$	5		$	154
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 accrual and NPNS contracts. As of September 30, 2014, if counterparty groups were to fail completely to perform on contracts, Ameren, Ameren Missouri and Ameren Illinois' maximum exposure was $6 million, $4 million, and $2 million, respectively. As of December 31, 2013, if counterparty groups were to fail completely to perform on contracts, Ameren, Ameren Missouri and Ameren Illinois' maximum exposure was $13 million, $12 million, and $1 million, respectively. The potential loss on counterparty exposures is reduced by the application of master trading and netting agreements and collateral held to the extent of reducing the exposure to zero. As of September 30, 2014, the potential loss after consideration of the application of master trading and netting agreements and collateral held for Ameren, Ameren Missouri and Ameren Illinois was $4 million, $3 million, and $1 million, respectively. As of December 31, 2013, the potential loss after consideration of the application of master trading and netting agreements and collateral held for Ameren, Ameren Missouri and Ameren Illinois was $6 million, $6 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 September 30, 2014, and December 31, 2013, 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 trading and netting agreements, assuming (1) the credit risk-related contingent features underlying these agreements were triggered on September 30, 2014, or December 31, 2013, respectively, 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)
2014
Ameren Missouri	$	62			$	2			$	57
Ameren Illinois	61				—				56
Ameren	$	123			$	2			$	113
2013
Ameren Missouri	$	70			$	2			$	67
Ameren Illinois	75				15				55
Ameren	$	145			$	17			$	122
(a)	Prior to consideration of master trading and netting agreements and including NPNS and accrual contract exposures.
(b)	As collateral requirements with certain counterparties are based on master trading and netting agreements, the aggregate amount of additional collateral required to be posted is determined after consideration of the effects of such agreements.

Fair_Value_Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 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 derivative 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 as of September 30, 2014:
			Fair Value									Weighted Average
			Assets			Liabilities			Valuation Technique(s)	Unobservable Input	Range
	Level 3 Derivative asset and liability - commodity contracts(a):
	Ameren	Fuel oils	$	3		$	(3	)	Option model	Volatilities(%)(b)	27-Feb	14
									Discounted cash flow	Counterparty credit risk(%)(c)(d)	0.25 - 1	0.72
										Ameren Missouri credit risk(%)(c)(d)	0.43	(e)
		Natural gas	1			—			Discounted cash flow	Nodal basis($/mmbtu)(c)	(0.10) - 0	-0.1
										Counterparty credit risk(%)(c)(d)	0.30 - 2	0.62
										Ameren Illinois credit risk(%)(c)(d)	0.43	(e)
		Power(f)	10			(129		)	Discounted cash flow	Average forward peak and off-peak pricing - forwards/swaps($/MWh)(c)	29 - 59	35
										Estimated auction price for FTRs($/MW)(b)	(1,853) - 2,087	199
										Nodal basis($/MWh)(c)	(6) - 0	-3
										Counterparty credit risk(%)(c)(d)	0.4	(e)
										Ameren Missouri and Ameren Illinois credit risk(%)(c)(d)	0.43	(e)
									Fundamental energy production model	Estimated future gas prices($/mmbtu)(b)	5-Apr	5
										Escalation rate(%)(b)(g)	2	(e)
									Contract price allocation	Estimated renewable energy credit costs($/credit)(b)	7-May	6
		Uranium	—			(3		)	Discounted cash flow	Average forward uranium pricing($/pound)(b)	35 - 41	36
	Ameren Missouri	Fuel oils	$	3		$	(3	)	Option model	Volatilities(%)(b)	27-Feb	14
									Discounted cash flow	Counterparty credit risk(%)(c)(d)	0.25 - 1	0.72
										Ameren Missouri credit risk(%)(c)(d)	0.43	(e)
		Power(f)	10			(5		)	Discounted cash flow	Average forward peak and off-peak pricing - forwards/swaps($/MWh)(c)	30 - 59	48
										Estimated auction price for FTRs($/MW)(b)	(1,853) - 2,087	199
										Counterparty credit risk(%)(c)(d)	0.4	(e)
										Ameren Missouri credit risk(%)(c)(d)	0.43	(e)
		Uranium	—			(3		)	Discounted cash flow	Average forward uranium pricing($/pound)(b)	35 - 41	36
	Ameren Illinois	Natural gas	$	1		$	—		Discounted cash flow	Nodal basis($/mmbtu)(c)	(0.10) - 0	-0.1
										Counterparty credit risk(%)(c)(d)	0.30 - 2	0.62
										Ameren Illinois credit risk(%)(c)(d)	0.43	(e)
		Power(f)	—			(124		)	Discounted cash flow	Average forward peak and off-peak pricing - forwards/swaps($/MWh)(b)	29 - 42	33
										Nodal basis($/MWh)(b)	(6) - 0	-3
										Ameren Illinois credit risk(%)(c)(d)	0.43	(e)
									Fundamental energy production model	Estimated future gas prices($/mmbtu)(b)	5-Apr	5
										Escalation rate(%)(b)(g)	2	(e)
									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)	Not applicable.
	(f)	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.
	(g)	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 Average
			Assets			Liabilities			Valuation Technique(s)	Unobservable Input	Range
	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 the first nine months of 2014 or 2013. At September 30, 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 September 30, 2014:
			Quoted Prices in				Significant Other				Significant Other				Total
			Active Markets for				Observable Inputs				Unobservable
			Identical Assets				(Level 2)				Inputs
			or Liabilities								(Level 3)
			(Level 1)
Assets:
Ameren	Derivative assets - commodity contracts(a):
	Fuel oils		$	—			$	—			$	3			$	3
	Natural gas		—				1				1				2
	Power		—				1				10				11
	Total derivative assets - commodity contracts		$	—			$	2			$	14			$	16
	Nuclear decommissioning trust fund:
	Cash and cash equivalents		$	1			$	—			$	—			$	1
	Equity securities:
	U.S. large capitalization		348				—				—				348
	Debt securities:
	Corporate bonds		—				60				—				60
	Municipal bonds		—				2				—				2
	U.S. treasury and agency securities		—				100				—				100
	Asset-backed securities		—				11				—				11
	Other		—				5				—				5
	Total nuclear decommissioning trust fund		$	349			$	178			$	—			$	527	(b)
	Total Ameren		$	349			$	180			$	14			$	543
Ameren	Derivative assets - commodity contracts(a):
Missouri	Fuel oils		$	—			$	—			$	3			$	3
	Power		—				1				10				11
	Total derivative assets - commodity contracts		$	—			$	1			$	13			$	14
	Nuclear decommissioning trust fund:
	Cash and cash equivalents		$	1			$	—			$	—			$	1
	Equity securities:
	U.S. large capitalization		348				—				—				348
	Debt securities:
	Corporate bonds		—				60				—				60
	Municipal bonds		—				2				—				2
	U.S. treasury and agency securities		—				100				—				100
	Asset-backed securities		—				11				—				11
	Other		—				5				—				5
	Total nuclear decommissioning trust fund		$	349			$	178			$	—			$	527	(b)
	Total Ameren Missouri		$	349			$	179			$	13			$	541
Ameren	Derivative assets - commodity contracts(a):
Illinois	Natural gas		$	—			$	1			$	1			$	2
Liabilities:
Ameren	Derivative liabilities - commodity contracts(a):
	Fuel oils		$	3			$	—			$	3			$	6
	Natural gas		2				26				—				28
	Power		—				1				129				130
	Uranium		—				—				3				3
	Total Ameren		$	5			$	27			$	135			$	167
Ameren	Derivative liabilities - commodity contracts(a):
Missouri	Fuel oils		$	3			$	—			$	3			$	6
	Natural gas		2				4				—				6
	Power		—				1				5				6
	Uranium		—				—				3				3
	Total Ameren Missouri		$	5			$	5			$	11			$	21
Ameren	Derivative liabilities - commodity contracts(a):
Illinois	Natural gas		$	—			$	22			$	—			$	22
	Power		—				—				124				124
	Total Ameren Illinois		$	—			$	22			$	124			$	146
(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 Other				Total
			Active Markets for				Observable Inputs				Unobservable
			Identical Assets				(Level 2)				Inputs
			or Liabilities								(Level 3)
			(Level 1)
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
	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	Derivative assets - commodity contracts(a):
Missouri	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	Derivative assets - commodity contracts(a):
Illinois	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	Derivative liabilities - commodity contracts(a):
Missouri	Fuel oils		$	—			$	—			$	3			$	3
	Natural gas		3				8				—				11
	Power		—				2				2				4
	Uranium		—				—				6				6
	Total Ameren Missouri		$	3			$	10			$	11			$	24
Ameren	Derivative liabilities - commodity contracts(a):
Illinois	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 three months ended September 30, 2014:
		Net derivative commodity contracts
Three Months		Ameren			Ameren			Ameren
		Missouri			Illinois
Fuel oils:
Beginning balance at July 1, 2014	$	2		$	(a)		$	2
Realized and unrealized gains (losses) included in regulatory assets/liabilities		(2	)		(a)			(2	)
Ending balance at September 30, 2014	$	—		$	(a)		$	—
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014	$	(2	)	$	(a)		$	(2	)
Natural gas:
Beginning balance at July 1, 2014	$	—		$	—		$	—
Realized and unrealized gains (losses) included in regulatory assets/liabilities		—			1			1
Ending balance at September 30, 2014	$	—		$	1		$	1
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014	$	—		$	—		$	—
Power:
Beginning balance at July 1, 2014	$	15		$	(103	)	$	(88	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities		(5	)		(23	)		(28	)
Settlements		(5	)		2			(3	)
Ending balance at September 30, 2014	$	5		$	(124	)	$	(119	)
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014	$	(6	)	$	(22	)	$	(28	)
Uranium:
Beginning balance at July 1, 2014	$	(7	)	$	(a)		$	(7	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities		3			(a)			3
Settlements		1			(a)			1
Ending balance at September 30, 2014	$	(3	)	$	(a)		$	(3	)
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014	$	3		$	(a)		$	3
(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 three months ended September 30, 2013:
		Net derivative commodity contracts
Three Months		Ameren			Ameren			Ameren
		Missouri			Illinois
Fuel oils:
Beginning balance at July 1, 2013	$	3		$	(a)		$	3
Realized and unrealized gains (losses) included in regulatory assets/liabilities		1			(a)			1
Purchases		1			(a)			1
Sales		(1	)		(a)			(1	)
Settlements		(1	)		(a)			(1	)
Ending balance at September 30, 2013	$	3		$	(a)		$	3
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2013	$	1		$	(a)		$	1
Natural gas:
Beginning balance at July 1, 2013	$	(1	)	$	2		$	1
Realized and unrealized gains (losses) included in regulatory assets/liabilities		—			(2	)		(2	)
Purchases		1			—			1
Ending balance at September 30, 2013	$	—		$	—		$	—
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2013	$	—		$	(1	)	$	(1	)
Power:
Beginning balance at July 1, 2013	$	37		$	(80	)	$	(43	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities		(3	)		(17	)		(20	)
Sales		1			—			1
Settlements		(6	)		3			(3	)
Transfers into Level 3		(1	)		—			(1	)
Ending balance at September 30, 2013	$	28		$	(94	)	$	(66	)
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2013	$	(2	)	$	(16	)	$	(18	)
Uranium:
Beginning balance at July 1, 2013	$	(3	)	$	(a)		$	(3	)
Purchases		(2	)		(a)			(2	)
Ending balance at September 30, 2013	$	(5	)	$	(a)		$	(5	)
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2013	$	(2	)	$	(a)		$	(2	)
(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 nine months ended September 30, 2014:
		Net derivative commodity contracts
Nine Months		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		(3	)		(a)			(3	)
Settlements		(2	)		(a)			(2	)
Ending balance at September 30, 2014	$	—		$	(a)		$	—
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014	$	(2	)	$	(a)		$	(2	)
Natural gas:
Beginning balance at January 1, 2014	$	—		$	—		$	—
Realized and unrealized gains (losses) included in regulatory assets/liabilities		—			1			1
Purchases		—			(1	)		(1	)
Settlements		—			1			1
Ending balance at September 30, 2014	$	—		$	1		$	1
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014	$	—		$	—		$	—
Power:
Beginning balance at January 1, 2014	$	19		$	(108	)	$	(89	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities		(23	)		(19	)		(42	)
Purchases		34			—			34
Settlements		(25	)		3			(22	)
Ending balance at September 30, 2014	$	5		$	(124	)	$	(119	)
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014	$	(3	)	$	(21	)	$	(24	)
Uranium:
Beginning balance at January 1, 2014	$	(6	)	$	(a)		$	(6	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities		(1	)		(a)			(1	)
Settlements		4			(a)			4
Ending balance at September 30, 2014	$	(3	)	$	(a)		$	(3	)
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014	$	—		$	(a)		$	—
(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 nine months ended September 30, 2013:
		Net derivative commodity contracts
Nine Months		Ameren			Ameren			Ameren
		Missouri			Illinois
Fuel oils:
Beginning balance at January 1, 2013	$	5		$	(a)		$	5
Realized and unrealized gains (losses) included in regulatory assets/liabilities		(1	)		(a)			(1	)
Purchases		2			(a)			2
Sales		(1	)		(a)			(1	)
Settlements		(2	)		(a)			(2	)
Ending balance at September 30, 2013	$	3		$	(a)		$	3
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 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 September 30, 2013	$	—		$	—		$	—
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2013	$	—		$	—		$	—
Power:
Beginning balance at January 1, 2013	$	11		$	(111	)	$	(100	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities		3			(2	)		1
Purchases		40			—			40
Sales		1			—			1
Settlements		(28	)		19			(9	)
Transfers into Level 3		(3	)		—			(3	)
Transfers out of Level 3		4			—			4
Ending balance at September 30, 2013	$	28		$	(94	)	$	(66	)
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2013	$	—		$	(7	)	$	(7	)
Uranium:
Beginning balance at January 1, 2013	$	(2	)	$	(a)		$	(2	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities		(2	)		(a)			(2	)
Purchases		(2	)		(a)			(2	)
Settlements		1			(a)			1
Ending balance at September 30, 2013	$	(5	)	$	(a)		$	(5	)
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 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 shown below. 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 three and nine months ended September 30, 2014, and 2013, there were no transfers between Level 1 and Level 2 related to derivative commodity contracts. For the three and nine months ended September 30, 2014 there were no transfers between Level 2 and Level 3 related to derivative commodity contracts. For the three months ended September 30, 2013 there were $(1) million of transfers out of Level 2 into Level 3 related to power contracts at Ameren and Ameren Missouri. For the nine months ended September 30, 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.
The Ameren Companies’ carrying amounts of cash and cash equivalents approximate fair value because of the short-term nature of these instruments and 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, capital lease obligations and preferred stock at September 30, 2014, and December 31, 2013:
	30-Sep-14								31-Dec-13
	Carrying				Fair				Carrying				Fair
	Amount				Value				Amount				Value
Ameren:(a)
Long-term debt and capital lease obligations (including current portion)	$	5,944			$	6,647			$	6,038			$	6,584
Preferred stock	142				122				142				118
Ameren Missouri:
Long-term debt and capital lease obligations (including current portion)	$	4,004			$	4,466			$	3,757			$	4,124
Preferred stock	80				73				80				71
Ameren Illinois:
Long-term debt	$	1,940			$	2,181			$	1,856			$	2,028
Preferred stock	62				49				62				47
(a)	Preferred stock is recorded in “Noncontrolling Interests” on the consolidated balance sheet.

Related_Party_Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2014
Related Party Transactions [Abstract]	'
RELATED PARTY TRANSACTIONS	'
	RELATED PARTY TRANSACTIONS
	Ameren (parent) and its subsidiaries have engaged in, and may in the future engage in, affiliate transactions in the normal course of business. These transactions primarily consist of power purchases and sales, services received or rendered, and borrowings and lendings.
	Transactions between affiliates are reported as intercompany transactions on their respective financial statements, but are eliminated in consolidation for Ameren’s financial statements. For a discussion of our material related party agreements, see Note 14 - Related Party Transactions under Part II, Item 8, of the Form 10-K and the money pool arrangements discussed in Note 3 - Short-term Debt and Liquidity of this report.
	Electric Power Supply Agreements
	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, in 2014, Ameren Missouri and Ameren Illinois entered into energy product agreements by which Ameren Missouri agreed to sell and Ameren Illinois agreed to purchase 168,400 megawatthours at an average price of $51 per megawatthour during the period of January 1, 2015, through February 28, 2017.
	The following table presents the impact on Ameren Missouri and Ameren Illinois of related party transactions for the three and nine months ended September 30, 2014, and 2013.
					Three Months				Nine Months
	Agreement	Income Statement			Ameren		Ameren		Ameren		Ameren
		Line Item			Missouri		Illinois		Missouri		Illinois
	Ameren Missouri power supply	Operating Revenues	2014	$	2	$	(a)	$	5	$	(a)
	agreements with Ameren Illinois		2013		(b)		(a)		1		(a)
	Ameren Missouri and Ameren Illinois	Operating Revenues	2014		6		(b)		15		1
	rent and facility services		2013		4		(b)		16		1
	Ameren Missouri and Ameren Illinois	Operating Revenues	2014		(b)		(b)		1		(b)
	miscellaneous support services		2013		1		(b)		1		2
	Total Operating Revenues		2014	$	8	$	(b)	$	21	$	1
			2013		5		(b)		18		3
	Ameren Illinois power supply	Purchased Power	2014	$	(a)	$	2	$	(a)	$	5
	agreements with Ameren Missouri		2013		(a)		(b)		(a)		1
	Ameren Illinois transmission	Purchased Power	2014		(a)		1		(a)		2
	services with ATXI		2013		(a)		1		(a)		2
	Total Purchased Power		2014	$	(a)	$	3	$	(a)	$	7
			2013		(a)		1		(a)		3
					Three Months				Nine Months
	Agreement	Income Statement			Ameren		Ameren		Ameren		Ameren
		Line Item			Missouri		Illinois		Missouri		Illinois
	Ameren Services support services	Other Operations and Maintenance	2014	$	25	$	26	$	90	$	80
	agreement		2013		25		22		85		70
	Insurance premiums(c)	Other Operations and Maintenance	2014		(b)		(a)		(b)		(a)
			2013		(b)		(a)		(b)		(a)
	Total Other Operations and		2014	$	25	$	26	$	90	$	80
	Maintenance Expenses		2013		25		22		85		70
	Money pool borrowings (advances)	Interest Charges	2014	$	(b)	$	(b)	$	(b)	$	(b)
			2013		(b)		(b)		(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	9 Months Ended
	Sep. 30, 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 the notes to our financial statements in this report and in our Form 10-K, will not have a material adverse effect on our results of operations, financial position, or liquidity.
	Reference is made to Note 1 - Summary of Significant Accounting Policies, Note 2 - Rate and Regulatory Matters, Note 14 - Related Party Transactions, Note 15 - Commitments and Contingencies, and Note 16 - Divestiture Transactions and Discontinued Operations under Part II, Item 8, of the Form 10-K. See also Note 1 - Summary of Significant Accounting Policies, Note 2 - Rate and Regulatory Matters, Note 8 - Related Party Transactions, Note 10 - Callaway Energy Center, and Note 12 - 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 September 30, 2014. The property coverage and the nuclear liability coverage must be renewed on April 1 and January 1, respectively, of each year. Both coverages were renewed in 2014.
	Type and Source of Coverage	Maximum  Coverages			Maximum Assessments
					for Single Incidents
	Public liability and nuclear worker liability:
	American Nuclear Insurers	$	375		$	—
	Pool participation	13,241		(a)	128		(b)
		$	13,616	(c)	$	128
	Property damage:
	NEIL	$	2,250	(d)	$	23	(e)
	European Mutual Association for Nuclear Insurance	500		(f)	—
		$	2,750		$	23
	Replacement power:
	NEIL	$	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 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 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 $327.6 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 and 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 and has reinsured this coverage with third-party insurance companies. See Note 8 - 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 five-year inflationary adjustment was effective September 10, 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 limit of $3.24 billion, or $1.83 billion, for events not involving radiation contamination within a 12-month period for coverage for such terrorist acts.
	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.
	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 at September 30, 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 September 30, 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 $17 million during the remainder of 2014 and $64 million in 2015 for these customer energy efficiency programs.
		Coal			Natural			Nuclear		Purchased		Methane		Other		Total
					Gas(a)			Fuel		Power(b)		Gas
	Ameren:(c)
	2014	$	151		$	93		$	62	$	62	$	1	$	88	$	457
	2015	635			225			56		190		3		156		1,265
	2016	659			127			69		105		4		76		1,040
	2017	682			80			59		66		4		50		941
	2018	111			41			61		55		5		51		324
	Thereafter	114			101			179		645		91		350		1,480
	Total	$	2,352		$	667		$	486	$	1,123	$	108	$	771	$	5,507
	Ameren Missouri:
	2014	$	151		$	16		$	62	$	4	$	1	$	60	$	294
	2015	635			39			56		21		3		110		864
	2016	659			21			69		21		4		39		813
	2017	682			13			59		21		4		26		805
	2018	111			8			61		21		5		27		233
	Thereafter	114			29			179		120		91		183		716
	Total	$	2,352		$	126		$	486	$	208	$	108	$	445	$	3,725
	Ameren Illinois:
	2014	$	—		$	77		$	—	$	58	$	—	$	9	$	144
	2015	—			186			—		169		—		28		383
	2016	—			106			—		84		—		24		214
	2017	—			67			—		45		—		24		136
	2018	—			33			—		34		—		24		91
	Thereafter	—			72			—		525		—		167		764
	Total	$	—		$	541		$	—	$	915	$	—	$	276	$	1,732
	(a)	Includes amounts for generation and for distribution.
	(b)	The purchased power amounts for Ameren and Ameren Illinois include twenty-year agreements for renewable energy credits that were entered into in December 2010 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 ongoing 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 energy centers. Significant new rules proposed or promulgated include the regulation of CO2 emissions from existing energy centers through the proposed Clean Power Plan and from new energy centers through the 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 energy centers; a regulation governing management of CCR and coal ash impoundments; the MATS, which require reduction of emissions of mercury, toxic metals, and acid gases from energy centers; revised NSPS for particulate matter, SO2, and NOx emissions from new sources; new effluent standards applicable to waste water discharges from energy centers 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. 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 September 30, 2014, Ameren and Ameren Missouri estimate capital expenditure investments of $325 million to $375 million through 2018 to comply with existing environmental regulations. This estimate assumes that CCR will continue to be regulated as nonhazardous. 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 electrostatic precipitator upgrades at the Labadie energy center and the installation of additional controls including mercury control technology at multiple energy centers within its coal-fired fleet through 2018. However, Ameren Missouri is evaluating 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 ten 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 emissions that result from burning fossil fuels. In 2005, the EPA issued regulations with respect to SO2 and NOx emissions (the CAIR). In December 2008, the United States Court of Appeals for the District of Columbia Circuit found various aspects of the regulations to be unlawful and remanded the CAIR to the EPA for further action, but allowed the CAIR's cap-and-trade programs to remain effective until they are replaced by the EPA. In July 2011, the EPA issued the CSAPR as the CAIR replacement. The CSAPR regulations were vacated by the United States Court of Appeals for the District of Columbia Circuit. The EPA appealed to the United States Supreme Court. In April 2014, the United States Supreme Court reversed the decision of the United States Court of Appeals for the District of Columbia Circuit and upheld the CSAPR. In October 2014, the United States Court of Appeals for the District of Columbia Circuit granted the EPA’s motion to lift the stay on CSPAR. The CSPAR will become effective on January 1, 2015, for SO2 and annual NOx reductions, and on May 1, 2015, for ozone season NOx reductions, with further reductions in 2017 and in subsequent years. The EPA did not revise the emission reductions previously included in 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 for 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 require 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 to April 2016 to comply with the MATS.
Emission Allowances
The Clean Air Act created marketable commodities called emission allowances under the acid rain program, the NOx budget trading program, the CAIR and the CSAPR. Ameren Missouri expects to have enough allowances for 2014 to avoid making external purchases to comply with the CAIR and the acid rain program.
Ameren and Ameren Missouri are reviewing the United States Court of Appeals for the District of Columbia Circuit’s decision in October 2014 lifting the stay on the CSAPR. As discussed above, the CSAPR allowance programs will begin in 2015. Ameren Missouri expects to have sufficient allowances for 2015 to avoid making external purchases to comply with CSAPR.
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 beginning in 2011 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 permit processes and NSR programs at stationary sources that are already subject to those programs, but may not apply operating permit processes and NSR programs to non-stationary sources solely as a result of their greenhouse gas emissions. Ameren Missouri is currently evaluating the decision and the impact, if any, on its operations.
In June 2013, the Obama administration announced that it had directed the EPA to set CO2 emissions standards for both new and existing power plants. The EPA published proposed regulations in January 2014 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 rates that the EPA calculated for each state. The EPA believes, based on their assumptions, that the Clean Power Plan would achieve a 30% decrease in 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 by June 2015. After the proposed rule is finalized, states will have from one to three years to develop compliance plans. States will be allowed to develop independent plans or join with other states to develop joint plans. Ameren Missouri is evaluating the proposed Clean Power Plan and the potential impact to its operations. 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, both of which may result in the amount and timing of CO2 emission reductions being revised.
Based on preliminary studies, if the proposed Clean Power Plan was to be made final, Ameren Missouri anticipates new or accelerated capital expenditures and increased fuel costs would be required 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 may require Ameren Missouri to construct combined cycle gas-fired and renewable energy centers, currently estimated to cost 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 gas-fired energy centers. Ameren Missouri expects all of these increased costs, which could begin in 2017, would 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 cost recovery in rates of these compliance costs. Ameren's and Ameren Missouri's earnings may benefit from increased investment to comply with greenhouse gas limitations to the extent 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 will defend itself vigorously. However, there can be no assurances that it will be successful in its efforts.
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 by September 30, 2015.
Ash Management
In May 2010, the EPA announced proposed new regulations regarding the management and disposal of CCR, which could affect future disposal and handling costs for CCR at Ameren Missouri’s coal-fired energy centers. Those proposed regulations include two options for managing CCRs, under either solid or hazardous waste regulations, but either alternative would allow for some continued beneficial uses, such as recycling CCR without classifying it as waste. The EPA announced that its April 2013 proposed revisions to the effluent limitations applicable to steam electric power plants would apply to ash ponds and CCR management and that it intended to align the effluent limitations with the CCR rules when finalized. The EPA is expected to issue regulations describing how it will regulate CCR by December 2014. Ameren Missouri is evaluating the proposed regulations to determine whether the current management of CCR, including beneficial reuse, and the use of the ash ponds should be altered. Ameren Missouri is evaluating the potential compliance costs associated with the proposed regulation of CCR impoundments and landfills, which could be material, if such regulations are adopted.
Remediation
We are involved in a number of remediation actions to clean up sites impacted 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 the federal or state governments as a potentially responsible party at several contaminated sites.
As of September 30, 2014, Ameren Missouri had completed remediation at the last remaining former MGP site for which remediation was required. Ameren Missouri does not have a rate rider mechanism that permits it to recover from utility customers remediation costs associated with former MGP sites.
As of September 30, 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. Based on current estimated plans, Ameren Illinois could substantially conclude remediation efforts at most of these sites by 2018. The ICC permits Ameren Illinois to recover remediation and litigation costs associated with its former MGP sites from its electric and natural gas utility customers through environmental remediation cost rate riders. To be recoverable, such costs must be prudently incurred and are subject to annual review by the ICC. As of September 30, 2014, Ameren Illinois estimated the obligation related to these former MGP sites at $254 million to $316 million. Ameren and Ameren Illinois recorded a liability of $254 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 fluctuate as investigation and 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 used an off-site landfill, which Ameren Illinois did not own, in connection with the former operation of an energy center. Ameren Illinois could be required to perform certain maintenance activities associated with that landfill. As of September 30, 2014, Ameren Illinois estimated the obligation related to the landfill 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 September 30, 2014, Ameren Illinois recorded a liability of $0.7 million to represent its estimate of the obligation for these sites.
Ameren Missouri is investigating and addressing two waste sites in Missouri as a result of federal agency mandates. One of the cleanup sites is a former coal tar distillery located in St. Louis, Missouri. In 2008, the EPA issued an administrative order to Ameren Missouri pertaining to this distillery operated by Koppers Company or its predecessor and successor companies. While Ameren Missouri is the current owner of the site, 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 September 30, 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. At the other federal agency-mandated cleanup site, Ameren Missouri was a customer of an electrical equipment repair and disposal company that previously operated a facility in Cape Girardeau, Missouri. A trust was established in the early 1990s by several businesses and governmental agencies to fund the investigation and cleanup of this site, which was completed in 2005. Ameren Missouri anticipates that this trust fund will be sufficient to complete the remaining adjacent off-site cleanup, and therefore, believes it has no liability at September 30, 2014, for this site.
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 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 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 September 30, 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 for this site, as no other amount within the range was a better estimate.
In December 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 September 30, 2014, Ameren Missouri estimated the obligation related to the cleanup at $2.2 million to $4.5 million. Ameren Missouri recorded a liability of $2.2 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. The rebuilt Taum Sauk energy center became fully operational in April 2010. Ameren Missouri had liability insurance coverage for the Taum Sauk incident, subject to certain limits and deductibles.
In June 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 the 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 request for appellate review of that order by the United States Court of Appeals for the Eight 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 the terms of that settlement, Ameren Missouri received a payment of $27 million. As of September 30, 2014, Ameren Missouri had an insurance receivable balance of $41 million and expects to ultimately 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 September 30, 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 the average number of parties being 81 as of September 30, 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 September 30, 2014:
Ameren		Ameren	Ameren		Total(a)
		Missouri	Illinois
1		48	62		75
(a)	Total does not equal the sum of the subsidiary unit lawsuits because some of the lawsuits name multiple Ameren entities as defendants.
As of September 30, 2014, Ameren, Ameren Missouri and Ameren Illinois had liabilities of $13 million, $5 million, and $8 million, respectively, recorded to represent their best estimate 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 cash expenditures in excess of the amount included in base electric rates are to be recovered from a trust fund that was established when Ameren acquired IP. At September 30, 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 customers within IP’s historical service territory.
Ameren Illinois Municipal Taxes
Ameren Illinois received tax liability notices from the city of O'Fallon, Illinois, relating to prior-period electric and natural gas municipal taxes. The city alleges that Ameren Illinois failed to collect prior-period taxes from more than 2,400 accounts, primarily in annexed areas, for the period 2004 through 2012. In July 2013, the O’Fallon city administrator issued an order stating that Ameren Illinois was liable to the city of O’Fallon for $4 million. In August 2013, Ameren Illinois filed an appeal and a stay of the O’Fallon city administrator’s order to the Circuit Court of St. Clair County. In addition, in December 2012, the city of Peoria issued a tax liability notice alleging that Ameren Illinois failed to collect prior-period municipal taxes from certain accounts. In September 2013, a hearing officer issued an order stating that Ameren Illinois was liable to the city of Peoria for $0.5 million. Ameren Illinois filed an appeal and a stay of the order to the Circuit Court of Peoria County. Also, in late 2012, five other cities issued tax liability notices alleging that Ameren Illinois failed to collect an immaterial amount of taxes from certain accounts. Ameren Illinois believes its defenses to the allegations are meritorious. As of September 30, 2014, Ameren Illinois estimated its obligation at $2 million to $5 million. Ameren Illinois recorded a liability of $2 million, which reflects potential settlements with the Illinois cities.

Callaway_Energy_Center

Callaway Energy Center	9 Months Ended
	Sep. 30, 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 paid the federal government for disposing of the spent nuclear fuel at one mill, or one-tenth of one cent, for each kilowatthour generated by those plants and sold. The NWPA also requires the DOE 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 has 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 has the capability for additional storage capacity for spent nuclear fuel generated through the end of the energy center’s current license. 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 2009, the federal government announced that a spent nuclear fuel repository at Yucca Mountain, Nevada was unworkable. The federal government took steps to terminate the Yucca Mountain program, while acknowledging its continuing obligation to dispose of utilities’ spent nuclear fuel. 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, co-located with the pilot facility. The plan also proposes to site a permanent geological repository by 2026, to characterize the site and to design and to license the repository by 2042, and to begin operation by 2048.
	In view of the federal government's efforts to terminate the Yucca Mountain program, 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 a proposal to the United States Congress to reduce the fee to zero, which became effective on May 16, 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 and 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, as well as certain NRC fees, and 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 to by the parties. In March 2014, Ameren Missouri submitted its 2013 costs to the DOE for reimbursement under the settlement agreement. Ameren Missouri expects to receive the 2013 cost reimbursement of approximately $15 million during the fourth quarter of 2014. This reimbursement is included in "Miscellaneous accounts and notes receivable" on Ameren’s and Ameren Missouri’s September 30, 2014 and December 31, 2013 respective balance sheets. Included in these reimbursements are costs related to a dry spent fuel storage facility 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 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 that the NRC has historically relied upon 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. In June 2012, a number of groups petitioned the NRC to suspend final licensing decisions in certain NRC licensing proceedings, including the Callaway license extension, until the NRC completed its proceedings on the vacated rules. In August 2012, the NRC stated that it would not issue licenses dependent on the vacated rules until it appropriately addressed the court's remand. In September 2012, the NRC directed its staff to issue a generic environmental impact statement and a final rule to address the court's ruling. In September 2014, the NRC published its final rule and generic environmental impact statement. On October 20, 2014, the final rule became effective and the NRC lifted its suspension on final licensing decisions.
	Electric utility rates 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. If Ameren Missouri's operating license extension application is approved by the NRC, a revised funding analysis will be prepared, and the rates charged to customers will be adjusted accordingly to reflect the operating license extension at the time the next triennial cost study and funding analysis is approved by the MoPSC. 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.

Retirement_Benefits

Retirement Benefits	9 Months Ended
	Sep. 30, 2014
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]	'
RETIREMENT BENEFITS	'
	RETIREMENT BENEFITS
	Ameren’s pension and postretirement plans are funded in compliance with income tax regulations and to meet federal funding or regulatory requirements. As a result, Ameren expects to fund its pension plans at a level equal to the greater of the pension expense or the legally required minimum contribution. Considering Ameren’s assumptions at September 30, 2014, the plan’s estimated investment performance through September 30, 2014, and Ameren’s pension funding policy, Ameren expects to make annual contributions of $40 million to $110 million in each of the next five years, with aggregate estimated contributions of $340 million. These amounts are estimates which may change with actual investment performance, changes in interest rates, any pertinent changes in government regulations, and any voluntary contributions. Our policy for postretirement benefits is primarily to fund the voluntary employees’ beneficiary association trusts to match the annual postretirement expense.
	The following table presents the components of the net periodic benefit cost (benefit) for Ameren’s pension and postretirement benefit plans for the three and nine months ended September 30, 2014, and 2013:
		Pension Benefits												Postretirement Benefits
		Three Months						Nine Months						Three Months						Nine Months
		2014			2013			2014			2013			2014			2013			2014			2013
	Service cost	$	20		$	23		$	60		$	69		$	5		$	6		$	14		$	17
	Interest cost	46			40			137			121			12			11			37			34
	Expected return on plan assets	(58		)	(54		)	(172		)	(162		)	(16		)	(16		)	(48		)	(47		)
	Amortization of:
	Prior service cost (benefit)	(1		)	(1		)	(1		)	(3		)	(2		)	(1		)	(4		)	(3		)
	Actuarial loss (gain)	13			23			37			69			(2		)	2			(5		)	6
	Net periodic benefit cost (benefit)(a)	$	20		$	31		$	61		$	94		$	(3	)	$	2		$	(6	)	$	7
	(a)	Includes $2 million and $8 million in total net costs for pension benefits for the three and nine months ended September 30, 2013, respectively, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income. Includes less than $1 million in total net costs for postretirement benefits for both the three and nine months ended September 30, 2013, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income.
	Ameren Missouri and Ameren Illinois are responsible for their respective shares of Ameren’s pension and postretirement costs. The following table presents the pension costs and the postretirement benefit costs incurred for the three and nine months ended September 30, 2014, and 2013:
		Pension Benefits												Postretirement Benefits
		Three Months						Nine Months						Three Months						Nine Months
		2014			2013			2014			2013			2014			2013			2014			2013
	Ameren Missouri	$	13		$	18		$	38		$	54		$ (a)			$	2		$	2		$	7
	Ameren Illinois	7			10			22			31			(3		)	(a)			(7		)	(a)
	Other(b)	(a)			3			1			9			(a)			(a)			(1		)	(a)
	Ameren(c)	$	20		$	31		$	61		$	94		$	(3	)	$	2		$	(6	)	$	7
	(a)	Less than $1 million.
	(b)	Includes $2 million and $8 million in total net costs for pension benefits for the three and nine months ended September 30, 2013, respectively, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income. Includes less than $1 million in total net costs for postretirement benefits for both the three and nine months ended September 30, 2013, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income.
	(c)	Includes amounts for Ameren registrants and nonregistrant subsidiaries.

Divestiture_Transactions_and_D

Divestiture Transactions and Discontinued Operations (Notes)	9 Months Ended
	Sep. 30, 2014
Discontinued Operations and Disposal Groups [Abstract]	'
DIVESTITURE TRANSACTIONS AND DISCONTINUED OPERATIONS	'
	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. 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, before consideration of a net working capital adjustment. The agreement with Rockland Capital required $17 million of the purchase price to be held in escrow until the two-year anniversary of the closing of the sale to fund certain indemnity obligations, if any, of Medina Valley. The Rockland Capital escrow receivable balance is reflected on Ameren's September 30, 2014, consolidated balance sheet in "Other assets." The corresponding payable due to Genco is reflected on Ameren's September 30, 2014, consolidated balance sheet in "Other deferred credits and liabilities." 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
	New AER and the Elgin, Gibson City, Grand Tower, Meredosia, and Hutsonville energy centers have been classified collectively in Ameren’s consolidated financial statements as discontinued operations for all periods presented in this report. The disposal groups have been aggregated in the disclosures below. See Note 16 - Divestiture Transactions and Discontinued Operations under Part II, Item 8, of the Form 10-K for additional information related to disposal groups. The following table presents the components of discontinued operations in Ameren's consolidated statement of income for the three and nine months ended September 30, 2014, and 2013:
		Three Months						Nine Months
		2014			2013			2014			2013
	Operating revenues	$	—		$	311		$	1		$	878
	Operating expenses	(1		)	(309		)	(4		)	(1,034		)	(a)
	Operating income (loss)	(1		)	2			(3		)	(156		)
	Other income (loss)	—			—			—			(1		)
	Interest charges	—			(9		)	—			(31		)
	Loss before income taxes	(1		)	(7		)	(3		)	(188		)
	Income tax (expense) benefit	—			4			—			(24		)
	Loss from discontinued operations, net of taxes	$	(1	)	$	(3	)	$	(3	)	$	(212	)
	(a)	Included a noncash pretax asset impairment charge of $175 million for the nine months ended September 30, 2013, to reduce the carrying value of the New AER disposal group to its estimated fair value less cost to sell.
	Ameren recorded a cumulative pretax charge to earnings of $175 million for the nine months ended September 30, 2013, to reduce the carrying value of the New AER disposal group to its estimated fair value less cost to sell. Also, 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, when it became apparent that the temporary difference would reverse. For the nine months ended September 30, 2013, this change in basis resulted in a cumulative discontinued operations deferred tax expense of $96 million. The deferred tax expense was partially offset by the then-expected tax benefits of $72 million related to the pretax loss from discontinued operations including the impairment charge recorded during the nine months ended September 30, 2013.
	Ameren’s results of operations for the nine months ended September 30, 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. The final working capital adjustment and a portion of the contingent liabilities were paid to IPH in the third quarter of 2014, resulting in a $13 million cash payment. 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, including the adoption of recently issued guidance from the IRS related to tangible property repairs and other matters. As a result, tax expense and benefits ultimately realized in discontinued operations may differ materially from those recorded as of September 30, 2014.
	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 September 30, 2014, and December 31, 2013:
		30-Sep-14			31-Dec-13
	Assets of discontinued operations
	Cash and cash equivalents	$	—		$	—
	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)	Includes 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 September 30, 2014, and December 31, 2013, respectively.
	Pursuant to the IPH transaction agreement, as amended, Ameren is obligated to pay up to $29 million for certain contingent liabilities as of September 30, 2014, which were included in "Other deferred credits and liabilities" on Ameren's September 30, 2014 consolidated balance sheet.
	The note receivable from Marketing Company related to the cash collateral support provided to New AER was $23 million and $18 million at September 30, 2014, and December 31, 2013, respectively, and was reflected on Ameren's consolidated balance sheet in "Other assets." This receivable is due to Ameren, with interest, on December 2, 2015, or sooner as cash collateral requirements are reduced. In addition, as of September 30, 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 $23 million, which includes $4 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 September 30, 2014 consolidated balance sheet, Ameren had a total of $141 million in guarantees outstanding for New AER that were not recorded on Ameren’s September 30, 2014 consolidated balance sheet, which included:
	•	$132 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 September 30, 2014, Ameren would have been required under its guarantees to provide $4 million to the counterparties.
	•	$9 million related to requirements for lease agreements and potential environmental obligations. If New AER had not fulfilled its lease obligation as of September 30, 2014, Ameren would have been required to provide approximately $8 million to the leasing counterparty.
	Additionally, at September 30, 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 September 30, 2014.

Segment_Information

Segment Information	9 Months Ended
	Sep. 30, 2014
Segment Reporting [Abstract]	'
SEGMENT INFORMATION	'
	SEGMENT INFORMATION
	Ameren has two reportable segments: Ameren Missouri and Ameren Illinois. The Ameren Missouri segment for both Ameren and Ameren Missouri includes all of the operations of Ameren Missouri’s business as described in Note 1 - Summary of Significant Accounting Policies. The Ameren Illinois segment for both Ameren and Ameren Illinois includes all of the operations of Ameren Illinois’ business as described in Note 1 - Summary of Significant Accounting Policies. The category called Other primarily includes Ameren (parent) activities, Ameren Services, and ATXI. In 2013, the Other category also included certain corporate activities previously included in the Merchant Generation segment.
	The following table presents information about the reported revenues and specified items reflected in Ameren’s net income attributable to Ameren Corporation from continuing operations for the three and nine months ended September 30, 2014, and 2013, and total assets of continuing operations as of September 30, 2014, and December 31, 2013.
	Three Months	Ameren		Ameren		Other			Intersegment			Ameren
		Missouri		Illinois					Eliminations
	2014
	External revenues	$	1,089	$	572	$	9		$	—		$	1,670
	Intersegment revenues	8		—		2			(10		)	—
	Net income (loss) attributable to Ameren Corporation from continuing operations	222		75		(3		)	—			294
	2013
	External revenues	$	1,088	$	547	$	3		$	—		$	1,638
	Intersegment revenues	5		—		1			(6		)	—
	Net income (loss) attributable to Ameren Corporation from continuing operations	238		77		(10		)	—			305
	Nine Months
	2014
	External revenues	$	2,793	$	1,864	$	26		$	—		$	4,683
	Intersegment revenues	21		1		3			(25		)	—
	Net income (loss) attributable to Ameren Corporation from continuing operations	395		156		(10		)	—			541
	2013
	External revenues	$	2,760	$	1,744	$	12		$	—		$	4,516
	Intersegment revenues	18		3		2			(23		)	—
	Net income (loss) attributable to Ameren Corporation from continuing operations	362		139		(37		)	—			464
	As of September 30, 2014:
	Total assets	$	13,179	$	7,983	$	810		$	(111	)	$	21,861	(a)
	As of December 31, 2013:
	Total assets	$	12,904	$	7,454	$	752		$	(233	)	$	20,877	(a)
	(a)    Excludes total assets from discontinued operations of $15 million and $165 million as of September 30, 2014, and December 31, 2013, respectively.

Summary_Of_Significant_Account1

Summary Of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2014
Accounting Policies [Abstract]	'
Consolidation	'
	The financial statements of Ameren are prepared on a consolidated basis, and therefore include the accounts of its majority-owned subsidiaries. All significant intercompany transactions have been eliminated. Ameren Missouri and Ameren Illinois have no subsidiaries, and therefore their financial statements are not prepared on a consolidated basis. 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. The results of operations of an interim period may not give a true indication of results that may be expected for a full year. These financial statements should be read in conjunction with the financial statements and the notes thereto included in the Form 10-K.
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.
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 April 24, 2014. The 2014 Incentive Plan provides for a maximum of 8 million common shares to be available for grant to eligible employees and directors, and 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, as of April 24, 2014, 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 and will therefore 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.
Intangible Assets	'
	Ameren and Ameren Missouri classify renewable energy credits and emission allowances as intangible assets. Ameren Illinois consumes renewable energy credits as they are purchased through the IPA procurement process and expenses them immediately. Ameren Missouri’s emission allowances are allocated by the EPA and therefore are recorded at zero cost. We evaluate intangible assets for impairment if events or changes in circumstances indicate that their carrying amount might be impaired.
	At September 30, 2014, Ameren’s and Ameren Missouri’s intangible assets consisted of renewable energy credits obtained through wind and solar power purchase agreements. The book values of both Ameren’s and Ameren Missouri’s renewable energy credits were $20 million and $22 million at September 30, 2014 and December 31, 2013, respectively.
	Ameren Missouri’s and Ameren Illinois’ renewable energy credits and Ameren Missouri’s emission allowances are charged to “Purchased power” expense and “Fuel” expense, respectively, as they are used in operations.
Excise Taxes	'
	Excise taxes levied on us are reflected on Ameren Missouri electric customer bills and on Ameren Missouri and Ameren Illinois natural gas customer bills. They 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 reflected on Ameren Illinois electric customer bills are imposed on the customer and are therefore not included in revenues and expenses. They are included in “Taxes accrued” on the balance sheet.
Uncertain Tax Positions	'
	With the adoption of new accounting guidance in the first quarter of 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 our balance sheets.
	Ameren’s federal income tax returns for the years 2011 and 2012 are before the Appeals Office of the IRS. It is reasonably possible that a settlement will be reached with the Appeals Office of the IRS in the next 12 months for the years 2011 and 2012. The potential settlement, which would primarily relate to uncertain tax positions associated with the timing of research tax deductions, is expected to result in a decrease in Ameren’s uncertain tax benefits of $6 million, all of which relates to Ameren Missouri and none of which will have a material impact on their respective results of operations or liquidity.
	Ameren’s federal income tax return for the year 2013 is currently under examination by the IRS and it is reasonably possible that a settlement will be reached with the IRS examination team in the next 12 months for that year. The potential settlement, which would relate to the timing of research tax deductions and the tax basis of certain leases related to the divestiture of the merchant generation business, is expected to result in a decrease in Ameren’s uncertain tax benefits of $73 million, of which $17 million relates to Ameren Missouri and $1 million relates to Ameren Illinois. Although we are unable to estimate the impact of any potential settlement at this time, up to $55 million of the Ameren total could increase net income from Ameren’s discontinued operations. Settlement of the remaining $18 million of uncertain tax positions at Ameren, as well as those positions at Ameren Missouri and Ameren Illinois, are associated with the timing of deductions and will not have a material impact on our results of operations.
	In addition, it is reasonably possible that other events will occur during the next 12 months that would cause the total amount of our unrecognized tax benefits to fluctuate. However, other than as described above, we do not believe any such fluctuations would be material to our results of operations, financial position, or liquidity.
	State income tax returns are generally subject to examination for a period of three years after filing of the return. We do not currently have material state income tax issues under examination, administrative appeals, or litigation. 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.
Asset Retirement Obligations	'
	AROs at Ameren, Ameren Missouri and Ameren Illinois increased at September 30, 2014, compared to December 31, 2013, to reflect the accretion of obligations to their fair value and an additional ARO at Ameren and Ameren Missouri of $2 million related to the retirement costs for a CCR storage facility, partially offset by immaterial settlements.
Noncontrolling Interest	'
	As of September 30, 2014, Ameren's noncontrolling interests were composed of the preferred stock not subject to mandatory redemption of Ameren Missouri and Ameren Illinois. All noncontrolling interests are classified as a component of equity separate from Ameren's equity on its consolidated balance sheet.
Accounting and Reporting Developments	'
	The following is a summary of recently adopted or issued authoritative accounting guidance relevant to the Ameren Companies.
	Presentation of an Unrecognized Tax Benefit
	In July 2013, FASB issued additional authoritative accounting guidance 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 to the extent a net operating loss carryforward, a similar tax loss, or a tax credit carryforward is available under the tax law. This guidance was effective for the Ameren Companies beginning in the first quarter of 2014. 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 the first quarter 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. For comparative purposes, the Ameren Companies reclassified the December 31, 2013 balances in accordance with the new guidance as discussed in the Uncertain Tax Positions section above. 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 Disposals of Components of an Entity
	In April 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 has 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 will be 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 would not be 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 May 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 to which the entity expects to be entitled 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 Ameren Companies are currently assessing the impacts of this guidance.

Derivative_Financial_Instrumen1

Derivative Financial Instruments Derivative Financial Instruments (Policies)	9 Months Ended
	Sep. 30, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]	'
Derivatives	'
	We use derivatives to manage the risk of changes in market prices for natural gas, diesel, power, and uranium. 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.

Retirement_Benefits_Retirement

Retirement Benefits Retirement Benefits (Policies)	9 Months Ended
	Sep. 30, 2014
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]	'
Retirement Benefits	'
	Ameren’s pension and postretirement plans are funded in compliance with income tax regulations and to meet federal funding or regulatory requirements. As a result, Ameren expects to fund its pension plans at a level equal to the greater of the pension expense or the legally required minimum contribution. Considering Ameren’s assumptions at September 30, 2014, the plan’s estimated investment performance through September 30, 2014, and Ameren’s pension funding policy, Ameren expects to make annual contributions of $40 million to $110 million in each of the next five years, with aggregate estimated contributions of $340 million. These amounts are estimates which may change with actual investment performance, changes in interest rates, any pertinent changes in government regulations, and any voluntary contributions. Our policy for postretirement benefits is primarily to fund the voluntary employees’ beneficiary association trusts to match the annual postretirement expense.

Summary_Of_Significant_Account2

Summary Of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2014
Accounting Policies [Abstract]	'
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 three and nine months ended September 30, 2014, and 2013:
		Three Months								Nine Months
		2014				2013				2014			2013
	Net income (loss) attributable to Ameren Corporation:
	Continuing operations	$	294			$	305			$	541		$	464
	Discontinued operations	(1		)		(3		)		(3		)	(212		)
	Net income attributable to Ameren Corporation	$	293			$	302			$	538		$	252
	Average common shares outstanding - basic	242.6				242.6				242.6			242.6
	Assumed settlement of performance share units	1.7				2.5				1.7			1.8
	Average common shares outstanding - diluted	244.3				245.1				244.3			244.4
	Earnings (loss) per common share – basic:
	Continuing operations	$	1.21			$	1.26			$	2.23		$	1.92
	Discontinued operations	—				(0.01		)		(0.01		)	(0.88		)
	Earnings per common share – basic	$	1.21			$	1.25			$	2.22		$	1.04
	Earnings (loss) per common share – diluted:
	Continuing operations	$	1.2			$	1.25			$	2.21		$	1.91
	Discontinued operations	—				(0.01		)		(0.01		)	(0.88		)
	Earnings per common share – diluted	$	1.2			$	1.24			$	2.2		$	1.03
Summary Of Nonvested Shares Related To Long-Term Incentive Plan	'
	A summary of nonvested performance share units at September 30, 2014, and changes during the nine months ended September 30, 2014, under the 2006 Incentive Plan and the 2014 Incentive Plan are presented below:
		Performance Share Units
		Share Units		Weighted-average Fair Value Per Share Unit at Grant Date
	Nonvested at January 1, 2014	1,218,544		$	33.23
	Granted(a)	685,026		38.9
	April Grants(b)	38,559		50.34
	Forfeitures	(65,847	)	33.82
	Vested(c)	(123,295	)	38.64
	Nonvested at September 30, 2014	1,752,987		$	35.42
	(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)	Share units vested due to the 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.
Schedule Of Amortization Expense Based On Usage Of Renewable Energy Credits And Emission Allowances	'
	The following table presents amortization expense based on usage of renewable energy credits and emission allowances, net of gains from sales, for Ameren, Ameren Missouri and Ameren Illinois, during the three and nine months ended September 30, 2014, and 2013:
			Three Months							Nine Months
		2014				2013				2014			2013
	Ameren Missouri	$	1			$	—			$	7		$	(a)
	Ameren Illinois		1				2				7			9
	Ameren	$	2			$	2			$	14		$	9
	(a)	Less than $1 million.
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 three and nine months ended September 30, 2014, and 2013:
		Three Months								Nine Months
		2014				2013				2014			2013
	Ameren Missouri	$	47			$	49			$	120		$	120
	Ameren Illinois	9				10				46			43
	Ameren	$	56			$	59			$	166		$	163
Schedule of Unrecognized Tax Benefits	'
	The following table presents the total amount of reserves for unrecognized tax benefits (detriments) related to uncertain tax positions as of September 30, 2014, and December 31, 2013:
		30-Sep-14				December 31,
						2013
	Ameren	$	97			$	90
	Ameren Missouri	35				31
	Ameren Illinois	1				(1		)
Schedule of Unrecognized Tax Benefits That Would Impact Effective Tax Rate	'
	The following table presents the amount of reserves for unrecognized tax benefits, included in the table above, related to uncertain tax positions that would impact results of operations, if recognized, as of September 30, 2014, and December 31, 2013:
		30-Sep-14				December 31,
						2013
	Ameren	$	55			$	54
	Ameren Missouri	3				3
	Ameren Illinois	—				—
Equity Changes Attributable To Noncontrolling Interest	'
	A reconciliation of the equity changes attributable to the noncontrolling interests at Ameren for the three and nine months ended September 30, 2014, and 2013, are shown below:
		Three Months								Nine Months
		2014				2013				2014			2013
	Noncontrolling interests, beginning of period	$	142			$	151		(a)	$	142		$	151		(a)
	Net income from continuing operations attributable to noncontrolling interests	2				2				5			5
	Dividends paid to noncontrolling interest holders	(2		)		(2		)		(5		)	(5		)
	Noncontrolling interests, end of period	$	142			$	151		(a)	$	142		$	151		(a)
	(a)	Included the 20% EEI ownership interest not owned by Ameren prior to the divestiture of New AER to IPH. Prior to the divestiture of New AER, the assets and liabilities of EEI were consolidated in Ameren’s balance sheet at a 100% ownership level and were included in “Assets of discontinued operations” and “Liabilities of discontinued operations,” respectively. The divestiture of New AER, which included EEI, was completed in the fourth quarter of 2013. See Note 12 - Divestiture Transactions and Discontinued Operations for additional information.

ShortTerm_Debt_And_Liquidity_S

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Tables)	9 Months Ended
	Sep. 30, 2014
Schedule of Commercial paper [Abstract]	'
Schedule of Short-term Debt [Table Text Block]	'
	The following table presents commercial paper outstanding at Ameren (parent), Ameren Missouri and Ameren Illinois as of September 30, 2014, and December 31, 2013. Ameren Illinois established a commercial paper program in May 2014.
		30-Sep-14				31-Dec-13
	Ameren (parent)	$	499			$	368
	Ameren Missouri	65				—
	Ameren Illinois	189				—
	Ameren Consolidated	$	753			$	368
	The following table summarizes the commercial paper activity and relevant interest rates under Ameren’s (parent), Ameren Missouri’s and Ameren Illinois’ commercial paper programs for the nine months ended September 30, 2014, and 2013:
			Ameren (parent)				Ameren Missouri			Ameren Illinois			Ameren Consolidated
	2014
	Average daily commercial paper outstanding		$	386			$	141		$	157		$	609
	Weighted-average interest rate		0.36		%		0.38		%	0.31		%	0.35		%
	Peak commercial paper during period(a)		$	531			$	495		$	300		$	907
	Peak interest rate		0.75		%		0.7		%	0.34		%	0.75		%
	2013
	Average daily commercial paper outstanding		$	26			$	—		$	—		$	26
	Weighted-average interest rate		0.52		%		—		%	—		%	0.52		%
	Peak commercial paper during period(a)		$	92			$	—		$	—		$	92
	Peak interest rate		0.85		%		—		%	—		%	0.85		%
	(a)	The timing of peak commercial paper issuances varies by company, and therefore the peak amounts presented by company might not equal the Ameren Consolidated peak commercial paper issuances for the period.

LongTerm_Debt_And_Equity_Finan1

Long-Term Debt And Equity Financings (Tables)

9 Months Ended

Sep. 30, 2014

Long-Term Debt And Equity Financings [Line Items]

'

Schedule Of Coverage Ratios

'

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 and dividend coverage ratios and bonds and preferred stock issuable as of September 30, 2014, at an assumed annual 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

4.6

$

3,304

≥2.5

126.3

$

2,823

Ameren Illinois

≥2.0

6.7

3,636

(d)

≥1.5

2.4

203

(e)

(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 $833 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.

(e)

Preferred stock issuable is restricted by the amount of preferred stock that is currently authorized by Ameren Illinois’ articles of incorporation.

Ameren Illinois Company

'

Long-Term Debt And Equity Financings [Line Items]

'

Debt Instrument 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:

Environmental improvement and pollution control revenue bonds

Principal Amount

5.90% Series 1993 due 2023(a)

$

32

5.70% 1994A Series due 2024(a)

36

5.95% 1993 Series C-1 due 2026

35

5.70% 1993 Series C-2 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.

Other_Income_and_Expenses_Tabl

Other Income and Expenses (Tables)	9 Months Ended
	Sep. 30, 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 for the three and nine months ended September 30, 2014, and 2013:
		Three Months				Nine Months
		2014		2013		2014		2013
	Ameren:(a)
	Miscellaneous income:
	Allowance for equity funds used during construction	$	10	$	10	$	26	$	26
	Interest income on industrial development revenue bonds	6		7		20		21
	Interest income	3		2		8		3
	Other	2		1		6		1
	Total miscellaneous income	$	21	$	20	$	60	$	51
	Miscellaneous expense:
	Donations	$	3	$	2	$	9	$	7
	Other	4		3		11		11
	Total miscellaneous expense	$	7	$	5	$	20	$	18
	Ameren Missouri:
	Miscellaneous income:
	Allowance for equity funds used during construction	$	9	$	8	$	24	$	22
	Interest income on industrial development revenue bonds	6		7		20		21
	Interest income	—		1		1		1
	Total miscellaneous income	$	15	$	16	$	45	$	44
	Miscellaneous expense:
	Donations	$	2	$	—	$	5	$	3
	Other	2		2		5		7
	Total miscellaneous expense	$	4	$	2	$	10	$	10
	Ameren Illinois:
	Miscellaneous income:
	Allowance for equity funds used during construction	$	1	$	2	$	2	$	4
	Interest income	2		1		5		2
	Other	1		1		5		1
	Total miscellaneous income	$	4	$	4	$	12	$	7
	Miscellaneous expense:
	Donations	$	—	$	—	$	3	$	3
	Other	2		3		4		4
	Total miscellaneous expense	$	2	$	3	$	7	$	7
	(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Derivative_Financial_Instrumen2

Derivative Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2014
Derivative Instruments and Hedging Activities Disclosure [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 September 30, 2014, and December 31, 2013. As of September 30, 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)	52		(b)		52		66		(b)		66
	Natural gas (in mmbtu)	23		102		125		28		108		136
	Power (in megawatthours)	1		11		12		3		11		14
	Uranium (pounds in thousands)	557		(b)		557		796		(b)		796
	(a)	Fuel oils consist of ultra-low-sulfur diesel, on-highway 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 September 30, 2014, and December 31, 2013:
		Balance Sheet Location		Ameren  Missouri				Ameren  Illinois				Ameren
	2014
	Fuel oils	Other current assets		$	3			$	—			$	3
	Natural gas	Other current assets		—				1				1
		Other assets		—				1				1
	Power	Other current assets		10				—				10
		Other assets		1				—				1
		Total assets		$	14			$	2			$	16
	Fuel oils	Other current liabilities		$	5			$	—			$	5
		Other deferred credits and liabilities		1				—				1
	Natural gas	Other current liabilities		3				16				19
		Other deferred credits and liabilities		3				6				9
	Power	Other current liabilities		6				8				14
		Other deferred credits and liabilities		—				116				116
	Uranium	Other current liabilities		2				—				2
		Other deferred credits and liabilities		1				—				1
		Total liabilities		$	21			$	146			$	167
	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	Other current liabilities		5				27				32
		Other deferred credits and liabilities		6				19				25
	Power	Other current liabilities		4				9				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
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 as regulatory assets or regulatory liabilities as of September 30, 2014, and December 31, 2013:
		Ameren				Ameren				Ameren
		Missouri				Illinois
	2014
	Fuel oils derivative contracts(a)	$	(5	)		$	—			$	(5	)
	Natural gas derivative contracts(b)	(6		)		(20		)		(26		)
	Power derivative contracts(c)	5				(124		)		(119		)
	Uranium derivative contracts(d)	(3		)		—				(3		)
	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 oil derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s transportation costs for coal through December 2017. Current losses deferred as regulatory assets include $4 million and $4 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 $1 million and $1 million at Ameren and Ameren Illinois, respectively. Current losses deferred as regulatory assets include $19 million, $3 million, and $16 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 $10 million and $10 million at Ameren and Ameren Missouri. Current losses deferred as regulatory assets include $14 million, $6 million, and $8 million at Ameren, Ameren Missouri and Ameren Illinois, respectively.
	(d)	Represents net losses on 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.
Offsetting Derivative Assets and Liabilities	'
	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 September 30, 2014, and December 31, 2013:
							Gross Amounts Not Offset in the Balance Sheet
	Commodity Contracts Eligible to be Offset		Gross Amounts Recognized in the Balance Sheet				Derivative Instruments				Cash Collateral Received/Posted(a)			Net
														Amount
	2014
	Assets:
	Ameren Missouri		$	14			$	7			$	—		$	7
	Ameren Illinois		2				1				—			1
	Ameren		$	16			$	8			$	—		$	8
	Liabilities:
	Ameren Missouri		$	21			$	7			$	5		$	9
	Ameren Illinois		146				1				—			145
	Ameren		$	167			$	8			$	5		$	154
	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.
Derivative Instruments With Credit Risk-Related Contingent Features	'
	The additional collateral required is the net liability position allowed under the master trading and netting agreements, assuming (1) the credit risk-related contingent features underlying these agreements were triggered on September 30, 2014, or December 31, 2013, respectively, 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)
	2014
	Ameren Missouri	$	62			$	2			$	57
	Ameren Illinois	61				—				56
	Ameren	$	123			$	2			$	113
	2013
	Ameren Missouri	$	70			$	2			$	67
	Ameren Illinois	75				15				55
	Ameren	$	145			$	17			$	122
	(a)	Prior to consideration of master trading and netting agreements and including NPNS and accrual contract exposures.
	(b)	As collateral requirements with certain counterparties are based on master trading and netting agreements, the aggregate amount of additional collateral required to be posted is determined after consideration of the effects of such agreements.

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2014
Fair Value Disclosures [Abstract]	'
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis, Valuation Techniques	'
	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 September 30, 2014:
			Fair Value									Weighted Average
			Assets			Liabilities			Valuation Technique(s)	Unobservable Input	Range
	Level 3 Derivative asset and liability - commodity contracts(a):
	Ameren	Fuel oils	$	3		$	(3	)	Option model	Volatilities(%)(b)	27-Feb	14
									Discounted cash flow	Counterparty credit risk(%)(c)(d)	0.25 - 1	0.72
										Ameren Missouri credit risk(%)(c)(d)	0.43	(e)
		Natural gas	1			—			Discounted cash flow	Nodal basis($/mmbtu)(c)	(0.10) - 0	-0.1
										Counterparty credit risk(%)(c)(d)	0.30 - 2	0.62
										Ameren Illinois credit risk(%)(c)(d)	0.43	(e)
		Power(f)	10			(129		)	Discounted cash flow	Average forward peak and off-peak pricing - forwards/swaps($/MWh)(c)	29 - 59	35
										Estimated auction price for FTRs($/MW)(b)	(1,853) - 2,087	199
										Nodal basis($/MWh)(c)	(6) - 0	-3
										Counterparty credit risk(%)(c)(d)	0.4	(e)
										Ameren Missouri and Ameren Illinois credit risk(%)(c)(d)	0.43	(e)
									Fundamental energy production model	Estimated future gas prices($/mmbtu)(b)	5-Apr	5
										Escalation rate(%)(b)(g)	2	(e)
									Contract price allocation	Estimated renewable energy credit costs($/credit)(b)	7-May	6
		Uranium	—			(3		)	Discounted cash flow	Average forward uranium pricing($/pound)(b)	35 - 41	36
	Ameren Missouri	Fuel oils	$	3		$	(3	)	Option model	Volatilities(%)(b)	27-Feb	14
									Discounted cash flow	Counterparty credit risk(%)(c)(d)	0.25 - 1	0.72
										Ameren Missouri credit risk(%)(c)(d)	0.43	(e)
		Power(f)	10			(5		)	Discounted cash flow	Average forward peak and off-peak pricing - forwards/swaps($/MWh)(c)	30 - 59	48
										Estimated auction price for FTRs($/MW)(b)	(1,853) - 2,087	199
										Counterparty credit risk(%)(c)(d)	0.4	(e)
										Ameren Missouri credit risk(%)(c)(d)	0.43	(e)
		Uranium	—			(3		)	Discounted cash flow	Average forward uranium pricing($/pound)(b)	35 - 41	36
	Ameren Illinois	Natural gas	$	1		$	—		Discounted cash flow	Nodal basis($/mmbtu)(c)	(0.10) - 0	-0.1
										Counterparty credit risk(%)(c)(d)	0.30 - 2	0.62
										Ameren Illinois credit risk(%)(c)(d)	0.43	(e)
		Power(f)	—			(124		)	Discounted cash flow	Average forward peak and off-peak pricing - forwards/swaps($/MWh)(b)	29 - 42	33
										Nodal basis($/MWh)(b)	(6) - 0	-3
										Ameren Illinois credit risk(%)(c)(d)	0.43	(e)
									Fundamental energy production model	Estimated future gas prices($/mmbtu)(b)	5-Apr	5
										Escalation rate(%)(b)(g)	2	(e)
									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)	Not applicable.
	(f)	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.
	(g)	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 Average
			Assets			Liabilities			Valuation Technique(s)	Unobservable Input	Range
	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 September 30, 2014:
				Quoted Prices in				Significant Other				Significant Other				Total
				Active Markets for				Observable Inputs				Unobservable
				Identical Assets				(Level 2)				Inputs
				or Liabilities								(Level 3)
				(Level 1)
	Assets:
	Ameren	Derivative assets - commodity contracts(a):
		Fuel oils		$	—			$	—			$	3			$	3
		Natural gas		—				1				1				2
		Power		—				1				10				11
		Total derivative assets - commodity contracts		$	—			$	2			$	14			$	16
		Nuclear decommissioning trust fund:
		Cash and cash equivalents		$	1			$	—			$	—			$	1
		Equity securities:
		U.S. large capitalization		348				—				—				348
		Debt securities:
		Corporate bonds		—				60				—				60
		Municipal bonds		—				2				—				2
		U.S. treasury and agency securities		—				100				—				100
		Asset-backed securities		—				11				—				11
		Other		—				5				—				5
		Total nuclear decommissioning trust fund		$	349			$	178			$	—			$	527	(b)
		Total Ameren		$	349			$	180			$	14			$	543
	Ameren	Derivative assets - commodity contracts(a):
	Missouri	Fuel oils		$	—			$	—			$	3			$	3
		Power		—				1				10				11
		Total derivative assets - commodity contracts		$	—			$	1			$	13			$	14
		Nuclear decommissioning trust fund:
		Cash and cash equivalents		$	1			$	—			$	—			$	1
		Equity securities:
		U.S. large capitalization		348				—				—				348
		Debt securities:
		Corporate bonds		—				60				—				60
		Municipal bonds		—				2				—				2
		U.S. treasury and agency securities		—				100				—				100
		Asset-backed securities		—				11				—				11
		Other		—				5				—				5
		Total nuclear decommissioning trust fund		$	349			$	178			$	—			$	527	(b)
		Total Ameren Missouri		$	349			$	179			$	13			$	541
	Ameren	Derivative assets - commodity contracts(a):
	Illinois	Natural gas		$	—			$	1			$	1			$	2
	Liabilities:
	Ameren	Derivative liabilities - commodity contracts(a):
		Fuel oils		$	3			$	—			$	3			$	6
		Natural gas		2				26				—				28
		Power		—				1				129				130
		Uranium		—				—				3				3
		Total Ameren		$	5			$	27			$	135			$	167
	Ameren	Derivative liabilities - commodity contracts(a):
	Missouri	Fuel oils		$	3			$	—			$	3			$	6
		Natural gas		2				4				—				6
		Power		—				1				5				6
		Uranium		—				—				3				3
		Total Ameren Missouri		$	5			$	5			$	11			$	21
	Ameren	Derivative liabilities - commodity contracts(a):
	Illinois	Natural gas		$	—			$	22			$	—			$	22
		Power		—				—				124				124
		Total Ameren Illinois		$	—			$	22			$	124			$	146
	(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 Other				Total
Active Markets for				Observable Inputs				Unobservable
Identical Assets				(Level 2)				Inputs
or Liabilities								(Level 3)
(Level 1)
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
	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	Derivative assets - commodity contracts(a):
Missouri	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	Derivative assets - commodity contracts(a):
Illinois	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	Derivative liabilities - commodity contracts(a):
Missouri	Fuel oils		$	—			$	—			$	3			$	3
	Natural gas		3				8				—				11
	Power		—				2				2				4
	Uranium		—				—				6				6
	Total Ameren Missouri		$	3			$	10			$	11			$	24
Ameren	Derivative liabilities - commodity contracts(a):
Illinois	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 Three 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 three months ended September 30, 2014:
			Net derivative commodity contracts
	Three Months		Ameren			Ameren			Ameren
			Missouri			Illinois
	Fuel oils:
	Beginning balance at July 1, 2014	$	2		$	(a)		$	2
	Realized and unrealized gains (losses) included in regulatory assets/liabilities		(2	)		(a)			(2	)
	Ending balance at September 30, 2014	$	—		$	(a)		$	—
	Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014	$	(2	)	$	(a)		$	(2	)
	Natural gas:
	Beginning balance at July 1, 2014	$	—		$	—		$	—
	Realized and unrealized gains (losses) included in regulatory assets/liabilities		—			1			1
	Ending balance at September 30, 2014	$	—		$	1		$	1
	Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014	$	—		$	—		$	—
	Power:
	Beginning balance at July 1, 2014	$	15		$	(103	)	$	(88	)
	Realized and unrealized gains (losses) included in regulatory assets/liabilities		(5	)		(23	)		(28	)
	Settlements		(5	)		2			(3	)
	Ending balance at September 30, 2014	$	5		$	(124	)	$	(119	)
	Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014	$	(6	)	$	(22	)	$	(28	)
	Uranium:
	Beginning balance at July 1, 2014	$	(7	)	$	(a)		$	(7	)
	Realized and unrealized gains (losses) included in regulatory assets/liabilities		3			(a)			3
	Settlements		1			(a)			1
	Ending balance at September 30, 2014	$	(3	)	$	(a)		$	(3	)
	Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014	$	3		$	(a)		$	3
	(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 three months ended September 30, 2013:
			Net derivative commodity contracts
	Three Months		Ameren			Ameren			Ameren
			Missouri			Illinois
	Fuel oils:
	Beginning balance at July 1, 2013	$	3		$	(a)		$	3
	Realized and unrealized gains (losses) included in regulatory assets/liabilities		1			(a)			1
	Purchases		1			(a)			1
	Sales		(1	)		(a)			(1	)
	Settlements		(1	)		(a)			(1	)
	Ending balance at September 30, 2013	$	3		$	(a)		$	3
	Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2013	$	1		$	(a)		$	1
	Natural gas:
	Beginning balance at July 1, 2013	$	(1	)	$	2		$	1
	Realized and unrealized gains (losses) included in regulatory assets/liabilities		—			(2	)		(2	)
	Purchases		1			—			1
	Ending balance at September 30, 2013	$	—		$	—		$	—
	Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2013	$	—		$	(1	)	$	(1	)
	Power:
	Beginning balance at July 1, 2013	$	37		$	(80	)	$	(43	)
	Realized and unrealized gains (losses) included in regulatory assets/liabilities		(3	)		(17	)		(20	)
	Sales		1			—			1
	Settlements		(6	)		3			(3	)
	Transfers into Level 3		(1	)		—			(1	)
	Ending balance at September 30, 2013	$	28		$	(94	)	$	(66	)
	Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2013	$	(2	)	$	(16	)	$	(18	)
	Uranium:
	Beginning balance at July 1, 2013	$	(3	)	$	(a)		$	(3	)
	Purchases		(2	)		(a)			(2	)
	Ending balance at September 30, 2013	$	(5	)	$	(a)		$	(5	)
	Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2013	$	(2	)	$	(a)		$	(2	)
	(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 nine months ended September 30, 2014:
			Net derivative commodity contracts
	Nine Months		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		(3	)		(a)			(3	)
Settlements		(2	)		(a)			(2	)
Ending balance at September 30, 2014	$	—		$	(a)		$	—
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014	$	(2	)	$	(a)		$	(2	)
Natural gas:
Beginning balance at January 1, 2014	$	—		$	—		$	—
Realized and unrealized gains (losses) included in regulatory assets/liabilities		—			1			1
Purchases		—			(1	)		(1	)
Settlements		—			1			1
Ending balance at September 30, 2014	$	—		$	1		$	1
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014	$	—		$	—		$	—
Power:
Beginning balance at January 1, 2014	$	19		$	(108	)	$	(89	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities		(23	)		(19	)		(42	)
Purchases		34			—			34
Settlements		(25	)		3			(22	)
Ending balance at September 30, 2014	$	5		$	(124	)	$	(119	)
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014	$	(3	)	$	(21	)	$	(24	)
Uranium:
Beginning balance at January 1, 2014	$	(6	)	$	(a)		$	(6	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities		(1	)		(a)			(1	)
Settlements		4			(a)			4
Ending balance at September 30, 2014	$	(3	)	$	(a)		$	(3	)
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014	$	—		$	(a)		$	—
(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 nine months ended September 30, 2013:
		Net derivative commodity contracts
Nine Months		Ameren			Ameren			Ameren
		Missouri			Illinois
Fuel oils:
Beginning balance at January 1, 2013	$	5		$	(a)		$	5
Realized and unrealized gains (losses) included in regulatory assets/liabilities		(1	)		(a)			(1	)
Purchases		2			(a)			2
Sales		(1	)		(a)			(1	)
Settlements		(2	)		(a)			(2	)
Ending balance at September 30, 2013	$	3		$	(a)		$	3
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 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 September 30, 2013	$	—		$	—		$	—
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2013	$	—		$	—		$	—
Power:
Beginning balance at January 1, 2013	$	11		$	(111	)	$	(100	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities		3			(2	)		1
Purchases		40			—			40
Sales		1			—			1
Settlements		(28	)		19			(9	)
Transfers into Level 3		(3	)		—			(3	)
Transfers out of Level 3		4			—			4
Ending balance at September 30, 2013	$	28		$	(94	)	$	(66	)
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2013	$	—		$	(7	)	$	(7	)
Uranium:
Beginning balance at January 1, 2013	$	(2	)	$	(a)		$	(2	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities		(2	)		(a)			(2	)
Purchases		(2	)		(a)			(2	)
Settlements		1			(a)			1
Ending balance at September 30, 2013	$	(5	)	$	(a)		$	(5	)
Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2013	$	(2	)	$	(a)		$	(2	)
(a)	Not applicable.
Schedule Of Carrying Amounts And Estimated Fair Values Of Long-Term Debt, Capital Lease Obligations And Preferred Stock	'
	The following table presents the carrying amounts and estimated fair values of our long-term debt, capital lease obligations and preferred stock at September 30, 2014, and December 31, 2013:
		30-Sep-14								31-Dec-13
		Carrying				Fair				Carrying				Fair
		Amount				Value				Amount				Value
	Ameren:(a)
	Long-term debt and capital lease obligations (including current portion)	$	5,944			$	6,647			$	6,038			$	6,584
	Preferred stock	142				122				142				118
	Ameren Missouri:
	Long-term debt and capital lease obligations (including current portion)	$	4,004			$	4,466			$	3,757			$	4,124
	Preferred stock	80				73				80				71
	Ameren Illinois:
	Long-term debt	$	1,940			$	2,181			$	1,856			$	2,028
	Preferred stock	62				49				62				47
	(a)	Preferred stock is recorded in “Noncontrolling Interests” on the consolidated balance sheet.

Related_Party_Transactions_Tab

Related Party Transactions (Tables)	9 Months Ended
	Sep. 30, 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 three and nine months ended September 30, 2014, and 2013.
					Three Months				Nine Months
	Agreement	Income Statement			Ameren		Ameren		Ameren		Ameren
		Line Item			Missouri		Illinois		Missouri		Illinois
	Ameren Missouri power supply	Operating Revenues	2014	$	2	$	(a)	$	5	$	(a)
	agreements with Ameren Illinois		2013		(b)		(a)		1		(a)
	Ameren Missouri and Ameren Illinois	Operating Revenues	2014		6		(b)		15		1
	rent and facility services		2013		4		(b)		16		1
	Ameren Missouri and Ameren Illinois	Operating Revenues	2014		(b)		(b)		1		(b)
	miscellaneous support services		2013		1		(b)		1		2
	Total Operating Revenues		2014	$	8	$	(b)	$	21	$	1
			2013		5		(b)		18		3
	Ameren Illinois power supply	Purchased Power	2014	$	(a)	$	2	$	(a)	$	5
	agreements with Ameren Missouri		2013		(a)		(b)		(a)		1
	Ameren Illinois transmission	Purchased Power	2014		(a)		1		(a)		2
	services with ATXI		2013		(a)		1		(a)		2
	Total Purchased Power		2014	$	(a)	$	3	$	(a)	$	7
			2013		(a)		1		(a)		3
					Three Months				Nine Months
	Agreement	Income Statement			Ameren		Ameren		Ameren		Ameren
		Line Item			Missouri		Illinois		Missouri		Illinois
	Ameren Services support services	Other Operations and Maintenance	2014	$	25	$	26	$	90	$	80
	agreement		2013		25		22		85		70
	Insurance premiums(c)	Other Operations and Maintenance	2014		(b)		(a)		(b)		(a)
			2013		(b)		(a)		(b)		(a)
	Total Other Operations and		2014	$	25	$	26	$	90	$	80
	Maintenance Expenses		2013		25		22		85		70
	Money pool borrowings (advances)	Interest Charges	2014	$	(b)	$	(b)	$	(b)	$	(b)
			2013		(b)		(b)		(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)	9 Months Ended
	Sep. 30, 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 September 30, 2014. The property coverage and the nuclear liability coverage must be renewed on April 1 and January 1, respectively, of each year. Both coverages were renewed in 2014.
	Type and Source of Coverage	Maximum  Coverages			Maximum Assessments
					for Single Incidents
	Public liability and nuclear worker liability:
	American Nuclear Insurers	$	375		$	—
	Pool participation	13,241		(a)	128		(b)
		$	13,616	(c)	$	128
	Property damage:
	NEIL	$	2,250	(d)	$	23	(e)
	European Mutual Association for Nuclear Insurance	500		(f)	—
		$	2,750		$	23
	Replacement power:
	NEIL	$	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 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 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 $327.6 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 and 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 and has reinsured this coverage with third-party insurance companies. See Note 8 - Related Party Transactions for more information on this affiliate transaction.
Long-term Purchase Commitment [Table Text Block]	'
	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 at September 30, 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 September 30, 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 $17 million during the remainder of 2014 and $64 million in 2015 for these customer energy efficiency programs.
		Coal			Natural			Nuclear		Purchased		Methane		Other		Total
					Gas(a)			Fuel		Power(b)		Gas
	Ameren:(c)
	2014	$	151		$	93		$	62	$	62	$	1	$	88	$	457
	2015	635			225			56		190		3		156		1,265
	2016	659			127			69		105		4		76		1,040
	2017	682			80			59		66		4		50		941
	2018	111			41			61		55		5		51		324
	Thereafter	114			101			179		645		91		350		1,480
	Total	$	2,352		$	667		$	486	$	1,123	$	108	$	771	$	5,507
	Ameren Missouri:
	2014	$	151		$	16		$	62	$	4	$	1	$	60	$	294
	2015	635			39			56		21		3		110		864
	2016	659			21			69		21		4		39		813
	2017	682			13			59		21		4		26		805
	2018	111			8			61		21		5		27		233
	Thereafter	114			29			179		120		91		183		716
	Total	$	2,352		$	126		$	486	$	208	$	108	$	445	$	3,725
	Ameren Illinois:
	2014	$	—		$	77		$	—	$	58	$	—	$	9	$	144
	2015	—			186			—		169		—		28		383
	2016	—			106			—		84		—		24		214
	2017	—			67			—		45		—		24		136
	2018	—			33			—		34		—		24		91
	Thereafter	—			72			—		525		—		167		764
	Total	$	—		$	541		$	—	$	915	$	—	$	276	$	1,732
	(a)	Includes amounts for generation and for distribution.
	(b)	The purchased power amounts for Ameren and Ameren Illinois include twenty-year agreements for renewable energy credits that were entered into in December 2010 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 Asbestors-Related Litigation Pending Lawsuits	'
	The following table presents the pending asbestos-related lawsuits filed against the Ameren Companies as of September 30, 2014:
	Ameren		Ameren	Ameren		Total(a)
			Missouri	Illinois
	1		48	62		75
	(a)	Total does not equal the sum of the subsidiary unit lawsuits because some of the lawsuits name multiple Ameren entities as defendants.

Retirement_Benefits_Tables

Retirement Benefits (Tables)	9 Months Ended
	Sep. 30, 2014
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]	'
Components Of Net Periodic Benefit Cost	'
	The following table presents the components of the net periodic benefit cost (benefit) for Ameren’s pension and postretirement benefit plans for the three and nine months ended September 30, 2014, and 2013:
		Pension Benefits												Postretirement Benefits
		Three Months						Nine Months						Three Months						Nine Months
		2014			2013			2014			2013			2014			2013			2014			2013
	Service cost	$	20		$	23		$	60		$	69		$	5		$	6		$	14		$	17
	Interest cost	46			40			137			121			12			11			37			34
	Expected return on plan assets	(58		)	(54		)	(172		)	(162		)	(16		)	(16		)	(48		)	(47		)
	Amortization of:
	Prior service cost (benefit)	(1		)	(1		)	(1		)	(3		)	(2		)	(1		)	(4		)	(3		)
	Actuarial loss (gain)	13			23			37			69			(2		)	2			(5		)	6
	Net periodic benefit cost (benefit)(a)	$	20		$	31		$	61		$	94		$	(3	)	$	2		$	(6	)	$	7
	(a)	Includes $2 million and $8 million in total net costs for pension benefits for the three and nine months ended September 30, 2013, respectively, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income. Includes less than $1 million in total net costs for postretirement benefits for both the three and nine months ended September 30, 2013, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income.
Summary Of Benefit Plan Costs Incurred	'
	Ameren Missouri and Ameren Illinois are responsible for their respective shares of Ameren’s pension and postretirement costs. The following table presents the pension costs and the postretirement benefit costs incurred for the three and nine months ended September 30, 2014, and 2013:
		Pension Benefits												Postretirement Benefits
		Three Months						Nine Months						Three Months						Nine Months
		2014			2013			2014			2013			2014			2013			2014			2013
	Ameren Missouri	$	13		$	18		$	38		$	54		$ (a)			$	2		$	2		$	7
	Ameren Illinois	7			10			22			31			(3		)	(a)			(7		)	(a)
	Other(b)	(a)			3			1			9			(a)			(a)			(1		)	(a)
	Ameren(c)	$	20		$	31		$	61		$	94		$	(3	)	$	2		$	(6	)	$	7
	(a)	Less than $1 million.
	(b)	Includes $2 million and $8 million in total net costs for pension benefits for the three and nine months ended September 30, 2013, respectively, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income. Includes less than $1 million in total net costs for postretirement benefits for both the three and nine months ended September 30, 2013, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income.
	(c)	Includes amounts for Ameren registrants and nonregistrant subsidiaries.

Divestiture_Transactions_and_D1

Divestiture Transactions and Discontinued Operations (Tables)	9 Months Ended
	Sep. 30, 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 carrying amounts of the components of assets and liabilities segregated on Ameren's consolidated balance sheets as discontinued operations at September 30, 2014, and December 31, 2013:
		30-Sep-14			31-Dec-13
	Assets of discontinued operations
	Cash and cash equivalents	$	—		$	—
	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)	Includes 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 September 30, 2014, and December 31, 2013, respectively.
	The following table presents the components of discontinued operations in Ameren's consolidated statement of income for the three and nine months ended September 30, 2014, and 2013:
		Three Months						Nine Months
		2014			2013			2014			2013
	Operating revenues	$	—		$	311		$	1		$	878
	Operating expenses	(1		)	(309		)	(4		)	(1,034		)	(a)
	Operating income (loss)	(1		)	2			(3		)	(156		)
	Other income (loss)	—			—			—			(1		)
	Interest charges	—			(9		)	—			(31		)
	Loss before income taxes	(1		)	(7		)	(3		)	(188		)
	Income tax (expense) benefit	—			4			—			(24		)
	Loss from discontinued operations, net of taxes	$	(1	)	$	(3	)	$	(3	)	$	(212	)
	(a)	Included a noncash pretax asset impairment charge of $175 million for the nine months ended September 30, 2013, to reduce the carrying value of the New AER disposal group to its estimated fair value less cost to sell.

Segment_Information_Tables

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2014
Segment Reporting [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 from continuing operations for the three and nine months ended September 30, 2014, and 2013, and total assets of continuing operations as of September 30, 2014, and December 31, 2013.
	Three Months	Ameren		Ameren		Other			Intersegment			Ameren
		Missouri		Illinois					Eliminations
	2014
	External revenues	$	1,089	$	572	$	9		$	—		$	1,670
	Intersegment revenues	8		—		2			(10		)	—
	Net income (loss) attributable to Ameren Corporation from continuing operations	222		75		(3		)	—			294
	2013
	External revenues	$	1,088	$	547	$	3		$	—		$	1,638
	Intersegment revenues	5		—		1			(6		)	—
	Net income (loss) attributable to Ameren Corporation from continuing operations	238		77		(10		)	—			305
	Nine Months
	2014
	External revenues	$	2,793	$	1,864	$	26		$	—		$	4,683
	Intersegment revenues	21		1		3			(25		)	—
	Net income (loss) attributable to Ameren Corporation from continuing operations	395		156		(10		)	—			541
	2013
	External revenues	$	2,760	$	1,744	$	12		$	—		$	4,516
	Intersegment revenues	18		3		2			(23		)	—
	Net income (loss) attributable to Ameren Corporation from continuing operations	362		139		(37		)	—			464
	As of September 30, 2014:
	Total assets	$	13,179	$	7,983	$	810		$	(111	)	$	21,861	(a)
	As of December 31, 2013:
	Total assets	$	12,904	$	7,454	$	752		$	(233	)	$	20,877	(a)
	(a)    Excludes total assets from discontinued operations of $15 million and $165 million as of September 30, 2014, and December 31, 2013, respectively.

Summary_Of_Significant_Account3

Summary Of Significant Accounting Policies (Narrative) (Detail) (USD $)

3 Months Ended

9 Months Ended

9 Months Ended

1 Months Ended

9 Months Ended

In Millions, except Per Share data, unless otherwise specified

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Dec. 31, 2013

Sep. 30, 2014

Dec. 31, 2013

Dec. 31, 2013

Apr. 30, 2014

Sep. 30, 2014

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Union Electric Company

Union Electric Company

Electric Energy, Inc

Performance Shares

Performance Shares

Power

Power

Natural Gas

Natural Gas

Union Electric Company

Ameren Illinois Company

Union Electric Company

Ameren Illinois Company

customer

customer

customer

customer

Basis Of Presentation And Significant Accounting Policies [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount

0

0

0

0

'

'

'

'

'

'

'

'

'

'

Asset Retirement Obligation, Liabilities Incurred

'

'

$2

'

'

$2

'

'

'

'

'

'

'

'

Number of customers

'

'

'

'

'

'

'

'

'

'

1,200,000

1,200,000

127,000

807,000

Maximum shares available for grants

8

'

8

'

'

'

'

'

'

'

'

'

'

'

Fair value of each share unit, per share

'

'

'

'

'

'

'

'

$50.34

[1]

$38.90

[2]

'

'

'

'

Closing common share price

'

'

'

'

$36.16

'

'

'

'

'

'

'

'

'

Performance period

'

'

'

'

'

'

'

'

'

'3 years

'

'

'

'

Risk free interest rate period

'

'

'

'

'

'

'

'

'

'3 years

'

'

'

'

Three-year risk-free rate

'

'

'

'

'

'

'

'

'

0.78%

'

'

'

'

Volatility rate, minimum

'

'

'

'

'

'

'

'

12.00%

12.00%

'

'

'

'

Volatility rate, maximum

'

'

'

'

'

'

'

'

18.00%

18.00%

'

'

'

'

Book value of renewable energy credits

$20

'

$20

'

$22

$20

$22

'

'

'

'

'

'

'

Percentage of EEI not owned by Ameren

'

'

'

'

'

'

'

20.00%

'

'

'

'

'

'

[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.

Summary_Of_Significant_Account4

Summary Of Significant Accounting Policies Schedule of Earnings Per Share Basic And Diluted (Details) (USD $)	3 Months Ended		9 Months Ended
In Millions, except Per Share data, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Schedule of Earnings Per Share Basic And Diluted [Abstract]	'	'	'	'
Continuing Operations	$294	$305	$541	$464
Discontinued Operations	-1	-3	-3	-212
Net Income (Loss) Attributable to Parent	$293	$302	$538	$252
Average Common Shares Outstanding - Basic	242.6	242.6	242.6	242.6
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	1.7	2.5	1.7	1.8
Average Common Shares Outstanding - Diluted	244.3	245.1	244.3	244.4
Continuing Operations	$1.21	$1.26	$2.23	$1.92
Discontinued Operations	'	($0.01)	($0.01)	($0.88)
Earnings Per Share, Basic	$1.21	$1.25	$2.22	$1.04
Continuing Operations	$1.20	$1.25	$2.21	$1.91
Discontinued Operations	'	($0.01)	($0.01)	($0.88)
Earnings Per Share, Diluted	$1.20	$1.24	$2.20	$1.03

Summary_Of_Significant_Account5

Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Summary of Nonvested Shares Related To Long-Term Incentive Plan) (Details) (Performance Shares, USD $)	1 Months Ended		9 Months Ended
	Apr. 30, 2014		Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'		'
Three-year risk-free rate	'		0.78%
Volatility rate, minimum	12.00%		12.00%
Volatility rate, maximum	18.00%		18.00%
Share Units	'		'
Share Units, Nonvested as of January 1, 2014	'		1,218,544
Share Units, Granted	38,559	[1]	685,026	[2]
Share Units, Forfeitures	'		-65,847
Share Units, Vested	'		-123,295	[3]
Share Units, Nonvested as of March 31, 2014	'		1,752,987
Weighted-average Fair Value per Unit at Grant Date	'		'
Weighted-average Fair Value per Unit, Nonvested as of January 1, 2014	'		$33.23
Weighted-average Fair Value per Unit, Grants	$50.34	[1]	$38.90	[2]
Weighted-average Fair Value per Unit, Forfeitures	'		$33.82
Weighted-average Fair Value per Unit, Vested	'		$38.64	[3]
Weighted-average Fair Value per Unit, Nonvested as of March 31, 2014	'		$35.42
Performance period	'		'3 years
Minimum	'		'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'		'
Three-year risk-free rate	0.76%		'
Maximum	'		'
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]	'		'
Three-year risk-free rate	0.79%		'
[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]	Share units vested due to the 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.

Summary_Of_Significant_Account6

Summary Of Significant Accounting Policies (Schedule Of Amortization Expense Based On Usage Of Renewable Energy Credits And Emission Allowances) (Detail) (REC and Emission Allowances, USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Finite-Lived Intangible Assets [Line Items]	'	'	'	'
Amortization of intangible assets	$2	$2	$14	$9
Union Electric Company	'	'	'	'
Finite-Lived Intangible Assets [Line Items]	'	'	'	'
Amortization of intangible assets	1	'	7	1	[1]
Ameren Illinois Company	'	'	'	'
Finite-Lived Intangible Assets [Line Items]	'	'	'	'
Amortization of intangible assets	$1	$2	$7	$9
[1]	Less than $1 million.

Summary_Of_Significant_Account7

Summary Of Significant Accounting Policies (Schedule Of Excise Taxes) (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Accounting Policies [Line Items]	'	'	'	'
Excise tax expense	$56	$59	$166	$163
Union Electric Company	'	'	'	'
Accounting Policies [Line Items]	'	'	'	'
Excise tax expense	47	49	120	120
Ameren Illinois Company	'	'	'	'
Accounting Policies [Line Items]	'	'	'	'
Excise tax expense	$9	$10	$46	$43

Summary_Of_Significant_Account8

Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Schedule of Unrecognized Tax Benefits (Detriments)) (Details) (USD $)

Sep. 30, 2014

Dec. 31, 2013

Sep. 30, 2014

Dec. 31, 2013

Sep. 30, 2014

Dec. 31, 2013

Oct. 31, 2014

Oct. 31, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

In Millions, unless otherwise specified

Union Electric Company

Union Electric Company

Ameren Illinois Company

Ameren Illinois Company

Subsequent Event [Member]

Subsequent Event [Member]

Tax Year 2013 [Member]

Tax Year 2013 [Member]

Tax Year 2013 [Member]

2011 and 2012 Tax Years [Member]

New Ameren Energy Resources Company, LLC

Union Electric Company

Union Electric Company

Ameren Illinois Company

Tax Year 2013 [Member]

Schedule of Unrecognized Tax Benefits (Detriments) [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

Unrecognized tax benefit recorded in Accumulated deferred income taxes, net

$89

$84

$13

$15

$2

'

'

'

'

'

'

'

'

Unrecognized tax benefits (detriments)

97

90

35

31

1

-1

'

'

'

'

'

'

'

Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities

'

'

'

'

'

'

16

9

'

'

'

'

'

Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit

$18

'

'

'

'

'

'

'

$73

$17

$1

$6

$55

Summary_Of_Significant_Account9

Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Schedule of Unrecognized Tax Benefits (Detriments) That Would Impact the Effective Tax Rate) (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule of Unrecognized Tax Benefits (Detriments) That Would Impact Effective Tax Rate [Line Items]	'	'
Unrecognized tax benefits (detriments) that would impact effective tax rate	$55	$54
Union Electric Company	'	'
Schedule of Unrecognized Tax Benefits (Detriments) That Would Impact Effective Tax Rate [Line Items]	'	'
Unrecognized tax benefits (detriments) that would impact effective tax rate	3	3
Ameren Illinois Company	'	'
Schedule of Unrecognized Tax Benefits (Detriments) That Would Impact Effective Tax Rate [Line Items]	'	'
Unrecognized tax benefits (detriments) that would impact effective tax rate	'	'

Recovered_Sheet1

Summary Of Significant Accounting Policies (Equity Changes Attributable To Noncontrolling Interest) (Detail) (USD $)	3 Months Ended				9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013		Dec. 31, 2013
									Electric Energy, Inc
Stockholders' Equity Attributable to Noncontrolling Interest [Roll Forward]	'		'		'		'		'
Noncontrolling interest, beginning of period	$142	[1]	$151	[1]	$142		$151	[1]	'
Net income from continuing operations attributable to noncontrolling interests	2		2		5		5		'
Dividends paid to noncontrolling interest holders	-2		-2		-5		-5		'
Noncontrolling interest, end of period	$142	[1]	$151	[1]	$142	[1]	$151	[1]	'
Percentage of EEI not owned by Ameren	'		'		'		'		20.00%
[1]	Included the 20% EEI ownership interest not owned by Ameren prior to the divestiture of New AER to IPH. Prior to the divestiture of New AER, the assets and liabilities of EEI were consolidated in Ameren’s balance sheet at a 100% ownership level and were included in “Assets of discontinued operations” and “Liabilities of discontinued operations,” respectively. The divestiture of New AER, which included EEI, was completed in the fourth quarter of 2013. See Note 12 - Divestiture Transactions and Discontinued Operations for additional information.

Rate_And_Regulatory_Matters_Na

Rate And Regulatory Matters (Narrative-Missouri) (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014	Dec. 31, 2013	Jul. 31, 2014	Sep. 30, 2014	Sep. 30, 2014	Feb. 28, 2014	Feb. 13, 2014
			Union Electric Company	Union Electric Company	Ameren Illinois Company	Ameren Illinois Company	Electric Distribution [Member]	Electric Distribution [Member]	Electric Distribution [Member]	Customer [Domain]	Customer [Domain]
							Union Electric Company	Union Electric Company	Union Electric Company	Electric Distribution [Member]	Electric Distribution [Member]
							Pending Rate Case [Member]	Pending Rate Case [Member]	Accounting Authority Order Request [Member]	Union Electric Company	Union Electric Company
Rate And Regulatory Matters [Line Items]	'	'	'	'	'	'	'	'	'	'	'
Public Utilities, Requested Rate Increase (Decrease), Amount	'	'	'	'	'	'	$264,000,000	'	'	'	'
Plant Additions to Rate Base	'	'	'	'	'	'	1,400,000,000	'	'	'	'
Requested Rate Increase Related to Net Energy Costs	'	'	'	'	'	'	127,000,000	'	'	'	'
Sharing Level For Fac	'	'	'	'	'	'	95.00%	'	'	'	'
Public Utilities, Requested Return on Equity, Percentage	'	'	'	'	'	'	10.40%	'	'	'	'
Public Utilities, Requested Equity Capital Structure, Percentage	'	'	'	'	'	'	51.60%	'	'	'	'
Rate Base	'	'	'	'	'	'	7,300,000,000	'	'	'	'
Number of Months to complete MoPSC Electric Service Proceeding	'	'	'	'	'	'	'11 months	'	'	'	'
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration	'	'	'	'	'	'	'	'15 years	'	'	'
Regulatory assets	$1,259,000,000	$1,240,000,000	$539,000,000	$534,000,000	$712,000,000	$701,000,000	'	'	$36,000,000	'	'
Number of Customers Filed Complaint Case	'	'	'	'	'	'	'	'	'	'	37
Contested return on equity	'	'	'	'	'	'	'	'	'	9.80%	'

Rate_And_Regulatory_Matters_Na1

Rate And Regulatory Matters (Narrative-Illinois) (Detail) (USD $)

Sep. 30, 2014

Dec. 31, 2013

Sep. 30, 2014

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Ameren Illinois Company

Ameren Illinois Company

Ameren Illinois Company

Ameren Illinois Company

Ieima [Member]

Ieima [Member]

Ieima [Member]

Icc Staff Recommendation [Member]

Other Intervenors [Member]

Administrative Law Judge [Member]

Minimum

Maximum

Pending Ferc Case [Member]

Pending Ferc Case [Member]

Pending Ferc Case [Member]

Gas Distribution [Member]

Gas Distribution [Member]

Ameren Illinois Company

Ameren Illinois Company

Ameren Illinois Company

Ieima [Member]

Ieima [Member]

Ieima [Member]

Ameren Illinois Company

Ameren Illinois Company

Midwest Independent Transmission System Operator, Inc [Member]

Midwest Independent Transmission System Operator, Inc [Member]

Minimum

Final Rate Order [Member]

Rate order appeal [Member]

Electric Distribution [Member]

Electric Distribution [Member]

Electric Distribution [Member]

Ameren Illinois Company

Ameren Illinois Company

Ameren Illinois Company

Pending Ferc Case [Member]

Pending Ferc Case [Member]

Ameren Illinois Company

Midwest Independent Transmission System Operator, Inc [Member]

IEMA Revenue Requirement Reconciliation [Member]

2013 IEMA Revenue Requirement Reconciliation [Member]

2012 IEMA Revenue Requirement Reconciliation [Member]

Electric Distribution [Member]

Electric Distribution [Member]

Electric Distribution [Member]

IEMA Revenue Requirement Reconciliation [Member]

IEMA Revenue Requirement Reconciliation [Member]

IEMA Revenue Requirement Reconciliation [Member]

Rate And Regulatory Matters [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Regulatory assets

$1,259,000,000

$1,240,000,000

$712,000,000

$701,000,000

'

'

$76,000,000

$64,000,000

'

'

'

'

'

'

'

'

'

Current regulatory liabilities

132,000,000

216,000,000

121,000,000

159,000,000

'

'

'

'

13,000,000

'

'

'

'

'

'

'

'

Public Utilities, Requested Rate Increase (Decrease), Amount

'

'

'

'

'

'

205,000,000

'

'

205,000,000

7,000,000

204,000,000

'

'

'

'

'

Authorized Increase in Revenue from Utility Service

'

'

'

'

32,000,000

'

'

'

'

'

'

'

'

'

'

'

'

Public Utilities, Approved Return on Equity, Percentage

'

'

'

'

9.10%

10.40%

'

'

'

'

'

'

'

'

12.38%

'

10.57%

Percent Of Capital Structure Composed Of Equity

'

'

'

'

51.70%

'

'

'

'

'

'

'

'

'

'

'

'

Rate Base

'

'

'

'

1,100,000,000

'

'

'

'

'

'

'

'

'

'

'

'

Loss Contingency, Estimate of Possible Loss

'

'

'

'

'

'

'

'

'

'

'

'

$4,000,000

$19,000,000

$14,000,000

$11,000,000

'

Rate_And_Regulatory_Matters_Na2

Rate And Regulatory Matters (Narrative-Federal) (Detail) (USD $)

Sep. 30, 2014

Dec. 31, 2013

Sep. 30, 2014

Dec. 31, 2013

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Sep. 30, 2014

Dec. 31, 2013

Sep. 30, 2014

Dec. 31, 2013

Dec. 31, 2012

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2014

Nov. 30, 2014

In Millions, unless otherwise specified

Ameren Illinois Company

Ameren Illinois Company

Ameren Illinois Company

Ameren Illinois Company

Union Electric Company

Union Electric Company

Union Electric Company

Union Electric Company

Union Electric Company

Wholesale Distribution Rate Case [Member]

Wholesale Distribution Rate Case [Member]

Wholesale Distribution Rate Case [Member]

Wholesale Distribution Rate Case [Member]

Entergy Refund [Member]

Entergy Refund [Member]

Midwest Independent Transmission System Operator, Inc [Member]

Midwest Independent Transmission System Operator, Inc [Member]

Midwest Independent Transmission System Operator, Inc [Member]

New England Transmission Owners [Member]

New England Transmission Owners [Member]

New England Transmission Owners [Member]

Estimated Annual Reduction to Revenues From MISO ROE [Member]

Estimated Annual Reduction to Revenues From MISO ROE [Member]

Subsequent Event [Member]

Pending Ferc Case [Member]

Pending Ferc Case [Member]

New Nuclear Energy Center COL [Member]

New Nuclear Energy Center COL [Member]

New Nuclear Energy Center COL [Member]

Ameren Illinois Company

Ameren Illinois Company

Union Electric Company

Union Electric Company

Pending Ferc Case [Member]

Pending Ferc Case [Member]

Pending Ferc Case [Member]

Pending Ferc Case [Member]

Pending Ferc Case [Member]

Pending Ferc Case [Member]

Midwest Independent Transmission System Operator, Inc [Member]

Midwest Independent Transmission System Operator, Inc [Member]

Minimum

Maximum

Minimum

Maximum

Minimum

Ameren Illinois Company

Minimum

Maximum

Pending Ferc Case [Member]

Pending Ferc Case [Member]

Ameren Illinois Company

Rate And Regulatory Matters [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Current regulatory liabilities

$132

$216

$121

$159

'

'

$11

$57

'

'

'

$24

$13

$24

$13

'

'

'

'

'

'

'

'

'

'

'

Loss Contingency, Estimate of Possible Loss

'

'

'

'

4

19

'

'

'

'

'

'

'

'

'

'

25

14

'

11

'

'

'

16

12

'

Customer Requested Rate on Equity

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

9.15%

'

'

'

'

'

'

'

'

Public Utilities, Approved Return on Equity, Percentage

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

12.38%

10.57%

'

11.14%

10.57%

11.74%

'

'

'

Incentive adder to FERC allowed base return on common equity

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

50

Proceeds from Legal Settlements

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

31

'

'

'

'

'

'

'

'

'

'

Investments in Power and Distribution Projects

'

'

'

'

'

'

'

'

$69

$80

$120

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

ShortTerm_Debt_And_Liquidity_N

Short-Term Debt And Liquidity (Narrative) (Detail) (USD $)	3 Months Ended		9 Months Ended
In Billions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Utilities [Member]	'	'	'	'
Line of Credit Facility [Line Items]	'	'	'	'
Short Term Debt, Weighted Average Interest Rate During Period	0.10%	0.05%	0.23%	0.08%
Credit Agreements 2012 [Member]	'	'	'	'
Line of Credit Facility [Line Items]	'	'	'	'
Line of credit facility, maximum borrowing capacity	1.3	'	1.3	'
Actual debt-to-capital ratio	0.49	'	0.49	'
Minimum ratio of consolidated funds from operations plus interest expense to consolidated interest expense as of balance sheet date	'	'	'2.0 to 1.0	'
Current ratio of consolidated funds from operations plus interest expense to consolidated interest expense as of balance sheet date	'	'	'6.1 to 1.0	'
Credit Agreements 2012 [Member] | Maximum	'	'	'	'
Line of Credit Facility [Line Items]	'	'	'	'
Actual debt-to-capital ratio	0.65	'	0.65	'
Missouri Credit Agreement 2012 [Member] | Union Electric Company	'	'	'	'
Line of Credit Facility [Line Items]	'	'	'	'
Actual debt-to-capital ratio	0.48	'	0.48	'
Illinois Credit Agreement 2012 [Member] | Ameren Illinois Company	'	'	'	'
Line of Credit Facility [Line Items]	'	'	'	'
Actual debt-to-capital ratio	0.46	'	0.46	'

ShortTerm_Debt_And_Liquidity_S1

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper outstanding) (Details) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Line of Credit Facility [Line Items]	'	'
Commercial paper outstanding	$753	$368
Ameren (parent)	'	'
Line of Credit Facility [Line Items]	'	'
Commercial paper outstanding	499	368
Union Electric Company	'	'
Line of Credit Facility [Line Items]	'	'
Commercial paper outstanding	65	'
Ameren Illinois Company	'	'
Line of Credit Facility [Line Items]	'	'
Commercial paper outstanding	$189	'

ShortTerm_Debt_And_Liquidity_S2

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper) (Details) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014		Sep. 30, 2013
Line of Credit Facility [Line Items]	'		'
Average Daily Commercial Paper Borrowings Outstanding	$609		$26
Weighted Average Interest Rate	0.35%		0.52%
Peak Short Term Borrowings	907	[1]	92	[1]
Peak Short Term Borrowings Interest Rate	0.75%		0.85%
Ameren (parent)	'		'
Line of Credit Facility [Line Items]	'		'
Average Daily Commercial Paper Borrowings Outstanding	386		26
Weighted Average Interest Rate	0.36%		0.52%
Peak Short Term Borrowings	531	[1]	92	[1]
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	141		'
Weighted Average Interest Rate	0.38%		'
Peak Short Term Borrowings	495	[1]	'	[1]
Peak Short Term Borrowings Interest Rate	0.70%		'
Ameren Illinois Company	'		'
Line of Credit Facility [Line Items]	'		'
Average Daily Commercial Paper Borrowings Outstanding	157		'
Weighted Average Interest Rate	0.31%		'
Peak Short Term Borrowings	$300	[1]	'	[1]
Peak Short Term Borrowings Interest Rate	0.34%		'
[1]	(a)The timing of peak commercial paper issuances varies by company, and therefore the peak amounts presented by company might not equal the Ameren Consolidated peak

LongTerm_Debt_And_Equity_Finan2

Long-Term Debt And Equity Financings (Narrative) (Detail) (USD $)

9 Months Ended

1 Months Ended

1 Months Ended

1 Months Ended

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2014

Sep. 30, 2013

Sep. 30, 2014

Sep. 30, 2014

31-May-14

15-May-14

Jan. 31, 2014

Apr. 30, 2014

31-May-14

15-May-14

Jun. 30, 2014

Union Electric Company

Union Electric Company

Ameren Missouri And Ameren Illinois [Member]

Ameren Illinois Company

Ameren Illinois Company

Ameren Illinois Company

Ameren Illinois Company

Senior Unsecured Notes8875 Due2014 [Member]

Senior Unsecured Notes8875 Due2014 [Member]

Environmental Improvement And Pollution Control Revenue Bonds [Member]

Secured Debt [Member]

Secured Debt [Member]

Secured Debt [Member]

Secured Debt [Member]

Federal Energy Regulatory Commission Restriction [Member]

Minimum

Parent Company [Member]

Parent Company [Member]

Ameren Illinois Company

Senior Secured Notes, 3.50%, Due 2024 [Member]

Senior Secured Notes550 Due2014 [Member]

Senior Secured Notes550 Due2014 [Member]

Senior Secured Notes, 4.30%, Due 2044 [Member] [Member]

Federal Energy Regulatory Commission Restriction [Member]

Union Electric Company

Union Electric Company

Union Electric Company

Ameren Illinois Company

Long-Term Debt And Equity Financings [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Repayments of Other Long-term Debt

$692,000,000

'

'

'

'

'

'

'

'

$425,000,000

'

'

'

'

'

'

Debt Instrument, Face Amount

'

'

'

'

'

1,000,000

'

'

'

'

'

'

350,000,000

'

'

250,000,000

Redemptions of Long-term Debt

'

'

104,000,000

'

'

163,000,000

'

'

'

'

'

163,000,000

'

104,000,000

'

'

Debt Instrument, Interest Rate, Stated Percentage

'

'

'

'

'

'

'

'

'

'

8.88%

'

3.50%

'

5.50%

4.30%

Assumed interest rate

'

'

'

'

5.00%

'

'

'

'

'

'

'

'

'

'

'

Dividend rate

'

'

'

'

6.00%

'

'

'

'

'

'

'

'

'

'

'

Common stock equity to total capitalization

'

'

'

'

'

'

'

54.00%

30.00%

'

'

'

'

'

'

'

Proceeds from Issuance of Secured Debt

'

'

'

'

'

'

'

'

'

'

'

'

$348,000,000

'

'

$246,000,000

LongTerm_Debt_And_Equity_Finan3

Long-Term Debt And Equity Financings Long-Term Debt and Equity Financings (Schedule of Debt Redemptions (Details) (Ameren Illinois Company, USD $)	9 Months Ended		1 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Jan. 31, 2014	Jan. 31, 2014		Jan. 31, 2014		Jan. 31, 2014	Jan. 31, 2014	Jan. 31, 2014	Jan. 31, 2014
			Environmental Improvement And Pollution Control Revenue Bonds [Member]	Environmental Improvement And Pollution Control Revenue Bonds [Member]		Environmental Improvement And Pollution Control Revenue Bonds [Member]		Environmental Improvement And Pollution Control Revenue Bonds [Member]	Environmental Improvement And Pollution Control Revenue Bonds [Member]	Environmental Improvement And Pollution Control Revenue Bonds [Member]	Environmental Improvement And Pollution Control Revenue Bonds [Member]
				Series1993590 Due2023 [Member]		Series1994A 570 Due 2024 [Member]		Series C11993595 Due2026 [Member]	Series C21993570 Due2026 [Member]	Series1998A 5 Point 40 Due 2028 [Member]	Series1998b540 Due2028 [Member]
Long-Term Debt And Equity Financings [Line Items]	'	'	'	'		'		'	'	'	'
Debt Instrument, Face Amount	$1,000,000	'	'	'		'		'	'	'	'
Redemptions of Long-term Debt	$163,000,000	'	$163,000,000	$32,000,000	[1]	$36,000,000	[1]	$35,000,000	$8,000,000	$19,000,000	$33,000,000
[1]	Less than $1 million principal amount of the bonds remain outstanding after redemption

LongTerm_Debt_And_Equity_Finan4

Long-Term Debt And Equity Financings (Schedule Of Covered Ratio) (Detail) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014
Union Electric Company	'
Long-Term Debt And Equity Financings [Line Items]	'
Bonds Issuable	3,304	[1]
Preferred Stock Issuable	2,823
Retired bond capacity	833
Union Electric Company | Actual Ratio [Member]	'
Long-Term Debt And Equity Financings [Line Items]	'
Restricted payment interest coverage ratio, Actual	4.6
Dividend Coverage Ratio	126.3
Ameren Illinois Company	'
Long-Term Debt And Equity Financings [Line Items]	'
Bonds Issuable	3,636	[1],[2]
Preferred Stock Issuable	203	[3]
Retired bond capacity	204
Ameren Illinois Company | Actual Ratio [Member]	'
Long-Term Debt And Equity Financings [Line Items]	'
Restricted payment interest coverage ratio, Actual	6.7
Dividend Coverage Ratio	2.4
Minimum | Union Electric Company | Minimum Required Ratio [Member]	'
Long-Term Debt And Equity Financings [Line Items]	'
Restricted payment interest coverage ratio, Actual	2	[4]
Dividend Coverage Ratio	2.5	[5]
Minimum | Ameren Illinois Company | Minimum Required Ratio [Member]	'
Long-Term Debt And Equity Financings [Line Items]	'
Restricted payment interest coverage ratio, Actual	2	[4]
Dividend Coverage Ratio	1.5	[5]
[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 $833 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]	Preferred stock issuable is restricted by the amount of preferred stock that is currently authorized by Ameren Illinois’ articles of incorporation.
[4]	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.
[5]	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_Deta

Other Income and Expenses (Detail) (USD $)	3 Months Ended				9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Other Nonoperating Income (Expense) [Line Items]	'		'		'		'
Allowance for equity funds used during construction	$10	[1]	$10	[1]	$26	[1]	$26	[1]
Interest income on industrial development revenue bonds	6	[1]	7	[1]	20	[1]	21	[1]
Interest and dividend income	3	[1]	2	[1]	8	[1]	3	[1]
Other	2	[1]	1	[1]	6	[1]	1	[1]
Total miscellaneous income	21	[1]	20	[1]	60	[1]	51	[1]
Donations	3	[1]	2	[1]	9	[1]	7	[1]
Other	4	[1]	3	[1]	11	[1]	11	[1]
Total miscellaneous expense	7	[1]	5	[1]	20	[1]	18	[1]
Union Electric Company	'		'		'		'
Other Nonoperating Income (Expense) [Line Items]	'		'		'		'
Allowance for equity funds used during construction	9		8		24		22
Interest income on industrial development revenue bonds	6		7		20		21
Interest and dividend income	'		1		1		1
Total miscellaneous income	15		16		45		44
Donations	2		'		5		3
Other	2		2		5		7
Total miscellaneous expense	4		2		10		10
Ameren Illinois Company	'		'		'		'
Other Nonoperating Income (Expense) [Line Items]	'		'		'		'
Allowance for equity funds used during construction	1		2		2		4
Interest and dividend income	2		1		5		2
Other	1		1		5		1
Total miscellaneous income	4		4		12		7
Donations	'		'		3		3
Other	2		3		4		4
Total miscellaneous expense	$2		$3		$7		$7
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Derivative_Financial_Instrumen3

Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail)	Sep. 30, 2014		Dec. 31, 2013
	gal		gal
Fuel Oils	'		'
Derivative [Line Items]	'		'
Quantity	52,000,000	[1]	66,000,000	[1]
Natural Gas	'		'
Derivative [Line Items]	'		'
Quantity	125,000,000		136,000,000
Power	'		'
Derivative [Line Items]	'		'
Quantity	12,000,000		14,000,000
Uranium	'		'
Derivative [Line Items]	'		'
Quantity	557,000		796,000
Union Electric Company | Fuel Oils	'		'
Derivative [Line Items]	'		'
Quantity	52,000,000	[1]	66,000,000	[1]
Union Electric Company | Natural Gas	'		'
Derivative [Line Items]	'		'
Quantity	23,000,000		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	557,000		796,000
Ameren Illinois Company | Natural Gas	'		'
Derivative [Line Items]	'		'
Quantity	102,000,000		108,000,000
Ameren Illinois Company | Power	'		'
Derivative [Line Items]	'		'
Quantity	11,000,000		11,000,000
[1]	Fuel oils consist of ultra-low-sulfur diesel, on-highway diesel, and crude oil.

Derivative_Financial_Instrumen4

Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) (Not Designated As Hedging Instrument [Member], USD $)	Sep. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Derivative [Line Items]	'	'
Derivative assets	$16	$34
Derivative liabilities	167	178
Fuel Oils | Other Current Assets [Member]	'	'
Derivative [Line Items]	'	'
Derivative assets	3	6
Fuel Oils | Other Assets [Member]	'	'
Derivative [Line Items]	'	'
Derivative assets	'	3
Fuel Oils | Other Current Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	5	2
Fuel Oils | Other Deferred Credits And Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	1	1
Natural Gas | Other Current Assets [Member]	'	'
Derivative [Line Items]	'	'
Derivative assets	1	2
Natural Gas | Other Assets [Member]	'	'
Derivative [Line Items]	'	'
Derivative assets	1	'
Natural Gas | Other Current Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	19	32
Natural Gas | Other Deferred Credits And Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	9	25
Power | Other Current Assets [Member]	'	'
Derivative [Line Items]	'	'
Derivative assets	10	23
Power | Other Assets [Member]	'	'
Derivative [Line Items]	'	'
Derivative assets	1	'
Power | Other Current Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	14	13
Power | Other Deferred Credits And Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	116	99
Uranium | Other Current Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	2	5
Uranium | Other Deferred Credits And Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	1	1
Union Electric Company	'	'
Derivative [Line Items]	'	'
Derivative assets	14	33
Derivative liabilities	21	24
Union Electric Company | Fuel Oils | Other Current Assets [Member]	'	'
Derivative [Line Items]	'	'
Derivative assets	3	6
Union Electric Company | Fuel Oils | Other Assets [Member]	'	'
Derivative [Line Items]	'	'
Derivative assets	'	3
Union Electric Company | Fuel Oils | Other Current Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	5	2
Union Electric Company | Fuel Oils | Other Deferred Credits And Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	1	1
Union Electric Company | Natural Gas | Other Current Assets [Member]	'	'
Derivative [Line Items]	'	'
Derivative assets	'	1
Union Electric Company | Natural Gas | Other Current Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	3	5
Union Electric Company | Natural Gas | Other Deferred Credits And Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	3	6
Union Electric Company | Power | Other Current Assets [Member]	'	'
Derivative [Line Items]	'	'
Derivative assets	10	23
Union Electric Company | Power | Other Assets [Member]	'	'
Derivative [Line Items]	'	'
Derivative assets	1	'
Union Electric Company | Power | Other Current Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	6	4
Union Electric Company | Uranium | Other Current Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	2	5
Union Electric Company | Uranium | Other Deferred Credits And Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	1	1
Ameren Illinois Company	'	'
Derivative [Line Items]	'	'
Derivative assets	2	1
Derivative liabilities	146	154
Ameren Illinois Company | Natural Gas | Other Current Assets [Member]	'	'
Derivative [Line Items]	'	'
Derivative assets	1	1
Ameren Illinois Company | Natural Gas | Other Assets [Member]	'	'
Derivative [Line Items]	'	'
Derivative assets	1	'
Ameren Illinois Company | Natural Gas | Other Current Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	16	27
Ameren Illinois Company | Natural Gas | Other Deferred Credits And Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	6	19
Ameren Illinois Company | Power | Other Current Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	8	9
Ameren Illinois Company | Power | Other Deferred Credits And Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	$116	$99

Derivative_Financial_Instrumen5

Derivative Financial Instruments (Cumulative Amount Of Pretax Net Gains (Losses) On All Derivative Instruments In OCI) (Detail) (USD $)	Sep. 30, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Derivative [Line Items]	'		'
Current gains deferred as regulatory liabilities	$132		$216
Current losses deferred as regulatory assets	199		156
Fuel Oils	'		'
Derivative [Line Items]	'		'
Current losses deferred as regulatory assets	4		'
Fuel Oils | Regulatory Liabilities Or Assets [Member]	'		'
Derivative [Line Items]	'		'
Cumulative deferred pretax gains (losses)	-5	[1]	2
Natural Gas	'		'
Derivative [Line Items]	'		'
Current gains deferred as regulatory liabilities	1		'
Current losses deferred as regulatory assets	19		'
Natural Gas | Regulatory Liabilities Or Assets [Member]	'		'
Derivative [Line Items]	'		'
Cumulative deferred pretax gains (losses)	-26	[2]	-55
Power	'		'
Derivative [Line Items]	'		'
Current gains deferred as regulatory liabilities	10		'
Current losses deferred as regulatory assets	14		'
Power | Regulatory Liabilities Or Assets [Member]	'		'
Derivative [Line Items]	'		'
Cumulative deferred pretax gains (losses)	-119	[3]	-89
Uranium	'		'
Derivative [Line Items]	'		'
Current losses deferred as regulatory assets	2		'
Uranium | Regulatory Liabilities Or Assets [Member]	'		'
Derivative [Line Items]	'		'
Cumulative deferred pretax gains (losses)	-3	[4]	-6
Union Electric Company	'		'
Derivative [Line Items]	'		'
Current gains deferred as regulatory liabilities	11		57
Current losses deferred as regulatory assets	137		118
Union Electric Company | Fuel Oils	'		'
Derivative [Line Items]	'		'
Current losses deferred as regulatory assets	4		'
Union Electric Company | Fuel Oils | Regulatory Liabilities Or Assets [Member]	'		'
Derivative [Line Items]	'		'
Cumulative deferred pretax gains (losses)	-5	[1]	2
Union Electric Company | Natural Gas	'		'
Derivative [Line Items]	'		'
Current losses deferred as regulatory assets	3		'
Union Electric Company | Natural Gas | Regulatory Liabilities Or Assets [Member]	'		'
Derivative [Line Items]	'		'
Cumulative deferred pretax gains (losses)	-6	[2]	-10
Union Electric Company | Power	'		'
Derivative [Line Items]	'		'
Current gains deferred as regulatory liabilities	10		'
Current losses deferred as regulatory assets	6		'
Union Electric Company | Power | Regulatory Liabilities Or Assets [Member]	'		'
Derivative [Line Items]	'		'
Cumulative deferred pretax gains (losses)	5	[3]	19
Union Electric Company | Uranium	'		'
Derivative [Line Items]	'		'
Current losses deferred as regulatory assets	2		'
Union Electric Company | Uranium | Regulatory Liabilities Or Assets [Member]	'		'
Derivative [Line Items]	'		'
Cumulative deferred pretax gains (losses)	-3	[4]	-6
Ameren Illinois Company	'		'
Derivative [Line Items]	'		'
Current gains deferred as regulatory liabilities	121		159
Current losses deferred as regulatory assets	62		38
Ameren Illinois Company | Natural Gas	'		'
Derivative [Line Items]	'		'
Current gains deferred as regulatory liabilities	1		'
Current losses deferred as regulatory assets	16		'
Ameren Illinois Company | Natural Gas | Regulatory Liabilities Or Assets [Member]	'		'
Derivative [Line Items]	'		'
Cumulative deferred pretax gains (losses)	-20	[2]	-45
Ameren Illinois Company | Power	'		'
Derivative [Line Items]	'		'
Current losses deferred as regulatory assets	8		'
Ameren Illinois Company | Power | Regulatory Liabilities Or Assets [Member]	'		'
Derivative [Line Items]	'		'
Cumulative deferred pretax gains (losses)	($124)	[3]	($108)
[1]	Represents net losses associated with fuel oil derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s transportation costs for coal through December 2017. Current losses deferred as regulatory assets include $4 million and $4 million at Ameren and Ameren Missouri, respectively.
[2]	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 $1 million and $1 million at Ameren and Ameren Illinois, respectively. Current losses deferred as regulatory assets include $19 million, $3 million, and $16 million at Ameren, Ameren Missouri and Ameren Illinois, respectively.
[3]	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 $10 million and $10 million at Ameren and Ameren Missouri. Current losses deferred as regulatory assets include $14 million, $6 million, and $8 million at Ameren, Ameren Missouri and Ameren Illinois, respectively.
[4]	Represents net losses on 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_Instrumen6

Derivative Financial Instruments (Offsetting Derivative Assets and Liabilities) (Details) (Commodity Contract, USD $)	Sep. 30, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Offsetting Assets and Liabilities [Line Items]	'		'
Gross Amounts Recognized in the Balance Sheet	$16	[1]	$34	[1]
Derivative Instruments	8		10
Net Amount	8		24
Gross Amounts Recognized in the Balance Sheet	167	[1]	178	[1]
Derivative Instruments	8		10
Cash Collateral Received/Posted	5	[2]	24	[2]
Net Amount	154		144
Union Electric Company	'		'
Offsetting Assets and Liabilities [Line Items]	'		'
Gross Amounts Recognized in the Balance Sheet	14	[1]	33	[1]
Derivative Instruments	7		9
Net Amount	7		24
Gross Amounts Recognized in the Balance Sheet	21	[1]	24	[1]
Derivative Instruments	7		9
Cash Collateral Received/Posted	5	[2]	9	[2]
Net Amount	9		6
Ameren Illinois Company	'		'
Offsetting Assets and Liabilities [Line Items]	'		'
Gross Amounts Recognized in the Balance Sheet	2		1
Derivative Instruments	1		1
Net Amount	1		0
Gross Amounts Recognized in the Balance Sheet	146	[1]	154	[1]
Derivative Instruments	1		1
Cash Collateral Received/Posted	'		15	[2]
Net Amount	$145		$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_Instrumen7

Derivative Financial Instruments (Maximum Exposure If Counterparties Fail To Perform On Contracts) (Detail) (USD $)	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014	Dec. 31, 2012
			Union Electric Company	Union Electric Company	Ameren Illinois Company	Ameren Illinois Company
Derivative [Line Items]	'	'	'	'	'	'
Maximum exposure to counterparties related to derivative contracts	$6	$13	$4	$12	$2	$1

Derivative_Financial_Instrumen8

Derivative Financial Instruments (Potential Loss On Counterparty Exposures) (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Concentration Risk [Line Items]	'	'
Potential loss on counterparty exposures related to derivative contracts	$4	$6
Ameren Illinois Company	'	'
Concentration Risk [Line Items]	'	'
Potential loss on counterparty exposures related to derivative contracts	1	'
Union Electric Company	'	'
Concentration Risk [Line Items]	'	'
Potential loss on counterparty exposures related to derivative contracts	$3	$6

Derivative_Financial_Instrumen9

Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Detail) (USD $)	Sep. 30, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Derivative [Line Items]	'		'
Aggregate Fair Value of Derivative Liabilities	$123	[1]	$145	[1]
Cash Collateral Posted	2		17
Potential Aggregate Amount of Additional Collateral Required	113	[2]	122	[2]
Union Electric Company	'		'
Derivative [Line Items]	'		'
Aggregate Fair Value of Derivative Liabilities	62	[1]	70	[1]
Cash Collateral Posted	2		2
Potential Aggregate Amount of Additional Collateral Required	57	[2]	67	[2]
Ameren Illinois Company	'		'
Derivative [Line Items]	'		'
Aggregate Fair Value of Derivative Liabilities	61	[1]	75	[1]
Cash Collateral Posted	'		15
Potential Aggregate Amount of Additional Collateral Required	$56	[2]	$55	[2]
[1]	Prior to consideration of master trading and netting agreements and including NPNS and accrual contract exposures.
[2]	As collateral requirements with certain counterparties are based on master trading and netting agreements, the aggregate amount of additional collateral required to be posted is determined after consideration of the effects of such agreements.

Fair_Value_Measurements_Schedu

Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Detail) (USD $)	9 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Natural Gas | Discounted Cash Flow | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.30%	[1],[2]	'
Nodal basis	-0.1	[1]	'
Credit risk	0.43%	[1],[2]	'
Natural Gas | Discounted Cash Flow | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	2.00%	[1],[2]	'
Nodal basis	0	[1]	'
Credit risk	0.43%	[1],[2]	'
Natural Gas | Discounted Cash Flow | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.62%	[1],[2]	'
Nodal basis	-0.1	[1]	'
Credit risk	0.43%	[1],[2]	'
Natural Gas | Derivative Assets	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative assets	1	[3]	'
Natural Gas | Ameren Illinois Company | Discounted Cash Flow | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.30%	[1],[2]	'
Nodal basis	-0.1	[1]	'
Credit risk	0.43%	[1],[2]	'
Natural Gas | Ameren Illinois Company | Discounted Cash Flow | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	2.00%	[1],[2]	'
Nodal basis	0	[1]	'
Credit risk	0.43%	[1],[2]	'
Natural Gas | Ameren Illinois Company | Discounted Cash Flow | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.62%	[1],[2]	'
Nodal basis	-0.1	[1]	'
Credit risk	0.43%	[1],[2]	'
Natural Gas | Ameren Illinois Company | Derivative Assets	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative assets	1	[3]	'
Fuel Oils | Discounted Cash Flow | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.25%	[1],[2]	0.26%	[1],[2]
Credit risk	0.43%	[1],[2]	'
Fuel Oils | Discounted Cash Flow | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	1.00%	[1],[2]	2.00%	[1],[2]
Credit risk	0.43%	[1],[2]	'
Fuel Oils | Discounted Cash Flow | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.72%	[1],[2]	1.00%	[1],[2]
Credit risk	0.43%	[1],[2]	'
Fuel Oils | Option Model | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Volatilities	2.00%	[4]	10.00%	[4]
Fuel Oils | Option Model | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Volatilities	27.00%	[4]	35.00%	[4]
Fuel Oils | Option Model | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Volatilities	14.00%	[4]	16.00%	[4]
Fuel Oils | Derivative Assets	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative assets	3	[3]	8	[3]
Fuel Oils | Union Electric Company | Discounted Cash Flow | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.25%	[1],[2]	0.26%	[1],[2]
Credit risk	0.43%	[1],[2]	'
Fuel Oils | Union Electric Company | Discounted Cash Flow | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	1.00%	[1],[2]	2.00%	[1],[2]
Credit risk	0.43%	[1],[2]	'
Fuel Oils | Union Electric Company | Discounted Cash Flow | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.72%	[1],[2]	1.00%	[1],[2]
Credit risk	0.43%	[1],[2]	'
Fuel Oils | Union Electric Company | Option Model | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Volatilities	2.00%	[4]	10.00%	[4]
Fuel Oils | Union Electric Company | Option Model | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Volatilities	27.00%	[4]	35.00%	[4]
Fuel Oils | Union Electric Company | Option Model | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Volatilities	14.00%	[4]	16.00%	[4]
Fuel Oils | Union Electric Company | Derivative Assets	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative assets	3	[3]	8	[3]
Fuel Oils | Derivative Liabilities	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative liabilities	-3	[3]	-3	[3]
Fuel Oils | Derivative Liabilities | Union Electric Company	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative liabilities	-3	[3]	-3	[3]
Power | Discounted Cash Flow | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.40%	[1],[2]	0.39%	[1],[2]
Nodal basis	-6	[1]	-3	[1]
Credit risk	0.43%	[1],[2]	2.00%	[1],[2]
Average forward peak and off-peak pricing	29	[1]	25	[1]
Estimated auction price	-1,853	[4]	-1,594	[4]
Power | Discounted Cash Flow | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.40%	[1],[2]	0.50%	[1],[2]
Nodal basis	0	[1]	-1	[1]
Credit risk	0.43%	[1],[2]	2.00%	[1],[2]
Average forward peak and off-peak pricing	59	[1]	51	[1]
Estimated auction price	2,087	[4]	945	[4]
Power | Discounted Cash Flow | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.40%	[1],[2]	0.42%	[1],[2]
Nodal basis	-3	[1]	-2	[1]
Credit risk	0.43%	[1],[2]	2.00%	[1],[2]
Average forward peak and off-peak pricing	35	[1]	32	[1]
Estimated auction price	199	[4]	305	[4]
Power | Fundamental Energy Production Model | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Escalation rate	2.00%	[4],[5]	3.00%	[4],[5]
Estimated future gas prices	4	[4]	4	[4]
Power | Fundamental Energy Production Model | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Escalation rate	2.00%	[4],[5]	4.00%	[4],[5]
Estimated future gas prices	5	[4]	5	[4]
Power | Fundamental Energy Production Model | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Escalation rate	2.00%	[4],[5]	4.00%	[4],[5]
Estimated future gas prices	5	[4]	5	[4]
Power | Contract Price Allocation | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Estimated renewable energy credit costs	5	[4]	5	[4]
Power | Contract Price Allocation | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Estimated renewable energy credit costs	7	[4]	7	[4]
Power | Contract Price Allocation | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Estimated renewable energy credit costs	6	[4]	6	[4]
Power | Derivative Assets	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative assets	10	[3],[6]	21	[3],[7]
Power | Union Electric Company | Discounted Cash Flow | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.40%	[1],[2]	0.39%	[1],[2]
Nodal basis	'		-3	[1]
Credit risk	0.43%	[1],[2]	2.00%	[1],[2]
Average forward peak and off-peak pricing	30	[1]	25	[1]
Estimated auction price	-1,853	[4]	-1,594	[4]
Power | Union Electric Company | Discounted Cash Flow | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.40%	[1],[2]	0.50%	[1],[2]
Nodal basis	'		-1	[1]
Credit risk	0.43%	[1],[2]	2.00%	[1],[2]
Average forward peak and off-peak pricing	59	[1]	51	[1]
Estimated auction price	2,087	[4]	945	[4]
Power | Union Electric Company | Discounted Cash Flow | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.40%	[1],[2]	0.42%	[1],[2]
Nodal basis	'		-2	[1]
Credit risk	0.43%	[1],[2]	2.00%	[1],[2]
Average forward peak and off-peak pricing	48	[1]	40	[1]
Estimated auction price	199	[4]	305	[4]
Power | Union Electric Company | Derivative Assets	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative assets	10	[3],[6]	21	[3],[7]
Power | Ameren Illinois Company | Discounted Cash Flow | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Nodal basis	-6	[4]	-4	[4]
Credit risk	0.43%	[1],[2]	2.00%	[1],[2]
Average forward peak and off-peak pricing	29	[4]	27	[4]
Power | Ameren Illinois Company | Discounted Cash Flow | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Nodal basis	0	[4]	0	[4]
Credit risk	0.43%	[1],[2]	2.00%	[1],[2]
Average forward peak and off-peak pricing	42	[4]	36	[4]
Power | Ameren Illinois Company | Discounted Cash Flow | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Nodal basis	-3	[4]	-2	[4]
Credit risk	0.43%	[1],[2]	2.00%	[1],[2]
Average forward peak and off-peak pricing	33	[4]	30	[4]
Power | Ameren Illinois Company | Fundamental Energy Production Model | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Escalation rate	2.00%	[4],[5]	3.00%	[4],[5]
Estimated future gas prices	4	[4]	4	[4]
Power | Ameren Illinois Company | Fundamental Energy Production Model | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Escalation rate	2.00%	[4],[5]	4.00%	[4],[5]
Estimated future gas prices	5	[4]	5	[4]
Power | Ameren Illinois Company | Fundamental Energy Production Model | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Escalation rate	2.00%	[4],[5]	4.00%	[4],[5]
Estimated future gas prices	5	[4]	5	[4]
Power | Ameren Illinois Company | Contract Price Allocation | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Estimated renewable energy credit costs	5	[4]	5	[4]
Power | Ameren Illinois Company | Contract Price Allocation | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Estimated renewable energy credit costs	7	[4]	7	[4]
Power | Ameren Illinois Company | Contract Price Allocation | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Estimated renewable energy credit costs	6	[4]	6	[4]
Power | Ameren Illinois Company | Derivative Assets	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative assets	'		'	[3],[7]
Power | Derivative Liabilities	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative liabilities	-129	[3],[6]	-110	[3],[7]
Power | Derivative Liabilities | Union Electric Company	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative liabilities	-5	[3],[6]	-2	[3],[7]
Power | Derivative Liabilities | Ameren Illinois Company	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative liabilities	-124	[3],[6]	-108	[3],[7]
Uranium | Discounted Cash Flow | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Average forward pricing	35	[4]	34	[4]
Uranium | Discounted Cash Flow | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Average forward pricing	41	[4]	41	[4]
Uranium | Discounted Cash Flow | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Average forward pricing	36	[4]	36	[4]
Uranium | Derivative Assets	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative assets	'	[3]	'	[3]
Uranium | Union Electric Company | Discounted Cash Flow | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Average forward pricing	35	[4]	34	[4]
Uranium | Union Electric Company | Discounted Cash Flow | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Average forward pricing	41	[4]	41	[4]
Uranium | Union Electric Company | Discounted Cash Flow | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Average forward pricing	36	[4]	36	[4]
Uranium | Union Electric Company | Derivative Assets	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative assets	'	[3]	'	[3]
Uranium | Derivative Liabilities	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative liabilities	-3	[3]	-6	[3]
Uranium | Derivative Liabilities | Union Electric Company	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative liabilities	-3	[3]	-6	[3]
[1]	Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.
[2]	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.
[3]	The derivative asset and liability balances are presented net of counterparty credit considerations.
[4]	Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.
[5]	Escalation rate applies to power prices 2026 and beyond.
[6]	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.
[7]	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) (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Valuation adjustments related to net derivative contracts, liabilities	$1	$3
Union Electric Company	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Valuation adjustments related to net derivative contracts, liabilities	1	1
Ameren Illinois Company	'	'
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]	'	'
Valuation adjustments related to net derivative contracts, liabilities	$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) (Detail) (USD $)	Sep. 30, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	$527	[1]	$494
Assets fair value	543	[2]	528	[2]
Excluded receivables, payables, and accrued income, net	2		'
Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	527	[1]	494
Assets fair value	541	[2]	527	[2]
Commodity Contract	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	16	[2]	34	[2]
Derivative liabilities	167	[2]	178	[2]
Commodity Contract | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	14	[2]	33	[2]
Derivative liabilities	21	[2]	24	[2]
Commodity Contract | Ameren Illinois Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	2		1
Derivative liabilities	146	[2]	154	[2]
Commodity Contract | Fuel Oils	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	3	[2]	9	[2]
Derivative liabilities	6	[2]	3	[2]
Commodity Contract | Fuel Oils | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	3	[2]	9	[2]
Derivative liabilities	6	[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	28	[2]	57	[2]
Commodity Contract | Natural Gas | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	'		1	[2]
Derivative liabilities	6	[2]	11	[2]
Commodity Contract | Natural Gas | Ameren Illinois Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	2	[2]	1	[2]
Derivative liabilities	22	[2]	46	[2]
Commodity Contract | Power	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	11	[2]	23	[2]
Derivative liabilities	130	[2]	112	[2]
Commodity Contract | Power | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	11	[2]	23	[2]
Derivative liabilities	6	[2]	4	[2]
Commodity Contract | Power | Ameren Illinois Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative liabilities	124	[2]	108	[2]
Commodity Contract | Uranium	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative liabilities	3	[2]	6	[2]
Commodity Contract | Uranium | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative liabilities	3	[2]	6	[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 | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	1		3
Equity Securities | U.S. large capitalization	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	348		332
Equity Securities | U.S. large capitalization | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	348		332
Debt Securities | Corporate bonds	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	60		52
Debt Securities | Corporate bonds | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	60		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 | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	2		2
Debt Securities | US treasury and government securities	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	100		94
Debt Securities | US treasury and government securities | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	100		94
Debt Securities | Asset-backed Securities	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	11		10
Debt Securities | Asset-backed Securities | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	11		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 | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	5		1
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	349		335
Assets fair value	349	[2]	336	[2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	349		335
Assets fair value	349	[2]	336	[2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	'	[2]	1	[2]
Derivative liabilities	5	[2]	3	[2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	'	[2]	1	[2]
Derivative liabilities	5	[2]	3	[2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Fuel Oils	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	'	[2]	1	[2]
Derivative liabilities	3	[2]	'
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Fuel Oils | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	'	[2]	1	[2]
Derivative liabilities	3	[2]	'
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Natural Gas	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative liabilities	2	[2]	3	[2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Natural Gas | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	'		'	[2]
Derivative liabilities	2	[2]	3	[2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Natural Gas | Ameren Illinois Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative liabilities	'		'	[2]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Cash and cash equivalents	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	1		3
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Cash and cash equivalents | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	1		3
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Equity Securities | U.S. large capitalization	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	348		332
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Equity Securities | U.S. large capitalization | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	348		332
Significant Other Observable Inputs (Level 2)	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	178		159
Assets fair value	180	[2]	163	[2]
Significant Other Observable Inputs (Level 2) | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	178		159
Assets fair value	179	[2]	162	[2]
Significant Other Observable Inputs (Level 2) | Commodity Contract	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	2	[2]	4	[2]
Derivative liabilities	27	[2]	56	[2]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	1	[2]	3	[2]
Derivative liabilities	5	[2]	10	[2]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Ameren Illinois Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative liabilities	22	[2]	46	[2]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Natural Gas	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	1	[2]	2	[2]
Derivative liabilities	26	[2]	54	[2]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Natural Gas | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	'		1	[2]
Derivative liabilities	4	[2]	8	[2]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Natural Gas | Ameren Illinois Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	1	[2]	1	[2]
Derivative liabilities	22	[2]	46	[2]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Power	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	1	[2]	2	[2]
Derivative liabilities	1	[2]	2	[2]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Power | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	1	[2]	2	[2]
Derivative liabilities	1	[2]	2	[2]
Significant Other Observable Inputs (Level 2) | Debt Securities | Corporate bonds	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	60		52
Significant Other Observable Inputs (Level 2) | Debt Securities | Corporate bonds | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	60		52
Significant Other Observable Inputs (Level 2) | Debt Securities | Municipal bonds	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	2		2
Significant Other Observable Inputs (Level 2) | Debt Securities | Municipal bonds | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	2		2
Significant Other Observable Inputs (Level 2) | Debt Securities | US treasury and government securities	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	100		94
Significant Other Observable Inputs (Level 2) | Debt Securities | US treasury and government securities | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	100		94
Significant Other Observable Inputs (Level 2) | Debt Securities | Asset-backed Securities	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	11		10
Significant Other Observable Inputs (Level 2) | Debt Securities | Asset-backed Securities | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	11		10
Significant Other Observable Inputs (Level 2) | Debt Securities | Other Debt Securities	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	5		1
Significant Other Observable Inputs (Level 2) | Debt Securities | Other Debt Securities | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	5		1
Significant Other Unobservable Inputs (Level 3)	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Assets fair value	14	[2]	29	[2]
Significant Other Unobservable Inputs (Level 3) | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Assets fair value	13	[2]	29	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	14	[2]	29	[2]
Derivative liabilities	135	[2]	119	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	13	[2]	29	[2]
Derivative liabilities	11	[2]	11	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Ameren Illinois Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative liabilities	124	[2]	108	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Fuel Oils	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	3	[2]	8	[2]
Derivative liabilities	3	[2]	3	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Fuel Oils | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	3	[2]	8	[2]
Derivative liabilities	3	[2]	3	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Natural Gas	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	1	[2]	'
Derivative liabilities	'	[2]	'
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Natural Gas | Ameren Illinois Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	1	[2]	'	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Power	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	10	[2]	21	[2]
Derivative liabilities	129	[2]	110	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Power | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	10	[2]	21	[2]
Derivative liabilities	5	[2]	2	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Power | Ameren Illinois Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative liabilities	124	[2]	108	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Uranium	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative liabilities	3	[2]	6	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Uranium | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative liabilities	$3	[2]	$6	[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 Three In The Fair Value Hierarchy) (Detail) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Fuel Oils	'	'	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'	'	'	'
Beginning balance	$2	$3	$5	$5
Included in regulatory assets/liabilities	-2	1	-3	-1
Purchases	'	1	'	2
Sales	'	-1	'	-1
Settlements	'	-1	-2	-2
Ending balance	0	3	0	3
Change in unrealized gains (losses) related to assets/liabilities held at period end	-2	1	-2	0
Uranium	'	'	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'	'	'	'
Beginning balance	-7	-3	-6	-2
Included in regulatory assets/liabilities	3	'	-1	-2
Purchases	'	-2	'	-2
Settlements	1	'	4	1
Ending balance	-3	-5	-3	-5
Change in unrealized gains (losses) related to assets/liabilities held at period end	3	-2	0	-2
Natural Gas	'	'	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'	'	'	'
Beginning balance	0	1	0	0
Included in regulatory assets/liabilities	1	-2	1	-1
Purchases	'	1	-1	1
Settlements	'	'	1	'
Ending balance	1	0	1	0
Change in unrealized gains (losses) related to assets/liabilities held at period end	'	-1	'	'
Power	'	'	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'	'	'	'
Beginning balance	-88	-43	-89	-100
Included in regulatory assets/liabilities	-28	-20	-42	1
Purchases	'	'	34	40
Sales	'	1	'	1
Settlements	-3	-3	-22	-9
Transfers into Level 3	'	-1	'	-3
Transfer out of Level 3	'	'	'	4
Ending balance	-119	-66	-119	-66
Change in unrealized gains (losses) related to assets/liabilities held at period end	-28	-18	-24	-7
Union Electric Company | Fuel Oils	'	'	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'	'	'	'
Beginning balance	2	3	5	5
Included in regulatory assets/liabilities	-2	1	-3	-1
Purchases	'	1	'	2
Sales	'	-1	'	-1
Settlements	'	-1	-2	-2
Ending balance	0	3	0	3
Change in unrealized gains (losses) related to assets/liabilities held at period end	-2	1	-2	0
Union Electric Company | Uranium	'	'	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'	'	'	'
Beginning balance	-7	-3	-6	-2
Included in regulatory assets/liabilities	3	'	-1	-2
Purchases	'	-2	'	-2
Settlements	1	'	4	1
Ending balance	-3	-5	-3	-5
Change in unrealized gains (losses) related to assets/liabilities held at period end	3	-2	0	-2
Union Electric Company | Natural Gas	'	'	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'	'	'	'
Beginning balance	0	-1	0	0
Included in regulatory assets/liabilities	0	'	'	'
Purchases	'	1	0	0
Ending balance	0	0	0	0
Change in unrealized gains (losses) related to assets/liabilities held at period end	'	'	'	'
Union Electric Company | Power	'	'	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'	'	'	'
Beginning balance	15	37	19	11
Included in regulatory assets/liabilities	-5	-3	-23	3
Purchases	'	'	34	40
Sales	'	1	'	1
Settlements	-5	-6	-25	-28
Transfers into Level 3	'	-1	'	-3
Transfer out of Level 3	'	'	'	4
Ending balance	5	28	5	28
Change in unrealized gains (losses) related to assets/liabilities held at period end	-6	-2	-3	'
Ameren Illinois Company | Natural Gas	'	'	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'	'	'	'
Beginning balance	0	2	0	0
Included in regulatory assets/liabilities	1	-2	1	-1
Purchases	'	0	-1	1
Settlements	'	'	1	'
Ending balance	1	0	1	0
Change in unrealized gains (losses) related to assets/liabilities held at period end	'	-1	'	'
Ameren Illinois Company | Power	'	'	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'	'	'	'
Beginning balance	-103	-80	-108	-111
Included in regulatory assets/liabilities	-23	-17	-19	-2
Settlements	2	3	3	19
Transfers into Level 3	'	'	'	'
Ending balance	-124	-94	-124	-94
Change in unrealized gains (losses) related to assets/liabilities held at period end	($22)	($16)	($21)	($7)

Fair_Value_Measurements_Schedu3

Fair Value Measurements (Schedule Of Transfers Between Fair Value Hierarchy Levels) (Detail) (USD $)

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

9 Months Ended

3 Months Ended

In Millions, unless otherwise specified

Sep. 30, 2013

Sep. 30, 2013

Mar. 31, 2012

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Sep. 30, 2013

Fuel Oils

Fuel Oils

Natural Gas

Power

Power

Power

Power

Union Electric Company

Union Electric Company

Union Electric Company

Union Electric Company

Ameren Illinois Company

Transfer Into Or Out Of Level One [Member]

Transfer Into Or Out Of Level One [Member]

Transfer Into/Out of Level 2

Transfer Into/Out of Level 2

Transfer Into/Out of Level 2

Power

Power

Power

Power

Power

Transfer Into/Out of Level 2

Transfer Into/Out of Level 2

Derivative [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

Assets Transfers into Level 3

'

'

'

'

($1)

($3)

($1)

($3)

($1)

($3)

($1)

($3)

'

Assets Transfers out of Level 3

'

'

'

'

'

4

'

4

'

4

'

4

'

Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Transfers, Net

($1)

'

'

'

'

'

'

'

'

'

'

'

'

Fair_Value_Measurements_Schedu4

Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Long-Term Debt And Preferred Stock) (Detail) (USD $)	Sep. 30, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Fair Value	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Long-term debt (including current portion)	$6,647	[1]	$6,584	[1]
Preferred stock	122	[1]	118	[1]
Fair Value | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Long-term debt (including current portion)	4,466		4,124
Preferred stock	73		71
Fair Value | Ameren Illinois Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Long-term debt (including current portion)	2,181		2,028
Preferred stock	49		47
Carrying Amount	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Long-term debt (including current portion)	5,944	[1]	6,038	[1]
Preferred stock	142	[1]	142	[1]
Carrying Amount | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Long-term debt (including current portion)	4,004		3,757
Preferred stock	80		80
Carrying Amount | Ameren Illinois Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Long-term debt (including current portion)	1,940		1,856
Preferred stock	$62		$62
[1]	Preferred stock is recorded in “Noncontrolling Interests” on the consolidated balance sheet.

Related_Party_Transactions_Sch

Related Party Transactions (Schedule of Related Party Transactions) (Detail) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Union Electric Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Revenues	$8,000,000		$5,000,000		$21,000,000		$18,000,000
Operating Expenses	25,000,000		25,000,000		90,000,000		85,000,000
Ameren Illinois Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Revenues	1,000,000	[1]	1,000,000	[1]	1,000,000		3,000,000
Operating Expenses	26,000,000		22,000,000		80,000,000		70,000,000
Purchased Power | Union Electric Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Expenses	1,000,000	[2]	1,000,000	[2]	1,000,000	[2]	1,000,000	[2]
Purchased Power | Ameren Illinois Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Expenses	3,000,000		1,000,000		7,000,000		3,000,000
Ameren Missouri Power Supply Agreements with Ameren Illinois | Union Electric Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Revenues	2,000,000		1,000,000	[1]	5,000,000		1,000,000
Ameren Missouri Power Supply Agreements with Ameren Illinois | Ameren Illinois Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Revenues	1,000,000	[2]	1,000,000	[2]	1,000,000	[2]	1,000,000	[2]
Ameren Missouri and Ameren Illinois Rent and Facility Services | Union Electric Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Revenues	6,000,000		4,000,000		15,000,000		16,000,000
Ameren Missouri and Ameren Illinois Rent and Facility Services | Ameren Illinois Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Revenues	1,000,000	[1]	1,000,000	[1]	1,000,000		1,000,000
Ameren Missouri and Ameren Illinois miscellaneous support services [Member] | Union Electric Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Revenues	1,000,000	[1]	1,000,000		1,000,000		1,000,000
Ameren Missouri and Ameren Illinois miscellaneous support services [Member] | Ameren Illinois Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Revenues	1,000,000	[1]	1,000,000	[1]	1,000,000	[1]	2,000,000
Ameren Illinois Power Supply Agreements with Ameren Missouri | Union Electric Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Expenses	1,000,000	[2]	1,000,000	[2]	1,000,000	[2]	1,000,000	[2]
Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Expenses	2,000,000		1,000,000	[1]	5,000,000		1,000,000
Ameren Illinois Transmission Agreements with ATXI | Union Electric Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Expenses	1,000,000	[2]	1,000,000	[2]	1,000,000	[2]	1,000,000	[2]
Ameren Illinois Transmission Agreements with ATXI | Ameren Illinois Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Expenses	1,000,000		1,000,000		2,000,000		2,000,000
Ameren Services Support Services Agreement | Union Electric Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Expenses	25,000,000		25,000,000		90,000,000		85,000,000
Ameren Services Support Services Agreement | Ameren Illinois Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Expenses	26,000,000		22,000,000		80,000,000		70,000,000
Insurance Premiums | Union Electric Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Expenses	1,000,000	[1],[3]	1,000,000	[1],[3]	1,000,000	[1],[3]	1,000,000	[1],[3]
Insurance Premiums | Ameren Illinois Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Expenses	1,000,000	[2],[3]	1,000,000	[2],[3]	1,000,000	[2],[3]	1,000,000	[2],[3]
Related Party Money Pool Interest | Union Electric Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Interest Charges	1,000,000	[1]	1,000,000	[1]	1,000,000	[1]	1,000,000	[1]
Related Party Money Pool Interest | Ameren Illinois Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Interest Charges	$1,000,000	[1]	$1,000,000	[1]	$1,000,000	[1]	$1,000,000	[1]
[1]	Amount less than $1 million.
[2]	Not applicable.
[3]	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) (Ameren Illinois Company, Ameren Illinois Power Supply Agreements with Ameren Missouri)	9 Months Ended
	Sep. 30, 2014
	MWh
Ameren Illinois Company | Ameren Illinois Power Supply Agreements with Ameren Missouri	'
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 Energy Center) (Detail) (USD $)	9 Months Ended
	Sep. 30, 2014
Commitments And Contingencies [Line Items]	'
Threshold for which a retrospective assessment for a covered loss is necessary	$375,000,000
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 in calendar year per reactor incident under Price Andersen 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 secondary weekly indemnity coverage for prolonged nuclear plant outage in excess of primary indemnity coverage	900,000
Inflationary adjustment prescribed by most recent Price-Anderson Act renewal, in years	'5 years
Aggregate nuclear power industry insurance policy limit for losses from terrorist attacks within twelve month period	3,240,000,000
Public Liability	'
Commitments And Contingencies [Line Items]	'
Maximum Coverages	13,616,000,000	[1]
Maximum Assessments for Single Incidents	128,000,000
Public Liability And Nuclear Worker Liability - American Nuclear Insurers [Member]	'
Commitments And Contingencies [Line Items]	'
Maximum Coverages	375,000,000
Public Liability And Nuclear Worker Liability - Pool Participation [Member]	'
Commitments And Contingencies [Line Items]	'
Maximum Coverages	13,241,000,000	[2]
Maximum Assessments for Single Incidents	128,000,000	[3]
Property Damage	'
Commitments And Contingencies [Line Items]	'
Maximum Coverages	2,750,000,000
Maximum Assessments for Single Incidents	23,000,000
Property Damage - Nuclear Electric Insurance Ltd [Member]	'
Commitments And Contingencies [Line Items]	'
Maximum Coverages	2,250,000,000	[4]
Maximum Assessments for Single Incidents	23,000,000	[5]
Property Damage European Mutual Association for Nuclear Insurance [Member]	'
Commitments And Contingencies [Line Items]	'
Maximum Coverages	500,000,000	[6]
Amount of primary property liability coverage	500,000,000
Replacement Power - Nuclear Electric Insurance Ltd [Member]	'
Commitments And Contingencies [Line Items]	'
Maximum Coverages	490,000,000	[7]
Maximum Assessments for Single Incidents	9,000,000	[5]
Amount of weekly indemnity coverage thereafter not exceeding policy limit	490,000,000
Sub-limit of for non-nuclear events	327,600,000
Replacement Power - Energy Risk Assurance Company [Member]	'
Commitments And Contingencies [Line Items]	'
Maximum Coverages	64,000,000	[8]
Amount of weekly indemnity coverage thereafter not exceeding policy limit	3,600,000
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 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]	provides $2.25 billion in property damage, decontamination, and premature decommissioning insurance.
[5]	All NEIL insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL.
[6]	European Mutual Association for Nuclear Insurance provides $500 million in excess of the $2.25 billion property coverage provided by NEIL.
[7]	Provides replacement power cost insurance in the event of a prolonged accidental outage. Weekly indemnity 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 $327.6 million.
[8]	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 and 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 and has reinsured this coverage with third-party insurance companies. See Note 8 - Related Party Transactions for more information on this affiliate transaction.

Commitments_And_Contingencies_2

Commitments And Contingencies (Other Obligations) (Detail) (USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014
Long-term Purchase Commitment [Line Items]	'
2014	$457	[1]
2015	1,265	[1]
2016	1,040	[1]
2017	941	[1]
2018	324	[1]
Thereafter	1,480	[1]
Total	5,507	[1]
Renewable Energy Credits Agreements, Term	'20 years
Union Electric Company	'
Long-term Purchase Commitment [Line Items]	'
2014	294
2015	864
2016	813
2017	805
2018	233
Thereafter	716
Total	3,725
Ameren Illinois Company	'
Long-term Purchase Commitment [Line Items]	'
2014	144
2015	383
2016	214
2017	136
2018	91
Thereafter	764
Total	1,732
Investment in Energy Efficiency Programs | Union Electric Company	'
Long-term Purchase Commitment [Line Items]	'
2014	17
2015	64
Coal	'
Long-term Purchase Commitment [Line Items]	'
2014	151	[1]
2015	635	[1]
2016	659	[1]
2017	682	[1]
2018	111	[1]
Thereafter	114	[1]
Total	2,352	[1]
Coal | Union Electric Company	'
Long-term Purchase Commitment [Line Items]	'
2014	151
2015	635
2016	659
2017	682
2018	111
Thereafter	114
Total	2,352
Coal | Ameren Illinois Company	'
Long-term Purchase Commitment [Line Items]	'
2014	'
2015	'
2016	'
2017	'
2018	'
Thereafter	'
Total	'
Natural Gas	'
Long-term Purchase Commitment [Line Items]	'
2014	93	[1],[2]
2015	225	[1],[2]
2016	127	[1],[2]
2017	80	[1],[2]
2018	41	[1],[2]
Thereafter	101	[1],[2]
Total	667	[1],[2]
Natural Gas | Union Electric Company	'
Long-term Purchase Commitment [Line Items]	'
2014	16	[2]
2015	39	[2]
2016	21	[2]
2017	13	[2]
2018	8	[2]
Thereafter	29	[2]
Total	126	[2]
Natural Gas | Ameren Illinois Company	'
Long-term Purchase Commitment [Line Items]	'
2014	77	[2]
2015	186	[2]
2016	106	[2]
2017	67	[2]
2018	33	[2]
Thereafter	72	[2]
Total	541	[2]
Nuclear Fuel	'
Long-term Purchase Commitment [Line Items]	'
2014	62	[1]
2015	56	[1]
2016	69	[1]
2017	59	[1]
2018	61	[1]
Thereafter	179	[1]
Total	486	[1]
Nuclear Fuel | Union Electric Company	'
Long-term Purchase Commitment [Line Items]	'
2014	62
2015	56
2016	69
2017	59
2018	61
Thereafter	179
Total	486
Nuclear Fuel | Ameren Illinois Company	'
Long-term Purchase Commitment [Line Items]	'
2014	'
2015	'
2016	'
2017	'
2018	'
Thereafter	'
Total	'
Purchased Power	'
Long-term Purchase Commitment [Line Items]	'
2014	62	[1],[3]
2015	190	[1],[3]
2016	105	[1],[3]
2017	66	[1],[3]
2018	55	[1],[3]
Thereafter	645	[1],[3]
Total	1,123	[1],[3]
Purchased Power | Union Electric Company	'
Long-term Purchase Commitment [Line Items]	'
2014	4
2015	21
2016	21
2017	21
2018	21
Thereafter	120
Total	208
Purchased Power | Ameren Illinois Company	'
Long-term Purchase Commitment [Line Items]	'
2014	58	[3]
2015	169	[3]
2016	84	[3]
2017	45	[3]
2018	34	[3]
Thereafter	525	[3]
Total	915	[3]
Methane Gas	'
Long-term Purchase Commitment [Line Items]	'
2014	1	[1]
2015	3	[1]
2016	4	[1]
2017	4	[1]
2018	5	[1]
Thereafter	91	[1]
Total	108	[1]
Methane Gas | Union Electric Company	'
Long-term Purchase Commitment [Line Items]	'
2014	1
2015	3
2016	4
2017	4
2018	5
Thereafter	91
Total	108
Methane Gas | Ameren Illinois Company	'
Long-term Purchase Commitment [Line Items]	'
2014	'
2015	'
2016	'
2017	'
2018	'
Thereafter	'
Total	'
Other Commitments	'
Long-term Purchase Commitment [Line Items]	'
2014	88	[1]
2015	156	[1]
2016	76	[1]
2017	50	[1]
2018	51	[1]
Thereafter	350	[1]
Total	771	[1]
Other Commitments | Union Electric Company	'
Long-term Purchase Commitment [Line Items]	'
2014	60
2015	110
2016	39
2017	26
2018	27
Thereafter	183
Total	445
Other Commitments | Ameren Illinois Company	'
Long-term Purchase Commitment [Line Items]	'
2014	9
2015	28
2016	24
2017	24
2018	24
Thereafter	167
Total	$276
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
[2]	Includes amounts for generation and for distribution.
[3]	The purchased power amounts for Ameren and Ameren Illinois include twenty-year agreements for renewable energy credits that were entered into in December 2010 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_3

Commitments And Contingencies (Environmental Matters) (Detail) (USD $)	9 Months Ended
	Sep. 30, 2014
Loss Contingencies [Line Items]	'
EPA Clean Power Plan CO2 Proposed Reductions	30.00%
Estimated Environmental Expenditures to Comply with Clean Power Plan	$2,000,000,000
Minimum	'
Loss Contingencies [Line Items]	'
Estimated capital costs to comply with existing and known federal and state air emissions regulations	325,000,000
Maximum	'
Loss Contingencies [Line Items]	'
Estimated capital costs to comply with existing and known federal and state air emissions regulations	375,000,000
Former Coal Tar Distillery | Union Electric Company	'
Loss Contingencies [Line Items]	'
Range of possible loss, minimum	2,000,000
Range of possible loss maximum	5,000,000
Accrual for environmental loss contingencies	2,000,000
Former Coal Ash Landfill | Ameren Illinois Company	'
Loss Contingencies [Line Items]	'
Range of possible loss, minimum	500,000
Range of possible loss maximum	6,000,000
Accrual for environmental loss contingencies	500,000
Manufactured Gas Plant	'
Loss Contingencies [Line Items]	'
Accrual for environmental loss contingencies	254,000,000
Manufactured Gas Plant | Ameren Illinois Company	'
Loss Contingencies [Line Items]	'
Number of remediation sites	44
Range of possible loss, minimum	254,000,000
Range of possible loss maximum	316,000,000
Accrual for environmental loss contingencies	254,000,000
Other Environmental | Ameren Illinois Company	'
Loss Contingencies [Line Items]	'
Accrual for environmental loss contingencies	700,000
Sauget Area Two | Union Electric Company	'
Loss Contingencies [Line Items]	'
Range of possible loss, minimum	1,000,000
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 Contingencies [Line Items]	'
Range of possible loss, minimum	2,200,000
Range of possible loss maximum	4,500,000
Accrual for environmental loss contingencies	$2,200,000

Commitments_And_Contingencies_4

Commitments And Contingencies (Pumped-Storage Hydroelectric Facility Breach) (Detail) (Union Electric Company, USD $)	9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014
Union Electric Company	'
Loss Contingencies [Line Items]	'
Litigation Settlement, Amount	$27
Insurance settlements receivable	$41

Commitments_And_Contingencies_5

Commitments And Contingencies (Asbestos-Related Litigation And Tax Exemptions And Credits) (Detail) (USD $)	9 Months Ended						1 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014		Sep. 30, 2014	Sep. 30, 2014	Sep. 30, 2014	Sep. 30, 2014	Sep. 30, 2013	Jul. 31, 2013
	Asbestos-Related		Asbestos-Related	Asbestos-Related	Asbestos-Related	Collectibility of Taxes	State and Local Jurisdiction [Member]	State and Local Jurisdiction [Member]
	defendant		Union Electric Company	Ameren Illinois Company	Ameren Corporation	Ameren Illinois Company	Collectibility of Taxes	Collectibility of Taxes
	lawsuit		lawsuit	lawsuit	lawsuit	customer	Ameren Illinois Company	Ameren Illinois Company
Loss Contingencies [Line Items]	'		'	'	'	'	'	'
Average number of total defendants named	81		'	'	'	'	'	'
Asbestos-related lawsuits were pending	75	[1]	48	62	1	'	'	'
Range of possible loss, minimum	$13		$5	$8	'	$2	'	'
Range of possible loss maximum	'		'	'	'	5	'	'
Loss contingency accrual	'		'	'	'	2	'	'
Percent of allowed cash expenditures in excess of base rates to be recovered through charges assessed to customers	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%		'	'	'	'	'	'
Public Utilities, Number of Customers	'		'	'	'	2,400	'	'
Loss Contingency, Damages Awarded, Value	'		'	'	'	'	$0.50	$4
[1]	Total does not equal the sum of the subsidiary unit lawsuits because some of the lawsuits name multiple Ameren entities as defendants.

Callaway_Energy_Center_Narrati

Callaway Energy Center (Narrative) (Detail) (USD $)	9 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
	mill
Nuclear Waste Matters [Line Items]	'	'	'	'
Miscellaneous accounts and notes receivable	$117	$196	'	'
Nuclear Plant [Member]	'	'	'	'
Nuclear Waste Matters [Line Items]	'	'	'	'
Number of mills charged for NWF fee	1	'	'	'
Miscellaneous accounts and notes receivable	15	15	'	'
Assumed life of plant, in years	'40 years	'	'	'
Annual decommissioning costs included in costs of service	'	$7	$7	$7
Minimum | Nuclear Plant [Member]	'	'	'	'
Nuclear Waste Matters [Line Items]	'	'	'	'
Number of mills charged for NWF fee	0	'	'	'

Retirement_Benefits_Narrative_

Retirement Benefits (Narrative) (Detail) (USD $)	Sep. 30, 2014
In Millions, unless otherwise specified
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'
Defined benefit plan, estimated future employer contributions in each of the next five years	$340
Minimum	'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'
Defined benefit plan, estimated future employer contributions in each of the next five years	40
Maximum	'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'
Defined benefit plan, estimated future employer contributions in each of the next five years	$110

Retirement_Benefits_Components

Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) (USD $)	3 Months Ended				9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Pension Plan, Defined Benefit [Member]	'		'		'		'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'		'		'		'
Service cost	$20		$23		$60		$69
Interest cost	46		40		137		121
Expected return on plan assets	-58		-54		-172		-162
Prior service cost (benefit)	-1		-1		-1		-3
Actuarial loss	13		23		37		69
Net periodic benefit cost	20		31	[1]	61		94	[1]
Postretirement Benefit Costs [Member]	'		'		'		'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'		'		'		'
Service cost	5		6		14		17
Interest cost	12		11		37		34
Expected return on plan assets	-16		-16		-48		-47
Prior service cost (benefit)	-2		-1		-4		-3
Actuarial loss	-2		2		-5		6
Net periodic benefit cost	-3		2	[1]	-6		7	[1]
Other Postretirement Benefit Plan, Defined Benefit [Member]	'		'		'		'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'		'		'		'
Net periodic benefit cost	-3	[2]	2	[2]	-6	[2]	7	[2]
Other Postretirement Benefit Plan, Defined Benefit [Member] | New Ameren Energy Resources Company, LLC	'		'		'		'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'		'		'		'
Net periodic benefit cost	'		1	[3]	'		1	[3]
Pension Plan, Defined Benefit [Member]	'		'		'		'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'		'		'		'
Net periodic benefit cost	20	[2]	31	[2]	61	[2]	94	[2]
Pension Plan, Defined Benefit [Member] | New Ameren Energy Resources Company, LLC	'		'		'		'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'		'		'		'
Net periodic benefit cost	'		$2		'		$8
[1]	ncludes $2 million and $8 million in total net costs for pension benefits for the three and nine months ended September 30, 2013, respectively, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income. Includes less than $1 million in total net costs for postretirement benefits for both the three and nine months ended September 30, 2013, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income
[2]	Includes amounts for Ameren registrants and nonregistrant subsidiaries.
[3]	Less than $1 million.

Retirement_Benefits_Summary_Of

Retirement Benefits (Summary Of Benefit Plan Costs Incurred) (Detail) (USD $)	3 Months Ended				9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Pension Plan, Defined Benefit [Member]	'		'		'		'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'		'		'		'
Net periodic benefit cost	$20	[1]	$31	[1]	$61	[1]	$94	[1]
Pension Plan, Defined Benefit [Member] | Union Electric Company	'		'		'		'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'		'		'		'
Net periodic benefit cost	13		18		38		54
Pension Plan, Defined Benefit [Member] | Ameren Illinois Company	'		'		'		'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'		'		'		'
Net periodic benefit cost	7		10		22		31
Pension Plan, Defined Benefit [Member] | Other	'		'		'		'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'		'		'		'
Net periodic benefit cost	1	[2]	3	[3]	1		9	[3]
Other Postretirement Benefit Plan, Defined Benefit [Member]	'		'		'		'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'		'		'		'
Net periodic benefit cost	-3	[1]	2	[1]	-6	[1]	7	[1]
Other Postretirement Benefit Plan, Defined Benefit [Member] | Union Electric Company	'		'		'		'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'		'		'		'
Net periodic benefit cost	1	[2]	2		2		7
Other Postretirement Benefit Plan, Defined Benefit [Member] | Ameren Illinois Company	'		'		'		'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'		'		'		'
Net periodic benefit cost	-3		1	[2]	-7		1	[2]
Other Postretirement Benefit Plan, Defined Benefit [Member] | Other	'		'		'		'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'		'		'		'
Net periodic benefit cost	1	[2]	1	[2],[3]	-1		1	[2],[3]
New Ameren Energy Resources Company, LLC | Pension Plan, Defined Benefit [Member]	'		'		'		'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'		'		'		'
Net periodic benefit cost	'		2		'		8
New Ameren Energy Resources Company, LLC | Other Postretirement Benefit Plan, Defined Benefit [Member]	'		'		'		'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'		'		'		'
Net periodic benefit cost	'		$1	[2]	'		$1	[2]
[1]	Includes amounts for Ameren registrants and nonregistrant subsidiaries.
[2]	Less than $1 million.
[3]	Includes $2 million and $8 million in total net costs for pension benefits for the three and nine months ended September 30, 2013, respectively, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income. Includes less than $1 million in total net costs for postretirement benefits for both the three and nine months ended September 30, 2013, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income.

Divestiture_Transactions_and_D2

Divestiture Transactions and Discontinued Operations (Narrative) (Details) (USD $)	9 Months Ended						1 Months Ended	3 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2014	Sep. 30, 2013	Mar. 14, 2013	Sep. 30, 2014	Dec. 31, 2013	Jan. 31, 2014	Dec. 31, 2013	Sep. 30, 2014	Sep. 30, 2014
		New Ameren Energy Resources Company, LLC	New Ameren Energy Resources Company, LLC	New Ameren Energy Resources Company, LLC	Notes Payable, Other Payables [Member]	Notes Payable, Other Payables [Member]	Elgin, Gibson City and Grand Tower Energy Centers	Elgin, Gibson City and Grand Tower Energy Centers	Ameren Energy Marketing Company [Member]	Guarantee Type, Other [Member]
					New Ameren Energy Resources Company, LLC	New Ameren Energy Resources Company, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]	'	'	'	'	'	'	'	'	'	'
Escrow Deposit, Reserve Period	'2 years	'	'	'	'	'	'	'	'	'
Proceeds from Sales of Business, Affiliate and Productive Assets	'	'	'	'	'	'	$168	$137.50	'	'
Proceeds from Asset Sale Held in Escrow	'	'	'	'	'	'	17	'	'	'
Impairment of assets to be disposed of	'	'	175	'	'	'	'	'	'	'
Discontinued operations deferred tax expense	'	'	96	'	'	'	'	'	'	'
Discontinued operations deferred tax benefit	'	'	72	'	'	'	'	'	'	'
Working Capital and Contingent Liability Payment	'	13	'	'	'	'	'	'	'	'
Range of possible loss maximum	'	29	'	'	'	'	'	'	'	'
Notes, Loans and Financing Receivable, Net, Noncurrent	'	'	'	'	23	18	'	'	'	'
Collateral to be posted if credit ratings are below investment grade	'	23	'	'	'	'	'	'	'	'
Buyer's indemnification guarantee obligation	'	'	'	25	'	'	'	'	'	'
Loss contingency accrual	'	29	'	'	'	'	'	'	'	'
Guarantees Outstanding	'	141	'	'	'	'	'	'	132	9
Guarantees, Maximum Exposure	'	'	'	'	'	'	'	'	4	8
Letters of Credit Outstanding, Amount	'	$9	'	'	'	'	'	'	'	'

Divestiture_Transactions_and_D3

Divestiture Transactions and Discontinued Operations (Components of Discontinued Operations in Consolidated Statement of Income) (Details) (USD $)	3 Months Ended		9 Months Ended
In Millions, unless otherwise specified	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]	'	'	'	'
Operating revenues	$0	$311	$1	$878
Operating expenses	-1	-309	-4	-1,034	[1]
Operating income (loss)	-1	2	-3	-156
Other income (loss)	'	'	'	-1
Interest charges	0	-9	'	-31
Loss before income taxes	-1	-7	-3	-188
Income tax (expense) benefit	0	4	'	-24
Loss from discontinued operations, net of taxes	-1	-3	-3	-212
New Ameren Energy Resources Company, LLC	'	'	'	'
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]	'	'	'	'
Impairment of Long-Lived Assets to be Disposed of	'	'	'	$175
[1]	Included a noncash pretax asset impairment charge of $175 million for the nine months ended September 30, 2013, to reduce the carrying value of the New AER disposal group to its estimated fair value less cost to sell.

Divestiture_Transactions_and_D4

Divestiture Transactions and Discontinued Operations (Components of Assets and Liabilities) (Details) (USD $)	Sep. 30, 2014		Dec. 31, 2013		Sep. 30, 2013
In Millions, unless otherwise specified
Assets of discontinued operations	'		'		'
Cash and cash equivalents	'		'		$25
Accounts receivable and unbilled revenue	'		5		'
Materials and supplies	'		5		'
Property and plant, net	'		142		'
Accumulated deferred income taxes, net(a)	15	[1]	13	[1]	'
Total assets of discontinued operations	15		165		'
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		'
Asset retirement obligations	385		369		'
Shutdown Of Meredosia And Hutsonville Energy Centers [Member]	'		'		'
Liabilities of discontinued operations	'		'		'
Asset retirement obligations	$32		$31		'
[1]	(a)Includes 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 September 30, 2014, and December 31, 2013, respectively

Segment_Information_Schedule_O

Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) (USD $)	3 Months Ended			9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013	Sep. 30, 2014		Sep. 30, 2013	Dec. 31, 2013
Segment Reporting Information [Line Items]	'		'	'		'	'
Number of Reportable Segments	'		'	2		'	'
External revenues	$1,670,000,000		$1,638,000,000	$4,683,000,000		$4,516,000,000	'
Net income (loss) attributable to Ameren Corporation from continuing operations	294,000,000		305,000,000	541,000,000		464,000,000	'
Total assets	21,876,000,000		'	21,876,000,000		'	21,042,000,000
Assets of discontinued operations (Note 12)	15,000,000		'	15,000,000		'	165,000,000
Union Electric Company	'		'	'		'	'
Segment Reporting Information [Line Items]	'		'	'		'	'
External revenues	1,089,000,000		1,088,000,000	2,793,000,000		2,760,000,000	'
Intersegment revenues	8,000,000		5,000,000	21,000,000		18,000,000	'
Net income (loss) attributable to Ameren Corporation from continuing operations	222,000,000		238,000,000	395,000,000		362,000,000	'
Total assets	13,179,000,000		'	13,179,000,000		'	12,904,000,000
Ameren Illinois Company	'		'	'		'	'
Segment Reporting Information [Line Items]	'		'	'		'	'
External revenues	572,000,000		547,000,000	1,864,000,000		1,744,000,000	'
Intersegment revenues	'		'	1,000,000		3,000,000	'
Net income (loss) attributable to Ameren Corporation from continuing operations	75,000,000		77,000,000	156,000,000		139,000,000	'
Total assets	7,983,000,000		'	7,983,000,000		'	7,454,000,000
Other	'		'	'		'	'
Segment Reporting Information [Line Items]	'		'	'		'	'
External revenues	9,000,000		3,000,000	26,000,000		12,000,000	'
Intersegment revenues	2,000,000		1,000,000	3,000,000		2,000,000	'
Net income (loss) attributable to Ameren Corporation from continuing operations	-3,000,000		-10,000,000	-10,000,000		-37,000,000	'
Total assets	810,000,000		'	810,000,000		'	752,000,000
Intersegment Eliminations	'		'	'		'	'
Segment Reporting Information [Line Items]	'		'	'		'	'
External revenues	'		'	'		'	'
Intersegment revenues	-10,000,000		-6,000,000	-25,000,000		-23,000,000	'
Net income (loss) attributable to Ameren Corporation from continuing operations	'		'	'		'	'
Total assets	-111,000,000		'	-111,000,000		'	-233,000,000
Consolidated, Continuing Operations	'		'	'		'	'
Segment Reporting Information [Line Items]	'		'	'		'	'
External revenues	1,670,000,000		1,638,000,000	4,683,000,000		4,516,000,000	'
Intersegment revenues	'		'	'		'	'
Net income (loss) attributable to Ameren Corporation from continuing operations	294,000,000		305,000,000	541,000,000		464,000,000	'
Total assets	$21,861,000,000	[1]	'	$21,861,000,000	[1]	'	$20,877,000,000	[1]
[1]	Excludes total assets from discontinued operations of $15 million and $165 million as of September 30, 2014, and December 31, 2013, respectively.