Union Electric (UEPEP) 10-Q 11 Aug 14 2014 Q2 Quarterly report Financial data

Document_and_Entity_Informatio

Document and Entity Information	6 Months Ended
	Jun. 30, 2014
Entity Information [Line Items]	'
Document Type	'10-Q
Amendment Flag	'false
Document Period End Date	30-Jun-14
Document Fiscal Year Focus	'2014
Document Fiscal Period Focus	'Q2
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-Jun-14
Document Fiscal Year Focus	'2014
Document Fiscal Period Focus	'Q2
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-Jun-14
Document Fiscal Year Focus	'2014
Document Fiscal Period Focus	'Q2
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				6 Months Ended
In Millions, except Per Share data, unless otherwise specified	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Operating Revenues:	'		'		'		'
Electric	$1,235		$1,228		$2,341		$2,316
Gas	184		175		672		562
Total operating revenues	1,419		1,403		3,013		2,878
Operating Expenses:	'		'		'		'
Fuel	198		213		402		426
Purchased power	111		121		223		272
Gas purchased for resale	79		72		383		302
Other operations and maintenance	412		447		832		846
Depreciation and amortization	183		178		364		353
Taxes other than income taxes	114		111		241		233
Total operating expenses	1,097		1,142		2,445		2,432
Operating Income	322		261		568		446
Other Income and Expenses:	'		'		'		'
Miscellaneous income	21	[1]	16	[1]	39	[1]	31	[1]
Miscellaneous expense	4	[1]	5	[1]	13	[1]	13	[1]
Total other income (expense)	17		11		26		18
Interest Charges	89		100		181		201
Income Before Income Taxes	250		172		413		263
Income Taxes	99		66		163		101
Income from Continuing Operations	151		106		250		162
Loss from Discontinued Operations, Net of Taxes (Note 12)	-1		-10		-2		-209
Net Income (Loss)	150		96		248		-47
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	3		10		3		10
Comprehensive Income (Loss)	152		101		248		-51
Less: Net Income (Loss) Attributable to Noncontrolling Interests:	'		'		'		'
Net Income from Continuing Operations Attributable to Noncontrolling Interests	1		1		3		3
Net Income (Loss):	'		'		'		'
Continuing Operations	150		105		247		159
Discontinued Operations	-1		-10		-2		-209
Net Income (Loss)	149		95		245		-50
Income (Loss) from Continuing Operations, Per Basic Share	$0.62		$0.44		$1.02		$0.66
Income (Loss) from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Basic Share	($0.01)		($0.05)		($0.01)		($0.87)
Earnings Per Share, Basic	$0.61		$0.39		$1.01		($0.21)
Dividends per Common Share	$0.40		$0.40		$0.80		$0.80
Average Common Shares Outstanding - Basic	242.6		242.6		242.6		242.6
Union Electric Company	'		'		'		'
Operating Revenues:	'		'		'		'
Electric	871		860		1,620		1,592
Gas	28		29		96		93
Other	1		'		1		'
Total operating revenues	900		889		1,717		1,685
Operating Expenses:	'		'		'		'
Fuel	198		213		402		426
Purchased power	28		41		61		67
Gas purchased for resale	11		11		51		48
Other operations and maintenance	222		253		449		474
Depreciation and amortization	117		113		233		224
Taxes other than income taxes	81		79		159		156
Total operating expenses	657		710		1,355		1,395
Operating Income	243		179		362		290
Other Income and Expenses:	'		'		'		'
Miscellaneous income	16		14		30		28
Miscellaneous expense	2		3		6		8
Total other income (expense)	14		11		24		20
Interest Charges	54		56		106		116
Income Before Income Taxes	203		134		280		194
Income Taxes	76		49		105		68
Net Income (Loss)	127		85		175		126
Other Comprehensive Income	0		0		0		0
Comprehensive Income (Loss)	127		85		175		126
Net Income (Loss):	'		'		'		'
Net Income (Loss)	127		85		175		126
Preferred Stock Dividends	1		1		2		2
Net Income Available to Common Stockholder	126		84		173		124
Ameren Illinois Company	'		'		'		'
Operating Revenues:	'		'		'		'
Electric	364		368		717		728
Gas	155		146		576		470
Other	'		2		'		2
Total operating revenues	519		516		1,293		1,200
Operating Expenses:	'		'		'		'
Purchased power	86		80		167		207
Gas purchased for resale	67		61		331		254
Other operations and maintenance	195		196		395		372
Depreciation and amortization	64		62		127		123
Taxes other than income taxes	32		30		78		72
Total operating expenses	444		429		1,098		1,028
Operating Income	75		87		195		172
Other Income and Expenses:	'		'		'		'
Miscellaneous income	5		2		8		3
Miscellaneous expense	1		1		5		4
Total other income (expense)	4		1		3		-1
Interest Charges	29		34		59		65
Income Before Income Taxes	50		54		139		106
Income Taxes	21		22		56		42
Net Income (Loss)	29		32		83		64
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	-1		-1		-2		-2
Comprehensive Income (Loss)	28		31		81		62
Net Income (Loss):	'		'		'		'
Net Income (Loss)	29		32		83		64
Preferred Stock Dividends	1		1		2		2
Net Income Available to Common Stockholder	$28		$31		$81		$62
[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		6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Pension and other postretirement benefit plan activity, tax expense (benefit)	$3	$8	$3	$8
Ameren Illinois Company	'	'	'	'
Pension and other postretirement benefit plan activity, tax expense (benefit)	'	'	($1)	($1)

Consolidated_Statement_of_Comp

Consolidated Statement of Comprehensive Income (Loss) (USD $)	3 Months Ended		6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Income from Continuing Operations	$151	$106	$250	$162
Other Comprehensive Income, Net of Taxes	'	'	'	'
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	3	10	3	10
Comprehensive Income from Continuing Operations	154	116	253	172
Less: Comprehensive Income from Continuing Operations Attributable to Noncontrolling Interests	1	1	3	3
Comprehensive Income from Continuing Operations Attributable to Ameren Corporation	153	115	250	169
Loss from Discontinued Operations, Net of Taxes	-1	-10	-2	-209
Other Comprehensive Loss from Discontinued Operations, Net of Taxes	'	-4	'	-11
Comprehensive Loss from Discontinued Operations Attributable to Ameren Corporation	-1	-14	-2	-220
Comprehensive Income (Loss)	$152	$101	$248	($51)

Consolidated_Statement_of_Comp1

Consolidated Statement of Comprehensive Income (Loss) Consolidated Statement of Income (Parenthetical) (USD $)	3 Months Ended		6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Statement of Comprehensive Income [Abstract]	'	'	'	'
Pension and other postretirement benefit plan activity, tax expense (benefit)	$3	$8	$3	$8

Consolidated_Balance_Sheet

Consolidated Balance Sheet (USD $)	Jun. 30, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Current Assets:	'		'
Cash and cash equivalents	$46		$30
Accounts receivable - trade (less allowance for doubtful accounts)	454		404
Unbilled revenue	299		304
Miscellaneous accounts and notes receivable	213		196
Materials and supplies	491		526
Current regulatory assets	202		156
Current accumulated deferred income taxes, net	177		106
Other current assets	68		85
Assets of discontinued operations (Note 12)	15		165
Total current assets	1,965		1,972
Property and Plant, Net	16,726		16,205
Investments and Other Assets:	'		'
Nuclear decommissioning trust fund	523		494
Goodwill	411		411
Intangible assets	19		22
Regulatory assets	1,213		1,240
Other assets	731		698
Total investments and other assets	2,897		2,865
TOTAL ASSETS	21,588		21,042
Current Liabilities:	'		'
Current maturities of long-term debt	119		534
Short-term Debt	793		368
Accounts and wages payable	575		806
Taxes accrued	132		55
Interest accrued	92		86
Current regulatory liabilities	218		216
Other current liabilities	350		351
Liabilities of discontinued operations (Note 12)	33		45
Total current liabilities	2,312		2,461
Long-term Debt, Net	5,825		5,504
Deferred Credits and Other Liabilities:	'		'
Accumulated deferred income taxes, net	3,526		3,250
Accumulated deferred investment tax credits	60		63
Regulatory liabilities	1,784		1,705
Asset retirement obligations	380		369
Pension and other postretirement benefits	463		466
Other deferred credits and liabilities	524		538
Total deferred credits and other liabilities	6,737		6,391
Commitments and Contingencies	'		'
Stockholders' Equity:	'		'
Common Stock	2		2
Other paid-in capital	5,607		5,632
Retained earnings	957		907
Accumulated other comprehensive income (loss)	6		3
Stockholder's equity	6,572		6,544
Noncontrolling Interest	142	[1]	142
Total equity	6,714		6,686
TOTAL LIABILITIES AND EQUITY	21,588		21,042
Union Electric Company	'		'
Current Assets:	'		'
Cash and cash equivalents	28		1
Accounts receivable - trade (less allowance for doubtful accounts)	217		191
Accounts receivable - affiliates	2		1
Unbilled revenue	214		168
Miscellaneous accounts and notes receivable	81		57
Materials and supplies	352		352
Current regulatory assets	141		118
Other current assets	82		71
Total current assets	1,117		959
Property and Plant, Net	10,599		10,452
Investments and Other Assets:	'		'
Nuclear decommissioning trust fund	523		494
Intangible assets	19		22
Regulatory assets	529		534
Other assets	416		443
Total investments and other assets	1,487		1,493
TOTAL ASSETS	13,203		12,904
Current Liabilities:	'		'
Current maturities of long-term debt	119		109
Borrowings from money pool	61		105
Short-term Debt	185		'
Accounts and wages payable	195		387
Accounts payable - affiliates	16		30
Taxes accrued	157		220
Interest accrued	73		57
Current regulatory liabilities	39		57
Other current liabilities	101		82
Total current liabilities	946		1,047
Long-term Debt, Net	3,885		3,648
Deferred Credits and Other Liabilities:	'		'
Accumulated deferred income taxes, net	2,613		2,524
Accumulated deferred investment tax credits	57		59
Regulatory liabilities	1,099		1,041
Asset retirement obligations	378		366
Pension and other postretirement benefits	172		189
Other deferred credits and liabilities	42		37
Total deferred credits and other liabilities	4,361		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	1,860		1,842
Stockholder's equity	4,011		3,993
TOTAL LIABILITIES AND EQUITY	13,203		12,904
Ameren Illinois Company	'		'
Current Assets:	'		'
Cash and cash equivalents	2		1
Accounts receivable - trade (less allowance for doubtful accounts)	221		201
Unbilled revenue	85		135
Miscellaneous accounts and notes receivable	7		13
Materials and supplies	138		174
Current regulatory assets	61		38
Current accumulated deferred income taxes, net	77		45
Other current assets	14		26
Total current assets	605		633
Property and Plant, Net	5,882		5,589
Investments and Other Assets:	'		'
Goodwill	411		411
Regulatory assets	677		701
Other assets	144		120
Total investments and other assets	1,232		1,232
TOTAL ASSETS	7,719		7,454
Current Liabilities:	'		'
Borrowings from money pool	'		56
Short-term Debt	105		'
Accounts and wages payable	209		243
Accounts payable - affiliates	25		18
Taxes accrued	18		23
Customer deposits	75		79
Current environmental remediation	47		43
Current regulatory liabilities	179		159
Other current liabilities	121		150
Total current liabilities	779		771
Long-term Debt, Net	1,940		1,856
Deferred Credits and Other Liabilities:	'		'
Accumulated deferred income taxes, net	1,205		1,116
Accumulated deferred investment tax credits	4		4
Regulatory liabilities	685		664
Pension and other postretirement benefits	215		197
Environmental remediation	212		232
Other deferred credits and liabilities	152		166
Total deferred credits and other liabilities	2,473		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	491		410
Accumulated other comprehensive income (loss)	9		11
Stockholder's equity	2,527		2,448
TOTAL LIABILITIES AND EQUITY	$7,719		$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.” 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 $)	Jun. 30, 2014	Dec. 31, 2013
In Millions, except Per Share data, unless otherwise specified
Accounts receivable - trade allowance for doubtful accounts	$23	$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	$16	$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 $)	6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014		Jun. 30, 2013
Cash Flows From Operating Activities:	'		'
Net income (loss)	$248		($47)
Loss from discontinued operations, net of taxes	2		209
Adjustments to reconcile net income (loss) to net cash provided by operating activities:	'		'
Depreciation and amortization	349		334
Amortization of nuclear fuel	47		29
Amortization of debt issuance costs and premium/discounts	11		12
Deferred income taxes and investment tax credits, net	178		70
Allowance for equity funds used during construction	-16	[1]	-16	[1]
Stock-based compensation costs	15		14
Other	-8		18
Changes in assets and liabilities:	'		'
Receivables	-62		-92
Materials and supplies	35		77
Accounts and wages payable	-180		-75
Taxes accrued	68		67
Assets, other	-68		49
Liabilities, other	3		9
Pension and other postretirement benefits	21		36
Counterparty collateral, net	15		35
Net cash provided by operating activities – continuing operations	658		729
Net cash provided by (used in) operating activities – discontinued operations	-4		39
Net cash provided by operating activities	654		768
Cash Flows From Investing Activities:	'		'
Capital expenditures	-883		-575
Nuclear fuel expenditures	-26		-25
Purchases of securities - nuclear decommissioning trust fund	-290		-97
Sales and maturities of securities - nuclear decommissioning trust fund	283		89
Proceeds from Notes Receivable	70		'
Contributions to Note Receivable	-78		'
Other	2		2
Net cash used in investing activities – continuing operations	-922		-606
Net cash provided by (used in) investing activities – discontinued operations	152		-31
Net cash used in investing activities	-770		-637
Cash Flows From Financing Activities:	'		'
Dividends on common stock	-194		-194
Dividends paid to noncontrolling interest holders	-3		-3
Short-term debt, net	425		25
Repayments of Other Long-term Debt	-692		'
Proceeds from Issuance of Long-term Debt	598		'
Capital Issuance Costs	-4		'
Advances received for construction	2		7
Net cash provided by (used in) financing activities – continuing operations	132		-165
Net cash used in financing activities – discontinued operations	'		'
Net cash provided by (used in) financing activities	132		-165
Net change in cash and cash equivalents	16		-34
Cash and cash equivalents at beginning of period	30		209
Cash and cash equivalents at end of period	46		175
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	46		150
Union Electric Company	'		'
Cash Flows From Operating Activities:	'		'
Net income (loss)	175		126
Adjustments to reconcile net income (loss) to net cash provided by operating activities:	'		'
Depreciation and amortization	220		208
Amortization of nuclear fuel	47		29
FAC prudence review charge	'		23
Amortization of debt issuance costs and premium/discounts	4		4
Deferred income taxes and investment tax credits, net	61		13
Allowance for equity funds used during construction	-15		-14
Changes in assets and liabilities:	'		'
Receivables	-97		-155
Materials and supplies	'		28
Accounts and wages payable	-163		-119
Taxes accrued	-65		79
Assets, other	-5		61
Liabilities, other	39		37
Pension and other postretirement benefits	11		18
Net cash provided by operating activities	212		338
Cash Flows From Investing Activities:	'		'
Capital expenditures	-375		-273
Nuclear fuel expenditures	-26		-25
Money pool advances, net	'		24
Purchases of securities - nuclear decommissioning trust fund	-290		-97
Sales and maturities of securities - nuclear decommissioning trust fund	283		89
Other	-5		-3
Net cash used in investing activities	-413		-285
Cash Flows From Financing Activities:	'		'
Dividends on common stock	-155		-180
Dividends on preferred stock	-2		-2
Money pool borrowings, net	-44		'
Redemptions of Long-term Debt	-104		'
Short-term debt, net	185		'
Proceeds from Issuance of Long-term Debt	350		'
Capital Issuance Costs	-2		'
Net cash provided by (used in) financing activities	228		-182
Net change in cash and cash equivalents	27		-129
Cash and cash equivalents at beginning of period	1		148
Cash and cash equivalents at end of period	28		19
Ameren Illinois Company	'		'
Cash Flows From Operating Activities:	'		'
Net income (loss)	83		64
Adjustments to reconcile net income (loss) to net cash provided by operating activities:	'		'
Depreciation and amortization	125		121
Amortization of debt issuance costs and premium/discounts	6		7
Deferred income taxes and investment tax credits, net	58		61
Other	-4		-4
Changes in assets and liabilities:	'		'
Receivables	36		62
Materials and supplies	36		50
Accounts and wages payable	2		46
Taxes accrued	-5		-6
Assets, other	-61		-4
Liabilities, other	3		-18
Pension and other postretirement benefits	7		15
Counterparty collateral, net	15		32
Net cash provided by operating activities	301		426
Cash Flows From Investing Activities:	'		'
Capital expenditures	-436		-283
Other	4		4
Net cash used in investing activities	-432		-279
Cash Flows From Financing Activities:	'		'
Dividends on common stock	'		-30
Dividends on preferred stock	-2		-2
Money pool borrowings, net	-56		-24
Redemptions of Long-term Debt	-163		'
Short-term debt, net	105		'
Proceeds from Issuance of Long-term Debt	248		'
Capital Issuance Costs	-2		'
Advances received for construction	2		7
Net cash provided by (used in) financing activities	132		-49
Net change in cash and cash equivalents	1		98
Cash and cash equivalents at beginning of period	1		'
Cash and cash equivalents at end of period	$2		$98
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Summary_Of_Significant_Account

Summary Of Significant Accounting Policies	6 Months Ended
	Jun. 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 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 developing 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
	There were no material differences between Ameren’s basic and diluted earnings per share amounts for the three and six months ended June 30, 2014, and 2013, caused by the assumed settlement of performance share units. The number of dilutive performance share units had an immaterial impact on earnings per share.
	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 June 30, 2014, and changes during the six months ended June 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)	683,591		38.9
	April Grants(b)	38,559		50.34
	Forfeitures	(65,847	)	33.82
	Vested(c)	(116,297	)	38.81
	Nonvested at June 30, 2014	1,758,550		$	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.
	(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. We evaluate intangible assets for impairment if events or changes in circumstances indicate that their carrying amount might be impaired.
	At June 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 value of Ameren’s and Ameren Missouri’s renewable energy credits was each $19 million at June 30, 2014. The book value of Ameren’s and Ameren Missouri’s renewable energy credits was each $22 million at December 31, 2013.
	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 six months ended June 30, 2014, and 2013:
			Three Months							Six Months
		2014				2013				2014			2013
	Ameren Missouri	$	—			$	—			$	6		$	(a)
	Ameren Illinois		3				3				6			7
	Ameren	$	3			$	3			$	12		$	7
	(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 six months ended June 30, 2014, and 2013:
		Three Months								Six Months
		2014				2013				2014			2013
	Ameren Missouri	$	39			$	38			$	73		$	71
	Ameren Illinois	11				11				37			33
	Ameren	$	50			$	49			$	110		$	104
	Uncertain Tax Positions
	The following table presents the amount of unrecognized tax benefits (detriments) related to uncertain tax positions as of June 30, 2014, and December 31, 2013:
		30-Jun-14				December 31,
						2013
	Ameren	$	94			$	90
	Ameren Missouri	34				31
	Ameren Illinois	—				(1		)
	With the adoption of new accounting guidance in the first quarter of 2014, unrecognized tax benefits are recorded in “Accumulated deferred income taxes, net” as a reduction to the deferred tax assets for net operating loss and tax credit carryforwards on Ameren’s, Ameren Missouri’s and Ameren Illinois’ respective balance sheets. Unrecognized tax benefits that exceed these carryforwards are recorded in “Other deferred credits and liabilities” on the respective balance sheets. At June 30, 2014, unrecognized tax benefits of $86 million, $13 million, and $- million were recorded in “Accumulated deferred income taxes, net” on Ameren's, Ameren Missouri's and Ameren Illinois' balance sheets, respectively. At December 31, 2013, unrecognized tax benefits of $84 million, $15 million, and $- million previously recorded in “Other deferred credits and liabilities” on the 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 amount of unrecognized tax benefits (detriments) related to uncertain tax positions as of June 30, 2014, and December 31, 2013, that would impact the effective tax rate, if recognized:
		30-Jun-14				December 31,
						2013
	Ameren	$	55			$	54
	Ameren Missouri	3				3
	Ameren Illinois	(1		)		—
	Ameren’s federal income tax returns for the years 2007 through 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 2007 through 2011. This settlement, which is primarily related to uncertain tax positions for research tax deductions, is expected to result in a decrease in uncertain tax benefits of $20 million and $13 million for Ameren and Ameren Missouri, respectively, none of which would impact their respective effective tax rates. In addition, it is reasonably possible that other events will occur during the next 12 months that would cause the total amount of unrecognized tax benefits for the Ameren Companies to fluctuate. However, the Ameren Companies do not believe any such fluctuations, including the decrease from the reasonably possible IRS Appeals Office settlement discussed above, would be material to their 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. The Ameren Companies 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 June 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 June 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 six months ended June 30, 2014, and 2013, are shown below:
	Three Months								Six 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	1				1				3			3
Dividends paid to noncontrolling interest holders	(1		)		(1		)		(3		)	(3		)
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.” 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 in “Accumulated deferred income taxes, net” as a reduction to the deferred tax assets for net operating loss and tax credit carryforwards on the respective 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	6 Months Ended
	Jun. 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
	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 anticipated 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 to meet more stringent environmental regulations, the replacement of the nuclear reactor vessel head at the Callaway energy center in order to ensure continued safe and dependable operations, two new substations in St. Louis, and the O’Fallon energy center, which will be Missouri’s largest investor-owned utility solar facility, 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 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 anticipated 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 changes 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.
	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, MIEC and 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.
	Earnings Complaint and Rate Shift Complaint Cases
	In February 2014, Ameren Missouri’s largest customer, Noranda, and 37 residential customers filed an earnings complaint case and a rate shift complaint case with 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. Noranda and the residential customers are currently requesting the MoPSC approve a $49 million reduction to Ameren Missouri’s annual revenue requirement. Included in Noranda’s request is a reduction of Ameren Missouri’s authorized return on common equity to 9.4%. The MoPSC staff filed testimony in this case that recommended no reduction to Ameren Missouri’s annual revenue requirement. The MoOPC and MIEC intervened in the earnings complaint case. The rate shift complaint case seeks to reduce Noranda's electric rates with an offsetting increase in electric rates for Ameren Missouri's other customers. While the rate shift proposal is revenue neutral to Ameren Missouri, Ameren Missouri does not believe that the proposed reduction to Noranda's electric rates, which would result in rates that are significantly below Ameren Missouri's cost of service, is appropriate or in the best interests of Ameren Missouri's other electric customers.
	While the MoPSC has no time requirement by which it must issue orders in these cases, it has adopted procedural schedules that Ameren Missouri expects would render a decision in the rate shift case during the third quarter of 2014, and in the earnings complaint case by September 26, 2014. Ameren Missouri does not believe that a reduction in electric service rates is justified and filed testimony that supports that position, which is consistent with Ameren Missouri’s July 2014 electric rate case filing.
	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 June 30, 2014, Ameren Illinois had recorded a regulatory asset of $42 million and $64 million, respectively, to reflect its expected 2014 and 2013 revenue requirement reconciliation adjustments, with interest. As of June 30, 2014, Ameren Illinois had recorded a regulatory liability of $35 million to reflect its 2012 revenue requirement reconciliation adjustment, with interest, which will be refunded to customers during 2014.
	In September 2012 and December 2012, the ICC issued orders in Ameren Illinois’ IEIMA performance-based formula rate filings. Ameren Illinois appealed both orders to the Appellate Court of the Fourth District of Illinois. The primary issues Ameren Illinois appealed were the rate treatment of accumulated deferred income taxes and vacation obligations as well as the calculation of Ameren Illinois’ capital structure. In December 2013, the appellate court rendered its decision upholding the ICC’s September and December 2012 orders. Ameren Illinois filed an appeal to the Illinois Supreme Court in March 2014. In May 2014, the Illinois Supreme Court denied Ameren Illinois’ appeal.
	In December 2013, the ICC issued an order in Ameren Illinois' annual 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 calculation of its capital structure and the rate treatment of accumulated deferred income taxes related to the transfer of former Ameren Missouri assets in Illinois to Ameren Illinois. Ameren Illinois will not pursue the calculation of its capital structure in its appeal as a result of the Illinois Supreme Court ruling discussed above in May 2014. Ameren Illinois will continue its appeal of the rate treatment of accumulated deferred income taxes.
	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 July 2014, the ICC staff submitted its calculation of the revenue requirement included in Ameren Illinois’ update filling. The ICC staff recommended adjustments that would result in a $202 million increase in Ameren Illinois’ electric delivery service revenue requirement. An 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 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 is also appealing the rate treatment of accumulated deferred income taxes related to the transfer of former Ameren Missouri assets 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.
	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. In March 2011, FERC issued an order authorizing the proposed rates to take effect, subject to refund when the final rates are determined. Ameren Illinois has reached settlements with four of its nine wholesale customers, which have been approved by FERC and for which refunds have been issued. The impasse with the remaining five wholesale customers is awaiting final FERC action. In November 2012, a FERC administrative law judge issued an initial decision, which is now pending before FERC. The timing of a decision from FERC is uncertain and subsequent appeals are possible. In accordance with the administrative law judge's initial decision, Ameren and Ameren Illinois have both included on their respective balance sheets in “Current regulatory liabilities” an estimate of $16 million and $13 million as of June 30, 2014, and December 31, 2013, respectively, for the refund due to the remaining wholesale customers relating to billings since March 2011.
	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's 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. During the three months ended June 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 return on common equity to 9.15%, as well as a limit on the common equity ratio, under the MISO tariff. Currently, the FERC-allowed return on common equity for MISO transmission owners is 12.38%. This complaint case could result in a reduction to Ameren Illinois' and ATXI's allowed return on common equity. That reduction could also result in a refund for transmission service revenues earned after the filing of the complaint case in November 2013. FERC has not issued an order in this case, and it is under no deadline to do so.
	In June 2014, FERC issued an order that reduced the base allowed return on common equity for New England transmission owners from 11.14% to 10.57% with rate incentives allowed up to 11.74%. The FERC order in the New England transmission owners’ case applied observable market data from October 2012 to March 2013 to determine the allowed return on common equity. Ameren believes some aspects of the FERC order in the New England transmission owners’ case may establish precedent in the pending MISO case. However, the calculation FERC used to establish the base allowed return on common equity, which is based on a unique time period for each complaint case, required multiple inputs based on observable market data specific to the utility industry and broader macroeconomic data, which are highly uncertain. Due to the wide range of potential outcomes and significant uncertainty regarding the inputs required in FERC’s calculation, the Ameren Companies cannot reasonably estimate the impact, if any, that a FERC ruling in the MISO complaint case could have on their allowed base return on common equity.
	If FERC lowered MISO’s allowed base return on equity 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 $9 million and $7 million, respectively, from the filing of the complaint case in November 2013 through June 30, 2014. The estimated ongoing annual reduction in revenues if the MISO return on equity was 10.57% for Ameren and Ameren Illinois would be $16 million and $12 million, respectively. Ameren Missouri would not expect a reduction of its allowed base return on common equity to result in a material impact to its financial statements. If Ameren and Ameren Illinois were to determine that a refund to their electric transmission customers is probable and can 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 orders 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 any expense is not probable, and therefore no liability has been recorded as of June 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 June 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 the costs incurred will be disallowed in rates, a charge to earnings would be recognized in the period in which that determination is made.
	Pumped-storage Hydroelectric Energy Center Relicensing
	In June 2008, Ameren Missouri filed a relicensing application with FERC to operate its Taum Sauk pumped-storage hydroelectric energy center. The existing FERC license expired on June 30, 2010. In July 2010, Ameren Missouri received a license extension that allowed Taum Sauk to continue operations until FERC issued a new license. In July 2014, FERC issued an order authorizing Ameren Missouri to operate its Taum Sauk pumped-storage hydroelectric energy center for an additional 30 years through July 2044.

ShortTerm_Debt_And_Liquidity

Short-Term Debt And Liquidity	6 Months Ended
	Jun. 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 six months ended June 30, 2014, but they were used to support commercial paper issuances and to issue letters of credit. As of June 30, 2014, 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 June 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 June 30, 2014, and December 31, 2013. Ameren Illinois established a commercial paper program in May 2014.
		30-Jun-14				31-Dec-13
	Ameren (parent)	$	503			$	368
	Ameren Missouri	185				—
	Ameren Illinois	105				—
	Ameren Consolidated	$	793			$	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 six months ended June 30, 2014, and 2013:
			Ameren (parent)				Ameren Missouri			Ameren Illinois			Ameren Consolidated
	2014
	Average daily commercial paper outstanding		$	328			$	146		$	242		$	607
	Weighted-average interest rate		0.32		%		0.31		%	0.32		%	0.32		%
	Peak commercial paper during period(a)		$	503			$	495		$	300		$	907
	Peak interest rate		0.35		%		0.7		%	0.34		%	0.7		%
	2013
	Average daily commercial paper outstanding		$	13			$	—		$	—		$	13
	Weighted-average interest rate		0.54		%		—		%	—		%	0.54		%
	Peak commercial paper during period(a)		$	78			$	—		$	—		$	78
	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 presents 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 June 30, 2014, the ratios of consolidated indebtedness to total consolidated capitalization, calculated in accordance with the provisions of the 2012 Credit Agreements, were 50%, 50% and 45%, 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 June 30, 2014, was 6.0 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 June 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 money pools only as a lender. Surplus internal funds are contributed to the money pool from participants. The primary sources of external funds for the money pool are the 2012 Credit Agreements and the commercial paper programs. The total amount available to the pool participants from the 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 money pool or remit funds from other external sources. The availability of funds is also determined by funding requirement limits established by regulatory authorizations. Participants receiving a loan under the money pool agreement must repay the principal amount of such loan, together with accrued interest. The rate of interest depends on the composition of internal and external funds in the money pool. The average interest rate for borrowing under the utility money pool for the three and six months ended June 30, 2014, was 0.19% and 0.29%, respectively (2013 - 0.07% and 0.09%, 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 six months ended June 30, 2014, and 2013.

LongTerm_Debt_And_Equity_Finan

Long-Term Debt And Equity Financings

6 Months Ended

Jun. 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 June 30, 2014, at an assumed annual interest rate of 6% and dividend rate of 7%.

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

4.7

$

3,168

≥2.5

130.8

$

2,508

Ameren Illinois

≥2.0

6.7

3,780

(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 June 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 June 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	6 Months Ended
	Jun. 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 (loss) for the three and six months ended June 30, 2014, and 2013:
		Three Months				Six Months
		2014		2013		2014		2013
	Ameren:(a)
	Miscellaneous income:
	Allowance for equity funds used during construction	$	9	$	8	$	16	$	16
	Interest income on industrial development revenue bonds	7		7		14		14
	Interest income	2		1		5		1
	Other	3		—		4		—
	Total miscellaneous income	$	21	$	16	$	39	$	31
	Miscellaneous expense:
	Donations	$	1	$	1	$	6	$	5
	Other	3		4		7		8
	Total miscellaneous expense	$	4	$	5	$	13	$	13
	Ameren Missouri:
	Miscellaneous income:
	Allowance for equity funds used during construction	$	8	$	7	$	15	$	14
	Interest income on industrial development revenue bonds	7		7		14		14
	Interest income	1		—		1		—
	Total miscellaneous income	$	16	$	14	$	30	$	28
	Miscellaneous expense:
	Donations	$	1	$	1	$	3	$	3
	Other	1		2		3		5
	Total miscellaneous expense	$	2	$	3	$	6	$	8
	Ameren Illinois:
	Miscellaneous income:
	Allowance for equity funds used during construction	$	1	$	1	$	1	$	2
	Interest income	1		1		3		1
	Other	3		—		4		—
	Total miscellaneous income	$	5	$	2	$	8	$	3
	Miscellaneous expense:
	Donations	$	—	$	—	$	3	$	3
	Other	1		1		2		1
	Total miscellaneous expense	$	1	$	1	$	5	$	4
	(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Derivative_Financial_Instrumen

Derivative Financial Instruments	6 Months Ended
	Jun. 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 June 30, 2014, and December 31, 2013. As of June 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)	51		(b)		51		66		(b)		66
	Natural gas (in mmbtu)	25		101		126		28		108		136
	Power (in megawatthours)	1		11		12		3		11		14
	Uranium (pounds in thousands)	627		(b)		627		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 June 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 June 30, 2014, and December 31, 2013:
		Balance Sheet Location		Ameren  Missouri				Ameren  Illinois				Ameren
	2014
	Fuel oils	Other current assets		$	5			$	—			$	5
		Other assets		2				—				2
	Natural gas	Other current assets		1				3				4
		Other assets		—				1				1
	Power	Other current assets		22				—				22
		Total assets		$	30			$	4			$	34
	Fuel oils	Other current liabilities		$	2			$	—			$	2
		Other deferred credits and liabilities		1				—				1
	Natural gas	Other current liabilities		4				16				20
		Other deferred credits and liabilities		2				8				10
	Power	Other current liabilities		6				7				13
		Other deferred credits and liabilities		—				96				96
	Uranium	Other current liabilities		5				—				5
		Other deferred credits and liabilities		2				—				2
		Total liabilities		$	22			$	127			$	149
	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 June 30, 2014, and December 31, 2013:
		Ameren				Ameren				Ameren
		Missouri				Illinois
	2014
	Fuel oils derivative contracts(a)	$	3			$	—			$	3
	Natural gas derivative contracts(b)	(5		)		(20		)		(25		)
	Power derivative contracts(c)	16				(103		)		(87		)
	Uranium derivative contracts(d)	(7		)		—				(7		)
	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 gains on fuel oils derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s transportation costs for coal through December 2017, as of June 30, 2014. Current gains deferred as regulatory liabilities include $2 million and $2 million at Ameren and Ameren Missouri, respectively, as of June 30, 2014. Current losses deferred as regulatory assets include $1 million and $1 million at Ameren and Ameren Missouri, respectively, as of June 30, 2014.
(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 2017, at Ameren Illinois, in each case as of June 30, 2014. Current gains deferred as regulatory liabilities include $4 million, $1 million, and $3 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of June 30, 2014. Current losses deferred as regulatory assets include $20 million, $4 million, and $16 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of June 30, 2014.
(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, in each case as of June 30, 2014. Current gains deferred as regulatory liabilities include $22 million and $22 million at Ameren and Ameren Missouri, respectively, as of June 30, 2014. Current losses deferred as regulatory assets include $13 million, $6 million, and $7 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of June 30, 2014.
(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, as of June 30, 2014. Current losses deferred as regulatory assets include $5 million and $5 million at Ameren and Ameren Missouri, respectively, as of June 30, 2014.
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 June 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		$	30			$	9			$	—		$	21
Ameren Illinois		4				3				—			1
Ameren		$	34			$	12			$	—		$	22
Liabilities:
Ameren Missouri		$	22			$	9			$	10		$	3
Ameren Illinois		127				3				—			124
Ameren		$	149			$	12			$	10		$	127
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 June 30, 2014, if counterparty groups were to fail completely to perform on contracts, Ameren, Ameren Missouri and Ameren Illinois' maximum exposure was $20 million, $11 million, and $9 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 June 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 $13 million, $7 million, and $6 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 June 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 June 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	$	60			$	2			$	53
Ameren Illinois	65				—				59
Ameren	$	125			$	2			$	112
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	6 Months Ended
	Jun. 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 June 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	$	5		$	(3	)	Option model	Volatilities(%)(b)	May-34	15
									Discounted cash flow	Counterparty credit risk(%)(c)(d)	0.25 - 0.91	0.63
		Power(e)	21			(109		)	Discounted cash flow	Average forward peak and off-peak pricing - forwards/swaps($/MWh)(c)	22 - 60	36
										Estimated auction price for FTRs($/MW)(b)	(1,716) - 2,024	443
										Nodal basis($/MWh)(c)	(6) - 0	-3
										Counterparty credit risk(%)(c)(d)	0.25	(f)
										Ameren Missouri and Ameren Illinois credit risk(%)(c)(d)	0.43	(f)
									Fundamental energy production model	Estimated future gas prices($/mmbtu)(b)	6-May	5
										Escalation rate(%)(b)(g)	3-Feb	3
									Contract price allocation	Estimated renewable energy credit costs($/credit)(b)	7-May	6
		Uranium	—			(7		)	Discounted cash flow	Average forward uranium pricing($/pound)(b)	28 - 33	29
	Ameren Missouri	Fuel oils	$	5		$	(3	)	Option model	Volatilities(%)(b)	May-34	15
									Discounted cash flow	Counterparty credit risk(%)(c)(d)	0.25 - 0.91	0.63
		Power(e)	21			(6		)	Discounted cash flow	Average forward peak and off-peak pricing - forwards/swaps($/MWh)(c)	22 - 60	48
										Estimated auction price for FTRs($/MW)(b)	(1,716) - 2,024	443
										Nodal basis($/MWh)(c)	(3) - (1)	-2
										Counterparty credit risk(%)(c)(d)	0.25	(f)
										Ameren Missouri credit risk(%)(c)(d)	0.43	(f)
		Uranium	—			(7		)	Discounted cash flow	Average forward uranium pricing($/pound)(b)	28 - 33	29
	Ameren Illinois	Power(e)	$	—		$	(103	)	Discounted cash flow	Average forward peak and off-peak pricing - forwards/swaps($/MWh)(b)	28 - 46	33
										Nodal basis($/MWh)(b)	(6) - 0	-3
										Ameren Illinois credit risk(%)(c)(d)	0.43	(f)
									Fundamental energy production model	Estimated future gas prices($/mmbtu)(b)	6-May	5
										Escalation rate(%)(b)(g)	3-Feb	3
									Contract price allocation	Estimated renewable energy credit costs($/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 2018. Valuations beyond 2018 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.
	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 six months of 2014 or 2013. At June 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 June 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		$	2			$	—			$	5			$	7
	Natural gas		—				5				—				5
	Power		—				1				21				22
	Total derivative assets - commodity contracts		$	2			$	6			$	26			$	34
	Nuclear decommissioning trust fund:
	Cash and cash equivalents		$	2			$	—			$	—			$	2
	Equity securities:
	U.S. large capitalization		344				—				—				344
	Debt securities:
	Corporate bonds		—				57				—				57
	Municipal bonds		—				2				—				2
	U.S. treasury and agency securities		—				98				—				98
	Asset-backed securities		—				12				—				12
	Other		—				6				—				6
	Total nuclear decommissioning trust fund		$	346			$	175			$	—			$	521		(b)
	Total Ameren		$	348			$	181			$	26			$	555
Ameren	Derivative assets - commodity contracts(a):
Missouri	Fuel oils		$	2			$	—			$	5			$	7
	Natural gas		—				1				—				1
	Power		—				1				21				22
	Total derivative assets - commodity contracts		$	2			$	2			$	26			$	30
	Nuclear decommissioning trust fund:
	Cash and cash equivalents		$	2			$	—			$	—			$	2
	Equity securities:
	U.S. large capitalization		344				—				—				344
	Debt securities:
	Corporate bonds		—				57				—				57
	Municipal bonds		—				2				—				2
	U.S. treasury and agency securities		—				98				—				98
	Asset-backed securities		—				12				—				12
	Other		—				6				—				6
	Total nuclear decommissioning trust fund		$	346			$	175			$	—			$	521		(b)
	Total Ameren Missouri		$	348			$	177			$	26			$	551
Ameren	Derivative assets - commodity contracts(a):
Illinois	Natural gas		$	—			$	4			$	—			$	4
Liabilities:
Ameren	Derivative liabilities - commodity contracts(a):
	Fuel oils		$	—			$	—			$	3			$	3
	Natural gas		2				28				—				30
	Power		—				—				109				109
	Uranium		—				—				7				7
	Total Ameren		$	2			$	28			$	119			$	149
Ameren	Derivative liabilities - commodity contracts(a):
Missouri	Fuel oils		$	—			$	—			$	3			$	3
	Natural gas		2				4				—				6
	Power		—				—				6				6
	Uranium		—				—				7				7
	Total Ameren Missouri		$	2			$	4			$	16			$	22
Ameren	Derivative liabilities - commodity contracts(a):
Illinois	Natural gas		$	—			$	24			$	—			$	24
	Power		—				—				103				103
	Total Ameren Illinois		$	—			$	24			$	103			$	127
(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 June 30, 2014:
		Net derivative commodity contracts
Three Months		Ameren			Ameren			Ameren
		Missouri			Illinois
Fuel oils:
Beginning balance at April 1, 2014	$	1		$	(a)		$	1
Realized and unrealized gains (losses) included in regulatory assets/liabilities		1			(a)			1
Ending balance at June 30, 2014	$	2		$	(a)		$	2
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2014	$	1		$	(a)		$	1
Natural gas:
Beginning balance at April 1, 2014	$	—		$	—		$	—
Purchases		—			1			1
Settlements		—			(1	)		(1	)
Ending balance at June 30, 2014	$	—		$	—		$	—
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2014	$	—		$	—		$	—
Power:
Beginning balance at April 1, 2014	$	10		$	(120	)	$	(110	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities		(13	)		16			3
Purchases		34			—			34
Settlements		(15	)		1			(14	)
Transfers out of Level 3		(1	)		—			(1	)
Ending balance at June 30, 2014	$	15		$	(103	)	$	(88	)
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2014	$	(1	)	$	15		$	14
Uranium:
Beginning balance at April 1, 2014	$	(5	)	$	(a)		$	(5	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities		(4	)		(a)			(4	)
Settlements		2			(a)			2
Ending balance at June 30, 2014	$	(7	)	$	(a)		$	(7	)
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2014	$	(4	)	$	(a)		$	(4	)
(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 June 30, 2013:
		Net derivative commodity contracts
Three Months		Ameren			Ameren			Ameren
		Missouri			Illinois
Fuel oils:
Beginning balance at April 1, 2013	$	5		$	(a)		$	5
Realized and unrealized gains (losses) included in regulatory assets/liabilities		(2	)		(a)			(2	)
Ending balance at June 30, 2013	$	3		$	(a)		$	3
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2013	$	(1	)	$	(a)		$	(1	)
Natural gas:
Beginning balance at April 1, 2013	$	—		$	2		$	2
Purchases		(1	)		—			(1	)
Ending balance at June 30, 2013	$	(1	)	$	2		$	1
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2013	$	(1	)	$	—		$	(1	)
Power:
Beginning balance at April 1, 2013	$	2		$	(81	)	$	(79	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities		1			1			2
Purchases		40			—			40
Settlements		(9	)		—			(9	)
Transfers out of Level 3		3			—			3
Ending balance at June 30, 2013	$	37		$	(80	)	$	(43	)
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2013	$	3		$	(4	)	$	(1	)
Uranium:
Beginning balance at April 1, 2013	$	(2	)	$	(a)		$	(2	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities		(2	)		(a)			(2	)
Settlements		1			(a)			1
Ending balance at June 30, 2013	$	(3	)	$	(a)		$	(3	)
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2013	$	(1	)	$	(a)		$	(1	)
(a)	Not applicable.
The following table summarizes the changes in the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy for the six months ended June 30, 2014:
		Net derivative commodity contracts
Six 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		(1	)		(a)			(1	)
Settlements		(2	)		(a)			(2	)
Ending balance at June 30, 2014	$	2		$	(a)		$	2
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2014	$	—		$	(a)		$	—
Natural gas:
Beginning balance at January 1, 2014	$	—		$	—		$	—
Purchases		—			(1	)		(1	)
Settlements		—			1			1
Ending balance at June 30, 2014	$	—		$	—		$	—
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2014	$	—		$	—		$	—
Power:
Beginning balance at January 1, 2014	$	19		$	(108	)	$	(89	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities		(18	)		4			(14	)
Purchases		34			—			34
Settlements		(20	)		1			(19	)
Ending balance at June 30, 2014	$	15		$	(103	)	$	(88	)
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2014	$	(3	)	$	1		$	(2	)
Uranium:
Beginning balance at January 1, 2014	$	(6	)	$	(a)		$	(6	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities		(4	)		(a)			(4	)
Settlements		3			(a)			3
Ending balance at June 30, 2014	$	(7	)	$	(a)		$	(7	)
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2014	$	(4	)	$	(a)		$	(4	)
(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 six months ended June 30, 2013:
		Net derivative commodity contracts
Six 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		(2	)		(a)			(2	)
Purchases		1			(a)			1
Settlements		(1	)		(a)			(1	)
Ending balance at June 30, 2013	$	3		$	(a)		$	3
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2013	$	(1	)	$	(a)		$	(1	)
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 June 30, 2013	$	(1	)	$	2		$	1
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2013	$	—		$	—		$	—
Power:
Beginning balance at January 1, 2013	$	11		$	(111	)	$	(100	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities		6			15			21
Purchases		40			—			40
Settlements		(22	)		16			(6	)
Transfers into Level 3		(2	)		—			(2	)
Transfers out of Level 3		4			—			4
Ending balance at June 30, 2013	$	37		$	(80	)	$	(43	)
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2013	$	—		$	15		$	15
Uranium:
Beginning balance at January 1, 2013	$	(2	)	$	(a)		$	(2	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities		(2	)		(a)			(2	)
Settlements		1			(a)			1
Ending balance at June 30, 2013	$	(3	)	$	(a)		$	(3	)
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2013	$	(1	)	$	(a)		$	(1	)
(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 six months ended June 30, 2014, and 2013, there were no transfers between Level 1 and Level 2 related to derivative commodity contracts. The following table summarizes all transfers between fair value hierarchy levels related to derivative commodity contracts for the three and six months ended June 30, 2014, and 2013:
	2014									2013
	Ameren Missouri			Ameren Illinois			Ameren			Ameren Missouri			Ameren Illinois			Ameren
Three Months
Transfers out of Level 3 / Transfers into Level 2 - Power	$	(1	)	$	—		$	(1	)	$	3		$	—		$	3
Six Months
Transfers into Level 3 / Transfers out of Level 2 - Power	$	—		$	—		$	—		$	(2	)	$	—		$	(2	)
Transfers out of Level 3 / Transfers into Level 2 - Power	—			—			—			4			—			4
Net fair value of Level 3 transfers	$	—		$	—		$	—		$	2		$	—		$	2
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 June 30, 2014, and December 31, 2013:
	30-Jun-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			$	6,648			$	6,038			$	6,584
Preferred stock	142				120				142				118
Ameren Missouri:
Long-term debt and capital lease obligations (including current portion)	$	4,004			$	4,464			$	3,757			$	4,124
Preferred stock	80				72				80				71
Ameren Illinois:
Long-term debt	$	1,940			$	2,184			$	1,856			$	2,028
Preferred stock	62				48				62				47
(a)	Preferred stock is recorded in “Noncontrolling Interests” on the consolidated balance sheet.

Related_Party_Transactions

Related Party Transactions	6 Months Ended
	Jun. 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.
	The following table presents the impact on Ameren Missouri and Ameren Illinois of related party transactions for the three and six months ended June 30, 2014, and 2013.
					Three Months				Six Months
	Agreement	Income Statement			Ameren		Ameren		Ameren		Ameren
		Line Item			Missouri		Illinois		Missouri		Illinois
	Ameren Missouri power supply	Operating Revenues	2014	$	3	$	(a)	$	3	$	(a)
	agreements with Ameren Illinois		2013		(b)		(a)		1		(a)
	Ameren Missouri and Ameren Illinois	Operating Revenues	2014		4		1		9		1
	rent and facility services		2013		5		(b)		11		1
	Ameren Missouri and Ameren Illinois	Operating Revenues	2014		1		(b)		1		(b)
	miscellaneous support services		2013		(b)		2		(b)		2
	Total Operating Revenues		2014	$	8	$	1	$	13	$	1
			2013		5		2		12		3
	Ameren Illinois power supply	Purchased Power	2014	$	(a)	$	3	$	(a)	$	3
	agreements with Ameren Missouri		2013		(a)		(b)		(a)		1
	Ameren Illinois transmission	Purchased Power	2014		(a)		(b)		(a)		1
	services with ATXI		2013		(a)		(b)		(a)		1
	Total Purchased Power		2014	$	(a)	$	3	$	(a)	$	4
			2013		(a)		(b)		(a)		2
	Ameren Services support services	Other Operations and Maintenance	2014	$	32	$	27	$	65	$	54
	agreement		2013		28		24		60		48
	Insurance premiums(c)	Other Operations and Maintenance	2014		(b)		(a)		(b)		(a)
			2013		(b)		(a)		(b)		(a)
	Total Other Operations and		2014	$	32	$	27	$	65	$	54
	Maintenance Expenses		2013		28		24		60		48
	Money pool borrowings (advances)	Interest Charges	2014	$	(b)	$	(b)	$	(b)	$	(b)
			2013		—		(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, property damage, and terrorism coverage.

Commitments_And_Contingencies

Commitments And Contingencies	6 Months Ended
	Jun. 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 June 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 $900,000 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. For a complete listing of our obligations and commitments, see Note 15 - Commitments and Contingencies under Part II, Item 8 of the Form 10-K.
	At June 30, 2014, total other obligations related to commitments for coal, natural gas, nuclear fuel, purchased power, methane gas, equipment, energy efficiency program expenditures and meter reading services, among other agreements, at Ameren, Ameren Missouri and Ameren Illinois were $6,068 million, $4,200 million, and $1,810 million, respectively.
	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 particularly 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 new and existing energy centers through the proposed Clean Power Plan; 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 our 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 these future regulations are unknown, the combined effects of the new and proposed environmental regulations would 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 our 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 June 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 expenditure investments 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 environmental compliance plan for 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. Ameren Missouri has not finalized a compliance plan for the proposed Clean Power Plan.
	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 law 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. Ameren and Ameren Missouri are continuing to review the United States Supreme Court’s decision and expect the EPA to issue CSAPR implementation guidance in the near future. 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. Assuming the EPA does not revise the emission reductions previously included in the CSAPR, 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, emission compliance can be achieved by averaging emissions from similar electric generating 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 continuing to review the United States Supreme Court’s decision upholding the CSAPR and will review any implementation guidance the EPA may issue regarding the CSAPR and its emission allowance program.
	Greenhouse Gas Regulation
	In December 2009, the EPA issued its “endangerment finding” under the Clean Air Act, which stated that greenhouse gas emissions, including CO2, endanger human health and welfare and that emissions of greenhouse gases from motor vehicles contribute to that endangerment. 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 these actions, we are 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 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 permitting processes and NSR programs at stationary sources that are already subject to those programs, but may not apply operating permitting 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 the Clean Power Plan, assuming it is adopted and implemented as proposed, would achieve a 30% decrease in CO2 emissions from 2005 levels by 2030. Beginning in 2020, the plan also has interim goals of aggressively reducing CO2 emissions. The EPA expects the proposed regulations to be finalized by June 2015. After the proposed regulations are 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 of CO2 emission reductions and the timing of the reductions being revised.
	Based on preliminary studies, if the proposed rules were 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 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, the proposed rule could result in the closure or alteration of the operation of some of its coal-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 our 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 our 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. The trial in this matter is currently scheduled to begin in 2015. 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 May 2014, the EPA announced a finalized rule applicable to cooling water intake structures at existing power plants. The rule imposes standards 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, however, the final rule will determine the schedule. 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 our 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 of 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 June 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. Costs are subject to annual review by the ICC.
	As of June 30, 2014, Ameren Missouri has one remaining former MGP site for which remediation is scheduled. Remediation is complete at the other Ameren Missouri former MGP sites. Ameren Missouri does not currently have a rate rider mechanism that permits it to recover from utility customers remediation costs associated with MGP sites.
	The following table presents, as of June 30, 2014, the estimated obligation to complete the remediation of these former MGP sites:
		Estimate						Recorded
	Low			High			Liability(a)
Ameren	$	259		$	318		$	259
Ameren Missouri	1			2			1
Ameren Illinois	258			316			258
(a)	Recorded liability represents the estimated minimum probable obligations, 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 June 30, 2014, Ameren Illinois estimated the obligation related to the cleanup 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 June 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 June 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, Ameren Missouri believes it has no liability at June 30, 2014, for this site.
Ameren Missouri also has a federal agency mandate to complete an investigation for a site in Illinois. In 2000, the EPA notified Ameren Missouri and numerous other companies, including Solutia, that former landfills and lagoons in Sauget, Illinois, 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 June 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 June 30, 2014, Ameren Missouri estimated the obligation related to the cleanup at $1 million to $4.5 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.
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, which is pending in the United States District Court for the Eastern District of Missouri, Ameren Missouri claims that the insurance company breached its duty to indemnify Ameren Missouri for the losses resulting from the incident.
In June 2014, Ameren Missouri reached a settlement with a 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 June 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 June 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 84 as of June 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 June 30, 2014:
Ameren		Ameren	Ameren		Total(a)
		Missouri	Illinois
1		49	59		73
(a)	Total does not equal the sum of the subsidiary unit lawsuits because some of the lawsuits name multiple Ameren entities as defendants.
At June 30, 2014, Ameren, Ameren Missouri and Ameren Illinois had liabilities of $12 million, $5 million, and $7 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 June 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 June 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	6 Months Ended
	Jun. 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 pay 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 collects one mill from its electric customers for each kilowatthour of electricity that it generates and sells from its Callaway energy center.
	Although both the NWPA and the standard contract stated that the federal government would begin to dispose of spent nuclear fuel by 1998, the federal government is not meeting its disposal obligation. Ameren Missouri has sufficient installed capacity at the Callaway energy center to store its spent nuclear fuel generated through 2020, and it has the capability for additional storage capacity for spent nuclear fuel generated through the end of the energy center’s current 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. It 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. In June 2011, the parties entered into a settlement agreement that provides for annual recovery of additional spent fuel storage and related costs incurred from 2010 through 2013 with the ability to extend the recovery period as mutually agreed to by the parties. On March 6, 2014, the parties entered into an addendum to the settlement agreement that extended the recovery period through December 31, 2016. 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 third quarter of 2014. This reimbursement is included in "Miscellaneous accounts and notes receivable" on Ameren’s and Ameren Missouri’s June 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 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, within two years, a generic environmental impact statement and a final rule to address the court's ruling. The current schedule provides for the NRC to publish the proposed generic environmental impact statement and waste confidence rule on September 13, 2014, and the final generic environmental impact statement and final waste confidence rule on October 3, 2014.
	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. Decommissioning costs are included in the costs of service used to establish electric rates for Ameren Missouri's customers. These costs amounted to $7 million in each of the years 2013, 2012, and 2011. 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 were filed with the MoPSC in September 2011. In October 2012, the MoPSC issued an order approving the stipulation and agreement between Ameren Missouri and the MoPSC staff that maintained the current rate of deposits to the trust fund and the rate of return assumptions used in the analysis. 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	6 Months Ended
	Jun. 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 June 30, 2014, the plan’s estimated investment performance through June 30, 2014, and Ameren’s pension funding policy, Ameren expects to make annual contributions of $20 million to $100 million in each of the next five years, with aggregate estimated contributions of $270 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 six months ended June 30, 2014, and 2013:
		Pension Benefits														Postretirement Benefits
		Three Months							Six Months							Three Months							Six Months
		2014			2013				2014			2013				2014			2013				2014			2013
	Service cost	$	19		$	22			$	40		$	46			$	4		$	5			$	9		$	11
	Interest cost	42			41				91			81				12			11				25			23
	Expected return on plan assets	(57		)	(54		)		(114		)	(108		)		(16		)	(15		)		(32		)	(31		)
	Amortization of:
	Prior service cost (benefit)	—			(1		)		—			(2		)		(1		)	(1		)		(2		)	(2		)
	Actuarial loss (gain)	12			24				24			46				(2		)	2				(3		)	4
	Net periodic benefit cost (benefit)	$	16		$	32		(a)	$	41		$	63		(a)	$	(3	)	$	2		(a)	$	(3	)	$	5		(a)
	(a)	The net periodic benefit cost includes $3 million and $6 million in total net costs for pension benefits, for the three and six months ended June 30, 2013, respectively, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income (loss). The net periodic benefit cost includes $1 million and $- million in total net costs for postretirement benefits, for the three and six months ended June 30, 2013, respectively, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income (loss).
	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 six months ended June 30, 2014, and 2013:
		Pension Benefits														Postretirement Benefits
		Three Months							Six Months							Three Months							Six Months
		2014			2013				2014			2013				2014			2013				2014			2013
	Ameren Missouri	$	8		$	18			$	25		$	36			$	1		$	2			$	2		$	5
	Ameren Illinois	7			11				15			21				(3		)	(1		)		(4		)	—
	Other	1			3			(b)	1			6			(b)	(1		)	1			(b)	(1		)	—			(b)
	Ameren(a)	$	16		$	32			$	41		$	63			$	(3	)	$	2			$	(3	)	$	5
	(a)	Includes amounts for Ameren registrants and nonregistrant subsidiaries.
	(b)	The net periodic benefit cost includes $3 million and $6 million in total net costs for pension benefits, for the three and six months ended June 30, 2013, respectively, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income (loss). The net periodic benefit cost includes $1 million and $- million in total net costs for postretirement benefits, for the three and six months ended June 30, 2013, respectively, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income (loss).

Divestiture_Transactions_and_D

Divestiture Transactions and Discontinued Operations (Notes)	6 Months Ended
	Jun. 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 provided that if the Elgin, Gibson City, and Grand Tower gas-fired energy centers were subsequently sold to a third party and Medina Valley receives proceeds within two years of the closing of the New AER divestiture, Medina Valley will pay Genco any proceeds from such sale, net of taxes and other expenses, in excess of the amounts previously paid to Genco, which totaled $137.5 million.
	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 June 30, 2014, consolidated balance sheet in "Other assets." The corresponding payable due to Genco is reflected on Ameren's June 30, 2014, consolidated balance sheet in "Other deferred credits and liabilities." An immaterial net working capital adjustment with Rockland Capital is expected to be finalized during the third quarter of 2014. 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 (loss) for the three and six months ended June 30, 2014, and 2013:
		Three Months						Six Months
		2014			2013			2014			2013
	Operating revenues	$	—		$	303		$	1		$	567
	Operating expenses	(1		)	(310		)	(3		)	(725		)	(a)
	Operating loss	(1		)	(7		)	(2		)	(158		)
	Other income (loss)	—			1			—			(1		)
	Interest charges	—			(11		)	—			(22		)
	Loss before income taxes	(1		)	(17		)	(2		)	(181		)
	Income tax (expense) benefit	—			7			—			(28		)
	Loss from discontinued operations, net of taxes	$	(1	)	$	(10	)	$	(2	)	$	(209	)
	(a)	Included a noncash pretax asset impairment charge of $168 million for the six months ended June 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 $168 million for the six months ended June 30, 2013, to reduce the carrying value of the New AER disposal group to its estimated fair value less cost to sell. During the three months ended March 31, 2013, Ameren adjusted the accumulated deferred income taxes on its consolidated balance sheet to reflect the excess of tax basis over financial reporting basis of its stock investment in AER, when it became apparent that the temporary difference would reverse. For the six months ended June 30, 2013, this change in basis resulted in a cumulative discontinued operations deferred tax expense of $97 million. The deferred tax expense was partially offset by the then-expected tax benefits of $69 million related to the pretax loss from discontinued operations including the impairment charge recorded during the six months ended June 30, 2013.
	During the three and six months ended June 30, 2014, Ameren recorded adjustments for its estimate of the New AER net working capital adjustment and for certain contingent liabilities associated with the New AER divestiture to IPH. Additionally, Ameren recognized the operating revenues and operating expenses associated with the Elgin, Gibson City, and Grand Tower energy centers prior to the completion of their sale to Rockland Capital on January 31, 2014. The operating expenses associated with the abandoned Meredosia and Hutsonville energy centers were also included in discontinued operations.
	Ameren’s results of operations for the six months ended June 30, 2014, and financial position as of June 30, 2014, reflect the final amount owed to IPH. 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 June 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 June 30, 2014, and December 31, 2013:
		30-Jun-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, respectively, at June 30, 2014, and December 31, 2013.
	Pursuant to the IPH transaction agreement, as amended, Ameren is obligated to pay up to $34 million for certain contingent liabilities. Of these liabilities, $29 million were included in "Other deferred credits and liabilities" and $5 million were included in "Accounts and wages payable" on Ameren's June 30, 2014 consolidated balance sheet.
	The note receivable from Marketing Company related to the cash collateral support provided to New AER was $26 million and $18 million at June 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. 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 (parent) 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 (parent) pursuant to which Dynegy will, among other things, guarantee IPH's indemnification obligations until December 2, 2015.
	In addition to the $34 million of contingent liabilities recorded on Ameren’s June 30, 2014 consolidated balance sheet, Ameren had a total of $147 million in guarantees outstanding for New AER that were not recorded on Ameren’s June 30, 2014 consolidated balance sheet, which included:
	•	$138 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 June 30, 2014, Ameren would have been required under its guarantees to provide approximately $10 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 June 30, 2014, Ameren would have been required to provide approximately $8 million to the leasing counterparty.
	Additionally, at June 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 June 30, 2014.

Segment_Information

Segment Information	6 Months Ended
	Jun. 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 six months ended June 30, 2014, and 2013, and total assets in continuing operations as of June 30, 2014, and December 31, 2013.
	Three Months	Ameren		Ameren		Other			Intersegment			Ameren
		Missouri		Illinois					Eliminations
	2014
	External revenues	$	893	$	518	$	8		$	—		$	1,419
	Intersegment revenues	7		1		—			(8		)	—
	Net income (loss) attributable to Ameren Corporation from continuing operations	126		28		(4		)	—			150
	2013
	External revenues	$	883	$	514	$	6		$	—		$	1,403
	Intersegment revenues	6		2		—			(8		)	—
	Net income (loss) attributable to Ameren Corporation from continuing operations	84		31		(10		)	—			105
	Six Months
	2014
	External revenues	$	1,704	$	1,292	$	17		$	—		$	3,013
	Intersegment revenues	13		1		1			(15		)	—
	Net income (loss) attributable to Ameren Corporation from continuing operations	173		81		(7		)	—			247
	2013
	External revenues	$	1,672	$	1,197	$	9		$	—		$	2,878
	Intersegment revenues	13		3		1			(17		)	—
	Net income (loss) attributable to Ameren Corporation from continuing operations	124		62		(27		)	—			159
	As of June 30, 2014:
	Total assets	$	13,203	$	7,719	$	773		$	(122	)	$	21,573	(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 June 30, 2014, and December 31, 2013, respectively.

Summary_Of_Significant_Account1

Summary Of Significant Accounting Policies (Policies)	6 Months Ended
	Jun. 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	'
	There were no material differences between Ameren’s basic and diluted earnings per share amounts for the three and six months ended June 30, 2014, and 2013, caused by the assumed settlement of performance share units. The number of dilutive performance share units had an immaterial impact on earnings per share.
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. We evaluate intangible assets for impairment if events or changes in circumstances indicate that their carrying amount might be impaired.
	At June 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 value of Ameren’s and Ameren Missouri’s renewable energy credits was each $19 million at June 30, 2014. The book value of Ameren’s and Ameren Missouri’s renewable energy credits was each $22 million at December 31, 2013.
	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 in “Accumulated deferred income taxes, net” as a reduction to the deferred tax assets for net operating loss and tax credit carryforwards on Ameren’s, Ameren Missouri’s and Ameren Illinois’ respective balance sheets. Unrecognized tax benefits that exceed these carryforwards are recorded in “Other deferred credits and liabilities” on the respective balance sheets. At June 30, 2014, unrecognized tax benefits of $86 million, $13 million, and $- million were recorded in “Accumulated deferred income taxes, net” on Ameren's, Ameren Missouri's and Ameren Illinois' balance sheets, respectively. At December 31, 2013, unrecognized tax benefits of $84 million, $15 million, and $- million previously recorded in “Other deferred credits and liabilities” on the respective balance sheets were reclassified to “Accumulated deferred income taxes, net” for comparative purposes.
	Ameren’s federal income tax returns for the years 2007 through 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 2007 through 2011. This settlement, which is primarily related to uncertain tax positions for research tax deductions, is expected to result in a decrease in uncertain tax benefits of $20 million and $13 million for Ameren and Ameren Missouri, respectively, none of which would impact their respective effective tax rates. In addition, it is reasonably possible that other events will occur during the next 12 months that would cause the total amount of unrecognized tax benefits for the Ameren Companies to fluctuate. However, the Ameren Companies do not believe any such fluctuations, including the decrease from the reasonably possible IRS Appeals Office settlement discussed above, would be material to their 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. The Ameren Companies 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 June 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 June 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 in “Accumulated deferred income taxes, net” as a reduction to the deferred tax assets for net operating loss and tax credit carryforwards on the respective 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)	6 Months Ended
	Jun. 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)	6 Months Ended
	Jun. 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 June 30, 2014, the plan’s estimated investment performance through June 30, 2014, and Ameren’s pension funding policy, Ameren expects to make annual contributions of $20 million to $100 million in each of the next five years, with aggregate estimated contributions of $270 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)	6 Months Ended
	Jun. 30, 2014
Accounting Policies [Abstract]	'
Summary Of Nonvested Shares Related To Long-Term Incentive Plan	'
	A summary of nonvested performance share units at June 30, 2014, and changes during the six months ended June 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)	683,591		38.9
	April Grants(b)	38,559		50.34
	Forfeitures	(65,847	)	33.82
	Vested(c)	(116,297	)	38.81
	Nonvested at June 30, 2014	1,758,550		$	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.
	(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 six months ended June 30, 2014, and 2013:
			Three Months							Six Months
		2014				2013				2014			2013
	Ameren Missouri	$	—			$	—			$	6		$	(a)
	Ameren Illinois		3				3				6			7
	Ameren	$	3			$	3			$	12		$	7
	(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 six months ended June 30, 2014, and 2013:
		Three Months								Six Months
		2014				2013				2014			2013
	Ameren Missouri	$	39			$	38			$	73		$	71
	Ameren Illinois	11				11				37			33
	Ameren	$	50			$	49			$	110		$	104
Schedule of Unrecognized Tax Benefits	'
	The following table presents the amount of unrecognized tax benefits (detriments) related to uncertain tax positions as of June 30, 2014, and December 31, 2013:
		30-Jun-14				December 31,
						2013
	Ameren	$	94			$	90
	Ameren Missouri	34				31
	Ameren Illinois	—				(1		)
Schedule of Unrecognized Tax Benefits That Would Impact Effective Tax Rate	'
	The following table presents the amount of unrecognized tax benefits (detriments) related to uncertain tax positions as of June 30, 2014, and December 31, 2013, that would impact the effective tax rate, if recognized:
		30-Jun-14				December 31,
						2013
	Ameren	$	55			$	54
	Ameren Missouri	3				3
	Ameren Illinois	(1		)		—
Equity Changes Attributable To Noncontrolling Interest	'
	A reconciliation of the equity changes attributable to the noncontrolling interests at Ameren for the three and six months ended June 30, 2014, and 2013, are shown below:
		Three Months								Six 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	1				1				3			3
	Dividends paid to noncontrolling interest holders	(1		)		(1		)		(3		)	(3		)
	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.” 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)	6 Months Ended
	Jun. 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 June 30, 2014, and December 31, 2013. Ameren Illinois established a commercial paper program in May 2014.
		30-Jun-14				31-Dec-13
	Ameren (parent)	$	503			$	368
	Ameren Missouri	185				—
	Ameren Illinois	105				—
	Ameren Consolidated	$	793			$	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 six months ended June 30, 2014, and 2013:
			Ameren (parent)				Ameren Missouri			Ameren Illinois			Ameren Consolidated
	2014
	Average daily commercial paper outstanding		$	328			$	146		$	242		$	607
	Weighted-average interest rate		0.32		%		0.31		%	0.32		%	0.32		%
	Peak commercial paper during period(a)		$	503			$	495		$	300		$	907
	Peak interest rate		0.35		%		0.7		%	0.34		%	0.7		%
	2013
	Average daily commercial paper outstanding		$	13			$	—		$	—		$	13
	Weighted-average interest rate		0.54		%		—		%	—		%	0.54		%
	Peak commercial paper during period(a)		$	78			$	—		$	—		$	78
	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)

6 Months Ended

Jun. 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 June 30, 2014, at an assumed annual interest rate of 6% and dividend rate of 7%.

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

4.7

$

3,168

≥2.5

130.8

$

2,508

Ameren Illinois

≥2.0

6.7

3,780

(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)	6 Months Ended
	Jun. 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 (loss) for the three and six months ended June 30, 2014, and 2013:
		Three Months				Six Months
		2014		2013		2014		2013
	Ameren:(a)
	Miscellaneous income:
	Allowance for equity funds used during construction	$	9	$	8	$	16	$	16
	Interest income on industrial development revenue bonds	7		7		14		14
	Interest income	2		1		5		1
	Other	3		—		4		—
	Total miscellaneous income	$	21	$	16	$	39	$	31
	Miscellaneous expense:
	Donations	$	1	$	1	$	6	$	5
	Other	3		4		7		8
	Total miscellaneous expense	$	4	$	5	$	13	$	13
	Ameren Missouri:
	Miscellaneous income:
	Allowance for equity funds used during construction	$	8	$	7	$	15	$	14
	Interest income on industrial development revenue bonds	7		7		14		14
	Interest income	1		—		1		—
	Total miscellaneous income	$	16	$	14	$	30	$	28
	Miscellaneous expense:
	Donations	$	1	$	1	$	3	$	3
	Other	1		2		3		5
	Total miscellaneous expense	$	2	$	3	$	6	$	8
	Ameren Illinois:
	Miscellaneous income:
	Allowance for equity funds used during construction	$	1	$	1	$	1	$	2
	Interest income	1		1		3		1
	Other	3		—		4		—
	Total miscellaneous income	$	5	$	2	$	8	$	3
	Miscellaneous expense:
	Donations	$	—	$	—	$	3	$	3
	Other	1		1		2		1
	Total miscellaneous expense	$	1	$	1	$	5	$	4
	(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Derivative_Financial_Instrumen2

Derivative Financial Instruments (Tables)	6 Months Ended
	Jun. 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 June 30, 2014, and December 31, 2013. As of June 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)	51		(b)		51		66		(b)		66
	Natural gas (in mmbtu)	25		101		126		28		108		136
	Power (in megawatthours)	1		11		12		3		11		14
	Uranium (pounds in thousands)	627		(b)		627		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 June 30, 2014, and December 31, 2013:
		Balance Sheet Location		Ameren  Missouri				Ameren  Illinois				Ameren
	2014
	Fuel oils	Other current assets		$	5			$	—			$	5
		Other assets		2				—				2
	Natural gas	Other current assets		1				3				4
		Other assets		—				1				1
	Power	Other current assets		22				—				22
		Total assets		$	30			$	4			$	34
	Fuel oils	Other current liabilities		$	2			$	—			$	2
		Other deferred credits and liabilities		1				—				1
	Natural gas	Other current liabilities		4				16				20
		Other deferred credits and liabilities		2				8				10
	Power	Other current liabilities		6				7				13
		Other deferred credits and liabilities		—				96				96
	Uranium	Other current liabilities		5				—				5
		Other deferred credits and liabilities		2				—				2
		Total liabilities		$	22			$	127			$	149
	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 June 30, 2014, and December 31, 2013:
		Ameren				Ameren				Ameren
		Missouri				Illinois
	2014
	Fuel oils derivative contracts(a)	$	3			$	—			$	3
	Natural gas derivative contracts(b)	(5		)		(20		)		(25		)
	Power derivative contracts(c)	16				(103		)		(87		)
	Uranium derivative contracts(d)	(7		)		—				(7		)
	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 gains on fuel oils derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s transportation costs for coal through December 2017, as of June 30, 2014. Current gains deferred as regulatory liabilities include $2 million and $2 million at Ameren and Ameren Missouri, respectively, as of June 30, 2014. Current losses deferred as regulatory assets include $1 million and $1 million at Ameren and Ameren Missouri, respectively, as of June 30, 2014.
	(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 2017, at Ameren Illinois, in each case as of June 30, 2014. Current gains deferred as regulatory liabilities include $4 million, $1 million, and $3 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of June 30, 2014. Current losses deferred as regulatory assets include $20 million, $4 million, and $16 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of June 30, 2014.
	(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, in each case as of June 30, 2014. Current gains deferred as regulatory liabilities include $22 million and $22 million at Ameren and Ameren Missouri, respectively, as of June 30, 2014. Current losses deferred as regulatory assets include $13 million, $6 million, and $7 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of June 30, 2014.
	(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, as of June 30, 2014. Current losses deferred as regulatory assets include $5 million and $5 million at Ameren and Ameren Missouri, respectively, as of June 30, 2014.
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 June 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		$	30			$	9			$	—		$	21
	Ameren Illinois		4				3				—			1
	Ameren		$	34			$	12			$	—		$	22
	Liabilities:
	Ameren Missouri		$	22			$	9			$	10		$	3
	Ameren Illinois		127				3				—			124
	Ameren		$	149			$	12			$	10		$	127
	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 June 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	$	60			$	2			$	53
	Ameren Illinois	65				—				59
	Ameren	$	125			$	2			$	112
	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)	6 Months Ended
	Jun. 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 June 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	$	5		$	(3	)	Option model	Volatilities(%)(b)	May-34	15
									Discounted cash flow	Counterparty credit risk(%)(c)(d)	0.25 - 0.91	0.63
		Power(e)	21			(109		)	Discounted cash flow	Average forward peak and off-peak pricing - forwards/swaps($/MWh)(c)	22 - 60	36
										Estimated auction price for FTRs($/MW)(b)	(1,716) - 2,024	443
										Nodal basis($/MWh)(c)	(6) - 0	-3
										Counterparty credit risk(%)(c)(d)	0.25	(f)
										Ameren Missouri and Ameren Illinois credit risk(%)(c)(d)	0.43	(f)
									Fundamental energy production model	Estimated future gas prices($/mmbtu)(b)	6-May	5
										Escalation rate(%)(b)(g)	3-Feb	3
									Contract price allocation	Estimated renewable energy credit costs($/credit)(b)	7-May	6
		Uranium	—			(7		)	Discounted cash flow	Average forward uranium pricing($/pound)(b)	28 - 33	29
	Ameren Missouri	Fuel oils	$	5		$	(3	)	Option model	Volatilities(%)(b)	May-34	15
									Discounted cash flow	Counterparty credit risk(%)(c)(d)	0.25 - 0.91	0.63
		Power(e)	21			(6		)	Discounted cash flow	Average forward peak and off-peak pricing - forwards/swaps($/MWh)(c)	22 - 60	48
										Estimated auction price for FTRs($/MW)(b)	(1,716) - 2,024	443
										Nodal basis($/MWh)(c)	(3) - (1)	-2
										Counterparty credit risk(%)(c)(d)	0.25	(f)
										Ameren Missouri credit risk(%)(c)(d)	0.43	(f)
		Uranium	—			(7		)	Discounted cash flow	Average forward uranium pricing($/pound)(b)	28 - 33	29
	Ameren Illinois	Power(e)	$	—		$	(103	)	Discounted cash flow	Average forward peak and off-peak pricing - forwards/swaps($/MWh)(b)	28 - 46	33
										Nodal basis($/MWh)(b)	(6) - 0	-3
										Ameren Illinois credit risk(%)(c)(d)	0.43	(f)
									Fundamental energy production model	Estimated future gas prices($/mmbtu)(b)	6-May	5
										Escalation rate(%)(b)(g)	3-Feb	3
									Contract price allocation	Estimated renewable energy credit costs($/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 2018. Valuations beyond 2018 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.
	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 June 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		$	2			$	—			$	5			$	7
		Natural gas		—				5				—				5
		Power		—				1				21				22
		Total derivative assets - commodity contracts		$	2			$	6			$	26			$	34
		Nuclear decommissioning trust fund:
		Cash and cash equivalents		$	2			$	—			$	—			$	2
		Equity securities:
		U.S. large capitalization		344				—				—				344
		Debt securities:
		Corporate bonds		—				57				—				57
		Municipal bonds		—				2				—				2
		U.S. treasury and agency securities		—				98				—				98
		Asset-backed securities		—				12				—				12
		Other		—				6				—				6
		Total nuclear decommissioning trust fund		$	346			$	175			$	—			$	521		(b)
		Total Ameren		$	348			$	181			$	26			$	555
	Ameren	Derivative assets - commodity contracts(a):
	Missouri	Fuel oils		$	2			$	—			$	5			$	7
		Natural gas		—				1				—				1
		Power		—				1				21				22
		Total derivative assets - commodity contracts		$	2			$	2			$	26			$	30
		Nuclear decommissioning trust fund:
		Cash and cash equivalents		$	2			$	—			$	—			$	2
		Equity securities:
		U.S. large capitalization		344				—				—				344
		Debt securities:
		Corporate bonds		—				57				—				57
		Municipal bonds		—				2				—				2
		U.S. treasury and agency securities		—				98				—				98
		Asset-backed securities		—				12				—				12
		Other		—				6				—				6
		Total nuclear decommissioning trust fund		$	346			$	175			$	—			$	521		(b)
		Total Ameren Missouri		$	348			$	177			$	26			$	551
	Ameren	Derivative assets - commodity contracts(a):
	Illinois	Natural gas		$	—			$	4			$	—			$	4
	Liabilities:
	Ameren	Derivative liabilities - commodity contracts(a):
		Fuel oils		$	—			$	—			$	3			$	3
		Natural gas		2				28				—				30
		Power		—				—				109				109
		Uranium		—				—				7				7
		Total Ameren		$	2			$	28			$	119			$	149
	Ameren	Derivative liabilities - commodity contracts(a):
	Missouri	Fuel oils		$	—			$	—			$	3			$	3
		Natural gas		2				4				—				6
		Power		—				—				6				6
		Uranium		—				—				7				7
		Total Ameren Missouri		$	2			$	4			$	16			$	22
	Ameren	Derivative liabilities - commodity contracts(a):
	Illinois	Natural gas		$	—			$	24			$	—			$	24
		Power		—				—				103				103
		Total Ameren Illinois		$	—			$	24			$	103			$	127
	(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 June 30, 2014:
			Net derivative commodity contracts
	Three Months		Ameren			Ameren			Ameren
			Missouri			Illinois
	Fuel oils:
	Beginning balance at April 1, 2014	$	1		$	(a)		$	1
	Realized and unrealized gains (losses) included in regulatory assets/liabilities		1			(a)			1
	Ending balance at June 30, 2014	$	2		$	(a)		$	2
	Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2014	$	1		$	(a)		$	1
	Natural gas:
	Beginning balance at April 1, 2014	$	—		$	—		$	—
	Purchases		—			1			1
	Settlements		—			(1	)		(1	)
	Ending balance at June 30, 2014	$	—		$	—		$	—
	Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2014	$	—		$	—		$	—
	Power:
	Beginning balance at April 1, 2014	$	10		$	(120	)	$	(110	)
	Realized and unrealized gains (losses) included in regulatory assets/liabilities		(13	)		16			3
	Purchases		34			—			34
	Settlements		(15	)		1			(14	)
	Transfers out of Level 3		(1	)		—			(1	)
	Ending balance at June 30, 2014	$	15		$	(103	)	$	(88	)
	Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2014	$	(1	)	$	15		$	14
	Uranium:
	Beginning balance at April 1, 2014	$	(5	)	$	(a)		$	(5	)
	Realized and unrealized gains (losses) included in regulatory assets/liabilities		(4	)		(a)			(4	)
	Settlements		2			(a)			2
	Ending balance at June 30, 2014	$	(7	)	$	(a)		$	(7	)
	Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2014	$	(4	)	$	(a)		$	(4	)
	(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 June 30, 2013:
			Net derivative commodity contracts
	Three Months		Ameren			Ameren			Ameren
			Missouri			Illinois
	Fuel oils:
	Beginning balance at April 1, 2013	$	5		$	(a)		$	5
	Realized and unrealized gains (losses) included in regulatory assets/liabilities		(2	)		(a)			(2	)
	Ending balance at June 30, 2013	$	3		$	(a)		$	3
	Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2013	$	(1	)	$	(a)		$	(1	)
	Natural gas:
	Beginning balance at April 1, 2013	$	—		$	2		$	2
	Purchases		(1	)		—			(1	)
	Ending balance at June 30, 2013	$	(1	)	$	2		$	1
	Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2013	$	(1	)	$	—		$	(1	)
	Power:
	Beginning balance at April 1, 2013	$	2		$	(81	)	$	(79	)
	Realized and unrealized gains (losses) included in regulatory assets/liabilities		1			1			2
	Purchases		40			—			40
	Settlements		(9	)		—			(9	)
	Transfers out of Level 3		3			—			3
	Ending balance at June 30, 2013	$	37		$	(80	)	$	(43	)
	Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2013	$	3		$	(4	)	$	(1	)
	Uranium:
	Beginning balance at April 1, 2013	$	(2	)	$	(a)		$	(2	)
	Realized and unrealized gains (losses) included in regulatory assets/liabilities		(2	)		(a)			(2	)
	Settlements		1			(a)			1
	Ending balance at June 30, 2013	$	(3	)	$	(a)		$	(3	)
	Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2013	$	(1	)	$	(a)		$	(1	)
	(a)	Not applicable.
	The following table summarizes the changes in the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy for the six months ended June 30, 2014:
			Net derivative commodity contracts
	Six 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		(1	)		(a)			(1	)
Settlements		(2	)		(a)			(2	)
Ending balance at June 30, 2014	$	2		$	(a)		$	2
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2014	$	—		$	(a)		$	—
Natural gas:
Beginning balance at January 1, 2014	$	—		$	—		$	—
Purchases		—			(1	)		(1	)
Settlements		—			1			1
Ending balance at June 30, 2014	$	—		$	—		$	—
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2014	$	—		$	—		$	—
Power:
Beginning balance at January 1, 2014	$	19		$	(108	)	$	(89	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities		(18	)		4			(14	)
Purchases		34			—			34
Settlements		(20	)		1			(19	)
Ending balance at June 30, 2014	$	15		$	(103	)	$	(88	)
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2014	$	(3	)	$	1		$	(2	)
Uranium:
Beginning balance at January 1, 2014	$	(6	)	$	(a)		$	(6	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities		(4	)		(a)			(4	)
Settlements		3			(a)			3
Ending balance at June 30, 2014	$	(7	)	$	(a)		$	(7	)
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2014	$	(4	)	$	(a)		$	(4	)
(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 six months ended June 30, 2013:
		Net derivative commodity contracts
Six 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		(2	)		(a)			(2	)
Purchases		1			(a)			1
Settlements		(1	)		(a)			(1	)
Ending balance at June 30, 2013	$	3		$	(a)		$	3
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2013	$	(1	)	$	(a)		$	(1	)
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 June 30, 2013	$	(1	)	$	2		$	1
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2013	$	—		$	—		$	—
Power:
Beginning balance at January 1, 2013	$	11		$	(111	)	$	(100	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities		6			15			21
Purchases		40			—			40
Settlements		(22	)		16			(6	)
Transfers into Level 3		(2	)		—			(2	)
Transfers out of Level 3		4			—			4
Ending balance at June 30, 2013	$	37		$	(80	)	$	(43	)
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2013	$	—		$	15		$	15
Uranium:
Beginning balance at January 1, 2013	$	(2	)	$	(a)		$	(2	)
Realized and unrealized gains (losses) included in regulatory assets/liabilities		(2	)		(a)			(2	)
Settlements		1			(a)			1
Ending balance at June 30, 2013	$	(3	)	$	(a)		$	(3	)
Change in unrealized gains (losses) related to assets/liabilities held at June 30, 2013	$	(1	)	$	(a)		$	(1	)
(a)	Not applicable.
Schedule Of Transfers Between Fair Value Hierarchy Levels	'
	The following table summarizes all transfers between fair value hierarchy levels related to derivative commodity contracts for the three and six months ended June 30, 2014, and 2013:
		2014									2013
		Ameren Missouri			Ameren Illinois			Ameren			Ameren Missouri			Ameren Illinois			Ameren
	Three Months
	Transfers out of Level 3 / Transfers into Level 2 - Power	$	(1	)	$	—		$	(1	)	$	3		$	—		$	3
	Six Months
	Transfers into Level 3 / Transfers out of Level 2 - Power	$	—		$	—		$	—		$	(2	)	$	—		$	(2	)
	Transfers out of Level 3 / Transfers into Level 2 - Power	—			—			—			4			—			4
	Net fair value of Level 3 transfers	$	—		$	—		$	—		$	2		$	—		$	2
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 June 30, 2014, and December 31, 2013:
		30-Jun-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			$	6,648			$	6,038			$	6,584
	Preferred stock	142				120				142				118
	Ameren Missouri:
	Long-term debt and capital lease obligations (including current portion)	$	4,004			$	4,464			$	3,757			$	4,124
	Preferred stock	80				72				80				71
	Ameren Illinois:
	Long-term debt	$	1,940			$	2,184			$	1,856			$	2,028
	Preferred stock	62				48				62				47
	(a)	Preferred stock is recorded in “Noncontrolling Interests” on the consolidated balance sheet.

Related_Party_Transactions_Tab

Related Party Transactions (Tables)	6 Months Ended
	Jun. 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 six months ended June 30, 2014, and 2013.
					Three Months				Six Months
	Agreement	Income Statement			Ameren		Ameren		Ameren		Ameren
		Line Item			Missouri		Illinois		Missouri		Illinois
	Ameren Missouri power supply	Operating Revenues	2014	$	3	$	(a)	$	3	$	(a)
	agreements with Ameren Illinois		2013		(b)		(a)		1		(a)
	Ameren Missouri and Ameren Illinois	Operating Revenues	2014		4		1		9		1
	rent and facility services		2013		5		(b)		11		1
	Ameren Missouri and Ameren Illinois	Operating Revenues	2014		1		(b)		1		(b)
	miscellaneous support services		2013		(b)		2		(b)		2
	Total Operating Revenues		2014	$	8	$	1	$	13	$	1
			2013		5		2		12		3
	Ameren Illinois power supply	Purchased Power	2014	$	(a)	$	3	$	(a)	$	3
	agreements with Ameren Missouri		2013		(a)		(b)		(a)		1
	Ameren Illinois transmission	Purchased Power	2014		(a)		(b)		(a)		1
	services with ATXI		2013		(a)		(b)		(a)		1
	Total Purchased Power		2014	$	(a)	$	3	$	(a)	$	4
			2013		(a)		(b)		(a)		2
	Ameren Services support services	Other Operations and Maintenance	2014	$	32	$	27	$	65	$	54
	agreement		2013		28		24		60		48
	Insurance premiums(c)	Other Operations and Maintenance	2014		(b)		(a)		(b)		(a)
			2013		(b)		(a)		(b)		(a)
	Total Other Operations and		2014	$	32	$	27	$	65	$	54
	Maintenance Expenses		2013		28		24		60		48
	Money pool borrowings (advances)	Interest Charges	2014	$	(b)	$	(b)	$	(b)	$	(b)
			2013		—		(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, property damage, and terrorism coverage.

Commitments_And_Contingencies_

Commitments And Contingencies (Tables)	6 Months Ended
	Jun. 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 June 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 $900,000 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.
Schedule of Estimated Obligations for Manufactured Gas Plant Remediation	'
	The following table presents, as of June 30, 2014, the estimated obligation to complete the remediation of these former MGP sites:
		Estimate						Recorded
		Low			High			Liability(a)
	Ameren	$	259		$	318		$	259
	Ameren Missouri	1			2			1
	Ameren Illinois	258			316			258
	(a)	Recorded liability represents the estimated minimum probable obligations, as no other amount within the range was a better estimate.
Schedule of Asbestors-Related Litigation Pending Lawsuits	'
	The following table presents the pending asbestos-related lawsuits filed against the Ameren Companies as of June 30, 2014:
	Ameren		Ameren	Ameren		Total(a)
			Missouri	Illinois
	1		49	59		73
	(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)	6 Months Ended
	Jun. 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 six months ended June 30, 2014, and 2013:
		Pension Benefits														Postretirement Benefits
		Three Months							Six Months							Three Months							Six Months
		2014			2013				2014			2013				2014			2013				2014			2013
	Service cost	$	19		$	22			$	40		$	46			$	4		$	5			$	9		$	11
	Interest cost	42			41				91			81				12			11				25			23
	Expected return on plan assets	(57		)	(54		)		(114		)	(108		)		(16		)	(15		)		(32		)	(31		)
	Amortization of:
	Prior service cost (benefit)	—			(1		)		—			(2		)		(1		)	(1		)		(2		)	(2		)
	Actuarial loss (gain)	12			24				24			46				(2		)	2				(3		)	4
	Net periodic benefit cost (benefit)	$	16		$	32		(a)	$	41		$	63		(a)	$	(3	)	$	2		(a)	$	(3	)	$	5		(a)
	(a)	The net periodic benefit cost includes $3 million and $6 million in total net costs for pension benefits, for the three and six months ended June 30, 2013, respectively, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income (loss). The net periodic benefit cost includes $1 million and $- million in total net costs for postretirement benefits, for the three and six months ended June 30, 2013, respectively, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income (loss).
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 six months ended June 30, 2014, and 2013:
		Pension Benefits														Postretirement Benefits
		Three Months							Six Months							Three Months							Six Months
		2014			2013				2014			2013				2014			2013				2014			2013
	Ameren Missouri	$	8		$	18			$	25		$	36			$	1		$	2			$	2		$	5
	Ameren Illinois	7			11				15			21				(3		)	(1		)		(4		)	—
	Other	1			3			(b)	1			6			(b)	(1		)	1			(b)	(1		)	—			(b)
	Ameren(a)	$	16		$	32			$	41		$	63			$	(3	)	$	2			$	(3	)	$	5
	(a)	Includes amounts for Ameren registrants and nonregistrant subsidiaries.
	(b)	The net periodic benefit cost includes $3 million and $6 million in total net costs for pension benefits, for the three and six months ended June 30, 2013, respectively, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income (loss). The net periodic benefit cost includes $1 million and $- million in total net costs for postretirement benefits, for the three and six months ended June 30, 2013, respectively, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income (loss).

Divestiture_Transactions_and_D1

Divestiture Transactions and Discontinued Operations (Tables)	6 Months Ended
	Jun. 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 June 30, 2014, and December 31, 2013:
		30-Jun-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, respectively, at June 30, 2014, and December 31, 2013.
	The following table presents the components of discontinued operations in Ameren's consolidated statement of income (loss) for the three and six months ended June 30, 2014, and 2013:
		Three Months						Six Months
		2014			2013			2014			2013
	Operating revenues	$	—		$	303		$	1		$	567
	Operating expenses	(1		)	(310		)	(3		)	(725		)	(a)
	Operating loss	(1		)	(7		)	(2		)	(158		)
	Other income (loss)	—			1			—			(1		)
	Interest charges	—			(11		)	—			(22		)
	Loss before income taxes	(1		)	(17		)	(2		)	(181		)
	Income tax (expense) benefit	—			7			—			(28		)
	Loss from discontinued operations, net of taxes	$	(1	)	$	(10	)	$	(2	)	$	(209	)
	(a)	Included a noncash pretax asset impairment charge of $168 million for the six months ended June 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)	6 Months Ended
	Jun. 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 six months ended June 30, 2014, and 2013, and total assets in continuing operations as of June 30, 2014, and December 31, 2013.
	Three Months	Ameren		Ameren		Other			Intersegment			Ameren
		Missouri		Illinois					Eliminations
	2014
	External revenues	$	893	$	518	$	8		$	—		$	1,419
	Intersegment revenues	7		1		—			(8		)	—
	Net income (loss) attributable to Ameren Corporation from continuing operations	126		28		(4		)	—			150
	2013
	External revenues	$	883	$	514	$	6		$	—		$	1,403
	Intersegment revenues	6		2		—			(8		)	—
	Net income (loss) attributable to Ameren Corporation from continuing operations	84		31		(10		)	—			105
	Six Months
	2014
	External revenues	$	1,704	$	1,292	$	17		$	—		$	3,013
	Intersegment revenues	13		1		1			(15		)	—
	Net income (loss) attributable to Ameren Corporation from continuing operations	173		81		(7		)	—			247
	2013
	External revenues	$	1,672	$	1,197	$	9		$	—		$	2,878
	Intersegment revenues	13		3		1			(17		)	—
	Net income (loss) attributable to Ameren Corporation from continuing operations	124		62		(27		)	—			159
	As of June 30, 2014:
	Total assets	$	13,203	$	7,719	$	773		$	(122	)	$	21,573	(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 June 30, 2014, and December 31, 2013, respectively.

Summary_Of_Significant_Account3

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

6 Months Ended

6 Months Ended

1 Months Ended

6 Months Ended

In Millions, except Per Share data, unless otherwise specified

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Dec. 31, 2013

Apr. 30, 2014

Jun. 30, 2014

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Union Electric Company

Union Electric Company

Ameren Illinois Company

Ameren Illinois 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]

'

'

'

'

'

'

'

'

'

'

'

'

'

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

'

'

'

'

'

'

'

'

'

'

'

'

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

19

22

19

22

'

'

'

'

'

'

'

'

'

Unrecognized tax benefit recorded in Accumulated deferred income taxes, net

86

84

13

15

'

'

'

'

'

'

'

'

'

Estimated unrecognized tax decreases resulting from settlements with taxing authorities

$20

'

$13

'

'

'

'

'

'

'

'

'

'

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.

Summary_Of_Significant_Account4

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		6 Months Ended
	Apr. 30, 2014		Jun. 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]	683,591	[2]
Share Units, Forfeitures	'		-65,847
Share Units, Vested	'		-116,297	[3]
Share Units, Nonvested as of March 31, 2014	'		1,758,550
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	'		$38.81	[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.
[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_Account5

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		6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Finite-Lived Intangible Assets [Line Items]	'	'	'	'
Amortization of intangible assets	$3	$3	$12	$7
Union Electric Company	'	'	'	'
Finite-Lived Intangible Assets [Line Items]	'	'	'	'
Amortization of intangible assets	'	'	6	1	[1]
Ameren Illinois Company	'	'	'	'
Finite-Lived Intangible Assets [Line Items]	'	'	'	'
Amortization of intangible assets	$3	$3	$6	$7
[1]	Less than $1 million.

Summary_Of_Significant_Account6

Summary Of Significant Accounting Policies (Schedule Of Excise Taxes) (Detail) (USD $)	3 Months Ended		6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Accounting Policies [Line Items]	'	'	'	'
Excise tax expense	$50	$49	$110	$104
Union Electric Company	'	'	'	'
Accounting Policies [Line Items]	'	'	'	'
Excise tax expense	39	38	73	71
Ameren Illinois Company	'	'	'	'
Accounting Policies [Line Items]	'	'	'	'
Excise tax expense	$11	$11	$37	$33

Summary_Of_Significant_Account7

Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Schedule of Unrecognized Tax Benefits (Detriments)) (Details) (USD $)	Jun. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Schedule of Unrecognized Tax Benefits (Detriments) [Line Items]	'	'
Unrecognized tax benefits (detriments)	$94	$90
Union Electric Company	'	'
Schedule of Unrecognized Tax Benefits (Detriments) [Line Items]	'	'
Unrecognized tax benefits (detriments)	34	31
Ameren Illinois Company	'	'
Schedule of Unrecognized Tax Benefits (Detriments) [Line Items]	'	'
Unrecognized tax benefits (detriments)	'	($1)

Summary_Of_Significant_Account8

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 $)	Jun. 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	($1)	'

Summary_Of_Significant_Account9

Summary Of Significant Accounting Policies (Equity Changes Attributable To Noncontrolling Interest) (Detail) (USD $)	3 Months Ended				6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 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	1		1		3		3		'
Dividends paid to noncontrolling interest holders	-1		-1		-3		-3		'
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.” 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 $)	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2014	Feb. 28, 2014	Feb. 13, 2014	Jul. 31, 2014
			Union Electric Company	Union Electric Company	Ameren Illinois Company	Ameren Illinois Company	Electric Distribution [Member]	Customer [Domain]	Customer [Domain]	Subsequent Event [Member]
							Union Electric Company	Electric Distribution [Member]	Electric Distribution [Member]	Electric Distribution [Member]
							Accounting Authority Order Request [Member]	Union Electric Company	Union Electric Company	Union Electric Company
									customer	Pending Rate Case [Member]
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
Regulatory assets	1,213,000,000	1,240,000,000	529,000,000	534,000,000	677,000,000	701,000,000	36,000,000	'	'	'
Number of Customers Filed Complaint Case	'	'	'	'	'	'	'	'	37	'
Contested return on equity	'	'	'	'	'	'	'	9.80%	'	'
Customer Requested Rate Decrease	'	'	'	'	'	'	'	$49,000,000	'	'
Customer Requested Rate on Equity	'	'	'	'	'	'	'	9.40%	'	'

Rate_And_Regulatory_Matters_Na1

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

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Ameren Illinois Company

Ameren Illinois Company

Ameren Illinois Company

Ameren Illinois Company

Ieima [Member]

Ieima [Member]

Ieima [Member]

Icc Staff Recommendation [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]

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

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]

IEMA Revenue Requirement Reconciliation [Member]

Rate And Regulatory Matters [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Regulatory assets

$1,213,000,000

$1,240,000,000

$677,000,000

$701,000,000

'

'

$42,000,000

$64,000,000

'

'

'

'

'

'

'

Current regulatory liabilities

218,000,000

216,000,000

179,000,000

159,000,000

'

'

'

'

35,000,000

'

'

'

'

'

'

Public Utilities, Requested Rate Increase (Decrease), Amount

'

'

'

'

'

'

205,000,000

'

'

202,000,000

'

'

'

'

'

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

$9,000,000

$7,000,000

'

Rate_And_Regulatory_Matters_Na2

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

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Dec. 31, 2013

Jun. 30, 2014

Dec. 31, 2013

Jan. 31, 2011

Jun. 30, 2014

Dec. 31, 2013

Dec. 31, 2012

Jun. 30, 2014

Jun. 30, 2008

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2014

Jun. 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]

Wholesale Distribution Rate Case [Member]

Entergy Refund [Member]

Entergy Refund [Member]

Ferc Relicensing [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]

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

Ameren Illinois Company

Union Electric Company

Union Electric Company

Taum Sauk Energy Center [Member]

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

appraiser

Minimum

Ameren Illinois Company

Minimum

Maximum

Pending Ferc Case [Member]

Pending Ferc Case [Member]

Ameren Illinois Company

Rate And Regulatory Matters [Line Items]

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of Customers, Reached Agreements

'

'

'

'

'

'

'

'

'

'

'

'

'

4

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of Wholesale Customers

'

'

'

'

'

'

'

'

'

'

'

'

'

9

'

'

'

'

'

'

'

'

'

'

'

'

'

Number of Customers, Remaining on Agreements

'

'

'

'

'

'

'

'

'

'

'

'

'

5

'

'

'

'

'

'

'

'

'

'

'

'

'

Current regulatory liabilities

$218

$216

$179

$159

'

'

$39

$57

'

'

'

$16

$13

'

$14

$13

'

'

'

'

'

'

'

'

'

'

'

Loss Contingency, Estimate of Possible Loss

'

'

'

'

4

19

'

'

'

'

'

'

'

'

'

'

'

25

'

9

'

7

'

'

'

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%

'

'

Proceeds from Legal Settlements

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

31

'

'

'

'

'

'

'

'

'

'

Investments in Power and Distribution Projects

'

'

'

'

'

'

'

'

$69

$80

$120

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

Number Of Years For Proposed Relicensing Application Filed With Ferc

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'

'30 years

'

'

'

'

'

'

'

'

ShortTerm_Debt_And_Liquidity_N

Short-Term Debt And Liquidity (Narrative) (Detail) (USD $)	3 Months Ended		6 Months Ended
In Billions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Utilities [Member]	'	'	'	'
Line of Credit Facility [Line Items]	'	'	'	'
Short Term Debt, Weighted Average Interest Rate During Period	0.19%	0.07%	0.29%	0.09%
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	0.5	'	0.5	'
Minimum ratio of consolidated funds from operations plus interest expense to consolidated interest expense as of balance sheet date	'	'	'2.0 to 1.0	'
Current ratio of consolidated funds from operations plus interest expense to consolidated interest expense as of balance sheet date	'	'	'6.0 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	0.5	'	0.5	'
Illinois Credit Agreement 2012 [Member] | Ameren Illinois Company	'	'	'	'
Line of Credit Facility [Line Items]	'	'	'	'
Actual debt-to-capital ratio	0.45	'	0.45	'

ShortTerm_Debt_And_Liquidity_S1

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper outstanding) (Details) (USD $)	Jun. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Line of Credit Facility [Line Items]	'	'
Commercial paper outstanding	$793	$368
Ameren (parent)	'	'
Line of Credit Facility [Line Items]	'	'
Commercial paper outstanding	503	368
Union Electric Company	'	'
Line of Credit Facility [Line Items]	'	'
Commercial paper outstanding	185	'
Ameren Illinois Company	'	'
Line of Credit Facility [Line Items]	'	'
Commercial paper outstanding	$105	'

ShortTerm_Debt_And_Liquidity_S2

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper) (Details) (USD $)	6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014		Jun. 30, 2013
Line of Credit Facility [Line Items]	'		'
Average Daily Commercial Paper Borrowings Outstanding	$607		$13
Weighted Average Interest Rate	0.32%		0.54%
Peak Short Term Borrowings	907	[1]	78	[1]
Peak Short Term Borrowings Interest Rate	0.70%		0.85%
Ameren (parent)	'		'
Line of Credit Facility [Line Items]	'		'
Average Daily Commercial Paper Borrowings Outstanding	328		13
Weighted Average Interest Rate	0.32%		0.54%
Peak Short Term Borrowings	503	[1]	78	[1]
Peak Short Term Borrowings Interest Rate	0.35%		0.85%
Union Electric Company	'		'
Line of Credit Facility [Line Items]	'		'
Average Daily Commercial Paper Borrowings Outstanding	146		'
Weighted Average Interest Rate	0.31%		'
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	242		'
Weighted Average Interest Rate	0.32%		'
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 $)

6 Months Ended

1 Months Ended

1 Months Ended

1 Months Ended

Jun. 30, 2014

Jun. 30, 2013

Jun. 30, 2014

Jun. 30, 2013

Jun. 30, 2014

Jun. 30, 2014

Jun. 30, 2013

Jun. 30, 2014

Jun. 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

'

'

'

'

6.00%

'

'

'

'

'

'

'

'

'

'

'

Dividend rate

'

'

'

'

7.00%

'

'

'

'

'

'

'

'

'

'

'

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 $)	6 Months Ended		1 Months Ended
	Jun. 30, 2014	Jun. 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 $)	6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014
Union Electric Company	'
Long-Term Debt And Equity Financings [Line Items]	'
Bonds Issuable	3,168	[1]
Preferred Stock Issuable	2,508
Retired bond capacity	833
Union Electric Company | Actual Ratio [Member]	'
Long-Term Debt And Equity Financings [Line Items]	'
Restricted payment interest coverage ratio, Actual	4.7
Dividend Coverage Ratio	130.8
Ameren Illinois Company	'
Long-Term Debt And Equity Financings [Line Items]	'
Bonds Issuable	3,780	[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				6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Other Nonoperating Income (Expense) [Line Items]	'		'		'		'
Allowance for equity funds used during construction	$9	[1]	$8	[1]	$16	[1]	$16	[1]
Interest income on industrial development revenue bonds	7	[1]	7	[1]	14	[1]	14	[1]
Interest and dividend income	2	[1]	1	[1]	5	[1]	1	[1]
Other	3	[1]	'	[1]	4	[1]	'	[1]
Total miscellaneous income	21	[1]	16	[1]	39	[1]	31	[1]
Donations	1	[1]	1	[1]	6	[1]	5	[1]
Other	3	[1]	4	[1]	7	[1]	8	[1]
Total miscellaneous expense	4	[1]	5	[1]	13	[1]	13	[1]
Union Electric Company	'		'		'		'
Other Nonoperating Income (Expense) [Line Items]	'		'		'		'
Allowance for equity funds used during construction	8		7		15		14
Interest income on industrial development revenue bonds	7		7		14		14
Interest and dividend income	1		'		1		'
Total miscellaneous income	16		14		30		28
Donations	1		1		3		3
Other	1		2		3		5
Total miscellaneous expense	2		3		6		8
Ameren Illinois Company	'		'		'		'
Other Nonoperating Income (Expense) [Line Items]	'		'		'		'
Allowance for equity funds used during construction	1		1		1		2
Interest and dividend income	1		1		3		1
Other	3		'		4		'
Total miscellaneous income	5		2		8		3
Donations	'		'		3		3
Other	1		1		2		1
Total miscellaneous expense	$1		$1		$5		$4
[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)	Jun. 30, 2014		Dec. 31, 2013
	gal		gal
Fuel Oils	'		'
Derivative [Line Items]	'		'
Quantity	51,000,000	[1]	66,000,000	[1]
Natural Gas	'		'
Derivative [Line Items]	'		'
Quantity	126,000,000		136,000,000
Power	'		'
Derivative [Line Items]	'		'
Quantity	12,000,000		14,000,000
Uranium	'		'
Derivative [Line Items]	'		'
Quantity	627,000		796,000
Union Electric Company | Fuel Oils	'		'
Derivative [Line Items]	'		'
Quantity	51,000,000	[1]	66,000,000	[1]
Union Electric Company | Natural Gas	'		'
Derivative [Line Items]	'		'
Quantity	25,000,000		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	627,000		796,000
Ameren Illinois Company | Natural Gas	'		'
Derivative [Line Items]	'		'
Quantity	101,000,000		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 $)	Jun. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Derivative [Line Items]	'	'
Derivative assets	$34	$34
Derivative liabilities	149	178
Fuel Oils | Other Current Assets [Member]	'	'
Derivative [Line Items]	'	'
Derivative assets	5	6
Fuel Oils | Other Assets [Member]	'	'
Derivative [Line Items]	'	'
Derivative assets	2	3
Fuel Oils | Other Current Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	2	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	4	2
Natural Gas | Other Assets [Member]	'	'
Derivative [Line Items]	'	'
Derivative assets	1	'
Natural Gas | Other Current Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	20	32
Natural Gas | Other Deferred Credits And Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	10	25
Power | Other Current Assets [Member]	'	'
Derivative [Line Items]	'	'
Derivative assets	22	23
Power | Other Current Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	13	13
Power | Other Deferred Credits And Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	96	99
Uranium | Other Current Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	5	5
Uranium | Other Deferred Credits And Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	2	1
Union Electric Company	'	'
Derivative [Line Items]	'	'
Derivative assets	30	33
Derivative liabilities	22	24
Union Electric Company | Fuel Oils | Other Current Assets [Member]	'	'
Derivative [Line Items]	'	'
Derivative assets	5	6
Union Electric Company | Fuel Oils | Other Assets [Member]	'	'
Derivative [Line Items]	'	'
Derivative assets	2	3
Union Electric Company | Fuel Oils | Other Current Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	2	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	1
Union Electric Company | Natural Gas | Other Current Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	4	5
Union Electric Company | Natural Gas | Other Deferred Credits And Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	2	6
Union Electric Company | Power | Other Current Assets [Member]	'	'
Derivative [Line Items]	'	'
Derivative assets	22	23
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	5	5
Union Electric Company | Uranium | Other Deferred Credits And Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	2	1
Ameren Illinois Company	'	'
Derivative [Line Items]	'	'
Derivative assets	4	1
Derivative liabilities	127	154
Ameren Illinois Company | Natural Gas | Other Current Assets [Member]	'	'
Derivative [Line Items]	'	'
Derivative assets	3	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	8	19
Ameren Illinois Company | Power | Other Current Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	7	9
Ameren Illinois Company | Power | Other Deferred Credits And Liabilities [Member]	'	'
Derivative [Line Items]	'	'
Derivative liabilities	$96	$99

Derivative_Financial_Instrumen5

Derivative Financial Instruments (Cumulative Amount Of Pretax Net Gains (Losses) On All Derivative Instruments In OCI) (Detail) (USD $)	Jun. 30, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Derivative [Line Items]	'		'
Current gains deferred as regulatory liabilities	$218		$216
Current losses deferred as regulatory assets	202		156
Fuel Oils	'		'
Derivative [Line Items]	'		'
Current gains deferred as regulatory liabilities	2		'
Current losses deferred as regulatory assets	1		'
Fuel Oils | Regulatory Liabilities Or Assets [Member]	'		'
Derivative [Line Items]	'		'
Cumulative deferred pretax gains (losses)	3	[1]	2
Natural Gas	'		'
Derivative [Line Items]	'		'
Current gains deferred as regulatory liabilities	4		'
Current losses deferred as regulatory assets	20		'
Natural Gas | Regulatory Liabilities Or Assets [Member]	'		'
Derivative [Line Items]	'		'
Cumulative deferred pretax gains (losses)	-25	[2]	-55
Power	'		'
Derivative [Line Items]	'		'
Current gains deferred as regulatory liabilities	22		'
Current losses deferred as regulatory assets	13		'
Power | Regulatory Liabilities Or Assets [Member]	'		'
Derivative [Line Items]	'		'
Cumulative deferred pretax gains (losses)	-87	[3]	-89
Uranium	'		'
Derivative [Line Items]	'		'
Current losses deferred as regulatory assets	5		'
Uranium | Regulatory Liabilities Or Assets [Member]	'		'
Derivative [Line Items]	'		'
Cumulative deferred pretax gains (losses)	-7	[4]	-6
Union Electric Company	'		'
Derivative [Line Items]	'		'
Current gains deferred as regulatory liabilities	39		57
Current losses deferred as regulatory assets	141		118
Union Electric Company | Fuel Oils	'		'
Derivative [Line Items]	'		'
Current gains deferred as regulatory liabilities	2		'
Current losses deferred as regulatory assets	1		'
Union Electric Company | Fuel Oils | Regulatory Liabilities Or Assets [Member]	'		'
Derivative [Line Items]	'		'
Cumulative deferred pretax gains (losses)	3	[1]	2
Union Electric Company | Natural Gas	'		'
Derivative [Line Items]	'		'
Current gains deferred as regulatory liabilities	1		'
Current losses deferred as regulatory assets	4		'
Union Electric Company | Natural Gas | Regulatory Liabilities Or Assets [Member]	'		'
Derivative [Line Items]	'		'
Cumulative deferred pretax gains (losses)	-5	[2]	-10
Union Electric Company | Power	'		'
Derivative [Line Items]	'		'
Current gains deferred as regulatory liabilities	22		'
Current losses deferred as regulatory assets	6		'
Union Electric Company | Power | Regulatory Liabilities Or Assets [Member]	'		'
Derivative [Line Items]	'		'
Cumulative deferred pretax gains (losses)	16	[3]	19
Union Electric Company | Uranium	'		'
Derivative [Line Items]	'		'
Current losses deferred as regulatory assets	5		'
Union Electric Company | Uranium | Regulatory Liabilities Or Assets [Member]	'		'
Derivative [Line Items]	'		'
Cumulative deferred pretax gains (losses)	-7	[4]	-6
Ameren Illinois Company	'		'
Derivative [Line Items]	'		'
Current gains deferred as regulatory liabilities	179		159
Current losses deferred as regulatory assets	61		38
Ameren Illinois Company | Natural Gas	'		'
Derivative [Line Items]	'		'
Current gains deferred as regulatory liabilities	3		'
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	7		'
Ameren Illinois Company | Power | Regulatory Liabilities Or Assets [Member]	'		'
Derivative [Line Items]	'		'
Cumulative deferred pretax gains (losses)	($103)	[3]	($108)
[1]	Represents net gains on fuel oils derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s transportation costs for coal through December 2017, as of June 30, 2014. Current gains deferred as regulatory liabilities include $2 million and $2 million at Ameren and Ameren Missouri, respectively, as of June 30, 2014. Current losses deferred as regulatory assets include $1 million and $1 million at Ameren and Ameren Missouri, respectively, as of June 30, 2014
[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 2017, at Ameren Illinois, in each case as of June 30, 2014. Current gains deferred as regulatory liabilities include $4 million, $1 million, and $3 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of June 30, 2014. Current losses deferred as regulatory assets include $20 million, $4 million, and $16 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of June 30, 2014.
[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, in each case as of June 30, 2014. Current gains deferred as regulatory liabilities include $22 million and $22 million at Ameren and Ameren Missouri, respectively, as of June 30, 2014. Current losses deferred as regulatory assets include $13 million, $6 million, and $7 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of June 30, 2014.
[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, as of June 30, 2014. Current losses deferred as regulatory assets include $5 million and $5 million at Ameren and Ameren Missouri, respectively, as of June 30, 2014.

Derivative_Financial_Instrumen6

Derivative Financial Instruments (Offsetting Derivative Assets and Liabilities) (Details) (Commodity Contract, USD $)	Jun. 30, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Offsetting Assets and Liabilities [Line Items]	'		'
Gross Amounts Recognized in the Balance Sheet	$34	[1]	$34	[1]
Derivative Instruments	12		10
Net Amount	22		24
Gross Amounts Recognized in the Balance Sheet	149	[1]	178	[1]
Derivative Instruments	12		10
Cash Collateral Received/Posted	10	[2]	24	[2]
Net Amount	127		144
Union Electric Company	'		'
Offsetting Assets and Liabilities [Line Items]	'		'
Gross Amounts Recognized in the Balance Sheet	30	[1]	33	[1]
Derivative Instruments	9		9
Net Amount	21		24
Gross Amounts Recognized in the Balance Sheet	22	[1]	24	[1]
Derivative Instruments	9		9
Cash Collateral Received/Posted	10	[2]	9	[2]
Net Amount	3		6
Ameren Illinois Company	'		'
Offsetting Assets and Liabilities [Line Items]	'		'
Gross Amounts Recognized in the Balance Sheet	4		1
Derivative Instruments	3		1
Net Amount	1		0
Gross Amounts Recognized in the Balance Sheet	127	[1]	154	[1]
Derivative Instruments	3		1
Cash Collateral Received/Posted	'		15	[2]
Net Amount	$124		$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 $)	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2014	Dec. 31, 2013	Jun. 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	$20	$13	$11	$12	$9	$1

Derivative_Financial_Instrumen8

Derivative Financial Instruments (Potential Loss On Counterparty Exposures) (Detail) (USD $)	Jun. 30, 2014	Dec. 31, 2013
In Millions, unless otherwise specified
Concentration Risk [Line Items]	'	'
Potential loss on counterparty exposures related to derivative contracts	$13	$6
Ameren Illinois Company	'	'
Concentration Risk [Line Items]	'	'
Potential loss on counterparty exposures related to derivative contracts	6	'
Union Electric Company	'	'
Concentration Risk [Line Items]	'	'
Potential loss on counterparty exposures related to derivative contracts	$7	$6

Derivative_Financial_Instrumen9

Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Detail) (USD $)	Jun. 30, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Derivative [Line Items]	'		'
Aggregate Fair Value of Derivative Liabilities	$125	[1]	$145	[1]
Cash Collateral Posted	2		17
Potential Aggregate Amount of Additional Collateral Required	112	[2]	122	[2]
Union Electric Company	'		'
Derivative [Line Items]	'		'
Aggregate Fair Value of Derivative Liabilities	60	[1]	70	[1]
Cash Collateral Posted	2		2
Potential Aggregate Amount of Additional Collateral Required	53	[2]	67	[2]
Ameren Illinois Company	'		'
Derivative [Line Items]	'		'
Aggregate Fair Value of Derivative Liabilities	65	[1]	75	[1]
Cash Collateral Posted	'		15
Potential Aggregate Amount of Additional Collateral Required	$59	[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 $)	6 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014		Dec. 31, 2013
Fuel Oils | Discounted Cash Flow | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.25%	[1],[2]	0.26%	[1],[2]
Fuel Oils | Discounted Cash Flow | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.91%	[1],[2]	2.00%	[1],[2]
Fuel Oils | Discounted Cash Flow | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.63%	[1],[2]	1.00%	[1],[2]
Fuel Oils | Option Model | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Volatilities	5.00%	[3]	10.00%	[3]
Fuel Oils | Option Model | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Volatilities	34.00%	[3]	35.00%	[3]
Fuel Oils | Option Model | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Volatilities	15.00%	[3]	16.00%	[3]
Fuel Oils | Derivative Assets	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative assets	5	[4]	8	[4]
Fuel Oils | Union Electric Company | Discounted Cash Flow | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.25%	[1],[2]	0.26%	[1],[2]
Fuel Oils | Union Electric Company | Discounted Cash Flow | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.91%	[1],[2]	2.00%	[1],[2]
Fuel Oils | Union Electric Company | Discounted Cash Flow | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.63%	[1],[2]	1.00%	[1],[2]
Fuel Oils | Union Electric Company | Option Model | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Volatilities	5.00%	[3]	10.00%	[3]
Fuel Oils | Union Electric Company | Option Model | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Volatilities	34.00%	[3]	35.00%	[3]
Fuel Oils | Union Electric Company | Option Model | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Volatilities	15.00%	[3]	16.00%	[3]
Fuel Oils | Union Electric Company | Derivative Assets	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative assets	5	[4]	8	[4]
Fuel Oils | Derivative Liabilities	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative liabilities	-3	[4]	-3	[4]
Fuel Oils | Derivative Liabilities | Union Electric Company	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative liabilities	-3	[4]	-3	[4]
Power | Discounted Cash Flow | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.25%	[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	22	[1]	25	[1]
Estimated auction price	-1,716	[3]	-1,594	[3]
Power | Discounted Cash Flow | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.25%	[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	60	[1]	51	[1]
Estimated auction price	2,024	[3]	945	[3]
Power | Discounted Cash Flow | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.25%	[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	36	[1]	32	[1]
Estimated auction price	443	[3]	305	[3]
Power | Fundamental Energy Production Model | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Escalation rate	2.00%	[3],[5]	3.00%	[3],[5]
Estimated future gas prices	5	[3]	4	[3]
Power | Fundamental Energy Production Model | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Escalation rate	3.00%	[3],[5]	4.00%	[3],[5]
Estimated future gas prices	6	[3]	5	[3]
Power | Fundamental Energy Production Model | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Escalation rate	3.00%	[3],[5]	4.00%	[3],[5]
Estimated future gas prices	5	[3]	5	[3]
Power | Contract Price Allocation | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Estimated renewable energy credit costs	5	[3]	5	[3]
Power | Contract Price Allocation | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Estimated renewable energy credit costs	7	[3]	7	[3]
Power | Contract Price Allocation | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Estimated renewable energy credit costs	6	[3]	6	[3]
Power | Derivative Assets	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative assets	21	[4],[6]	21	[4],[7]
Power | Union Electric Company | Discounted Cash Flow | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.25%	[1],[2]	0.39%	[1],[2]
Nodal basis	-3	[1]	-3	[1]
Credit risk	0.43%	[1],[2]	2.00%	[1],[2]
Average forward peak and off-peak pricing	22	[1]	25	[1]
Estimated auction price	-1,716	[3]	-1,594	[3]
Power | Union Electric Company | Discounted Cash Flow | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.25%	[1],[2]	0.50%	[1],[2]
Nodal basis	-1	[1]	-1	[1]
Credit risk	0.43%	[1],[2]	2.00%	[1],[2]
Average forward peak and off-peak pricing	60	[1]	51	[1]
Estimated auction price	2,024	[3]	945	[3]
Power | Union Electric Company | Discounted Cash Flow | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Counterparty credit risk	0.25%	[1],[2]	0.42%	[1],[2]
Nodal basis	-2	[1]	-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	443	[3]	305	[3]
Power | Union Electric Company | Derivative Assets	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative assets	21	[4],[6]	21	[4],[7]
Power | Ameren Illinois Company | Discounted Cash Flow | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Nodal basis	-6	[3]	-4	[3]
Credit risk	0.43%	[1],[2]	2.00%	[1],[2]
Average forward peak and off-peak pricing	28	[3]	27	[3]
Power | Ameren Illinois Company | Discounted Cash Flow | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Nodal basis	0	[3]	0	[3]
Credit risk	0.43%	[1],[2]	2.00%	[1],[2]
Average forward peak and off-peak pricing	46	[3]	36	[3]
Power | Ameren Illinois Company | Discounted Cash Flow | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Nodal basis	-3	[3]	-2	[3]
Credit risk	0.43%	[1],[2]	2.00%	[1],[2]
Average forward peak and off-peak pricing	33	[3]	30	[3]
Power | Ameren Illinois Company | Fundamental Energy Production Model | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Escalation rate	2.00%	[3],[5]	3.00%	[3],[5]
Estimated future gas prices	5	[3]	4	[3]
Power | Ameren Illinois Company | Fundamental Energy Production Model | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Escalation rate	3.00%	[3],[5]	4.00%	[3],[5]
Estimated future gas prices	6	[3]	5	[3]
Power | Ameren Illinois Company | Fundamental Energy Production Model | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Escalation rate	3.00%	[3],[5]	4.00%	[3],[5]
Estimated future gas prices	5	[3]	5	[3]
Power | Ameren Illinois Company | Contract Price Allocation | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Estimated renewable energy credit costs	5	[3]	5	[3]
Power | Ameren Illinois Company | Contract Price Allocation | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Estimated renewable energy credit costs	7	[3]	7	[3]
Power | Ameren Illinois Company | Contract Price Allocation | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Estimated renewable energy credit costs	6	[3]	6	[3]
Power | Ameren Illinois Company | Derivative Assets	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative assets	'		'	[4],[7]
Power | Derivative Liabilities	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative liabilities	-109	[4],[6]	-110	[4],[7]
Power | Derivative Liabilities | Union Electric Company	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative liabilities	-6	[4],[6]	-2	[4],[7]
Power | Derivative Liabilities | Ameren Illinois Company	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative liabilities	-103	[4],[6]	-108	[4],[7]
Uranium | Discounted Cash Flow | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Average forward pricing	28	[3]	34	[3]
Uranium | Discounted Cash Flow | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Average forward pricing	33	[3]	41	[3]
Uranium | Discounted Cash Flow | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Average forward pricing	29	[3]	36	[3]
Uranium | Derivative Assets	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative assets	'	[4]	'	[4]
Uranium | Union Electric Company | Discounted Cash Flow | Minimum	'		'
Fair Value Inputs [Abstract]	'		'
Average forward pricing	28	[3]	34	[3]
Uranium | Union Electric Company | Discounted Cash Flow | Maximum	'		'
Fair Value Inputs [Abstract]	'		'
Average forward pricing	33	[3]	41	[3]
Uranium | Union Electric Company | Discounted Cash Flow | Weighted Average	'		'
Fair Value Inputs [Abstract]	'		'
Average forward pricing	29	[3]	36	[3]
Uranium | Union Electric Company | Derivative Assets	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative assets	'	[4]	'	[4]
Uranium | Derivative Liabilities	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative liabilities	-7	[4]	-6	[4]
Uranium | Derivative Liabilities | Union Electric Company	'		'
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]	'		'
Derivative liabilities	-7	[4]	-6	[4]
[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]	Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.
[4]	The derivative asset and liability balances are presented net of counterparty credit considerations.
[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 $)	Jun. 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 $)	Jun. 30, 2014		Dec. 31, 2013
In Millions, unless otherwise specified
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	$521	[1]	$494
Assets fair value	555	[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	521	[1]	494
Assets fair value	551	[2]	527	[2]
Commodity Contract	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	34	[2]	34	[2]
Derivative liabilities	149	[2]	178	[2]
Commodity Contract | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	30	[2]	33	[2]
Derivative liabilities	22	[2]	24	[2]
Commodity Contract | Ameren Illinois Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	4		1
Derivative liabilities	127	[2]	154	[2]
Commodity Contract | Fuel Oils	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	7	[2]	9	[2]
Derivative liabilities	3	[2]	3	[2]
Commodity Contract | Fuel Oils | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	7	[2]	9	[2]
Derivative liabilities	3	[2]	3	[2]
Commodity Contract | Natural Gas	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	5	[2]	2	[2]
Derivative liabilities	30	[2]	57	[2]
Commodity Contract | Natural Gas | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	1	[2]	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	4	[2]	1	[2]
Derivative liabilities	24	[2]	46	[2]
Commodity Contract | Power	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	22	[2]	23	[2]
Derivative liabilities	109	[2]	112	[2]
Commodity Contract | Power | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	22	[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	103	[2]	108	[2]
Commodity Contract | Uranium	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative liabilities	7	[2]	6	[2]
Commodity Contract | Uranium | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative liabilities	7	[2]	6	[2]
Cash and cash equivalents	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	2		3
Cash and cash equivalents | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	2		3
Equity Securities | U.S. large capitalization	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	344		332
Equity Securities | U.S. large capitalization | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	344		332
Debt Securities | Corporate bonds	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	57		52
Debt Securities | Corporate bonds | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	57		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	98		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	98		94
Debt Securities | Asset-backed Securities	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	12		10
Debt Securities | Asset-backed Securities | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	12		10
Debt Securities | Other Debt Securities	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	6		1
Debt Securities | Other Debt Securities | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	6		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	346		335
Assets fair value	348	[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	346		335
Assets fair value	348	[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	[2]	1	[2]
Derivative liabilities	2	[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	[2]	1	[2]
Derivative liabilities	2	[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	[2]	1	[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	[2]	1	[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	2		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	2		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	344		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	344		332
Significant Other Observable Inputs (Level 2)	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Nuclear Decommissioning Trust Fund	175		159
Assets fair value	181	[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	175		159
Assets fair value	177	[2]	162	[2]
Significant Other Observable Inputs (Level 2) | Commodity Contract	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	6	[2]	4	[2]
Derivative liabilities	28	[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	2	[2]	3	[2]
Derivative liabilities	4	[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	24	[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	5	[2]	2	[2]
Derivative liabilities	28	[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]	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	4	[2]	1	[2]
Derivative liabilities	24	[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	'		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	'		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	57		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	57		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	98		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	98		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	12		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	12		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	6		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	6		1
Significant Other Unobservable Inputs (Level 3)	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Assets fair value	26	[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	26	[2]	29	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	26	[2]	29	[2]
Derivative liabilities	119	[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	26	[2]	29	[2]
Derivative liabilities	16	[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	103	[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	5	[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	5	[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 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	'		'	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Power	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative assets	21	[2]	21	[2]
Derivative liabilities	109	[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	21	[2]	21	[2]
Derivative liabilities	6	[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	103	[2]	108	[2]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Uranium	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Derivative liabilities	7	[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	$7	[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		6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Fuel Oils	'	'	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'	'	'	'
Beginning balance	$1	$5	$5	$5
Included in regulatory assets/liabilities	1	-2	-1	-2
Purchases	'	'	'	1
Settlements	'	'	-2	-1
Ending balance	2	3	2	3
Change in unrealized gains (losses) related to assets/liabilities held at period end	1	-1	'	-1
Uranium	'	'	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'	'	'	'
Beginning balance	-5	-2	-6	-2
Included in regulatory assets/liabilities	-4	-2	-4	-2
Settlements	2	1	3	1
Ending balance	-7	-3	-7	-3
Change in unrealized gains (losses) related to assets/liabilities held at period end	-4	-1	-4	-1
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
Purchases	1	-1	-1	0
Settlements	-1	'	1	'
Ending balance	0	1	0	1
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	-110	-79	-89	-100
Included in regulatory assets/liabilities	3	2	-14	21
Purchases	34	40	34	40
Settlements	-14	-9	-19	-6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers Into Level 3	'	'	'	-2
Transfer out of Level 3	-1	3	'	4
Ending balance	-88	-43	-88	-43
Change in unrealized gains (losses) related to assets/liabilities held at period end	14	-1	-2	15
Union Electric Company | Fuel Oils	'	'	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'	'	'	'
Beginning balance	1	5	5	5
Included in regulatory assets/liabilities	1	-2	-1	-2
Purchases	'	'	'	1
Settlements	'	'	-2	-1
Ending balance	2	3	2	3
Change in unrealized gains (losses) related to assets/liabilities held at period end	1	-1	'	-1
Union Electric Company | Uranium	'	'	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'	'	'	'
Beginning balance	-5	-2	-6	-2
Included in regulatory assets/liabilities	-4	-2	-4	-2
Settlements	2	1	3	1
Ending balance	-7	-3	-7	-3
Change in unrealized gains (losses) related to assets/liabilities held at period end	-4	-1	-4	-1
Union Electric Company | Natural Gas	'	'	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'	'	'	'
Beginning balance	0	0	0	0
Purchases	0	-1	0	-1
Ending balance	0	-1	0	-1
Change in unrealized gains (losses) related to assets/liabilities held at period end	'	-1	'	'
Union Electric Company | Power	'	'	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'	'	'	'
Beginning balance	10	2	19	11
Included in regulatory assets/liabilities	-13	1	-18	6
Purchases	34	40	34	40
Settlements	-15	-9	-20	-22
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers Into Level 3	'	'	'	-2
Transfer out of Level 3	-1	3	'	4
Ending balance	15	37	15	37
Change in unrealized gains (losses) related to assets/liabilities held at period end	-1	3	-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
Purchases	1	0	-1	1
Settlements	-1	'	1	'
Ending balance	0	2	0	2
Change in unrealized gains (losses) related to assets/liabilities held at period end	'	'	'	'
Ameren Illinois Company | Power	'	'	'	'
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]	'	'	'	'
Beginning balance	-120	-81	-108	-111
Included in regulatory assets/liabilities	16	1	4	15
Settlements	1	'	1	16
Transfer out of Level 3	'	'	'	'
Ending balance	-103	-80	-103	-80
Change in unrealized gains (losses) related to assets/liabilities held at period end	$15	($4)	$1	$15

Fair_Value_Measurements_Schedu3

Fair Value Measurements (Schedule Of Transfers Between Fair Value Hierarchy Levels) (Detail) (USD $)	3 Months Ended		6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2013
Derivative [Line Items]	'	'	'
Net fair value of Level 3 transfers	'	'	$2
Power	'	'	'
Derivative [Line Items]	'	'	'
Assets Transfers out of Level 3	-1	3	4
Power | Transfer Into/Out of Level 2	'	'	'
Derivative [Line Items]	'	'	'
Assets Transfers into Level 3	'	'	-2
Assets Transfers out of Level 3	-1	3	4
Union Electric Company	'	'	'
Derivative [Line Items]	'	'	'
Net fair value of Level 3 transfers	'	'	2
Union Electric Company | Power	'	'	'
Derivative [Line Items]	'	'	'
Assets Transfers out of Level 3	-1	3	4
Union Electric Company | Power | Transfer Into/Out of Level 2	'	'	'
Derivative [Line Items]	'	'	'
Assets Transfers into Level 3	'	'	-2
Assets Transfers out of Level 3	-1	3	4
Ameren Illinois Company	'	'	'
Derivative [Line Items]	'	'	'
Net fair value of Level 3 transfers	'	'	'
Ameren Illinois Company | Power	'	'	'
Derivative [Line Items]	'	'	'
Assets Transfers out of Level 3	'	'	'
Ameren Illinois Company | Power | Transfer Into/Out of Level 2	'	'	'
Derivative [Line Items]	'	'	'
Assets Transfers into Level 3	'	'	'
Assets Transfers out of Level 3	'	'	'

Fair_Value_Measurements_Schedu4

Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Long-Term Debt And Preferred Stock) (Detail) (USD $)	Jun. 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)	$6,648	[1]	$6,584	[1]
Preferred stock	120	[1]	118	[1]
Fair Value | Union Electric Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Long-term debt (including current portion)	4,464		4,124
Preferred stock	72		71
Fair Value | Ameren Illinois Company	'		'
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]	'		'
Long-term debt (including current portion)	2,184		2,028
Preferred stock	48		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				6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Union Electric Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Revenues	$8,000,000		$5,000,000		$13,000,000		$12,000,000
Operating Expenses	32,000,000		28,000,000		65,000,000		60,000,000
Ameren Illinois Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Revenues	1,000,000		2,000,000		1,000,000		3,000,000
Operating Expenses	27,000,000		24,000,000		54,000,000		48,000,000
Purchased Power [Member] | Union Electric Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Expenses	1,000,000	[1]	1,000,000	[1]	1,000,000	[1]	1,000,000	[1]
Purchased Power [Member] | Ameren Illinois Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Expenses	3,000,000		1,000,000	[2]	4,000,000		2,000,000
Ameren Missouri Power Supply Agreements with Ameren Illinois | Union Electric Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Revenues	3,000,000		1,000,000	[2]	3,000,000		1,000,000
Ameren Missouri Power Supply Agreements with Ameren Illinois | Ameren Illinois Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Revenues	1,000,000	[1]	1,000,000	[1]	1,000,000	[1]	1,000,000	[1]
Ameren Missouri and Ameren Illinois Rent and Facility Services | Union Electric Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Revenues	4,000,000		5,000,000		9,000,000		11,000,000
Ameren Missouri and Ameren Illinois Rent and Facility Services | Ameren Illinois Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Revenues	1,000,000		1,000,000	[2]	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		1,000,000	[2]	1,000,000		1,000,000	[2]
Ameren Missouri and Ameren Illinois miscellaneous support services [Member] | Ameren Illinois Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Revenues	1,000,000	[2]	2,000,000		1,000,000	[2]	2,000,000
Ameren Illinois Power Supply Agreements with Ameren Missouri | Union Electric Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Expenses	1,000,000	[1]	1,000,000	[1]	1,000,000	[1]	1,000,000	[1]
Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Expenses	3,000,000		1,000,000	[2]	3,000,000		1,000,000
Ameren Illinois Transmission Agreements with ATXI | Union Electric Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Expenses	1,000,000	[1]	1,000,000	[1]	1,000,000	[1]	1,000,000	[1]
Ameren Illinois Transmission Agreements with ATXI | Ameren Illinois Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Expenses	1,000,000	[2]	1,000,000	[2]	1,000,000		1,000,000
Ameren Services Support Services Agreement | Union Electric Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Expenses	32,000,000		28,000,000		65,000,000		60,000,000
Ameren Services Support Services Agreement | Ameren Illinois Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Operating Expenses	27,000,000		24,000,000		54,000,000		48,000,000
Insurance Premiums | Union Electric 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]
Insurance Premiums | Ameren Illinois 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]
Related Party Money Pool Interest | Union Electric Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Interest Charges	1,000,000	[2]	1,000,000	[2]	1,000,000	[2]	1,000,000	[2]
Related Party Money Pool Interest | Ameren Illinois Company	'		'		'		'
Related Party Transaction [Line Items]	'		'		'		'
Interest Charges	$1,000,000	[2]	$1,000,000	[2]	$1,000,000	[2]	$1,000,000	[2]
[1]	Not applicable.
[2]	Amount less than $1 million.
[3]	Represents insurance premiums paid to Missouri Energy Risk Assurance Company LLC, an affiliate, for replacement power, property damage, and terrorism coverage.

Commitments_And_Contingencies_1

Commitments And Contingencies (Callaway Energy Center) (Detail) (USD $)	6 Months Ended
	Jun. 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 $900,000 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 $)	Jun. 30, 2014
In Millions, unless otherwise specified
Long-term Purchase Commitment [Line Items]	'
Other obligations	$6,068
Union Electric Company	'
Long-term Purchase Commitment [Line Items]	'
Other obligations	4,200
Ameren Illinois Company	'
Long-term Purchase Commitment [Line Items]	'
Other obligations	$1,810

Commitments_And_Contingencies_3

Commitments And Contingencies (Environmental Matters) (Detail) (USD $)	6 Months Ended
	Jun. 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]	'
Range of possible loss, minimum	259,000,000
Range of possible loss maximum	318,000,000
Accrual for environmental loss contingencies	259,000,000	[1]
Manufactured Gas Plant | Union Electric Company	'
Loss Contingencies [Line Items]	'
Number of remediation sites	1
Range of possible loss, minimum	1,000,000
Range of possible loss maximum	2,000,000
Accrual for environmental loss contingencies	1,000,000	[1]
Manufactured Gas Plant | Ameren Illinois Company	'
Loss Contingencies [Line Items]	'
Number of remediation sites	44
Range of possible loss, minimum	258,000,000
Range of possible loss maximum	316,000,000
Accrual for environmental loss contingencies	258,000,000	[1]
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	1,000,000
Range of possible loss maximum	4,500,000
Accrual for environmental loss contingencies	$1,000,000
[1]	Recorded liability represents the estimated minimum probable obligations, as no other amount within the range was a better estimate.

Commitments_And_Contingencies_4

Commitments And Contingencies (Pumped-Storage Hydroelectric Facility Breach) (Detail) (Union Electric Company, USD $)	6 Months Ended
In Millions, unless otherwise specified	Jun. 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 $)	6 Months Ended						1 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014		Jun. 30, 2014	Jun. 30, 2014	Jun. 30, 2014	Jun. 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	84		'	'	'	'	'	'
Asbestos-related lawsuits were pending	73	[1]	49	59	1	'	'	'
Range of possible loss, minimum	$12		$5	$7	'	$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 $)	6 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
	mill
Nuclear Waste Matters [Line Items]	'	'	'	'
Miscellaneous accounts and notes receivable	$213	$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 $)	Jun. 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	$270
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	20
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	$100

Retirement_Benefits_Components

Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) (USD $)	3 Months Ended				6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Pension Plan, Defined Benefit [Member]	'		'		'		'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'		'		'		'
Service cost	$19		$22		$40		$46
Interest cost	42		41		91		81
Expected return on plan assets	-57		-54		-114		-108
Prior service cost (benefit)	'		-1		'		-2
Actuarial loss	12		24		24		46
Net periodic benefit cost	16		32	[1]	41		63	[1]
Postretirement Benefit Costs [Member]	'		'		'		'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'		'		'		'
Service cost	4		5		9		11
Interest cost	12		11		25		23
Expected return on plan assets	-16		-15		-32		-31
Prior service cost (benefit)	-1		-1		-2		-2
Actuarial loss	-2		2		-3		4
Net periodic benefit cost	-3		2	[1]	-3		5	[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]	-3	[2]	5	[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		'		'
Pension Plan, Defined Benefit [Member]	'		'		'		'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'		'		'		'
Net periodic benefit cost	16	[2]	32	[2]	41	[2]	63	[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	'		$3		'		$6
[1]	The net periodic benefit cost includes $3 million and $6 million in total net costs for pension benefits, for the three and six months ended June 30, 2013, respectively, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income (loss). The net periodic benefit cost includes $1 million and $- million in total net costs for postretirement benefits, for the three and six months ended June 30, 2013, respectively, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income (loss).
[2]	Includes amounts for Ameren registrants and nonregistrant subsidiaries

Retirement_Benefits_Summary_Of

Retirement Benefits (Summary Of Benefit Plan Costs Incurred) (Detail) (USD $)	3 Months Ended				6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013
Pension Plan, Defined Benefit [Member]	'		'		'		'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'		'		'		'
Net periodic benefit cost	$16	[1]	$32	[1]	$41	[1]	$63	[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	8		18		25		36
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		11		15		21
Pension Plan, Defined Benefit [Member] | Other	'		'		'		'
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]	'		'		'		'
Net periodic benefit cost	1		3	[2]	1		6	[2]
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]	-3	[1]	5	[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		5
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		-4		'
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]	-1		'	[2]
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	'		3		'		6
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		'		'
[1]	Includes amounts for Ameren registrants and nonregistrant subsidiaries
[2]	The net periodic benefit cost includes $3 million and $6 million in total net costs for pension benefits, for the three and six months ended June 30, 2013, respectively, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income (loss). The net periodic benefit cost includes $1 million and $- million in total net costs for postretirement benefits, for the three and six months ended June 30, 2013, respectively, which were included in “Loss from discontinued operations, net of taxes” on Ameren’s consolidated statement of income (loss).

Divestiture_Transactions_and_D2

Divestiture Transactions and Discontinued Operations (Narrative) (Details) (USD $)

3 Months Ended

6 Months Ended

6 Months Ended

1 Months Ended

3 Months Ended

In Millions, unless otherwise specified

Jun. 30, 2014

Jun. 30, 2013

Jun. 30, 2014

Jun. 30, 2013

Dec. 31, 2013

Jun. 30, 2013

Jun. 30, 2014

Mar. 14, 2013

Jun. 30, 2014

Dec. 31, 2013

Jan. 31, 2014

Dec. 31, 2013

Jun. 30, 2014

Jun. 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

'

'

'

'

'

168

'

'

'

'

'

'

'

'

Discontinued operations deferred tax expense

'

'

'

'

'

97

'

'

'

'

'

'

'

'

Discontinued operations deferred tax benefit

'

'

'

'

'

69

'

'

'

'

'

'

'

'

Loss from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest

1

10

2

209

'

'

'

'

'

'

'

'

'

'

Asset retirement obligations

380

'

380

'

369

'

'

'

'

'

'

'

'

'

Range of possible loss maximum

'

'

'

'

'

'

34

'

'

'

'

'

'

'

Other deferred credits and liabilities

524

'

524

'

538

'

29

'

'

'

'

'

'

'

Accounts and wages payable

575

'

575

'

806

'

5

'

'

'

'

'

'

'

Notes, Loans and Financing Receivable, Net, Noncurrent

'

'

'

'

'

'

'

'

26

18

'

'

'

'

Buyer's indemnification guarantee obligation

'

'

'

'

'

'

'

25

'

'

'

'

'

'

Loss contingency accrual

'

'

'

'

'

'

34

'

'

'

'

'

'

'

Guarantees Outstanding

'

'

'

'

'

'

147

'

'

'

'

'

138

9

Guarantees, Maximum Exposure

'

'

'

'

'

'

'

'

'

'

'

'

10

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		6 Months Ended
In Millions, unless otherwise specified	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]	'	'	'	'
Operating revenues	$0	$303	$1	$567
Operating expenses	-1	-310	-3	-725	[1]
Operating loss	-1	-7	-2	-158
Other income (loss)	'	1	'	-1
Interest charges	0	-11	'	-22
Loss before income taxes	-1	-17	-2	-181
Income tax (expense) benefit	0	7	'	-28
Loss from discontinued operations, net of taxes	-1	-10	-2	-209
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	'	'	'	$168
[1]	Included a noncash pretax asset impairment charge of $168 million for the six months ended June 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 $)	Jun. 30, 2014		Dec. 31, 2013		Jun. 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	380		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, respectively, at June 30, 2014, and December 31, 2013.

Segment_Information_Schedule_O

Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) (USD $)	3 Months Ended			6 Months Ended
	Jun. 30, 2014		Jun. 30, 2013	Jun. 30, 2014		Jun. 30, 2013	Dec. 31, 2013
Segment Reporting Information [Line Items]	'		'	'		'	'
Number of Reportable Segments	'		'	2		'	'
External revenues	$1,419,000,000		$1,403,000,000	$3,013,000,000		$2,878,000,000	'
Net income (loss) attributable to Ameren Corporation from continuing operations	150,000,000		105,000,000	247,000,000		159,000,000	'
Total assets	21,588,000,000		'	21,588,000,000		'	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	893,000,000		883,000,000	1,704,000,000		1,672,000,000	'
Intersegment revenues	7,000,000		6,000,000	13,000,000		13,000,000	'
Net income (loss) attributable to Ameren Corporation from continuing operations	126,000,000		84,000,000	173,000,000		124,000,000	'
Total assets	13,203,000,000		'	13,203,000,000		'	12,904,000,000
Ameren Illinois Company	'		'	'		'	'
Segment Reporting Information [Line Items]	'		'	'		'	'
External revenues	518,000,000		514,000,000	1,292,000,000		1,197,000,000	'
Intersegment revenues	1,000,000		2,000,000	1,000,000		3,000,000	'
Net income (loss) attributable to Ameren Corporation from continuing operations	28,000,000		31,000,000	81,000,000		62,000,000	'
Total assets	7,719,000,000		'	7,719,000,000		'	7,454,000,000
Other	'		'	'		'	'
Segment Reporting Information [Line Items]	'		'	'		'	'
External revenues	8,000,000		6,000,000	17,000,000		9,000,000	'
Intersegment revenues	'		'	1,000,000		1,000,000	'
Net income (loss) attributable to Ameren Corporation from continuing operations	-4,000,000		-10,000,000	-7,000,000		-27,000,000	'
Total assets	773,000,000		'	773,000,000		'	752,000,000
Intersegment Eliminations	'		'	'		'	'
Segment Reporting Information [Line Items]	'		'	'		'	'
External revenues	'		'	'		'	'
Intersegment revenues	-8,000,000		-8,000,000	-15,000,000		-17,000,000	'
Net income (loss) attributable to Ameren Corporation from continuing operations	'		'	'		'	'
Total assets	-122,000,000		'	-122,000,000		'	-233,000,000
Consolidated, Continuing Operations	'		'	'		'	'
Segment Reporting Information [Line Items]	'		'	'		'	'
External revenues	1,419,000,000		1,403,000,000	3,013,000,000		2,878,000,000	'
Intersegment revenues	'		'	'		'	'
Net income (loss) attributable to Ameren Corporation from continuing operations	150,000,000		105,000,000	247,000,000		159,000,000	'
Total assets	$21,573,000,000	[1]	'	$21,573,000,000	[1]	'	$20,877,000,000	[1]
[1]	Excludes total assets from discontinued operations of $15 million and $165 million as of June 30, 2014, and December 31, 2013, respectively.