Ameren (AEE) 10-Q11 May 152015 Q1 Quarterly reportFinancial data

Company Profile

Document_and_Entity_Informatio

Document and Entity Information	3 Months Ended
Document and Entity Information	Mar. 31, 2015	Apr. 30, 2015
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	FALSE
Document Period End Date	31-Mar-15
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Trading Symbol	AEE
Entity Registrant Name	AMEREN CORP
Entity Central Index Key	1002910
Current Fiscal Year End Date	-19
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		242,634,798
Union Electric Company
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	FALSE
Document Period End Date	31-Mar-15
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Entity Registrant Name	UNION ELECTRIC CO
Entity Central Index Key	100826
Current Fiscal Year End Date	-19
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		102,123,834
Ameren Illinois Company
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	FALSE
Document Period End Date	31-Mar-15
Document Fiscal Year Focus	2015
Document Fiscal Period Focus	Q1
Entity Registrant Name	AMEREN ILLINOIS CO
Entity Central Index Key	18654
Current Fiscal Year End Date	-19
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		25,452,373

Consolidated_Statement_of_Inco

Consolidated Statement of Income (Loss) (USD $)	3 Months Ended
In Millions, except Per Share data, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Operating Revenues:
Electric	$1,143		$1,106
Gas	413		488
Total operating revenues	1,556		1,594
Operating Expenses:
Fuel	206		204
Purchased power	139		114
Gas purchased for resale	236		304
Other operations and maintenance	401		418
Depreciation and amortization	193		181
Taxes other than income taxes	125		127
Total operating expenses	1,300		1,348
Operating Income	256		246
Other Income and Expenses:
Miscellaneous income	19	[1]	18	[1]
Miscellaneous expense	11	[1]	9	[1]
Total other income (expense)	8		9
Interest Charges	88		92
Income Before Income Taxes	176		163
Income Taxes	66		64
Income from Continuing Operations	110		99
Loss from Discontinued Operations, Net of Taxes (Note 12)	0		-1
Net Income (Loss)	110		98
Comprehensive Income	108		96
Less: Net Income (Loss) Attributable to Noncontrolling Interests:
Net Income from Continuing Operations Attributable to Noncontrolling Interests	2		2
Net Income (Loss):
Continuing Operations	108		97
Discontinued Operations	0		-1
Net Income (Loss)	108		96
Earnings Per Share, Basic [Abstract]
Continuing Operations	$0.45		$0.40
Discontinued Operations	$0		$0
Earnings Per Share, Basic	$0.45		$0.40
Dividends per Common Share	$0.41		$0.40
Average Common Shares Outstanding - Basic	242.6		242.6
Union Electric Company
Operating Revenues:
Electric	742		749
Gas	58		68
Total operating revenues	800		817
Operating Expenses:
Fuel	206		204
Purchased power	39		35
Gas purchased for resale	31		40
Other operations and maintenance	211		225
Depreciation and amortization	118		116
Taxes other than income taxes	80		78
Total operating expenses	685		698
Operating Income	115		119
Other Income and Expenses:
Miscellaneous income	11		14
Miscellaneous expense	3		4
Total other income (expense)	8		10
Interest Charges	55		52
Income Before Income Taxes	68		77
Income Taxes	26		29
Net Income (Loss)	42		48
Other Comprehensive Income	0		0
Comprehensive Income	42		48
Net Income (Loss):
Net Income (Loss)	42		48
Earnings Per Share, Basic [Abstract]
Preferred Stock Dividends	1		1
Net Income Available to Common Stockholder	41		47
Ameren Illinois Company
Operating Revenues:
Electric	390		353
Gas	355		421
Total operating revenues	745		774
Operating Expenses:
Purchased power	102		81
Gas purchased for resale	205		264
Other operations and maintenance	202		200
Depreciation and amortization	73		63
Taxes other than income taxes	43		46
Total operating expenses	625		654
Operating Income	120		120
Other Income and Expenses:
Miscellaneous income	7		3
Miscellaneous expense	5		4
Total other income (expense)	2		-1
Interest Charges	33		30
Income Before Income Taxes	89		89
Income Taxes	35		35
Net Income (Loss)	54		54
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	-1		-1
Comprehensive Income	53		53
Net Income (Loss):
Net Income (Loss)	54		54
Earnings Per Share, Basic [Abstract]
Preferred Stock Dividends	1		1
Net Income Available to Common Stockholder	$53		$53

[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Consolidated_Statement_of_Inco1

Consolidated Statement of Income (Loss) (Parenthetical) (Ameren Illinois Company, USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
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
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Income from Continuing Operations	$110	$99
Other Comprehensive Income from Continuing Operations, Net Of Taxes	0	0
Other Comprehensive Income from Continuing Operations, Net of Taxes
Comprehensive Income from Continuing Operations	110	99
Less: Comprehensive Income from Continuing Operations Attributable to Noncontrolling Interests	2	2
Comprehensive Income from Continuing Operations Attributable to Ameren Corporation	108	97
Loss from Discontinued Operations, Net of Taxes	0	-1
Other Comprehensive Loss from Discontinued Operations, Net of Taxes	0	0
Comprehensive Loss from Discontinued Operations Attributable to Ameren Corporation	0	-1
Comprehensive Income	$108	$96

Consolidated_Balance_Sheet

Consolidated Balance Sheet (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Current Assets:
Cash and cash equivalents	$6	$5
Accounts receivable - trade (less allowance for doubtful accounts)	524	423
Unbilled revenue	212	265
Miscellaneous accounts and notes receivable	100	81
Materials and supplies	449	524
Current regulatory assets	265	295
Current accumulated deferred income taxes, net	331	352
Other current assets	91	86
Assets of discontinued operations (Note 12)	15	15
Total current assets	1,993	2,046
Property and Plant, Net	17,700	17,424
Investments and Other Assets:
Nuclear decommissioning trust fund	558	549
Goodwill	411	411
Regulatory assets	1,577	1,582
Other assets	645	664
Total investments and other assets	3,191	3,206
TOTAL ASSETS	22,884	22,676
Current Liabilities:
Current maturities of long-term debt	380	120
Short-term Debt	955	714
Accounts and wages payable	434	711
Taxes accrued	79	46
Interest accrued	94	85
Current regulatory liabilities	107	106
Other current liabilities	437	434
Liabilities of discontinued operations (Note 12)	34	33
Total current liabilities	2,520	2,249
Long-term Debt, Net	5,860	6,120
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	3,964	3,923
Accumulated deferred investment tax credits	65	64
Regulatory liabilities	1,897	1,850
Asset retirement obligations	500	396
Pension and other postretirement benefits	708	705
Other deferred credits and liabilities	524	514
Total deferred credits and other liabilities	7,658	7,452
Commitments and Contingencies
Stockholders' Equity:
Common Stock	2	2
Other paid-in capital	5,600	5,617
Retained earnings	1,111	1,103
Accumulated other comprehensive income (loss)	-9	-9
Stockholder's equity	6,704	6,713
Noncontrolling Interest	142	142
Total equity	6,846	6,855
TOTAL LIABILITIES AND EQUITY	22,884	22,676
Union Electric Company
Current Assets:
Cash and cash equivalents	1	1
Accounts receivable - trade (less allowance for doubtful accounts)	202	190
Accounts receivable - affiliates	3	65
Unbilled revenue	120	146
Miscellaneous accounts and notes receivable	42	35
Materials and supplies	361	347
Current regulatory assets	158	163
Other current assets	77	92
Total current assets	964	1,039
Property and Plant, Net	10,959	10,867
Investments and Other Assets:
Nuclear decommissioning trust fund	558	549
Regulatory assets	687	695
Other assets	387	391
Total investments and other assets	1,632	1,635
TOTAL ASSETS	13,555	13,541
Current Liabilities:
Current maturities of long-term debt	380	120
Borrowings from money pool	61	0
Short-term Debt	140	97
Accounts and wages payable	182	405
Accounts payable - affiliates	59	56
Taxes accrued	69	32
Interest accrued	51	58
Current regulatory liabilities	32	18
Other current liabilities	114	117
Total current liabilities	1,088	903
Long-term Debt, Net	3,619	3,879
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	2,821	2,806
Accumulated deferred investment tax credits	62	61
Regulatory liabilities	1,169	1,147
Asset retirement obligations	493	389
Pension and other postretirement benefits	277	274
Other deferred credits and liabilities	32	30
Total deferred credits and other liabilities	4,854	4,707
Commitments and Contingencies
Stockholders' Equity:
Common Stock	511	511
Other paid-in capital	1,784	1,569
Preferred stock	80	80
Retained earnings	1,619	1,892
Stockholder's equity	3,994	4,052
TOTAL LIABILITIES AND EQUITY	13,555	13,541
Ameren Illinois Company
Current Assets:
Cash and cash equivalents	0	1
Advances to money pool	33	0
Accounts receivable - trade (less allowance for doubtful accounts)	304	212
Accounts receivable - affiliates	5	22
Unbilled revenue	92	119
Miscellaneous accounts and notes receivable	12	9
Materials and supplies	88	177
Current regulatory assets	105	129
Current accumulated deferred income taxes, net	159	160
Other current assets	13	15
Total current assets	811	844
Property and Plant, Net	6,272	6,165
Investments and Other Assets:
Goodwill	411	411
Regulatory assets	883	883
Other assets	79	78
Total investments and other assets	1,373	1,372
TOTAL ASSETS	8,456	8,381
Current Liabilities:
Borrowings from money pool	0	15
Short-term Debt	0	32
Accounts and wages payable	193	207
Accounts payable - affiliates	49	50
Taxes accrued	41	17
Interest accrued	43	24
Customer deposits	76	77
Mark-to-market derivative liabilities	43	42
Current environmental remediation	46	52
Current regulatory liabilities	69	84
Other current liabilities	100	100
Total current liabilities	660	700
Long-term Debt, Net	2,241	2,241
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,421	1,408
Regulatory liabilities	727	703
Pension and other postretirement benefits	278	277
Environmental remediation	200	199
Other deferred credits and liabilities	216	192
Total deferred credits and other liabilities	2,842	2,779
Commitments and Contingencies
Stockholders' Equity:
Common Stock	0	0
Other paid-in capital	1,980	1,980
Preferred stock	62	62
Retained earnings	664	611
Accumulated other comprehensive income (loss)	7	8
Stockholder's equity	2,713	2,661
TOTAL LIABILITIES AND EQUITY	$8,456	$8,381

Consolidated_Balance_Sheet_Par

Consolidated Balance Sheet (Parenthetical) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, except Per Share data, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Accounts receivable - trade allowance for doubtful accounts	$23	$21
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	8	8
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	$15	$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 $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Cash Flows From Operating Activities:
Net income (loss)	$110		$98
Loss from discontinued operations, net of taxes	0		1
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	195		176
Amortization of nuclear fuel	23		24
Amortization of debt issuance costs and premium/discounts	5		5
Deferred income taxes and investment tax credits, net	59		84
Allowance for equity funds used during construction	-5	[1]	-7	[1]
Stock-based compensation costs	8		9
Other	-11		-1
Changes in assets and liabilities:
Receivables	-48		-86
Materials and supplies	75		102
Accounts and wages payable	-215		-183
Taxes accrued	33		18
Regulatory assets and liabilities	62		-40
Assets, other	14		10
Liabilities, other	-33		-11
Pension and other postretirement benefits	27		30
Counterparty collateral, net	-2		10
Net cash provided by operating activities b continuing operations	297		239
Net cash provided by operating activities b discontinued operations	1		0
Net cash provided by operating activities	298		239
Cash Flows From Investing Activities:
Capital expenditures	-417		-442
Nuclear fuel expenditures	-17		-10
Purchases of securities - nuclear decommissioning trust fund	-84		-186
Sales and maturities of securities - nuclear decommissioning trust fund	79		182
Proceeds from Notes Receivable	5		56
Contributions to Note Receivable	-5		-65
Net cash used in investing activities b continuing operations	-439		-465
Net cash provided by investing activities b discontinued operations	0		152
Net cash used in investing activities	-439		-313
Cash Flows From Financing Activities:
Dividends on common stock	-99		-97
Dividends paid to noncontrolling interest holders	-2		-2
Short-term debt, net	241		332
Redemptions of Long-term Debt	0		-163
Other	2
Net cash provided by (used in) financing activities b continuing operations	142		70
Net cash used in financing activities - discontinued operations	0		0
Net cash provided by financing activities	142		70
Net change in cash and cash equivalents	1		-4
Cash and cash equivalents at beginning of period	5		30
Cash and cash equivalents at beginning of period	5
Cash and cash equivalents at end of period	6		26
Union Electric Company
Cash Flows From Operating Activities:
Net income (loss)	42		48
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	121		112
Amortization of nuclear fuel	23		24
Amortization of debt issuance costs and premium/discounts	2		2
Deferred income taxes and investment tax credits, net	21		30
Allowance for equity funds used during construction	-4		-7
Changes in assets and liabilities:
Receivables	60		2
Materials and supplies	-14		14
Accounts and wages payable	-171		-153
Taxes accrued	40		30
Regulatory assets and liabilities	27		-28
Assets, other	3		5
Liabilities, other	-5		2
Pension and other postretirement benefits	12		15
Net cash provided by operating activities	157		96
Cash Flows From Investing Activities:
Capital expenditures	-145		-188
Nuclear fuel expenditures	-17		-10
Purchases of securities - nuclear decommissioning trust fund	-84		-186
Sales and maturities of securities - nuclear decommissioning trust fund	79		182
Other	-2		-2
Net cash used in investing activities	-169		-204
Cash Flows From Financing Activities:
Dividends on common stock	-315		-77
Dividends on preferred stock	-1		-1
Short-term debt, net	43		290
Money pool borrowings, net	61		-105
Capital contribution from parent	224
Net cash provided by financing activities	12		107
Net change in cash and cash equivalents			-1
Cash and cash equivalents at beginning of period	1		1
Cash and cash equivalents at end of period	1		0
Ameren Illinois Company
Cash Flows From Operating Activities:
Net income (loss)	54		54
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization	72		62
Amortization of debt issuance costs and premium/discounts	4		3
Deferred income taxes and investment tax credits, net	13		36
Allowance for equity funds used during construction	-1
Other	-3		-2
Changes in assets and liabilities:
Receivables	-41		-94
Materials and supplies	89		88
Accounts and wages payable	-11		14
Taxes accrued	24		-1
Regulatory assets and liabilities	33		-11
Assets, other	6		5
Liabilities, other	4		16
Pension and other postretirement benefits	11		10
Counterparty collateral, net	-1		12
Net cash provided by operating activities	254		192
Cash Flows From Investing Activities:
Capital expenditures	-174		-215
Money pool advances, net	-33		0
Other	0		1
Net cash used in investing activities	-207		-214
Cash Flows From Financing Activities:
Dividends on preferred stock	-1		-1
Short-term debt, net	-32
Money pool borrowings, net	-15		186
Redemptions of Long-term Debt	0		-163
Net cash provided by financing activities	-48		22
Net change in cash and cash equivalents	-1		0
Cash and cash equivalents at beginning of period	1		1
Cash and cash equivalents at end of period	$0		$1

[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Summary_Of_Significant_Account

Summary Of Significant Accounting Policies	3 Months Ended
Summary Of Significant Accounting Policies	Mar. 31, 2015
Accounting Policies [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
	General
	Ameren, headquartered in St. Louis, Missouri, is a public utility holding company under PUHCA 2005, administered by the FERC. Ameren’s primary assets are its equity interests in its subsidiaries, including Ameren Missouri and Ameren Illinois. Ameren’s subsidiaries are separate, independent legal entities with separate businesses, assets, and liabilities. Dividends on Ameren’s common stock and the payment of 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 Illinois Company, doing business as Ameren Illinois, operates rate-regulated electric and natural gas transmission and distribution businesses in Illinois.
	Ameren has various other subsidiaries that conduct activities such as the provision of shared services. Ameren also has a subsidiary, ATXI, that operates a FERC rate-regulated electric transmission business. ATXI is developing MISO-approved electric transmission projects, including the Illinois Rivers, Spoon River, and Mark Twain projects. Ameren is also pursuing reliability projects within Ameren Missouri's and Ameren Illinois' service territories as well as competitive electric transmission investment opportunities outside of these territories, including investments outside of MISO.
	The operating results, assets, and liabilities of 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. 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 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.
	Asset Retirement Obligations
	The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the three months ended March 31, 2015:


		Ameren				Ameren		Ameren
		Missouri				Illinois(a)		Ameren
	Balance at December 31, 2014	$	389			$	7	$	396
	Balance at December 31, 2014	$	389			$	7	$	396
	Accretion in 2015(b)	5				(c)		5
	Accretion in 2015(b)	5				(c)		5
	Change in estimates(d)	99				(c)		99
	Change in estimates(d)	99				(c)		99
	Balance at March 31, 2015	$	493			$	7	$	500
	Balance at March 31, 2015	$	493			$	7	$	500

	(a)	Included in “Other deferred credits and liabilities” on the balance sheet.

	(b)	Accretion expense was recorded as an increase to regulatory assets at Ameren Missouri and Ameren Illinois.

	(c)	Less than $1 million.

	(d)	The ARO increase also resulted in a corresponding increase recorded to “Property and Plant, Net.” Ameren Missouri’s estimates related to its Callaway energy center decommissioning costs changed to reflect increased costs from the 2015 cost study and funding analysis, extension of the estimated operating life until 2044, and a reduction in the discount rate assumption. See Note 10 - Callaway Energy Center for additional information.
	In addition, during the second quarter of 2015, Ameren and Ameren Missouri each expect to record an increase to their ARO related to retirement costs for CCR storage facilities of between $90 million and $120 million, with a corresponding increase to “Property and Plant, Net.” This increase is a result of the EPA’s new regulations for the management and disposal of CCR, which were published in April 2015. See Note 9 - Commitments and Contingencies in this report for additional information.
	Stock-based Compensation
	A summary of nonvested performance share units at March 31, 2015, and changes during the three months ended March 31, 2015, under the 2006 Incentive Plan and the 2014 Incentive Plan are presented below:


		Number of Performance Share Units		Weighted-average Fair Value Per Performance Share Unit
	Nonvested at January 1, 2015	1,162,377		$	35.35
	Nonvested at January 1, 2015	1,162,377		$	35.35
	Granted(a)	566,332		52.88
	Granted(a)	566,332		52.88
	Forfeitures	(1,944	)	34.75
	Forfeitures	(1,944	)	34.75
	Vested(b)	(68,411	)	47.88
	Vested(b)	(68,411	)	47.88
	Nonvested at March 31, 2015	1,658,354		$	40.82
	Nonvested at March 31, 2015	1,658,354		$	40.82

	(a)	Performance share units granted to certain executive and nonexecutive officers and other eligible employees in 2015 under the 2014 Incentive Plan.

	(b)	Performance 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 performance share unit awarded in 2015 under the 2014 Incentive Plan was determined to be $52.88, which was based on Ameren’s closing common share price of $46.13 at December 31, 2014, 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, 2015. The simulations can produce a greater fair value for the performance 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 1.10%, 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.
	Excise Taxes
	Ameren Missouri and Ameren Illinois collect certain excise taxes from customers that are levied on the sale or distribution of natural gas and electricity. Excise taxes are levied on Ameren Missouri’s electric and natural gas businesses and on Ameren Illinois’ natural gas business and are recorded gross in “Operating Revenues - Electric,” “Operating Revenues - Gas” and “Operating Expenses - Taxes other than income taxes” on the statement of income or the statement of income and comprehensive income. Excise taxes for electric service in Illinois are levied on the customer and are therefore not included in Ameren Illinois’ revenues and expenses. 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 months ended March 31, 2015 and 2014:


		Three Months
		2015				2014
	Ameren Missouri	$	34			$	34
	Ameren Missouri	$	34			$	34
	Ameren Illinois	23				26
	Ameren Illinois	23				26
Ameren	$	57			$	60
Ameren	$	57			$	60
Uncertain Tax Positions
The following table presents the total amount of reserves for unrecognized tax benefits (detriments) related to uncertain tax positions as of March 31, 2015, and December 31, 2014:


	31-Mar-15				31-Dec-14
Ameren	$	53			$	54
Ameren	$	53			$	54
Ameren Missouri	(1		)		—
Ameren Missouri	(1		)		—
Ameren Illinois	(1		)		(1		)
The following table presents the amount of reserves for unrecognized tax benefits, included in the table above, related to uncertain tax positions that, if recognized, would impact results of operations as of March 31, 2015, and December 31, 2014:


	31-Mar-15				31-Dec-14
Ameren	$	52			$	52
Ameren	$	52			$	52
Ameren Missouri	(1		)		—
Ameren Missouri	(1		)		—
Ameren Illinois	(1		)		(1		)
In March 2015, a settlement was reached with the IRS for tax year 2012. Since there were no uncertain tax positions related to the 2012 tax year as of December 31, 2014, this settlement did not impact the amount of recorded unrecognized tax benefits.
Ameren’s federal income tax return for the tax year 2013 is currently under examination by the IRS. It is reasonably possible that a settlement will be reached with the IRS in the next 12 months, which will result in a reduction of Ameren’s unrecognized tax benefits of $53 million related to discontinued operations.
In addition, it is reasonably possible that other events will occur during the next 12 months that would cause the total amount of our unrecognized tax benefits to fluctuate. However, other than as described above, we do not believe any such fluctuations would be material to our results of operations, financial position, or liquidity.
State income tax returns are generally subject to examination for a period of three years after filing. We do not currently have material state income tax issues under examination, administrative appeal, 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.
Earnings Per Share
There were no material differences between Ameren’s basic and diluted earnings per share amounts for the three months ended March 31, 2015 and 2014. The assumed settlement of dilutive performance share units had an immaterial impact on earnings per share.
Accounting and Reporting Developments
Below is a summary of recently adopted or issued authoritative accounting guidance relevant to the Ameren Companies.
Presentation of Debt Issuance Costs
In April 2015, FASB issued authoritative accounting guidance to simplify the presentation of debt issuance costs. The guidance requires debt issuance costs to be presented in the balance sheet as a reduction to the associated debt liability. Currently, debt issuance costs are presented as a component of “Other assets” on the Ameren Companies’ balance sheets. The guidance will be effective for the Ameren Companies in the first quarter of 2016 and applied retrospectively. The guidance will not affect the Ameren Companies' results of operations, financial position, or liquidity, as this guidance is presentation-related only.

Rate_And_Regulatory_Matters

Rate And Regulatory Matters	3 Months Ended
Rate And Regulatory Matters	Mar. 31, 2015
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
	2015 Electric Rate Order
	In April 2015, the MoPSC issued an order approving an increase in Ameren Missouri’s annual revenues for electric service of $122 million, including $109 million related to the increase in net energy costs above the net energy costs included in base rates previously authorized by the MoPSC. The remaining increase of $13 million approved by the order was for non-energy costs. The revenue increase was based on a 9.53% return on common equity, a capital structure composed of 51.8% common equity, and a rate base of $7.0 billion to reflect investments through December 31, 2014. Rate changes consistent with the order will become effective on May 30, 2015.
	The order approved Ameren Missouri’s request for continued use of the FAC; however, it changed the FAC to exclude substantially all transmission charges and revenues. In addition, the order did not approve the continued use of the regulatory tracking mechanisms for storm costs and vegetation management and infrastructure inspection costs. The order did approve the continued use of the regulatory tracking mechanisms for pension and postretirement benefits, renewable energy standard cost, solar rebates, and uncertain tax positions that the MoPSC authorized in earlier electric rate orders.
	In addition, the order approved a reduction to Noranda’s electric rates with an offsetting increase in electric rates for Ameren Missouri’s other customers. The rate shift will be revenue neutral to Ameren Missouri. Ameren Missouri supplies electricity to Noranda’s aluminum smelter in southeast Missouri under a 15-year agreement, which is subject to termination as early as May 31, 2020, and on each May 31 thereafter, upon at least five years notice by either party. Termination of the agreement by Ameren Missouri would require MoPSC approval.
	Ameren Missouri will request a rehearing on several aspects of the MoPSC’s order, including the allowed return on common equity and the elimination of recovery of changes in transmission charges and revenues through the FAC. The MoOPC and the intervenor parties in this case may similarly seek rehearing or subsequently appeal any aspect of the order. Ameren Missouri cannot predict whether any such application for rehearing or appeal will be filed, or the outcome if so filed.
	Accounting Authority Order
	In November 2013, the MoPSC issued an accounting authority order that allowed Ameren Missouri to seek recovery of fixed costs totaling $36 million that were not previously recovered from Noranda as a result of the loss of load caused by a severe 2009 ice storm in a future electric rate case. In its April 2015 electric rate order, the MoPSC did not approve recovery of these fixed costs. Ameren Missouri had not recorded any revenue associated with this accounting authority order and will not record a charge to earnings based on the outcome of the MoPSC’s April 2015 electric rate order.
	MEEIA Filing
	In December 2014, Ameren Missouri filed an energy efficiency plan with the MoPSC under the MEEIA. This filing proposed a three-year plan that includes a portfolio of customer energy efficiency programs along with a cost recovery mechanism. If the plan is approved, beginning in January 2016, Ameren Missouri intends to invest $135 million over three years in the proposed customer energy efficiency programs. Ameren Missouri requested continued use of a MEEIA rider, which allows it to collect from or refund to customers any difference in the actual amounts incurred and the amounts collected from customers for the MEEIA program costs and net shared benefits. In addition, Ameren Missouri requested incentives to earn additional revenues by achieving certain customer energy efficiency goals, including $25 million if 100% of its customer energy efficiency goals are achieved during the three-year period. Ameren Missouri must achieve at least 70% of its customer energy efficiency goals before it earns any incentive award.
	Illinois
	IEIMA
	Under the provisions of the IEIMA, Ameren Illinois’ electric delivery service rates are subject to an annual revenue requirement reconciliation to its actual costs.Throughout each year, Ameren Illinois records a regulatory asset or a regulatory liability and a corresponding increase or decrease to operating revenues for any differences between the revenue requirement reflected in customer rates for that year and its estimate of the probable increase or decrease in the revenue requirement expected to ultimately be approved by the ICC based on that year's actual costs incurred. As of March 31, 2015, Ameren Illinois had recorded regulatory assets of $8 million, $101 million, and $52 million, to reflect its expected 2015, 2014 and 2013 revenue requirement reconciliation adjustments, with interest, respectively. Ameren Illinois is collecting the 2013 revenue requirement reconciliation adjustment from customers during 2015.
	In April 2015, Ameren Illinois filed with the ICC its annual electric delivery service formula rate update to establish the revenue requirement used for 2016 rates. Pending ICC approval, Ameren Illinois’ update filing will result in a $110 million increase in Ameren Illinois’ electric delivery service revenue requirement, beginning in January 2016. This update reflects an increase to the annual formula rate based on 2014 actual costs and expected net plant additions for 2015, an increase to include the 2014 revenue requirement reconciliation adjustment, and a decrease for the conclusion of the 2013 revenue requirement reconciliation adjustment, which will be fully collected from customers in 2015.
	In April 2015, the IEIMA’s formula rate framework was extended until the end of 2019, with further extensions possible through 2022.
	2015 Natural Gas Delivery Service Rate Case
	In January 2015, Ameren Illinois filed a request with the ICC seeking approval to increase its annual revenues for natural gas delivery service by $53 million. The request was based on a 10.25% return on common equity, a capital structure composed of 50% common equity, and a rate base of $1.2 billion. In an attempt to reduce regulatory lag, Ameren Illinois used a 2016 future test year in this proceeding. Included in the request was a proposal to implement a decoupling rider mechanism for residential and small nonresidential customers. The decoupling rider would ensure that changes in natural gas sales volumes do not affect Ameren Illinois' annual natural gas revenues for these rate classes.
	A decision by the ICC in this proceeding is required by December 2015, with new rates expected to be effective in January 2016. Ameren Illinois cannot predict the level of any delivery service rate changes the ICC may approve, whether the ICC will approve the decoupling rider, or whether any rate changes that may eventually be approved will be sufficient to enable Ameren Illinois to recover its costs and to earn a reasonable return on investments when the rate changes go into effect.
	2013 Natural Gas Delivery Service Rate Order
	In December 2013, the ICC issued a rate order that approved an increase in Ameren Illinois’ revenues for natural gas delivery service based on a 9.1% return on common equity. The rate changes became effective January 1, 2014. In March 2014, Ameren Illinois filed with the Appellate Court of the Fourth District of Illinois an appeal of the allowed return on common equity included in the ICC's order. Ameren Illinois sought a 10.4% return on common equity in this rate case. A decision is expected in 2015.
	2015 ICC Purchased Power Reconciliation
	In January 2015, the ICC issued an order that approved Ameren Illinois' reconciliation of revenues collected under its purchased power rider mechanism and Ameren Illinois' related cumulative power usage cost. In the first quarter of 2015, based on the January 2015 order, both Ameren and Ameren Illinois recorded a $15 million increase to electric revenues for the recovery of this cumulative power usage cost from electric customers.
	ATXI Transmission Project
	In August 2014, ATXI made a filing with the ICC requesting a certificate of public convenience and necessity and project approval for the Spoon River project, a MISO-approved transmission line project located in northwest Illinois. A decision is expected from the ICC in 2015. A certificate of public convenience and necessity is required before ATXI can proceed with right-of-way acquisition.
	Federal
	Ameren Illinois Electric Transmission Rate Refund
	In July 2012, the FERC issued an order concluding that Ameren Illinois improperly included acquisition premiums, including goodwill, in determining the common equity used in its electric transmission formula rate and thereby inappropriately recovered a higher amount from its electric transmission customers. The order required Ameren Illinois to make refunds to customers for such improperly included amounts.
	Ameren Illinois submitted a refund report in November 2012, which concluded that no refund was warranted. Several wholesale customers filed a protest with the FERC regarding that conclusion. In June 2013, the FERC issued an order that rejected Ameren Illinois' November 2012 refund report and provided guidance as to the filing of a new refund report. In July 2013, Ameren Illinois filed a revised refund report based on the guidance provided in the June 2013 order, which also concluded that no refund was warranted. In June 2014, the FERC issued an order establishing settlement procedures and, if necessary, hearing procedures regarding Ameren Illinois’ July 2013 refund report.
	In March 2015, Ameren Illinois reached a settlement agreement with the wholesale customers that resolves the issues in this proceeding. The settlement agreement requires FERC approval. Upon approval by the FERC, the settlement agreement will require Ameren Illinois to make refunds and payments of $8 million to transmission customers. It will also require Ameren Illinois to take other actions, such as reducing common equity for electric transmission ratemaking purposes on a prospective basis. There is no date by which the FERC must act with respect to the settlement agreement. Ameren Illinois estimates the maximum refund obligation through March 31, 2015, to be $23 million. Ameren Illinois’ March 31, 2015 and December 31, 2014 balance sheets included an $8 million and a $7 million current liability, respectively, for its estimate of the probable refund to transmission customers. 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 was made.
	FERC Complaint Cases
	Currently, the FERC-allowed base return on common equity for MISO transmission owners is 12.38%. In November 2013, a customer group filed a complaint case with the FERC seeking a reduction in the allowed base return on common equity for the FERC-regulated MISO transmission rate base under the MISO tariff to 9.15%. The FERC scheduled the case for hearing proceedings, requiring an initial decision to be issued no later than November 30, 2015. As the maximum FERC-allowed refund period for the November 2013 complaint case ended in February 2015, another customer complaint case was filed in February 2015. The February 2015 complaint case seeks a reduction in the allowed base return on common equity for the FERC-regulated MISO transmission rate base under the MISO tariff to 8.67%.
	In 2014, the FERC issued orders in a proceeding, in which the Ameren Companies were not involved, reducing the allowed base return on common equity for New England transmission owners from 11.14% to 10.57%, with rate incentives allowed up to 11.74%. The FERC order in the New England transmission owners’ case applied observable market data from October 2012 to March 2013 to determine the allowed base return on common equity. The evidence and the calculation used in the New England transmission owners’ case may guide the FERC’s decision in the MISO complaint cases discussed above. The FERC calculation will establish the allowed base return on common equity, which specifies a unique time period for each complaint case, and will require multiple inputs based on observable market data specific to the utility industry and broader macroeconomic data. In January 2015, the settlement judge for the initial MISO complaint case ordered that July 13, 2015, be the cut-off date for the observable market data to be used in the calculation of the allowed base return on common equity. Based on the information in these orders, Ameren and Ameren Illinois recorded current liabilities on their respective balance sheets as of March 31, 2015, and December 31, 2014, representing their estimate of the refunds from the refund effective date of November 12, 2013, through the respective balance sheet date. A 50 basis point reduction in the FERC-allowed return on common equity would reduce Ameren's and Ameren Illinois' 2015 earnings by an estimated $4 million and $2 million, respectively, based on 2015 projected rate base. Ameren Missouri did not record a liability as of March 31, 2015, and does not expect that a reduction in the FERC-allowed base return on common equity for MISO transmission owners would be material to its results of operations, financial position, or liquidity.
	Based on a November 2014 request, the FERC approved an incentive adder of up to 50 basis points on the allowed base return on common equity for our participation in an RTO. The incentive adder became effective on January 6, 2015. The FERC also approved our request to defer collection of the incentive adder until the issuance of the final order addressing the initial MISO complaint case.
	Ameren Missouri Power Purchase Agreement with Entergy
	Beginning in 2005, the FERC issued a series of orders addressing a complaint filed in 2001 by the Louisiana Public Service Commission against Entergy and certain of its affiliates. The complaint alleged unjust and unreasonable cost allocations. As a result of the FERC orders, Entergy began billing Ameren Missouri in 2007 for additional charges under a 165-megawatt power purchase agreement, which expired August 31, 2009. In May 2012, the FERC issued an order stating that Entergy should not have included additional charges to Ameren Missouri under the power purchase agreement. Pursuant to the order, in June 2012, Entergy paid Ameren Missouri $31 million. In November 2013, Entergy filed an appeal of the FERC's May 2012 order with the United States Court of Appeals for the District of Columbia Circuit. In March 2015, the United States Court of Appeals for the District of Columbia Circuit upheld the FERC’s May 2012 order. Ameren Missouri believes the outstanding issues relating to its power purchase agreement with Entergy have been resolved and will not result in a charge to earnings.
	Combined Construction and Operating License
	In 2008, Ameren Missouri filed an application with the NRC for a COL for a new nuclear unit at Ameren Missouri's existing Callaway County, Missouri, energy center site. In 2009, Ameren Missouri suspended its efforts to build a new nuclear unit at the Callaway site, and the NRC suspended review of the COL application. The suspended status of the COL application currently extends through the end of 2015.
	Ameren Missouri estimates the total cost to obtain a COL for the Callaway site to be approximately $100 million. As of March 31, 2015, Ameren Missouri had capitalized investments of $69 million for the development of a new nuclear energy center. Ameren is currently evaluating all potential nuclear technologies in order to maintain an option for nuclear power in the future.
	All of Ameren Missouri's capitalized investments for the development of a new nuclear energy center will remain capitalized while management pursues options to maximize the value of its investment. If efforts to license additional nuclear generation are abandoned, if the NRC does not extend the COL application suspended status, or if management concludes it is probable that the costs incurred will be disallowed in rates, a charge to earnings would be recognized in the period in which that determination is made.

ShortTerm_Debt_And_Liquidity

Short-Term Debt And Liquidity	3 Months Ended
	Mar. 31, 2015
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 December 11, 2019, were not utilized for direct borrowings during the three months ended March 31, 2015, but were used to support commercial paper issuances and to issue letters of credit. Based on letters of credit issued under the 2012 Credit Agreements, as well as commercial paper outstanding, the aggregate amount of credit capacity available under the 2012 Credit Agreements to Ameren (parent), Ameren Missouri, and Ameren Illinois, collectively, at March 31, 2015, was $1.1 billion.
	Commercial Paper
	The following table presents commercial paper outstanding at Ameren (parent), Ameren Missouri, and Ameren Illinois as of March 31, 2015, and December 31, 2014.


		31-Mar-15				31-Dec-14
	Ameren (parent)	$	815			$	585

	Ameren Missouri	140				97

	Ameren Illinois	—				32

	Ameren Consolidated	$	955			$	714

	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 three months ended March 31, 2015 and 2014. Ameren Illinois established a commercial paper program in May 2014.


			Ameren				Ameren			Ameren			Ameren Consolidated
			(parent)				Missouri			Illinois
	2015
	Average daily commercial paper outstanding		$	691			$	151		$	10		$	852

	Weighted-average interest rate		0.55		%		0.49		%	0.44		%	0.53		%
	Peak commercial paper during period(a)		$	815			$	243		$	39		$	955

	Peak interest rate		0.7		%		0.6		%	0.6		%	0.7		%
	2014
	Average daily commercial paper outstanding		$	339			$	200		$	—		$	539

	Weighted-average interest rate		0.45		%		0.45		%	—		%	0.45		%
	Peak commercial paper during period(a)		$	452			$	290		$	—		$	700

	Peak interest rate		0.75		%		0.7		%	—		%	0.75		%

	(a)	The timing of peak commercial paper issuances varies by company, and therefore the peak amounts presented by company might not equal the Ameren Consolidated peak commercial paper issuances for the period.
	Indebtedness Provisions and Other Covenants
	The information below is a summary of the Ameren Companies’ compliance with indebtedness provisions and other covenants within the 2012 Credit Agreements. See Note 4 - Short-term Debt and Liquidity under Part II, Item 8, in the Form 10-K for a detailed description of these provisions.
	The 2012 Credit Agreements contain nonfinancial covenants, including restrictions on the ability to incur liens, to transact with affiliates, to dispose of assets, to make investments in or transfer assets to its affiliates, and to merge with other entities. The 2012 Credit Agreements require each of Ameren, Ameren Missouri, and Ameren Illinois to maintain consolidated indebtedness of not more than 65% of its consolidated total capitalization pursuant to a defined calculation set forth in the agreements. As of March 31, 2015, the ratios of consolidated indebtedness to total consolidated capitalization, calculated in accordance with the provisions of the 2012 Credit Agreements, were 51%, 50%, and 46%, for Ameren, Ameren Missouri, and Ameren Illinois, respectively. In addition, under the 2012 Illinois Credit Agreement and, by virtue of the cross-default provisions of the 2012 Missouri Credit Agreement, under the 2012 Missouri Credit Agreement, Ameren is required to maintain a ratio of consolidated funds from operations plus interest expense to consolidated interest expense of at least 2.0 to 1.0. However, the interest coverage requirement only applies at such times as Ameren does not have a senior long-term unsecured credit rating of at least Baa3 from Moody’s or BBB- from S&P. As of March 31, 2015, Ameren exceeded the rating requirements; therefore, the interest coverage requirement was not applicable. Failure of a borrower to satisfy a financial covenant constitutes an immediate default under the applicable 2012 Credit Agreement.
	The 2012 Credit Agreements contain default provisions that apply separately to each borrower; provided, however, that a default of Ameren Missouri or Ameren Illinois under the applicable 2012 Credit Agreement will also be deemed to constitute a default of Ameren under such agreement. Defaults include a cross-default of such borrower under any other agreement covering outstanding indebtedness of such borrower and certain subsidiaries (other than project finance subsidiaries and nonmaterial subsidiaries) in excess of $75 million in the aggregate (including under the other 2012 Credit Agreement). However, under the default provisions of the 2012 Credit Agreements, any default of Ameren under any 2012 Credit Agreement that results solely from a default of Ameren Missouri or Ameren Illinois thereunder does not result in a cross-default of Ameren under the other 2012 Credit Agreement. Further, the 2012 Credit Agreement default provisions provide that an Ameren default under any of the 2012 Credit Agreements does not constitute a default by Ameren Missouri or Ameren Illinois.
	None of the Ameren Companies' credit agreements or financing 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 March 31, 2015.
	Money Pools
	Ameren has money pool agreements with and among its subsidiaries to coordinate and provide for certain short-term cash and working capital requirements.
	Ameren Missouri, Ameren Illinois, and Ameren Services may participate in the utility money pool as both lenders and borrowers. Ameren may participate in the utility money pool only as a lender. Surplus internal funds are contributed to the utility money pool from participants. The primary sources of external funds for the utility money pool are the 2012 Credit Agreements and the commercial paper programs. The total amount available to the pool participants from the utility money pool at any given time is reduced by the amount of borrowings made by participants, but is increased to the extent that the pool participants advance surplus funds to the utility money pool or remit funds from other external sources. The availability of funds is also determined by funding requirement limits established by regulatory authorizations. Participants receiving a loan under the utility money pool agreement must repay the principal amount of such loan, together with accrued interest. The rate of interest depends on the composition of internal and external funds in the utility money pool. The average interest rate for borrowing under the utility money pool for the three months ended March 31, 2015 and 2014, was 0.08% and 0.39%, 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 months ended March 31, 2015 and 2014.

LongTerm_Debt_And_Equity_Finan

Long-Term Debt And Equity Financings

3 Months Ended

Mar. 31, 2015

Long-Term Debt And Equity Financings [Abstract]

LONG-TERM DEBT AND EQUITY FINANCINGS

LONG-TERM DEBT AND EQUITY

Ameren Missouri

In March 2015, Ameren Missouri received cash capital contributions of $224 million from Ameren (parent).

In April 2015, Ameren Missouri issued $250 million of 3.65% senior secured notes due April 15, 2045, with interest payable semiannually on April 15 and October 15 of each year, beginning October 15, 2015. Ameren Missouri received proceeds of $247 million, which were used to repay outstanding short-term debt, including short-term debt that Ameren Missouri incurred in connection with the repayment of $114 million of its 4.75% senior secured notes that matured on April 1, 2015.

Indenture Provisions and Other Covenants

Ameren Missouri’s and Ameren Illinois’ indentures, credit facilities, 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 March 31, 2015, at an assumed annual interest rate of 5% and dividend rate of 6%.

Required Interest

Actual Interest

Bonds Issuable(b)

Required Dividend

Actual Dividend

Preferred Stock

Coverage Ratio(a)

Coverage Ratio

Coverage Ratio(c)

Coverage Ratio

Issuable

Ameren Missouri

≥2.0

4.6

3,386

≥2.5

113.4

2,530

Ameren Illinois

≥2.0

6.6

3,423

(d)

≥1.5

2.8

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 $832 million and $204 million at Ameren Missouri and Ameren Illinois, respectively.

(c)

Coverage required on the annual dividend on preferred stock outstanding and to be issued, as required in the respective company’s articles of incorporation.

(d)

Amount of bonds issuable by Ameren Illinois based on unfunded property additions and retired bonds solely under the former IP mortgage indenture. The amount of bonds issuable by Ameren Illinois is also subject to the lien restrictions contained in the 2012 Illinois Credit Agreement.

(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.” The FERC has consistently interpreted the provision to allow dividends to be paid as long as (1) the source of the dividends is clearly disclosed, (2) the dividends are not excessive, and (3) there is no self-dealing on the part of corporate officials. At a minimum, Ameren believes that dividends can be paid by its subsidiaries that are public utilities from retained earnings. In addition, under Illinois law, Ameren Illinois may not pay any dividend on its stock, unless, among other things, its earnings and earned surplus are sufficient to declare and pay a dividend after provision is made for reasonable and proper reserves, or unless Ameren Illinois has specific authorization from the ICC.

Ameren Illinois’ articles of incorporation require dividend payments on its common stock to be based on ratios of common stock to total capitalization and other provisions related to certain operating expenses and accumulations of earned surplus. Ameren Illinois committed to the FERC to maintain a minimum of 30% equity in its capital structure. As of March 31, 2015, Ameren Illinois had 54% of equity in its capital structure.

In order for the Ameren Companies to issue securities in the future, we have to comply with all applicable requirements in effect at the time of any such issuances.

Off-Balance-Sheet Arrangements

At March 31, 2015, 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	3 Months Ended
	Mar. 31, 2015
Other Nonoperating Income (Expense) [Abstract]
OTHER INCOME AND EXPENSES	OTHER INCOME AND EXPENSES
	The following table presents the components of “Other Income and Expenses” in the Ameren Companies’ statements of income for the three months ended March 31, 2015 and 2014:


		Three Months
		2015		2014
	Ameren:(a)
	Miscellaneous income:
	Allowance for equity funds used during construction	$	5	$	7

	Interest income on industrial development revenue bonds	7		7

	Interest income	4		2

	Other	3		2

	Total miscellaneous income	$	19	$	18

	Miscellaneous expense:
	Donations	$	8	$	5

	Other	3		4

	Total miscellaneous expense	$	11	$	9

	Ameren Missouri:
	Miscellaneous income:
	Allowance for equity funds used during construction	$	4	$	7

	Interest income on industrial development revenue bonds	7		7

	Total miscellaneous income	$	11	$	14

	Miscellaneous expense:
	Donations	$	2	$	2

	Other	1		2

	Total miscellaneous expense	$	3	$	4

	Ameren Illinois:
	Miscellaneous income:
	Allowance for equity funds used during construction	$	1	$	—

	Interest income	4		2

	Other	2		1

	Total miscellaneous income	$	7	$	3

	Miscellaneous expense:
	Donations	$	3	$	3

	Other	2		1

	Total miscellaneous expense	$	5	$	4


	(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Derivative_Financial_Instrumen

Derivative Financial Instruments	3 Months Ended
	Mar. 31, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
DERIVATIVE FINANCIAL INSTRUMENTS	NOTE 6 - DERIVATIVE FINANCIAL INSTRUMENTS
	We use derivatives to manage the risk of changes in market prices for natural gas, power, and uranium, as well as the risk of changes in rail transportation surcharges through fuel oil hedges. Such price fluctuations may cause the following:

	•	an unrealized appreciation or depreciation of our contracted commitments to purchase or sell when purchase or sale prices under the commitments are compared with current commodity prices;

	•	market values of natural gas and uranium inventories that differ from the cost of those commodities in inventory; and

	•	actual cash outlays for the purchase of these commodities that differ from anticipated cash outlays.
	The derivatives that we use to hedge these risks are governed by our risk management policies for forward contracts, futures, options, and swaps. Our net positions are continually assessed within our structured hedging programs to determine whether new or offsetting transactions are required. The goal of the hedging program is generally to mitigate financial risks while ensuring that sufficient volumes are available to meet our requirements. Contracts we enter into as part of our risk management program may be settled financially, settled by physical delivery, or net settled with the counterparty.
	The following table presents open gross commodity contract volumes by commodity type for derivative assets and liabilities as of March 31, 2015, and December 31, 2014. As of March 31, 2015, 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)
		2015						2014
	Commodity	Ameren Missouri		Ameren Illinois		Ameren		Ameren Missouri		Ameren Illinois		Ameren
	Fuel oils (in gallons)(a)	40		(b)		40		50		(b)		50

	Natural gas (in mmbtu)	29		121		150		28		108		136

	Power (in megawatthours)	1		10		11		1		11		12

	Uranium (pounds in thousands)	349		(b)		349		332		(b)		332


	(a)	Fuel oils consist of heating oil and ultra-low-sulfur diesel.

	(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 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 March 31, 2015, and December 31, 2014, 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 a master netting arrangement. The Ameren Companies did not elect to adopt this guidance for any eligible commodity contracts.
	The following table presents the carrying value and balance sheet location of all derivative commodity contracts, none of which were designated as hedging instruments, as of March 31, 2015, and December 31, 2014:


		Balance Sheet Location		Ameren				Ameren				Ameren
				Missouri				Illinois
	2015
	Fuel oils	Other current assets		$	1			$	—			$	1

	Natural gas	Other current assets		—				1				1

	Power	Other current assets		7				—				7

		Total assets		$	8			$	1			$	9

	Fuel oils	Other current liabilities		$	21			$	—			$	21

		Other deferred credits and liabilities		7				—				7

	Natural gas	MTM derivative liabilities		(a)				32				(a)

		Other current liabilities		6				—				38

		Other deferred credits and liabilities		8				18				26

	Power	MTM derivative liabilities		(a)				11				(a)

		Other current liabilities		1				—				12

		Other deferred credits and liabilities		—				153				153

	Uranium	Other current liabilities		1				—				1

		Total liabilities		$	44			$	214			$	258

2014
Fuel oils	Other current assets		$	2			$	—			$	2

Natural gas	Other current assets		1				1				2

Power	Other current assets		15				—				15

	Total assets		$	18			$	1			$	19

Fuel oils	Other current liabilities		$	22			$	—			$	22

	Other deferred credits and liabilities		7				—				7

Natural gas	MTM derivative liabilities		(a)				31				(a)

	Other current liabilities		6				—				37

	Other deferred credits and liabilities		6				13				19

Power	MTM derivative liabilities		(a)				11				(a)

	Other current liabilities		3				—				14

	Other deferred credits and liabilities		—				131				131

Uranium	Other current liabilities		2				—				2

	Total liabilities		$	46			$	186			$	232


(a)	Balance sheet line item not applicable to registrant.

The following table presents the cumulative amount of pretax net gains (losses) on all derivative instruments deferred in regulatory assets or regulatory liabilities as of March 31, 2015, and December 31, 2014:


	Ameren				Ameren				Ameren
	Missouri				Illinois
2015
Fuel oils derivative contracts(a)	$	(27	)		$	—			$	(27	)

Natural gas derivative contracts(b)	(14		)		(49		)		(63		)
Power derivative contracts(c)	6				(164		)		(158		)

Uranium derivative contracts(d)	(1		)		—				(1		)

2014
Fuel oils derivative contracts	$	(29	)		$	—			$	(29	)

Natural gas derivative contracts	(11		)		(43		)		(54		)
Power derivative contracts	12				(142		)		(130		)

Uranium derivative contracts	(2		)		—				(2		)


(a)	Represents net losses associated with fuel oils derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s rail transportation surcharges for coal through December 2017. Current losses deferred as regulatory assets include $20 million at Ameren and Ameren Missouri.

(b)	Represents net losses associated with natural gas derivative contracts. These contracts are a partial hedge of natural gas requirements through October 2019 at Ameren and Ameren Missouri and through October 2018 at Ameren Illinois. Current gains deferred as regulatory liabilities include $1 million at Ameren and Ameren Illinois, respectively. Current losses deferred as regulatory assets include $38 million, $6 million, and $32 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively.

(c)	Represents net gains (losses) associated with power derivative contracts. These contracts are a partial hedge of power price requirements through May 2032 at Ameren and Ameren Illinois and through December 2016 at Ameren Missouri. Current gains deferred as regulatory liabilities include $7 million at Ameren and Ameren Missouri. Current losses deferred as regulatory assets include $12 million, $1 million, and $11 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively.

(d)	Represents net losses on uranium derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s uranium requirements through December 2016. Current losses deferred as regulatory assets include $1 million at Ameren and Ameren Missouri.
Derivative instruments are subject to various credit-related losses in the event of nonperformance by counterparties to the transaction. Exchange-traded contracts are supported by the financial and credit quality of the clearing members of the respective exchanges and have nominal credit risk. In all other transactions, we are exposed to credit risk. Our credit risk management program involves establishing credit limits and collateral requirements for counterparties, using master netting arrangements, and reporting daily exposure to senior management.
We believe that entering into master netting arrangements mitigates the level of financial loss that could result from default by allowing net settlement of derivative assets and liabilities. We generally enter into the following master netting arrangements: (1) the International Swaps and Derivatives Association Agreement, a standardized financial natural gas and electric contract; (2) the Master Power Purchase and Sale Agreement, created by the Edison Electric Institute and the National Energy Marketers Association, a standardized contract for the purchase and sale of wholesale power; and (3) the North American Energy Standards Board Inc. Agreement, a standardized contract for the purchase and sale of natural gas. These master netting arrangements allow the counterparties to net settle sale and purchase transactions. Further, collateral requirements are calculated at the master netting arrangement level by counterparty.
The following table provides the recognized gross derivative balances and the net amounts of those derivatives subject to an enforceable master netting arrangement or similar agreement as of March 31, 2015, and December 31, 2014:


						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
2015
Assets:
Ameren Missouri		$	8			$	4			$	—			$	4

Ameren Illinois		1				—				—				1

Ameren		$	9			$	4			$	—			$	5

Liabilities:
Ameren Missouri		$	44			$	4			$	4			$	36

Ameren Illinois		214				—				2				212

Ameren		$	258			$	4			$	6			$	248

2014
Assets:
Ameren Missouri		$	18			$	5			$	—			$	13

Ameren Illinois		1				—				—				1

Ameren		$	19			$	5			$	—			$	14

Liabilities:
Ameren Missouri		$	46			$	5			$	5			$	36

Ameren Illinois		186				—				—				186

Ameren		$	232			$	5			$	5			$	222


(a)	Cash collateral received reduces gross asset balances and is included in “Other current liabilities” and “Other deferred credits and liabilities” on the balance sheet. Cash collateral posted reduces gross liability balances and is included in “Other current assets” and “Other assets” on the balance sheet.
Concentrations of Credit Risk
In determining our concentrations of credit risk related to derivative instruments, we review our individual counterparties and categorize each counterparty into groupings according to the primary business in which each engages. We calculate maximum exposures based on the gross fair value of financial instruments, including NPNS and other accrual contracts. As of March 31, 2015, if counterparty groups were to fail completely to perform on contracts, Ameren, Ameren Missouri, and Ameren Illinois' maximum exposure was $6 million, $5 million, and $1 million, respectively. The potential loss on counterparty exposures is reduced by the application of master netting arrangements and collateral held, to the extent of reducing the exposure to zero. As of March 31, 2015, the potential loss after consideration of the application of master netting arrangements and collateral held for Ameren, Ameren Missouri, and Ameren Illinois was $5 million, $5 million, and $- million, respectively.
Derivative Instruments with Credit Risk-Related Contingent Features
Our commodity contracts contain collateral provisions tied to the Ameren Companies’ credit ratings. If we were to experience an adverse change in our credit ratings, or if a counterparty with reasonable grounds for uncertainty regarding performance of an obligation requested adequate assurance of performance, additional collateral postings might be required. The following table presents, as of March 31, 2015, the aggregate fair value of all derivative instruments with credit risk-related contingent features in a gross liability position, the cash collateral posted, and the aggregate amount of additional collateral that could be required to be posted with counterparties. The additional collateral required is the net liability position allowed under the master netting arrangements, assuming (1) the credit risk-related contingent features underlying these arrangements were triggered on March 31, 2015, 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)
2015
Ameren Missouri	$	87			$	6			$	83

Ameren Illinois	80				2				74

Ameren	$	167			$	8			$	157


(a)	Prior to consideration of master netting arrangements and including NPNS and other accrual contract exposures.

(b)	As collateral requirements with certain counterparties are based on master netting arrangements, the aggregate amount of additional collateral required to be posted is determined after consideration of the effects of such arrangements.

Fair_Value_Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2015
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 three hierarchy levels. See Note 8 - Fair Value Measurements under Part II, Item 8, of the Form 10-K for information related to hierarchy levels. 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 March 31, 2015:


			Fair Value									Weighted Average
			Assets			Liabilities			Valuation Technique(s)	Unobservable Input	Range
	Level 3 Derivative asset and liability - commodity contracts(a):
	Ameren	Fuel oils	$	1		$	(7	)	Option model	Volatilities(%)(b)	33 - 80	42

									Discounted cash flow	Ameren Missouri credit risk(%)(b)(c)	0.4	(d)
		Natural gas	1			(1		)	Option model	Volatilities(%)(e)	Jun-39	33

										Nodal basis($/mmbtu)(e)	(0.40) - (0.20)	-0.3
									Discounted cash flow	Nodal basis($/mmbtu)(e)	(0.40) - (0.10)	-0.3
										Counterparty credit risk(%)(b)(c)	0.31 - 12.07	2.47
										Ameren Missouri and Ameren Illinois credit risk(%)(b)(c)	0.4	(d)
		Power(f)	5			(165		)	Discounted cash flow	Average forward peak and off-peak pricing - forwards/swaps($/MWh)(g)	22 - 46	32

										Estimated auction price for FTRs($/MW)(e)	(597) - 1,922	153
										Nodal basis($/MWh)(e)	(11) - 0	-3
										Counterparty credit risk(%)(b)(c)	0.29 - 10.98	5.84
										Ameren Missouri and Ameren Illinois credit risk(%)(b)(c)	0.4	(d)
									Fundamental energy production model	Estimated future gas prices($/mmbtu)(e)	5-Mar	4
										Escalation rate(%)(e)(h)	1	(d)
									Contract price allocation	Estimated renewable energy credit costs($/credit)(e)	7-May	6
		Uranium	—			(1		)	Discounted cash flow	Average forward uranium pricing($/pound)(e)	40 - 43	40

	Ameren Missouri	Fuel oils	$	1		$	(7	)	Option model	Volatilities(%)(b)	33 - 80	42

									Discounted cash flow	Ameren Missouri credit risk(%)(b)(c)	0.4	(d)
		Natural gas	—			(1		)	Option model	Volatilities(%)(e)	Jun-39	33

										Nodal basis($/mmbtu)(e)	(0.40) - (0.20)	-0.3
									Discounted cash flow	Nodal basis($/mmbtu)(e)	-0.1	(d)
										Counterparty credit risk(%)(b)(c)	0.54 - 12.07	5
										Ameren Missouri credit risk(%)(b)(c)	0.4	(d)
		Power(f)	5			(1		)	Discounted cash flow	Average forward peak and off-peak pricing - forwards/swaps($/MWh)(b)	24 - 46	36

										Estimated auction price for FTRs($/MW)(e)	(597) - 1,922	153
										Nodal basis($/MWh)(b)	(11) - (4)	-8
										Counterparty credit risk(%)(b)(c)	0.29 - 10.98	5.84
		Uranium	—			(1		)	Discounted cash flow	Average forward uranium pricing($/pound)(e)	40 - 43	40

	Ameren Illinois	Natural gas	$	1		$	—		Discounted cash flow	Nodal basis($/mmbtu)(e)	(0.40) - (0.10)	-0.3

										Counterparty credit risk(%)(b)(c)	0.31 - 2.28	1.28
										Ameren Illinois credit risk(%)(b)(c)	0.4	(d)
		Power(f)	—			(164		)	Discounted cash flow	Average forward peak and off-peak pricing - forwards/swaps($/MWh)(e)	22 - 38	31

										Nodal basis($/MWh)(e)	(6) - 0	-3
										Ameren Illinois credit risk(%)(b)(c)	0.4	(d)
									Fundamental energy production model	Estimated future gas prices($/mmbtu)(e)	5-Mar	4
										Escalation rate(%)(e)(h)	1	(d)
									Contract price allocation	Estimated renewable energy credit costs($/credit)(e)	7-May	6

	(a)	The derivative asset and liability balances are presented net of counterparty credit considerations.

	(b)	Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.

	(c)	Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances.

	(d)	Not applicable.

(e)	Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.

(f)	Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2018. Valuations beyond 2018 use fundamentally modeled pricing by month for peak and off-peak demand.

(g)	The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes due to their opposing positions. As such, refer to the power sensitivity analysis for each company above.

(h)	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, 2014:


		Fair Value										Weighted
		Assets			Liabilities				Valuation Technique(s)	Unobservable Input	Range	Average
Level 3 Derivative asset and liability – commodity contracts(a):
Ameren	Fuel oils	$	2		$	(8	)		Option model	Volatilities(%)(b)	Mar-39	32

									Discounted cash flow	Ameren Missouri credit risk(%)(b)(c)	0.43	(d)
										Escalation rate(%)(e)(f)	5	(d)
	Natural Gas	1			(2		)		Option model	Volatilities(%)(b)	31 - 144	63

										Nodal basis($/mmbtu)(e)	(0.40) - 0	-0.2
									Discounted cash flow	Nodal basis($/mmbtu)(e)	(0.40) - 0.10	-0.2
										Counterparty credit risk(%)(b)(c)	0.43 - 13	3
										Ameren Missouri and Ameren Illinois credit risk(%)(b)(c)	0.43	(d)
	Power(g)	11			(144		)		Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps($/MWh)(h)	27 - 50	32

										Estimated auction price for FTRs($/MW)(e)	(1,833) - 2,743	171
										Nodal basis($/MWh)(e)	(6) - 0	-2
										Counterparty credit risk(%)(b)(c)	0.26	(d)
										Ameren Missouri and Ameren Illinois credit risk(%)(b)(c)	0.43	(d)
									Fundamental energy production model	Estimated future gas prices($/mmbtu)(e)	5-Apr	4
										Escalation rate(%)(e)(i)	0 - 1	1
									Contract price allocation	Estimated renewable energy credit costs($/credit)(e)	7-May	6
	Uranium	—			(2		)		Discounted cash flow	Average forward uranium pricing($/pound)(e)	35 - 40	36

Ameren Missouri	Fuel oils	$	2		$	(8	)		Option model	Volatilities(%)(b)	Mar-39	32

									Discounted cash flow	Ameren Missouri credit risk(%)(b)(c)	0.43	(d)
										Escalation rate(%)(e)(f)	5	(d)
	Natural Gas	—			(1		)		Option model	Volatilities(%)(b)	31 - 144	53

										Nodal basis($/mmbtu)(e)	(0.40) - 0	-0.3
									Discounted cash flow	Nodal basis($/mmbtu)(e)	-0.1	(d)
										Counterparty credit risk(%)(b)(c)	0.57 - 13	5
										Ameren Missouri credit risk(%)(b)(c)	0.43	(d)
	Power(g)	11			(2		)		Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps($/MWh)(b)	27 - 50	32

										Estimated auction price for FTRs($/MW)(e)	(1,833) - 2,743	171
										Counterparty credit risk(%)(b)(c)	0.26	(d)
										Ameren Missouri credit risk(%)(b)(c)	0.43	(d)
	Uranium	—			(2		)		Discounted cash flow	Average forward uranium pricing($/pound)(e)	35 - 40	36

Ameren Illinois	Natural Gas	$	1		$	(1	)		Option model	Volatilities(%)(b)	50 - 144	94

										Nodal basis($/mmbtu)(e)	(0.10) - 0	-0.1
									Discounted cash flow	Nodal basis($/mmbtu)(e)	(0.40) - 0.10	-0.2
										Counterparty credit risk(%)(b)(c)	0.43 - 2	0.83
										Ameren Illinois credit risk(%)(b)(c)	0.43	(d)
	Power(g)	—			(142		)		Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps($/MWh)(e)	27 - 38	32

										Nodal basis($/MWh)(e)	(6) - 0	-2
										Ameren Illinois credit risk(%)(b)(c)	0.43	(d)
									Fundamental energy production model	Estimated future gas prices($/mmbtu)(e)	5-Apr	4
										Escalation rate(%)(e)(i)	0 - 1	1
									Contract price allocation	Estimated renewable energy credit costs($/credit)(e)	7-May	6

(a)	The derivative asset and liability balances are presented net of counterparty credit considerations.

(b)	Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.

(c)	Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances.

(d)	Not applicable.

(e)	Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.

(f)	Escalation rate applies to fuel oil prices 2017 and beyond.

(g)	Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2018. Valuations beyond 2018 use fundamentally modeled pricing by month for peak and off-peak demand.

(h)	The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes due to their opposing positions. As such, refer to the power sensitivity analysis for each company above.

(i)	Escalation rate applies to power prices 2026 and beyond.
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 quarter of 2015 or 2014. At March 31, 2015, 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, 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.
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 March 31, 2015:


			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			$	1

	Natural gas		—				—				1				1

	Power		—				2				5				7

	Total derivative assets - commodity contracts		$	—			$	2			$	7			$	9

	Nuclear decommissioning trust fund:
	Cash and cash equivalents		$	2			$	—			$	—			$	2

	Equity securities:
	U.S. large capitalization		368				—				—				368

	Debt securities:
	Corporate bonds		—				65				—				65

	U.S. treasury and agency securities		—				101				—				101

	Other		—				20				—				20

	Total nuclear decommissioning trust fund		$	370			$	186			$	—			$	556	(b)

	Total Ameren		$	370			$	188			$	7			$	565

Ameren	Derivative assets - commodity contracts(a):
Missouri	Fuel oils		$	—			$	—			$	1			$	1

	Power		—				2				5				7

	Total derivative assets - commodity contracts		$	—			$	2			$	6			$	8

	Nuclear decommissioning trust fund:
	Cash and cash equivalents		$	2			$	—			$	—			$	2

	Equity securities:
	U.S. large capitalization		368				—				—				368

	Debt securities:
	Corporate bonds		—				65				—				65

	U.S. treasury and agency securities		—				101				—				101

	Other		—				20				—				20

	Total nuclear decommissioning trust fund		$	370			$	186			$	—			$	556	(b)

	Total Ameren Missouri		$	370			$	188			$	6			$	564

Ameren	Derivative assets - commodity contracts(a):
Illinois	Natural gas		$	—			$	—			$	1			$	1

Liabilities:
Ameren	Derivative liabilities - commodity contracts(a):
	Fuel oils		$	21			$	—			$	7			$	28

	Natural gas		—				63				1				64

	Power		—				—				165				165

	Uranium		—				—				1				1

	Total Ameren		$	21			$	63			$	174			$	258

Ameren	Derivative liabilities - commodity contracts(a):
Missouri	Fuel oils		$	21			$	—			$	7			$	28

	Natural gas		—				13				1				14

	Power		—				—				1				1

	Uranium		—				—				1				1

	Total Ameren Missouri		$	21			$	13			$	10			$	44

Ameren	Derivative liabilities - commodity contracts(a):
Illinois	Natural gas		$	—			$	50			$	—			$	50

	Power		—				—				164				164

	Total Ameren Illinois		$	—			$	50			$	164			$	214


(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, 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			$	2

	Natural gas		—				1				1				2

	Power		—				4				11				15

	Total derivative assets - commodity contracts		$	—			$	5			$	14			$	19

	Nuclear decommissioning trust fund:
	Cash and cash equivalents		$	1			$	—			$	—			$	1

	Equity securities:
	U.S. large capitalization		364				—				—				364

	Debt securities:
	Corporate bonds		—				63				—				63

	U.S. treasury and agency securities		—				102				—				102

	Other		—				17				—				17

	Total nuclear decommissioning trust fund		$	365			$	182			$	—			$	547	(b)

	Total Ameren		$	365			$	187			$	14			$	566

Ameren	Derivative assets - commodity contracts(a):
Missouri	Fuel oils		$	—			$	—			$	2			$	2

	Natural gas		—				1				—				1

	Power		—				4				11				15

	Total derivative assets - commodity contracts		$	—			$	5			$	13			$	18

	Nuclear decommissioning trust fund:
	Cash and cash equivalents		$	1			$	—			$	—			$	1

	Equity securities:
	U.S. large capitalization		364				—				—				364

	Debt securities:
	Corporate bonds		—				63				—				63

	U.S. treasury and agency securities		—				102				—				102

	Other		—				17				—				17

	Total nuclear decommissioning trust fund		$	365			$	182			$	—			$	547	(b)

	Total Ameren Missouri		$	365			$	187			$	13			$	565

Ameren	Derivative assets - commodity contracts(a):
Illinois	Natural gas		$	—			$	—			$	1			$	1

Liabilities:
Ameren	Derivative liabilities - commodity contracts(a):
	Fuel oils		$	21			$	—			$	8			$	29

	Natural gas		1				53				2				56

	Power		—				1				144				145

	Uranium		—				—				2				2

	Total Ameren		$	22			$	54			$	156			$	232

Ameren	Derivative liabilities - commodity contracts(a):
Missouri	Fuel oils		$	21			$	—			$	8			$	29

	Natural gas		1				10				1				12

	Power		—				1				2				3

	Uranium		—				—				2				2

	Total Ameren Missouri		$	22			$	11			$	13			$	46

Ameren	Derivative liabilities - commodity contracts(a):
Illinois	Natural gas		$	—			$	43			$	1			$	44

	Power		—				—				142				142

	Total Ameren Illinois		$	—			$	43			$	143			$	186


(a)	The derivative asset and liability balances are presented net of counterparty credit considerations.

(b)	Balance excludes $2 million of receivables, payables, and accrued income, net.
The following table 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 March 31, 2015:


		Net derivative commodity contracts
		Ameren			Ameren			Ameren
		Missouri			Illinois
Fuel oils:
Beginning balance at January 1, 2015	$	(6	)	$	(a)		$	(6	)

Realized and unrealized gains (losses) included in regulatory assets/liabilities		(1	)		(a)			(1	)

Settlements		1			(a)			1

Ending balance at March 31, 2015	$	(6	)	$	(a)		$	(6	)

Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2015	$	(3	)	$	(a)		$	(3	)

Natural gas:
Beginning balance at January 1, 2015	$	(1	)	$	—		$	(1	)

Purchases		—			1			1

Ending balance at March 31, 2015	$	(1	)	$	1		$	—

Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2015	$	—		$	—		$	—

Power:
Beginning balance at January 1, 2015	$	9		$	(142	)	$	(133	)

Realized and unrealized gains (losses) included in regulatory assets/liabilities		(2	)		(25	)		(27	)
Settlements		(3	)		3			—

Ending balance at March 31, 2015	$	4		$	(164	)	$	(160	)

Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2015	$	—		$	(24	)	$	(24	)

Uranium:
Beginning balance at January 1, 2015	$	(2	)	$	(a)		$	(2	)

Realized and unrealized gains (losses) included in regulatory assets/liabilities		1			(a)			1

Ending balance at March 31, 2015	$	(1	)	$	(a)		$	(1	)

Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2015	$	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 three months ended March 31, 2014:


		Net derivative commodity contracts
		Ameren			Ameren			Ameren
		Missouri			Illinois
Fuel oils:
Beginning balance at January 1, 2014	$	5		$	(a)		$	5

Realized and unrealized gains (losses) included in regulatory assets/liabilities		(2	)		(a)			(2	)

Settlements		(2	)		(a)			(2	)

Ending balance at March 31, 2014	$	1		$	(a)		$	1

Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2014	$	(1	)	$	(a)		$	(1	)

Natural gas:
Beginning balance at January 1, 2014	$	—		$	—		$	—

Purchases		—			(2	)		(2	)

Settlements		—			2			2

Ending balance at March 31, 2014	$	—		$	—		$	—

Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2014	$	—		$	—		$	—

Power:
Beginning balance at January 1, 2014	$	19		$	(108	)	$	(89	)

Realized and unrealized gains (losses) included in regulatory assets/liabilities		(5	)		(12	)		(17	)
Settlements		(5	)		—			(5	)

Transfers out of Level 3		1			—			1

Ending balance at March 31, 2014	$	10		$	(120	)	$	(110	)

Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2014	$	(1	)	$	(14	)	$	(15	)
Uranium:
Beginning balance at January 1, 2014	$	(6	)	$	(a)		$	(6	)

Settlements		1			(a)			1

Ending balance at March 31, 2014	$	(5	)	$	(a)		$	(5	)

Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2014	$	—		$	(a)		$	—


(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 similar financial trades observable on electronic exchanges between the periods. Any reclassifications are reported as transfers out of Level 3 at the fair value measurement reported at the beginning of the period in which the changes occur. For the three months ended March 31, 2015 and 2014, there were no transfers between Level 1 and Level 2 or between Level 2 and Level 3 related to derivative commodity contracts, with the exception of $1 million of transfers out of Level 3 into Level 2 related to power contracts at Ameren and Ameren Missouri for the three months ended March 31, 2014.
The Ameren Companies’ carrying amounts of cash and cash equivalents approximate fair value because of the short-term nature of these instruments. They are considered to be Level 1 in the fair value hierarchy. 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 March 31, 2015, and December 31, 2014:


	31-Mar-15								31-Dec-14
	Carrying				Fair				Carrying				Fair
	Amount				Value				Amount				Value
Ameren:(a)
Long-term debt and capital lease obligations (including current portion)	$	6,240			$	7,127			$	6,240			$	7,135

Preferred stock	142				123				142				122

Ameren Missouri:
Long-term debt and capital lease obligations (including current portion)	$	3,999			$	4,574			$	3,999			$	4,518

Preferred stock	80				74				80				73

Ameren Illinois:
Long-term debt	$	2,241			$	2,553			$	2,241			$	2,517

Preferred stock	62				49				62				49


(a)	Preferred stock is recorded in “Noncontrolling Interests” on the consolidated balance sheet.

Related_Party_Transactions

Related Party Transactions	3 Months Ended
	Mar. 31, 2015
Related Party Transactions [Abstract]
RELATED PARTY TRANSACTIONS	RELATED PARTY TRANSACTIONS
	Ameren (parent) and its subsidiaries have engaged in, and may in the future engage in, affiliate transactions in the normal course of business. These transactions primarily consist of power purchases and sales, services received or rendered, and borrowings and lendings.
	Transactions between affiliates are reported as intercompany transactions on their respective financial statements but are eliminated in consolidation for Ameren’s financial statements. For a discussion of our material related party agreements, see Note 14 - Related Party Transactions under Part II, Item 8, of the Form 10-K and the money pool arrangements discussed in Note 3 - Short-term Debt and Liquidity of this report.
	Electric Power Supply Agreements
	In April 2015, Ameren Illinois used an RFP process, administered by the IPA, to procure energy products from June 1, 2015, through May 31, 2018. Ameren Missouri was among the winning suppliers in the energy product RFP process. As a result, in 2015, Ameren Missouri and Ameren Illinois entered into energy product agreements by which Ameren Missouri agreed to sell and Ameren Illinois agreed to purchase 667,000 megawatthours at an average price of $36 per megawatthour during the period of June 1, 2015, through June 30, 2017.
	The following table presents the impact on Ameren Missouri and Ameren Illinois of related party transactions for the three months ended March 31, 2015 and 2014.


					Three Months
	Agreement	Income Statement			Ameren		Ameren
		Line Item			Missouri		Illinois
	Ameren Missouri power supply	Operating Revenues	2015	$	1	$	(a)

	agreements with Ameren Illinois		2014		(b)		(a)

	Ameren Missouri and Ameren Illinois	Operating Revenues	2015		6		1

	rent and facility services		2014		5		(b)

	Ameren Missouri and Ameren Illinois	Operating Revenues	2015		(b)		(b)

	miscellaneous support services		2014		(b)		(b)

	Total Operating Revenues		2015	$	7	$	1

			2014		5		(b)

	Ameren Illinois power supply	Purchased Power	2015	$	(a)	$	1

	agreements with Ameren Missouri		2014		(a)		(b)

	Ameren Illinois transmission	Purchased Power	2015		(a)		1

	services with ATXI		2014		(a)		1

	Total Purchased Power		2015	$	(a)	$	2

			2014		(a)		1

	Ameren Services support services	Other Operations and Maintenance	2015	$	34	$	29

	agreement		2014		33		27

	Total Other Operations and		2015	$	34	$	29

	Maintenance Expenses		2014		33		27

	Money pool borrowings (advances)	Interest Charges/ Miscellaneous	2015	$	(b)	$	(b)

		Income	2014		(b)		(b)


	(a)	Not applicable.

	(b)	Amount less than $1 million.

Commitments_And_Contingencies

Commitments And Contingencies	3 Months Ended
	Mar. 31, 2015
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 March 31, 2015. 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 2015.


	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	$	2,750		(d)	$	26	(e)

	European Mutual Association for Nuclear Insurance	500			(f)	—

		$	3,250			$	26

	Replacement power:
	NEIL	$	490		(g)	$	9	(e)


	(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 for both radiation and nonradiation events. An additional $500 million is provided for radiation events only for a total of $2.75 billion.

	(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 $2.75 billion and $2.25 billion property coverage for radiation and nonradiation events, respectively, 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 twelve weeks of an outage, plus up to $3.6 million per week for a minimum of 71 weeks thereafter for a total not exceeding the policy limit of $490 million. Nonradiation events are sub-limited to $328 million.
	The Price-Anderson Act is a federal law that limits the liability for claims from an incident involving any licensed United States commercial nuclear energy center. The limit is based on the number of licensed reactors. The limit of liability and the maximum potential annual payments are adjusted at least every five years for inflation to reflect changes in the Consumer Price Index. The most recent five-year inflationary adjustment became effective in September 2013. Owners of a nuclear reactor cover this exposure through a combination of private insurance and mandatory participation in a financial protection pool, as established by the Price-Anderson Act.
	Losses resulting from terrorist attacks on nuclear facilities are covered under NEIL’s policies, subject to an industrywide aggregate policy limit of $3.24 billion within a 12-month period, or $1.83 billion for events not involving radiation contamination.
	If losses from a nuclear incident at the Callaway energy center exceed the limits of, or are not covered by, insurance, or if coverage is unavailable, Ameren Missouri is at risk for any uninsured losses. If a serious nuclear incident were to occur, it could have a material adverse effect on Ameren’s and Ameren Missouri’s results of operations, financial position, or liquidity.
	Other Obligations
	To supply a portion of the fuel requirements of Ameren Missouri’s energy centers, Ameren Missouri has entered into various long-term commitments for the procurement of coal, natural gas, nuclear fuel, and methane gas. Additionally, Ameren Missouri and Ameren Illinois 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 March 31, 2015, total other obligations related to commitments for coal, natural gas, nuclear fuel, purchased power, methane gas, equipment, customer energy efficiency program expenditures and meter reading services, among other agreements, at Ameren, Ameren Missouri, and Ameren Illinois were $5,326 million, $3,577 million, and $1,702 million, respectively.
	In April 2015, Ameren Illinois used an RFP process, administered by the IPA, to procure energy products from June 1, 2015, through May 31, 2018. Ameren Illinois contracted to purchase approximately 5,526,000 megawatthours of energy products for $185 million during this period.
	Environmental Matters
	We are subject to various environmental laws and regulations enforced by federal, state, and local authorities. From the beginning phases of siting and development to the operation of existing or new electric generation, transmission and distribution facilities and natural gas storage, transmission and distribution facilities, our activities involve compliance with diverse environmental laws and regulations. These laws and regulations address emissions, discharges to water, water usage, impacts to air, land, and water, and chemical and waste handling. Complex and lengthy processes are required to obtain and renew approvals, permits, or licenses for new, existing or modified facilities. Additionally, the use and handling of various chemicals or hazardous materials require release prevention plans and emergency response procedures.
	The EPA is developing and implementing environmental regulations that will have a significant impact on the electric utility industry. Over time, compliance with these regulations could be costly for certain companies, including Ameren Missouri, that operate coal-fired power plants. Significant new rules proposed or promulgated include the regulation of CO2 emissions from existing power plants through the proposed Clean Power Plan and from new power plants through the revised NSPS; the CSAPR, which requires further reductions of SO2 emissions and NOx emissions from power plants; a regulation governing management of CCR and CCR landfills and impoundments; the MATS, which require reduction of emissions of mercury, toxic metals, and acid gases from power plants; revised NSPS for particulate matter, SO2, and NOx emissions from new sources; new effluent standards applicable to waste water discharges from power plants and new regulations under the Clean Water Act that could require significant capital expenditures, such as modifications to water intake structures or new cooling towers at Ameren Missouri’s energy centers. Certain of these new and proposed regulations, if adopted, are likely to be challenged through litigation, so their ultimate implementation, as well as the timing of any such implementation, is uncertain. Although many details of the future regulations are unknown, the combined effects of the new and proposed environmental regulations could result in significant capital expenditures and increased operating costs for Ameren and Ameren Missouri. The EPA also periodically reviews and revises national ambient air quality standards, including those standards associated with particulate matter, ozone, and emissions from power plants such as SO2, NOx. Compliance with all of these environmental laws and regulations could be prohibitively expensive, result in the closure or alteration of the operation of some of Ameren Missouri’s energy centers, or require capital investment. Ameren and Ameren Missouri expect these costs would be recoverable through rates, subject to MoPSC prudence review, but the nature and timing of costs, as well as the applicable regulatory framework, could result in regulatory lag.
	As of March 31, 2015, Ameren and Ameren Missouri estimate capital expenditure investments of $350 million to $400 million through 2019 to comply with existing environmental regulations. Considerable uncertainty remains in this estimate. The actual amount of capital investments required to comply with existing environmental regulations may vary substantially from the above estimate due to uncertainty as to the precise compliance strategies that will be used and their ultimate cost, among other things. This estimate does not include the impacts of the proposed Clean Power Plan’s reduction in emissions of CO2, but does include our preliminary estimate of the capital expenditures required for the CCR regulations that were published in April 2015, both of which are discussed below.
	Ameren Missouri's current plan for compliance with existing environmental regulations for air emissions includes burning ultra-low-sulfur coal and installing new or optimizing existing pollution control equipment. Ameren Missouri has two scrubbers at its Sioux energy center, which are used to reduce SO2 emissions and other pollutants. Ameren Missouri's compliance plan assumes the installation of additional controls, including mercury control technology at multiple energy centers within its coal-fired fleet through 2019. However, Ameren Missouri continues to evaluate its operations and options to determine how to comply with the CSAPR, the MATS, and other recently finalized or proposed EPA regulations. Ameren Missouri may be required to install additional pollution controls within the next six to 10 years. As the Clean Power Plan is still subject to revision by the EPA and implementation by the states, Ameren Missouri has not finalized a compliance plan for the proposed rule.
	The following sections describe the more significant new or proposed environmental laws and rules and environmental enforcement and remediation matters that affect or could affect our operations.
	Clean Air Act
	Both federal and state laws require significant reductions in SO2 and NOx through either emission source reductions or the use and retirement of emission allowances. The CSAPR became effective on January 1, 2015, for SO2 and annual NOx reductions, and on May 1, 2015, for ozone season NOx reductions. There will be further emission reduction requirements in 2017 and potentially more in subsequent years. Ameren Missouri expects to have sufficiently reduced emissions and have sufficient allowances to avoid making external purchases to comply with CSAPR for 2015. To achieve compliance with CSAPR, Ameren Missouri operates two scrubbers at its Sioux energy center and burns ultra-low sulfur coal. Ameren Missouri does not expect to make additional capital investments to comply with the current CSAPR requirements. However, Ameren Missouri expects to incur additional operations and maintenance costs to lower its emissions at one or more of its energy centers for compliance with the CSAPR in future years. These higher operations and maintenance costs are expected to be collected from customers through the FAC or higher base rates.
	In December 2011, the EPA issued the MATS under the Clean Air Act, which requires reductions in emissions of mercury and other hazardous air pollutants, such as acid gases, trace metals, and hydrogen chloride. The United States Supreme Court is considering challenges to the MATS, with a decision expected in June 2015. The MATS do not require a specific control technology to achieve the emission reductions. The MATS apply to each unit at a coal-fired power plant. However, in certain cases, compliance can be achieved by averaging emissions from similar units at the same power plant. Compliance was required by April 2015 or, with a case-by-case extension, by April 2016. Ameren Missouri's Labadie and Meramec energy centers were granted extensions and expect to comply with the MATS by April 2016. Ameren Missouri expects to make additional capital investments at its Labadie and Meramec energy centers to comply with the MATS. These capital expenditure investments are included in Ameren's and Ameren Missouri's estimate above. In addition, Ameren Missouri is incurring additional operations and maintenance costs to lower its emissions at its energy centers in compliance with the MATS. These higher operations and maintenance costs are expected to be collected from customers through the FAC or higher base rates.
	In December 2014, the EPA published its proposal to strengthen the 2008 national ambient air quality standard for ozone. A final standard is expected in October 2015, after which states that do not meet the standard must develop and implement plans to achieve compliance. Ameren Missouri is currently evaluating the proposed standard and the possible effects on its operations.
	Greenhouse Gas Regulation
	Greenhouse gas emissions from stationary sources, such as power plants, are subject to regulation under the Clean Air Act. As a result, Ameren Missouri is required to consider the emissions of greenhouse gases in any air permit application.
	In January 2014, the EPA published proposed regulations that would set revised CO2 emissions standards for new power plants. The proposed standards would establish separate emissions limits for new natural-gas-fired plants and new coal-fired plants. In June 2014, the EPA proposed the Clean Power Plan, which sets forth CO2 emissions standards that would be applicable to existing power plants. The proposed Clean Power Plan would require each state to develop plans to achieve CO2 emission standards that the EPA calculated for each state. The EPA believes that the Clean Power Plan would achieve a 30% reduction in the nation's existing power plant CO2 emissions from 2005 levels by 2030. The proposed rule also has interim goals of aggressively reducing CO2 emissions by 2020. The EPA expects the proposed rule will be finalized in 2015. If the proposed rule is finalized, states would have one to three years to develop compliance plans. States would be allowed to develop independent plans or to join with other states to develop joint plans. Ameren Missouri is evaluating the proposed Clean Power Plan and the potential impact to its operations, including those related to electric system reliability. Significant uncertainty exists regarding the standard for existing power plants, as the finalized rule could be different from the proposed rule and will be subject to legal challenges, either of which could result in the amount and timing of CO2 emission standards being revised.
	If the proposed Clean Power Plan were to be implemented in its current form, Ameren Missouri may need to incur new or accelerated capital expenditures and increased fuel costs in order to achieve compliance. As proposed, the Clean Power Plan would require states, including Missouri and Illinois, to submit compliance plans as early as 2016. Compliance plans might require Ameren Missouri to construct natural gas-fired combined cycle generation and renewable generation, at a currently estimated cost of approximately $2 billion by 2020, that Ameren Missouri believes would otherwise not be necessary to meet the energy needs of its customers. Additionally, Missouri’s implementation of the proposed rules, if adopted, could result in the closure or alteration of the operation of some of Ameren Missouri’s coal and natural gas-fired energy centers, which could result in increased operating costs or impairment of assets. Ameren Missouri expects substantially all of these increased costs, which could begin in 2017, to be recoverable, subject to MoPSC prudence review, through substantially higher electric rates charged to its customers.
	Future federal and state legislation or regulations that mandate limits on the emission of greenhouse gases may result in significant increases in capital expenditures and operating costs, which could lead to increased liquidity needs and higher financing costs. These compliance costs could be prohibitive at some of Ameren Missouri’s energy centers, which could result in the impairment of long-lived assets if costs are not recovered through rates. Mandatory limits on the emission of greenhouse gases could increase costs for Ameren Missouri’s customers or have a material adverse effect on Ameren's and Ameren Missouri's results of operations, financial position, and liquidity if regulators delay or deny recovery in rates of these compliance costs. The cost of Ameren Illinois’ purchased power and gas purchased for resale could increase; however, Ameren Illinois expects these costs would be recovered from customers with no material adverse effect on its results of operations, financial position, or liquidity. Ameren's and Ameren Missouri's earnings might benefit from increased investment to comply with greenhouse gas limitations to the extent that the investments are reflected and recovered timely in rates charged to customers.
	NSR and Clean Air Litigation
	In January 2011, the Department of Justice, on behalf of the EPA, filed a complaint against Ameren Missouri in the United States District Court for the Eastern District of Missouri. The EPA's complaint, as amended in October 2013, alleges that in performing projects at its Rush Island coal-fired energy center in 2007 and 2010, Ameren Missouri violated provisions of the Clean Air Act and Missouri law. In January 2012, the district court granted, in part, Ameren Missouri's motion to dismiss various aspects of the EPA's penalty claims. The EPA's claims for unspecified injunctive relief remain. Ameren Missouri believes its defenses are meritorious and is defending itself vigorously. However, there can be no assurances that it will be successful in its efforts.
The ultimate resolution of this matter could have a material adverse effect on the future results of operations, financial position, and liquidity of Ameren and Ameren Missouri. A resolution could result in increased capital expenditures for the installation of pollution control equipment, increased operations and maintenance expenses, and penalties. We are unable to predict the ultimate resolution of these matters or the costs that might be incurred.
Clean Water Act
In August 2014, the EPA published the final rule applicable to cooling water intake structures at existing power plants. The rule requires a case-by-case evaluation and plan for reducing the mortality of aquatic organisms impinged on the facility’s intake screens or entrained through the plant's cooling water system. Implementation of this rule will be administered through each power plant’s water discharge permitting process. All coal-fired and nuclear energy centers at Ameren Missouri are subject to this rule. The rule could have an adverse effect on Ameren’s and Ameren Missouri’s results of operations, financial position, and liquidity if its implementation requires the installation of cooling towers or extensive modifications to the cooling water systems at our energy centers and if those investments are not recovered timely in electric rates charged to its customers.
In April 2013, the EPA announced its proposal to revise the effluent limitation guidelines applicable to steam electric generating units under the Clean Water Act. Effluent limitation guidelines are national standards for wastewater discharges to surface water that are based on the effectiveness of available control technology. The EPA's proposed rule raised several compliance options that would prohibit effluent discharges of certain, but not all, waste streams and impose more stringent limitations on certain components in wastewater discharges from power plants. If the rule is enacted as proposed, Ameren Missouri would be subject to the revised limitations beginning as early as July 1, 2017, but no later than July 1, 2022. The EPA is expected to issue final guidelines in September 2015.
Ash Management
In December 2014, the EPA issued regulations regarding the management and disposal of CCR, which will affect future CCR disposal and handling costs at Ameren Missouri's energy centers. The final CCR regulations were published in April 2015. The regulations allow for the management of CCR as a solid waste, as well as for its continued beneficial uses, such as recycling, which could reduce the amount to be disposed. The regulations also established criteria regarding the structural integrity, location, and operation of CCR impoundments and landfills. They require groundwater monitoring and closure of impoundments if the groundwater standards are not achieved. Ameren Missouri is currently evaluating the regulations to determine their impact on current management of CCR and the costs associated with compliance. Ameren Missouri also expects to record an increase to its ARO associated with CCR storage facilities as a result of the new regulations. See Note 1 - Summary of Significant Accounting Policies in this report for additional information. Ameren Missouri's capital expenditure plan includes the cost of constructing landfills as part of its environmental compliance plan. Ameren Missouri expects certain of its ash ponds could be closed within the next five years.
The new regulations do not apply to inactive ash ponds at plants no longer in operation, such as Ameren’s Meredosia and Hutsonville energy centers.
Remediation
We are involved in a number of remediation actions to clean up sites affected by hazardous substances, as required by federal and state law. Such laws require that responsible parties fund remediation actions regardless of their degree of fault, the legality of original disposal, or the ownership of a disposal site. Ameren Missouri and Ameren Illinois have each been identified by federal or state governments as a potentially responsible party at several contaminated sites.
As of March 31, 2015, Ameren Illinois owned or was otherwise responsible for 44 former MGP sites in Illinois, which are in various stages of investigation, evaluation, remediation, and closure. Ameren Illinois estimates it could substantially conclude remediation efforts at most of these sites by 2018. The ICC allows Ameren Illinois to recover remediation and litigation costs associated with its former MGP sites from its electric and natural gas utility customers through environmental adjustment rate riders. To be recoverable, such costs must be prudently incurred. Costs are subject to annual review by the ICC. As of March 31, 2015, Ameren Illinois estimated the obligation related to these former MGP sites at $245 million to $314 million. Ameren and Ameren Illinois recorded a liability of $245 million to represent their estimated minimum obligation for these sites, as no other amount within the range was a better estimate.
The scope and extent to which these former MGP sites are remediated may increase as remediation efforts continue. Considerable uncertainty remains in these estimates, as many factors can influence the ultimate actual costs, including site specific unanticipated underground structures, the degree to which groundwater is encountered, regulatory changes, local ordinances, and site accessibility. The actual costs may vary substantially from these estimates.
Ameren Illinois formerly used an off-site landfill, which Ameren Illinois did not own, in connection with the operation of a previously-owned energy center. Ameren Illinois could be required to perform certain maintenance activities at that landfill, which is now closed. As of March 31, 2015, Ameren Illinois estimated the obligation related to the landfill at $0.5 million to $6 million. Ameren Illinois recorded a liability of $0.5 million to represent its estimated minimum obligation for this site, as no other amount within the range was a better estimate. Ameren Illinois is also responsible for the cleanup of some underground storage tanks and a water treatment plant in Illinois. As of March 31, 2015, Ameren Illinois recorded a liability of $0.7 million to represent its best estimate of the obligation for these sites.
In 2008, the EPA issued an administrative order to Ameren Missouri pertaining to a former coal tar distillery operated by Koppers Company or its predecessor and successor companies. While Ameren Missouri is the current owner of the site, which is located in St. Louis, Missouri, it did not conduct any of the manufacturing operations involving coal tar or its byproducts. Ameren Missouri, along with two other potentially responsible parties, have completed site investigation activities and have submitted their findings to the EPA. As of March 31, 2015, Ameren Missouri estimated its obligation at $2 million to $5 million. Ameren Missouri recorded a liability of $2 million to represent its estimated minimum obligation, as no other amount within the range was a better estimate.
Ameren Missouri also participated in the investigation of various sites located in Sauget, Illinois. In 2000, the EPA notified Ameren Missouri and numerous other companies, including Solutia, Inc., that former landfills and lagoons at those sites may contain soil and groundwater contamination. These sites are known as Sauget Area 2. From about 1926 until 1976, Ameren Missouri operated an energy center adjacent to Sauget Area 2. Ameren Missouri currently owns a parcel of property at Sauget Area 2 that was once used as a landfill.
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 March 31, 2015, 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 March 31, 2015, Ameren Missouri estimated the obligation related to the cleanup at $2.3 million to $4.5 million. Ameren Missouri recorded a liability of $2.3 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. The breach resulted in significant flooding in the local area, which damaged a state park. Ameren Missouri had liability insurance coverage for the Taum Sauk incident, subject to certain limits and deductibles.
In 2010, Ameren Missouri sued an insurance company that was providing Ameren Missouri with liability coverage on the date of the Taum Sauk incident. In the litigation, Ameren Missouri claims that the insurance company breached its duty to indemnify Ameren Missouri for losses resulting from the incident. In September 2014, the United States District Court for the Eastern District of Missouri ordered the case to be transferred to the United States District Court for the Southern District of New York for trial. The transfer order has been stayed pending resolution of Ameren Missouri’s October 2014 appeal of that order to the United States Court of Appeals for the Eighth Circuit.
As of March 31, 2015, Ameren Missouri had an insurance receivable balance of $41 million. The insurance claim was $53 million as of March 31, 2015. Ameren Missouri expects to collect this receivable from the remaining insurance company in the pending litigation described above. This receivable is included in “Other assets” on Ameren’s and Ameren Missouri’s balance sheets as of March 31, 2015. Ameren's and Ameren Missouri's results of operations, financial position, and liquidity could be adversely affected if Ameren Missouri's remaining insurance receivable balance is not collected.
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 77 as the average number of parties as of March 31, 2015. 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 March 31, 2015:


Ameren		Ameren		Ameren		Total(a)
		Missouri		Illinois
1		42		53		65

(a)	Total does not equal the sum of the subsidiary unit lawsuits because some of the lawsuits name multiple Ameren entities as defendants.
As of March 31, 2015, 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 March 31, 2015, 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.

Callaway_Energy_Center

Callaway Energy Center	3 Months Ended
Callaway Energy Center	Mar. 31, 2015
Nuclear Waste Matters [Abstract]
CALLAWAY ENERGY CENTER	CALLAWAY ENERGY CENTER
	Under the NWPA, the DOE is responsible for disposing of spent nuclear fuel from the Callaway energy center and other commercial nuclear energy centers. Under the NWPA, Ameren and other utilities that own and operate those energy centers are responsible for paying the disposal costs. The NWPA established the fee that these utilities pay the federal government for disposing of the spent nuclear fuel at one mill, or one-tenth of one cent, for each kilowatthour generated and sold by those plants. The NWPA also requires the DOE annually to review the nuclear waste fee against the cost of the nuclear waste disposal program and to propose to the United States Congress any fee adjustment necessary to offset the costs of the program. As required by the NWPA, Ameren Missouri and other utilities have entered into standard contracts with the federal government. The government, represented by the DOE, is responsible for implementing these provisions of the NWPA. Consistent with the NWPA and its standard contract, Ameren Missouri had historically collected one mill from its electric customers for each kilowatthour of electricity that it generated and sold from its Callaway energy center. Because the federal government is not meeting its disposal obligation, the collection of this fee is currently suspended.
	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 to add additional dry cask storage capacity. The DOE's delay in carrying out its obligation to dispose of spent nuclear fuel from the Callaway energy center is not expected to adversely affect the continued operations of the energy center.
	In January 2013, the DOE issued its plan for the management and disposal of spent nuclear fuel. The DOE's plan calls for a pilot interim storage facility to begin operation with an initial focus on accepting spent nuclear fuel from shutdown reactor sites by 2021. By 2025, a larger interim storage facility would be available. The plan also proposes to begin operation of a permanent storage facility by 2048.
	As a result of the DOE's failure to begin to dispose of spent nuclear fuel from commercial nuclear energy centers and fulfill its contractual obligations, Ameren Missouri and other nuclear energy center owners have also sued the DOE to recover costs incurred for ongoing storage of their spent fuel. Ameren Missouri filed a breach of contract lawsuit to recover costs that it incurred through 2009. The lawsuit sought reimbursement for the cost of reracking the Callaway energy center’s spent fuel pool, for certain NRC fees, and for Missouri ad valorem taxes that Ameren Missouri would not have incurred had the DOE performed its contractual obligations. The parties entered into a settlement agreement that provides for annual recovery of additional spent fuel storage and related costs incurred from 2010 through 2016, with the ability to extend the recovery period as mutually agreed upon by the parties. Included in these reimbursements are costs related to a dry spent fuel storage facility that Ameren Missouri is constructing at its Callaway energy center. Ameren Missouri intends to begin transferring spent fuel assemblies to this facility in 2015. Ameren Missouri will continue to apply for reimbursement from the DOE for the cost to construct and operate the dry spent fuel storage facility along with related allowable costs.
	In March 2015, the NRC approved Ameren Missouri’s application to extend its Callaway energy center's operating license from 2024 to 2044.
	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 the expected life of the nuclear energy center. Amounts collected from customers are deposited into the external nuclear decommissioning trust fund to provide for the Callaway energy center’s decommissioning. It is assumed that the Callaway energy center site will be decommissioned through the immediate dismantlement method and removed from service. Ameren and Ameren Missouri have recorded an ARO for the Callaway energy center decommissioning costs at fair value, which represents the present value of estimated future cash outflows. Annual decommissioning costs of $7 million are included in the costs used to establish electric rates for Ameren Missouri's customers. Every three years, the MoPSC requires Ameren Missouri to file an updated cost study and funding analysis for decommissioning its Callaway energy center. Following the NRC’s decision to approve Ameren Missouri’s operating license extension application, an updated cost study and a revised funding analysis were filed with the MoPSC on April 1, 2015. Ameren Missouri’s April 2015 filing supported no change to the decommissioning cost included in electric service rates. There is no time requirement by which the MoPSC must issue an order regarding the decommissioning cost included in Ameren Missouri’s electric service rates. 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	3 Months Ended
	Mar. 31, 2015
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]
RETIREMENT BENEFITS	RETIREMENT BENEFITS
	Ameren’s pension 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 March 31, 2015, the plan’s estimated investment performance through March 31, 2015, and Ameren’s pension funding policy, Ameren expects to make annual contributions of $25 million to $115 million through 2019, with aggregate estimated contributions of $290 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. Separately, 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 months ended March 31, 2015 and 2014:


		Pension Benefits						Postretirement Benefits
		Three Months							Three Months
		2015			2014				2015			2014
	Service cost	$	24		$	21			$	5		$	5

	Interest cost	44			49				12			13

	Expected return on plan assets	(62		)	(57		)		(17		)	(16		)
	Amortization of:
	Prior service benefit	—			—				(1		)	(1		)

	Actuarial loss (gain)	18			12				1			(1		)

	Net periodic benefit cost	$	24		$	25			$	—		$	—

	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 months ended March 31, 2015 and 2014:


		Pension Benefits						Postretirement Benefits
		Three Months							Three Months
		2015			2014				2015			2014
	Ameren Missouri	$	15		$	17			$	1		$	1

	Ameren Illinois	9			8				(1		)	(1		)

	Ameren(a)	$	24		$	25			$	—		$	—


	(a)	Includes amounts for Ameren registrants and nonregistrant subsidiaries.

Divestiture_Transactions_and_D

Divestiture Transactions and Discontinued Operations (Notes)	3 Months Ended
	Mar. 31, 2015
Discontinued Operations and Disposal Groups [Abstract]
DIVESTITURE TRANSACTIONS AND DISCONTINUED OPERATIONS	DIVESTITURE TRANSACTIONS AND DISCONTINUED OPERATIONS
	On December 2, 2013, Ameren completed the divestiture of New AER to IPH in accordance with the transaction agreement between Ameren and IPH dated March 14, 2013, as amended by a letter agreement dated December 2, 2013. The transaction agreement with IPH, as amended, provides that if the Elgin, Gibson City, and Grand Tower gas-fired energy centers are subsequently sold by Medina Valley and if Medina Valley receives additional proceeds from such sale, Medina Valley will pay Genco any proceeds from such sale, net of taxes and other expenses, in excess of the $137.5 million previously paid to Genco.
	On January 31, 2014, Medina Valley completed the sale of the Elgin, Gibson City, and Grand Tower gas-fired energy centers to Rockland Capital for a total purchase price of $168 million, before consideration of a net working capital adjustment. The agreement with Rockland Capital required $17 million of the purchase price to be held in escrow until January 31, 2016, to fund certain indemnity obligations, if any, of Medina Valley. The Rockland Capital escrow receivable balance and the corresponding payable due to Genco is reflected on Ameren's March 31, 2015 consolidated balance sheet in "Other current assets" and in "Other current liabilities," respectively. Medina Valley expects to pay Genco any remaining portion of the escrow balance on January 31, 2016. Ameren did not record a gain from its sale of the Elgin, Gibson City, and Grand Tower gas-fired energy centers.
	Discontinued Operations Presentation
	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 final tax basis of the AER disposal group and the related tax benefit resulting from the transaction with IPH are dependent upon the resolution of tax matters under audit. It is reasonably possible that in the next 12 months these tax audits will be completed. As a result, tax expense and benefits ultimately realized from the divestitures, including the final resolution of Ameren’s uncertain tax positions, may differ materially from those recorded as of and for the three months ended March 31, 2015. The assets and liabilities of Ameren’s discontinued operations at March 31, 2015, and December 31, 2014, consist primarily of AROs and related deferred income tax assets associated with the abandoned Meredosia and Hutsonville energy centers.
	Pursuant to the IPH transaction agreement, as amended, Ameren is obligated to pay up to $25 million for certain contingent liabilities as of March 31, 2015, which were included in "Other current liabilities" on Ameren's March 31, 2015 consolidated balance sheet.
	The note receivable from Marketing Company related to the cash collateral support provided to New AER was $12 million at March 31, 2015, and December 31, 2014, and was reflected on Ameren's consolidated balance sheet in "Miscellaneous accounts and notes receivable.” This receivable is due to Ameren, with interest, on December 2, 2015, or sooner as cash collateral requirements are reduced. In addition, as of March 31, 2015, if Ameren’s credit ratings had been below investment grade, Ameren could have been required to post additional cash collateral in support of New AER in the amount of $20 million, which includes $10 million currently covered by Ameren guarantees. This cash collateral support is part of Ameren’s obligation to provide certain limited credit support to New AER until December 2, 2015, as discussed below.
	Ameren Guarantees and Letters of Credit
	The IPH transaction agreement, as amended, requires Ameren to maintain its financial obligations with respect to all credit support provided to New AER as of the December 2, 2013 closing date of the divestiture. Ameren must also provide such additional credit support as required by contracts entered into prior to the closing date, in each case until December 2, 2015. IPH shall indemnify Ameren for any payments Ameren makes pursuant to these credit support obligations if the counterparty does not return the posted collateral to Ameren. IPH's indemnification obligation is secured by certain AERG and Genco assets. In addition, Dynegy has provided a limited guarantee of $25 million to Ameren pursuant to which Dynegy will, among other things, guarantee IPH's indemnification obligations until December 2, 2015.
	In addition to the $25 million of contingent liabilities recorded on Ameren’s March 31, 2015 consolidated balance sheet, Ameren had a total of $105 million in guarantees outstanding for New AER that were not recorded on Ameren’s March 31, 2015 consolidated balance sheet, which included:

	•	$103 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 March 31, 2015, Ameren would have been required under its guarantees to provide $10 million to the counterparties.

	•	$2 million related to requirements for lease agreements and potential environmental obligations. If New AER had not fulfilled its lease obligation as of March 31, 2015, Ameren would have been required to provide $1 million to the lease counterparty.
	Additionally, at March 31, 2015, Ameren had issued letters of credit totaling $9 million as credit support on behalf of New AER.
	Ameren has not recorded a liability 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 March 31, 2015.

Segment_Information

Segment Information	3 Months Ended
	Mar. 31, 2015
Segment Reporting [Abstract]
SEGMENT INFORMATION	SEGMENT INFORMATION
	Ameren has two reportable segments: Ameren Missouri and Ameren Illinois. Ameren Missouri and Ameren Illinois each have one reportable segment. The Ameren Missouri segment for both Ameren and Ameren Missouri includes all 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 company activities, Ameren Services, and ATXI.
	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 months ended March 31, 2015 and 2014, and total assets of continuing operations as of March 31, 2015, and December 31, 2014.


	Three Months	Ameren		Ameren		Other			Intersegment			Ameren
		Missouri		Illinois					Eliminations
	2015
	External revenues	$	793	$	744	$	19		$	—		$	1,556

	Intersegment revenues	7		1		1			(9		)	—

	Net income attributable to Ameren Corporation from continuing operations	41		53		14			—			108

	2014
	External revenues	$	811	$	774	$	9		$	—		$	1,594

	Intersegment revenues	6		—		1			(7		)	—

	Net income (loss) attributable to Ameren Corporation from continuing operations	47		53		(3		)	—			97

	As of March 31, 2015:
	Total assets	$	13,555	$	8,456	$	1,082		$	(224	)	$	22,869	(a)

	As of December 31, 2014:
	Total assets	$	13,541	$	8,381	$	942		$	(203	)	$	22,661	(a)

	(a) Excludes total assets from discontinued operations of $15 million as of March 31, 2015, and December 31, 2014.

Summary_Of_Significant_Account1

Summary Of Significant Accounting Policies (Policies)	3 Months Ended
Summary Of Significant Accounting Policies (Policies)	Mar. 31, 2015
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 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.
Consolidation	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.
Excise Taxes	Excise taxes are levied on Ameren Missouri’s electric and natural gas businesses and on Ameren Illinois’ natural gas business and are recorded gross in “Operating Revenues - Electric,” “Operating Revenues - Gas” and “Operating Expenses - Taxes other than income taxes” on the statement of income or the statement of income and comprehensive income. Excise taxes for electric service in Illinois are levied on the customer and are therefore not included in Ameren Illinois’ revenues and expenses.
Uncertain Tax Positions	Ameren’s federal income tax return for the tax year 2013 is currently under examination by the IRS. It is reasonably possible that a settlement will be reached with the IRS in the next 12 months, which will result in a reduction of Ameren’s unrecognized tax benefits of $53 million related to discontinued operations.
	In addition, it is reasonably possible that other events will occur during the next 12 months that would cause the total amount of our unrecognized tax benefits to fluctuate. However, other than as described above, we do not believe any such fluctuations would be material to our results of operations, financial position, or liquidity.
	State income tax returns are generally subject to examination for a period of three years after filing. We do not currently have material state income tax issues under examination, administrative appeal, 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.
Accounting and Reporting Developments	Below is a summary of recently adopted or issued authoritative accounting guidance relevant to the Ameren Companies.
	Presentation of Debt Issuance Costs
	In April 2015, FASB issued authoritative accounting guidance to simplify the presentation of debt issuance costs. The guidance requires debt issuance costs to be presented in the balance sheet as a reduction to the associated debt liability. Currently, debt issuance costs are presented as a component of “Other assets” on the Ameren Companies’ balance sheets. The guidance will be effective for the Ameren Companies in the first quarter of 2016 and applied retrospectively. The guidance will not affect the Ameren Companies' results of operations, financial position, or liquidity, as this guidance is presentation-related only.

Derivative_Financial_Instrumen1

Derivative Financial Instruments Derivative Financial Instruments (Policies)	3 Months Ended
	Mar. 31, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivatives
	•	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)	3 Months Ended
Retirement Benefits Retirement Benefits (Policies)	Mar. 31, 2015
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]
Retirement Benefits	Ameren’s pension 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 March 31, 2015, the plan’s estimated investment performance through March 31, 2015, and Ameren’s pension funding policy, Ameren expects to make annual contributions of $25 million to $115 million through 2019, with aggregate estimated contributions of $290 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. Separately, 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)	3 Months Ended
Summary Of Significant Accounting Policies (Tables)	Mar. 31, 2015
Accounting Policies [Abstract]
Schedule of Change in Asset Retirement Obligation [Table Text Block]	The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the three months ended March 31, 2015:


		Ameren				Ameren			Ameren
		Missouri				Illinois(a)			Ameren
	Balance at December 31, 2014	$	389			$	7		$	396
	Balance at December 31, 2014	$	389			$	7		$	396
	Accretion in 2015(b)	5				(c)			5
	Accretion in 2015(b)	5				(c)			5
	Change in estimates(d)	99				(c)			99
	Change in estimates(d)	99				(c)			99
	Balance at March 31, 2015	$	493			$	7		$	500
	Balance at March 31, 2015	$	493			$	7		$	500

	(a)	Included in “Other deferred credits and liabilities” on the balance sheet.

	(b)	Accretion expense was recorded as an increase to regulatory assets at Ameren Missouri and Ameren Illinois.

	(c)	Less than $1 million.

	(d)	The ARO increase also resulted in a corresponding increase recorded to “Property and Plant, Net.” Ameren Missouri’s estimates related to its Callaway energy center decommissioning costs changed to reflect increased costs from the 2015 cost study and funding analysis, extension of the estimated operating life until 2044, and a reduction in the discount rate assumption. See Note 10 - Callaway Energy Center for additional information.
Summary Of Nonvested Shares Related To Long-Term Incentive Plan	A summary of nonvested performance share units at March 31, 2015, and changes during the three months ended March 31, 2015, under the 2006 Incentive Plan and the 2014 Incentive Plan are presented below:


		Number of Performance Share Units		Weighted-average Fair Value Per Performance Share Unit
	Nonvested at January 1, 2015	1,162,377		$	35.35
	Nonvested at January 1, 2015	1,162,377		$	35.35
	Granted(a)	566,332		52.88
	Granted(a)	566,332		52.88
	Forfeitures	(1,944	)	34.75
	Forfeitures	(1,944	)	34.75
	Vested(b)	(68,411	)	47.88
	Vested(b)	(68,411	)	47.88
	Nonvested at March 31, 2015	1,658,354		$	40.82
	Nonvested at March 31, 2015	1,658,354		$	40.82

	(a)	Performance share units granted to certain executive and nonexecutive officers and other eligible employees in 2015 under the 2014 Incentive Plan.

	(b)	Performance 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 Unrecognized Tax Benefits	The following table presents the total amount of reserves for unrecognized tax benefits (detriments) related to uncertain tax positions as of March 31, 2015, and December 31, 2014:


		31-Mar-15				31-Dec-14
	Ameren	$	53			$	54
	Ameren	$	53			$	54
	Ameren Missouri	(1		)		—
	Ameren Missouri	(1		)		—
	Ameren Illinois	(1		)		(1		)
Schedule of Unrecognized Tax Benefits That Would Impact Effective Tax Rate	The following table presents the amount of reserves for unrecognized tax benefits, included in the table above, related to uncertain tax positions that, if recognized, would impact results of operations as of March 31, 2015, and December 31, 2014:


		31-Mar-15				31-Dec-14
	Ameren	$	52			$	52
	Ameren	$	52			$	52
	Ameren Missouri	(1		)		—
	Ameren Missouri	(1		)		—
	Ameren Illinois	(1		)		(1		)
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 months ended March 31, 2015 and 2014:


		Three Months
		2015				2014
	Ameren Missouri	$	34			$	34
	Ameren Missouri	$	34			$	34
	Ameren Illinois	23				26
	Ameren Illinois	23				26
	Ameren	$	57			$	60
	Ameren	$	57			$	60

ShortTerm_Debt_And_Liquidity_S

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Tables)	3 Months Ended
	Mar. 31, 2015
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 March 31, 2015, and December 31, 2014.


		31-Mar-15				31-Dec-14
	Ameren (parent)	$	815			$	585

	Ameren Missouri	140				97

	Ameren Illinois	—				32

	Ameren Consolidated	$	955			$	714

	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 three months ended March 31, 2015 and 2014. Ameren Illinois established a commercial paper program in May 2014.


			Ameren				Ameren			Ameren			Ameren Consolidated
			(parent)				Missouri			Illinois
	2015
	Average daily commercial paper outstanding		$	691			$	151		$	10		$	852

	Weighted-average interest rate		0.55		%		0.49		%	0.44		%	0.53		%
	Peak commercial paper during period(a)		$	815			$	243		$	39		$	955

	Peak interest rate		0.7		%		0.6		%	0.6		%	0.7		%
	2014
	Average daily commercial paper outstanding		$	339			$	200		$	—		$	539

	Weighted-average interest rate		0.45		%		0.45		%	—		%	0.45		%
	Peak commercial paper during period(a)		$	452			$	290		$	—		$	700

	Peak interest rate		0.75		%		0.7		%	—		%	0.75		%

	(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)

3 Months Ended

Mar. 31, 2015

Long-Term Debt And Equity Financings [Line Items]

Schedule Of Coverage Ratios

Indenture Provisions and Other Covenants

Required Interest

Actual Interest

Bonds Issuable(b)

Required Dividend

Actual Dividend

Preferred Stock

Coverage Ratio(a)

Coverage Ratio

Coverage Ratio(c)

Coverage Ratio

Issuable

Ameren Missouri

≥2.0

4.6

3,386

≥2.5

113.4

2,530

Ameren Illinois

≥2.0

6.6

3,423

(d)

≥1.5

2.8

203

(e)

(a)

(b)

(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)

(e)

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

Other_Income_and_Expenses_Tabl

Other Income and Expenses (Tables)	3 Months Ended
	Mar. 31, 2015
Other Nonoperating Income (Expense) [Abstract]
Other Income And Expenses	The following table presents the components of “Other Income and Expenses” in the Ameren Companies’ statements of income for the three months ended March 31, 2015 and 2014:


		Three Months
		2015		2014
	Ameren:(a)
	Miscellaneous income:
	Allowance for equity funds used during construction	$	5	$	7

	Interest income on industrial development revenue bonds	7		7

	Interest income	4		2

	Other	3		2

	Total miscellaneous income	$	19	$	18

	Miscellaneous expense:
	Donations	$	8	$	5

	Other	3		4

	Total miscellaneous expense	$	11	$	9

	Ameren Missouri:
	Miscellaneous income:
	Allowance for equity funds used during construction	$	4	$	7

	Interest income on industrial development revenue bonds	7		7

	Total miscellaneous income	$	11	$	14

	Miscellaneous expense:
	Donations	$	2	$	2

	Other	1		2

	Total miscellaneous expense	$	3	$	4

	Ameren Illinois:
	Miscellaneous income:
	Allowance for equity funds used during construction	$	1	$	—

	Interest income	4		2

	Other	2		1

	Total miscellaneous income	$	7	$	3

	Miscellaneous expense:
	Donations	$	3	$	3

	Other	2		1

	Total miscellaneous expense	$	5	$	4


	(a)	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Derivative_Financial_Instrumen2

Derivative Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2015
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 March 31, 2015, and December 31, 2014. As of March 31, 2015, 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)
		2015						2014
	Commodity	Ameren Missouri		Ameren Illinois		Ameren		Ameren Missouri		Ameren Illinois		Ameren
	Fuel oils (in gallons)(a)	40		(b)		40		50		(b)		50

	Natural gas (in mmbtu)	29		121		150		28		108		136

	Power (in megawatthours)	1		10		11		1		11		12

	Uranium (pounds in thousands)	349		(b)		349		332		(b)		332


	(a)	Fuel oils consist of heating oil and ultra-low-sulfur diesel.

	(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 March 31, 2015, and December 31, 2014:


		Balance Sheet Location		Ameren				Ameren				Ameren
				Missouri				Illinois
	2015
	Fuel oils	Other current assets		$	1			$	—			$	1

	Natural gas	Other current assets		—				1				1

	Power	Other current assets		7				—				7

		Total assets		$	8			$	1			$	9

	Fuel oils	Other current liabilities		$	21			$	—			$	21

		Other deferred credits and liabilities		7				—				7

	Natural gas	MTM derivative liabilities		(a)				32				(a)

		Other current liabilities		6				—				38

		Other deferred credits and liabilities		8				18				26

	Power	MTM derivative liabilities		(a)				11				(a)

		Other current liabilities		1				—				12

		Other deferred credits and liabilities		—				153				153

	Uranium	Other current liabilities		1				—				1

		Total liabilities		$	44			$	214			$	258

	2014
	Fuel oils	Other current assets		$	2			$	—			$	2

	Natural gas	Other current assets		1				1				2

	Power	Other current assets		15				—				15

		Total assets		$	18			$	1			$	19

	Fuel oils	Other current liabilities		$	22			$	—			$	22

		Other deferred credits and liabilities		7				—				7

	Natural gas	MTM derivative liabilities		(a)				31				(a)

		Other current liabilities		6				—				37

		Other deferred credits and liabilities		6				13				19

	Power	MTM derivative liabilities		(a)				11				(a)

		Other current liabilities		3				—				14

		Other deferred credits and liabilities		—				131				131

	Uranium	Other current liabilities		2				—				2

		Total liabilities		$	46			$	186			$	232


(a)	Balance sheet line item not applicable to registrant.
Cumulative Pretax Net Gains (Losses) On All Derivative Instruments In OCI	The following table presents the cumulative amount of pretax net gains (losses) on all derivative instruments deferred in regulatory assets or regulatory liabilities as of March 31, 2015, and December 31, 2014:


		Ameren				Ameren				Ameren
		Missouri				Illinois
	2015
	Fuel oils derivative contracts(a)	$	(27	)		$	—			$	(27	)

	Natural gas derivative contracts(b)	(14		)		(49		)		(63		)
	Power derivative contracts(c)	6				(164		)		(158		)

	Uranium derivative contracts(d)	(1		)		—				(1		)

	2014
	Fuel oils derivative contracts	$	(29	)		$	—			$	(29	)

	Natural gas derivative contracts	(11		)		(43		)		(54		)
	Power derivative contracts	12				(142		)		(130		)

	Uranium derivative contracts	(2		)		—				(2		)


	(a)	Represents net losses associated with fuel oils derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s rail transportation surcharges for coal through December 2017. Current losses deferred as regulatory assets include $20 million at Ameren and Ameren Missouri.

	(b)	Represents net losses associated with natural gas derivative contracts. These contracts are a partial hedge of natural gas requirements through October 2019 at Ameren and Ameren Missouri and through October 2018 at Ameren Illinois. Current gains deferred as regulatory liabilities include $1 million at Ameren and Ameren Illinois, respectively. Current losses deferred as regulatory assets include $38 million, $6 million, and $32 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively.

	(c)	Represents net gains (losses) associated with power derivative contracts. These contracts are a partial hedge of power price requirements through May 2032 at Ameren and Ameren Illinois and through December 2016 at Ameren Missouri. Current gains deferred as regulatory liabilities include $7 million at Ameren and Ameren Missouri. Current losses deferred as regulatory assets include $12 million, $1 million, and $11 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively.

	(d)	Represents net losses on uranium derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s uranium requirements through December 2016. Current losses deferred as regulatory assets include $1 million at Ameren and Ameren Missouri.
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 March 31, 2015, and December 31, 2014:


							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
	2015
	Assets:
	Ameren Missouri		$	8			$	4			$	—		$	4

	Ameren Illinois		1				—				—			1

	Ameren		$	9			$	4			$	—		$	5

	Liabilities:
	Ameren Missouri		$	44			$	4			$	4		$	36

	Ameren Illinois		214				—				2			212

	Ameren		$	258			$	4			$	6		$	248

	2014
	Assets:
	Ameren Missouri		$	18			$	5			$	—		$	13

	Ameren Illinois		1				—				—			1

	Ameren		$	19			$	5			$	—		$	14

	Liabilities:
	Ameren Missouri		$	46			$	5			$	5		$	36

	Ameren Illinois		186				—				—			186

	Ameren		$	232			$	5			$	5		$	222


	(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 netting arrangements, assuming (1) the credit risk-related contingent features underlying these arrangements were triggered on March 31, 2015, 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)
	2015
	Ameren Missouri	$	87			$	6			$	83

	Ameren Illinois	80				2				74

	Ameren	$	167			$	8			$	157


	(a)	Prior to consideration of master netting arrangements and including NPNS and other accrual contract exposures.

	(b)	As collateral requirements with certain counterparties are based on master netting arrangements, the aggregate amount of additional collateral required to be posted is determined after consideration of the effects of such arrangements.

Fair_Value_Measurements_Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2015
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 March 31, 2015:


			Fair Value									Weighted Average
			Assets			Liabilities			Valuation Technique(s)	Unobservable Input	Range
	Level 3 Derivative asset and liability - commodity contracts(a):
	Ameren	Fuel oils	$	1		$	(7	)	Option model	Volatilities(%)(b)	33 - 80	42

									Discounted cash flow	Ameren Missouri credit risk(%)(b)(c)	0.4	(d)
		Natural gas	1			(1		)	Option model	Volatilities(%)(e)	Jun-39	33

										Nodal basis($/mmbtu)(e)	(0.40) - (0.20)	-0.3
									Discounted cash flow	Nodal basis($/mmbtu)(e)	(0.40) - (0.10)	-0.3
										Counterparty credit risk(%)(b)(c)	0.31 - 12.07	2.47
										Ameren Missouri and Ameren Illinois credit risk(%)(b)(c)	0.4	(d)
		Power(f)	5			(165		)	Discounted cash flow	Average forward peak and off-peak pricing - forwards/swaps($/MWh)(g)	22 - 46	32

										Estimated auction price for FTRs($/MW)(e)	(597) - 1,922	153
										Nodal basis($/MWh)(e)	(11) - 0	-3
										Counterparty credit risk(%)(b)(c)	0.29 - 10.98	5.84
										Ameren Missouri and Ameren Illinois credit risk(%)(b)(c)	0.4	(d)
									Fundamental energy production model	Estimated future gas prices($/mmbtu)(e)	5-Mar	4
										Escalation rate(%)(e)(h)	1	(d)
									Contract price allocation	Estimated renewable energy credit costs($/credit)(e)	7-May	6
		Uranium	—			(1		)	Discounted cash flow	Average forward uranium pricing($/pound)(e)	40 - 43	40

	Ameren Missouri	Fuel oils	$	1		$	(7	)	Option model	Volatilities(%)(b)	33 - 80	42

									Discounted cash flow	Ameren Missouri credit risk(%)(b)(c)	0.4	(d)
		Natural gas	—			(1		)	Option model	Volatilities(%)(e)	Jun-39	33

										Nodal basis($/mmbtu)(e)	(0.40) - (0.20)	-0.3
									Discounted cash flow	Nodal basis($/mmbtu)(e)	-0.1	(d)
										Counterparty credit risk(%)(b)(c)	0.54 - 12.07	5
										Ameren Missouri credit risk(%)(b)(c)	0.4	(d)
		Power(f)	5			(1		)	Discounted cash flow	Average forward peak and off-peak pricing - forwards/swaps($/MWh)(b)	24 - 46	36

										Estimated auction price for FTRs($/MW)(e)	(597) - 1,922	153
										Nodal basis($/MWh)(b)	(11) - (4)	-8
										Counterparty credit risk(%)(b)(c)	0.29 - 10.98	5.84
		Uranium	—			(1		)	Discounted cash flow	Average forward uranium pricing($/pound)(e)	40 - 43	40

	Ameren Illinois	Natural gas	$	1		$	—		Discounted cash flow	Nodal basis($/mmbtu)(e)	(0.40) - (0.10)	-0.3

										Counterparty credit risk(%)(b)(c)	0.31 - 2.28	1.28
										Ameren Illinois credit risk(%)(b)(c)	0.4	(d)
		Power(f)	—			(164		)	Discounted cash flow	Average forward peak and off-peak pricing - forwards/swaps($/MWh)(e)	22 - 38	31

										Nodal basis($/MWh)(e)	(6) - 0	-3
										Ameren Illinois credit risk(%)(b)(c)	0.4	(d)
									Fundamental energy production model	Estimated future gas prices($/mmbtu)(e)	5-Mar	4
										Escalation rate(%)(e)(h)	1	(d)
									Contract price allocation	Estimated renewable energy credit costs($/credit)(e)	7-May	6

	(a)	The derivative asset and liability balances are presented net of counterparty credit considerations.

	(b)	Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.

	(c)	Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances.

	(d)	Not applicable.

	(e)	Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.

(f)	Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2018. Valuations beyond 2018 use fundamentally modeled pricing by month for peak and off-peak demand.

(g)	The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes due to their opposing positions. As such, refer to the power sensitivity analysis for each company above.

(h)	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, 2014:


		Fair Value										Weighted
		Assets			Liabilities				Valuation Technique(s)	Unobservable Input	Range	Average
Level 3 Derivative asset and liability – commodity contracts(a):
Ameren	Fuel oils	$	2		$	(8	)		Option model	Volatilities(%)(b)	Mar-39	32

									Discounted cash flow	Ameren Missouri credit risk(%)(b)(c)	0.43	(d)
										Escalation rate(%)(e)(f)	5	(d)
	Natural Gas	1			(2		)		Option model	Volatilities(%)(b)	31 - 144	63

										Nodal basis($/mmbtu)(e)	(0.40) - 0	-0.2
									Discounted cash flow	Nodal basis($/mmbtu)(e)	(0.40) - 0.10	-0.2
										Counterparty credit risk(%)(b)(c)	0.43 - 13	3
										Ameren Missouri and Ameren Illinois credit risk(%)(b)(c)	0.43	(d)
	Power(g)	11			(144		)		Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps($/MWh)(h)	27 - 50	32

										Estimated auction price for FTRs($/MW)(e)	(1,833) - 2,743	171
										Nodal basis($/MWh)(e)	(6) - 0	-2
										Counterparty credit risk(%)(b)(c)	0.26	(d)
										Ameren Missouri and Ameren Illinois credit risk(%)(b)(c)	0.43	(d)
									Fundamental energy production model	Estimated future gas prices($/mmbtu)(e)	5-Apr	4
										Escalation rate(%)(e)(i)	0 - 1	1
									Contract price allocation	Estimated renewable energy credit costs($/credit)(e)	7-May	6
	Uranium	—			(2		)		Discounted cash flow	Average forward uranium pricing($/pound)(e)	35 - 40	36

Ameren Missouri	Fuel oils	$	2		$	(8	)		Option model	Volatilities(%)(b)	Mar-39	32

									Discounted cash flow	Ameren Missouri credit risk(%)(b)(c)	0.43	(d)
										Escalation rate(%)(e)(f)	5	(d)
	Natural Gas	—			(1		)		Option model	Volatilities(%)(b)	31 - 144	53

										Nodal basis($/mmbtu)(e)	(0.40) - 0	-0.3
									Discounted cash flow	Nodal basis($/mmbtu)(e)	-0.1	(d)
										Counterparty credit risk(%)(b)(c)	0.57 - 13	5
										Ameren Missouri credit risk(%)(b)(c)	0.43	(d)
	Power(g)	11			(2		)		Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps($/MWh)(b)	27 - 50	32

										Estimated auction price for FTRs($/MW)(e)	(1,833) - 2,743	171
										Counterparty credit risk(%)(b)(c)	0.26	(d)
										Ameren Missouri credit risk(%)(b)(c)	0.43	(d)
	Uranium	—			(2		)		Discounted cash flow	Average forward uranium pricing($/pound)(e)	35 - 40	36

Ameren Illinois	Natural Gas	$	1		$	(1	)		Option model	Volatilities(%)(b)	50 - 144	94

										Nodal basis($/mmbtu)(e)	(0.10) - 0	-0.1
									Discounted cash flow	Nodal basis($/mmbtu)(e)	(0.40) - 0.10	-0.2
										Counterparty credit risk(%)(b)(c)	0.43 - 2	0.83
										Ameren Illinois credit risk(%)(b)(c)	0.43	(d)
	Power(g)	—			(142		)		Discounted cash flow	Average forward peak and off-peak pricing – forwards/swaps($/MWh)(e)	27 - 38	32

										Nodal basis($/MWh)(e)	(6) - 0	-2
										Ameren Illinois credit risk(%)(b)(c)	0.43	(d)
									Fundamental energy production model	Estimated future gas prices($/mmbtu)(e)	5-Apr	4
										Escalation rate(%)(e)(i)	0 - 1	1
									Contract price allocation	Estimated renewable energy credit costs($/credit)(e)	7-May	6

(a)	The derivative asset and liability balances are presented net of counterparty credit considerations.

(b)	Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.

(c)	Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances.

(d)	Not applicable.

(e)	Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.

(f)	Escalation rate applies to fuel oil prices 2017 and beyond.

(g)	Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2018. Valuations beyond 2018 use fundamentally modeled pricing by month for peak and off-peak demand.

(h)	The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes due to their opposing positions. As such, refer to the power sensitivity analysis for each company above.

(i)	Escalation rate applies to power prices 2026 and beyond.
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 March 31, 2015:


				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			$	1

		Natural gas		—				—				1				1

		Power		—				2				5				7

		Total derivative assets - commodity contracts		$	—			$	2			$	7			$	9

		Nuclear decommissioning trust fund:
		Cash and cash equivalents		$	2			$	—			$	—			$	2

		Equity securities:
		U.S. large capitalization		368				—				—				368

		Debt securities:
		Corporate bonds		—				65				—				65

		U.S. treasury and agency securities		—				101				—				101

		Other		—				20				—				20

		Total nuclear decommissioning trust fund		$	370			$	186			$	—			$	556		(b)

		Total Ameren		$	370			$	188			$	7			$	565

	Ameren	Derivative assets - commodity contracts(a):
	Missouri	Fuel oils		$	—			$	—			$	1			$	1

		Power		—				2				5				7

		Total derivative assets - commodity contracts		$	—			$	2			$	6			$	8

		Nuclear decommissioning trust fund:
		Cash and cash equivalents		$	2			$	—			$	—			$	2

		Equity securities:
		U.S. large capitalization		368				—				—				368

		Debt securities:
		Corporate bonds		—				65				—				65

		U.S. treasury and agency securities		—				101				—				101

		Other		—				20				—				20

		Total nuclear decommissioning trust fund		$	370			$	186			$	—			$	556		(b)

		Total Ameren Missouri		$	370			$	188			$	6			$	564

	Ameren	Derivative assets - commodity contracts(a):
	Illinois	Natural gas		$	—			$	—			$	1			$	1

	Liabilities:
	Ameren	Derivative liabilities - commodity contracts(a):
	Fuel oils		$	21			$	—			$	7			$	28

	Natural gas		—				63				1				64

	Power		—				—				165				165

	Uranium		—				—				1				1

	Total Ameren		$	21			$	63			$	174			$	258

Ameren	Derivative liabilities - commodity contracts(a):
Missouri	Fuel oils		$	21			$	—			$	7			$	28

	Natural gas		—				13				1				14

	Power		—				—				1				1

	Uranium		—				—				1				1

	Total Ameren Missouri		$	21			$	13			$	10			$	44

Ameren	Derivative liabilities - commodity contracts(a):
Illinois	Natural gas		$	—			$	50			$	—			$	50

	Power		—				—				164				164

	Total Ameren Illinois		$	—			$	50			$	164			$	214


(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, 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			$	2

	Natural gas		—				1				1				2

	Power		—				4				11				15

	Total derivative assets - commodity contracts		$	—			$	5			$	14			$	19

	Nuclear decommissioning trust fund:
	Cash and cash equivalents		$	1			$	—			$	—			$	1

	Equity securities:
	U.S. large capitalization		364				—				—				364

	Debt securities:
	Corporate bonds		—				63				—				63

	U.S. treasury and agency securities		—				102				—				102

	Other		—				17				—				17

	Total nuclear decommissioning trust fund		$	365			$	182			$	—			$	547		(b)

	Total Ameren		$	365			$	187			$	14			$	566

Ameren	Derivative assets - commodity contracts(a):
Missouri	Fuel oils		$	—			$	—			$	2			$	2

	Natural gas		—				1				—				1

	Power		—				4				11				15

	Total derivative assets - commodity contracts		$	—			$	5			$	13			$	18

	Nuclear decommissioning trust fund:
	Cash and cash equivalents		$	1			$	—			$	—			$	1

	Equity securities:
	U.S. large capitalization		364				—				—				364

	Debt securities:
	Corporate bonds		—				63				—				63

	U.S. treasury and agency securities		—				102				—				102

	Other		—				17				—				17

	Total nuclear decommissioning trust fund		$	365			$	182			$	—			$	547		(b)

	Total Ameren Missouri		$	365			$	187			$	13			$	565

Ameren	Derivative assets - commodity contracts(a):
Illinois	Natural gas		$	—			$	—			$	1			$	1

Liabilities:
Ameren	Derivative liabilities - commodity contracts(a):
	Fuel oils		$	21			$	—			$	8			$	29

	Natural gas		1				53				2				56

	Power		—				1				144				145

	Uranium		—				—				2				2

	Total Ameren		$	22			$	54			$	156			$	232

Ameren	Derivative liabilities - commodity contracts(a):
Missouri	Fuel oils		$	21			$	—			$	8			$	29

	Natural gas		1				10				1				12

	Power		—				1				2				3

	Uranium		—				—				2				2

	Total Ameren Missouri		$	22			$	11			$	13			$	46

Ameren	Derivative liabilities - commodity contracts(a):
Illinois	Natural gas		$	—			$	43			$	1			$	44

	Power		—				—				142				142

	Total Ameren Illinois		$	—			$	43			$	143			$	186


(a)	The derivative asset and liability balances are presented net of counterparty credit considerations.

(b)	Balance excludes $2 million of receivables, payables, and accrued income, net.
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 March 31, 2015:


			Net derivative commodity contracts
			Ameren			Ameren			Ameren
			Missouri			Illinois
	Fuel oils:
	Beginning balance at January 1, 2015	$	(6	)	$	(a)		$	(6	)

	Realized and unrealized gains (losses) included in regulatory assets/liabilities		(1	)		(a)			(1	)

	Settlements		1			(a)			1

	Ending balance at March 31, 2015	$	(6	)	$	(a)		$	(6	)

	Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2015	$	(3	)	$	(a)		$	(3	)

	Natural gas:
	Beginning balance at January 1, 2015	$	(1	)	$	—		$	(1	)

	Purchases		—			1			1

	Ending balance at March 31, 2015	$	(1	)	$	1		$	—

	Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2015	$	—		$	—		$	—

	Power:
	Beginning balance at January 1, 2015	$	9		$	(142	)	$	(133	)

	Realized and unrealized gains (losses) included in regulatory assets/liabilities		(2	)		(25	)		(27	)
	Settlements		(3	)		3			—

	Ending balance at March 31, 2015	$	4		$	(164	)	$	(160	)

	Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2015	$	—		$	(24	)	$	(24	)

	Uranium:
	Beginning balance at January 1, 2015	$	(2	)	$	(a)		$	(2	)

	Realized and unrealized gains (losses) included in regulatory assets/liabilities		1			(a)			1

	Ending balance at March 31, 2015	$	(1	)	$	(a)		$	(1	)

	Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2015	$	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 three months ended March 31, 2014:


			Net derivative commodity contracts
			Ameren			Ameren			Ameren
			Missouri			Illinois
	Fuel oils:
	Beginning balance at January 1, 2014	$	5		$	(a)		$	5

	Realized and unrealized gains (losses) included in regulatory assets/liabilities		(2	)		(a)			(2	)

	Settlements		(2	)		(a)			(2	)

	Ending balance at March 31, 2014	$	1		$	(a)		$	1

	Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2014	$	(1	)	$	(a)		$	(1	)

Natural gas:
Beginning balance at January 1, 2014	$	—		$	—		$	—

Purchases		—			(2	)		(2	)

Settlements		—			2			2

Ending balance at March 31, 2014	$	—		$	—		$	—

Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2014	$	—		$	—		$	—

Power:
Beginning balance at January 1, 2014	$	19		$	(108	)	$	(89	)

Realized and unrealized gains (losses) included in regulatory assets/liabilities		(5	)		(12	)		(17	)
Settlements		(5	)		—			(5	)

Transfers out of Level 3		1			—			1

Ending balance at March 31, 2014	$	10		$	(120	)	$	(110	)

Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2014	$	(1	)	$	(14	)	$	(15	)
Uranium:
Beginning balance at January 1, 2014	$	(6	)	$	(a)		$	(6	)

Settlements		1			(a)			1

Ending balance at March 31, 2014	$	(5	)	$	(a)		$	(5	)

Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2014	$	—		$	(a)		$	—


(a)	Not applicable.
Schedule Of Carrying Amounts And Estimated Fair Values Of Long-Term Debt, Capital Lease Obligations And Preferred Stock	The following table presents the carrying amounts and estimated fair values of our long-term debt, capital lease obligations and preferred stock at March 31, 2015, and December 31, 2014:


		31-Mar-15								31-Dec-14
		Carrying				Fair				Carrying				Fair
		Amount				Value				Amount				Value
	Ameren:(a)
	Long-term debt and capital lease obligations (including current portion)	$	6,240			$	7,127			$	6,240			$	7,135

	Preferred stock	142				123				142				122

	Ameren Missouri:
	Long-term debt and capital lease obligations (including current portion)	$	3,999			$	4,574			$	3,999			$	4,518

	Preferred stock	80				74				80				73

	Ameren Illinois:
	Long-term debt	$	2,241			$	2,553			$	2,241			$	2,517

	Preferred stock	62				49				62				49


	(a)	Preferred stock is recorded in “Noncontrolling Interests” on the consolidated balance sheet.

Related_Party_Transactions_Tab

Related Party Transactions (Tables)	3 Months Ended
	Mar. 31, 2015
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 months ended March 31, 2015 and 2014.


					Three Months
	Agreement	Income Statement			Ameren		Ameren
		Line Item			Missouri		Illinois
	Ameren Missouri power supply	Operating Revenues	2015	$	1	$	(a)

	agreements with Ameren Illinois		2014		(b)		(a)

	Ameren Missouri and Ameren Illinois	Operating Revenues	2015		6		1

	rent and facility services		2014		5		(b)

	Ameren Missouri and Ameren Illinois	Operating Revenues	2015		(b)		(b)

	miscellaneous support services		2014		(b)		(b)

	Total Operating Revenues		2015	$	7	$	1

			2014		5		(b)

	Ameren Illinois power supply	Purchased Power	2015	$	(a)	$	1

	agreements with Ameren Missouri		2014		(a)		(b)

	Ameren Illinois transmission	Purchased Power	2015		(a)		1

	services with ATXI		2014		(a)		1

	Total Purchased Power		2015	$	(a)	$	2

			2014		(a)		1

	Ameren Services support services	Other Operations and Maintenance	2015	$	34	$	29

	agreement		2014		33		27

	Total Other Operations and		2015	$	34	$	29

	Maintenance Expenses		2014		33		27

	Money pool borrowings (advances)	Interest Charges/ Miscellaneous	2015	$	(b)	$	(b)

		Income	2014		(b)		(b)


	(a)	Not applicable.

	(b)	Amount less than $1 million.

Commitments_And_Contingencies_

Commitments And Contingencies (Tables)	3 Months Ended
	Mar. 31, 2015
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 March 31, 2015. 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 2015.


	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	$	2,750	(d)	$	26	(e)

	European Mutual Association for Nuclear Insurance	500		(f)	—

		$	3,250		$	26

	Replacement power:
	NEIL	$	490	(g)	$	9	(e)


	(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 for both radiation and nonradiation events. An additional $500 million is provided for radiation events only for a total of $2.75 billion.

	(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 $2.75 billion and $2.25 billion property coverage for radiation and nonradiation events, respectively, 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 twelve weeks of an outage, plus up to $3.6 million per week for a minimum of 71 weeks thereafter for a total not exceeding the policy limit of $490 million. Nonradiation events are sub-limited to $328 million.
Schedule of Asbestors-Related Litigation Pending Lawsuits	The following table presents the pending asbestos-related lawsuits filed against the Ameren Companies as of March 31, 2015:


	Ameren		Ameren	Ameren		Total(a)
			Missouri	Illinois
	1		42	53		65

	(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)	3 Months Ended
	Mar. 31, 2015
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 months ended March 31, 2015 and 2014:


		Pension Benefits						Postretirement Benefits
		Three Months							Three Months
		2015			2014				2015			2014
	Service cost	$	24		$	21			$	5		$	5

	Interest cost	44			49				12			13

	Expected return on plan assets	(62		)	(57		)		(17		)	(16		)
	Amortization of:
	Prior service benefit	—			—				(1		)	(1		)

	Actuarial loss (gain)	18			12				1			(1		)

	Net periodic benefit cost	$	24		$	25			$	—		$	—

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 months ended March 31, 2015 and 2014:


		Pension Benefits						Postretirement Benefits
		Three Months							Three Months
		2015			2014				2015			2014
	Ameren Missouri	$	15		$	17			$	1		$	1

	Ameren Illinois	9			8				(1		)	(1		)

	Ameren(a)	$	24		$	25			$	—		$	—


	(a)	Includes amounts for Ameren registrants and nonregistrant subsidiaries.

Segment_Information_Tables

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2015
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 months ended March 31, 2015 and 2014, and total assets of continuing operations as of March 31, 2015, and December 31, 2014.


	Three Months	Ameren		Ameren		Other			Intersegment			Ameren
		Missouri		Illinois					Eliminations
	2015
	External revenues	$	793	$	744	$	19		$	—		$	1,556

	Intersegment revenues	7		1		1			(9		)	—

	Net income attributable to Ameren Corporation from continuing operations	41		53		14			—			108

	2014
	External revenues	$	811	$	774	$	9		$	—		$	1,594

	Intersegment revenues	6		—		1			(7		)	—

	Net income (loss) attributable to Ameren Corporation from continuing operations	47		53		(3		)	—			97

	As of March 31, 2015:
	Total assets	$	13,555	$	8,456	$	1,082		$	(224	)	$	22,869	(a)

	As of December 31, 2014:
	Total assets	$	13,541	$	8,381	$	942		$	(203	)	$	22,661	(a)

	(a) Excludes total assets from discontinued operations of $15 million as of March 31, 2015, and December 31, 2014.

Summary_Of_Significant_Account3

Summary Of Significant Accounting Policies (Narrative) (Detail) (USD $)	3 Months Ended
In Millions, except Per Share data, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Basis Of Presentation And Significant Accounting Policies [Line Items]
Closing common share price			$46.13
Performance Shares
Basis Of Presentation And Significant Accounting Policies [Line Items]
Fair value of each share unit, per share	$52.88	[1]
Performance period	3 years
Risk free interest rate period	3 years
Three-year risk-free rate	1.10%
Volatility rate, minimum	12.00%
Volatility rate, maximum	18.00%
Minimum
Basis Of Presentation And Significant Accounting Policies [Line Items]
Costs Incurred, Asset Retirement Obligation Incurred	$90
Minimum \| Union Electric Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Costs Incurred, Asset Retirement Obligation Incurred	90
Maximum
Basis Of Presentation And Significant Accounting Policies [Line Items]
Costs Incurred, Asset Retirement Obligation Incurred	120
Maximum \| Union Electric Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Costs Incurred, Asset Retirement Obligation Incurred	$120

[1]	Performance share units granted to certain executive and nonexecutive officers and other eligible employees in 2015 under the 2014 Incentive Plan.

Summary_Of_Significant_Account4

Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Schedule of Asset Retirement Obligations) (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	$500		$396
Asset Retirement Obligation, Accretion Expense	5	[1]
Asset Retirement Obligation, Revision of Estimate	99	[2]
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	493		389
Asset Retirement Obligation, Accretion Expense	5	[1]
Asset Retirement Obligation, Revision of Estimate	99	[2]
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	7	[3]	7	[3]
Asset Retirement Obligation, Accretion Expense	1	[1],[4]
Asset Retirement Obligation, Revision of Estimate	$1	[2],[4]

[1]	Accretion expense was recorded as an increase to regulatory assets at Ameren Missouri and Ameren Illinois.
[2]	The ARO increase also resulted in a corresponding increase recorded to bProperty and Plant, Net.b Ameren Missouribs estimates related to its Callaway energy center decommissioning costs changed to reflect increased costs from the 2015 cost study and funding analysis, extension of the estimated operating life until 2044, and a reduction in the discount rate assumption. See Note 10 - Callaway Energy Center for additional information
[3]	Included in bOther deferred credits and liabilitiesb on the balance sheet
[4]	Less than $1 million.

Summary_Of_Significant_Account5

Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Summary of Nonvested Shares Related To Long-Term Incentive Plan) (Details) (Performance Shares, USD $)	3 Months Ended
	Mar. 31, 2015
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Three-year risk-free rate	1.10%
Volatility rate, minimum	12.00%
Volatility rate, maximum	18.00%
Share Units
Share Units, Nonvested as of January 1, 2014	1,162,377
Share Units, Granted	566,332	[1]
Share Units, Forfeitures	-1,944
Share Units, Vested	-68,411	[2]
Share Units, Nonvested as of March 31, 2014	1,658,354
Weighted-average Fair Value per Unit at Grant Date
Weighted-average Fair Value per Unit, Nonvested as of January 1, 2014	$35.35
Weighted-average Fair Value per Unit, Grants	$52.88	[1]
Weighted-average Fair Value per Unit, Forfeitures	$34.75
Weighted-average Fair Value per Unit, Vested	$47.88	[2]
Weighted-average Fair Value per Unit, Nonvested as of March 31, 2014	$40.82
Performance period	3 years

[1]	Performance share units granted to certain executive and nonexecutive officers and other eligible employees in 2015 under the 2014 Incentive Plan.
[2]	(b)Performance share units vested due to the attainment of retirement eligibility by certain employees. Actual shares issued for retirement-eligible employees will vary depending on actual performance over the three-year measurement period.

Summary_Of_Significant_Account6

Summary Of Significant Accounting Policies (Schedule Of Excise Taxes) (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Accounting Policies [Line Items]
Excise tax expense	$57	$60
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	34	34
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$23	$26

Summary_Of_Significant_Account7

Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Schedule of Unrecognized Tax Benefits (Detriments)) (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Unrecognized Tax Benefits (Detriments) [Line Items]
Unrecognized tax benefits (detriments)	$53	$54
Union Electric Company
Schedule of Unrecognized Tax Benefits (Detriments) [Line Items]
Unrecognized tax benefits (detriments)	-1	0
Ameren Illinois Company
Schedule of Unrecognized Tax Benefits (Detriments) [Line Items]
Unrecognized tax benefits (detriments)	-1	-1
2013 Tax Year [Member]
Schedule of Unrecognized Tax Benefits (Detriments) [Line Items]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	$53

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 $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Unrecognized Tax Benefits (Detriments) That Would Impact Effective Tax Rate [Line Items]
Unrecognized tax benefits (detriments) that would impact effective tax rate	$52	$52
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	-1	0
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)	($1)

Rate_And_Regulatory_Matters_Na

Rate And Regulatory Matters (Narrative-Missouri) (Detail) (USD $)	3 Months Ended		1 Months Ended
	Dec. 31, 2014	Mar. 31, 2015	Apr. 30, 2015
Rate And Regulatory Matters [Line Items]
Regulatory assets	$1,582,000,000	1,577,000,000
Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	695,000,000	687,000,000
Electric Distribution [Member] \| Union Electric Company \| MEEIA [Member]
Rate And Regulatory Matters [Line Items]
Energy Efficiency Program Spending	135,000,000
Incentive Award if Energy Efficiency Goals Are Achieved	25,000,000
Achieved Percentage of Energy Efficiency Earnings For Incentive Award	100.00%
Incentive Award if Energy Efficiency Goals Are Achieved, Period	3 years
Minimum Percentage of Energy Efficiency Goal Achievement For Company To Be Eligible For Incentive Award	70.00%
Electric Distribution [Member] \| Union Electric Company \| Final Rate Order [Member]
Rate And Regulatory Matters [Line Items]
Long-term Contract for Purchase of Electric Power, Date of Contract Expiration		15 years
Electric Distribution [Member] \| Union Electric Company \| Accounting Authority Order Request [Member]
Rate And Regulatory Matters [Line Items]
Regulatory assets		36,000,000
Subsequent Event [Member] \| Electric Distribution [Member] \| Union Electric Company \| Final Rate Order [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount			122,000,000
Public Utilities, Approved Return on Equity, Percentage			9.53%
Public Utilities, Approved Equity Capital Structure, Percentage			51.80%
Rate Base			7,000,000,000
Subsequent Event [Member] \| Electric Distribution - Net Energy Costs [Member] \| Union Electric Company \| Final Rate Order [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount			109,000,000
Subsequent Event [Member] \| Electric Distribution - Non-Energy Costs [Member] \| Union Electric Company \| Final Rate Order [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount			$13,000,000

Rate_And_Regulatory_Matters_Na1

Rate And Regulatory Matters (Narrative-Illinois) (Detail) (USD $)	3 Months Ended	1 Months Ended	12 Months Ended	1 Months Ended
	Mar. 31, 2015	Jan. 31, 2015	Dec. 31, 2014	Apr. 30, 2015
Rate And Regulatory Matters [Line Items]
Regulatory assets	$1,577,000,000		1,582,000,000
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	883,000,000		883,000,000
Ameren Illinois Company \| Electric Distribution [Member] \| Final Rate Order [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	15,000,000
Ameren Illinois Company \| Gas Distribution [Member] \| Pending Rate Case [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		53,000,000
Public Utilities, Requested Return on Equity, Percentage		10.25%
Public Utilities, Requested Equity Capital Structure, Percentage		50.00%
Rate Base		1,200,000,000
Ameren Illinois Company \| Gas Distribution [Member] \| Final Rate Order [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Return on Equity, Percentage			9.10%
Ameren Illinois Company \| Gas Distribution [Member] \| Rate order appeal [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Return on Equity, Percentage			10.40%
IEIMA [Member] \| Ameren Illinois Company \| Electric Distribution [Member] \| 2015 IEMA Revenue Requirement Reconciliation [Member]
Rate And Regulatory Matters [Line Items]
Regulatory assets	8,000,000
IEIMA [Member] \| Ameren Illinois Company \| Electric Distribution [Member] \| 2015 IEMA Revenue Requirement Reconciliation [Member] \| Subsequent Event [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount				110,000,000
IEIMA [Member] \| Ameren Illinois Company \| Electric Distribution [Member] \| 2014 IEMA Revenue Requirement Reconciliation [Member]
Rate And Regulatory Matters [Line Items]
Regulatory assets	101,000,000
IEIMA [Member] \| Ameren Illinois Company \| Electric Distribution [Member] \| 2013 IEMA Revenue Requirement Reconciliation [Member]
Rate And Regulatory Matters [Line Items]
Regulatory assets	$52,000,000

Rate_And_Regulatory_Matters_Na2

Rate And Regulatory Matters (Narrative-Federal) (Detail) (USD $)	12 Months Ended	1 Months Ended		3 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Dec. 31, 2012	Feb. 28, 2015	Nov. 30, 2014	Mar. 31, 2015	Dec. 31, 2014
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities				$107	106
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities				69	84
Ameren Illinois Company \| Pending Ferc Case [Member]
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities				8	7
Ameren Illinois Company \| Pending Ferc Case [Member] \| Minimum
Rate And Regulatory Matters [Line Items]
Loss Contingency, Estimate of Possible Loss				8
Ameren Illinois Company \| Pending Ferc Case [Member] \| Maximum
Rate And Regulatory Matters [Line Items]
Loss Contingency, Estimate of Possible Loss				23
Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities				32	18
Union Electric Company \| New Nuclear Energy Center COL [Member]
Rate And Regulatory Matters [Line Items]
Investments in Power and Distribution Projects				69
Union Electric Company \| New Nuclear Energy Center COL [Member] \| Minimum
Rate And Regulatory Matters [Line Items]
Investments in Power and Distribution Projects				100
Entergy Refund [Member] \| Union Electric Company
Rate And Regulatory Matters [Line Items]
Proceeds from Legal Settlements	31
Midwest Independent Transmission System Operator, Inc [Member] \| Pending Ferc Case [Member]
Rate And Regulatory Matters [Line Items]
Rate Of Return On Common Equity				12.38%
Customer Requested Rate on Equity		8.67%		9.15%
Basis point reduction in FERC allowed base return on common equity				0.50%
Earnings reduction in FERC allowed base return on common equity				4
Incentive adder to FERC allowed base return on common equity			0.50%
Midwest Independent Transmission System Operator, Inc [Member] \| Pending Ferc Case [Member] \| Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Earnings reduction in FERC allowed base return on common equity				$2
New England Transmission Owners [Member] \| Pending Ferc Case [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Return on Equity, Percentage					11.14%
New England Transmission Owners [Member] \| Pending Ferc Case [Member] \| Minimum
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Return on Equity, Percentage					10.57%
New England Transmission Owners [Member] \| Pending Ferc Case [Member] \| Maximum
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Return on Equity, Percentage					11.74%

ShortTerm_Debt_And_Liquidity_N

Short-Term Debt And Liquidity (Narrative) (Detail) (USD $)	3 Months Ended
Short-Term Debt And Liquidity (Narrative) (Detail) (USD $)	Mar. 31, 2015	Mar. 31, 2014
Utilities [Member]
Line of Credit Facility [Line Items]
Short Term Debt, Weighted Average Interest Rate During Period	0.08%	0.39%
Credit Agreements 2012 [Member]
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	1,100,000,000
Actual debt-to-capital ratio	0.51
Minimum ratio of consolidated funds from operations plus interest expense to consolidated interest expense as of balance sheet date	2.0 to 1.0
Line of Credit Facility, Covenant Terms, Default Provision, Maximum Indebtedness	75,000,000
Credit Agreements 2012 [Member] \| Maximum
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.65
Missouri Credit Agreement 2012 [Member] \| Union Electric Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.5
Illinois Credit Agreement 2012 [Member] \| Ameren Illinois Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.46

ShortTerm_Debt_And_Liquidity_S1

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper outstanding) (Details) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Line of Credit Facility [Line Items]
Commercial paper outstanding	$955	$714
Ameren (parent)
Line of Credit Facility [Line Items]
Commercial paper outstanding	815	585
Union Electric Company
Line of Credit Facility [Line Items]
Commercial paper outstanding	140	97
Ameren Illinois Company
Line of Credit Facility [Line Items]
Commercial paper outstanding	$0	$32

ShortTerm_Debt_And_Liquidity_S2

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper) (Details) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$852		$539
Weighted Average Interest Rate	0.53%		0.45%
Peak Short Term Borrowings	955	[1]	700	[1]
Peak Short Term Borrowings Interest Rate	0.70%		0.75%
Ameren (parent)
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	691		339
Weighted Average Interest Rate	0.55%		0.45%
Peak Short Term Borrowings	815	[1]	452	[1]
Peak Short Term Borrowings Interest Rate	0.70%		0.75%
Union Electric Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	151		200
Weighted Average Interest Rate	0.49%		0.45%
Peak Short Term Borrowings	243	[1]	290	[1]
Peak Short Term Borrowings Interest Rate	0.60%		0.70%
Ameren Illinois Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	10		0
Weighted Average Interest Rate	0.44%		0.00%
Peak Short Term Borrowings	$39	[1]	$0	[1]
Peak Short Term Borrowings Interest Rate	0.60%		0.00%

[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 commercial paper issuances for the period.

LongTerm_Debt_And_Equity_Finan2

Long-Term Debt And Equity Financings (Narrative) (Detail) (USD $)	3 Months Ended		1 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Apr. 30, 2015
Long-Term Debt And Equity Financings [Line Items]
Redemptions of Long-term Debt	$0	$163,000,000
Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Capital contribution from parent	224,000,000
Ameren Missouri And Ameren Illinois [Member]
Long-Term Debt And Equity Financings [Line Items]
Assumed interest rate	5.00%
Dividend rate	6.00%
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Redemptions of Long-term Debt	0	163,000,000
Ameren Illinois Company \| Federal Energy Regulatory Commission Restriction [Member]
Long-Term Debt And Equity Financings [Line Items]
Common stock equity to total capitalization	54.00%
Ameren Illinois Company \| Minimum \| Federal Energy Regulatory Commission Restriction [Member]
Long-Term Debt And Equity Financings [Line Items]
Common stock equity to total capitalization	30.00%
Subsequent Event [Member] \| Secured Debt [Member] \| Senior Secured Notes, 3.65%, Due 2045 [Member] \| Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount			250,000,000
Debt Instrument, Interest Rate, Stated Percentage			3.65%
Proceeds from Issuance of Secured Debt			247,000,000
Subsequent Event [Member] \| Secured Debt [Member] \| Senior Secured Notes475 Due2015 [Member] \| Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Redemptions of Long-term Debt			$114,000,000
Debt Instrument, Interest Rate, Stated Percentage			4.75%

LongTerm_Debt_And_Equity_Finan3

Long-Term Debt And Equity Financings (Schedule Of Covered Ratio) (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015
Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Bonds Issuable	3,386	[1]
Preferred Stock Issuable	2,530
Retired bond capacity	832
Union Electric Company \| Actual Ratio [Member]
Long-Term Debt And Equity Financings [Line Items]
Restricted payment interest coverage ratio, Actual	4.6
Dividend Coverage Ratio	113.4
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Bonds Issuable	3,423	[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	6.6
Dividend Coverage Ratio	2.8
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 $832 million and $204 million at Ameren Missouri and Ameren Illinois, respectively.
[2]	Amount of bonds issuable by Ameren Illinois based on unfunded property additions and retired bonds solely under the former IP mortgage indenture. The amount of bonds issuable by Ameren Illinois is also subject to the lien restrictions contained in the 2012 Illinois Credit Agreement.
[3]	Preferred stock issuable is restricted by the amount of preferred stock that is currently authorized by Ameren Illinoisb 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 companybs articles of incorporation.

Other_Income_and_Expenses_Deta

Other Income and Expenses (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$5	[1]	$7	[1]
Interest income on industrial development revenue bonds	7	[1]	7	[1]
Interest and dividend income	4	[1]	2	[1]
Other	3	[1]	2	[1]
Total miscellaneous income	19	[1]	18	[1]
Donations	8	[1]	5	[1]
Other	3	[1]	4	[1]
Total miscellaneous expense	11	[1]	9	[1]
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	4		7
Interest income on industrial development revenue bonds	7		7
Total miscellaneous income	11		14
Donations	2		2
Other	1		2
Total miscellaneous expense	3		4
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	1
Interest and dividend income	4		2
Other	2		1
Total miscellaneous income	7		3
Donations	3		3
Other	2		1
Total miscellaneous expense	$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)	Mar. 31, 2015		Dec. 31, 2014
	gal		gal
Fuel Oils
Derivative [Line Items]
Quantity	40,000,000	[1]	50,000,000	[1]
Natural Gas
Derivative [Line Items]
Quantity	150,000,000		136,000,000
Power
Derivative [Line Items]
Quantity	11,000,000		12,000,000
Uranium
Derivative [Line Items]
Quantity	349,000		332,000
Union Electric Company \| Fuel Oils
Derivative [Line Items]
Quantity	40,000,000	[1]	50,000,000	[1]
Union Electric Company \| Natural Gas
Derivative [Line Items]
Quantity	29,000,000		28,000,000
Union Electric Company \| Power
Derivative [Line Items]
Quantity	1,000,000		1,000,000
Union Electric Company \| Uranium
Derivative [Line Items]
Quantity	349,000		332,000
Ameren Illinois Company \| Natural Gas
Derivative [Line Items]
Quantity	121,000,000		108,000,000
Ameren Illinois Company \| Power
Derivative [Line Items]
Quantity	10,000,000		11,000,000

[1]	Fuel oils consist of heating oil and ultra-low-sulfur diesel.

Derivative_Financial_Instrumen4

Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) (Not Designated As Hedging Instrument [Member], USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Derivative assets	$9	$19
Derivative liabilities	258	232
Fuel Oils \| Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	1	2
Fuel Oils \| Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	21	22
Fuel Oils \| Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	7	7
Natural Gas \| Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	1	2
Natural Gas \| Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	38	37
Natural Gas \| Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	26	19
Power \| Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	7	15
Power \| Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	12	14
Power \| Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	153	131
Uranium \| Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	1	2
Union Electric Company
Derivative [Line Items]
Derivative assets	8	18
Derivative liabilities	44	46
Union Electric Company \| Fuel Oils \| Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	1	2
Union Electric Company \| Fuel Oils \| Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	21	22
Union Electric Company \| Fuel Oils \| Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	7	7
Union Electric Company \| Natural Gas \| Other Current Assets [Member]
Derivative [Line Items]
Derivative assets		1
Union Electric Company \| Natural Gas \| Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	6	6
Union Electric Company \| Natural Gas \| Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	8	6
Union Electric Company \| Power \| Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	7	15
Union Electric Company \| Power \| Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	1	3
Union Electric Company \| Uranium \| Other Current Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	1	2
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	1	1
Derivative liabilities	214	186
Ameren Illinois Company \| Natural Gas \| Other Current Assets [Member]
Derivative [Line Items]
Derivative assets	1	1
Ameren Illinois Company \| Natural Gas \| Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	18	13
Ameren Illinois Company \| Natural Gas \| Mark To Market Derivative Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	32	31
Ameren Illinois Company \| Power \| Other Deferred Credits And Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	153	131
Ameren Illinois Company \| Power \| Mark To Market Derivative Liabilities [Member]
Derivative [Line Items]
Derivative liabilities	$11	$11

Derivative_Financial_Instrumen5

Derivative Financial Instruments (Cumulative Amount Of Pretax Net Gains (Losses) On All Derivative Instruments In OCI) (Detail) (USD $)	Mar. 31, 2015		Dec. 31, 2014
In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Derivative [Line Items]
Current gains deferred as regulatory liabilities	$107		$106
Current losses deferred as regulatory assets	265		295
Fuel Oils
Derivative [Line Items]
Current losses deferred as regulatory assets	20
Fuel Oils \| Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	-27	[1]	-29
Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current losses deferred as regulatory assets	38
Natural Gas \| Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	-63	[2]	-54
Power
Derivative [Line Items]
Current gains deferred as regulatory liabilities	7
Current losses deferred as regulatory assets	12
Power \| Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	-158	[3]	-130
Uranium
Derivative [Line Items]
Current losses deferred as regulatory assets	1
Uranium \| Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	-1	[4]	-2
Union Electric Company
Derivative [Line Items]
Current gains deferred as regulatory liabilities	32		18
Current losses deferred as regulatory assets	158		163
Union Electric Company \| Fuel Oils
Derivative [Line Items]
Current losses deferred as regulatory assets	20
Union Electric Company \| Fuel Oils \| Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	-27	[1]	-29
Union Electric Company \| Natural Gas
Derivative [Line Items]
Current losses deferred as regulatory assets	6
Union Electric Company \| Natural Gas \| Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	-14	[2]	-11
Union Electric Company \| Power
Derivative [Line Items]
Current gains deferred as regulatory liabilities	7
Current losses deferred as regulatory assets	1
Union Electric Company \| Power \| Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	6	[3]	12
Union Electric Company \| Uranium
Derivative [Line Items]
Current losses deferred as regulatory assets	1
Union Electric Company \| Uranium \| Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	-1	[4]	-2
Ameren Illinois Company
Derivative [Line Items]
Current gains deferred as regulatory liabilities	69		84
Current losses deferred as regulatory assets	105		129
Ameren Illinois Company \| Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current losses deferred as regulatory assets	32
Ameren Illinois Company \| Natural Gas \| Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	-49	[2]	-43
Ameren Illinois Company \| Power
Derivative [Line Items]
Current losses deferred as regulatory assets	11
Ameren Illinois Company \| Power \| Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	($164)	[3]	($142)

[1]	Represents net losses associated with fuel oils derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouribs rail transportation surcharges for coal through December 2017. Current losses deferred as regulatory assets include $20 million at Ameren and Ameren Missouri.
[2]	Represents net losses associated with natural gas derivative contracts. These contracts are a partial hedge of natural gas requirements through October 2019 at Ameren and Ameren Missouri and through October 2018 at Ameren Illinois. Current gains deferred as regulatory liabilities include $1 million at Ameren and Ameren Illinois, respectively. Current losses deferred as regulatory assets include $38 million, $6 million, and $32 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively.
[3]	Represents net gains (losses) associated with power derivative contracts. These contracts are a partial hedge of power price requirements through May 2032 at Ameren and Ameren Illinois and through December 2016 at Ameren Missouri. Current gains deferred as regulatory liabilities include $7 million at Ameren and Ameren Missouri. Current losses deferred as regulatory assets include $12 million, $1 million, and $11 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively.
[4]	Represents net losses on uranium derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouribs uranium requirements through December 2016. Current losses deferred as regulatory assets include $1 million at Ameren and Ameren Missouri.

Derivative_Financial_Instrumen6

Derivative Financial Instruments (Offsetting Derivative Assets and Liabilities) (Details) (Commodity Contract, USD $)	Mar. 31, 2015		Dec. 31, 2014
In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	$9	[1]	$19	[1]
Derivative Instruments	4		5
Net Amount	5		14
Gross Amounts Recognized in the Balance Sheet	258	[1]	232	[1]
Derivative Instruments	4		5
Cash Collateral Received/Posted	6	[2]	5	[2]
Net Amount	248		222
Union Electric Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	8	[1]	18	[1]
Derivative Instruments	4		5
Net Amount	4		13
Gross Amounts Recognized in the Balance Sheet	44	[1]	46	[1]
Derivative Instruments	4		5
Cash Collateral Received/Posted	4	[2]	5	[2]
Net Amount	36		36
Ameren Illinois Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet	1		1
Net Amount	1		1
Gross Amounts Recognized in the Balance Sheet	214	[1]	186	[1]
Cash Collateral Received/Posted	2	[2]
Net Amount	$212		$186

[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 bOther current liabilitiesb and bOther deferred credits and liabilitiesb on the balance sheet. Cash collateral posted reduces gross liability balances and is included in bOther current assetsb and bOther assetsb on the balance sheet.

Derivative_Financial_Instrumen7

Derivative Financial Instruments (Maximum Exposure If Counterparties Fail To Perform On Contracts) (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	$6
Union Electric Company
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	5
Ameren Illinois Company
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	$1

Derivative_Financial_Instrumen8

Derivative Financial Instruments (Potential Loss On Counterparty Exposures) (Detail) (USD $)	Mar. 31, 2015
In Millions, unless otherwise specified	Mar. 31, 2015
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$5
Union Electric Company
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$5

Derivative_Financial_Instrumen9

Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Detail) (USD $)	Mar. 31, 2015
In Millions, unless otherwise specified	Mar. 31, 2015
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	$167	[1]
Cash Collateral Posted	8
Potential Aggregate Amount of Additional Collateral Required	157	[2]
Union Electric Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	87	[1]
Cash Collateral Posted	6
Potential Aggregate Amount of Additional Collateral Required	83	[2]
Ameren Illinois Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	80	[1]
Cash Collateral Posted	2
Potential Aggregate Amount of Additional Collateral Required	$74	[2]

[1]	Prior to consideration of master netting arrangements and including NPNS and other accrual contract exposures.
[2]	As collateral requirements with certain counterparties are based on master netting arrangements, the aggregate amount of additional collateral required to be posted is determined after consideration of the effects of such arrangements.

Fair_Value_Measurements_Schedu

Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Detail) (USD $)	3 Months Ended		12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Natural Gas \| Discounted Cash Flow \| Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.31%	[1],[2]	0.43%	[1],[2]
Nodal basis	-0.4	[3]	-0.4	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Natural Gas \| Discounted Cash Flow \| Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	12.07%	[1],[2]	13.00%	[1],[2]
Nodal basis	-0.1	[3]	0.1	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Natural Gas \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	2.47%	[1],[2]	3.00%	[1],[2]
Nodal basis	-0.3	[3]	-0.2	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Natural Gas \| Option Model \| Minimum
Fair Value Inputs [Abstract]
Volatilities	6.00%	[3]	31.00%	[1]
Nodal basis	-0.4	[3]	-0.4	[3]
Natural Gas \| Option Model \| Maximum
Fair Value Inputs [Abstract]
Volatilities	39.00%	[3]	144.00%	[1]
Nodal basis	-0.2	[3]	0	[3]
Natural Gas \| Option Model \| Weighted Average
Fair Value Inputs [Abstract]
Volatilities	33.00%	[3]	63.00%	[1]
Nodal basis	-0.3	[3]	-0.2	[3]
Natural Gas \| Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	$1	[4]	$1	[4]
Natural Gas \| Union Electric Company \| Discounted Cash Flow \| Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.54%	[1],[2]	0.57%	[1],[2]
Nodal basis	-0.1	[3]	-0.1	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Natural Gas \| Union Electric Company \| Discounted Cash Flow \| Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	12.07%	[1],[2]	13.00%	[1],[2]
Nodal basis	-0.1	[3]	-0.1	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Natural Gas \| Union Electric Company \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	5.00%	[1],[2]	5.00%	[1],[2]
Nodal basis	-0.1	[3]	-0.1	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Natural Gas \| Union Electric Company \| Option Model \| Minimum
Fair Value Inputs [Abstract]
Volatilities	6.00%	[3]	31.00%	[1]
Nodal basis	-0.4	[3]	-0.4	[3]
Natural Gas \| Union Electric Company \| Option Model \| Maximum
Fair Value Inputs [Abstract]
Volatilities	39.00%	[3]	144.00%	[1]
Nodal basis	-0.2	[3]	0	[3]
Natural Gas \| Union Electric Company \| Option Model \| Weighted Average
Fair Value Inputs [Abstract]
Volatilities	33.00%	[3]	53.00%	[1]
Nodal basis	-0.3	[3]	-0.3	[3]
Natural Gas \| Ameren Illinois Company \| Discounted Cash Flow \| Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.31%	[1],[2]	0.43%	[1],[2]
Nodal basis	-0.4	[3]	-0.4	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Natural Gas \| Ameren Illinois Company \| Discounted Cash Flow \| Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	2.28%	[1],[2]	2.00%	[1],[2]
Nodal basis	-0.1	[3]	0.1	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Natural Gas \| Ameren Illinois Company \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	1.28%	[1],[2]	0.83%	[1],[2]
Nodal basis	-0.3	[3]	-0.2	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Natural Gas \| Ameren Illinois Company \| Option Model \| Minimum
Fair Value Inputs [Abstract]
Volatilities			50.00%	[1]
Nodal basis			-0.1	[3]
Natural Gas \| Ameren Illinois Company \| Option Model \| Maximum
Fair Value Inputs [Abstract]
Volatilities			144.00%	[1]
Nodal basis			0	[3]
Natural Gas \| Ameren Illinois Company \| Option Model \| Weighted Average
Fair Value Inputs [Abstract]
Volatilities			94.00%	[1]
Nodal basis			-0.1	[3]
Natural Gas \| Ameren Illinois Company \| Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	1	[4]	1	[4]
Natural Gas \| Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	-1	[4]	-2	[4]
Natural Gas \| Derivative Liabilities \| Union Electric Company
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	-1	[4]	-1	[4]
Natural Gas \| Derivative Liabilities \| Ameren Illinois Company
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities			-1	[4]
Fuel Oils \| Discounted Cash Flow \| Minimum
Fair Value Inputs [Abstract]
Escalation rate			5.00%	[3],[5]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Fuel Oils \| Discounted Cash Flow \| Maximum
Fair Value Inputs [Abstract]
Escalation rate			5.00%	[3],[5]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Fuel Oils \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs [Abstract]
Escalation rate			5.00%	[3],[5]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Fuel Oils \| Option Model \| Minimum
Fair Value Inputs [Abstract]
Volatilities	33.00%	[1]	3.00%	[1]
Fuel Oils \| Option Model \| Maximum
Fair Value Inputs [Abstract]
Volatilities	80.00%	[1]	39.00%	[1]
Fuel Oils \| Option Model \| Weighted Average
Fair Value Inputs [Abstract]
Volatilities	42.00%	[1]	32.00%	[1]
Fuel Oils \| Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	1	[4]	2	[4]
Fuel Oils \| Union Electric Company \| Discounted Cash Flow \| Minimum
Fair Value Inputs [Abstract]
Escalation rate			5.00%	[3],[5]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Fuel Oils \| Union Electric Company \| Discounted Cash Flow \| Maximum
Fair Value Inputs [Abstract]
Escalation rate			5.00%	[3],[5]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Fuel Oils \| Union Electric Company \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs [Abstract]
Escalation rate			5.00%	[3],[5]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Fuel Oils \| Union Electric Company \| Option Model \| Minimum
Fair Value Inputs [Abstract]
Volatilities	33.00%	[1]	3.00%	[1]
Fuel Oils \| Union Electric Company \| Option Model \| Maximum
Fair Value Inputs [Abstract]
Volatilities	80.00%	[1]	39.00%	[1]
Fuel Oils \| Union Electric Company \| Option Model \| Weighted Average
Fair Value Inputs [Abstract]
Volatilities	42.00%	[1]	32.00%	[1]
Fuel Oils \| Union Electric Company \| Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	1	[4]	2	[4]
Fuel Oils \| Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	-7	[4]	-8	[4]
Fuel Oils \| Derivative Liabilities \| Union Electric Company
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	-7	[4]	-8	[4]
Power \| Discounted Cash Flow \| Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.29%	[1],[2]	0.26%	[1],[2]
Nodal basis	-11	[3]	-6	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Average forward peak and off-peak pricing	22	[6]	27	[7]
Estimated auction price	-597	[3]	-1,833	[3]
Power \| Discounted Cash Flow \| Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	10.98%	[1],[2]	0.26%	[1],[2]
Nodal basis	0	[3]	0	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Average forward peak and off-peak pricing	46	[6]	50	[7]
Estimated auction price	1,922	[3]	2,743	[3]
Power \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	5.84%	[1],[2]	0.26%	[1],[2]
Nodal basis	-3	[3]	-2	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Average forward peak and off-peak pricing	32	[6]	32	[7]
Estimated auction price	153	[3]	171	[3]
Power \| Fundamental Energy Production Model \| Minimum
Fair Value Inputs [Abstract]
Escalation rate	1.00%	[3],[8]	0.00%	[3],[9]
Estimated future gas prices	3	[3]	4	[3]
Power \| Fundamental Energy Production Model \| Maximum
Fair Value Inputs [Abstract]
Escalation rate	1.00%	[3],[8]	1.00%	[3],[9]
Estimated future gas prices	5	[3]	5	[3]
Power \| Fundamental Energy Production Model \| Weighted Average
Fair Value Inputs [Abstract]
Escalation rate	1.00%	[3],[8]	1.00%	[3],[9]
Estimated future gas prices	4	[3]	4	[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	5	[10],[4]	11	[11],[4]
Power \| Union Electric Company \| Discounted Cash Flow \| Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.29%	[1],[2]	0.26%	[1],[2]
Nodal basis	-11	[1]
Credit risk			0.43%	[1],[2]
Average forward peak and off-peak pricing	24	[1]	27	[1]
Estimated auction price	-597	[3]	-1,833	[3]
Power \| Union Electric Company \| Discounted Cash Flow \| Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	10.98%	[1],[2]	0.26%	[1],[2]
Nodal basis	-4	[1]
Credit risk			0.43%	[1],[2]
Average forward peak and off-peak pricing	46	[1]	50	[1]
Estimated auction price	1,922	[3]	2,743	[3]
Power \| Union Electric Company \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	5.84%	[1],[2]	0.26%	[1],[2]
Nodal basis	-8	[1]
Credit risk			0.43%	[1],[2]
Average forward peak and off-peak pricing	36	[1]	32	[1]
Estimated auction price	153	[3]	171	[3]
Power \| Union Electric Company \| Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	5	[10],[4]	11	[11],[4]
Power \| Ameren Illinois Company \| Discounted Cash Flow \| Minimum
Fair Value Inputs [Abstract]
Nodal basis	-6	[3]	-6	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Average forward peak and off-peak pricing	22	[3]
Average forward pricing			27	[3]
Power \| Ameren Illinois Company \| Discounted Cash Flow \| Maximum
Fair Value Inputs [Abstract]
Nodal basis	0	[3]	0	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Average forward peak and off-peak pricing	38	[3]
Average forward pricing			38	[3]
Power \| Ameren Illinois Company \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs [Abstract]
Nodal basis	-3	[3]	-2	[3]
Credit risk	0.40%	[1],[2]	0.43%	[1],[2]
Average forward peak and off-peak pricing	31	[3]
Average forward pricing			32	[3]
Power \| Ameren Illinois Company \| Fundamental Energy Production Model \| Minimum
Fair Value Inputs [Abstract]
Escalation rate	1.00%	[3],[8]	0.00%	[3],[9]
Estimated future gas prices	3	[3]	4	[3]
Power \| Ameren Illinois Company \| Fundamental Energy Production Model \| Maximum
Fair Value Inputs [Abstract]
Escalation rate	1.00%	[3],[8]	1.00%	[3],[9]
Estimated future gas prices	5	[3]	5	[3]
Power \| Ameren Illinois Company \| Fundamental Energy Production Model \| Weighted Average
Fair Value Inputs [Abstract]
Escalation rate	1.00%	[3],[8]	1.00%	[3],[9]
Estimated future gas prices	4	[3]	4	[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 \| Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	-165	[10],[4]	-144	[11],[4]
Power \| Derivative Liabilities \| Union Electric Company
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	-1	[10],[4]	-2	[11],[4]
Power \| Derivative Liabilities \| Ameren Illinois Company
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	-164	[10],[4]	-142	[11],[4]
Uranium \| Discounted Cash Flow \| Minimum
Fair Value Inputs [Abstract]
Average forward pricing	40	[3]	35	[3]
Uranium \| Discounted Cash Flow \| Maximum
Fair Value Inputs [Abstract]
Average forward pricing	43	[3]	40	[3]
Uranium \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs [Abstract]
Average forward pricing	40	[3]	36	[3]
Uranium \| Union Electric Company \| Discounted Cash Flow \| Minimum
Fair Value Inputs [Abstract]
Average forward pricing	40	[3]	35	[3]
Uranium \| Union Electric Company \| Discounted Cash Flow \| Maximum
Fair Value Inputs [Abstract]
Average forward pricing	43	[3]	40	[3]
Uranium \| Union Electric Company \| Discounted Cash Flow \| Weighted Average
Fair Value Inputs [Abstract]
Average forward pricing	40	[3]	36	[3]
Uranium \| Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	-1	[4]	-2	[4]
Uranium \| Derivative Liabilities \| Union Electric Company
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	($1)	[4]	($2)	[4]

[1]	(b)Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.
[2]	(c)Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances.
[3]	(e)Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.
[4]	(a)The derivative asset and liability balances are presented net of counterparty credit considerations.
[5]	(f)Escalation rate applies to fuel oil prices 2017 and beyond.
[6]	(g)The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes due to their opposing positions. As such, refer to the power sensitivity analysis for each company above.
[7]	(h)The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes due to their opposing positions. As such, refer to the power sensitivity analysis for each company above.
[8]	(h)Escalation rate applies to power prices 2026 and beyond.
[9]	(i)Escalation rate applies to power prices 2026 and beyond.
[10]	(f)Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2018. Valuations beyond 2018 use fundamentally modeled pricing by month for peak and off-peak demand.
[11]	(g)Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2018. Valuations beyond 2018 use fundamentally modeled pricing by month for peak and off-peak demand.

Fair_Value_Measurements_Narrat

Fair Value Measurements (Narrative) (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Valuation adjustments related to net derivative contracts, liabilities	$1	$1
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	$1

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 $)	Mar. 31, 2015		Dec. 31, 2014
In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	$556	[1]	$547	[1]
Assets fair value	565	[2]	566	[2]
Excluded receivables, payables, and accrued income, net	2		2
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	556	[1]	547	[1]
Assets fair value	564	[2]	565	[2]
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	9	[2]	19	[2]
Derivative liabilities	258	[2]	232	[2]
Commodity Contract \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	8	[2]	18	[2]
Derivative liabilities	44	[2]	46	[2]
Commodity Contract \| Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1		1
Derivative liabilities	214	[2]	186	[2]
Commodity Contract \| Fuel Oils
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	[2]	2	[2]
Derivative liabilities	28	[2]	29	[2]
Commodity Contract \| Fuel Oils \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	[2]	2	[2]
Derivative liabilities	28	[2]	29	[2]
Commodity Contract \| Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	[2]	2	[2]
Derivative liabilities	64	[2]	56	[2]
Commodity Contract \| Natural Gas \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets			1	[2]
Derivative liabilities	14	[2]	12	[2]
Commodity Contract \| Natural Gas \| Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	[2]	1	[2]
Derivative liabilities	50	[2]	44	[2]
Commodity Contract \| Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	7	[2]	15	[2]
Derivative liabilities	165	[2]	145	[2]
Commodity Contract \| Power \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	7	[2]	15	[2]
Derivative liabilities	1	[2]	3	[2]
Commodity Contract \| Power \| Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	164	[2]	142	[2]
Commodity Contract \| Uranium
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	1	[2]	2	[2]
Commodity Contract \| Uranium \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	1	[2]	2	[2]
Cash and cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	2		1
Cash and cash equivalents \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	2		1
Equity Securities \| U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	368		364
Equity Securities \| U.S. large capitalization \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	368		364
Debt Securities \| Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	65		63
Debt Securities \| Corporate bonds \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	65		63
Debt Securities \| US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	101		102
Debt Securities \| US treasury and government securities \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	101		102
Debt Securities \| Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	20		17
Debt Securities \| Other Debt Securities \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	20		17
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	370		365
Assets fair value	370	[2]	365	[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	370		365
Assets fair value	370	[2]	365	[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 liabilities	21	[2]	22	[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 liabilities	21	[2]	22	[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 liabilities	21	[2]	21	[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 liabilities	21	[2]	21	[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			1	[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 liabilities			1	[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		1
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		1
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	368		364
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	368		364
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	186		182
Assets fair value	188	[2]	187	[2]
Significant Other Observable Inputs (Level 2) \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	186		182
Assets fair value	188	[2]	187	[2]
Significant Other Observable Inputs (Level 2) \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	[2]	5	[2]
Derivative liabilities	63	[2]	54	[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]	5	[2]
Derivative liabilities	13	[2]	11	[2]
Significant Other Observable Inputs (Level 2) \| Commodity Contract \| Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	50	[2]	43	[2]
Significant Other Observable Inputs (Level 2) \| Commodity Contract \| Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets			1	[2]
Derivative liabilities	63	[2]	53	[2]
Significant Other Observable Inputs (Level 2) \| Commodity Contract \| Natural Gas \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets			1	[2]
Derivative liabilities	13	[2]	10	[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 liabilities	50	[2]	43	[2]
Significant Other Observable Inputs (Level 2) \| Commodity Contract \| Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	[2]	4	[2]
Derivative liabilities			1	[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	2	[2]	4	[2]
Derivative liabilities			1	[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	65		63
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	65		63
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	101		102
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	101		102
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	20		17
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	20		17
Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets fair value	7	[2]	14	[2]
Significant Other Unobservable Inputs (Level 3) \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets fair value	6	[2]	13	[2]
Significant Other Unobservable Inputs (Level 3) \| Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	7	[2]	14	[2]
Derivative liabilities	174	[2]	156	[2]
Significant Other Unobservable Inputs (Level 3) \| Commodity Contract \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	6	[2]	13	[2]
Derivative liabilities	10	[2]	13	[2]
Significant Other Unobservable Inputs (Level 3) \| Commodity Contract \| Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	164	[2]	143	[2]
Significant Other Unobservable Inputs (Level 3) \| Commodity Contract \| Fuel Oils
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	[2]	2	[2]
Derivative liabilities	7	[2]	8	[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	1	[2]	2	[2]
Derivative liabilities	7	[2]	8	[2]
Significant Other Unobservable Inputs (Level 3) \| Commodity Contract \| Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	[2]	1	[2]
Derivative liabilities	1	[2]	2	[2]
Significant Other Unobservable Inputs (Level 3) \| Commodity Contract \| Natural Gas \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	1	[2]	1	[2]
Significant Other Unobservable Inputs (Level 3) \| Commodity Contract \| Natural Gas \| Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	[2]	1	[2]
Derivative liabilities			1	[2]
Significant Other Unobservable Inputs (Level 3) \| Commodity Contract \| Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	5	[2]	11	[2]
Derivative liabilities	165	[2]	144	[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	5	[2]	11	[2]
Derivative liabilities	1	[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	164	[2]	142	[2]
Significant Other Unobservable Inputs (Level 3) \| Commodity Contract \| Uranium
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	1	[2]	2	[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	$1	[2]	$2	[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
In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Fuel Oils
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	($6)	$5
Included in regulatory assets/liabilities	-1	-2
Settlements	1	-2
Ending balance	-6	1
Change in unrealized gains (losses) related to assets/liabilities held at period end	-3	-1
Uranium
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	-2	-6
Included in regulatory assets/liabilities	1
Settlements		1
Ending balance	-1	-5
Change in unrealized gains (losses) related to assets/liabilities held at period end	1
Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	-1	0
Purchases	1	-2
Settlements		2
Ending balance	0	0
Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	-133	-89
Included in regulatory assets/liabilities	-27	-17
Settlements		-5
Transfer out of Level 3		-1
Ending balance	-160	-110
Change in unrealized gains (losses) related to assets/liabilities held at period end	-24	-15
Union Electric Company \| Fuel Oils
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	-6	5
Included in regulatory assets/liabilities	-1	-2
Settlements	1	-2
Ending balance	-6	1
Change in unrealized gains (losses) related to assets/liabilities held at period end	-3	-1
Union Electric Company \| Uranium
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	-2	-6
Included in regulatory assets/liabilities	1
Settlements		1
Ending balance	-1	-5
Change in unrealized gains (losses) related to assets/liabilities held at period end	1
Union Electric Company \| Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	-1	0
Purchases	0	0
Settlements		0
Ending balance	-1	0
Union Electric Company \| Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	9	19
Included in regulatory assets/liabilities	-2	-5
Settlements	-3	-5
Transfer out of Level 3		-1
Ending balance	4	10
Change in unrealized gains (losses) related to assets/liabilities held at period end		-1
Ameren Illinois Company \| Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	0
Purchases	1	-2
Settlements		2
Ending balance	1	0
Ameren Illinois Company \| Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	-142	-108
Included in regulatory assets/liabilities	-25	-12
Settlements	3
Ending balance	-164	-120
Change in unrealized gains (losses) related to assets/liabilities held at period end	($24)	($14)

Fair_Value_Measurements_Schedu3

Fair Value Measurements (Schedule Of Transfers Between Fair Value Hierarchy Levels) (Detail) (Power, USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2014
Derivative [Line Items]
Assets Transfers out of Level 3	$1
Union Electric Company
Derivative [Line Items]
Assets Transfers out of Level 3	$1

Fair_Value_Measurements_Schedu4

Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Long-Term Debt And Preferred Stock) (Detail) (USD $)	Mar. 31, 2015		Dec. 31, 2014
In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)	$7,127	[1]	$7,135	[1]
Preferred stock	123	[1]	122	[1]
Fair Value \| Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)	4,574		4,518
Preferred stock	74		73
Fair Value \| Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)	2,553		2,517
Preferred stock	49		49
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)	6,240	[1]	6,240	[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)	3,999		3,999
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)	2,241		2,241
Preferred stock	$62		$62

[1]	Preferred stock is recorded in bNoncontrolling Interestsb on the consolidated balance sheet.

Related_Party_Transactions_Sch

Related Party Transactions (Schedule of Related Party Transactions) (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	$7		$5
Operating Expenses	34		33
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1		1
Operating Expenses	29		27
Purchased Power \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	2		1
Ameren Missouri Power Supply Agreements with Ameren Illinois \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	1		1	[1]
Ameren Missouri and Ameren Illinois Rent and Facility Services \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	6		5
Ameren Missouri and Ameren Illinois Rent and Facility Services \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1		1	[1]
Ameren Missouri and Ameren Illinois miscellaneous support services [Member] \| Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	1	[1]	1	[1]
Ameren Missouri and Ameren Illinois miscellaneous support services [Member] \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	[1]	1	[1]
Ameren Illinois Power Supply Agreements with Ameren Missouri \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	1		1	[1]
Ameren Illinois Transmission Agreements with ATXI \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	1		1
Ameren Services Support Services Agreement \| Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	34		33
Ameren Services Support Services Agreement \| Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	29		27
Related Party Money Pool Interest \| Union Electric Company
Related Party Transaction [Line Items]
Interest Charges	1	[1]	1	[1]
Related Party Money Pool Interest \| Ameren Illinois Company
Related Party Transaction [Line Items]
Interest Charges	$1	[1]	$1	[1]

[1]	Amount less than $1 million.

Related_Party_Transactions_Nar

Related Party Transactions Narrative (Details) (Ameren Illinois Company, Ameren Illinois Power Supply Agreements with Ameren Missouri)	3 Months Ended
	Mar. 31, 2015
	MWh
Ameren Illinois Company \| Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power	667,000
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate	36

Commitments_And_Contingencies_1

Commitments And Contingencies (Callaway Energy Center) (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015
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 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
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
Maximum Assessments for Single Incidents	0
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	3,250,000,000
Maximum Assessments for Single Incidents	26,000,000
Property Damage - Nuclear Electric Insurance Ltd [Member]
Commitments And Contingencies [Line Items]
Maximum Coverages	2,750,000,000	[4]
Maximum Assessments for Single Incidents	26,000,000	[5]
Property Damage European Mutual Association for Nuclear Insurance [Member]
Commitments And Contingencies [Line Items]
Maximum Coverages	500,000,000	[6]
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	328,000,000
Non-radiation event [Member]
Commitments And Contingencies [Line Items]
Maximum Coverages	2,250,000,000
Aggregate nuclear power industry insurance policy limit for losses from terrorist attacks within twelve month period	1,830,000,000
Radiation Event [Member]
Commitments And Contingencies [Line Items]
Maximum Coverages	$500,000,000

[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 for both radiation and nonradiation events.
[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 $2.75 billion and $2.25 billion property coverage for radiation and nonradiation events, respectively, 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 twelve weeks of an outage, plus up to $3.6 million per week for a minimum of 71 weeks thereafter for a total not exceeding the policy limit of $490 million. Nonradiation events are sub-limited to $328 million.

Commitments_And_Contingencies_2

Commitments And Contingencies (Other Obligations) (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015
	MWh
Long-term Purchase Commitment [Line Items]
Total Other Obligations	$5,326
Union Electric Company
Long-term Purchase Commitment [Line Items]
Total Other Obligations	3,577
Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Total Other Obligations	1,702
Amount of Megawatthours	5,526,000
Long-term Purchase Commitment, Amount	$185

Commitments_And_Contingencies_3

Commitments And Contingencies (Environmental Matters) (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015
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	350,000,000
Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	400,000,000
Former Coal Tar Distillery \| Union Electric Company
Loss Contingencies [Line Items]
Range of possible loss, minimum	2,000,000
Range of possible loss maximum	5,000,000
Accrual for environmental loss contingencies	2,000,000
Former Coal Ash Landfill \| Ameren Illinois Company
Loss Contingencies [Line Items]
Range of possible loss, minimum	500,000
Range of possible loss maximum	6,000,000
Accrual for environmental loss contingencies	500,000
Manufactured Gas Plant
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	245,000,000
Manufactured Gas Plant \| Ameren Illinois Company
Loss Contingencies [Line Items]
Number of remediation sites	44
Range of possible loss, minimum	245,000,000
Range of possible loss maximum	314,000,000
Accrual for environmental loss contingencies	245,000,000
Other Environmental \| Ameren Illinois Company
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	700,000
Sauget Area Two \| Union Electric Company
Loss Contingencies [Line Items]
Range of possible loss, minimum	1,000,000
Range of possible loss maximum	2,500,000
Accrual for environmental loss contingencies	1,000,000
Substation in St Charles, Missouri \| Union Electric Company
Loss Contingencies [Line Items]
Range of possible loss, minimum	2,300,000
Range of possible loss maximum	4,500,000
Accrual for environmental loss contingencies	$2,300,000

Commitments_And_Contingencies_4

Commitments And Contingencies (Pumped-Storage Hydroelectric Facility Breach) (Detail) (Union Electric Company, USD $)	Mar. 31, 2015
In Millions, unless otherwise specified	Mar. 31, 2015
Union Electric Company
Loss Contingencies [Line Items]
Insurance settlements receivable	$41
Insurance Claim	$53

Commitments_And_Contingencies_5

Commitments And Contingencies (Asbestos-Related Litigation) (Detail) (Asbestos-Related, USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015
	defendant
	lawsuit
Loss Contingencies [Line Items]
Average number of total defendants named	77
Asbestos-related lawsuits were pending	65	[1]
Range of possible loss, minimum	$12
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%
Union Electric Company
Loss Contingencies [Line Items]
Asbestos-related lawsuits were pending	42
Range of possible loss, minimum	5
Ameren Illinois Company
Loss Contingencies [Line Items]
Asbestos-related lawsuits were pending	53
Range of possible loss, minimum	$7
Ameren Corporation
Loss Contingencies [Line Items]
Asbestos-related lawsuits were pending	1

[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 $)	3 Months Ended	12 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2014
	mill	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2014
Nuclear Waste Matters [Line Items]
Miscellaneous accounts and notes receivable	$100			$81
Nuclear Plant [Member]
Nuclear Waste Matters [Line Items]
Number of mills charged for NWF fee	1
Miscellaneous accounts and notes receivable				15
Annual decommissioning costs included in costs of service	$7	$7	$7

Retirement_Benefits_Narrative_

Retirement Benefits (Narrative) (Detail) (USD $)	Mar. 31, 2015
In Millions, unless otherwise specified	Mar. 31, 2015
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	$290
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	25
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	$115

Retirement_Benefits_Components

Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$24		$21
Interest cost	44		49
Expected return on plan assets	-62		-57
Prior service cost (benefit)			0
Actuarial loss	18		12
Net periodic benefit cost	24	[1]	25	[1]
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	5		5
Interest cost	12		13
Expected return on plan assets	-17		-16
Prior service cost (benefit)	-1		-1
Actuarial loss	1		-1
Net periodic benefit cost	$0	[1]	$0	[1]

[1]	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
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014
Pension Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$24	[1]	$25	[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	15		17
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	9		8
Other Postretirement Benefit Plan, Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	0	[1]	0	[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		1
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	($1)		($1)

[1]	Includes amounts for Ameren registrants and nonregistrant subsidiaries.

Divestiture_Transactions_and_D1

Divestiture Transactions and Discontinued Operations (Narrative) (Details) (USD $)	1 Months Ended	3 Months Ended
In Millions, unless otherwise specified	Jan. 31, 2014	Dec. 31, 2013	Mar. 31, 2015	Dec. 31, 2014	Mar. 14, 2013
New Ameren Energy Resources Company, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Notes, Loans and Financing Receivable, Net, Current			$12	$12
Range of possible loss maximum			25
Collateral to be posted if credit ratings are below investment grade			20
Buyer's indemnification guarantee obligation					25
Loss contingency accrual			25
Guarantees Outstanding			105
Letters of Credit Outstanding, Amount			9
Elgin, Gibson City and Grand Tower Energy Centers
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Sales of Business, Affiliate and Productive Assets	168	137.5
Proceeds from Asset Sale Held in Escrow	17
Ameren Energy Marketing Company [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Guarantees Outstanding			103
Guarantees, Maximum Exposure			10
Guarantee Type, Other [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Guarantees Outstanding			2
Guarantees, Maximum Exposure			$1

Segment_Information_Schedule_O

Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) (USD $)	3 Months Ended
In Millions, unless otherwise specified	Mar. 31, 2015		Mar. 31, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Number of Reportable Segments	2
External revenues	$1,556		$1,594
Net income (loss) attributable to Ameren Corporation from continuing operations	108		97
Total assets	22,884			22,676
Assets of discontinued operations (Note 12)	15			15
Union Electric Company
Segment Reporting Information [Line Items]
External revenues	793		811
Intersegment revenues	7		6
Net income (loss) attributable to Ameren Corporation from continuing operations	41		47
Total assets	13,555			13,541
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	744		774
Intersegment revenues	1
Net income (loss) attributable to Ameren Corporation from continuing operations	53		53
Total assets	8,456			8,381
Other
Segment Reporting Information [Line Items]
External revenues	19		9
Intersegment revenues	1		1
Net income (loss) attributable to Ameren Corporation from continuing operations	14		-3
Total assets	1,082			942
Intersegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues	-9		-7
Total assets	-224			-203
Consolidated, Continuing Operations
Segment Reporting Information [Line Items]
External revenues	1,556		1,594
Net income (loss) attributable to Ameren Corporation from continuing operations	108		97
Total assets	$22,869	[1]		$22,661	[1]

[1]	Excludes total assets from discontinued operations of $15 million as of MarchB 31, 2015, and December 31, 2014.