Union Electric (UEPEM) 10-Q 10 May 16 2016 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2016	Apr. 29, 2016
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2016
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus	Q1
Trading Symbol	AEE
Entity Registrant Name	AMEREN CORP
Entity Central Index Key	1,002,910
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		242,634,798
Union Electric Company
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2016
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus	Q1
Entity Registrant Name	UNION ELECTRIC CO
Entity Central Index Key	100,826
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		102,123,834
Ameren Illinois Company
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2016
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus	Q1
Entity Registrant Name	AMEREN ILLINOIS CO
Entity Central Index Key	18,654
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		25,452,373

Consolidated Statement of Incom

Consolidated Statement of Income (Loss) - USD ($) shares in Millions, $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Operating Revenues:
Electric		$ 1,102	$ 1,143
Gas		332	413
Total operating revenues		1,434	1,556
Operating Expenses:
Fuel		203	206
Purchased power		138	139
Gas purchased for resale		152	236
Other operations and maintenance		400	401
Depreciation and amortization		207	193
Taxes other than income taxes		114	125
Total operating expenses		1,214	1,300
Operating Income		220	256
Other Income and Expenses:
Miscellaneous income	[1]	20	19
Miscellaneous expense	[1]	7	11
Total other income (expense)		13	8
Interest Charges		95	88
Income Before Income Taxes		138	176
Income Taxes		31	66
Income from Continuing Operations		107	110
Income (Loss) from Discontinued Operations, Net of Taxes		0	0
Net Income (Loss)		107	110
Pension and other postretirement benefit plan activity, net of income taxes (benefit)		(2)	0
Comprehensive Income		103	108
Less: Net Income (Loss) Attributable to Noncontrolling Interests:
Net Income from Continuing Operations Attributable to Noncontrolling Interests		2	2
Net Income (Loss):
Continuing Operations		105	108
Discontinued Operations		0	0
Net Income (Loss)		$ 105	$ 108
Earnings Per Share, Basic [Abstract]
Continuing Operations		$ 0.43	$ 0.45
Discontinued Operations		0	0
Earnings Per Share, Basic		0.43	0.45
Dividends per Common Share		$ 0.425	$ 0.41
Average Common Shares Outstanding - Basic		242.6	242.6
Union Electric Company
Operating Revenues:
Electric		$ 694	$ 742
Gas		47	58
Total operating revenues		741	800
Operating Expenses:
Fuel		203	206
Purchased power		42	39
Gas purchased for resale		21	31
Other operations and maintenance		212	211
Depreciation and amortization		127	118
Taxes other than income taxes		73	80
Total operating expenses		678	685
Operating Income		63	115
Other Income and Expenses:
Miscellaneous income		15	11
Miscellaneous expense		2	3
Total other income (expense)		13	8
Interest Charges		52	55
Income Before Income Taxes		24	68
Income Taxes		9	26
Net Income (Loss)		15	42
Other Comprehensive Income		0	0
Comprehensive Income		15	42
Net Income (Loss):
Net Income (Loss)		15	42
Earnings Per Share, Basic [Abstract]
Preferred Stock Dividends		1	1
Net Income Available to Common Stockholder		14	41
Ameren Illinois Company
Operating Revenues:
Electric		392	390
Gas		285	355
Total operating revenues		677	745
Operating Expenses:
Purchased power		104	102
Gas purchased for resale		131	205
Other operations and maintenance		194	202
Depreciation and amortization		77	73
Taxes other than income taxes		38	43
Total operating expenses		544	625
Operating Income		133	120
Other Income and Expenses:
Miscellaneous income		5	7
Miscellaneous expense		5	5
Total other income (expense)			2
Interest Charges		35	33
Income Before Income Taxes		98	89
Income Taxes		38	35
Net Income (Loss)		60	54
Pension and other postretirement benefit plan activity, net of income taxes (benefit)		(1)	(1)
Comprehensive Income		59	53
Net Income (Loss):
Net Income (Loss)		60	54
Earnings Per Share, Basic [Abstract]
Preferred Stock Dividends		1	1
Net Income Available to Common Stockholder		$ 59	$ 53
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Consolidated Statement of Inco3

Consolidated Statement of Income (Loss) (Parenthetical) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Pension and other postretirement benefit plan activity, tax expense (benefit)	$ 1	$ 0
Ameren Illinois Company
Pension and other postretirement benefit plan activity, tax expense (benefit)	$ (1)	$ (1)

Consolidated Statement of Compr

Consolidated Statement of Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Statement of Comprehensive Income [Abstract]
Income from Continuing Operations	$ 107	$ 110
Other Comprehensive Income from Continuing Operations, Net of Taxes
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	(2)	0
Comprehensive Income from Continuing Operations	105	110
Less: Comprehensive Income from Continuing Operations Attributable to Noncontrolling Interests	2	2
Comprehensive Income	$ 103	$ 108

Consolidated Statement of Comp5

Consolidated Statement of Comprehensive Income (Loss) Consolidated Statement of Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Statement of Comprehensive Income [Abstract]
Pension and other postretirement benefit plan activity, tax expense (benefit)	$ 1	$ 0

Consolidated Balance Sheet

Consolidated Balance Sheet - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Current Assets:
Cash and cash equivalents	$ 13	$ 292
Accounts receivable - trade (less allowance for doubtful accounts)	428	388
Unbilled revenue	186	239
Miscellaneous accounts and notes receivable	56	98
Materials and supplies	483	538
Current regulatory assets	215	260
Other current assets	63	88
Assets of discontinued operations	14	14
Total current assets	1,458	1,917
Property and Plant, Net	19,000	18,799
Investments and Other Assets:
Nuclear decommissioning trust fund	567	556
Goodwill	411	411
Regulatory assets	1,376	1,382
Other assets	573	575
Total investments and other assets	2,927	2,924
TOTAL ASSETS	23,385	23,640
Current Liabilities:
Current maturities of long-term debt	135	395
Short-term Debt	581	301
Accounts and wages payable	429	777
Taxes accrued	77	43
Interest accrued	99	89
Customer deposits	98	100
Current regulatory liabilities	87	80
Other current liabilities	305	279
Liabilities of discontinued operations	28	29
Total current liabilities	1,839	2,093
Long-term Debt, Net	6,881	6,880
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	3,928	3,885
Accumulated deferred investment tax credits	59	60
Regulatory liabilities	1,931	1,905
Asset retirement obligations	625	618
Pension and other postretirement benefits	581	580
Other deferred credits and liabilities	530	531
Total deferred credits and other liabilities	$ 7,654	$ 7,579
Commitments and Contingencies
Stockholders' Equity:
Common Stock	$ 2	$ 2
Other paid-in capital	5,539	5,616
Retained earnings	1,333	1,331
Accumulated other comprehensive income (loss)	(5)	(3)
Stockholder's equity	6,869	6,946
Noncontrolling Interest	142	142
Total equity	7,011	7,088
TOTAL LIABILITIES AND EQUITY	23,385	23,640
Union Electric Company
Current Assets:
Cash and cash equivalents	0	199
Advances to money pool	0	36
Accounts receivable - trade (less allowance for doubtful accounts)	150	174
Accounts receivable - affiliates	16	54
Unbilled revenue	105	128
Miscellaneous accounts and notes receivable	44	78
Materials and supplies	389	387
Current regulatory assets	55	89
Other current assets	29	41
Total current assets	788	1,186
Property and Plant, Net	11,181	11,183
Investments and Other Assets:
Nuclear decommissioning trust fund	567	556
Regulatory assets	591	605
Other assets	316	321
Total investments and other assets	1,474	1,482
TOTAL ASSETS	13,443	13,851
Current Liabilities:
Current maturities of long-term debt	6	266
Short-term Debt	165	0
Accounts and wages payable	174	417
Accounts payable - affiliates	59	56
Taxes accrued	64	31
Interest accrued	50	59
Current regulatory liabilities	15	28
Other current liabilities	132	120
Total current liabilities	665	977
Long-term Debt, Net	3,845	3,844
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	2,857	2,844
Accumulated deferred investment tax credits	57	58
Regulatory liabilities	1,181	1,172
Asset retirement obligations	619	612
Pension and other postretirement benefits	236	234
Other deferred credits and liabilities	27	28
Total deferred credits and other liabilities	$ 4,977	$ 4,948
Commitments and Contingencies
Stockholders' Equity:
Common Stock	$ 511	$ 511
Other paid-in capital	1,822	1,822
Preferred stock	80	80
Retained earnings	1,543	1,669
Stockholder's equity	3,956	4,082
TOTAL LIABILITIES AND EQUITY	13,443	13,851
Ameren Illinois Company
Current Assets:
Cash and cash equivalents	0	71
Advances to money pool	58	0
Accounts receivable - trade (less allowance for doubtful accounts)	262	204
Accounts receivable - affiliates	10	22
Unbilled revenue	81	111
Miscellaneous accounts and notes receivable	12	19
Materials and supplies	94	151
Current regulatory assets	156	167
Other current assets	13	15
Total current assets	686	760
Property and Plant, Net	6,956	6,848
Investments and Other Assets:
Goodwill	411	411
Regulatory assets	777	771
Other assets	114	113
Total investments and other assets	1,302	1,295
TOTAL ASSETS	8,944	8,903
Current Liabilities:
Current maturities of long-term debt	129	129
Short-term Debt	0	0
Accounts and wages payable	180	249
Accounts payable - affiliates	55	66
Taxes accrued	12	13
Interest accrued	44	28
Customer deposits	67	69
Mark-to-market derivative liabilities	51	45
Current environmental remediation	38	28
Current regulatory liabilities	55	39
Other current liabilities	82	86
Total current liabilities	713	752
Long-term Debt, Net	2,343	2,342
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,518	1,480
Accumulated deferred investment tax credits	2	2
Regulatory liabilities	749	732
Pension and other postretirement benefits	271	271
Environmental remediation	187	205
Other deferred credits and liabilities	236	222
Total deferred credits and other liabilities	$ 2,963	$ 2,912
Commitments and Contingencies
Stockholders' Equity:
Common Stock	$ 0	$ 0
Other paid-in capital	2,005	2,005
Preferred stock	62	62
Retained earnings	854	825
Accumulated other comprehensive income (loss)	4	5
Stockholder's equity	2,925	2,897
TOTAL LIABILITIES AND EQUITY	$ 8,944	$ 8,903

Consolidated Balance Sheet (Par

Consolidated Balance Sheet (Parenthetical) - USD ($) shares in Millions, $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Accounts receivable - trade allowance for doubtful accounts	$ 21	$ 19
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	$ 6	$ 7
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	$ 12
Common stock, no par value	$ 0	$ 0
Common stock, shares authorized	45	45
Common stock, shares outstanding	25.5	25.5

Consolidated Statement of Cash

Consolidated Statement of Cash Flows - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Cash Flows From Operating Activities:
Net income (loss)		$ 107	$ 110
(Income) from discontinued operations, net of taxes		0	0
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization		210	195
Amortization of nuclear fuel		24	23
Amortization of debt issuance costs and premium/discounts		6	5
Deferred income taxes and investment tax credits, net		42	59
Allowance for equity funds used during construction	[1]	(8)	(5)
Share-based compensation costs		6	8
Other		(3)	(11)
Changes in assets and liabilities:
Receivables		55	(48)
Materials and supplies		55	75
Accounts and wages payable		(246)	(215)
Taxes accrued		30	35
Regulatory assets and liabilities		81	62
Assets, other		4	14
Liabilities, other		(25)	(21)
Pension and other postretirement benefits		9	27
Counterparty collateral, net		3	(2)
Net cash provided by operating activities – continuing operations		350	311
Net cash provided by (used in) operating activities – discontinued operations		(1)	1
Net cash provided by operating activities		349	312
Cash Flows From Investing Activities:
Capital expenditures		(496)	(417)
Nuclear fuel expenditures		(21)	(17)
Purchases of securities - nuclear decommissioning trust fund		(130)	(84)
Sales and maturities of securities - nuclear decommissioning trust fund		125	79
Proceeds from Notes Receivable		0	5
Contributions to Note Receivable		0	(5)
Other		(2)	0
Net cash used in investing activities – continuing operations		(524)	(439)
Net cash provided by investing activities – discontinued operations		14	0
Net cash used in investing activities		(510)	(439)
Cash Flows From Financing Activities:
Dividends on common stock		(103)	(99)
Dividends paid to noncontrolling interest holders		(2)	(2)
Short-term debt, net		280	241
Maturity of Long-term Debt		(260)	0
Employee payroll taxes related to share-based payments		(32)	(12)
Other		(1)	0
Net cash provided by (used in) financing activities – continuing operations		(118)	128
Net change in cash and cash equivalents		(279)	1
Cash and cash equivalents at beginning of period		292	5
Cash and cash equivalents at end of period		13	6
Union Electric Company
Cash Flows From Operating Activities:
Net income (loss)		15	42
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization		130	121
Amortization of nuclear fuel		24	23
Amortization of debt issuance costs and premium/discounts		2	2
Deferred income taxes and investment tax credits, net		9	21
Allowance for equity funds used during construction		(7)	(4)
Changes in assets and liabilities:
Receivables		81	60
Materials and supplies		(2)	(14)
Accounts and wages payable		(172)	(171)
Taxes accrued		31	40
Regulatory assets and liabilities		45	27
Assets, other		5	3
Liabilities, other		3	(5)
Pension and other postretirement benefits		5	12
Net cash provided by operating activities		169	157
Cash Flows From Investing Activities:
Capital expenditures		(178)	(145)
Nuclear fuel expenditures		(21)	(17)
Purchases of securities - nuclear decommissioning trust fund		(130)	(84)
Sales and maturities of securities - nuclear decommissioning trust fund		125	79
Money pool advances, net		36	0
Other		(2)	(2)
Net cash used in investing activities		(170)	(169)
Cash Flows From Financing Activities:
Dividends on common stock		(140)	(315)
Dividends on preferred stock		(1)	(1)
Short-term debt, net		165	43
Money pool borrowings, net		0	61
Maturity of Long-term Debt		(260)	0
Capital contribution from parent		38	224
Net cash provided by (used in) financing activities		(198)	12
Net change in cash and cash equivalents		(199)	0
Cash and cash equivalents at beginning of period		199	1
Cash and cash equivalents at end of period		0	1
Ameren Illinois Company
Cash Flows From Operating Activities:
Net income (loss)		60	54
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization		77	72
Amortization of debt issuance costs and premium/discounts		4	4
Deferred income taxes and investment tax credits, net		37	13
Allowance for equity funds used during construction		(1)	(1)
Other		(3)	(3)
Changes in assets and liabilities:
Receivables		(22)	(41)
Materials and supplies		57	89
Accounts and wages payable		(33)	(11)
Taxes accrued		(3)	24
Regulatory assets and liabilities		32	33
Assets, other		7	5
Liabilities, other		12	4
Pension and other postretirement benefits		4	11
Net cash provided by operating activities		229	254
Cash Flows From Investing Activities:
Capital expenditures		(211)	(174)
Money pool advances, net		(58)	(33)
Net cash used in investing activities		(269)	(207)
Cash Flows From Financing Activities:
Dividends on common stock		(30)	0
Dividends on preferred stock		(1)	(1)
Short-term debt, net		0	(32)
Money pool borrowings, net		0	(15)
Net cash provided by (used in) financing activities		(31)	(48)
Net change in cash and cash equivalents		(71)	(1)
Cash and cash equivalents at beginning of period		71	1
Cash and cash equivalents at end of period		$ 0	$ 0
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Summary Of Significant Accounti

Summary Of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2016
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. 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 projects to improve electric transmission system reliability 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. Unless otherwise stated, these notes to Ameren’s financial statements exclude discontinued operations for all periods presented. See Note 12 - Discontinued Operations in this report and Note 16 - Divestiture Transactions and Discontinued Operations under Part II, Item 8, of the Form 10-K for additional information. Ameren’s financial statements 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 statement 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 AROs at Ameren, Ameren Missouri, and Ameren Illinois increased at March 31, 2016, compared to December 31, 2015, to reflect the accretion of obligations to their fair value, partially offset by immaterial settlements. Share-based Compensation A summary of nonvested performance share units at March 31, 2016 , and changes during the three months ended March 31, 2016 , under the 2006 Incentive Plan and the 2014 Incentive Plan are presented below: Performance Share Units Share Units Weighted-average Fair Value per Share Unit Nonvested at January 1, 2016 1,024,870 $ 46.08 Granted (a) 580,737 44.13 Forfeitures (12,315 ) 45.12 Vested (b) (8,265 ) 42.91 Nonvested at March 31, 2016 1,585,027 $ 45.39 (a) Performance share units granted to certain executive and nonexecutive officers and other eligible employees 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 2016 under the 2014 Incentive Plan was determined to be $44.13 , which was based on Ameren’s closing common share price of $43.23 at December 31, 2015 , and lattice simulations. Lattice simulations are used to estimate expected share payout based on Ameren’s total shareholder return for a three -year performance period relative to the designated peer group beginning January 1, 2016. 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 1.31% , volatility of 15% to 20% 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, 2016 and 2015 : Three Months 2016 2015 Ameren Missouri $ 30 $ 34 Ameren Illinois 20 23 Ameren $ 50 $ 57 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, 2016 and 2015. The assumed settlement of dilutive performance share units had an immaterial impact on earnings per share. The calculation of diluted earnings per share reflected the adoption of FASB guidance related to employee share-based payment accounting discussed below. Accounting and Reporting Developments Below is a summary of recently issued authoritative accounting standards relevant to the Ameren Companies. Revenue from Contracts with Customers In May 2014, the FASB issued authoritative accounting guidance that changes the criteria for recognizing revenue from a contract with a customer. The underlying principle of the guidance is that an entity will recognize revenue for the transfer of promised goods or services to customers at an amount that the entity expects to be entitled to in exchange for those goods or services. The guidance also requires additional disclosures to enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. Entities can apply the guidance retrospectively to each reporting period presented or retrospectively by recording a cumulative effect adjustment to retained earnings in the period of initial adoption. The Ameren Companies are currently assessing the impacts of this guidance on their results of operations, financial position, and disclosures, including their accounting for contributions in aid of construction and similar arrangements, as well as the transition method that they will use to adopt the guidance. In August 2015, the FASB deferred the effective date of this revenue guidance to the first quarter of 2018, with an option for entities to early adopt in the first quarter of 2017. The Ameren Companies do not expect to early adopt this guidance. Amendments to the Consolidation Analysis In February 2015, the FASB issued authoritative accounting guidance that amends the consolidation analysis for variable interest entities and voting interest entities. The new guidance affects (1) limited partnerships, similar legal entities, and certain investment funds, (2) the evaluation of fees paid to a decision maker or service provider as a variable interest, (3) how fee arrangements impact the primary beneficiary determination, and (4) the evaluation of related party relationships on the primary beneficiary determination. The adoption of this guidance in the first quarter of 2016 did not impact the Ameren Companies’ results of operations, financial position, cash flows, or disclosures. Leases In February 2016, the FASB issued authoritative accounting guidance that will require an entity to recognize assets and liabilities arising from a lease. Consistent with current GAAP, the recognition, measurement, and presentation of expenses and cash flows arising from a lease will depend primarily on its classification as a finance or operating lease. The guidance also requires additional disclosures to enable users of financial statements to understand the amount, timing, and uncertainty of cash flows arising from leases. The guidance will be effective for the Ameren Companies in the first quarter of 2019 with an option for entities to early adopt. Upon adoption, the Ameren Companies will recognize and measure operating leases on their respective balance sheets at the beginning of the earliest period presented. The Ameren Companies are currently assessing the impacts of this guidance on their results of operations, financial position, cash flows, and disclosures. Improvements to Employee Share-Based Payment Accounting In March 2016, the FASB issued authoritative accounting guidance that simplifies the accounting for share-based payment transactions, including the income tax consequences, the calculation of diluted earnings per share, the classification of awards as either equity or liabilities, and the classification on the statement of cash flows. Ameren determines for each performance share unit award whether the difference between the deduction for tax purposes and the compensation cost recognized for financial reporting purposes results in either an excess tax benefit or an excess tax deficit. Previously, excess tax benefits were recognized in "Other paid-in capital" on Ameren’s consolidated balance sheet, and in certain cases, excess tax deficits were recognized in “Income taxes” on Ameren’s consolidated income statement. The new guidance increases income statement volatility by requiring all excess tax benefits and deficits to be recognized in “Income taxes,” and treated as discrete items in the period in which they occur. Ameren adopted this guidance in the first quarter of 2016, which resulted in recognition of a $21 million income tax benefit during the period. For the three months ended March 31, 2015, Ameren reclassified, for comparison purposes, $2 million of excess tax benefits on the statement of cash flows from financing to operating activity, and $12 million of employee payroll taxes related to share-based payments from operating to financing activity.

Rate And Regulatory Matters

Rate And Regulatory Matters	3 Months Ended
	Mar. 31, 2016
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 MEEIA 2013 The MEEIA 2013 performance incentive allowed Ameren Missouri an opportunity to earn additional revenues by achieving certain customer energy efficiency goals, including $19 million if 100% of the goals were achieved during the three -year period, with the potential to earn a larger performance incentive if Ameren Missouri’s energy savings exceeded those goals. In November 2015, the MoPSC issued an order that clarified how an input used in the calculation of the performance incentive would be determined. Ameren Missouri filed an appeal of the order with the Missouri Court of Appeals, Western District. If the Missouri Court of Appeals upholds the MoPSC order, the MEEIA 2013 performance incentive will be significantly less than the performance incentive calculated using Ameren Missouri’s interpretation. A decision from the Missouri Court of Appeals is expected in 2016. Separately, an order from the MoPSC determining the MEEIA 2013 performance incentive is also expected in 2016. Ameren Missouri has not recorded any revenues associated with the MEEIA 2013 performance incentive. Ameren Missouri believes it will ultimately be found to have exceeded 100% of the customer energy efficiency goals, and it therefore expects to recognize revenues relating to the MEEIA 2013 performance incentive of at least $19 million in 2016. Noranda Ameren Missouri supplies electricity to Noranda’s aluminum smelter located in southeast Missouri under a long-term power supply agreement. In the first quarter of 2016, Noranda idled production at its aluminum smelter. In addition, Noranda filed voluntary petitions for a court-supervised restructuring process under Chapter 11 of the United States Bankruptcy Code. Noranda stated it would maintain the flexibility to restart operations at the smelter should conditions allow. Ameren Missouri has been working with Noranda, legislators, and other stakeholders on a potential legislative solution to support Noranda’s operations. For utility service through March 31, 2016, Noranda prepaid an amount to Ameren Missouri in excess of its utility service usage. Ameren Missouri expects to be paid in full for utility services provided to Noranda. In its April 2015 electric rate order, the MoPSC approved a rate design that established $78 million in annual revenues, net of fuel and purchased power costs, as Noranda’s portion of Ameren Missouri’s revenue requirement. The portion of Ameren Missouri’s annual revenue requirement reflected in Noranda’s electric rate is based on the smelter using approximately 4.2 million megawatthours annually, which is almost 100% of its operating capacity. Ameren Missouri’s rates, including those for Noranda, are seasonal. Noranda’s summer base rate (June through September) is $45.78 per megawatthour, and its winter base rate (October through May) is $31.11 per megawatthour. In 2016, actual sales volumes to Noranda will be significantly below the sales volumes reflected in rates. As a result, Ameren Missouri will not fully recover its revenue requirement until rates are adjusted by the MoPSC in a future electric rate case to accurately reflect Noranda’s actual sales volumes. As a result of Noranda’s idled production described above, Ameren Missouri is applying a provision in its FAC tariff that, under certain circumstances, allows Ameren Missouri to retain a portion of the revenues from any off-system sales it makes as a result of reduced tariff sales to Noranda. The current market price of electricity is less than Noranda’s electric rate, and Ameren Missouri expects market prices to remain below Noranda’s electric rate during 2016. Accordingly, this FAC-tariff provision will not enable Ameren Missouri to fully recover its revenue requirement under current market conditions. Ameren Missouri expects to file an electric rate case in 2016 to reflect additional infrastructure investments and rising costs, including depreciation, transmission service, and property tax expenses, and expects the resulting new rates to reflect Noranda’s actual sales volumes, which would prospectively eliminate the impact of the current revenue shortfall from Noranda sales levels. The rate case would take place over a period of up to 11 months from the date of filing. Ameren Missouri may seek recovery of lost revenues in a filing with the MoPSC to recover certain costs incurred but not contemporaneously recovered by rate revenues as a result of Noranda's reduced operations. Ameren Missouri will continue to monitor Noranda’s sales volumes and to evaluate regulatory and legislative options that might mitigate adverse financial impacts. The reduction in Noranda’s sales volumes have adversely affected and will continue to adversely affect Ameren’s and Ameren Missouri’s results of operations, financial condition, and liquidity until customer rates are adjusted in a future rate case. ATXI Transmission Projects The Mark Twain project is a MISO-approved 95 -mile transmission line located in northeast Missouri. In April 2016, the MoPSC granted ATXI a certificate of convenience and necessity for the Mark Twain project. Starting construction under the certificate is subject to ATXI obtaining assents from the five counties where the line will be constructed. The Mark Twain project is expected to be completed in 2018. Extended difficulties in obtaining the assents could delay the completion date. Illinois IEIMA Under the provisions of the IEIMA's performance-based formula rate-making framework, which currently extends through 2019, Ameren Illinois’ electric delivery service rates are subject to an annual revenue requirement reconciliation to its actual recoverable 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 recoverable costs incurred. As of March 31, 2016 , Ameren Illinois had recorded regulatory assets of $9 million , $65 million , and $82 million , to reflect its expected 2016 and 2015 revenue requirement reconciliation adjustments and the approved 2014 revenue requirement reconciliation adjustment, with interest, respectively. In April 2016, Ameren Illinois filed with the ICC its annual electric delivery service formula rate update to establish the revenue requirement used for 2017 rates. Pending ICC approval, Ameren Illinois’ update filing will result in a $14 million decrease in Ameren Illinois’ electric delivery service revenue requirement, beginning in January 2017. This update reflects an increase to the annual formula rate based on 2015 actual costs and expected net plant additions for 2016, an increase to include the 2015 revenue requirement reconciliation adjustment, and a decrease for the conclusion of the 2014 revenue requirement reconciliation adjustment, which will be fully collected from customers in 2016 , consistent with the ICC’s December 2015 annual update filing order. As of December 31, 2015, Ameren Illinois had recorded a regulatory asset of $103 million related to the approved 2014 revenue requirement reconciliation adjustment. Federal FERC Complaint Cases In November 2013, a customer group filed a complaint case with the FERC seeking a reduction in the allowed base return on common equity for the FERC-regulated transmission rate base under the MISO tariff from 12.38% to 9.15% . In December 2015, an administrative law judge issued an initial decision in the November 2013 complaint case that would lower the allowed base return on common equity to 10.32% and would require customer refunds to be issued for the 15-month period ending February 2015. The allowed base return on common equity in the initial decision was based on observable market data for the six months ended June 30, 2015. The FERC is expected to issue a final order on the November 2013 complaint case in the fourth quarter of 2016. Because 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 transmission rate base under the MISO tariff to 8.67% . The initial decision from an administrative law judge in the February 2015 complaint case, which will subsequently require FERC approval, is expected to be issued in the second quarter of 2016. On January 6, 2015, a FERC-approved incentive adder of up to 50 basis points on the allowed base return on common equity for our participation in an RTO became effective. Beginning with its January 6, 2015 effective date, the incentive adder will reduce any refund to customers relating to a reduction of the allowed base return on common equity from the complaint cases discussed above. As of March 31, 2016 , Ameren and Ameren Illinois had current regulatory liabilities of $55 million and $37 million , respectively, representing their estimates of the potential refunds from the November 12, 2013 refund effective date through March 31, 2016. Ameren and Ameren Illinois recorded liabilities to reflect the allowed base return on common equity in the initial decision for the November 2013 complaint case refund period and the observable market data for the six months ended December 31, 2015, for the February 2015 complaint case refund period. Ameren’s and Ameren Illinois’ liabilities also reflect the January 6, 2015 incentive adder discussed above. Ameren Missouri did not record a liability as of March 31, 2016 , and it 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.

Short-Term Debt And Liquidity

Short-Term Debt And Liquidity	3 Months Ended
	Mar. 31, 2016
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 Missouri Credit Agreement and the Illinois Credit Agreement, both of which expire on December 11, 2019, were not utilized for direct borrowings during the three months ended March 31, 2016 , but were used to support commercial paper issuances and to issue letters of credit. Based on letters of credit issued under the Credit Agreements, as well as commercial paper outstanding, the aggregate amount of credit capacity available under the Credit Agreements to Ameren (parent), Ameren Missouri, and Ameren Illinois, collectively, at March 31, 2016 , was $1.5 billion . Commercial Paper The following table presents commercial paper outstanding at Ameren (parent), Ameren Missouri, and Ameren Illinois as of March 31, 2016 , and December 31, 2015 : March 31, 2016 December 31, 2015 Ameren (parent) $ 416 $ 301 Ameren Missouri 165 — Ameren Illinois — — Ameren Consolidated $ 581 $ 301 The following table summarizes the borrowing 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, 2016 and 2015: Ameren (parent) Ameren Missouri Ameren Illinois Ameren Consolidated 2016 Average daily commercial paper outstanding $ 349 $ 68 $ — $ 417 Weighted-average interest rate 0.82 % 0.80 % — % 0.81 % Peak commercial paper during period (a) $ 482 $ 208 $ — $ 581 Peak interest rate 0.95 % 0.85 % — % 0.95 % 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 0.70 % 0.60 % 0.60 % 0.70 % (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 financial covenants in the 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 Credit Agreements also contain nonfinancial covenants, including restrictions on the ability to incur liens, to transact with affiliates, to dispose of assets, to make investments in or transfer assets to its affiliates, and to merge with other entities. The Credit Agreements require Ameren, Ameren Missouri, and Ameren Illinois to each 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, 2016 , the ratios of consolidated indebtedness to consolidated total capitalization, calculated in accordance with the provisions of the Credit Agreements, were 52% , 49% , and 46% , for Ameren, Ameren Missouri, and Ameren Illinois, respectively. In addition, under the Credit Agreements, if Ameren does not have a senior long-term unsecured credit rating of at least Baa3 from Moody’s or BBB- from S&P, 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. As of March 31, 2016 , Ameren’s senior long-term unsecured credit rating exceeded the minimum 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 Credit Agreement. The 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 Credit Agreement will also be deemed to constitute a default of Ameren under such agreement. Defaults include a cross-default resulting from a default of such borrower under any other agreement covering outstanding indebtedness of such borrower and certain subsidiaries (other than project finance subsidiaries and nonmaterial subsidiaries) in excess of $75 million in the aggregate (including under the other Credit Agreement). However, under the default provisions of the Credit Agreements, any default of Ameren under any 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 Credit Agreement. Further, the Credit Agreement default provisions provide that an Ameren default under any of the 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 covenants in their credit agreements at March 31, 2016 . 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 ATXI may participate in the utility money pool as both lenders and borrowers. Ameren and Ameren Services may participate in the utility money pool only as lenders. 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 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 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, 2016 and 2015 , was 0.47% and 0.08% , respectively. See Note 8 - Related Party Transactions for the amount of interest income and expense from the money pool arrangements recorded by the Ameren Companies for the three months ended March 31, 2016 and 2015 .

Long-Term Debt And Equity Finan

Long-Term Debt And Equity Financings	3 Months Ended
	Mar. 31, 2016
Long-Term Debt And Equity Financings [Abstract]
LONG-TERM DEBT AND EQUITY FINANCINGS	LONG-TERM DEBT AND EQUITY FINANCINGS Ameren Missouri In February 2016, $260 million principal amount of Ameren Missouri's 5.40% senior secured notes matured and was repaid with cash on hand and commercial paper borrowings. 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, dividend coverage ratios, and bonds and preferred stock issuable as of March 31, 2016 , at an assumed annual interest rate of 5% and dividend rate of 6% . Required Interest Coverage Ratio (a) Actual Interest Coverage Ratio Bonds Issuable (b) Required Dividend Coverage Ratio (c) Actual Dividend Coverage Ratio Preferred Stock Issuable Ameren Missouri ≥2.0 3.8 $ 3,809 ≥2.5 95.8 $ 2,128 Ameren Illinois ≥2.0 6.4 3,642 (d) ≥1.5 2.6 203 (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 $1,206 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 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, 2016 , Ameren Illinois had 51% 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, 2016 , none of the Ameren Companies had off-balance-sheet financing arrangements, other than operating leases entered into in the ordinary course of business, letters of credit, and Ameren parent guarantee arrangements on behalf of its subsidiaries. None of the Ameren Companies expect to engage in any significant off-balance-sheet financing arrangements in the near future.

Other Income and Expenses

Other Income and Expenses	3 Months Ended
	Mar. 31, 2016
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, 2016 and 2015 : Three Months 2016 2015 Ameren: (a) Miscellaneous income: Allowance for equity funds used during construction $ 8 $ 5 Interest income on industrial development revenue bonds 7 7 Interest income 4 4 Other 1 3 Total miscellaneous income $ 20 $ 19 Miscellaneous expense: Donations $ 5 $ 8 Other 2 3 Total miscellaneous expense $ 7 $ 11 Ameren Missouri: Miscellaneous income: Allowance for equity funds used during construction $ 7 $ 4 Interest income on industrial development revenue bonds 7 7 Other 1 — Total miscellaneous income $ 15 $ 11 Miscellaneous expense: Donations $ 1 $ 2 Other 1 1 Total miscellaneous expense $ 2 $ 3 Ameren Illinois: Miscellaneous income: Allowance for equity funds used during construction $ 1 $ 1 Interest income 4 4 Other — 2 Total miscellaneous income $ 5 $ 7 Miscellaneous expense: Donations $ 4 $ 3 Other 1 2 Total miscellaneous expense $ 5 $ 5 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Derivative Financial Instrument

Derivative Financial Instruments	3 Months Ended
	Mar. 31, 2016
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, 2016 , and December 31, 2015 . As of March 31, 2016 , these contracts ran through October 2018, March 2021, May 2032, and January 2019 for fuel oils, natural gas, power, and uranium, respectively. Quantity (in millions, except as indicated) 2016 2015 Commodity Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils (in gallons) (a) 29 (b) 29 35 (b) 35 Natural gas (in mmbtu) 29 150 179 30 151 181 Power (in megawatthours) 1 9 10 1 10 11 Uranium (pounds in thousands) 428 (b) 428 494 (b) 494 (a) Consists of ultra-low-sulfur diesel products. (b) Not applicable. Authoritative accounting guidance regarding derivative instruments requires that all contracts considered to be derivative instruments be recorded on the balance sheet at their fair values, unless the NPNS exception applies. See Note 7 - Fair Value Measurements for a discussion of our methods of assessing the fair value of derivative instruments. Many of our physical contracts, such as our purchased power contracts, qualify for the NPNS exception to derivative accounting rules. The revenue or expense on NPNS contracts is recognized at the contract price upon physical delivery. If we determine that a contract meets the definition of a derivative and is not eligible for the NPNS exception, we review the contract to determine whether the resulting gains or losses 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, 2016 , and December 31, 2015 , all contracts 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 or similar agreement. The Ameren Companies did not elect to adopt this guidance for any eligible derivative instruments. 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, 2016 , and December 31, 2015 : Balance Sheet Location Ameren Missouri Ameren Illinois Ameren 2016 Natural gas Other assets $ — $ 1 $ 1 Power Other current assets 6 — 6 Total assets (a) $ 6 $ 1 $ 7 Fuel oils Other current liabilities $ 19 $ — $ 19 Other deferred credits and liabilities 6 — 6 Natural gas MTM derivative liabilities (b) 36 (b) Other current liabilities 7 — 43 Other deferred credits and liabilities 7 15 22 Power MTM derivative liabilities (b) 15 (b) Other current liabilities — — 15 Other deferred credits and liabilities — 172 172 Uranium Other current liabilities 2 — 2 Other deferred credits and liabilities 2 — 2 Total liabilities (c) $ 43 $ 238 $ 281 2015 Natural gas Other current assets $ — $ 1 $ 1 Other assets 1 — 1 Power Other current assets 16 — 16 Total assets (a) $ 17 $ 1 $ 18 Fuel oils Other current liabilities $ 22 $ — $ 22 Other deferred credits and liabilities 7 — 7 Natural gas MTM derivative liabilities (b) 32 (b) Other current liabilities 6 — 38 Other deferred credits and liabilities 8 18 26 Power MTM derivative liabilities (b) 13 (b) Other current liabilities — — 13 Other deferred credits and liabilities — 157 157 Uranium Other current liabilities 1 — 1 Total liabilities (c) $ 44 $ 220 $ 264 (a) Because all contracts qualifying for hedge accounting receive regulatory deferral, the cumulative amount of pretax net gains on all derivative instruments is deferred as a regulatory liability. (b) Balance sheet line item not applicable to registrant. (c) Because all contracts qualifying for hedge accounting receive regulatory deferral, the cumulative amount of pretax net losses on all derivative instruments is deferred as a regulatory asset. 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 or similar agreements, and reporting daily exposure to senior management. We believe that entering into master netting arrangements or similar agreements mitigates the level of financial loss that could result from default by allowing net settlement of derivative assets and liabilities. We generally enter into the following master 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 or similar agreement level by counterparty. The following table provides the recognized gross derivative balances and the net amounts of those derivatives subject to an enforceable master netting arrangement or similar agreement as of March 31, 2016 , and December 31, 2015 : 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 2016 Assets: Ameren Missouri $ 6 $ 2 $ — $ 4 Ameren Illinois 1 1 — — Ameren $ 7 $ 3 $ — $ 4 Liabilities: Ameren Missouri $ 43 $ 2 $ 8 $ 33 Ameren Illinois 238 1 2 235 Ameren $ 281 $ 3 $ 10 $ 268 2015 Assets: Ameren Missouri $ 17 $ 1 $ — $ 16 Ameren Illinois 1 — — 1 Ameren $ 18 $ 1 $ — $ 17 Liabilities: Ameren Missouri $ 44 $ 1 $ 8 $ 35 Ameren Illinois 220 — 3 217 Ameren $ 264 $ 1 $ 11 $ 252 (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. These exposures are calculated on a gross basis, which include affiliate exposure not eliminated at the consolidated Ameren level. The potential loss on counterparty exposures may be reduced or eliminated by the application of master netting arrangements or similar agreements and collateral held. As of March 31, 2016 , if counterparty groups were to fail completely to perform on contracts, the Ameren Companies’ maximum exposure would have been immaterial with or without consideration of the application of master netting arrangements or similar agreements and collateral held. 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 our ability to satisfy an obligation requested adequate assurance of performance, additional collateral postings might be required. The following table presents, as of March 31, 2016 , 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 counterparties could require. The additional collateral required is the net liability position allowed under the master netting arrangements or similar agreements, assuming (1) the credit risk-related contingent features underlying these arrangements were triggered on March 31, 2016 , and (2) those counterparties with rights to do so requested collateral. Aggregate Fair Value of Derivative Liabilities (a) Cash Collateral Posted Potential Aggregate Amount of Additional Collateral Required (b) 2016 Ameren Missouri $ 88 $ 7 $ 76 Ameren Illinois 77 2 71 Ameren $ 165 $ 9 $ 147 (a) Before consideration of master netting arrangements or similar agreements and including NPNS and other accrual contract exposures. (b) As collateral requirements with certain counterparties are based on master netting arrangements or similar agreements, 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, 2016
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	FAIR VALUE MEASUREMENTS Fair value is defined as the 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 utilized by the Ameren Companies for the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy for the periods ended March 31, 2016 , and December 31, 2015 : Fair Value Weighted Average Assets Liabilities Valuation Technique(s) Unobservable Input Range Level 3 Derivative asset and liability - commodity contracts (a) : 2016 Natural gas $ — $ (1 ) Discounted cash flow Nodal basis($/mmbtu) (b) (0.60) - 0 (0.50) Counterparty credit risk(%) (c)(d) 0.40 - 6 0.93 Ameren Illinois credit risk(%) (c)(d) 0.40 (e) Power (f) 6 (187 ) Discounted cash flow Average forward peak and off-peak pricing - forwards/swaps($/MWh) (g) 18 - 37 27 Estimated auction price for FTRs($/MW) (b) (818) - 1,062 67 Nodal basis($/MWh) (g) (10) - (1) (2) Counterparty credit risk(%) (c)(d) 0.55 (e) Ameren Illinois credit risk(%) (c)(d) 0.40 (e) Fundamental energy production model Estimated future gas prices($/mmbtu) (b) 3 - 5 4 Escalation rate(%) (b)(h) 4 (e) Contract price allocation Estimated renewable energy credit costs($/credit) (b) 5 - 7 6 Uranium — (4 ) Option model Volatilities(%) (b) 20 (e) Discounted cash flow Average forward uranium pricing($/pound) (b) 28 - 33 31 Ameren Missouri credit risk(%) (c)(d) 0.40 (e) 2015 Natural gas $ 1 $ (1 ) Option model Volatilities(%) (b) 35 – 55 45 Nodal basis($/mmbtu) (c) (0.30) – 0 (0.20) Discounted cash flow Nodal basis($/mmbtu) (b) (0.10) – 0 (0.10) Counterparty credit risk(%) (c)(d) 0.40 – 12 7 Ameren Missouri credit risk(%) (c)(d) 0.40 (e) Power (f) 16 (170 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps($/MWh) (g) 22 – 39 29 Estimated auction price for FTRs($/MW) (b) (270) – 2,057 211 Nodal basis($/MWh) (g) (10) – (1) (3) Counterparty credit risk(%) (c)(d) 0.86 (e) Ameren Illinois credit risk(%) (c)(d) 0.40 (e) Fundamental energy production model Estimated future gas prices($/mmbtu) (b) 3 – 4 4 Escalation rate(%) (b)(h) 3 (e) Contract price allocation Estimated renewable energy credit costs($/credit) (b) 5 – 7 6 Uranium — (1 ) Option model Volatilities(%) (b) 20 (e) Discounted cash flow Average forward uranium pricing($/pound) (b) 35 – 42 37 Ameren Missouri credit risk(%) (c)(d) 0.40 (e) 6 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement. (c) Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement. (d) Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances. (e) Not applicable. (f) Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2020. Valuations beyond 2020 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. (h) Escalation rate applies to power prices 2031 and beyond for March 31, 2016 and to power prices 2026 and beyond for December 31, 2015. 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 2016 or 2015 . At March 31, 2016 , and December 31, 2015 , the counterparty default risk valuation adjustment related to derivative contracts was immaterial for Ameren, Ameren Missouri, and Ameren Illinois. 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, 2016 : Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Total Assets: Ameren Derivative assets - commodity contracts (a) : Natural gas $ — $ 1 $ — $ 1 Power — — 6 6 Total derivative assets - commodity contracts $ — $ 1 $ 6 $ 7 Nuclear decommissioning trust fund: Cash and cash equivalents $ 2 $ — $ — $ 2 Equity securities: U.S. large capitalization 369 — — 369 Debt securities: U.S. treasury and agency securities — 121 — 121 Corporate bonds — 56 — 56 Other — 18 — 18 Total nuclear decommissioning trust fund $ 371 $ 195 $ — $ 566 (b) Total Ameren $ 371 $ 196 $ 6 $ 573 Ameren Derivative assets - commodity contracts (a) : Missouri Power $ — $ — $ 6 $ 6 Nuclear decommissioning trust fund: Cash and cash equivalents $ 2 $ — $ — $ 2 Equity securities: U.S. large capitalization 369 — — 369 Debt securities: U.S. treasury and agency securities — 121 — 121 Corporate bonds — 56 — 56 Other — 18 — 18 Total nuclear decommissioning trust fund $ 371 $ 195 $ — $ 566 (b) Total Ameren Missouri $ 371 $ 195 $ 6 $ 572 Ameren Derivative assets - commodity contracts (a) : Illinois Natural gas $ — $ 1 $ — $ 1 Liabilities: Ameren Derivative liabilities - commodity contracts (a) : Fuel oils $ 25 $ — $ — $ 25 Natural gas 1 63 1 65 Power — — 187 187 Uranium — — 4 4 Total Ameren $ 26 $ 63 $ 192 $ 281 Ameren Derivative liabilities - commodity contracts (a) : Missouri Fuel oils $ 25 $ — $ — $ 25 Natural gas 1 13 — 14 Uranium — — 4 4 Total Ameren Missouri $ 26 $ 13 $ 4 $ 43 Ameren Derivative liabilities - commodity contracts (a) : Illinois Natural gas $ — $ 50 $ 1 $ 51 Power — — 187 187 Total Ameren Illinois $ — $ 50 $ 188 $ 238 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Balance excludes $ 1 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, 2015 : Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Total Assets: Ameren Derivative assets - commodity contracts (a) : Natural gas $ — $ 1 $ 1 $ 2 Power — — 16 16 Total derivative assets - commodity contracts $ — $ 1 $ 17 $ 18 Nuclear decommissioning trust fund: Cash and cash equivalents $ 4 $ — $ — $ 4 Equity securities: U.S. large capitalization 364 — — 364 Debt securities: U.S. treasury and agency securities — 109 — 109 Corporate bonds — 58 — 58 Other — 22 — 22 Total nuclear decommissioning trust fund $ 368 $ 189 $ — $ 557 (b) Total Ameren $ 368 $ 190 $ 17 $ 575 Ameren Derivative assets - commodity contracts (a) : Missouri Natural gas $ — $ — $ 1 $ 1 Power — — 16 16 Total derivative assets - commodity contracts $ — $ — $ 17 $ 17 Nuclear decommissioning trust fund: Cash and cash equivalents $ 4 $ — $ — $ 4 Equity securities: U.S. large capitalization 364 — — 364 Debt securities: U.S. treasury and agency securities — 109 — 109 Corporate bonds — 58 — 58 Other — 22 — 22 Total nuclear decommissioning trust fund $ 368 $ 189 $ — $ 557 (b) Total Ameren Missouri $ 368 $ 189 $ 17 $ 574 Ameren Derivative assets - commodity contracts (a) : Illinois Natural gas $ — $ 1 $ — $ 1 Liabilities: Ameren Derivative liabilities - commodity contracts (a) : Fuel oils $ 29 $ — $ — $ 29 Natural gas 1 62 1 64 Power — — 170 170 Uranium — — 1 1 Total Ameren $ 30 $ 62 $ 172 $ 264 Ameren Derivative liabilities - commodity contracts (a) : Missouri Fuel oils $ 29 $ — $ — $ 29 Natural gas — 13 1 14 Uranium — — 1 1 Total Ameren Missouri $ 29 $ 13 $ 2 $ 44 Ameren Derivative liabilities - commodity contracts (a) : Illinois Natural gas $ 1 $ 49 $ — $ 50 Power — — 170 170 Total Ameren Illinois $ 1 $ 49 $ 170 $ 220 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Balance excludes $(1) 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, 2016 : Net derivative commodity contracts Ameren Missouri Ameren Illinois Ameren Natural gas: Beginning balance at January 1, 2016 $ — $ — $ — Settlements — (1 ) (1 ) Ending balance at March 31, 2016 $ — $ (1 ) $ (1 ) Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2016 $ — $ — $ — Power: Beginning balance at January 1, 2016 $ 16 $ (170 ) $ (154 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (3 ) (21 ) (24 ) Settlements (7 ) 4 (3 ) Ending balance at March 31, 2016 $ 6 $ (187 ) $ (181 ) Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2016 $ — $ (19 ) $ (19 ) Uranium: Beginning balance at January 1, 2016 $ (1 ) $ (a) $ (1 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (3 ) (a) (3 ) Ending balance at March 31, 2016 $ (4 ) $ (a) $ (4 ) Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2016 $ (3 ) $ (a) $ (3 ) (a) Not applicable. The following table summarizes the changes in the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy for the three months ended March 31, 2015 : Net derivative commodity contracts Ameren Missouri Ameren Illinois Ameren 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 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 to be 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, 2016 , and December 31, 2015 : March 31, 2016 December 31, 2015 Carrying Amount Fair Value Carrying Amount Fair Value Ameren: Long-term debt and capital lease obligations (including current portion) $ 7,016 $ 7,744 $ 7,275 $ 7,814 Preferred stock (a) 142 126 142 125 Ameren Missouri: Long-term debt and capital lease obligations (including current portion) $ 3,851 $ 4,280 $ 4,110 $ 4,449 Preferred stock 80 75 80 75 Ameren Illinois: Long-term debt (including current portion) $ 2,472 $ 2,744 $ 2,471 $ 2,665 Preferred stock 62 51 62 50 (a) Preferred stock is recorded in “Noncontrolling Interests” on the consolidated balance sheet.

Related Party Transactions

Related Party Transactions	3 Months Ended
	Mar. 31, 2016
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 Agreement In April 2016, Ameren Illinois conducted a procurement event, administered by the IPA, to purchase energy products through May 31, 2019. Ameren Missouri was among the winning suppliers in this event. As a result Ameren Missouri and Ameren Illinois entered into an energy product agreement by which Ameren Missouri agreed to sell and Ameren Illinois agreed to purchase 375,200 megawatthours at an average price of $34.71 per megawatthour during the period of June 1, 2017, through September 30, 2018. The following table presents the impact on Ameren Missouri and Ameren Illinois of related party transactions for the three months ended March 31, 2016 and 2015 : Three Months Agreement Income Statement Line Item Ameren Missouri Ameren Illinois Ameren Missouri power supply Operating Revenues 2016 $ 9 $ (a) agreements with Ameren Illinois 2015 1 (a) Ameren Missouri and Ameren Illinois Operating Revenues 2016 6 1 rent and facility services 2015 6 1 Ameren Missouri and Ameren Illinois Operating Revenues 2016 (b) (b) miscellaneous support services 2015 (b) (b) Total Operating Revenues 2016 $ 15 $ 1 2015 7 1 Ameren Illinois power supply Purchased Power 2016 $ (a) $ 9 agreements with Ameren Missouri 2015 (a) 1 Ameren Illinois transmission Purchased Power 2016 (a) (b) services with ATXI 2015 (a) 1 Total Purchased Power 2016 $ (a) $ 9 2015 (a) 2 Ameren Services support services Other Operations and Maintenance 2016 $ 34 $ 31 agreement 2015 34 29 Money pool borrowings (advances) Interest Charges/ Miscellaneous Income 2016 $ (b) $ (b) 2015 (b) (b) (a) Not applicable. (b) Amount less than $1 million.

Commitments And Contingencies

Commitments And Contingencies	3 Months Ended
	Mar. 31, 2016
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 2 - Rate and Regulatory Matters, Note 8 - Related Party Transactions, and Note 10 - Callaway Energy Center. Callaway Energy Center The following table presents insurance coverage at Ameren Missouri’s Callaway energy center at March 31, 2016 . 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 2016. Type and Source of Coverage Maximum Coverages Maximum Assessments for Single Incidents Public liability and nuclear worker liability: American Nuclear Insurers $ 375 $ — Pool participation 13,114 (a) 127 (b) $ 13,489 (c) $ 127 Property damage: NEIL $ 2,750 (d) $ 30 (e) European Mutual Association for Nuclear Insurance 450 (f) — $ 3,200 $ 30 Replacement power: NEIL $ 490 (g) $ 7 (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 $127 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 radiation events. NEIL provides $2.3 billion in property damage for nonradiation events. An additional $500 million is provided for radiation events only. The total provided by NEIL for radiation and nonradiation events is $2.75 billion and $2.3 billion , respectively. (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 $450 million in excess of the $2.75 billion and $2.3 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 have entered into various long-term commitments for purchased power and natural gas for distribution. At March 31, 2016 , total obligations related to commitments for coal, natural gas, nuclear fuel, purchased power, methane gas, equipment, and meter reading services, among other agreements, at Ameren, Ameren Missouri, and Ameren Illinois were $4,566 million , $2,786 million , and $1,729 million , respectively. For additional information regarding our obligations and commitments at December 31, 2015, see Note 15 - Commitments and Contingencies under Part II, Item 8 of the Form 10-K. In April 2016, Ameren Illinois conducted a procurement event, administered by the IPA, to purchase energy products through May 31, 2019. In this event, Ameren Illinois contracted to purchase approximately 3,609,800 megawatthours of energy products for $105 million , from June 1, 2016, through May 31, 2019. See Note 8 - Related Party Transactions in this report for additional information regarding the energy product agreement between Ameren Missouri and Ameren Illinois as a result of this procurement event. Environmental Matters We are subject to various environmental laws and regulations enforced by federal, state, and local authorities. Such requirements can impact the siting, development and operation of new and existing generation, transmission, distribution and natural gas storage facilities. Such requirements can encompass emissions, discharges to water, water usage, impacts to air, land, and water, and chemical and waste handling. Complex and lengthy approval processes are required to obtain, modify or renew permits and licenses for new or existing facilities. Additionally, the use and handling of various chemicals or hazardous materials at some of our facilities require release prevention plans and emergency response procedures. The EPA has promulgated several environmental regulations that will have a significant impact on the electric utility industry. Over time, compliance with these regulations could be costly for Ameren Missouri, which operates coal-fired power plants. Significant new rules include the regulation of CO 2 emissions from existing power plants through the Clean Power Plan and from new power plants through the revised NSPS; the CSAPR, which requires further reductions of SO 2 emissions and NO x emissions from power plants; a regulation governing management and storage of CCR; the MATS, which require reduction of emissions of mercury, toxic metals, and acid gases from power plants; revised NSPS for particulate matter, SO 2 , and NO x emissions from new sources; new effluent standards applicable to wastewater 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. The EPA also periodically reviews and revises national ambient air quality standards, including those standards associated with emissions from power plants, such as particulate matter, ozone, SO 2 and NO x . Certain of these new regulations are being or 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 future regulations are unknown, the individual or combined effects of new environmental regulations could result in significant capital expenditures and increased operating costs for Ameren and Ameren Missouri. 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 that these costs would be recoverable through rates, subject to MoPSC prudence review, but the nature and timing of costs and their recovery could result in regulatory lag. 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 and Ameren Missouri estimate that they will need to make capital expenditures of $600 million to $700 million in the aggregate from 2016 through 2020 in order to comply with existing environmental regulations. Ameren Missouri may be required to install additional air emissions controls within the next six to 10 years. This estimate includes capital expenditures required for the CCR regulations, the rule applicable to cooling water intake structures at existing power plants under the Clean Water Act, and the effluent limitation guidelines applicable to steam electric generating units under the Clean Water Act, all of which are discussed below. These estimates do not include the impacts of the Clean Power Plan discussed below. Considerable uncertainty remains in these estimates. The actual amount of capital expenditures 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. The following sections describe the more significant new environmental laws and rules and environmental enforcement and remediation matters that affect or could affect our operations. Clean Air Act Federal and state laws require significant reductions in SO 2 and NO x through either emission source reductions or the use and retirement of emission allowances. The CSAPR became effective in 2015. There will be further emission reduction requirements in 2017 and potentially more in subsequent years. To achieve compliance with CSAPR, Ameren Missouri burns ultra-low-sulfur coal, operates two scrubbers at its Sioux energy center, and optimizes other existing pollution control equipment. Ameren Missouri does not expect to make additional capital investments to comply with the current CSAPR requirements. However, Ameren Missouri expects to incur additional costs to lower its emissions at one or more of its energy centers to comply with the CSAPR in future years. These higher costs are expected to be recovered from customers through the FAC or higher base rates. CO 2 Emissions Standards The Clean Power Plan, which sets forth CO 2 emissions standards applicable to existing power plants, was issued by the EPA in August 2015 but stayed by the United States Supreme Court in February 2016 pending the outcome of various appeals, as discussed below. If upheld, the Clean Power Plan would require Missouri and Illinois to reduce CO 2 emissions from power plants within their states significantly below 2005 levels by 2030. The rule contains interim compliance periods commencing in 2022 that would require each state to demonstrate progress in achieving its CO 2 reduction target. Ameren is evaluating the Clean Power Plan's potential impacts to its operations, including those related to electric system reliability, and to its level of investment in customer energy efficiency programs, renewable energy, and other forms of generation investment. Significant uncertainty exists regarding the impact of the Clean Power Plan, as its implementation will depend upon plans to be developed by the states. Numerous legal challenges are pending, which could result in the rule being declared invalid or the nature and timing of CO 2 emissions reductions being revised. In February 2016, the United States Supreme Court stayed the Clean Power Plan and all implementation requirements until such time as legal appeals are concluded. The District of Columbia Circuit Court of Appeals has scheduled hearings for June 2016 on the legality of the rule. A decision by the District of Columbia Circuit Court of Appeals is expected to be issued in 2016 and subsequent appeals to the United States Supreme Court are likely. Appeals are expected to take several years to conclude. We cannot predict the outcome of such legal challenges or their impact on our results of operations, financial position, or liquidity. If the rule is ultimately upheld and implemented in substantially similar form to the rule when issued, compliance measures 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 and require Ameren Missouri to make new or accelerated capital expenditures. Ameren Missouri expects substantially all of these increased costs to be recoverable, subject to MoPSC prudence review, through higher rates to customers, which could be significant. Also, in 2015, the EPA issued final regulations that set CO 2 emissions standards for new power plants. These new standards establish separate emissions limits for new natural-gas-fired combined cycle plants and new coal-fired plants. Federal and state legislation or regulations that mandate limits on the emission of CO 2 may result in significant increases in capital expenditures and operating costs, which could lead to increased liquidity needs and higher financing costs. Mandatory limits on the emission of CO 2 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 CO 2 emission limitations to the extent that the investments are reflected and recovered on a timely basis 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. Ameren Missouri anticipates that a trial of this case will occur in 2016. 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 results of operations, financial position, and liquidity of Ameren and Ameren Missouri. A resolution of this matter could result in increased capital expenditures for the installation of pollution control equipment and increased operations and maintenance expenses. We are unable to predict the ultimate resolution of these matters or the costs that might be incurred. Clean Water Act In 2014, the EPA issued its final rule applicable to cooling water intake structures at existing power plants. The rule requires a case-by-case evaluation and plan for reducing aquatic organisms impinged on the facility’s intake screens or entrained through the plant's cooling water system. All of Ameren Missouri’s coal-fired and nuclear energy centers are subject to this rule. Each of Ameren Missouri’s affected energy centers will become subject to the revised limitations when the energy center renews its water discharge permit. These permits are scheduled to be renewed between 2018 and 2023. 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 on a timely basis in electric rates charged to Ameren Missouri's customers. In 2015, the EPA issued its final rule under the Clean Water Act to revise the effluent limitation guidelines applicable to steam electric generating units. Effluent limitation guidelines are national standards for water discharges that are based on the effectiveness of available control technology. The EPA's rule prohibits effluent discharges of certain waste streams and imposes more stringent limitations on certain components in water discharges from power plants. All of Ameren Missouri’s coal-fired energy centers are subject to this rule, which will be applied when each energy center renews its water discharge permit beginning as early as 2018. Ameren Missouri is evaluating the final rule, which became effective in January 2016, and the possible effects on its operations. 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 extensive changes in the water discharge systems at Ameren Missouri’s energy centers and if these investments are not recovered on a timely basis through electric rates charged to Ameren Missouri’s customers. Ash Management In 2015, the EPA issued regulations regarding the management and disposal of CCR. These regulations will affect CCR disposal and handling costs at Ameren Missouri's energy centers. 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 establish 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's capital expenditure plan includes the cost of constructing landfills as part of its environmental compliance plan. The new regulations do not apply to ash ponds at plants no longer in operation, such as Ameren’s Meredosia and Hutsonville energy centers. Remediation The Ameren Companies are involved in a number of remediation actions to clean up sites impacted from the use or disposal of materials containing hazardous substances. Federal and state laws can require responsible parties to 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, 2016 , 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 by 2025. 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. Costs are subject to annual review by the ICC. As of March 31, 2016 , Ameren Illinois estimated the obligation related to these former MGP sites at $224 million to $313 million . Ameren and Ameren Illinois recorded a liability of $224 million to represent the 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 because many site specific factors can influence the ultimate actual costs, including 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 a third-party owned landfill in connection with the operation of a previously-owned energy center. While no litigation is pending, Ameren Illinois could be required to perform certain maintenance activities at that landfill, which is now closed. As of March 31, 2016 , Ameren Illinois estimated the obligation related to this site at $0.5 million to $6 million . Ameren Illinois recorded a liability of $0.5 million to represent its estimated minimum obligation for this site, as no other amount within the range was a better estimate. Ameren Illinois is also responsible for the cleanup of some underground storage tanks and a water treatment plant in Illinois. As of March 31, 2016 , 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 three companies including Ameren Missouri pertaining to a former coal tar distillery in St. Louis, Missouri operated by Koppers Company or its predecessor and successor companies. While Ameren Missouri is the current owner of the site, it did not conduct any of the manufacturing operations involving coal tar or its byproducts. Site investigation activities have been concluded and reports have been submitted to the EPA for review and approval. As of March 31, 2016 , 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 for this site, as no other amount within the range was a better estimate. Ameren Missouri also participated in the investigation of various sites known as Sauget Area 2 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. 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 by others 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, 2016 , 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, 2016 , Ameren Missouri estimated and recorded a $0.6 million liability related to the site. Although monitoring will continue for some time, no significant remediation measures are anticipated. 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. Ameren Missouri Municipal Taxes The cities of Creve Coeur and Winchester, Missouri, on behalf of themselves and other municipalities in Ameren Missouri’s service area, filed a class action lawsuit in November 2011, against Ameren Missouri in the Circuit Court of St. Louis County, Missouri. The lawsuit alleges that Ameren Missouri failed to collect and pay gross receipts taxes or license fees on certain revenues. Ameren and Ameren Missouri recorded immaterial liabilities on their respective balance sheets as of March 31, 2016 and December 31, 2015, representing their estimate of taxes and fees due as a result of this lawsuit. Ameren Missouri believes its defenses are meritorious and is defending itself vigorously; however, there can be no assurances that Ameren Missouri will be successful in its efforts. The ultimate resolution of any unpaid municipal tax or fees could have a material adverse effect on the results of operations, financial position, and liquidity of Ameren and Ameren Missouri.

Callaway Energy Center

Callaway Energy Center	3 Months Ended
	Mar. 31, 2016
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 to review the nuclear waste fee annually 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 DOE. 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 DOE would begin to dispose of spent nuclear fuel by 1998, the DOE is not meeting its disposal obligation. 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. 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 sued the DOE to recover costs, such as certain NRC fees and Missouri ad valorem taxes, incurred for ongoing storage of their spent fuel. The lawsuit resulted in a settlement agreement that provides for annual recovery of additional spent fuel storage and related costs. Ameren Missouri will continue to apply for reimbursement from the DOE for allowable costs associated with the ongoing storage of spent fuel. 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. In April 2016, the MoPSC approved no change in electric service rates for decommissioning costs based on Ameren Missouri’s updated cost study and funding analysis filed in April 2015. 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. If the assumed return on trust assets is not earned, Ameren Missouri believes that it is probable that any such earnings deficiency will be recovered in rates.

Retirement Benefits

Retirement Benefits	3 Months Ended
	Mar. 31, 2016
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, 2016 , the plan’s estimated investment performance through March 31, 2016 , and Ameren’s pension funding policy, Ameren expects to make annual contributions of $40 million to $70 million through 2020, with aggregate estimated contributions of $280 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) incurred for Ameren’s pension and postretirement benefit plans for the three months ended March 31, 2016 and 2015 : Pension Benefits Postretirement Benefits Three Months Three Months 2016 2015 2016 2015 Service cost $ 20 $ 24 $ 5 $ 5 Interest cost 47 44 12 12 Expected return on plan assets (63 ) (62 ) (18 ) (17 ) Amortization of: Prior service benefit — — (1 ) (1 ) Actuarial loss (gain) 9 18 (3 ) 1 Net periodic benefit cost (benefit) $ 13 $ 24 $ (5 ) $ — 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 (benefit) incurred for the three months ended March 31, 2016 and 2015 : Pension Benefits Postretirement Benefits Three Months Three Months 2016 2015 2016 2015 Ameren Missouri (a) $ 8 $ 15 $ (1 ) $ 1 Ameren Illinois 5 9 (4 ) (1 ) Ameren (a)(b) $ 13 $ 24 $ (5 ) $ — (a) Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. (b) Includes amounts for Ameren registrants and nonregistrant subsidiaries.

Divestiture Transactions and Di

Divestiture Transactions and Discontinued Operations	3 Months Ended
	Mar. 31, 2016
Discontinued Operations and Disposal Groups [Abstract]
DIVESTITURE TRANSACTIONS AND DISCONTINUED OPERATIONS	DISCONTINUED OPERATIONS On January 31, 2014, Medina Valley completed the sale of the Elgin, Gibson City, and Grand Tower gas-fired energy centers to Rockland Capital for a total purchase price of $168 million . The agreement with Rockland Capital required a portion of the purchase price to be held in escrow until January 31, 2016, to fund certain indemnity obligations, if any, of Medina Valley. Medina Valley received the amount held in escrow from Rockland Capital in the first quarter of 2016 and paid Genco its portion of the escrow amount in April 2016. See Note 16 - Divestiture Transactions and Discontinued Operations under Part II, Item 8, of the Form 10-K for additional information related to discontinued operations.

Segment Information

Segment Information	3 Months Ended
	Mar. 31, 2016
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 net income attributable to Ameren common shareholders from continuing operations for the three months ended March 31, 2016 and 2015 , and total assets of continuing operations as of March 31, 2016 , and December 31, 2015 : Three Months Ameren Missouri Ameren Illinois Other Intersegment Eliminations Ameren 2016 External revenues $ 726 $ 676 $ 32 $ — $ 1,434 Intersegment revenues 15 1 — (16 ) — Net income attributable to Ameren common shareholders from continuing operations 14 59 32 — 105 2015 External revenues $ 793 $ 744 $ 19 $ — $ 1,556 Intersegment revenues 7 1 1 (9 ) — Net income attributable to Ameren common shareholders from continuing operations 41 53 14 — 108 As of March 31, 2016: Total assets $ 13,443 $ 8,944 $ 1,214 $ (230 ) $ 23,371 (a) As of December 31, 2015: Total assets $ 13,851 $ 8,903 $ 1,139 $ (267 ) $ 23,626 (a) (a) Excludes total assets from discontinued operations of $14 million as of March 31, 2016 , and December 31, 2015.

Summary Of Significant Accoun22

Summary Of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2016
Accounting Policies [Abstract]
Consolidation	Ameren’s financial statements 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 statement 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.
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, 2016 and 2015. The assumed settlement of dilutive performance share units had an immaterial impact on earnings per share. The calculation of diluted earnings per share reflected the adoption of FASB guidance related to employee share-based payment accounting discussed below.
Accounting and Reporting Developments	Below is a summary of recently issued authoritative accounting standards relevant to the Ameren Companies. Revenue from Contracts with Customers In May 2014, the FASB issued authoritative accounting guidance that changes the criteria for recognizing revenue from a contract with a customer. The underlying principle of the guidance is that an entity will recognize revenue for the transfer of promised goods or services to customers at an amount that the entity expects to be entitled to in exchange for those goods or services. The guidance also requires additional disclosures to enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. Entities can apply the guidance retrospectively to each reporting period presented or retrospectively by recording a cumulative effect adjustment to retained earnings in the period of initial adoption. The Ameren Companies are currently assessing the impacts of this guidance on their results of operations, financial position, and disclosures, including their accounting for contributions in aid of construction and similar arrangements, as well as the transition method that they will use to adopt the guidance. In August 2015, the FASB deferred the effective date of this revenue guidance to the first quarter of 2018, with an option for entities to early adopt in the first quarter of 2017. The Ameren Companies do not expect to early adopt this guidance. Amendments to the Consolidation Analysis In February 2015, the FASB issued authoritative accounting guidance that amends the consolidation analysis for variable interest entities and voting interest entities. The new guidance affects (1) limited partnerships, similar legal entities, and certain investment funds, (2) the evaluation of fees paid to a decision maker or service provider as a variable interest, (3) how fee arrangements impact the primary beneficiary determination, and (4) the evaluation of related party relationships on the primary beneficiary determination. The adoption of this guidance in the first quarter of 2016 did not impact the Ameren Companies’ results of operations, financial position, cash flows, or disclosures. Leases In February 2016, the FASB issued authoritative accounting guidance that will require an entity to recognize assets and liabilities arising from a lease. Consistent with current GAAP, the recognition, measurement, and presentation of expenses and cash flows arising from a lease will depend primarily on its classification as a finance or operating lease. The guidance also requires additional disclosures to enable users of financial statements to understand the amount, timing, and uncertainty of cash flows arising from leases. The guidance will be effective for the Ameren Companies in the first quarter of 2019 with an option for entities to early adopt. Upon adoption, the Ameren Companies will recognize and measure operating leases on their respective balance sheets at the beginning of the earliest period presented. The Ameren Companies are currently assessing the impacts of this guidance on their results of operations, financial position, cash flows, and disclosures. Improvements to Employee Share-Based Payment Accounting In March 2016, the FASB issued authoritative accounting guidance that simplifies the accounting for share-based payment transactions, including the income tax consequences, the calculation of diluted earnings per share, the classification of awards as either equity or liabilities, and the classification on the statement of cash flows. Ameren determines for each performance share unit award whether the difference between the deduction for tax purposes and the compensation cost recognized for financial reporting purposes results in either an excess tax benefit or an excess tax deficit. Previously, excess tax benefits were recognized in "Other paid-in capital" on Ameren’s consolidated balance sheet, and in certain cases, excess tax deficits were recognized in “Income taxes” on Ameren’s consolidated income statement. The new guidance increases income statement volatility by requiring all excess tax benefits and deficits to be recognized in “Income taxes,” and treated as discrete items in the period in which they occur. Ameren adopted this guidance in the first quarter of 2016, which resulted in recognition of a $21 million income tax benefit during the period. For the three months ended March 31, 2015, Ameren reclassified, for comparison purposes, $2 million of excess tax benefits on the statement of cash flows from financing to operating activity, and $12 million of employee payroll taxes related to share-based payments from operating to financing activity.

Derivative Financial Instrume23

Derivative Financial Instruments Derivative Financial Instruments (Policies)	3 Months Ended
	Mar. 31, 2016
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
	Mar. 31, 2016
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, 2016 , the plan’s estimated investment performance through March 31, 2016 , and Ameren’s pension funding policy, Ameren expects to make annual contributions of $40 million to $70 million through 2020, with aggregate estimated contributions of $280 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 Accoun25

Summary Of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2016
Accounting Policies [Abstract]
Summary Of Nonvested Shares Related To Long-Term Incentive Plan	A summary of nonvested performance share units at March 31, 2016 , and changes during the three months ended March 31, 2016 , under the 2006 Incentive Plan and the 2014 Incentive Plan are presented below: Performance Share Units Share Units Weighted-average Fair Value per Share Unit Nonvested at January 1, 2016 1,024,870 $ 46.08 Granted (a) 580,737 44.13 Forfeitures (12,315 ) 45.12 Vested (b) (8,265 ) 42.91 Nonvested at March 31, 2016 1,585,027 $ 45.39 (a) Performance share units granted to certain executive and nonexecutive officers and other eligible employees 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 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, 2016 and 2015 : Three Months 2016 2015 Ameren Missouri $ 30 $ 34 Ameren Illinois 20 23 Ameren $ 50 $ 57

Short-Term Debt And Liquidity S

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Tables)	3 Months Ended
	Mar. 31, 2016
Schedule of Commercial paper [Abstract]
Schedule of Short-term Debt	The following table presents commercial paper outstanding at Ameren (parent), Ameren Missouri, and Ameren Illinois as of March 31, 2016 , and December 31, 2015 : March 31, 2016 December 31, 2015 Ameren (parent) $ 416 $ 301 Ameren Missouri 165 — Ameren Illinois — — Ameren Consolidated $ 581 $ 301 The following table summarizes the borrowing 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, 2016 and 2015: Ameren (parent) Ameren Missouri Ameren Illinois Ameren Consolidated 2016 Average daily commercial paper outstanding $ 349 $ 68 $ — $ 417 Weighted-average interest rate 0.82 % 0.80 % — % 0.81 % Peak commercial paper during period (a) $ 482 $ 208 $ — $ 581 Peak interest rate 0.95 % 0.85 % — % 0.95 % 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 0.70 % 0.60 % 0.60 % 0.70 % (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.

Long-Term Debt And Equity Fin27

Long-Term Debt And Equity Financings (Tables)	3 Months Ended
	Mar. 31, 2016
Long-Term Debt And Equity Financings [Line Items]
Schedule Of Coverage Ratios	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, dividend coverage ratios, and bonds and preferred stock issuable as of March 31, 2016 , at an assumed annual interest rate of 5% and dividend rate of 6% . Required Interest Coverage Ratio (a) Actual Interest Coverage Ratio Bonds Issuable (b) Required Dividend Coverage Ratio (c) Actual Dividend Coverage Ratio Preferred Stock Issuable Ameren Missouri ≥2.0 3.8 $ 3,809 ≥2.5 95.8 $ 2,128 Ameren Illinois ≥2.0 6.4 3,642 (d) ≥1.5 2.6 203 (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 $1,206 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 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.

Other Income and Expenses (Tabl

Other Income and Expenses (Tables)	3 Months Ended
	Mar. 31, 2016
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, 2016 and 2015 : Three Months 2016 2015 Ameren: (a) Miscellaneous income: Allowance for equity funds used during construction $ 8 $ 5 Interest income on industrial development revenue bonds 7 7 Interest income 4 4 Other 1 3 Total miscellaneous income $ 20 $ 19 Miscellaneous expense: Donations $ 5 $ 8 Other 2 3 Total miscellaneous expense $ 7 $ 11 Ameren Missouri: Miscellaneous income: Allowance for equity funds used during construction $ 7 $ 4 Interest income on industrial development revenue bonds 7 7 Other 1 — Total miscellaneous income $ 15 $ 11 Miscellaneous expense: Donations $ 1 $ 2 Other 1 1 Total miscellaneous expense $ 2 $ 3 Ameren Illinois: Miscellaneous income: Allowance for equity funds used during construction $ 1 $ 1 Interest income 4 4 Other — 2 Total miscellaneous income $ 5 $ 7 Miscellaneous expense: Donations $ 4 $ 3 Other 1 2 Total miscellaneous expense $ 5 $ 5 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Derivative Financial Instrume29

Derivative Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 2016
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, 2016 , and December 31, 2015 . As of March 31, 2016 , these contracts ran through October 2018, March 2021, May 2032, and January 2019 for fuel oils, natural gas, power, and uranium, respectively. Quantity (in millions, except as indicated) 2016 2015 Commodity Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils (in gallons) (a) 29 (b) 29 35 (b) 35 Natural gas (in mmbtu) 29 150 179 30 151 181 Power (in megawatthours) 1 9 10 1 10 11 Uranium (pounds in thousands) 428 (b) 428 494 (b) 494 (a) Consists of ultra-low-sulfur diesel products. (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, 2016 , and December 31, 2015 : Balance Sheet Location Ameren Missouri Ameren Illinois Ameren 2016 Natural gas Other assets $ — $ 1 $ 1 Power Other current assets 6 — 6 Total assets (a) $ 6 $ 1 $ 7 Fuel oils Other current liabilities $ 19 $ — $ 19 Other deferred credits and liabilities 6 — 6 Natural gas MTM derivative liabilities (b) 36 (b) Other current liabilities 7 — 43 Other deferred credits and liabilities 7 15 22 Power MTM derivative liabilities (b) 15 (b) Other current liabilities — — 15 Other deferred credits and liabilities — 172 172 Uranium Other current liabilities 2 — 2 Other deferred credits and liabilities 2 — 2 Total liabilities (c) $ 43 $ 238 $ 281 2015 Natural gas Other current assets $ — $ 1 $ 1 Other assets 1 — 1 Power Other current assets 16 — 16 Total assets (a) $ 17 $ 1 $ 18 Fuel oils Other current liabilities $ 22 $ — $ 22 Other deferred credits and liabilities 7 — 7 Natural gas MTM derivative liabilities (b) 32 (b) Other current liabilities 6 — 38 Other deferred credits and liabilities 8 18 26 Power MTM derivative liabilities (b) 13 (b) Other current liabilities — — 13 Other deferred credits and liabilities — 157 157 Uranium Other current liabilities 1 — 1 Total liabilities (c) $ 44 $ 220 $ 264 (a) Because all contracts qualifying for hedge accounting receive regulatory deferral, the cumulative amount of pretax net gains on all derivative instruments is deferred as a regulatory liability. (b) Balance sheet line item not applicable to registrant. (c) Because all contracts qualifying for hedge accounting receive regulatory deferral, the cumulative amount of pretax net losses on all derivative instruments is deferred as a regulatory asset.
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, 2016 , and December 31, 2015 : 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 2016 Assets: Ameren Missouri $ 6 $ 2 $ — $ 4 Ameren Illinois 1 1 — — Ameren $ 7 $ 3 $ — $ 4 Liabilities: Ameren Missouri $ 43 $ 2 $ 8 $ 33 Ameren Illinois 238 1 2 235 Ameren $ 281 $ 3 $ 10 $ 268 2015 Assets: Ameren Missouri $ 17 $ 1 $ — $ 16 Ameren Illinois 1 — — 1 Ameren $ 18 $ 1 $ — $ 17 Liabilities: Ameren Missouri $ 44 $ 1 $ 8 $ 35 Ameren Illinois 220 — 3 217 Ameren $ 264 $ 1 $ 11 $ 252 (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 following table presents, as of March 31, 2016 , 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 counterparties could require. The additional collateral required is the net liability position allowed under the master netting arrangements or similar agreements, assuming (1) the credit risk-related contingent features underlying these arrangements were triggered on March 31, 2016 , and (2) those counterparties with rights to do so requested collateral. Aggregate Fair Value of Derivative Liabilities (a) Cash Collateral Posted Potential Aggregate Amount of Additional Collateral Required (b) 2016 Ameren Missouri $ 88 $ 7 $ 76 Ameren Illinois 77 2 71 Ameren $ 165 $ 9 $ 147 (a) Before consideration of master netting arrangements or similar agreements and including NPNS and other accrual contract exposures. (b) As collateral requirements with certain counterparties are based on master netting arrangements or similar agreements, 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, 2016
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 utilized by the Ameren Companies for the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy for the periods ended March 31, 2016 , and December 31, 2015 : Fair Value Weighted Average Assets Liabilities Valuation Technique(s) Unobservable Input Range Level 3 Derivative asset and liability - commodity contracts (a) : 2016 Natural gas $ — $ (1 ) Discounted cash flow Nodal basis($/mmbtu) (b) (0.60) - 0 (0.50) Counterparty credit risk(%) (c)(d) 0.40 - 6 0.93 Ameren Illinois credit risk(%) (c)(d) 0.40 (e) Power (f) 6 (187 ) Discounted cash flow Average forward peak and off-peak pricing - forwards/swaps($/MWh) (g) 18 - 37 27 Estimated auction price for FTRs($/MW) (b) (818) - 1,062 67 Nodal basis($/MWh) (g) (10) - (1) (2) Counterparty credit risk(%) (c)(d) 0.55 (e) Ameren Illinois credit risk(%) (c)(d) 0.40 (e) Fundamental energy production model Estimated future gas prices($/mmbtu) (b) 3 - 5 4 Escalation rate(%) (b)(h) 4 (e) Contract price allocation Estimated renewable energy credit costs($/credit) (b) 5 - 7 6 Uranium — (4 ) Option model Volatilities(%) (b) 20 (e) Discounted cash flow Average forward uranium pricing($/pound) (b) 28 - 33 31 Ameren Missouri credit risk(%) (c)(d) 0.40 (e) 2015 Natural gas $ 1 $ (1 ) Option model Volatilities(%) (b) 35 – 55 45 Nodal basis($/mmbtu) (c) (0.30) – 0 (0.20) Discounted cash flow Nodal basis($/mmbtu) (b) (0.10) – 0 (0.10) Counterparty credit risk(%) (c)(d) 0.40 – 12 7 Ameren Missouri credit risk(%) (c)(d) 0.40 (e) Power (f) 16 (170 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps($/MWh) (g) 22 – 39 29 Estimated auction price for FTRs($/MW) (b) (270) – 2,057 211 Nodal basis($/MWh) (g) (10) – (1) (3) Counterparty credit risk(%) (c)(d) 0.86 (e) Ameren Illinois credit risk(%) (c)(d) 0.40 (e) Fundamental energy production model Estimated future gas prices($/mmbtu) (b) 3 – 4 4 Escalation rate(%) (b)(h) 3 (e) Contract price allocation Estimated renewable energy credit costs($/credit) (b) 5 – 7 6 Uranium — (1 ) Option model Volatilities(%) (b) 20 (e) Discounted cash flow Average forward uranium pricing($/pound) (b) 35 – 42 37 Ameren Missouri credit risk(%) (c)(d) 0.40 (e) 6 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement. (c) Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement. (d) Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances. (e) Not applicable. (f) Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2020. Valuations beyond 2020 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. (h) Escalation rate applies to power prices 2031 and beyond for March 31, 2016 and to power prices 2026 and beyond for December 31, 2015.
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, 2016 : Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Total Assets: Ameren Derivative assets - commodity contracts (a) : Natural gas $ — $ 1 $ — $ 1 Power — — 6 6 Total derivative assets - commodity contracts $ — $ 1 $ 6 $ 7 Nuclear decommissioning trust fund: Cash and cash equivalents $ 2 $ — $ — $ 2 Equity securities: U.S. large capitalization 369 — — 369 Debt securities: U.S. treasury and agency securities — 121 — 121 Corporate bonds — 56 — 56 Other — 18 — 18 Total nuclear decommissioning trust fund $ 371 $ 195 $ — $ 566 (b) Total Ameren $ 371 $ 196 $ 6 $ 573 Ameren Derivative assets - commodity contracts (a) : Missouri Power $ — $ — $ 6 $ 6 Nuclear decommissioning trust fund: Cash and cash equivalents $ 2 $ — $ — $ 2 Equity securities: U.S. large capitalization 369 — — 369 Debt securities: U.S. treasury and agency securities — 121 — 121 Corporate bonds — 56 — 56 Other — 18 — 18 Total nuclear decommissioning trust fund $ 371 $ 195 $ — $ 566 (b) Total Ameren Missouri $ 371 $ 195 $ 6 $ 572 Ameren Derivative assets - commodity contracts (a) : Illinois Natural gas $ — $ 1 $ — $ 1 Liabilities: Ameren Derivative liabilities - commodity contracts (a) : Fuel oils $ 25 $ — $ — $ 25 Natural gas 1 63 1 65 Power — — 187 187 Uranium — — 4 4 Total Ameren $ 26 $ 63 $ 192 $ 281 Ameren Derivative liabilities - commodity contracts (a) : Missouri Fuel oils $ 25 $ — $ — $ 25 Natural gas 1 13 — 14 Uranium — — 4 4 Total Ameren Missouri $ 26 $ 13 $ 4 $ 43 Ameren Derivative liabilities - commodity contracts (a) : Illinois Natural gas $ — $ 50 $ 1 $ 51 Power — — 187 187 Total Ameren Illinois $ — $ 50 $ 188 $ 238 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Balance excludes $ 1 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, 2015 : Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Total Assets: Ameren Derivative assets - commodity contracts (a) : Natural gas $ — $ 1 $ 1 $ 2 Power — — 16 16 Total derivative assets - commodity contracts $ — $ 1 $ 17 $ 18 Nuclear decommissioning trust fund: Cash and cash equivalents $ 4 $ — $ — $ 4 Equity securities: U.S. large capitalization 364 — — 364 Debt securities: U.S. treasury and agency securities — 109 — 109 Corporate bonds — 58 — 58 Other — 22 — 22 Total nuclear decommissioning trust fund $ 368 $ 189 $ — $ 557 (b) Total Ameren $ 368 $ 190 $ 17 $ 575 Ameren Derivative assets - commodity contracts (a) : Missouri Natural gas $ — $ — $ 1 $ 1 Power — — 16 16 Total derivative assets - commodity contracts $ — $ — $ 17 $ 17 Nuclear decommissioning trust fund: Cash and cash equivalents $ 4 $ — $ — $ 4 Equity securities: U.S. large capitalization 364 — — 364 Debt securities: U.S. treasury and agency securities — 109 — 109 Corporate bonds — 58 — 58 Other — 22 — 22 Total nuclear decommissioning trust fund $ 368 $ 189 $ — $ 557 (b) Total Ameren Missouri $ 368 $ 189 $ 17 $ 574 Ameren Derivative assets - commodity contracts (a) : Illinois Natural gas $ — $ 1 $ — $ 1 Liabilities: Ameren Derivative liabilities - commodity contracts (a) : Fuel oils $ 29 $ — $ — $ 29 Natural gas 1 62 1 64 Power — — 170 170 Uranium — — 1 1 Total Ameren $ 30 $ 62 $ 172 $ 264 Ameren Derivative liabilities - commodity contracts (a) : Missouri Fuel oils $ 29 $ — $ — $ 29 Natural gas — 13 1 14 Uranium — — 1 1 Total Ameren Missouri $ 29 $ 13 $ 2 $ 44 Ameren Derivative liabilities - commodity contracts (a) : Illinois Natural gas $ 1 $ 49 $ — $ 50 Power — — 170 170 Total Ameren Illinois $ 1 $ 49 $ 170 $ 220 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Balance excludes $(1) 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, 2016 : Net derivative commodity contracts Ameren Missouri Ameren Illinois Ameren Natural gas: Beginning balance at January 1, 2016 $ — $ — $ — Settlements — (1 ) (1 ) Ending balance at March 31, 2016 $ — $ (1 ) $ (1 ) Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2016 $ — $ — $ — Power: Beginning balance at January 1, 2016 $ 16 $ (170 ) $ (154 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (3 ) (21 ) (24 ) Settlements (7 ) 4 (3 ) Ending balance at March 31, 2016 $ 6 $ (187 ) $ (181 ) Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2016 $ — $ (19 ) $ (19 ) Uranium: Beginning balance at January 1, 2016 $ (1 ) $ (a) $ (1 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (3 ) (a) (3 ) Ending balance at March 31, 2016 $ (4 ) $ (a) $ (4 ) Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2016 $ (3 ) $ (a) $ (3 ) (a) Not applicable. The following table summarizes the changes in the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy for the three months ended March 31, 2015 : Net derivative commodity contracts Ameren Missouri Ameren Illinois Ameren 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.
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, 2016 , and December 31, 2015 : March 31, 2016 December 31, 2015 Carrying Amount Fair Value Carrying Amount Fair Value Ameren: Long-term debt and capital lease obligations (including current portion) $ 7,016 $ 7,744 $ 7,275 $ 7,814 Preferred stock (a) 142 126 142 125 Ameren Missouri: Long-term debt and capital lease obligations (including current portion) $ 3,851 $ 4,280 $ 4,110 $ 4,449 Preferred stock 80 75 80 75 Ameren Illinois: Long-term debt (including current portion) $ 2,472 $ 2,744 $ 2,471 $ 2,665 Preferred stock 62 51 62 50 (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, 2016
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, 2016 and 2015 : Three Months Agreement Income Statement Line Item Ameren Missouri Ameren Illinois Ameren Missouri power supply Operating Revenues 2016 $ 9 $ (a) agreements with Ameren Illinois 2015 1 (a) Ameren Missouri and Ameren Illinois Operating Revenues 2016 6 1 rent and facility services 2015 6 1 Ameren Missouri and Ameren Illinois Operating Revenues 2016 (b) (b) miscellaneous support services 2015 (b) (b) Total Operating Revenues 2016 $ 15 $ 1 2015 7 1 Ameren Illinois power supply Purchased Power 2016 $ (a) $ 9 agreements with Ameren Missouri 2015 (a) 1 Ameren Illinois transmission Purchased Power 2016 (a) (b) services with ATXI 2015 (a) 1 Total Purchased Power 2016 $ (a) $ 9 2015 (a) 2 Ameren Services support services Other Operations and Maintenance 2016 $ 34 $ 31 agreement 2015 34 29 Money pool borrowings (advances) Interest Charges/ Miscellaneous Income 2016 $ (b) $ (b) 2015 (b) (b) (a) Not applicable. (b) Amount less than $1 million.

Commitments And Contingencies (

Commitments And Contingencies (Tables)	3 Months Ended
	Mar. 31, 2016
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, 2016 . 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 2016. Type and Source of Coverage Maximum Coverages Maximum Assessments for Single Incidents Public liability and nuclear worker liability: American Nuclear Insurers $ 375 $ — Pool participation 13,114 (a) 127 (b) $ 13,489 (c) $ 127 Property damage: NEIL $ 2,750 (d) $ 30 (e) European Mutual Association for Nuclear Insurance 450 (f) — $ 3,200 $ 30 Replacement power: NEIL $ 490 (g) $ 7 (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 $127 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 radiation events. NEIL provides $2.3 billion in property damage for nonradiation events. An additional $500 million is provided for radiation events only. The total provided by NEIL for radiation and nonradiation events is $2.75 billion and $2.3 billion , respectively. (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 $450 million in excess of the $2.75 billion and $2.3 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 .

Retirement Benefits (Tables)

Retirement Benefits (Tables)	3 Months Ended
	Mar. 31, 2016
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) incurred for Ameren’s pension and postretirement benefit plans for the three months ended March 31, 2016 and 2015 : Pension Benefits Postretirement Benefits Three Months Three Months 2016 2015 2016 2015 Service cost $ 20 $ 24 $ 5 $ 5 Interest cost 47 44 12 12 Expected return on plan assets (63 ) (62 ) (18 ) (17 ) Amortization of: Prior service benefit — — (1 ) (1 ) Actuarial loss (gain) 9 18 (3 ) 1 Net periodic benefit cost (benefit) $ 13 $ 24 $ (5 ) $ —
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 (benefit) incurred for the three months ended March 31, 2016 and 2015 : Pension Benefits Postretirement Benefits Three Months Three Months 2016 2015 2016 2015 Ameren Missouri (a) $ 8 $ 15 $ (1 ) $ 1 Ameren Illinois 5 9 (4 ) (1 ) Ameren (a)(b) $ 13 $ 24 $ (5 ) $ — (a) Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. (b) Includes amounts for Ameren registrants and nonregistrant subsidiaries.

Segment Information (Tables)

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2016
Segment Reporting [Abstract]
Schedule Of Segment Reporting Information By Segment	The following table presents information about the reported revenues and net income attributable to Ameren common shareholders from continuing operations for the three months ended March 31, 2016 and 2015 , and total assets of continuing operations as of March 31, 2016 , and December 31, 2015 : Three Months Ameren Missouri Ameren Illinois Other Intersegment Eliminations Ameren 2016 External revenues $ 726 $ 676 $ 32 $ — $ 1,434 Intersegment revenues 15 1 — (16 ) — Net income attributable to Ameren common shareholders from continuing operations 14 59 32 — 105 2015 External revenues $ 793 $ 744 $ 19 $ — $ 1,556 Intersegment revenues 7 1 1 (9 ) — Net income attributable to Ameren common shareholders from continuing operations 41 53 14 — 108 As of March 31, 2016: Total assets $ 13,443 $ 8,944 $ 1,214 $ (230 ) $ 23,371 (a) As of December 31, 2015: Total assets $ 13,851 $ 8,903 $ 1,139 $ (267 ) $ 23,626 (a) (a) Excludes total assets from discontinued operations of $14 million as of March 31, 2016 , and December 31, 2015.

Summary Of Significant Accoun35

Summary Of Significant Accounting Policies (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Basis Of Presentation And Significant Accounting Policies [Line Items]
Income Tax Expense (Benefit)	$ 21
Excess Tax Benefit from Share-based Compensation, Operating Activities	2
Employee Service Share-based Compensation, Tax Benefit from Compensation Expense	$ 32	$ 12

Summary Of Significant Accoun36

Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Summary of Nonvested Shares Related To Long-Term Incentive Plan) (Details) - $ / shares		3 Months Ended
		Mar. 31, 2016	Dec. 31, 2015
Weighted-average Fair Value per Unit at Grant Date
Closing common share price			$ 43.23
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Three-year risk-free rate		1.31%
Volatility rate, minimum		15.00%
Volatility rate, maximum		20.00%
Share Units
Share Units, Nonvested as of beginning of year		1,024,870
Share Units, Granted	[1]	580,737
Share Units, Forfeitures		(12,315)
Share Units, Vested	[2]	(8,265)
Share Units, Nonvested as of Current period end		1,585,027
Weighted-average Fair Value per Unit at Grant Date
Weighted-average Fair Value per Unit, Nonvested as of Beginning of year		$ 46.08
Weighted-average Fair Value per Unit, Grants	[1]	44.13
Weighted-average Fair Value per Unit, Forfeitures		45.12
Weighted-average Fair Value per Unit, Vested	[2]	42.91
Weighted-average Fair Value per Unit, Nonvested as of Current period end		$ 45.39
Performance period		3 years
Risk free interest rate period		3 years
[1]	Performance share units granted to certain executive and nonexecutive officers and other eligible employees 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 Accoun37

Summary Of Significant Accounting Policies (Schedule Of Excise Taxes) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Accounting Policies [Line Items]
Excise tax expense	$ 50	$ 57
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	30	34
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$ 20	$ 23

Rate And Regulatory Matters (Na

Rate And Regulatory Matters (Narrative-Missouri) (Detail)	3 Months Ended	12 Months Ended
	Mar. 31, 2016USD ($)mi	Dec. 31, 2015USD ($)MWh
Electric Distribution | Union Electric Company | MEEIA
Rate And Regulatory Matters [Line Items]
Incentive Award if Energy Efficiency Goals Are Achieved	$ 19,000,000
Achieved Percentage of Energy Efficiency Earnings For Incentive Award	100.00%
Incentive Award if Energy Efficiency Goals Are Achieved, Period of Recognition	3 years
Mark Twain Project | Ameren Transmission Company of Illinois
Rate And Regulatory Matters [Line Items]
Transmission Line Miles | mi	95
Noranda [Member] | Union Electric Company
Rate And Regulatory Matters [Line Items]
Revenue Requirement		$ 78,000,000
Noranda Annual Megawatthours | MWh		4,200,000
Noranda Operating Capacity		100.00%
Noranda Summer Base Rate		$ 45.78
Noranda Winter Base Rate		$ 31.11
Number of Months to complete MoPSC Electric Service Proceeding	11 months

Rate And Regulatory Matters (39

Rate And Regulatory Matters (Narrative-Illinois) (Detail) - USD ($) $ in Millions	1 Months Ended
	Apr. 30, 2016	Mar. 31, 2016	Dec. 31, 2015
Rate And Regulatory Matters [Line Items]
Regulatory assets		$ 1,376	$ 1,382
Current regulatory assets		215	260
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets		777	771
Current regulatory assets		156	167
2016 IEIMA Revenue Requirement Reconciliation | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Regulatory assets		9
2015 IEIMA Revenue Requirement Reconciliation | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Regulatory assets		65
2014 IEIMA Revenue Requirement Reconciliation | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Current regulatory assets		$ 82	$ 103
Pending Rate Case | IEIMA | Ameren Illinois Company | Electric Distribution | Subsequent Event
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 14

Rate And Regulatory Matters (40

Rate And Regulatory Matters (Narrative-Federal) (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Feb. 28, 2015	Mar. 31, 2016	Dec. 31, 2015
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		$ 87	$ 80
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		$ 55	$ 39
Midwest Independent Transmission System Operator, Inc | Administrative Law Judge [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Return on Equity, Percentage		10.32%
Midwest Independent Transmission System Operator, Inc | Pending Ferc Case
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Return on Equity, Percentage		12.38%
Customer Requested Rate on Equity	8.67%	9.15%
Current regulatory liabilities		$ 55
Incentive adder to FERC allowed base return on common equity		0.50%
Midwest Independent Transmission System Operator, Inc | Pending Ferc Case | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		$ 37

Short-Term Debt And Liquidity (

Short-Term Debt And Liquidity (Narrative) (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Utilities
Line of Credit Facility [Line Items]
Short Term Debt, Weighted Average Interest Rate During Period	0.47%	0.08%
Credit Agreements 2012
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,500,000,000
Actual debt-to-capital ratio	0.52
Line of Credit Facility, Covenant Terms, Default Provision, Maximum Indebtedness	$ 75,000,000
Credit Agreements 2012 | Maximum
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.65
Interest Coverage Requirement	200.00%
Credit Agreements 2012 | Union Electric Company | Maximum
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.65
Credit Agreements 2012 | Ameren Illinois Company | Maximum
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.65
Missouri Credit Agreement 2012 | Union Electric Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.49
Illinois Credit Agreement 2012 | Ameren Illinois Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.46

Short-Term Debt And Liquidity42

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper outstanding) (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Line of Credit Facility [Line Items]
Commercial paper outstanding	$ 581	$ 301
Ameren (parent)
Line of Credit Facility [Line Items]
Commercial paper outstanding	416	301
Union Electric Company
Line of Credit Facility [Line Items]
Commercial paper outstanding	165	0
Ameren Illinois Company
Line of Credit Facility [Line Items]
Commercial paper outstanding	$ 0	$ 0

Short-Term Debt And Liquidity43

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 417	$ 852
Weighted Average Interest Rate		0.81%	0.53%
Peak Short Term Borrowings	[1]	$ 581	$ 955
Peak Short Term Borrowings Interest Rate		0.95%	0.70%
Ameren (parent)
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 349	$ 691
Weighted Average Interest Rate		0.82%	0.55%
Peak Short Term Borrowings	[1]	$ 482	$ 815
Peak Short Term Borrowings Interest Rate		0.95%	0.70%
Union Electric Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 68	$ 151
Weighted Average Interest Rate		0.80%	0.49%
Peak Short Term Borrowings	[1]	$ 208	$ 243
Peak Short Term Borrowings Interest Rate		0.85%	0.60%
Ameren Illinois Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 0	$ 10
Weighted Average Interest Rate		0.00%	0.44%
Peak Short Term Borrowings	[1]	$ 0	$ 39
Peak Short Term Borrowings Interest Rate		0.00%	0.60%
[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.

Long-Term Debt And Equity Fin44

Long-Term Debt And Equity Financings (Narrative) (Detail)	3 Months Ended
	Mar. 31, 2016USD ($)
Ameren Missouri And Ameren Illinois
Long-Term Debt And Equity Financings [Line Items]
Assumed interest rate	5.00%
Dividend rate	6.00%
Secured Debt | Senior Secured Notes540 Due2016 [Member] | Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount	$ 260,000,000
Debt Instrument, Interest Rate, Stated Percentage	5.40%
Federal Energy Regulatory Commission Restriction [Member] | Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Common stock equity to total capitalization	51.00%
Federal Energy Regulatory Commission Restriction [Member] | Ameren Illinois Company | Minimum
Long-Term Debt And Equity Financings [Line Items]
Common stock equity to total capitalization	30.00%

Long-Term Debt And Equity Fin45

Long-Term Debt And Equity Financings (Schedule Of Covered Ratio) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2016USD ($)
Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Bonds Issuable	$ 3,809	[1]
Preferred Stock Issuable	2,128
Retired bond capacity	1,206
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Bonds Issuable	3,642	[1],[2]
Preferred Stock Issuable	203	[3]
Retired bond capacity	$ 204
Minimum Required Ratio [Member] | Minimum | Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Restricted payment interest coverage ratio, Actual	2	[4]
Dividend Coverage Ratio	2.5	[5]
Minimum Required Ratio [Member] | Minimum | Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Restricted payment interest coverage ratio, Actual	2	[4]
Dividend Coverage Ratio	1.5	[5]
Actual Ratio [Member] | Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Restricted payment interest coverage ratio, Actual	3.8
Dividend Coverage Ratio	95.8
Actual Ratio [Member] | Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Restricted payment interest coverage ratio, Actual	6.4
Dividend Coverage Ratio	2.6
[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 $1,206 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 Illinois Credit Agreement.
[3]	Preferred stock issuable is restricted by the amount of preferred stock that is currently authorized by Ameren Illinois’ articles of incorporation.
[4]	Coverage required on the annual interest charges on first mortgage bonds outstanding and to be issued. Coverage is not required in certain cases when additional first mortgage bonds are issued on the basis of retired bonds.
[5]	Coverage required on the annual dividend on preferred stock outstanding and to be issued, as required in the respective company’s articles of incorporation.

Other Income and Expenses (Deta

Other Income and Expenses (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	[1]	$ 8	$ 5
Interest income on industrial development revenue bonds	[1]	7	7
Interest and dividend income	[1]	4	4
Other	[1]	1	3
Total miscellaneous income	[1]	20	19
Donations	[1]	5	8
Other	[1]	2	3
Total miscellaneous expense	[1]	7	11
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		7	4
Interest income on industrial development revenue bonds		7	7
Other		1	0
Total miscellaneous income		15	11
Donations		1	2
Other		1	1
Total miscellaneous expense		2	3
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		1	1
Interest and dividend income		4	4
Other		0	2
Total miscellaneous income		5	7
Donations		4	3
Other		1	2
Total miscellaneous expense		$ 5	$ 5
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Derivative Financial Instrume47

Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) lb in Thousands, gal in Millions, MWh in Millions, MMBTU in Millions		Mar. 31, 2016MWhgalMMBTUlb	Dec. 31, 2015MWhgalMMBTUlb
Fuel Oils
Derivative [Line Items]
Quantity | gal	[1]	29	35
Natural Gas
Derivative [Line Items]
Quantity | MMBTU		179	181
Power
Derivative [Line Items]
Quantity | MWh		10	11
Uranium
Derivative [Line Items]
Quantity | lb		428	494
Union Electric Company | Fuel Oils
Derivative [Line Items]
Quantity | gal	[1]	29	35
Union Electric Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU		29	30
Union Electric Company | Power
Derivative [Line Items]
Quantity | MWh		1	1
Union Electric Company | Uranium
Derivative [Line Items]
Quantity | lb		428	494
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU		150	151
Ameren Illinois Company | Power
Derivative [Line Items]
Quantity | MWh		9	10
[1]	of ultra-low-sulfur diesel products.

Derivative Financial Instrume48

Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions		Mar. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Derivative assets	[1]	$ 7	$ 18
Derivative liabilities	[2]	281	264
Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		19	22
Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		6	7
Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets			1
Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets		1	1
Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		43	38
Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		22	26
Power | Other Current Assets
Derivative [Line Items]
Derivative assets		6	16
Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		15	13
Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		172	157
Uranium | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		2	1
Uranium | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		2
Union Electric Company
Derivative [Line Items]
Derivative assets	[1]	6	17
Derivative liabilities	[2]	43	44
Union Electric Company | Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		19	22
Union Electric Company | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		6	7
Union Electric Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets		0	1
Union Electric Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		7	6
Union Electric Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		7	8
Union Electric Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets		6	16
Union Electric Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		0	0
Union Electric Company | Uranium | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		2	1
Union Electric Company | Uranium | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		2
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	[1]	1	1
Derivative liabilities	[2]	238	220
Ameren Illinois Company | Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		0	0
Ameren Illinois Company | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		0	0
Ameren Illinois Company | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets			1
Ameren Illinois Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets		1
Ameren Illinois Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		0	0
Ameren Illinois Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		15	18
Ameren Illinois Company | Natural Gas | Mark To Market Derivative Liabilities
Derivative [Line Items]
Derivative liabilities		36	32
Ameren Illinois Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets		0	0
Ameren Illinois Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		0	0
Ameren Illinois Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		172	157
Ameren Illinois Company | Power | Mark To Market Derivative Liabilities
Derivative [Line Items]
Derivative liabilities		15	13
Ameren Illinois Company | Uranium | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		0	$ 0
Ameren Illinois Company | Uranium | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		$ 0
[1]	Because all contracts qualifying for hedge accounting receive regulatory deferral, the cumulative amount of pretax net gains on all derivative instruments is deferred as a regulatory liability.
[2]	Because all contracts qualifying for hedge accounting receive regulatory deferral, the cumulative amount of pretax net losses on all derivative instruments is deferred as a regulatory asset.

Derivative Financial Instrume49

Derivative Financial Instruments (Offsetting Derivative Assets and Liabilities) (Details) - Commodity Contract - USD ($) $ in Millions		Mar. 31, 2016		Dec. 31, 2015
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet		$ 7	[1]	$ 18	[2]
Derivative Instruments		3		1
Derivative, Collateral, Obligation to Return Cash	[3]	0		0
Net Amount		4		17
Gross Amounts Recognized in the Balance Sheet		281	[1]	264	[2]
Derivative Instruments		3		1
Cash Collateral Received/Posted	[3]	10		11
Net Amount		268		252
Union Electric Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet		6		17	[2]
Derivative Instruments		2		1
Derivative, Collateral, Obligation to Return Cash	[3]	0		0
Net Amount		4		16
Gross Amounts Recognized in the Balance Sheet		43	[1]	44	[2]
Derivative Instruments		2		1
Cash Collateral Received/Posted	[3]	8		8
Net Amount		33		35
Ameren Illinois Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet		1		1
Derivative Instruments		1		0
Derivative, Collateral, Obligation to Return Cash	[3]	0		0
Net Amount		0		1
Gross Amounts Recognized in the Balance Sheet		238	[1]	220	[2]
Derivative Instruments		1		0
Cash Collateral Received/Posted	[3]	2		3
Net Amount		$ 235		$ 217
[1]	The derivative asset and liability balances are presented net of counterparty credit considerations.
[2]	The derivative asset and liability balances are presented net of counterparty credit considerations.
[3]	Cash collateral received reduces gross asset balances and is included in “Other current liabilities” and “Other deferred credits and liabilities” on the balance sheet. Cash collateral posted reduces gross liability balances and is included in “Other current assets” and “Other assets” on the balance sheet.

Derivative Financial Instrume50

Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Detail) $ in Millions	Mar. 31, 2016USD ($)
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	$ 165	[1]
Cash Collateral Posted	9
Potential Aggregate Amount of Additional Collateral Required	147	[2]
Union Electric Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	88	[1]
Cash Collateral Posted	7
Potential Aggregate Amount of Additional Collateral Required	76	[2]
Ameren Illinois Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	77	[1]
Cash Collateral Posted	2
Potential Aggregate Amount of Additional Collateral Required	$ 71	[2]
[1]	a)Before consideration of master netting arrangements or similar agreements and including NPNS and other accrual contract exposures.
[2]	As collateral requirements with certain counterparties are based on master netting arrangements or similar agreements, 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) $ in Millions		3 Months Ended		12 Months Ended
		Mar. 31, 2016USD ($)$ / credit$ / MMBTU$ / lb$ / MWh$ / MW		Dec. 31, 2015USD ($)$ / credit$ / MMBTU$ / lb$ / MWh$ / MW
Natural Gas | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	[1],[2]	0.40%		0.40%
Nodal basis	[3]	(0.60)		(0.10)
Credit risk	[1],[2]	0.40%		0.40%
Natural Gas | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	[1],[2]	6.00%		12.00%
Nodal basis	[3]	0		0
Credit risk	[1],[2]	0.40%		0.40%
Natural Gas | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	[1],[2]	0.93%		7.00%
Nodal basis	[3]	(0.50)		(0.10)
Credit risk	[1],[2]	0.40%		0.40%
Natural Gas | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities	[3]			35.00%
Nodal basis	[2]			(0.30)
Natural Gas | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities	[3]			55.00%
Nodal basis	[2]			0
Natural Gas | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities	[3]			45.00%
Nodal basis	[2]			(0.20)
Natural Gas | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	[4]	$ 0		$ 1
Derivative liabilities | $	[4]	$ (1)		$ (1)
Power | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	[1],[2]	0.55%		0.86%
Nodal basis | $ / MWh		(10)	[5]	(10)	[6]
Credit risk	[1],[2]	0.40%		0.40%
Average forward peak and off-peak pricing | $ / MWh	[6]	18		22
Estimated auction price | $ / MW	[3]	(818)		(270)
Power | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	[1],[2]	0.55%		0.86%
Nodal basis | $ / MWh		(1)	[5]	(1)	[6]
Credit risk	[1],[2]	0.40%		0.40%
Average forward peak and off-peak pricing | $ / MWh	[6]	37		39
Estimated auction price | $ / MW	[3]	1,062		2,057
Power | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	[1],[2]	0.55%		0.86%
Nodal basis | $ / MWh		(2)	[5]	(3)	[6]
Credit risk	[1],[2]	0.40%		0.40%
Average forward peak and off-peak pricing | $ / MWh	[6]	27		29
Estimated auction price | $ / MW	[3]	67		211
Power | Fundamental Energy Production Model | Minimum
Fair Value Inputs [Abstract]
Escalation rate	[3],[7]	4.00%		3.00%
Estimated future gas prices	[3]	3		3
Power | Fundamental Energy Production Model | Maximum
Fair Value Inputs [Abstract]
Escalation rate	[3],[7]	4.00%		3.00%
Estimated future gas prices	[3]	5		4
Power | Fundamental Energy Production Model | Weighted Average
Fair Value Inputs [Abstract]
Escalation rate	[3],[7]	4.00%		3.00%
Estimated future gas prices	[3]	4		4
Power | Contract Price Allocation | Minimum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit		5	[5]	5	[3]
Power | Contract Price Allocation | Maximum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit		7	[5]	7	[3]
Power | Contract Price Allocation | Weighted Average
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit		6	[5]	6	[3]
Power | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	[4],[8]	$ 6		$ 16
Power | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	[4],[8]	$ (187)		$ (170)
Uranium | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Credit risk	[1],[2]	0.40%		0.40%
Average forward pricing | $ / lb	[3]	28		35
Uranium | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Credit risk	[1],[2]	0.40%		0.40%
Average forward pricing | $ / lb	[3]	33		42
Uranium | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Credit risk	[1],[2]	0.40%		0.40%
Average forward pricing | $ / lb	[3]	31		37
Uranium | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities	[3]	2000.00%		20.00%
Uranium | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities	[3]	2000.00%		20.00%
Uranium | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities	[3]	2000.00%		20.00%
Uranium | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	[4]	$ 0		$ 0
Uranium | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	[4]	$ (4)		$ (1)
[1]	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.
[2]	Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.
[3]	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]	Not applicable.
[6]	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.
[7]	Escalation rate applies to power prices 2031 and beyond for March 31, 2016 and to power prices 2026 and beyond for December 31, 2015.
[8]	Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2020. Valuations beyond 2020 use fundamentally modeled pricing by month for peak and off-peak demand.

Fair Value Measurements (Sche52

Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions		Mar. 31, 2016		Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		$ 566	[1]	$ 557	[2]
Assets fair value		573	[3]	575	[4]
Excluded receivables, payables, and accrued income, net		1		(1)
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		566	[1]	557	[2]
Assets fair value		572	[3]	574	[4]
Excluded receivables, payables, and accrued income, net		1		(1)
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		7	[3]	18	[4]
Derivative liabilities		281	[3]	264	[4]
Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		6		17	[4]
Derivative liabilities		43	[3]	44	[4]
Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1		1
Derivative liabilities		238	[3]	220	[4]
Cash and cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		2		4
Cash and cash equivalents | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		2		4
Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		369		364
Equity Securities | U.S. large capitalization | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		369		364
Debt Securities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		56		58
Debt Securities | Corporate bonds | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		56		58
Debt Securities | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		121		109
Debt Securities | US treasury and government securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		121		109
Debt Securities | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		18		22
Debt Securities | Other Debt Securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		18		22
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		371		368
Assets fair value		371	[3]	368	[4]
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		371		368
Assets fair value		371	[3]	368	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Derivative liabilities		26	[3]	30	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			0
Derivative liabilities		26	[3]	29	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0	[3]	1
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		4
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		4
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		369		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		369		364
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Debt Securities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Debt Securities | Corporate bonds | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Debt Securities | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Debt Securities | US treasury and government securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Debt Securities | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Debt Securities | Other Debt Securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		195		189
Assets fair value		196	[3]	190	[4]
Significant Other Observable Inputs (Level 2) | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		195		189
Assets fair value		195	[3]	189	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	1	[4]
Derivative liabilities		63	[3]	62	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			0
Derivative liabilities		13	[3]	13	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		50	[3]	49	[4]
Significant Other Observable Inputs (Level 2) | Cash and cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Observable Inputs (Level 2) | Cash and cash equivalents | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Observable Inputs (Level 2) | Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Observable Inputs (Level 2) | Equity Securities | U.S. large capitalization | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Observable Inputs (Level 2) | Debt Securities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		56		58
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		56		58
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		121		109
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		121		109
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		18		22
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		18		22
Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Assets fair value		6	[3]	17	[4]
Significant Other Unobservable Inputs (Level 3) | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Assets fair value		6	[3]	17	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		6	[3]	17	[4]
Derivative liabilities		192	[3]	172	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			17
Derivative liabilities		4	[3]	2	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		188	[3]	170	[4]
Significant Other Unobservable Inputs (Level 3) | Cash and cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Unobservable Inputs (Level 3) | Cash and cash equivalents | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Unobservable Inputs (Level 3) | Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Unobservable Inputs (Level 3) | Equity Securities | U.S. large capitalization | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Unobservable Inputs (Level 3) | Debt Securities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Unobservable Inputs (Level 3) | Debt Securities | Corporate bonds | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Unobservable Inputs (Level 3) | Debt Securities | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Unobservable Inputs (Level 3) | Debt Securities | US treasury and government securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Unobservable Inputs (Level 3) | Debt Securities | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Unobservable Inputs (Level 3) | Debt Securities | Other Debt Securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Uranium | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		4	[3]	1	[4]
Uranium | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		4	[3]	1	[4]
Uranium | 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		0	[3]	0	[4]
Uranium | 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		0	[3]	0	[4]
Uranium | Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0	[3]	0	[4]
Uranium | Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0	[3]	0	[4]
Uranium | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		4	[3]	1	[4]
Uranium | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		4	[3]	1	[4]
Fuel Oils | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		25	[3]	29	[4]
Fuel Oils | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		25	[3]	29	[4]
Fuel Oils | 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		25	[3]	29	[4]
Fuel Oils | 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		25	[3]	29	[4]
Fuel Oils | Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0	[3]	0	[4]
Fuel Oils | Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0	[3]	0	[4]
Fuel Oils | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0	[3]	0	[4]
Fuel Oils | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0	[3]	0	[4]
Power | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		6	[3]	16	[4]
Derivative liabilities		187	[3]	170	[4]
Power | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		6	[3]	16	[4]
Power | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		187	[3]	170	[4]
Power | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Derivative liabilities		0	[3]	0	[4]
Power | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Power | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0	[3]	0	[4]
Power | Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Derivative liabilities		0	[3]	0	[4]
Power | Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Power | Significant Other Observable Inputs (Level 2) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0	[3]	0
Power | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		6	[3]	16	[4]
Derivative liabilities		187	[3]	170	[4]
Power | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		6	[3]	16	[4]
Power | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		187	[3]	170	[4]
Natural Gas | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	2	[4]
Derivative liabilities		65	[3]	64	[4]
Natural Gas | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			1
Derivative liabilities		14	[3]	14	[4]
Natural Gas | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	1	[4]
Derivative liabilities		51	[3]	50	[4]
Natural Gas | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Derivative liabilities		1	[3]	1	[4]
Natural Gas | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			0
Derivative liabilities		1	[3]	0	[4]
Natural Gas | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Derivative liabilities		0	[3]	1	[4]
Natural Gas | Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	1	[4]
Derivative liabilities		63	[3]	62	[4]
Natural Gas | Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			0
Derivative liabilities		13	[3]	13	[4]
Natural Gas | Significant Other Observable Inputs (Level 2) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	1	[4]
Derivative liabilities		50	[3]	49	[4]
Natural Gas | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	1	[4]
Derivative liabilities		1	[3]	1	[4]
Natural Gas | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			1
Derivative liabilities		0	[3]	1	[4]
Natural Gas | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Derivative liabilities		$ 1	[3]	$ 0	[4]
[1]	Balance excludes $1 million of receivables, payables, and accrued income, net.
[2]	Balance excludes $(1) million of receivables, payables, and accrued income, net.
[3]	The derivative asset and liability balances are presented net of counterparty credit considerations.
[4]	The derivative asset and liability balances are presented net of counterparty credit considerations.

Fair Value Measurements (Sche53

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 ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Fuel Oils
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		$ (6)
Included in regulatory assets/liabilities		(1)
Settlements		1
Ending balance		(6)
Change in unrealized gains (losses) related to assets/liabilities held at period end		(3)
Uranium
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ (1)	(2)
Included in regulatory assets/liabilities	(3)	1
Ending balance	(4)	(1)
Change in unrealized gains (losses) related to assets/liabilities held at period end	(3)	1
Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	(1)
Purchases		1
Settlements	(1)
Ending balance	(1)	0
Change in unrealized gains (losses) related to assets/liabilities held at period end	0	0
Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(154)	(133)
Included in regulatory assets/liabilities	(24)	(27)
Settlements	(3)	0
Ending balance	(181)	(160)
Change in unrealized gains (losses) related to assets/liabilities held at period end	(19)	(24)
Union Electric Company | Fuel Oils
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance		(6)
Included in regulatory assets/liabilities		(1)
Settlements		1
Ending balance		(6)
Change in unrealized gains (losses) related to assets/liabilities held at period end		(3)
Union Electric Company | Uranium
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(1)	(2)
Included in regulatory assets/liabilities	(3)	1
Ending balance	(4)	(1)
Change in unrealized gains (losses) related to assets/liabilities held at period end	(3)	1
Union Electric Company | Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	(1)
Purchases		0
Settlements	0
Ending balance	0	(1)
Change in unrealized gains (losses) related to assets/liabilities held at period end	0	0
Union Electric Company | Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	16	9
Included in regulatory assets/liabilities	(3)	(2)
Settlements	(7)	(3)
Ending balance	6	4
Change in unrealized gains (losses) related to assets/liabilities held at period end	0	0
Ameren Illinois Company | Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	0
Purchases		1
Settlements	(1)
Ending balance	(1)	1
Change in unrealized gains (losses) related to assets/liabilities held at period end	0	0
Ameren Illinois Company | Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(170)	(142)
Included in regulatory assets/liabilities	(21)	(25)
Settlements	4	3
Ending balance	(187)	(164)
Change in unrealized gains (losses) related to assets/liabilities held at period end	$ (19)	$ (24)

Fair Value Measurements (Sche54

Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Long-Term Debt And Preferred Stock) (Detail) - USD ($) $ in Millions		Mar. 31, 2016	Dec. 31, 2015
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)	[1]	$ 7,744	$ 7,814
Preferred stock	[1]	126	125
Fair Value | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)		4,280	4,449
Preferred stock		75	75
Fair Value | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)		2,744	2,665
Preferred stock		51	50
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)	[1]	7,016	7,275
Preferred stock	[1]	142	142
Carrying Amount | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)		3,851	4,110
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)		2,472	2,471
Preferred stock		$ 62	$ 62
[1]	Preferred stock is recorded in “Noncontrolling Interests” on the consolidated balance sheet.

Related Party Transactions (Sch

Related Party Transactions (Schedule of Related Party Transactions) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016		Mar. 31, 2015
Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues		$ 15		$ 7
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues		1		1
Purchased Power | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses		9		2
Ameren Missouri Power Supply Agreements with Ameren Illinois | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues		9		1
Ameren Missouri and Ameren Illinois Rent and Facility Services | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues		6		6
Ameren Missouri and Ameren Illinois Rent and Facility Services | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues		1		1
Ameren Missouri and Ameren Illinois miscellaneous support services [Member] | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	[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
Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses		9		1
Ameren Illinois Transmission Agreements with ATXI | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses		1	[1]	1
Ameren Services Support Services Agreement | Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses		34		34
Ameren Services Support Services Agreement | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses		31		29
Related Party Money Pool Interest | Union Electric Company
Related Party Transaction [Line Items]
Interest Charges	[1]	1		1
Related Party Money Pool Interest | Ameren Illinois Company
Related Party Transaction [Line Items]
Interest Charges	[1]	$ 1		$ 1
[1]	Amount less than $1 million.

Related Party Transactions Narr

Related Party Transactions Narrative (Details) - April 2016 Procurement [Member] - Ameren Illinois Company - Ameren Illinois Power Supply Agreements with Ameren Missouri	3 Months Ended
	Mar. 31, 2016MWh$ / MWh
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power | MWh	375,200
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate | $ / MWh	34.71

Commitments And Contingencies57

Commitments And Contingencies (Callaway Energy Center) (Detail)	3 Months Ended
	Mar. 31, 2016USD ($)
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	127,000,000
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	13,489,000,000	[1]
Maximum Assessments for Single Incidents	127,000,000
Public Liability And Nuclear Worker Liability - American Nuclear Insurers
Commitments And Contingencies [Line Items]
Maximum Coverages	375,000,000
Maximum Assessments for Single Incidents	0
Public Liability And Nuclear Worker Liability - Pool Participation
Commitments And Contingencies [Line Items]
Maximum Coverages	13,114,000,000	[2]
Maximum Assessments for Single Incidents	127,000,000	[3]
Property Damage
Commitments And Contingencies [Line Items]
Maximum Coverages	3,200,000,000
Maximum Assessments for Single Incidents	30,000,000
Property Damage - Nuclear Electric Insurance Ltd
Commitments And Contingencies [Line Items]
Maximum Coverages	2,750,000,000	[4]
Maximum Assessments for Single Incidents	30,000,000	[5]
Property Damage European Mutual Association for Nuclear Insurance
Commitments And Contingencies [Line Items]
Maximum Coverages	450,000,000	[6]
Replacement Power - Nuclear Electric Insurance Ltd
Commitments And Contingencies [Line Items]
Maximum Coverages	490,000,000	[7]
Maximum Assessments for Single Incidents	7,000,000	[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
Commitments And Contingencies [Line Items]
Maximum Coverages	2,300,000,000
Aggregate nuclear power industry insurance policy limit for losses from terrorist attacks within twelve month period	1,830,000,000
Radiation Event
Commitments And Contingencies [Line Items]
Maximum Coverages	2,250,000,000
Additional Insurance aggregate maximum coverage	$ 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 $127 million per incident for each licensed reactor it operates with a maximum of $19 million per incident to be paid in a calendar year for each reactor. This limit is subject to change to account for the effects of inflation and changes in the number of licensed reactors.
[2]	Provided through mandatory participation in an industrywide retrospective premium assessment program.
[3]	Retrospective premium under the Price-Anderson Act. This is subject to retrospective assessment with respect to a covered loss in excess of $375 million in the event of an incident at any licensed United States commercial reactor, payable at $19 million per year.
[4]	NEIL provides $2.25 billion in property damage, decontamination, and premature decommissioning insurance for radiation events. NEIL provides $2.3 billion in property damage for nonradiation events. An additional $500 million is provided for radiation events only. The total provided by NEIL for radiation and nonradiation events is $2.75 billion and $2.3 billion, respectively.
[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 $450 million in excess of the $2.75 billion and $2.3 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 Contingencies58

Commitments And Contingencies (Other Obligations) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2016USD ($)MWh
Long-term Purchase Commitment [Line Items]
Total Other Obligations	$ 4,566
Union Electric Company
Long-term Purchase Commitment [Line Items]
Total Other Obligations	2,786
Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Total Other Obligations	$ 1,729
April 2016 Procurement [Member] | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Amount of Megawatthours | MWh	3,609,800
Long-term Purchase Commitment, Amount	$ 105

Commitments And Contingencies59

Commitments And Contingencies (Environmental Matters) (Detail) $ in Millions	Mar. 31, 2016USD ($)site
Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 600
Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 700
Union Electric Company
Loss Contingencies [Line Items]
Number of Energy Center Scrubbers	2
Manufactured Gas Plant
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 224
Manufactured Gas Plant | Ameren Illinois Company
Loss Contingencies [Line Items]
Number of remediation sites | site	44
Accrual for environmental loss contingencies	$ 224
Manufactured Gas Plant | Ameren Illinois Company | Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	224
Manufactured Gas Plant | Ameren Illinois Company | Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	313
Former Coal Ash Landfill | Ameren Illinois Company
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	0.5
Former Coal Ash Landfill | Ameren Illinois Company | Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	0.5
Former Coal Ash Landfill | Ameren Illinois Company | Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	6
Other Environmental | Ameren Illinois Company
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	0.7
Former Coal Tar Distillery | Union Electric Company
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	2
Former Coal Tar Distillery | Union Electric Company | Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	2
Former Coal Tar Distillery | Union Electric Company | Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	5
Sauget Area Two | Union Electric Company
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	1
Sauget Area Two | Union Electric Company | Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	1
Sauget Area Two | Union Electric Company | Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	2.5
Substation in St Charles, Missouri | Union Electric Company
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 0.6

Callaway Energy Center (Narrati

Callaway Energy Center (Narrative) (Detail) - Nuclear Plant $ in Millions	3 Months Ended
	Mar. 31, 2016USD ($)mill
Nuclear Waste Matters [Line Items]
Number of mills charged for NWF fee | mill	1
Annual decommissioning costs included in costs of service | $	$ 7

Retirement Benefits (Narrative)

Retirement Benefits (Narrative) (Detail) $ in Millions	Mar. 31, 2016USD ($)
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	$ 280
Minimum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, estimated future employer contributions in each of the next five years	40
Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, estimated future employer contributions in each of the next five years	$ 70

Retirement Benefits (Components

Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		$ 20	$ 24
Interest cost		47	44
Expected return on plan assets		(63)	(62)
Prior service cost (benefit)			0
Actuarial loss		9	18
Net periodic benefit cost	[1],[2]	13	24
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		5	5
Interest cost		12	12
Expected return on plan assets		(18)	(17)
Prior service cost (benefit)		(1)	(1)
Actuarial loss		(3)	1
Net periodic benefit cost	[1],[2]	$ (5)	$ 0
[1]	(a)Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates
[2]	(b)Includes amounts for Ameren registrants and nonregistrant subsidiaries.

Retirement Benefits (Summary Of

Retirement Benefits (Summary Of Benefit Plan Costs Incurred) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	[1],[2]	$ 13	$ 24
Pension Plan | Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost		8	15
Pension Plan | Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost		5	9
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	[1],[2]	(5)	0
Other Postretirement Benefit Plan, Defined Benefit | 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 | Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost		$ (4)	$ (1)
[1]	(a)Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates
[2]	(b)Includes amounts for Ameren registrants and nonregistrant subsidiaries.

Divestiture Transactions and 64

Divestiture Transactions and Discontinued Operations Divestiture Transactions and Discontinued Operations (Narrative) (Details) $ in Millions	1 Months Ended
	Jan. 31, 2014USD ($)
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

Segment Information (Schedule O

Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) $ in Millions		3 Months Ended
		Mar. 31, 2016USD ($)	Mar. 31, 2015USD ($)	Dec. 31, 2015USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments		2
External revenues		$ 1,434	$ 1,556
Net income (loss) attributable to Ameren Common Shareholders from continuing operations		105	108
Total assets		23,385		$ 23,640
Assets of discontinued operations		$ 14		14
Union Electric Company
Segment Reporting Information [Line Items]
Number of Reportable Segments		1
External revenues		$ 726	793
Intersegment revenues		15	7
Net income (loss) attributable to Ameren Common Shareholders from continuing operations		14	41
Total assets		$ 13,443		13,851
Ameren Illinois Company
Segment Reporting Information [Line Items]
Number of Reportable Segments		1
External revenues		$ 676	744
Intersegment revenues		1	1
Net income (loss) attributable to Ameren Common Shareholders from continuing operations		59	53
Total assets		8,944		8,903
Other
Segment Reporting Information [Line Items]
External revenues		32	19
Intersegment revenues		0	1
Net income (loss) attributable to Ameren Common Shareholders from continuing operations		32	14
Total assets		1,214		1,139
Intersegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues		(16)	(9)
Total assets		(230)		(267)
Consolidated, Continuing Operations
Segment Reporting Information [Line Items]
External revenues		1,434	1,556
Net income (loss) attributable to Ameren Common Shareholders from continuing operations		105	$ 108
Total assets	[1]	$ 23,371		$ 23,626
[1]	Excludes total assets from discontinued operations of $14 million as of March 31, 2016, and December 31, 2015.