Ameren (AEE) 10-Q 6 Nov 15 2015 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2015	Oct. 30, 2015
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2015
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	Q3
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	Sep. 30, 2015
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	Q3
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	Sep. 30, 2015
Document Fiscal Year Focus	2,015
Document Fiscal Period Focus	Q3
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		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Operating Revenues:
Electric		$ 1,700	$ 1,523	$ 4,093	$ 3,864
Gas		133	147	697	819
Total operating revenues		1,833	1,670	4,790	4,683
Operating Expenses:
Fuel		259	236	670	638
Purchased power		153	114	393	340
Gas purchased for resale		38	49	320	432
Other operations and maintenance		428	402	1,256	1,231
Provision for Callaway construction and operating license		0	0	69	0
Depreciation and amortization		201	187	594	551
Taxes other than income taxes		128	121	369	362
Total operating expenses		1,207	1,109	3,671	3,554
Operating Income		626	561	1,119	1,129
Other Income and Expenses:
Miscellaneous income	[1]	19	21	54	60
Miscellaneous expense	[1]	5	7	22	20
Total other income (expense)		14	14	32	40
Interest Charges		87	85	264	266
Income Before Income Taxes		553	490	887	903
Income Taxes		208	194	333	357
Income from Continuing Operations		345	296	554	546
Income (Loss) from Discontinued Operations, Net of Taxes (Note 12)		0	(1)	52	(3)
Net Income (Loss)		345	295	606	543
Pension and other postretirement benefit plan activity, net of income taxes (benefit)		0	0	4	3
Comprehensive Income		343	293	605	541
Less: Net Income (Loss) Attributable to Noncontrolling Interests:
Net Income from Continuing Operations Attributable to Noncontrolling Interests		2	2	5	5
Net Income (Loss):
Continuing Operations		343	294	549	541
Discontinued Operations		0	(1)	52	(3)
Net Income (Loss)		$ 343	$ 293	$ 601	$ 538
Earnings Per Share, Basic [Abstract]
Continuing Operations		$ 1.42	$ 1.21	$ 2.27	$ 2.23
Discontinued Operations		0	0	0.21	(0.01)
Earnings Per Share, Basic		1.42	1.21	2.48	2.22
Earnings Per Share, Diluted [Abstract]
Continuing Operations		1.41	1.20	2.26	2.21
Discontinued Operations		0	0	0.21	(0.01)
Earnings Per Share, Diluted		1.41	1.20	2.47	2.20
Dividends per Common Share		$ 0.41	$ 0.40	$ 1.23	$ 1.20
Average Common Shares Outstanding - Basic		242.6	242.6	242.6	242.6
Average Common Shares Outstanding - Diluted		243.9	244.3	243.8	244.3
Union Electric Company
Operating Revenues:
Electric		$ 1,151	$ 1,076	$ 2,752	$ 2,696
Gas		19	21	101	117
Other Revenue, Net		1		2	1
Total operating revenues		1,171	1,097	2,855	2,814
Operating Expenses:
Fuel		259	236	670	638
Purchased power		29	27	87	91
Gas purchased for resale		5	7	43	58
Other operations and maintenance		233	226	673	672
Provision for Callaway construction and operating license		0	0	69	0
Depreciation and amortization		125	118	367	351
Taxes other than income taxes		97	89	262	248
Total operating expenses		748	703	2,171	2,058
Operating Income		423	394	684	756
Other Income and Expenses:
Miscellaneous income		14	15	37	45
Miscellaneous expense		3	4	8	10
Total other income (expense)		11	11	29	35
Interest Charges		54	53	164	159
Income Before Income Taxes		380	352	549	632
Income Taxes		140	129	205	234
Net Income (Loss)		240	223	344	398
Other Comprehensive Income		0	0	0	0
Comprehensive Income		240	223	344	398
Net Income (Loss):
Net Income (Loss)		240	223	344	398
Earnings Per Share, Diluted [Abstract]
Preferred Stock Dividends		1	1	3	3
Net Income Available to Common Stockholder		239	222	341	395
Ameren Illinois Company
Operating Revenues:
Electric		540	445	1,316	1,162
Gas		115	127	597	703
Total operating revenues		655	572	1,913	1,865
Operating Expenses:
Purchased power		128	89	317	256
Gas purchased for resale		33	43	277	374
Other operations and maintenance		202	185	606	580
Depreciation and amortization		74	66	220	193
Taxes other than income taxes		29	31	101	109
Total operating expenses		466	414	1,521	1,512
Operating Income		189	158	392	353
Other Income and Expenses:
Miscellaneous income		4	4	15	12
Miscellaneous expense		3	2	10	7
Total other income (expense)		1	2	5	5
Interest Charges		33	31	99	90
Income Before Income Taxes		157	129	298	268
Income Taxes		59	54	114	110
Net Income (Loss)		98	75	184	158
Pension and other postretirement benefit plan activity, net of income taxes (benefit)		0	0	(2)	(2)
Comprehensive Income		98	75	182	156
Net Income (Loss):
Net Income (Loss)		98	75	184	158
Earnings Per Share, Diluted [Abstract]
Preferred Stock Dividends				2	2
Net Income Available to Common Stockholder		$ 98	$ 75	$ 182	$ 156
[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		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension and other postretirement benefit plan activity, tax expense (benefit)	$ 0	$ 0	$ 4	$ 3
Ameren Illinois Company
Pension and other postretirement benefit plan activity, tax expense (benefit)	$ (1)	$ (1)	$ (2)	$ (2)

Consolidated Statement of Compr

Consolidated Statement of Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Statement of Comprehensive Income [Abstract]
Income from Continuing Operations	$ 345	$ 296	$ 554	$ 546
Other Comprehensive Income from Continuing Operations, Net of Taxes
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	0	0	4	3
Comprehensive Income from Continuing Operations	345	296	558	549
Less: Comprehensive Income from Continuing Operations Attributable to Noncontrolling Interests	2	2	5	5
Comprehensive Income from Continuing Operations Attributable to Ameren Common Stockholders	343	294	553	544
Income (Loss) from Discontinued Operations, Net of Taxes	0	(1)	52	(3)
Other Comprehensive Income from Discontinued Operations, Net of Taxes	0	0	0	0
Comprehensive Income (Loss) from Discontinued Operations Attributable to Ameren Common Stockholders	0	(1)	52	(3)
Comprehensive Income	$ 343	$ 293	$ 605	$ 541

Consolidated Statement of Comp5

Consolidated Statement of Comprehensive Income (Loss) Consolidated Statement of Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Statement of Comprehensive Income [Abstract]
Pension and other postretirement benefit plan activity, tax expense (benefit)	$ 0	$ 0	$ 4	$ 3

Consolidated Balance Sheet

Consolidated Balance Sheet - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Current Assets:
Cash and cash equivalents	$ 72	$ 5
Accounts receivable - trade (less allowance for doubtful accounts)	508	423
Unbilled revenue	234	265
Miscellaneous accounts and notes receivable	113	81
Materials and supplies	548	524
Current regulatory assets	163	295
Current accumulated deferred income taxes, net	225	352
Other current assets	103	86
Assets of discontinued operations (Note 12)	17	15
Total current assets	1,983	2,046
Property and Plant, Net	18,307	17,424
Investments and Other Assets:
Nuclear decommissioning trust fund	534	549
Goodwill	411	411
Regulatory assets	1,578	1,582
Other assets	646	664
Total investments and other assets	3,169	3,206
TOTAL ASSETS	23,459	22,676
Current Liabilities:
Current maturities of long-term debt	395	120
Short-term Debt	783	714
Accounts and wages payable	525	711
Taxes accrued	160	46
Interest accrued	103	85
Current regulatory liabilities	89	106
Other current liabilities	404	434
Liabilities of discontinued operations (Note 12)	30	33
Total current liabilities	2,489	2,249
Long-term Debt, Net	5,981	6,120
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	4,084	3,923
Accumulated deferred investment tax credits	62	64
Regulatory liabilities	1,894	1,850
Asset retirement obligations	597	396
Pension and other postretirement benefits	666	705
Other deferred credits and liabilities	530	514
Total deferred credits and other liabilities	$ 7,833	$ 7,452
Commitments and Contingencies
Stockholders' Equity:
Common Stock	$ 2	$ 2
Other paid-in capital	5,612	5,617
Retained earnings	1,405	1,103
Accumulated other comprehensive income (loss)	(5)	(9)
Stockholder's equity	7,014	6,713
Noncontrolling Interest	142	142
Total equity	7,156	6,855
TOTAL LIABILITIES AND EQUITY	23,459	22,676
Union Electric Company
Current Assets:
Cash and cash equivalents	69	1
Advances to money pool	250	0
Accounts receivable - trade (less allowance for doubtful accounts)	260	190
Accounts receivable - affiliates	8	65
Unbilled revenue	148	146
Miscellaneous accounts and notes receivable	62	35
Materials and supplies	374	347
Current regulatory assets	97	163
Other current assets	57	92
Total current assets	1,325	1,039
Property and Plant, Net	11,041	10,867
Investments and Other Assets:
Nuclear decommissioning trust fund	534	549
Regulatory assets	646	695
Other assets	395	391
Total investments and other assets	1,575	1,635
TOTAL ASSETS	13,941	13,541
Current Liabilities:
Current maturities of long-term debt	266	120
Short-term Debt	0	97
Accounts and wages payable	187	405
Accounts payable - affiliates	35	56
Taxes accrued	273	32
Interest accrued	66	58
Current regulatory liabilities	41	18
Other current liabilities	116	117
Total current liabilities	984	903
Long-term Debt, Net	3,869	3,879
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	2,858	2,806
Accumulated deferred investment tax credits	59	61
Regulatory liabilities	1,166	1,147
Asset retirement obligations	590	389
Pension and other postretirement benefits	267	274
Other deferred credits and liabilities	30	30
Total deferred credits and other liabilities	$ 4,970	$ 4,707
Commitments and Contingencies
Stockholders' Equity:
Common Stock	$ 511	$ 511
Other paid-in capital	1,784	1,569
Preferred stock	80	80
Retained earnings	1,743	1,892
Stockholder's equity	4,118	4,052
TOTAL LIABILITIES AND EQUITY	13,941	13,541
Ameren Illinois Company
Current Assets:
Cash and cash equivalents	0	1
Accounts receivable - trade (less allowance for doubtful accounts)	229	212
Accounts receivable - affiliates	1	22
Unbilled revenue	86	119
Miscellaneous accounts and notes receivable	11	9
Materials and supplies	174	177
Current regulatory assets	65	129
Current accumulated deferred income taxes, net	50	160
Other current assets	16	15
Total current assets	632	844
Property and Plant, Net	6,615	6,165
Investments and Other Assets:
Goodwill	411	411
Regulatory assets	922	883
Other assets	76	78
Total investments and other assets	1,409	1,372
TOTAL ASSETS	8,656	8,381
Current Liabilities:
Current maturities of long-term debt	129	0
Short-term Debt	0	32
Borrowings from money pool	122	15
Accounts and wages payable	251	207
Accounts payable - affiliates	36	50
Taxes accrued	7	17
Interest accrued	39	24
Customer deposits	70	77
Mark-to-market derivative liabilities	38	42
Current environmental remediation	35	52
Current regulatory liabilities	37	84
Other current liabilities	90	100
Total current liabilities	854	700
Long-term Debt, Net	2,112	2,241
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,412	1,408
Regulatory liabilities	728	703
Pension and other postretirement benefits	282	277
Environmental remediation	203	199
Other deferred credits and liabilities	224	192
Total deferred credits and other liabilities	$ 2,849	$ 2,779
Commitments and Contingencies
Stockholders' Equity:
Common Stock	$ 0	$ 0
Other paid-in capital	1,980	1,980
Preferred stock	62	62
Retained earnings	793	611
Accumulated other comprehensive income (loss)	6	8
Stockholder's equity	2,841	2,661
TOTAL LIABILITIES AND EQUITY	$ 8,656	$ 8,381

Consolidated Balance Sheet (Par

Consolidated Balance Sheet (Parenthetical) - USD ($) shares in Millions, $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Accounts receivable - trade allowance for doubtful accounts	$ 20	$ 21
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	400	400
Common stock, shares outstanding	242.6	242.6
Union Electric Company
Accounts receivable - trade allowance for doubtful accounts	$ 7	$ 8
Common stock, par value	$ 5	$ 5
Common stock, shares authorized	150	150
Common stock, shares outstanding	102.1	102.1
Ameren Illinois Company
Accounts receivable - trade allowance for doubtful accounts	$ 13	$ 13
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		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014
Cash Flows From Operating Activities:
Net income (loss)		$ 606	$ 543
(Income) loss from discontinued operations, net of taxes		(52)	3
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Provision for Callaway construction and operating license		69	0
Depreciation and amortization		582	526
Amortization of nuclear fuel		71	70
Amortization of debt issuance costs and premium/discounts		16	16
Deferred income taxes and investment tax credits, net		318	370
Allowance for equity funds used during construction	[1]	(19)	(26)
Stock-based compensation costs		20	20
Other		(8)	(9)
Changes in assets and liabilities:
Receivables		(71)	16
Materials and supplies		(23)	(34)
Accounts and wages payable		(172)	(187)
Taxes accrued		114	100
Regulatory assets and liabilities		74	(216)
Assets, other		20	44
Liabilities, other		(41)	(21)
Pension and other postretirement benefits		29	(27)
Counterparty collateral, net		0	20
Net cash provided by operating activities – continuing operations		1,533	1,208
Net cash used in operating activities – discontinued operations		(5)	(5)
Net cash provided by operating activities		1,528	1,203
Cash Flows From Investing Activities:
Capital expenditures		(1,332)	(1,310)
Nuclear fuel expenditures		(30)	(28)
Purchases of securities - nuclear decommissioning trust fund		(301)	(365)
Sales and maturities of securities - nuclear decommissioning trust fund		290	354
Proceeds from Notes Receivable		12	79
Contributions to Note Receivable		(8)	(84)
Other		7	3
Net cash used in investing activities – continuing operations		(1,362)	(1,351)
Net cash provided by investing activities – discontinued operations		0	139
Net cash used in investing activities		(1,362)	(1,212)
Cash Flows From Financing Activities:
Dividends on common stock		(298)	(291)
Dividends paid to noncontrolling interest holders		(5)	(5)
Short-term debt, net		69	385
Redemptions of Long-term Debt		(114)	(692)
Issuances of Long-term Debt		249	598
Capital issuance costs		(2)	(4)
Other		2	1
Net cash provided by (used in) financing activities – continuing operations		(99)	(8)
Net cash used in financing activities - discontinued operations		0	0
Net cash used in financing activities		(99)	(8)
Net change in cash and cash equivalents		67	(17)
Cash and cash equivalents at beginning of period		5	30
Cash and cash equivalents at end of period		72	13
Union Electric Company
Cash Flows From Operating Activities:
Net income (loss)		344	398
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Provision for Callaway construction and operating license		69	0
Depreciation and amortization		356	329
Amortization of nuclear fuel		71	70
Amortization of debt issuance costs and premium/discounts		5	5
Deferred income taxes and investment tax credits, net		88	139
Allowance for equity funds used during construction		(16)	(24)
Other		1	1
Changes in assets and liabilities:
Receivables		(51)	(76)
Materials and supplies		(26)	3
Accounts and wages payable		(177)	(151)
Taxes accrued		243	(22)
Regulatory assets and liabilities		101	(78)
Assets, other		6	44
Liabilities, other		11	30
Pension and other postretirement benefits		15	(8)
Net cash provided by operating activities		1,040	660
Cash Flows From Investing Activities:
Capital expenditures		(444)	(548)
Nuclear fuel expenditures		(30)	(28)
Purchases of securities - nuclear decommissioning trust fund		(301)	(365)
Sales and maturities of securities - nuclear decommissioning trust fund		290	354
Money pool advances, net		(250)	0
Other		(4)	(6)
Net cash used in investing activities		(739)	(593)
Cash Flows From Financing Activities:
Dividends on common stock		(490)	(268)
Dividends on preferred stock		(3)	(3)
Short-term debt, net		(97)	65
Money pool borrowings, net		0	(105)
Redemptions of Long-term Debt		(114)	(104)
Issuances of Long-term Debt		249	350
Capital contribution from parent		224	0
Capital issuance costs		(2)	(2)
Net cash used in financing activities		(233)	(67)
Net change in cash and cash equivalents		68	0
Cash and cash equivalents at beginning of period		1	1
Cash and cash equivalents at end of period		69	1
Ameren Illinois Company
Cash Flows From Operating Activities:
Net income (loss)		184	158
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization		218	190
Amortization of debt issuance costs and premium/discounts		11	10
Deferred income taxes and investment tax credits, net		108	136
Allowance for equity funds used during construction		(3)	(2)
Other		(7)	(6)
Changes in assets and liabilities:
Receivables		45	80
Materials and supplies		3	(37)
Accounts and wages payable		11	1
Taxes accrued		(10)	(5)
Regulatory assets and liabilities		(31)	(135)
Assets, other		7	6
Liabilities, other		(13)	(4)
Pension and other postretirement benefits		13	(12)
Counterparty collateral, net		2	14
Net cash provided by operating activities		541	396
Cash Flows From Investing Activities:
Capital expenditures		(620)	(633)
Other		5	6
Net cash used in investing activities		(615)	(627)
Cash Flows From Financing Activities:
Dividends on preferred stock		(2)	(2)
Short-term debt, net		(32)	189
Money pool borrowings, net		107	(40)
Redemptions of Long-term Debt		0	(163)
Issuances of Long-term Debt		0	248
Capital issuance costs		0	(2)
Proceeds from Advances for Construction		0	1
Net cash used in financing activities		73	231
Net change in cash and cash equivalents		(1)	0
Cash and cash equivalents at beginning of period		1	1
Cash and cash equivalents at end of period		$ 0	$ 1
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Summary Of Significant Accounti

Summary Of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2015
Accounting Policies [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES General Ameren, headquartered in St. Louis, Missouri, is a public utility holding company under PUHCA 2005. 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. The operating results, assets, and liabilities of the Elgin, Gibson City, Grand Tower, Meredosia, and Hutsonville energy centers have been presented separately as discontinued operations for all periods presented in this report. Unless otherwise stated, these notes to Ameren’s financial statements exclude discontinued operations for all periods presented. See Note 12 - Divestiture Transactions and Discontinued Operations in this report for additional information regarding the discontinued operations presentation and Note 16 - Divestiture Transactions and Discontinued Operations under Part II, Item 8, of the Form 10-K for additional information regarding Ameren’s divestiture of New AER in December 2013. 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 presentation of our results. The preparation of financial statements in conformity with GAAP requires management to make certain estimates and assumptions. Such estimates and assumptions affect reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the dates of financial statements, and the reported amounts of revenues and expenses during the reported periods. Actual results could differ from those estimates. The results of operations of an interim period may not give a true indication of results that may be expected for a full year. These financial statements should be read in conjunction with the financial statements and the notes thereto included in the Form 10-K. Asset Retirement Obligations The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the nine months ended September 30, 2015 : Ameren Missouri Ameren Illinois (a) Ameren Balance at December 31, 2014 $ 389 $ 7 $ 396 Liabilities incurred (b) 3 — 3 Liabilities settled (1 ) (c) (1 ) Accretion in 2015 (d) 17 (c) 17 Change in estimates (e) 182 (c) 182 Balance at September 30, 2015 $ 590 $ 7 $ 597 (a) Included in “Other deferred credits and liabilities” on the balance sheet. (b) Ameren and Ameren Missouri recorded a new ARO of $3 million related to the Callaway energy center’s dry spent fuel storage facility. See Note 10 - Callaway Energy Center for additional information. (c) Less than $1 million. (d) Accretion expense was recorded as an increase to regulatory assets. (e) The ARO increase resulted in a corresponding increase recorded to “Property and Plant, Net.” During 2015, Ameren and Ameren Missouri increased their AROs related to the decommissioning of the Callaway energy center by $99 million to reflect the 2015 cost study and funding analysis filed with the MoPSC, extension of the estimated operating life until 2044, and a reduction in the discount rate assumption. See Note 10 - Callaway Energy Center for additional information. In addition, as a result of new federal regulations, Ameren and Ameren Missouri recorded an increase of $79 million to their AROs associated with CCR storage facilities. See Note 9 - Commitments and Contingencies for additional information. Ameren and Ameren Missouri also increased their AROs by $4 million due to a change in the estimated retirement dates of the Meramec and Rush Island energy centers as a result of the MoPSC’s April 2015 electric rate order. Stock-based Compensation A summary of nonvested performance share units at September 30, 2015 , and changes during the nine months ended September 30, 2015 , under the 2006 Incentive Plan and the 2014 Incentive Plan are presented below: Number of Performance Share Units Weighted-average Fair Value Per Performance Share Unit Nonvested at January 1, 2015 1,162,377 $ 35.35 Granted (a) 569,892 52.88 Forfeitures (1,944 ) 34.75 Vested (b) (92,892 ) 45.97 Nonvested at September 30, 2015 1,637,433 $ 40.85 (a) Performance share units granted to certain executive and nonexecutive officers and other eligible employees in 2015 under the 2014 Incentive Plan. (b) Performance share units vested due to the attainment of retirement eligibility by certain employees. Actual shares issued for retirement-eligible employees will vary depending on actual performance over the three -year measurement period. The fair value of each performance share unit awarded in 2015 under the 2014 Incentive Plan was determined to be $52.88 , which was based on Ameren’s closing common share price of $46.13 at December 31, 2014 , and lattice simulations. Lattice simulations are used to estimate expected share payout based on Ameren’s total stockholder return for a three -year performance period relative to the designated peer group beginning January 1, 2015. The simulations can produce a greater fair value for the performance share unit than the applicable closing common share price because they include the weighted payout scenarios in which an increase in the share price has occurred. The significant assumptions used to calculate fair value also included a three -year risk-free rate of 1.10% , volatility of 12% to 18% for the peer group, and Ameren’s attainment of a three-year average earnings per share threshold during the performance period. Excise Taxes Ameren Missouri and Ameren Illinois collect certain excise taxes from customers that are levied on the sale or distribution of natural gas and electricity. Excise taxes are levied on Ameren Missouri’s electric and natural gas businesses and on Ameren Illinois’ natural gas business and are recorded gross in “Operating Revenues - Electric,” “Operating Revenues - Gas” and “Operating Expenses - Taxes other than income taxes” on the statement of income or the statement of income and comprehensive income. Excise taxes for electric service in Illinois are levied on the customer and are therefore not included in Ameren Illinois’ revenues and expenses. The following table presents excise taxes recorded in “Operating Revenues - Electric,” “Operating Revenues - Gas” and “Operating Expenses - Taxes other than income taxes” for the three and nine months ended September 30, 2015 and 2014 : Three Months Nine Months 2015 2014 2015 2014 Ameren Missouri $ 52 $ 47 $ 127 $ 120 Ameren Illinois 9 9 42 46 Ameren $ 61 $ 56 $ 169 $ 166 Uncertain Tax Positions The following table presents the total amount of reserves for unrecognized tax benefits (detriments) related to uncertain tax positions as of September 30, 2015 , and December 31, 2014 : September 30, 2015 December 31, 2014 Ameren $ — $ 54 Ameren Missouri — — Ameren Illinois — (1 ) The following table presents the amount of reserves for unrecognized tax benefits, included in the table above, related to uncertain tax positions that, if recognized, would have impacted results of operations as of December 31, 2014 : December 31, 2014 Ameren $ 52 Ameren Missouri — Ameren Illinois (1 ) In June 2015, a settlement was reached with the IRS for the 2013 tax year. This settlement resolved the uncertain tax position associated with the final tax basis of New AER and the related tax benefit resulting from the divested merchant generation business. The settlement resulted in a reduction of Ameren’s unrecognized tax benefits of $53 million and an increase to net income from discontinued operations. State income tax returns are generally subject to examination for a period of three years after filing. We do not currently have material state income tax issues under examination, administrative appeal, or litigation. The state impact of any federal changes remains subject to examination by various states for up to one year after formal notification to the states. Earnings Per Share Basic earnings per share is computed by dividing net income attributable to Ameren common stockholders by the weighted-average number of common shares outstanding during the period. Earnings per diluted share is computed by dividing net income attributable to Ameren common stockholders by the weighted-average number of diluted common shares outstanding during the period. Earnings per diluted share reflects the potential dilution that would occur if certain stock-based performance share units were settled. The number of performance share units assumed to be settled was 1.3 million and 1.2 million in the three and nine months ended September 30, 2015 , respectively, and 1.7 million in both of the year-ago periods. There were no potentially dilutive securities excluded from the earnings per diluted share calculations for the three and nine months ended September 30, 2015 and 2014. 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 2014, FASB issued authoritative accounting guidance to clarify the principles for recognizing revenue and to develop a common revenue standard for GAAP. The guidance requires an entity to recognize an amount of revenue for the transfer of promised goods or services to customers that reflects the consideration which the entity expects to be entitled to in exchange for those goods or services. The guidance also requires additional disclosures to enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. In August 2015, FASB deferred the effective date of this revenue standard to the first quarter of 2018, with an option for entities to early adopt in the first quarter of 2017. The guidance allows entities to choose one of two transition methods, either by applying the guidance retrospectively to each reporting period presented or by recording a cumulative effect adjustment to retained earnings in the period of initial adoption. The Ameren Companies are currently assessing the impacts of this guidance on their results of operations, financial positions and disclosures, as well as the transition method that they will use to adopt the guidance. Presentation of Debt Issuance Costs In April 2015, FASB issued authoritative accounting guidance to simplify the presentation of debt issuance costs. The guidance requires debt issuance costs to be presented in the balance sheet as a reduction to the associated debt liability. Currently, debt issuance costs are presented as a component of “Other assets” on the Ameren Companies’ balance sheets. As of September 30, 2015, Ameren, Ameren Missouri, and Ameren Illinois had debt issuance costs of $35 million , $15 million , and $19 million , respectively. The Ameren Companies expect to early adopt this standard in the fourth quarter of 2015. The guidance will be applied retrospectively, and will not affect the Ameren Companies' results of operations or cash flows.

Rate And Regulatory Matters

Rate And Regulatory Matters	9 Months Ended
	Sep. 30, 2015
Public Utilities, General Disclosures [Abstract]
RATE AND REGULATORY MATTERS	RATE AND REGULATORY MATTERS Below is a summary of updates to significant regulatory proceedings and related lawsuits. See also Note 2 - Rate and Regulatory Matters under Part II, Item 8, of the Form 10-K. We are unable to predict the ultimate outcome of these matters, the timing of the final decisions of the various agencies and courts, or the impact on our results of operations, financial position, or liquidity. Missouri 2015 Electric Rate Order In April 2015, the MoPSC issued an order approving an increase in Ameren Missouri’s annual revenues for electric service of $122 million , including $109 million related to the increase in net energy costs above those included in base rates previously authorized by the MoPSC. The revenue increase was based on a 9.53% return on common equity, a capital structure composed of 51.8% common equity, and a rate base of $7.0 billion to reflect investments through December 31, 2014. Rate changes consistent with the order became effective on May 30, 2015. The order approved Ameren Missouri’s request for continued use of the FAC; however, it changed the FAC to exclude all transmission revenues and substantially all transmission charges. In addition, the order did not approve the continued use of the regulatory tracking mechanisms for storm costs and vegetation management and infrastructure inspection costs. These changes to Ameren Missouri’s recovery mechanisms are expected to contribute to regulatory lag. The order did approve the continued use of the regulatory tracking mechanisms for pension and other postretirement benefits, renewable energy standard costs, solar rebates, and uncertain tax positions that the MoPSC authorized in prior electric rate orders. In addition, the order approved a reduction to Noranda’s electric rates with an offsetting increase in electric rates for Ameren Missouri’s other customers. The rate shift is revenue neutral to Ameren Missouri. In June 2015, Ameren Missouri filed an appeal with the Missouri Court of Appeals, Western District, of the reduction to Noranda’s electric rates included in the MoPSC’s order. The outcome of this appeal is not expected to impact Ameren Missouri’s results of operations, financial position, or liquidity. MEEIA Filing The MEEIA established a regulatory framework that, among other things, requires the MoPSC to ensure that a utility’s financial incentives are aligned to help customers use energy more efficiently, to provide timely cost recovery, and to provide earnings opportunities associated with cost-effective energy efficiency programs. Missouri does not have a law mandating energy efficiency programs. In August 2012, the MoPSC approved Ameren Missouri’s customer energy efficiency programs, net shared benefits, and performance incentive for 2013 through 2015. The 2013 through 2015 plan anticipated Ameren Missouri would invest up to $147 million in customer energy efficiency programs, realize $100 million of net shared benefits, and be eligible for a performance incentive that would allow it the potential to earn additional revenues by achieving certain customer energy efficiency goals, including $19 million if 100% of the goals are achieved during the three -year period, with the potential to earn a larger performance incentive if Ameren Missouri’s energy savings exceed those goals. From January 2013 through September 2015, Ameren Missouri invested $110 million in customer energy efficiency programs and realized $134 million of net shared benefits. In June 2015, the MoPSC staff filed a complaint case with the MoPSC regarding the method and inputs used in calculating the performance incentive. If the MoPSC agrees with the MoPSC staff’s interpretation of the August 2012 MEEIA order, the performance incentive recognized in 2016 would be significantly less than the performance incentive calculated using Ameren Missouri’s interpretation. Ameren Missouri has not recorded revenues associated with the performance incentive. However, regardless of the MoPSC’s decision in the complaint case, Ameren Missouri believes it will exceed 100% of the customer energy efficiency goals, subject to MoPSC review, and therefore expects to recognize revenues of at least $19 million in 2016. There is no date by which the MoPSC must issue a decision in this complaint case. In October 2015, the MoPSC rejected Ameren Missouri’s MEEIA energy efficiency plan for 2016 through 2018, which included a portfolio of customer energy efficiency programs along with a rider to collect the program costs, net shared benefits, and a performance incentive from customers. Ameren Missouri is studying the MoPSC’s October 2015 order and evaluating whether to file another proposed energy efficiency plan with the MoPSC. Noranda Contract Notification Ameren Missouri supplies electricity to Noranda’s aluminum smelter in southeast Missouri under a long-term power supply agreement. In May 2015, Ameren Missouri notified Noranda of its intent to terminate the agreement effective June 1, 2020. If Ameren Missouri wanted to cease providing electricity to Noranda, Ameren Missouri would also be required to obtain approval from the MoPSC. Sales to Noranda represented 5% of Ameren Missouri’s total electric revenue in 2014. ATXI Transmission Projects In May 2015, the MoPSC granted ATXI a certificate of convenience and necessity for the seven -mile portion of the Illinois Rivers project located in Missouri. In June 2015, ATXI made a filing with the MoPSC requesting a certificate of convenience and necessity for the Mark Twain project. The Mark Twain project is a MISO-approved 100 -mile transmission line located in northeast Missouri. A decision is expected from the MoPSC in 2016. Illinois IEIMA Under the provisions of the IEIMA’s formula rate 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 September 30, 2015 , Ameren Illinois had recorded regulatory assets of $52 million , $103 million , and $14 million , to reflect its expected 2015, 2014, and 2013 revenue requirement reconciliation adjustments, with interest, respectively. Ameren Illinois is collecting the 2013 revenue requirement reconciliation adjustment from customers during 2015. Ameren Illinois’ annual electric delivery service formula rate update to establish customer rates for 2016 is currently pending before the ICC. If the ICC approves as filed, the annual update filing would result in a $109 million increase in Ameren Illinois’ electric delivery service revenue requirement, beginning in January 2016. This update reflects an increase to the annual formula rate based on 2014 actual recoverable costs and expected net plant additions for 2015, an increase to include the 2014 revenue requirement reconciliation adjustment, and a decrease for the conclusion of the 2013 revenue requirement reconciliation adjustment, which will be fully collected from customers in 2015. In October 2015, the ICC staff submitted its calculation of Ameren Illinois’ revenue requirement. The ICC staff recommended adjustments that would result in a $107 million increase in Ameren Illinois’ electric delivery service revenue requirement. An ICC decision on this update filing is required by December 2015. 2015 Natural Gas Delivery Service Rate Case In January 2015, Ameren Illinois filed a request with the ICC seeking approval to increase its annual revenues for natural gas delivery service. In an attempt to reduce regulatory lag, Ameren Illinois used a 2016 future test year in this proceeding. Additionally, the request included a proposal to implement a volume balancing adjustment for residential and small nonresidential customers. The volume balancing adjustment would increase or decrease revenues as weather deviates from normal conditions to ensure that weather-related changes in natural gas sales volumes do not result in an over or under collection of natural gas revenues for these rate classes. This case includes a capital structure composed of 50% common equity and a rate base of $1.2 billion . In July 2015, Ameren Illinois, the ICC staff, and certain other intervenors filed a stipulation and agreement with the ICC that would result in rates that are based on a return on common equity of 9.6% . The agreement does not address the positions of all of the parties in the rate case. Based on the terms in the agreement and the unresolved positions in the case, Ameren Illinois' request seeks an annual revenue increase of $45 million , which is consistent with the ICC staff’s recommendation and the administrative law judges' proposed order issued in November 2015. The administrative law judges' order also proposed the approval of the volume balancing adjustment. A decision by the ICC in this proceeding is required by December 2015, with new rates expected to be effective in January 2016. Ameren Illinois cannot predict the level of any delivery service rate changes the ICC may approve, whether the ICC will approve the volume balancing adjustment, or if the ICC will approve the agreement between Ameren Illinois, the ICC staff, and certain other intervenors. In addition, Ameren Illinois cannot predict whether any rate changes that may eventually be approved will be sufficient to enable Ameren Illinois to recover its costs and to earn a reasonable return on investments when the rate changes go into effect. 2015 ICC Purchased Power Reconciliation In January 2015, the ICC issued an order that approved Ameren Illinois' reconciliation of revenues collected under its purchased power rider mechanism and Ameren Illinois' related cumulative power usage cost. In the first quarter of 2015, based on the January 2015 order, both Ameren and Ameren Illinois recorded a $15 million increase to electric revenues for the recovery of this cumulative power usage cost from electric customers. ATXI Transmission Project The Spoon River project is a MISO-approved 46 -mile transmission line to be constructed in northwest Illinois. In September 2015, the ICC granted a certificate of public convenience and necessity and project approval for the Spoon River project. Federal Ameren Illinois Electric Transmission Rate Refund In July 2012, the FERC issued an order concluding that Ameren Illinois improperly included acquisition premiums, including goodwill, in determining the common equity used in its electric transmission formula rate and thereby inappropriately recovered a higher amount from its electric transmission customers. The order required Ameren Illinois to make refunds to customers for such improperly included amounts. In July 2015, the FERC approved a settlement agreement between Ameren Illinois and the affected customers. The settlement agreement required Ameren Illinois to make refunds and payments of $8 million to electric transmission customers, all of which was paid by September 30, 2015. The settlement agreement also requires Ameren Illinois to take other actions, such as reducing common equity for electric transmission ratemaking purposes on a prospective basis. FERC Complaint Cases In November 2013, a customer group filed a complaint case with the FERC seeking a reduction in the allowed base return on common equity for the FERC-regulated MISO transmission rate base under the MISO tariff to 9.15% . Currently, the FERC-allowed base return on common equity for MISO transmission owners is 12.38% . The FERC scheduled the case for hearing proceedings, requiring a proposed order from its administrative law judge to be issued no later than November 30, 2015, which will subsequently require FERC approval. The FERC has previously utilized a calculation to establish the allowed base return on common equity, which requires multiple inputs based on observable market data specific to the utility industry and broader macroeconomic data spanning unique time periods for each return on equity complaint case. We expect observable market data for the six months ended February 11, 2015, will be used in the November 2013 complaint case. As the maximum FERC-allowed refund period for the November 2013 complaint case ended in February 2015, another customer complaint case was filed in February 2015. The February 2015 complaint case seeks a reduction in the allowed base return on common equity for the FERC-regulated MISO transmission rate base under the MISO tariff to 8.67% . The FERC scheduled the February 2015 complaint case for hearing proceedings, requiring a proposed order from its administrative law judge to be issued no later than June 30, 2016, which will subsequently require FERC approval. We expect observable market data for the six months ended December 31, 2015, will be used in the February 2015 complaint case. As of September 30, 2015, Ameren and Ameren Illinois had current regulatory liabilities of $36 million and $25 million , respectively, representing their estimates of the potential refunds from the November 12, 2013 refund effective date. Ameren’s and Ameren Illinois’ recorded liabilities reflect their interpretation of the method and inputs used in the FERC’s calculation to establish the allowed base return on common equity, based on observable market data for the six months ended February 11, 2015, with respect to the November 2013 complaint case refund period, and based on observable market data through September 30, 2015 , with respect to the February 2015 complaint case refund period. Ameren’s and Ameren Illinois’ liabilities also reflect the incentive adder discussed below, which became effective in early January 2015. Ameren Missouri did not record a liability as of September 30, 2015 , and does not expect that a reduction in the FERC-allowed base return on common equity for MISO transmission owners would be material to its results of operations, financial position, or liquidity. A 50 basis point reduction in the FERC-allowed return on common equity would reduce Ameren's and Ameren Illinois' annual earnings by an estimated $4 million and $2 million , respectively, based on 2015 projected rate base. 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. Upon the issuance of the final order addressing the initial MISO complaint case, beginning with its January 6, 2015 effective date, the incentive adder will reduce any refund to customers relating to a reduction of the base return on common equity. Combined Construction and Operating License In 2008, Ameren Missouri filed an application with the NRC for a COL for a second nuclear unit at Ameren Missouri's existing Callaway County, Missouri, energy center site. In 2009, Ameren Missouri suspended its efforts to build a second nuclear unit at its existing Callaway site, and the NRC suspended review of the COL application. Prior to suspending its efforts, Ameren Missouri had capitalized $69 million related to the project. Due primarily to changes in vendor support for licensing efforts at the NRC, Ameren Missouri’s assessment of long-term capacity needs, declining costs of alternative generation technologies, and the regulatory framework in Missouri, Ameren Missouri discontinued its efforts to license and build a second nuclear unit at its existing Callaway site. As a result of this decision, in the second quarter of 2015, Ameren and Ameren Missouri recognized a $69 million noncash pretax provision for all of the previously capitalized costs of the COL. Ameren Missouri has withdrawn its COL application with the NRC.

Short-Term Debt And Liquidity

Short-Term Debt And Liquidity	9 Months Ended
	Sep. 30, 2015
Line of Credit Facility [Abstract]
SHORT-TERM DEBT AND LIQUIDITY	SHORT-TERM DEBT AND LIQUIDITY The liquidity needs of the Ameren Companies are typically supported through the use of available cash, drawings under committed credit agreements, commercial paper issuances, or, in the case of Ameren Missouri and Ameren Illinois, short-term intercompany borrowings. The 2012 Missouri Credit Agreement and the 2012 Illinois Credit Agreement, both of which expire on December 11, 2019, were not utilized for direct borrowings during the nine months ended September 30, 2015 , but were used to support commercial paper issuances and to issue letters of credit. Based on letters of credit issued under the 2012 Credit Agreements, as well as commercial paper outstanding, the aggregate amount of credit capacity available under the 2012 Credit Agreements to Ameren (parent), Ameren Missouri, and Ameren Illinois, collectively, at September 30, 2015 , was $1.3 billion . Commercial Paper The following table presents commercial paper outstanding at Ameren (parent), Ameren Missouri, and Ameren Illinois as of September 30, 2015 , and December 31, 2014 : September 30, 2015 December 31, 2014 Ameren (parent) $ 783 $ 585 Ameren Missouri — 97 Ameren Illinois — 32 Ameren Consolidated $ 783 $ 714 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 nine months ended September 30, 2015 and 2014: Ameren (parent) Ameren Missouri Ameren Illinois Ameren Consolidated 2015 Average daily commercial paper outstanding $ 770 $ 56 $ 6 $ 832 Weighted-average interest rate 0.56 % 0.50 % 0.44 % 0.56 % Peak commercial paper during period (a) $ 874 $ 294 $ 48 $ 1,108 Peak interest rate 0.70 % 0.60 % 0.60 % 0.70 % 2014 Average daily commercial paper outstanding $ 386 $ 141 $ 157 $ 609 Weighted-average interest rate 0.36 % 0.38 % 0.31 % 0.35 % Peak commercial paper during period (a) $ 531 $ 495 $ 300 $ 907 Peak interest rate 0.75 % 0.70 % 0.34 % 0.75 % (a) The timing of peak commercial paper issuances varies by company, and therefore the peak amounts presented by company might not equal the Ameren Consolidated peak commercial paper issuances for the period. Indebtedness Provisions and Other Covenants The information below is a summary of the Ameren Companies’ compliance with financial covenants in the 2012 Credit Agreements. See Note 4 - Short-term Debt and Liquidity under Part II, Item 8, in the Form 10-K for a detailed description of these provisions. The 2012 Credit Agreements also contain nonfinancial covenants, including restrictions on the ability to incur liens, to transact with affiliates, to dispose of assets, to make investments in or transfer assets to its affiliates, and to merge with other entities. The 2012 Credit Agreements require each of Ameren, Ameren Missouri, and Ameren Illinois to maintain consolidated indebtedness of not more than 65% of its consolidated total capitalization pursuant to a defined calculation set forth in the agreements. As of September 30, 2015 , the ratios of consolidated indebtedness to consolidated total capitalization, calculated in accordance with the 2012 Credit Agreements, were 50% , 48% , and 46% , for Ameren, Ameren Missouri, and Ameren Illinois, respectively. In addition, under the 2012 Illinois Credit Agreement and, by virtue of the cross-default provisions of the 2012 Missouri Credit Agreement, under the 2012 Missouri Credit Agreement, Ameren is required to maintain a ratio of consolidated funds from operations plus interest expense to consolidated interest expense of at least 2.0 to 1.0. However, the interest coverage requirement only applies at such times as Ameren does not have a senior long-term unsecured credit rating of at least Baa3 from Moody’s or BBB- from S&P. As of September 30, 2015 , Ameren exceeded the rating requirements; therefore, the interest coverage requirement was not applicable. Failure of a borrower to satisfy a financial covenant constitutes an immediate default under the applicable 2012 Credit Agreement. The 2012 Credit Agreements contain default provisions that apply separately to each borrower; provided, however, that a default of Ameren Missouri or Ameren Illinois under the applicable 2012 Credit Agreement will also be deemed to constitute a default of Ameren under such agreement. Defaults include a cross-default of such borrower under any other agreement covering outstanding indebtedness of such borrower and certain subsidiaries (other than project finance subsidiaries and nonmaterial subsidiaries) in excess of $75 million in the aggregate (including under the other 2012 Credit Agreement). However, under the default provisions of the 2012 Credit Agreements, any default of Ameren under any 2012 Credit Agreement that results solely from a default of Ameren Missouri or Ameren Illinois thereunder does not result in a cross-default of Ameren under the other 2012 Credit Agreement. Further, the 2012 Credit Agreement default provisions provide that an Ameren default under any of the 2012 Credit Agreements does not constitute a default by Ameren Missouri or Ameren Illinois. None of the Ameren Companies' credit agreements or financing arrangements contain credit rating triggers that would cause a default or acceleration of repayment of outstanding balances. The Ameren Companies were in compliance with the covenants in their credit agreements at September 30, 2015 . Money Pools Ameren has money pool agreements with and among its subsidiaries to coordinate and provide for certain short-term cash and working capital requirements. Ameren Missouri, Ameren Illinois, and 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 2012 Credit Agreements and the commercial paper programs. The total amount available to the pool participants from the utility money pool at any given time is reduced by the amount of borrowings made by participants, but is increased to the extent that the pool participants advance surplus funds to the utility money pool or remit funds from other external sources. The availability of funds is also determined by funding requirement limits established by regulatory authorizations. Participants receiving a loan under the utility money pool 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 and lending under the utility money pool for the three and nine months ended September 30, 2015 , was 0.10% and 0.09% , respectively ( 2014 - 0.10% and 0.23% , respectively). See Note 8 - Related Party Transactions for the amount of interest income and expense from the money pool arrangements recorded by the Ameren Companies for the three and nine months ended September 30, 2015 and 2014 .

Long-Term Debt And Equity Finan

Long-Term Debt And Equity Financings	9 Months Ended
	Sep. 30, 2015
Long-Term Debt And Equity Financings [Abstract]
LONG-TERM DEBT AND EQUITY FINANCINGS	LONG-TERM DEBT AND EQUITY Ameren Missouri In March 2015, Ameren Missouri received cash capital contributions of $224 million from Ameren (parent). In April 2015, Ameren Missouri issued $250 million of 3.65% senior secured notes due April 15, 2045, with interest payable semiannually on April 15 and October 15 of each year, beginning October 15, 2015. Ameren Missouri received proceeds of $247 million , which were used to repay outstanding short-term debt, including short-term debt that Ameren Missouri incurred in connection with the repayment of $114 million of its 4.75% senior secured notes that matured on April 1, 2015. Indenture Provisions and Other Covenants Ameren Missouri’s and Ameren Illinois’ indentures, credit facilities, and articles of incorporation include covenants and provisions related to issuances of first mortgage bonds and preferred stock. Ameren Missouri and Ameren Illinois are required to meet certain ratios to issue additional first mortgage bonds and preferred stock. A failure to achieve these ratios would not result in a default under these covenants and provisions, but would restrict the companies’ ability to issue bonds or preferred stock. The following table summarizes the required and actual interest coverage ratios and dividend coverage ratios, and bonds and preferred stock issuable as of September 30, 2015 , 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 3.7 $ 3,338 ≥2.5 99.3 $ 2,206 Ameren Illinois ≥2.0 7.0 3,659 (d) ≥1.5 3.0 203 (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 $946 million and $204 million at Ameren Missouri and Ameren Illinois, respectively. (c) Coverage required on the annual dividend on preferred stock outstanding and to be issued, as required in the respective company’s articles of incorporation. (d) Amount of bonds issuable by Ameren Illinois based on unfunded property additions and retired bonds solely under the former IP mortgage indenture. The amount of bonds issuable by Ameren Illinois is also subject to the lien restrictions contained in the 2012 Illinois Credit Agreement. (e) Preferred stock issuable is restricted by the amount of preferred stock that is currently authorized by Ameren Illinois’ articles of incorporation. Ameren Missouri and Ameren Illinois and certain other Ameren subsidiaries are subject to Section 305(a) of the Federal Power Act, which makes it unlawful for any officer or director of a public utility, as defined in the Federal Power Act, to participate in the making or paying of any dividend from any funds “properly included in capital account.” The FERC has consistently interpreted the provision to allow dividends to be paid as long as (1) the source of the dividends is clearly disclosed, (2) the dividends are not excessive, and (3) there is no self-dealing on the part of corporate officials. At a minimum, Ameren believes that dividends can be paid by its subsidiaries that are public utilities from retained earnings. In addition, under Illinois law, Ameren Illinois may not pay any dividend on its stock, unless, among other things, its earnings and earned surplus are sufficient to declare and pay a dividend after provision is made for reasonable and proper reserves, or unless Ameren Illinois has specific authorization from the ICC. Ameren Illinois’ articles of incorporation require dividend payments on its common stock to be based on ratios of common stock to total capitalization and other provisions related to certain operating expenses and accumulations of earned surplus. Ameren Illinois committed to the FERC to maintain a minimum of 30% equity in its capital structure. As of September 30, 2015 , Ameren Illinois had 52% 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 September 30, 2015 , none of the Ameren Companies had any off-balance-sheet financing arrangements, other than operating leases entered into in the ordinary course of business. None of the Ameren Companies expect to engage in any significant off-balance-sheet financing arrangements in the near future. See Note 12 - Divestiture Transactions and Discontinued Operations for Ameren (parent) guarantees and letters of credit issued to support New AER based on the transaction agreement with IPH.

Other Income and Expenses

Other Income and Expenses	9 Months Ended
	Sep. 30, 2015
Other Nonoperating Income (Expense) [Abstract]
OTHER INCOME AND EXPENSES	OTHER INCOME AND EXPENSES The following table presents the components of “Other Income and Expenses” in the Ameren Companies’ statements of income for the three and nine months ended September 30, 2015 and 2014 : Three Months Nine Months 2015 2014 2015 2014 Ameren: (a) Miscellaneous income: Allowance for equity funds used during construction $ 8 $ 10 $ 19 $ 26 Interest income on industrial development revenue bonds 7 6 20 20 Interest income 4 3 12 8 Other — 2 3 6 Total miscellaneous income $ 19 $ 21 $ 54 $ 60 Miscellaneous expense: Donations $ — $ 3 $ 10 $ 9 Other 5 4 12 11 Total miscellaneous expense $ 5 $ 7 $ 22 $ 20 Ameren Missouri: Miscellaneous income: Allowance for equity funds used during construction $ 7 $ 9 $ 16 $ 24 Interest income on industrial development revenue bonds 7 6 20 20 Interest income — — 1 1 Total miscellaneous income $ 14 $ 15 $ 37 $ 45 Miscellaneous expense: Donations $ — $ 2 $ 3 $ 5 Other 3 2 5 5 Total miscellaneous expense $ 3 $ 4 $ 8 $ 10 Ameren Illinois: Miscellaneous income: Allowance for equity funds used during construction $ 1 $ 1 $ 3 $ 2 Interest income 3 2 10 5 Other — 1 2 5 Total miscellaneous income $ 4 $ 4 $ 15 $ 12 Miscellaneous expense: Donations $ — $ — $ 4 $ 3 Other 3 2 6 4 Total miscellaneous expense $ 3 $ 2 $ 10 $ 7 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Derivative Financial Instrument

Derivative Financial Instruments	9 Months Ended
	Sep. 30, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
DERIVATIVE FINANCIAL INSTRUMENTS	NOTE 6 - DERIVATIVE FINANCIAL INSTRUMENTS We use derivatives to manage the risk of changes in market prices for natural gas, power, and uranium, as well as the risk of changes in rail transportation surcharges through fuel oil hedges. Such price fluctuations may cause the following: • an unrealized appreciation or depreciation of our contracted commitments to purchase or sell when purchase or sale prices under the commitments are compared with current commodity prices; • market values of natural gas and uranium inventories that differ from the cost of those commodities in inventory; and • actual cash outlays for the purchase of these commodities that differ from anticipated cash outlays. The derivatives that we use to hedge these risks are governed by our risk management policies for forward contracts, futures, options, and swaps. Our net positions are continually assessed within our structured hedging programs to determine whether new or offsetting transactions are required. The goal of the hedging program is generally to mitigate financial risks while ensuring that sufficient volumes are available to meet our requirements. Contracts we enter into as part of our risk management program may be settled financially, settled by physical delivery, or net settled with the counterparty. The following table presents open gross commodity contract volumes by commodity type for derivative assets and liabilities as of September 30, 2015 , and December 31, 2014 . As of September 30, 2015 , these contracts ran through October 2017, March 2021, May 2032, and December 2016 for fuel oils, natural gas, power, and uranium, respectively. Quantity (in millions, except as indicated) 2015 2014 Commodity Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils (in gallons) (a) 33 (b) 33 50 (b) 50 Natural gas (in mmbtu) 30 152 182 28 108 136 Power (in megawatthours) 1 10 11 1 11 12 Uranium (pounds in thousands) 299 (b) 299 332 (b) 332 (a) Fuel oils consist of heating oil and ultra-low-sulfur diesel. (b) Not applicable. Authoritative accounting guidance regarding derivative instruments requires that all contracts considered to be derivative instruments be recorded on the balance sheet at their fair values, unless the NPNS exception applies. See Note 7 - Fair Value Measurements for 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 does not qualify for the NPNS exception, we review the contract to determine if it qualifies for hedge accounting. We also consider whether gains or losses resulting from such derivatives qualify for regulatory deferral. Derivative contracts that qualify for regulatory deferral are recorded at fair value, with changes in fair value recorded as regulatory assets or regulatory liabilities in the period in which the change occurs. We believe derivative losses and gains deferred as regulatory assets and regulatory liabilities are probable of recovery or refund through future rates charged to customers. Regulatory assets and regulatory liabilities are amortized to operating income as related losses and gains are reflected in rates charged to customers. Therefore, gains and losses on these derivatives have no effect on operating income. As of September 30, 2015 , and December 31, 2014 , all contracts that qualify for hedge accounting received regulatory deferral. Authoritative accounting guidance permits companies to offset fair value amounts recognized for the right to reclaim cash collateral (a receivable) or the obligation to return cash collateral (a liability) against fair value amounts recognized for derivative instruments that are executed with the same counterparty under a master netting arrangement 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 instruments, none of which were designated as hedging instruments under hedge accounting, as of September 30, 2015 , and December 31, 2014 : Balance Sheet Location Ameren Missouri Ameren Illinois Ameren 2015 Natural gas Other current assets $ 1 $ — $ 1 Power Other current assets 22 — 22 Other assets 1 — 1 Total assets $ 24 $ — $ 24 Fuel oils Other current liabilities $ 19 $ — $ 19 Other deferred credits and liabilities 6 — 6 Natural gas MTM derivative liabilities (a) 26 (a) Other current liabilities 5 — 31 Other deferred credits and liabilities 9 18 27 Power MTM derivative liabilities (a) 12 (a) Other current liabilities 1 — 13 Other deferred credits and liabilities — 158 158 Uranium Other current liabilities 1 — 1 Total liabilities $ 41 $ 214 $ 255 2014 Fuel oils Other current assets $ 2 $ — $ 2 Natural gas Other current assets 1 1 2 Power Other current assets 15 — 15 Total assets $ 18 $ 1 $ 19 Fuel oils Other current liabilities $ 22 $ — $ 22 Other deferred credits and liabilities 7 — 7 Natural gas MTM derivative liabilities (a) 31 (a) Other current liabilities 6 — 37 Other deferred credits and liabilities 6 13 19 Power MTM derivative liabilities (a) 11 (a) Other current liabilities 3 — 14 Other deferred credits and liabilities — 131 131 Uranium Other current liabilities 2 — 2 Total liabilities $ 46 $ 186 $ 232 (a) Balance sheet line item not applicable to registrant. The following table presents the cumulative amount of pretax net gains (losses) on all derivative instruments deferred in regulatory assets or regulatory liabilities as of September 30, 2015 , and December 31, 2014 : Ameren Missouri Ameren Illinois Ameren 2015 Fuel oils derivative contracts (a) $ (25 ) $ — $ (25 ) Natural gas derivative contracts (b) (13 ) (44 ) (57 ) Power derivative contracts (c) 22 (170 ) (148 ) Uranium derivative contracts (d) (1 ) — (1 ) 2014 Fuel oils derivative contracts $ (29 ) $ — $ (29 ) Natural gas derivative contracts (11 ) (43 ) (54 ) Power derivative contracts 12 (142 ) (130 ) Uranium derivative contracts (2 ) — (2 ) (a) Represents net losses associated with fuel oils derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s rail transportation surcharges for coal through December 2017. Losses deferred as current regulatory assets include $19 million at Ameren and Ameren Missouri. (b) Represents net losses associated with natural gas derivative contracts. These contracts are a partial hedge of natural gas requirements through March 2021 at Ameren and Ameren Missouri and through October 2019 at Ameren Illinois. Gains deferred as current regulatory liabilities include $1 million at Ameren and Ameren Missouri. Losses deferred as current regulatory assets include $31 million , $5 million , and $26 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively. (c) Represents net gains (losses) associated with power derivative contracts. These contracts are a partial hedge of power price requirements through May 2032 at Ameren and Ameren Illinois and through December 2016 at Ameren Missouri. Gains deferred as current regulatory liabilities include $22 million at Ameren and Ameren Missouri. Losses deferred as current regulatory assets include $13 million , $1 million , and $12 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively. (d) Represents net losses on uranium derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s uranium requirements through January 2017. Losses deferred as current regulatory assets include $1 million at Ameren and Ameren Missouri. Derivative instruments are subject to various credit-related losses in the event of nonperformance by counterparties to the transaction. Exchange-traded contracts are supported by the financial and credit quality of the clearing members of the respective exchanges and have nominal credit risk. In all other transactions, we are exposed to credit risk. Our credit risk management program involves establishing credit limits and collateral requirements for counterparties, using master netting arrangements 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 September 30, 2015 , and December 31, 2014 : Gross Amounts Not Offset in the Balance Sheet Commodity Contracts Eligible to be Offset Gross Amounts Recognized in the Balance Sheet Derivative Instruments Cash Collateral Received/Posted (a) Net Amount 2015 Assets: Ameren Missouri $ 24 $ 2 $ — $ 22 Ameren Illinois — — — — Ameren $ 24 $ 2 $ — $ 22 Liabilities: Ameren Missouri $ 41 $ 2 $ 6 $ 33 Ameren Illinois 214 — 1 213 Ameren $ 255 $ 2 $ 7 $ 246 2014 Assets: Ameren Missouri $ 18 $ 5 $ — $ 13 Ameren Illinois 1 — — 1 Ameren $ 19 $ 5 $ — $ 14 Liabilities: Ameren Missouri $ 46 $ 5 $ 5 $ 36 Ameren Illinois 186 — — 186 Ameren $ 232 $ 5 $ 5 $ 222 (a) Cash collateral received reduces gross asset balances and is included in “Other current liabilities” and “Other deferred credits and liabilities” on the balance sheet. Cash collateral posted reduces gross liability balances and is included in “Other current assets” and “Other assets” on the balance sheet. Concentrations of Credit Risk In determining our concentrations of credit risk related to derivative instruments, we review our individual counterparties and categorize each counterparty into groupings according to the primary business in which each engages. We calculate maximum exposures based on the gross fair value of financial instruments, including NPNS and other accrual contracts. These exposures are presented on a gross basis, which include affiliate exposure not eliminated at the consolidated Ameren level. As of September 30, 2015 , if counterparty groups were to fail completely to perform on contracts, Ameren, Ameren Missouri, and Ameren Illinois' maximum exposures were $12 million , $12 million , and $- million, respectively. 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 September 30, 2015 , the potential loss after consideration of the application of master netting arrangements or similar agreements and collateral held for Ameren, Ameren Missouri, and Ameren Illinois was $11 million , $11 million , and $- million, respectively. Derivative Instruments with Credit Risk-Related Contingent Features Our commodity contracts contain collateral provisions tied to the Ameren Companies’ credit ratings. If we were to experience an adverse change in our credit ratings, or if a counterparty with reasonable grounds for uncertainty regarding performance of an obligation requested adequate assurance of performance, additional collateral postings might be required. The following table presents, as of September 30, 2015 , the aggregate fair value of all derivative instruments with credit risk-related contingent features in a gross liability position, the cash collateral posted, and the aggregate amount of additional collateral that could be required to be posted with counterparties. The additional collateral required is the net liability position allowed under the master netting arrangements or similar agreements, assuming (1) the credit risk-related contingent features underlying these arrangements were triggered on September 30, 2015 , 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) 2015 Ameren Missouri $ 88 $ 7 $ 75 Ameren Illinois 82 1 75 Ameren $ 170 $ 8 $ 150 (a) Prior to 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	9 Months Ended
	Sep. 30, 2015
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	FAIR VALUE MEASUREMENTS Fair value is defined as the exchange price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. We use various methods to determine fair value, including market, income, and cost approaches. With these approaches, we adopt certain assumptions that market participants would use in pricing the asset or liability, including assumptions about market risk or the risks inherent in the inputs to the valuation. Inputs to valuation can be readily observable, market-corroborated, or unobservable. We use valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs. Authoritative accounting guidance established a fair value hierarchy that prioritizes the inputs used to measure fair value. All financial assets and liabilities carried at fair value are classified and disclosed in one of three hierarchy levels. See Note 8 - Fair Value Measurements under Part II, Item 8, of the Form 10-K for information related to hierarchy levels. We perform an analysis each quarter to determine the appropriate hierarchy level of the assets and liabilities subject to fair value measurements. Financial assets and liabilities are classified in their entirety according to the lowest level of input that is significant to the fair value measurement. All assets and liabilities whose fair value measurement is based on significant unobservable inputs are classified as Level 3. The following table describes the valuation techniques and unobservable inputs for the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy as of September 30, 2015 : Fair Value Weighted Average Assets Liabilities Valuation Technique(s) Unobservable Input Range Level 3 Derivative asset and liability - commodity contracts (a) : Ameren Fuel oils $ — $ (1 ) Option model Volatilities(%) (e) 63 (d) Discounted cash flow Ameren Missouri credit risk(%) (b)(c) 0.40 (d) Natural gas — (1 ) Option model Volatilities(%) (e) 20 - 44 33 Nodal basis($/mmbtu) (b) (0.30) - (0.10) (0.20) Discounted cash flow Nodal basis($/mmbtu) (e) (1.40) - 0.10 (0.20) Counterparty credit risk(%) (b)(c) 0.40 - 12.07 6.60 Ameren Missouri credit risk(%) (b)(c) 0.40 (d) Power (f) 23 (171 ) Discounted cash flow Average forward peak and off-peak pricing - forwards/swaps($/MWh) (g) 26 - 40 29 Estimated auction price for FTRs($/MW) (e) (480) - 2,333 193 Nodal basis($/MWh) (e) (10) - (1) (3) Counterparty credit risk(%) (b)(c) 0.46 - 0.84 0.73 Ameren Illinois credit risk(%) (b)(c) 0.40 (d) Fundamental energy production model Estimated future gas prices($/mmbtu) (e) 3 - 4 4 Escalation rate(%) (e)(h) 2 - 3 2 Contract price allocation Estimated renewable energy credit costs($/credit) (e) 5 - 7 6 Uranium — (1 ) Discounted cash flow Average forward uranium pricing($/pound) (e) 37 - 39 37 Ameren Missouri Fuel oils $ — $ (1 ) Option model Volatilities(%) (e) 63 (d) Discounted cash flow Ameren Missouri credit risk(%) (b)(c) 0.40 (d) Natural gas — (1 ) Option model Volatilities(%) (e) 20 - 44 33 Nodal basis($/mmbtu) (b) (0.30) - (0.10) (0.20) Discounted cash flow Nodal basis($/mmbtu) (e) (1.40) - 0.10 (0.20) Counterparty credit risk(%) (b)(c) 0.40 - 12.07 6.60 Ameren Missouri credit risk(%) (b)(c) 0.40 (d) Power (f) 23 (1 ) Discounted cash flow Average forward peak and off-peak pricing - forwards/swaps($/MWh) (b) 26 - 40 34 Estimated auction price for FTRs($/MW) (e) (480) - 2,333 193 Nodal basis($/MWh) (b) (10) - (5) (9) Counterparty credit risk(%) (b)(c) 0.46 - 0.84 0.73 Uranium — (1 ) Discounted cash flow Average forward uranium pricing($/pound) (e) 37 - 39 37 Ameren Illinois Power (f) $ — $ (170 ) Discounted cash flow Average forward peak and off-peak pricing - forwards/swaps($/MWh) (e) 27 - 33 30 Nodal basis($/MWh) (e) (6) - (1) (3) Ameren Illinois credit risk(%) (b)(c) 0.40 (d) Fundamental energy production model Estimated future gas prices($/mmbtu) (e) 3 - 4 4 Escalation rate(%) (e)(h) 2 - 3 2 Contract price allocation Estimated renewable energy credit costs($/credit) (e) 5 - 7 6 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement. (c) Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances. (d) Not applicable. (e) Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement. (f) Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2019. Valuations beyond 2019 use fundamentally modeled pricing by month for peak and off-peak demand. (g) The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes due to their opposing positions. As such, refer to the power sensitivity analysis for each company above. (h) Escalation rate applies to power prices 2026 and beyond. The following table describes the valuation techniques and unobservable inputs for the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy as of December 31, 2014 : Fair Value Weighted Assets Liabilities Valuation Technique(s) Unobservable Input Range Average Level 3 Derivative asset and liability – commodity contracts (a) : Ameren Fuel oils $ 2 $ (8 ) Option model Volatilities(%) (b) 3 - 39 32 Discounted cash flow Ameren Missouri credit risk(%) (b)(c) 0.43 (d) Escalation rate(%) (e)(f) 5 (d) Natural Gas 1 (2 ) Option model Volatilities(%) (b) 31 - 144 63 Nodal basis($/mmbtu) (e) (0.40) - 0 (0.20) Discounted cash flow Nodal basis($/mmbtu) (e) (0.40) - 0.10 (0.20) Counterparty credit risk(%) (b)(c) 0.43 - 13 3 Ameren Missouri and Ameren Illinois credit risk(%) (b)(c) 0.43 (d) Power (g) 11 (144 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps($/MWh) (h) 27 - 50 32 Estimated auction price for FTRs($/MW) (e) (1,833) - 2,743 171 Nodal basis($/MWh) (e) (6) - 0 (2) Counterparty credit risk(%) (b)(c) 0.26 (d) Ameren Missouri and Ameren Illinois credit risk(%) (b)(c) 0.43 (d) Fundamental energy production model Estimated future gas prices($/mmbtu) (e) 4 - 5 4 Escalation rate(%) (e)(i) 0 - 1 1 Contract price allocation Estimated renewable energy credit costs($/credit) (e) 5 - 7 6 Uranium — (2 ) Discounted cash flow Average forward uranium pricing($/pound) (e) 35 - 40 36 Ameren Missouri Fuel oils $ 2 $ (8 ) Option model Volatilities(%) (b) 3 - 39 32 Discounted cash flow Ameren Missouri credit risk(%) (b)(c) 0.43 (d) Escalation rate(%) (e)(f) 5 (d) Natural Gas — (1 ) Option model Volatilities(%) (b) 31 - 144 53 Nodal basis($/mmbtu) (e) (0.40) - 0 (0.30) Discounted cash flow Nodal basis($/mmbtu) (e) (0.10) (d) Counterparty credit risk(%) (b)(c) 0.57 - 13 5 Ameren Missouri credit risk(%) (b)(c) 0.43 (d) Power (g) 11 (2 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps($/MWh) (b) 27 - 50 32 Estimated auction price for FTRs($/MW) (e) (1,833) - 2,743 171 Counterparty credit risk(%) (b)(c) 0.26 (d) Ameren Missouri credit risk(%) (b)(c) 0.43 (d) Uranium — (2 ) Discounted cash flow Average forward uranium pricing($/pound) (e) 35 - 40 36 Ameren Illinois Natural Gas $ 1 $ (1 ) Option model Volatilities(%) (b) 50 - 144 94 Nodal basis($/mmbtu) (e) (0.10) - 0 (0.10) Discounted cash flow Nodal basis($/mmbtu) (e) (0.40) - 0.10 (0.20) Counterparty credit risk(%) (b)(c) 0.43 - 2 0.83 Ameren Illinois credit risk(%) (b)(c) 0.43 (d) Power (g) — (142 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps($/MWh) (e) 27 - 38 32 Nodal basis($/MWh) (e) (6) - 0 (2) Ameren Illinois credit risk(%) (b)(c) 0.43 (d) Fundamental energy production model Estimated future gas prices($/mmbtu) (e) 4 - 5 4 Escalation rate(%) (e)(i) 0 - 1 1 Contract price allocation Estimated renewable energy credit costs($/credit) (e) 5 - 7 6 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement. (c) Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances. (d) Not applicable. (e) Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement. (f) Escalation rate applies to fuel oil prices 2017 and beyond. (g) Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2018. Valuations beyond 2018 use fundamentally modeled pricing by month for peak and off-peak demand. (h) The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes due to their opposing positions. As such, refer to the power sensitivity analysis for each company above. (i) Escalation rate applies to power prices 2026 and beyond. In accordance with applicable authoritative accounting guidance, we consider nonperformance risk in our valuation of derivative instruments by analyzing the credit standing of our counterparties and considering any counterparty credit enhancements (e.g., collateral). The guidance also requires that the fair value measurement of liabilities reflect the nonperformance risk of the reporting entity, as applicable. Therefore, we have factored the impact of our credit standing, as well as any potential credit enhancements, into the fair value measurement of both derivative assets and derivative liabilities. Included in our valuation, and based on current market conditions, is a valuation adjustment for counterparty default derived from market data such as the price of credit default swaps, bond yields, and credit ratings. No gains or losses related to valuation adjustments for counterparty default risk were recorded at Ameren, Ameren Missouri, or Ameren Illinois in the first nine months of 2015 or 2014 . At September 30, 2015 , the counterparty default risk liability valuation adjustment related to derivative contracts totaled $1 million , less than $1 million , and $1 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively. At December 31, 2014 , the counterparty default risk liability valuation adjustment related to derivative contracts totaled $1 million , less than $1 million , and $1 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively. The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of September 30, 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 Power — — 23 23 Total derivative assets - commodity contracts $ — $ 1 $ 23 $ 24 Nuclear decommissioning trust fund: Cash and cash equivalents $ 3 $ — $ — $ 3 Equity securities: U.S. large capitalization 338 — — 338 Debt securities: Corporate bonds — 61 — 61 U.S. treasury and agency securities — 109 — 109 Other — 24 — 24 Total nuclear decommissioning trust fund $ 341 $ 194 $ — $ 535 (b) Total Ameren $ 341 $ 195 $ 23 $ 559 Ameren Derivative assets - commodity contracts (a) : Missouri Natural gas — 1 — 1 Power — — 23 23 Total derivative assets - commodity contracts $ — $ 1 $ 23 $ 24 Nuclear decommissioning trust fund: Cash and cash equivalents $ 3 $ — $ — $ 3 Equity securities: U.S. large capitalization 338 — — 338 Debt securities: Corporate bonds — 61 — 61 U.S. treasury and agency securities — 109 — 109 Other — 24 — 24 Total nuclear decommissioning trust fund $ 341 $ 194 $ — $ 535 (b) Total Ameren Missouri $ 341 $ 195 $ 23 $ 559 Liabilities: Ameren Derivative liabilities - commodity contracts (a) : Fuel oils $ 24 $ — $ 1 $ 25 Natural gas 1 56 1 58 Power — — 171 171 Uranium — — 1 1 Total Ameren $ 25 $ 56 $ 174 $ 255 Ameren Derivative liabilities - commodity contracts (a) : Missouri Fuel oils $ 24 $ — $ 1 $ 25 Natural gas 1 12 1 14 Power — — 1 1 Uranium — — 1 1 Total Ameren Missouri $ 25 $ 12 $ 4 $ 41 Ameren Derivative liabilities - commodity contracts (a) : Illinois Natural gas $ — $ 44 $ — $ 44 Power — — 170 170 Total Ameren Illinois $ — $ 44 $ 170 $ 214 (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, 2014 : 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) : Fuel oils $ — $ — $ 2 $ 2 Natural gas — 1 1 2 Power — 4 11 15 Total derivative assets - commodity contracts $ — $ 5 $ 14 $ 19 Nuclear decommissioning trust fund: Cash and cash equivalents $ 1 $ — $ — $ 1 Equity securities: U.S. large capitalization 364 — — 364 Debt securities: Corporate bonds — 63 — 63 U.S. treasury and agency securities — 102 — 102 Other — 17 — 17 Total nuclear decommissioning trust fund $ 365 $ 182 $ — $ 547 (b) Total Ameren $ 365 $ 187 $ 14 $ 566 Ameren Derivative assets - commodity contracts (a) : Missouri Fuel oils $ — $ — $ 2 $ 2 Natural gas — 1 — 1 Power — 4 11 15 Total derivative assets - commodity contracts $ — $ 5 $ 13 $ 18 Nuclear decommissioning trust fund: Cash and cash equivalents $ 1 $ — $ — $ 1 Equity securities: U.S. large capitalization 364 — — 364 Debt securities: Corporate bonds — 63 — 63 U.S. treasury and agency securities — 102 — 102 Other — 17 — 17 Total nuclear decommissioning trust fund $ 365 $ 182 $ — $ 547 (b) Total Ameren Missouri $ 365 $ 187 $ 13 $ 565 Ameren Derivative assets - commodity contracts (a) : Illinois Natural gas $ — $ — $ 1 $ 1 Liabilities: Ameren Derivative liabilities - commodity contracts (a) : Fuel oils $ 21 $ — $ 8 $ 29 Natural gas 1 53 2 56 Power — 1 144 145 Uranium — — 2 2 Total Ameren $ 22 $ 54 $ 156 $ 232 Ameren Derivative liabilities - commodity contracts (a) : Missouri Fuel oils $ 21 $ — $ 8 $ 29 Natural gas 1 10 1 12 Power — 1 2 3 Uranium — — 2 2 Total Ameren Missouri $ 22 $ 11 $ 13 $ 46 Ameren Derivative liabilities - commodity contracts (a) : Illinois Natural gas $ — $ 43 $ 1 $ 44 Power — — 142 142 Total Ameren Illinois $ — $ 43 $ 143 $ 186 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Balance excludes $2 million of receivables, payables, and accrued income, net. The following table summarizes the changes in the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy for the three months ended September 30, 2015 : Net derivative commodity contracts Ameren Missouri Ameren Illinois Ameren Fuel oils: Beginning balance at July 1, 2015 $ (1 ) $ (a) $ (1 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (1 ) (a) (1 ) Settlements 1 (a) 1 Ending balance at September 30, 2015 $ (1 ) $ (a) $ (1 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2015 $ — $ (a) $ — Natural gas: Beginning balance at July 1, 2015 $ — $ (1 ) $ (1 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (1 ) 1 — Ending balance at September 30, 2015 $ (1 ) $ — $ (1 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2015 $ — $ — $ — Power: Beginning balance at July 1, 2015 $ 27 $ (165 ) $ (138 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities 2 (8 ) (6 ) Settlements (7 ) 3 (4 ) Ending balance at September 30, 2015 $ 22 $ (170 ) $ (148 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2015 $ 1 $ (7 ) $ (6 ) Uranium: Beginning balance at July 1, 2015 $ (2 ) $ (a) $ (2 ) Settlements 1 (a) 1 Ending balance at September 30, 2015 $ (1 ) $ (a) $ (1 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2015 $ — $ (a) $ — (a) Not applicable. The following table summarizes the changes in the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy for the three months ended September 30, 2014 : Net derivative commodity contracts Ameren Missouri Ameren Illinois Ameren Fuel oils: Beginning balance at July 1, 2014 $ 2 $ (a) $ 2 Realized and unrealized gains (losses) included in regulatory assets/liabilities (2 ) (a) (2 ) Ending balance at September 30, 2014 $ — $ (a) $ — Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014 $ (2 ) $ (a) $ (2 ) Natural gas: Beginning balance at July 1, 2014 $ — $ — $ — Realized and unrealized gains (losses) included in regulatory assets/liabilities — 1 1 Ending balance at September 30, 2014 $ — $ 1 $ 1 Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014 $ — $ — $ — Power: Beginning balance at July 1, 2014 $ 15 $ (103 ) $ (88 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (5 ) (23 ) (28 ) Settlements (5 ) 2 (3 ) Ending balance at September 30, 2014 $ 5 $ (124 ) $ (119 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014 $ (6 ) $ (22 ) $ (28 ) Uranium: Beginning balance at July 1, 2014 $ (7 ) $ (a) $ (7 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities 3 (a) 3 Settlements 1 (a) 1 Ending balance at September 30, 2014 $ (3 ) $ (a) $ (3 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014 $ 3 $ (a) $ 3 (a) Not applicable. The following table summarizes the changes in the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy for the nine months ended September 30, 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 4 (a) 4 Transfers out of Level 3 2 (a) 2 Ending balance at September 30, 2015 $ (1 ) $ (a) $ (1 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2015 $ — $ (a) $ — Natural gas: Beginning balance at January 1, 2015 $ (1 ) $ — $ (1 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (1 ) 1 — Settlements 1 (1 ) — Ending balance at September 30, 2015 $ (1 ) $ — $ (1 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2015 $ — $ — $ — Power: Beginning balance at January 1, 2015 $ 9 $ (142 ) $ (133 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities — (37 ) (37 ) Purchases 29 — 29 Settlements (16 ) 9 (7 ) Ending balance at September 30, 2015 $ 22 $ (170 ) $ (148 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2015 $ 1 $ (35 ) $ (34 ) Uranium: Beginning balance at January 1, 2015 $ (2 ) $ (a) $ (2 ) Settlements 1 (a) 1 Ending balance at September 30, 2015 $ (1 ) $ (a) $ (1 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2015 $ — $ (a) $ — (a) Not applicable. The following table summarizes the changes in the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy for the nine months ended September 30, 2014 : Net derivative commodity contracts Ameren Missouri Ameren Illinois Ameren Fuel oils: Beginning balance at January 1, 2014 $ 5 $ (a) $ 5 Realized and unrealized gains (losses) included in regulatory assets/liabilities (3 ) (a) (3 ) Settlements (2 ) (a) (2 ) Ending balance at September 30, 2014 $ — $ (a) $ — Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014 $ (2 ) $ (a) $ (2 ) Natural gas: Beginning balance at January 1, 2014 $ — $ — $ — Realized and unrealized gains (losses) included in regulatory assets/liabilities — 1 1 Purchases — (1 ) (1 ) Settlements — 1 1 Ending balance at September 30, 2014 $ — $ 1 $ 1 Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014 $ — $ — $ — Power: Beginning balance at January 1, 2014 $ 19 $ (108 ) $ (89 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (23 ) (19 ) (42 ) Purchases 34 — 34 Settlements (25 ) 3 (22 ) Ending balance at September 30, 2014 $ 5 $ (124 ) $ (119 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014 $ (3 ) $ (21 ) $ (24 ) Uranium: Beginning balance at January 1, 2014 $ (6 ) $ (a) $ (6 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (1 ) (a) (1 ) Settlements 4 (a) 4 Ending balance at September 30, 2014 $ (3 ) $ (a) $ (3 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014 $ — $ (a) $ — (a) Not applicable. Transfers in or out of Level 3 represent either (1) existing assets and liabilities that were previously categorized as a higher level, but were recategorized to Level 3 because the inputs to the model became unobservable during the period or (2) existing assets and liabilities that were previously classified as Level 3, but were recategorized to a higher level because the lowest significant input became observable during the period. Transfers between Level 1 and Level 3 for fuel oils were primarily caused by changes in availability of similar financial trades observable on electronic exchanges between the periods. Any reclassifications are reported as transfers out of Level 3 at the fair value measurement reported at the beginning of the period in which the changes occur. For the three and nine months ended September 30, 2015 , and 2014 , there were no transfers between Level 1, Level 2, and Level 3 related to derivative commodity contracts, with the exception of $2 million of transfers out of Level 3 into Level 1 related to fuel oil contracts at Ameren and Ameren Missouri for the nine months ended September 30, 2015. 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 September 30, 2015 , and December 31, 2014 : September 30, 2015 December 31, 2014 Carrying Amount Fair Value Carrying Amount Fair Value Ameren: (a) Long-term debt and capital lease obligations (including current portion) $ 6,376 $ 6,916 $ 6,240 $ 7,135 Preferred stock 142 124 142 122 Ameren Missouri: Long-term debt and capital lease obligations (including current portion) $ 4,135 $ 4,488 $ 3,999 $ 4,518 Preferred stock 80 75 80 73 Ameren Illinois: Long-term debt $ 2,241 $ 2,428 $ 2,241 $ 2,517 Preferred stock 62 49 62 49 (a) Preferred stock is recorded in “Noncontrolling Interests” on the consolidated balance sheet.

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2015
Related Party Transactions [Abstract]
RELATED PARTY TRANSACTIONS	RELATED PARTY TRANSACTIONS Ameren (parent) and its subsidiaries have engaged in, and may in the future engage in, affiliate transactions in the normal course of business. These transactions primarily consist of power purchases and sales, services received or rendered, and borrowings and lendings. Transactions between affiliates are reported as intercompany transactions on their respective financial statements but are eliminated in consolidation for Ameren’s financial statements. For a discussion of our material related party agreements, see Note 14 - Related Party Transactions under Part II, Item 8, of the Form 10-K and the money pool arrangements discussed in Note 3 - Short-term Debt and Liquidity of this report. Electric Power Supply Agreements In April and September 2015, Ameren Illinois conducted procurement events, administered by the IPA, to purchase energy products through May 31, 2018. Ameren Missouri was among the winning suppliers in these events. As a result, in April 2015, Ameren Missouri and Ameren Illinois entered into energy product agreements by which Ameren Missouri agreed to sell and Ameren Illinois agreed to purchase 667,000 megawatthours at an average price of $36 per megawatthour during the period of June 1, 2015, through June 30, 2017. In September 2015, Ameren Missouri and Ameren Illinois entered into energy product agreements by which Ameren Missouri agreed to sell and Ameren Illinois agreed to purchase 339,000 megawatthours at an average price of $38 per megawatthour during the period of November 1, 2015 through May 31, 2018. The following table presents the impact on Ameren Missouri and Ameren Illinois of related party transactions for the three and nine months ended September 30, 2015 and 2014 . Three Months Nine Months Agreement Income Statement Line Item Ameren Missouri Ameren Illinois Ameren Missouri Ameren Illinois Ameren Missouri power supply Operating Revenues 2015 $ 4 $ (a) $ 9 $ (a) agreements with Ameren Illinois 2014 2 (a) 5 (a) Ameren Missouri and Ameren Illinois Operating Revenues 2015 6 1 19 3 rent and facility services 2014 6 (b) 15 1 Ameren Missouri and Ameren Illinois Operating Revenues 2015 1 (b) 2 (b) miscellaneous support services 2014 (b) (b) 1 (b) Total Operating Revenues 2015 $ 11 $ 1 $ 30 $ 3 2014 8 (b) 21 1 Ameren Illinois power supply Purchased Power 2015 $ (a) $ 4 $ (a) $ 9 agreements with Ameren Missouri 2014 (a) 2 (a) 5 Ameren Illinois transmission Purchased Power 2015 (a) 1 (a) 2 services with ATXI 2014 (a) 1 (a) 2 Total Purchased Power 2015 $ (a) $ 5 $ (a) $ 11 2014 (a) 3 (a) 7 Ameren Services support services Other Operations and Maintenance 2015 $ 30 $ 28 $ 96 $ 87 agreement 2014 25 26 90 80 Money pool borrowings (advances) Interest Charges/ Miscellaneous Income 2015 $ (b) $ (b) $ (b) $ (b) 2014 (b) (b) (b) (b) (a) Not applicable. (b) Amount less than $1 million.

Commitments And Contingencies

Commitments And Contingencies	9 Months Ended
	Sep. 30, 2015
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	COMMITMENTS AND CONTINGENCIES We are involved in legal, tax and regulatory proceedings before various courts, regulatory commissions, authorities and governmental agencies with respect to matters that arise in the ordinary course of business, some of which involve substantial amounts of money. We believe that the final disposition of these proceedings, except as otherwise disclosed in the notes to our financial statements in this report and in our Form 10-K, will not have a material adverse effect on our results of operations, financial position, or liquidity. Reference is made to Note 1 - Summary of Significant Accounting Policies, Note 2 - Rate and Regulatory Matters, Note 14 - Related Party Transactions, Note 15 - Commitments and Contingencies, and Note 16 - Divestiture Transactions and Discontinued Operations under Part II, Item 8, of the Form 10-K. See also Note 1 - Summary of Significant Accounting Policies, Note 2 - Rate and Regulatory Matters, Note 8 - Related Party Transactions, Note 10 - Callaway Energy Center, and Note 12 - Divestiture Transactions and Discontinued Operations in this report. Callaway Energy Center The following table presents insurance coverage at Ameren Missouri’s Callaway energy center at September 30, 2015 . The property coverage and the nuclear liability coverage must be renewed on April 1 and January 1, respectively, of each year. Both coverages were renewed in 2015. Type and Source of Coverage Maximum Coverages Maximum Assessments for Single Incidents Public liability and nuclear worker liability: American Nuclear Insurers $ 375 $ — Pool participation 12,986 (a) 127 (b) $ 13,361 (c) $ 127 Property damage: NEIL $ 2,750 (d) $ 27 (e) European Mutual Association for Nuclear Insurance 500 (f) — $ 3,250 $ 27 Replacement power: NEIL $ 490 (g) $ 10 (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 both radiation and nonradiation events. An additional $500 million is provided for radiation events only for a total of $2.75 billion . (e) All NEIL insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL. (f) European Mutual Association for Nuclear Insurance provides $500 million in excess of the $2.75 billion and $2.25 billion property coverage for radiation and nonradiation events, respectively, provided by NEIL. (g) Provides replacement power cost insurance in the event of a prolonged accidental outage. Weekly indemnity up to $4.5 million for 52 weeks, which commences after the first twelve weeks of an outage, plus up to $3.6 million per week for a minimum of 71 weeks thereafter for a total not exceeding the policy limit of $490 million . Nonradiation events are sub-limited to $328 million . The Price-Anderson Act is a federal law that limits the liability for claims from an incident involving any licensed United States commercial nuclear energy center. The limit is based on the number of licensed reactors. The limit of liability and the maximum potential annual payments are adjusted at least every five years for inflation to reflect changes in the Consumer Price Index. The most recent five-year inflationary adjustment became effective in September 2013. Owners of a nuclear reactor cover this exposure through a combination of private insurance and mandatory participation in a financial protection pool, as established by the Price-Anderson Act. Losses resulting from terrorist attacks on nuclear facilities are covered under NEIL’s policies, subject to an industrywide aggregate policy limit of $3.24 billion within a 12-month period, or $1.83 billion for events not involving radiation contamination. If losses from a nuclear incident at the Callaway energy center exceed the limits of, or are not covered by, insurance, or if coverage is unavailable, Ameren Missouri is at risk for any uninsured losses. If a serious nuclear incident were to occur, it could have a material adverse effect on Ameren’s and Ameren Missouri’s results of operations, financial position, or liquidity. Other Obligations To supply a portion of the fuel requirements of Ameren Missouri’s energy centers, Ameren Missouri has entered into various long-term commitments for the procurement of coal, natural gas, nuclear fuel, and methane gas. Additionally, Ameren Missouri and Ameren Illinois have entered into various long-term commitments for purchased power and natural gas for distribution. For a complete listing of our obligations and commitments, see Note 15 - Commitments and Contingencies under Part II, Item 8 of the Form 10-K. At September 30, 2015 , total other 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 $4,832 million , $2,913 million , and $1,888 million , respectively. In April and September 2015, Ameren Illinois conducted procurement events, administered by the IPA, to purchase energy products through May 31, 2018. In the April procurement event, Ameren Illinois contracted to purchase approximately 5,526,000 megawatthours of energy products for $185 million from June 1, 2015, through May 31, 2018. In the September procurement event, Ameren Illinois contracted to purchase approximately 2,592,000 megawatt hours of energy products for $83 million from November 1, 2015, through May 31, 2018. The results of both procurement events are included in Ameren’s and Ameren Illinois’ obligations discussed above. See Note 8 - Related Party Transactions in this report for additional information regarding energy product agreements between Ameren Missouri and Ameren Illinois as a result of these procurement events. Environmental Matters We are subject to various environmental laws and regulations enforced by federal, state, and local authorities. From the beginning phases of siting and development to the operation of existing or new electric generation, transmission and distribution facilities and natural gas storage, transmission and distribution facilities, our activities involve compliance with diverse environmental laws and regulations. These laws and regulations address emissions, discharges to water, water usage, impacts to air, land, and water, and chemical and waste handling. Complex and lengthy processes are required to obtain and renew approvals, permits, or licenses for new, existing or modified facilities. Additionally, the use and handling of various chemicals or hazardous materials require release prevention plans and emergency response procedures. The EPA is developing and implementing environmental regulations that will have a significant impact on the electric utility industry. Over time, compliance with these regulations could be costly for certain companies, including Ameren Missouri, that operate coal-fired power plants. Significant new rules proposed or promulgated include the regulation of 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 of CCR and CCR landfills and impoundments; the MATS, which require reduction of emissions of mercury, toxic metals, and acid gases from power plants; revised NSPS for particulate matter, SO 2 , and NO x emissions from new sources; new effluent standards applicable to waste water discharges from power plants and new regulations under the Clean Water Act that could require significant capital expenditures, such as modifications to water intake structures or new cooling towers at Ameren Missouri’s energy centers. 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 . The EPA issued a more stringent national ambient air quality standard for ozone on October 1, 2015. Certain of these new regulations 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 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 these costs would be recoverable through rates, subject to MoPSC prudence review, but the nature and timing of costs, as well as the applicable regulatory framework, could result in regulatory lag. 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 $350 million to $400 million in the aggregate from 2015 through 2019 in order to comply with existing environmental regulations. Ameren Missouri may be required to install additional pollution controls within the next six to 10 years. This estimate includes our capital expenditures required for the CCR regulations that were published in 2015, compliance with the MATS, and the rule applicable to cooling water intake structures at existing power plants under the Clean Water Act, all of which are discussed below. This estimate does not include the impacts of the Clean Power Plan and the new effluent limitation guidelines applicable to steam electric generating units under the Clean Water Act, both of which are discussed below. Considerable uncertainty remains in this estimate. 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 on January 1, 2015, for SO 2 and annual NO x reductions, and on May 1, 2015, for ozone season NO x reductions. There will be further emission reduction requirements in 2017 and potentially more in subsequent years. Ameren Missouri expects to have sufficiently reduced emissions and have sufficient allowances to avoid purchasing allowances to comply with CSAPR for 2015. To achieve compliance with CSAPR, Ameren Missouri operates two scrubbers at its Sioux energy center and burns ultra-low sulfur coal. Ameren Missouri does not expect to make additional capital investments to comply with the current CSAPR requirements. However, Ameren Missouri expects to incur additional operations and maintenance costs to lower its emissions at one or more of its energy centers for compliance with the CSAPR in future years. These higher operations and maintenance costs are expected to be collected from customers through the FAC or higher base rates. In December 2011, the EPA issued the MATS under the Clean Air Act, which requires reductions in emissions of mercury and other hazardous air pollutants, such as acid gases, trace metals, and hydrogen chloride. The MATS do not require a specific control technology to achieve the emission reductions. The MATS apply to each unit at a coal-fired power plant. However, in certain cases, compliance can be achieved by averaging emissions from similar units at the same power plant. Compliance was required by April 2015 or, with a case-by-case extension, by April 2016. Ameren Missouri's Labadie and Meramec energy centers were granted extensions. In June 2015, the United States Supreme Court remanded a case challenging the MATS to the United States Court of Appeals for the District of Columbia Circuit for further consideration and ruled that the EPA was required to consider costs before deciding whether regulation of emissions of mercury and other hazardous air pollutants from power plants was appropriate and necessary. Ameren Missouri expects to make additional capital investments at its Labadie and Meramec energy centers to comply with the MATS by April 2016. These capital expenditure investments are included in Ameren's and Ameren Missouri's estimate of capital expenditures to comply with existing environmental regulations discussed above. In addition, Ameren Missouri is incurring additional operations and maintenance costs to lower its emissions at its energy centers in compliance with the MATS. These higher operations and maintenance costs are being collected from customers through the FAC or higher base rates. Greenhouse Gas Regulation The finalized version of the Clean Power Plan, which sets forth CO 2 emissions standards that will be applicable to existing power plants, was published in the Federal Register in October 2015 and will become effective in December 2015. The finalized regulations differ significantly from the proposed rule that the EPA issued in June 2014. Under the Clean Power Plan, Ameren Missouri expects to incur increased net fuel and operating costs, and new or accelerated capital expenditures, in addition to making modifications to existing operations in order to achieve compliance. Missouri and Illinois are each required to reduce their greenhouse gas emissions significantly below 2005 levels by 2030. States are required to submit preliminary compliance plans to the EPA as early as 2016, although extensions until 2018 are available. The rule contains interim compliance periods commencing in 2022 that require each state to demonstrate progress in achieving its greenhouse gas reduction target. Ameren Missouri is evaluating the Clean Power Plan and its potential implementation by Missouri and Illinois. This assessment includes the potential impacts to its operations, including those related to electric system reliability and 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 the finalized rule is different from the proposed rule and 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. We cannot predict the outcome of the legal challenges or their impact on our results of operations, financial position or liquidity. 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. 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 August 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 plants and new coal-fired plants. Federal and state legislation or regulations that mandate limits on the emission of greenhouse gases may result in significant increases in capital expenditures and operating costs, which could lead to increased liquidity needs and higher financing costs. Mandatory limits on the emission of greenhouse gases could increase costs for Ameren Missouri’s customers or have a material adverse effect on Ameren's and Ameren Missouri's results of operations, financial position, and liquidity if regulators delay or deny recovery in rates of these compliance costs. The cost of Ameren Illinois’ purchased power and gas purchased for resale could increase; however, Ameren Illinois expects these costs would be recovered from customers with no material adverse effect on its results of operations, financial position, or liquidity. Ameren's and Ameren Missouri's earnings might benefit from increased investment to comply with greenhouse gas limitations to the extent that the investments are reflected and recovered timely in rates charged to customers. NSR and Clean Air Litigation In January 2011, the Department of Justice, on behalf of the EPA, filed a complaint against Ameren Missouri in the United States District Court for the Eastern District of Missouri. The EPA's complaint, as amended in October 2013, alleges that in performing projects at its Rush Island coal-fired energy center in 2007 and 2010, Ameren Missouri violated provisions of the Clean Air Act and Missouri law. Ameren Missouri anticipates 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 future 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, increased operations and maintenance expenses, and penalties. We are unable to predict the ultimate resolution of these matters or the costs that might be incurred. Clean Water Act In August 2014, the EPA published the final rule applicable to cooling water intake structures at existing power plants. The rule requires a case-by-case evaluation and plan for reducing aquatic organisms impinged on the facility’s intake screens or entrained through the plant's cooling water system. Implementation of this rule will be administered through each power plant’s water discharge permitting process. All of Ameren Missouri’s coal-fired and nuclear energy centers are subject to this rule. The rule could have an adverse effect on Ameren’s and Ameren Missouri’s results of operations, financial position, and liquidity if its implementation requires the installation of cooling towers or extensive modifications to the cooling water systems at our energy centers and if those investments are not recovered timely in electric rates charged to its customers. In September 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 wastewater discharges that are based on the effectiveness of available control technology. The EPA's rule prohibits effluent discharges of certain, but not all, waste streams and imposes more stringent limitations on certain components in wastewater discharges from power plants. All of Ameren Missouri’s coal-fired energy centers are subject to this rule. Each of Ameren Missouri’s energy centers will become subject to the revised limitations when it renews its wastewater discharge permit. These permits are scheduled to be renewed between 2018 and 2023. Ameren Missouri is evaluating the final rule, which will become effective in January 2016, and the possible effects on its operations. Ash Management The EPA issued regulations in 2015 regarding the management and disposal of CCR, which will affect future 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. During the second quarter of 2015, Ameren and Ameren Missouri recorded an increase to their AROs associated with CCR storage facilities as a result of the new regulations. See Note 1 - Summary of Significant Accounting Policies in this report for additional information. Ameren Missouri's capital expenditure plan includes the cost of constructing landfills as part of its environmental compliance plan, with expenditures beginning in 2015. Ameren Missouri expects certain of its ash ponds could be closed within the next five years. The new regulations do not apply to inactive ash ponds at plants no longer in operation, such as Ameren’s Meredosia and Hutsonville energy centers. Remediation The Ameren Companies are involved in a number of remediation actions to clean up sites affected by hazardous substances, as required by federal and state law. Such laws require that responsible parties fund remediation actions regardless of their degree of fault, the legality of original disposal, or the ownership of a disposal site. Ameren Missouri and Ameren Illinois have each been identified by federal or state governments as a potentially responsible party at several contaminated sites. As of September 30, 2015 , Ameren Illinois owned or was otherwise responsible for 44 former MGP sites in Illinois, which are in various stages of investigation, evaluation, remediation, and closure. Ameren Illinois estimates it could substantially conclude remediation efforts 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. To be recoverable, such costs must be prudently incurred. Costs are subject to annual review by the ICC. As of September 30, 2015 , Ameren Illinois estimated the obligation related to these former MGP sites at $237 million to $309 million . Ameren and Ameren Illinois recorded a liability of $237 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, as many factors can influence the ultimate actual costs, including site specific unanticipated underground structures, the degree to which groundwater is encountered, regulatory changes, local ordinances, and site accessibility. The actual costs may vary substantially from these estimates. Ameren Illinois formerly used an off-site landfill, which Ameren Illinois did not own, in connection with the operation of a previously-owned energy center. Ameren Illinois could be required to perform certain maintenance activities at that landfill, which is now closed. As of September 30, 2015 , Ameren Illinois estimated the obligation related to the landfill at $0.5 million to $6 million . Ameren Illinois recorded a liability of $0.5 million to represent its estimated minimum obligation for this site, as no other amount within the range was a better estimate. Ameren Illinois is also responsible for the cleanup of some underground storage tanks and a water treatment plant in Illinois. As of September 30, 2015 , Ameren Illinois recorded a liability of $0.7 million to represent its best estimate of the obligation for these sites. In 2008, the EPA issued an administrative order to Ameren Missouri pertaining to a former coal tar distillery 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. Ameren Missouri, along with two other potentially responsible parties, have completed site investigation activities and have submitted their findings to the EPA. As of September 30, 2015 , Ameren Missouri estimated its obligation at $2 million to $5 million . Ameren Missouri recorded a liability of $2 million to represent its estimated minimum obligation, as no other amount within the range was a better estimate. Ameren Missouri also participated in the investigation of various sites located in Sauget, Illinois. In 2000, the EPA notified Ameren Missouri and numerous other companies, including Solutia, Inc., that former landfills and lagoons at those sites may contain soil and groundwater contamination. These sites are known as Sauget Area 2. From about 1926 until 1976, Ameren Missouri operated an energy center adjacent to Sauget Area 2. Ameren Missouri currently owns a parcel of property at Sauget Area 2 that was once used as a landfill. In December 2013, the EPA issued its record of decision for Sauget Area 2 approving the investigation and the remediation alternatives recommended by the potentially responsible parties. Further negotiation among the potentially responsible parties will determine how to fund the implementation of the EPA-approved cleanup remedies. As of September 30, 2015 , Ameren Missouri estimated its obligation related to Sauget Area 2 at $1 million to $2.5 million . Ameren Missouri recorded a liability of $1 million to represent its estimated minimum obligation for this site, as no other amount within the range was a better estimate. In December 2012, Ameren Missouri signed an administrative order with the EPA and agreed to investigate soil and groundwater conditions at an Ameren Missouri-owned substation in St. Charles, Missouri. As of September 30, 2015 , Ameren Missouri estimated and recorded a $0.8 million liability related to the remaining cleanup of the site. Our operations or those of our predecessor companies involve the use of, disposal of, and in appropriate circumstances, the cleanup of substances regulated under environmental laws. We are unable to determine whether such practices will result in future environmental commitments or will affect our results of operations, financial position, or liquidity. Pumped-storage Hydroelectric Facility Breach In December 2005, there was a breach of the upper reservoir at Ameren Missouri's Taum Sauk pumped-storage hydroelectric energy center. The breach resulted in significant flooding in the local area, which damaged a state park. Ameren Missouri had liability insurance coverage for the Taum Sauk incident, subject to certain limits and deductibles. In 2010, Ameren Missouri sued an insurance company that was providing Ameren Missouri with liability coverage on the date of the Taum Sauk incident. In the litigation, which is pending in the United States District Court for the Southern District of New York, Ameren Missouri claims that the insurance company breached its duty to indemnify Ameren Missouri for losses resulting from the incident. As of September 30, 2015 , Ameren Missouri had a net insurance receivable balance of $41 million . The insurance claim was $54 million as of September 30, 2015 . Ameren Missouri expects to collect this receivable from the insurance company in the pending litigation described above. This receivable is included in “Other assets” on Ameren’s and Ameren Missouri’s balance sheets as of September 30, 2015 . Ameren's and Ameren Missouri's results of operations, financial position, and liquidity could be adversely affected if Ameren Missouri's insurance receivable balance is not collected. Asbestos-related Litigation Ameren, Ameren Missouri, and Ameren Illinois have been named, along with numerous other parties, in a number of lawsuits filed by plaintiffs claiming varying degrees of injury from asbestos exposure at our current and former energy centers. Most have been filed in the Circuit Court of Madison County, Illinois. The total number of defendants named in each case varies, with 73 as the average number of parties as of September 30, 2015 . Each lawsuit seeks unspecified damages that, if awarded at trial, typically would be shared among the various defendants. The following table presents the pending asbestos-related lawsuits filed against the Ameren Companies as of September 30, 2015 : Ameren Ameren Missouri Ameren Illinois Total (a) 1 27 40 53 (a) Total does not equal the sum of the subsidiary unit lawsuits because some of the lawsuits name multiple Ameren entities as defendants. As of September 30, 2015 , Ameren, Ameren Missouri, and Ameren Illinois had “Other current liabilities” of $8 million , $3 million , and $5 million , respectively, recorded to represent their best estimate of their obligations related to asbestos claims. Ameren Illinois has a tariff rider to recover the costs of IP asbestos-related litigation claims, subject to the following terms: 90% of cash expenditures in excess of the amount included in base electric rates are to be recovered from a trust fund that was established when Ameren acquired IP. At September 30, 2015 , the trust fund balance was $22 million , including accumulated interest. If cash expenditures are less than the amount in base rates, Ameren Illinois will contribute 90% of the difference to the trust fund. Once the trust fund is depleted, 90% of allowed cash expenditures in excess of base rates will be recovered through charges assessed to customers under the tariff rider. The rider will permit recovery from customers within IP’s historical service territory.

Callaway Energy Center

Callaway Energy Center	9 Months Ended
	Sep. 30, 2015
Nuclear Waste Matters [Abstract]
CALLAWAY ENERGY CENTER	CALLAWAY ENERGY CENTER Under the NWPA, the DOE is responsible for disposing of spent nuclear fuel from the Callaway energy center and other commercial nuclear energy centers. Under the NWPA, Ameren and other utilities that own and operate those energy centers are responsible for paying the disposal costs. The NWPA established the fee that these utilities pay the federal government for disposing of the spent nuclear fuel at one mill, or one-tenth of one cent, for each kilowatthour generated and sold by those plants. The NWPA also requires the DOE annually to review the nuclear waste fee against the cost of the nuclear waste disposal program and to propose to the United States Congress any fee adjustment necessary to offset the costs of the program. As required by the NWPA, Ameren Missouri and other utilities have entered into standard contracts with the 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. Ameren Missouri has sufficient installed capacity at the Callaway energy center to store its spent nuclear fuel generated through the end of the energy center’s operating license in 2044. 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 incurred from 2010 through 2016, with the ability to extend the recovery period as mutually agreed upon by the parties. Included in these reimbursements are costs related to the dry spent fuel storage facility at the Callaway energy center, to which Ameren Missouri began transferring spent fuel assemblies in 2015. Ameren Missouri will continue to apply for reimbursement from the DOE for allowable costs associated with the dry spent fuel storage facility. Electric utility rates charged to customers provide for the recovery of the Callaway energy center's decommissioning costs, which include decontamination, dismantling, and site restoration costs, over the expected life of the nuclear energy center. Amounts collected from customers are deposited into the external nuclear decommissioning trust fund to provide for the Callaway energy center’s decommissioning. It is assumed that the Callaway energy center site will be decommissioned through the immediate dismantlement method and removed from service. Ameren and Ameren Missouri have recorded an ARO for the Callaway energy center decommissioning costs at fair value, which represents the present value of estimated future cash outflows. Annual decommissioning costs of $7 million are included in the costs used to establish electric rates for Ameren Missouri's customers. Every three years, the MoPSC requires Ameren Missouri to file an updated cost study and funding analysis for decommissioning its Callaway energy center. Following the NRC’s decision in March 2015 to extend the Callaway energy center’s operating license from 2024 to 2044, an updated cost study and a revised funding analysis were filed with the MoPSC in April 2015. Ameren Missouri’s April 2015 filing supported no change in electric service rates. There is no time requirement by which the MoPSC must issue an order regarding the decommissioning cost included in Ameren Missouri’s electric service rates. If the assumed return on trust assets is not earned, we believe that it is probable that any such earnings deficiency will be recovered in rates. The fair value of the trust fund for Ameren Missouri's Callaway energy center is reported as "Nuclear decommissioning trust fund" in Ameren's and Ameren Missouri's balance sheets. This amount is legally restricted and may be used only to fund the costs of nuclear decommissioning. Changes in the fair value of the trust fund are recorded as an increase or decrease to the nuclear decommissioning trust fund, with an offsetting adjustment to the related regulatory liability.

Retirement Benefits

Retirement Benefits	9 Months Ended
	Sep. 30, 2015
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]
RETIREMENT BENEFITS	RETIREMENT BENEFITS Ameren’s pension plans are funded in compliance with income tax regulations and to meet federal funding or regulatory requirements. As a result, Ameren expects to fund its pension plans at a level equal to the greater of the pension expense or the legally required minimum contribution. Considering Ameren’s assumptions at September 30, 2015 , the plan’s estimated investment performance through September 30, 2015 , and Ameren’s pension funding policy, Ameren expects to make annual contributions of $50 million to $110 million through 2019, with aggregate estimated contributions of $350 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 and nine months ended September 30, 2015 and 2014 : Pension Benefits Postretirement Benefits Three Months Nine Months Three Months Nine Months 2015 2014 2015 2014 2015 2014 2015 2014 Service cost $ 23 $ 20 $ 69 $ 60 $ 6 $ 5 $ 17 $ 14 Interest cost 43 46 130 137 12 12 36 37 Expected return on plan assets (62 ) (58 ) (186 ) (172 ) (17 ) (16 ) (51 ) (48 ) Amortization of: Prior service benefit — (1 ) — (1 ) (1 ) (2 ) (3 ) (4 ) Actuarial loss (gain) 19 13 56 37 1 (2 ) 4 (5 ) Settlement loss — — 1 — — — — — Net periodic benefit cost (benefit) $ 23 $ 20 $ 70 $ 61 $ 1 $ (3 ) $ 3 $ (6 ) 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 and nine months ended September 30, 2015 and 2014 : Pension Benefits Postretirement Benefits Three Months Nine Months Three Months Nine Months 2015 2014 2015 2014 2015 2014 2015 2014 Ameren Missouri (a) $ 14 $ 13 $ 42 $ 38 $ 2 $ — $ 6 $ 2 Ameren Illinois 9 7 28 22 — (3 ) (2 ) (7 ) Other — — — 1 (1 ) — (1 ) (1 ) Ameren (a)(b) $ 23 $ 20 $ 70 $ 61 $ 1 $ (3 ) $ 3 $ (6 ) (a) The amounts above do 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 built into rates. (b) Includes amounts for Ameren registrants and nonregistrant subsidiaries.

Divestiture Transactions and Di

Divestiture Transactions and Discontinued Operations	9 Months Ended
	Sep. 30, 2015
Discontinued Operations and Disposal Groups [Abstract]
DIVESTITURE TRANSACTIONS AND DISCONTINUED OPERATIONS	DIVESTITURE TRANSACTIONS AND DISCONTINUED OPERATIONS On December 2, 2013, Ameren completed the divestiture of New AER to IPH in accordance with the transaction agreement between Ameren and IPH dated March 14, 2013, as amended by a letter agreement dated December 2, 2013. The transaction agreement with IPH, as amended, provided that if the Elgin, Gibson City, and Grand Tower gas-fired energy centers were subsequently sold by Medina Valley and if Medina Valley received additional proceeds from such sale, Medina Valley would pay Genco any proceeds from such sale, net of taxes and other expenses, in excess of the $137.5 million previously paid to Genco. On January 31, 2014, Medina Valley completed the sale of the Elgin, Gibson City, and Grand Tower gas-fired energy centers to Rockland Capital for a total purchase price of $168 million . The agreement with Rockland Capital requires 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. The Rockland Capital escrow receivable balance and the corresponding payable due to Genco, both totaling $14 million , are reflected on Ameren's September 30, 2015 consolidated balance sheet in "Other current assets" and in "Other current liabilities," respectively. Medina Valley expects to pay Genco any amount it receives from the escrow on January 31, 2016. Ameren did not record a gain from its sale of the Elgin, Gibson City, and Grand Tower gas-fired energy centers. Discontinued Operations Presentation See Note 16 - Divestiture Transactions and Discontinued Operations under Part II, Item 8, of the Form 10-K for additional information related to disposal groups. All matters related to the final tax basis of New AER and the related tax benefit resulting from the divested merchant generation business have been resolved with the completion of the IRS’ audit for 2013. During the second quarter of 2015, based on the completion of the IRS audit, Ameren removed a reserve for unrecognized tax benefits recorded in 2013 and recognized a tax benefit from discontinued operations. See Note 1 - Summary of Significant Accounting Policies for additional information regarding the Ameren Companies’ uncertain tax positions. The following table presents the components of discontinued operations in Ameren's consolidated statement of income for the three and nine months ended September 30, 2015 and 2014 : Three Months Nine Months 2015 2014 2015 2014 Operating revenues $ — $ — $ — $ 1 Operating benefits (expenses) (1 ) (1 ) 2 (4 ) Operating income (loss) before income tax (1 ) (1 ) 2 (3 ) Income tax benefit 1 — 50 — Income (loss) from discontinued operations, net of taxes $ — $ (1 ) $ 52 $ (3 ) The following table presents the carrying amounts of the components of assets and liabilities of Ameren’s discontinued operations, which consist primarily of AROs and related deferred income tax assets associated with the abandoned Meredosia and Hutsonville energy centers, at September 30, 2015 , and December 31, 2014: September 30, 2015 December 31, 2014 Assets of discontinued operations Accumulated deferred income taxes, net $ 17 $ 15 Total assets of discontinued operations $ 17 $ 15 Liabilities of discontinued operations Accounts payable and other current obligations $ 1 $ 1 Asset retirement obligations 29 32 Total liabilities of discontinued operations $ 30 $ 33 Pursuant to the IPH transaction agreement, as amended, Ameren is obligated to pay up to $25 million for certain liabilities, which were included in "Other current liabilities" on Ameren's September 30, 2015 consolidated balance sheet as the payment is expected to be made by the end of 2015. Included in “Miscellaneous accounts and notes receivable” on Ameren’s consolidated balance sheet is a note receivable from Marketing Company related to the cash collateral support provided to New AER. The note receivable balance was $8 million and $12 million at September 30, 2015, and December 31, 2014, respectively. This note receivable is due to Ameren, with interest, on December 2, 2015, or sooner as cash collateral requirements are reduced. In addition, as of September 30, 2015, if Ameren’s credit ratings had been below investment grade, Ameren could have been required to post additional cash collateral in support of New AER in the amount of $21 million , which includes $6 million currently covered by Ameren guarantees discussed below. This cash collateral support is part of Ameren’s obligation to provide certain limited credit support to New AER until December 2, 2015, as discussed below. Ameren Guarantees and Letters of Credit The IPH transaction agreement, as amended, requires Ameren to maintain its financial obligations with respect to all credit support provided to New AER as of the December 2, 2013 closing date of the divestiture. Ameren must also provide such additional credit support as required by contracts entered into prior to the closing date, in each case until December 2, 2015. IPH shall indemnify Ameren for any payments Ameren makes pursuant to these credit support obligations if the counterparty does not return the posted collateral to Ameren. IPH's indemnification obligation is secured by certain AERG and Genco assets. In addition, Dynegy has provided a limited guarantee of $25 million to Ameren pursuant to which Dynegy will, among other things, guarantee IPH's indemnification obligations until December 2, 2015. In addition to the $25 million of liabilities recorded on Ameren’s September 30, 2015 consolidated balance sheet, Ameren had a total of $74 million in guarantees outstanding for New AER that were not recorded on Ameren’s September 30, 2015 consolidated balance sheet. Almost all of these guarantees support Marketing Company for physically and financially settled power transactions with its counterparties that were in place at the December 2, 2013 closing of the divestiture, as well as for Marketing Company's clearing broker and other service agreements. If Marketing Company did not fulfill its obligations to these counterparties who had active open positions as of September 30, 2015 , Ameren would have been required under its guarantees to provide $ 6 million to the counterparties. Also, at September 30, 2015 , Ameren had issued letters of credit totaling $9 million as credit support on behalf of New AER. Ameren has not recorded a liability for these contingent obligations because it does not believe a payment with respect to any of these guarantees or letters of credit was probable as of September 30, 2015 .

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2015
Segment Reporting [Abstract]
SEGMENT INFORMATION	SEGMENT INFORMATION Ameren has two reportable segments: Ameren Missouri and Ameren Illinois. Ameren Missouri and Ameren Illinois each have one reportable segment. The Ameren Missouri segment for both Ameren and Ameren Missouri includes all of the operations of Ameren Missouri’s business as described in Note 1 - Summary of Significant Accounting Policies. The Ameren Illinois segment for both Ameren and Ameren Illinois includes all of the operations of Ameren Illinois’ business as described in Note 1 - Summary of Significant Accounting Policies. The category called Other primarily includes Ameren parent company activities, Ameren Services, and ATXI. The following table presents information about the reported revenues and net income attributable to Ameren common stockholders from continuing operations for the three and nine months ended September 30, 2015 and 2014 , and total assets of continuing operations as of September 30, 2015 , and December 31, 2014 : Three Months Ameren Missouri Ameren Illinois Other Intersegment Eliminations Ameren 2015 External revenues $ 1,160 $ 654 $ 19 $ — $ 1,833 Intersegment revenues 11 1 1 (13 ) — Net income attributable to Ameren common stockholders from continuing operations 239 98 6 — 343 2014 External revenues $ 1,089 $ 572 $ 9 $ — $ 1,670 Intersegment revenues 8 — 2 (10 ) — Net income (loss) attributable to Ameren common stockholders from continuing operations 222 75 (3 ) — 294 Nine Months 2015 External revenues $ 2,825 $ 1,910 $ 55 $ — $ 4,790 Intersegment revenues 30 3 2 (35 ) — Net income attributable to Ameren common stockholders from continuing operations 341 182 26 — 549 2014 External revenues $ 2,793 $ 1,864 $ 26 $ — $ 4,683 Intersegment revenues 21 1 3 (25 ) — Net income (loss) attributable to Ameren common stockholders from continuing operations 395 156 (10 ) — 541 As of September 30, 2015: Total assets $ 13,941 $ 8,656 $ 1,358 $ (513 ) $ 23,442 (a) As of December 31, 2014: Total assets $ 13,541 $ 8,381 $ 942 $ (203 ) $ 22,661 (a) (a) Excludes total assets from discontinued operations of $17 million and $15 million as of September 30, 2015 and December 31, 2014, respectively.

Summary Of Significant Accoun22

Summary Of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2015
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 presentation of our results. The preparation of financial statements in conformity with GAAP requires management to make certain estimates and assumptions. Such estimates and assumptions affect reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the dates of financial statements, and the reported amounts of revenues and expenses during the reported periods. Actual results could differ from those estimates. The results of operations of an interim period may not give a true indication of results that may be expected for a full year. These financial statements should be read in conjunction with the financial statements and the notes thereto included in the Form 10-K.
Excise Taxes	Excise taxes are levied on Ameren Missouri’s electric and natural gas businesses and on Ameren Illinois’ natural gas business and are recorded gross in “Operating Revenues - Electric,” “Operating Revenues - Gas” and “Operating Expenses - Taxes other than income taxes” on the statement of income or the statement of income and comprehensive income. Excise taxes for electric service in Illinois are levied on the customer and are therefore not included in Ameren Illinois’ revenues and expenses.
Uncertain Tax Positions	State income tax returns are generally subject to examination for a period of three years after filing. We do not currently have material state income tax issues under examination, administrative appeal, or litigation. The state impact of any federal changes remains subject to examination by various states for up to one year after formal notification to the states.
Earnings Per Share	Basic earnings per share is computed by dividing net income attributable to Ameren common stockholders by the weighted-average number of common shares outstanding during the period. Earnings per diluted share is computed by dividing net income attributable to Ameren common stockholders by the weighted-average number of diluted common shares outstanding during the period. Earnings per diluted share reflects the potential dilution that would occur if certain stock-based performance share units were settled.
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 2014, FASB issued authoritative accounting guidance to clarify the principles for recognizing revenue and to develop a common revenue standard for GAAP. The guidance requires an entity to recognize an amount of revenue for the transfer of promised goods or services to customers that reflects the consideration which the entity expects to be entitled to in exchange for those goods or services. The guidance also requires additional disclosures to enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. In August 2015, FASB deferred the effective date of this revenue standard to the first quarter of 2018, with an option for entities to early adopt in the first quarter of 2017. The guidance allows entities to choose one of two transition methods, either by applying the guidance retrospectively to each reporting period presented or by recording a cumulative effect adjustment to retained earnings in the period of initial adoption. The Ameren Companies are currently assessing the impacts of this guidance on their results of operations, financial positions and disclosures, as well as the transition method that they will use to adopt the guidance. Presentation of Debt Issuance Costs In April 2015, FASB issued authoritative accounting guidance to simplify the presentation of debt issuance costs. The guidance requires debt issuance costs to be presented in the balance sheet as a reduction to the associated debt liability. Currently, debt issuance costs are presented as a component of “Other assets” on the Ameren Companies’ balance sheets. As of September 30, 2015, Ameren, Ameren Missouri, and Ameren Illinois had debt issuance costs of $35 million , $15 million , and $19 million , respectively. The Ameren Companies expect to early adopt this standard in the fourth quarter of 2015. The guidance will be applied retrospectively, and will not affect the Ameren Companies' results of operations or cash flows.

Derivative Financial Instrume23

Derivative Financial Instruments Derivative Financial Instruments (Policies)	9 Months Ended
	Sep. 30, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivatives	• an unrealized appreciation or depreciation of our contracted commitments to purchase or sell when purchase or sale prices under the commitments are compared with current commodity prices; • market values of natural gas and uranium inventories that differ from the cost of those commodities in inventory; and • actual cash outlays for the purchase of these commodities that differ from anticipated cash outlays. The derivatives that we use to hedge these risks are governed by our risk management policies for forward contracts, futures, options, and swaps. Our net positions are continually assessed within our structured hedging programs to determine whether new or offsetting transactions are required. The goal of the hedging program is generally to mitigate financial risks while ensuring that sufficient volumes are available to meet our requirements. Contracts we enter into as part of our risk management program may be settled financially, settled by physical delivery, or net settled with the counterparty.

Retirement Benefits Retirement

Retirement Benefits Retirement Benefits (Policies)	9 Months Ended
	Sep. 30, 2015
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]
Retirement Benefits	Ameren’s pension plans are funded in compliance with income tax regulations and to meet federal funding or regulatory requirements. As a result, Ameren expects to fund its pension plans at a level equal to the greater of the pension expense or the legally required minimum contribution. Considering Ameren’s assumptions at September 30, 2015 , the plan’s estimated investment performance through September 30, 2015 , and Ameren’s pension funding policy, Ameren expects to make annual contributions of $50 million to $110 million through 2019, with aggregate estimated contributions of $350 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)	9 Months Ended
	Sep. 30, 2015
Accounting Policies [Abstract]
Schedule of Change in Asset Retirement Obligation	The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the nine months ended September 30, 2015 : Ameren Missouri Ameren Illinois (a) Ameren Balance at December 31, 2014 $ 389 $ 7 $ 396 Liabilities incurred (b) 3 — 3 Liabilities settled (1 ) (c) (1 ) Accretion in 2015 (d) 17 (c) 17 Change in estimates (e) 182 (c) 182 Balance at September 30, 2015 $ 590 $ 7 $ 597 (a) Included in “Other deferred credits and liabilities” on the balance sheet. (b) Ameren and Ameren Missouri recorded a new ARO of $3 million related to the Callaway energy center’s dry spent fuel storage facility. See Note 10 - Callaway Energy Center for additional information. (c) Less than $1 million. (d) Accretion expense was recorded as an increase to regulatory assets. (e) The ARO increase resulted in a corresponding increase recorded to “Property and Plant, Net.” During 2015, Ameren and Ameren Missouri increased their AROs related to the decommissioning of the Callaway energy center by $99 million to reflect the 2015 cost study and funding analysis filed with the MoPSC, extension of the estimated operating life until 2044, and a reduction in the discount rate assumption. See Note 10 - Callaway Energy Center for additional information. In addition, as a result of new federal regulations, Ameren and Ameren Missouri recorded an increase of $79 million to their AROs associated with CCR storage facilities. See Note 9 - Commitments and Contingencies for additional information. Ameren and Ameren Missouri also increased their AROs by $4 million due to a change in the estimated retirement dates of the Meramec and Rush Island energy centers as a result of the MoPSC’s April 2015 electric rate order.
Summary Of Nonvested Shares Related To Long-Term Incentive Plan	A summary of nonvested performance share units at September 30, 2015 , and changes during the nine months ended September 30, 2015 , under the 2006 Incentive Plan and the 2014 Incentive Plan are presented below: Number of Performance Share Units Weighted-average Fair Value Per Performance Share Unit Nonvested at January 1, 2015 1,162,377 $ 35.35 Granted (a) 569,892 52.88 Forfeitures (1,944 ) 34.75 Vested (b) (92,892 ) 45.97 Nonvested at September 30, 2015 1,637,433 $ 40.85 (a) Performance share units granted to certain executive and nonexecutive officers and other eligible employees in 2015 under the 2014 Incentive Plan. (b) Performance share units vested due to the attainment of retirement eligibility by certain employees. Actual shares issued for retirement-eligible employees will vary depending on actual performance over the three -year measurement period.
Schedule of Excise Taxes	The following table presents excise taxes recorded in “Operating Revenues - Electric,” “Operating Revenues - Gas” and “Operating Expenses - Taxes other than income taxes” for the three and nine months ended September 30, 2015 and 2014 : Three Months Nine Months 2015 2014 2015 2014 Ameren Missouri $ 52 $ 47 $ 127 $ 120 Ameren Illinois 9 9 42 46 Ameren $ 61 $ 56 $ 169 $ 166
Schedule of Unrecognized Tax Benefits	The following table presents the total amount of reserves for unrecognized tax benefits (detriments) related to uncertain tax positions as of September 30, 2015 , and December 31, 2014 : September 30, 2015 December 31, 2014 Ameren $ — $ 54 Ameren Missouri — — Ameren Illinois — (1 )
Schedule of Unrecognized Tax Benefits That Would Impact Effective Tax Rate	The following table presents the amount of reserves for unrecognized tax benefits, included in the table above, related to uncertain tax positions that, if recognized, would have impacted results of operations as of December 31, 2014 : December 31, 2014 Ameren $ 52 Ameren Missouri — Ameren Illinois (1 )

Short-Term Debt And Liquidity S

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Tables)	9 Months Ended
	Sep. 30, 2015
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 September 30, 2015 , and December 31, 2014 : September 30, 2015 December 31, 2014 Ameren (parent) $ 783 $ 585 Ameren Missouri — 97 Ameren Illinois — 32 Ameren Consolidated $ 783 $ 714 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 nine months ended September 30, 2015 and 2014: Ameren (parent) Ameren Missouri Ameren Illinois Ameren Consolidated 2015 Average daily commercial paper outstanding $ 770 $ 56 $ 6 $ 832 Weighted-average interest rate 0.56 % 0.50 % 0.44 % 0.56 % Peak commercial paper during period (a) $ 874 $ 294 $ 48 $ 1,108 Peak interest rate 0.70 % 0.60 % 0.60 % 0.70 % 2014 Average daily commercial paper outstanding $ 386 $ 141 $ 157 $ 609 Weighted-average interest rate 0.36 % 0.38 % 0.31 % 0.35 % Peak commercial paper during period (a) $ 531 $ 495 $ 300 $ 907 Peak interest rate 0.75 % 0.70 % 0.34 % 0.75 % (a) The timing of peak commercial paper issuances varies by company, and therefore the peak amounts presented by company might not equal the Ameren Consolidated peak commercial paper issuances for the period.

Long-Term Debt And Equity Fin27

Long-Term Debt And Equity Financings (Tables)	9 Months Ended
	Sep. 30, 2015
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 and dividend coverage ratios, and bonds and preferred stock issuable as of September 30, 2015 , 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 3.7 $ 3,338 ≥2.5 99.3 $ 2,206 Ameren Illinois ≥2.0 7.0 3,659 (d) ≥1.5 3.0 203 (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 $946 million and $204 million at Ameren Missouri and Ameren Illinois, respectively. (c) Coverage required on the annual dividend on preferred stock outstanding and to be issued, as required in the respective company’s articles of incorporation. (d) Amount of bonds issuable by Ameren Illinois based on unfunded property additions and retired bonds solely under the former IP mortgage indenture. The amount of bonds issuable by Ameren Illinois is also subject to the lien restrictions contained in the 2012 Illinois Credit Agreement. (e) Preferred stock issuable is restricted by the amount of preferred stock that is currently authorized by Ameren Illinois’ articles of incorporation.

Other Income and Expenses (Tabl

Other Income and Expenses (Tables)	9 Months Ended
	Sep. 30, 2015
Other Nonoperating Income (Expense) [Abstract]
Other Income And Expenses	The following table presents the components of “Other Income and Expenses” in the Ameren Companies’ statements of income for the three and nine months ended September 30, 2015 and 2014 : Three Months Nine Months 2015 2014 2015 2014 Ameren: (a) Miscellaneous income: Allowance for equity funds used during construction $ 8 $ 10 $ 19 $ 26 Interest income on industrial development revenue bonds 7 6 20 20 Interest income 4 3 12 8 Other — 2 3 6 Total miscellaneous income $ 19 $ 21 $ 54 $ 60 Miscellaneous expense: Donations $ — $ 3 $ 10 $ 9 Other 5 4 12 11 Total miscellaneous expense $ 5 $ 7 $ 22 $ 20 Ameren Missouri: Miscellaneous income: Allowance for equity funds used during construction $ 7 $ 9 $ 16 $ 24 Interest income on industrial development revenue bonds 7 6 20 20 Interest income — — 1 1 Total miscellaneous income $ 14 $ 15 $ 37 $ 45 Miscellaneous expense: Donations $ — $ 2 $ 3 $ 5 Other 3 2 5 5 Total miscellaneous expense $ 3 $ 4 $ 8 $ 10 Ameren Illinois: Miscellaneous income: Allowance for equity funds used during construction $ 1 $ 1 $ 3 $ 2 Interest income 3 2 10 5 Other — 1 2 5 Total miscellaneous income $ 4 $ 4 $ 15 $ 12 Miscellaneous expense: Donations $ — $ — $ 4 $ 3 Other 3 2 6 4 Total miscellaneous expense $ 3 $ 2 $ 10 $ 7 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Derivative Financial Instrume29

Derivative Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2015
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Open Gross Derivative Volumes By Commodity Type	The following table presents open gross commodity contract volumes by commodity type for derivative assets and liabilities as of September 30, 2015 , and December 31, 2014 . As of September 30, 2015 , these contracts ran through October 2017, March 2021, May 2032, and December 2016 for fuel oils, natural gas, power, and uranium, respectively. Quantity (in millions, except as indicated) 2015 2014 Commodity Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils (in gallons) (a) 33 (b) 33 50 (b) 50 Natural gas (in mmbtu) 30 152 182 28 108 136 Power (in megawatthours) 1 10 11 1 11 12 Uranium (pounds in thousands) 299 (b) 299 332 (b) 332 (a) Fuel oils consist of heating oil and ultra-low-sulfur diesel. (b) Not applicable.
Derivative Instruments Carrying Value	The following table presents the carrying value and balance sheet location of all derivative instruments, none of which were designated as hedging instruments under hedge accounting, as of September 30, 2015 , and December 31, 2014 : Balance Sheet Location Ameren Missouri Ameren Illinois Ameren 2015 Natural gas Other current assets $ 1 $ — $ 1 Power Other current assets 22 — 22 Other assets 1 — 1 Total assets $ 24 $ — $ 24 Fuel oils Other current liabilities $ 19 $ — $ 19 Other deferred credits and liabilities 6 — 6 Natural gas MTM derivative liabilities (a) 26 (a) Other current liabilities 5 — 31 Other deferred credits and liabilities 9 18 27 Power MTM derivative liabilities (a) 12 (a) Other current liabilities 1 — 13 Other deferred credits and liabilities — 158 158 Uranium Other current liabilities 1 — 1 Total liabilities $ 41 $ 214 $ 255 2014 Fuel oils Other current assets $ 2 $ — $ 2 Natural gas Other current assets 1 1 2 Power Other current assets 15 — 15 Total assets $ 18 $ 1 $ 19 Fuel oils Other current liabilities $ 22 $ — $ 22 Other deferred credits and liabilities 7 — 7 Natural gas MTM derivative liabilities (a) 31 (a) Other current liabilities 6 — 37 Other deferred credits and liabilities 6 13 19 Power MTM derivative liabilities (a) 11 (a) Other current liabilities 3 — 14 Other deferred credits and liabilities — 131 131 Uranium Other current liabilities 2 — 2 Total liabilities $ 46 $ 186 $ 232 (a) Balance sheet line item not applicable to registrant.
Cumulative Pretax Net Gains (Losses) On All Derivative Instruments In OCI	The following table presents the cumulative amount of pretax net gains (losses) on all derivative instruments deferred in regulatory assets or regulatory liabilities as of September 30, 2015 , and December 31, 2014 : Ameren Missouri Ameren Illinois Ameren 2015 Fuel oils derivative contracts (a) $ (25 ) $ — $ (25 ) Natural gas derivative contracts (b) (13 ) (44 ) (57 ) Power derivative contracts (c) 22 (170 ) (148 ) Uranium derivative contracts (d) (1 ) — (1 ) 2014 Fuel oils derivative contracts $ (29 ) $ — $ (29 ) Natural gas derivative contracts (11 ) (43 ) (54 ) Power derivative contracts 12 (142 ) (130 ) Uranium derivative contracts (2 ) — (2 ) (a) Represents net losses associated with fuel oils derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s rail transportation surcharges for coal through December 2017. Losses deferred as current regulatory assets include $19 million at Ameren and Ameren Missouri. (b) Represents net losses associated with natural gas derivative contracts. These contracts are a partial hedge of natural gas requirements through March 2021 at Ameren and Ameren Missouri and through October 2019 at Ameren Illinois. Gains deferred as current regulatory liabilities include $1 million at Ameren and Ameren Missouri. Losses deferred as current regulatory assets include $31 million , $5 million , and $26 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively. (c) Represents net gains (losses) associated with power derivative contracts. These contracts are a partial hedge of power price requirements through May 2032 at Ameren and Ameren Illinois and through December 2016 at Ameren Missouri. Gains deferred as current regulatory liabilities include $22 million at Ameren and Ameren Missouri. Losses deferred as current regulatory assets include $13 million , $1 million , and $12 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively. (d) Represents net losses on uranium derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s uranium requirements through January 2017. Losses deferred as current regulatory assets include $1 million at Ameren and Ameren Missouri.
Offsetting Derivative Assets and Liabilities	The following table provides the recognized gross derivative balances and the net amounts of those derivatives subject to an enforceable master netting arrangement or similar agreement as of September 30, 2015 , and December 31, 2014 : Gross Amounts Not Offset in the Balance Sheet Commodity Contracts Eligible to be Offset Gross Amounts Recognized in the Balance Sheet Derivative Instruments Cash Collateral Received/Posted (a) Net Amount 2015 Assets: Ameren Missouri $ 24 $ 2 $ — $ 22 Ameren Illinois — — — — Ameren $ 24 $ 2 $ — $ 22 Liabilities: Ameren Missouri $ 41 $ 2 $ 6 $ 33 Ameren Illinois 214 — 1 213 Ameren $ 255 $ 2 $ 7 $ 246 2014 Assets: Ameren Missouri $ 18 $ 5 $ — $ 13 Ameren Illinois 1 — — 1 Ameren $ 19 $ 5 $ — $ 14 Liabilities: Ameren Missouri $ 46 $ 5 $ 5 $ 36 Ameren Illinois 186 — — 186 Ameren $ 232 $ 5 $ 5 $ 222 (a) Cash collateral received reduces gross asset balances and is included in “Other current liabilities” and “Other deferred credits and liabilities” on the balance sheet. Cash collateral posted reduces gross liability balances and is included in “Other current assets” and “Other assets” on the balance sheet.
Derivative Instruments With Credit Risk-Related Contingent Features	The following table presents, as of September 30, 2015 , the aggregate fair value of all derivative instruments with credit risk-related contingent features in a gross liability position, the cash collateral posted, and the aggregate amount of additional collateral that could be required to be posted with counterparties. The additional collateral required is the net liability position allowed under the master netting arrangements or similar agreements, assuming (1) the credit risk-related contingent features underlying these arrangements were triggered on September 30, 2015 , 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) 2015 Ameren Missouri $ 88 $ 7 $ 75 Ameren Illinois 82 1 75 Ameren $ 170 $ 8 $ 150 (a) Prior to 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)	9 Months Ended
	Sep. 30, 2015
Fair Value Disclosures [Abstract]
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis, Valuation Techniques	The following table describes the valuation techniques and unobservable inputs for the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy as of September 30, 2015 : Fair Value Weighted Average Assets Liabilities Valuation Technique(s) Unobservable Input Range Level 3 Derivative asset and liability - commodity contracts (a) : Ameren Fuel oils $ — $ (1 ) Option model Volatilities(%) (e) 63 (d) Discounted cash flow Ameren Missouri credit risk(%) (b)(c) 0.40 (d) Natural gas — (1 ) Option model Volatilities(%) (e) 20 - 44 33 Nodal basis($/mmbtu) (b) (0.30) - (0.10) (0.20) Discounted cash flow Nodal basis($/mmbtu) (e) (1.40) - 0.10 (0.20) Counterparty credit risk(%) (b)(c) 0.40 - 12.07 6.60 Ameren Missouri credit risk(%) (b)(c) 0.40 (d) Power (f) 23 (171 ) Discounted cash flow Average forward peak and off-peak pricing - forwards/swaps($/MWh) (g) 26 - 40 29 Estimated auction price for FTRs($/MW) (e) (480) - 2,333 193 Nodal basis($/MWh) (e) (10) - (1) (3) Counterparty credit risk(%) (b)(c) 0.46 - 0.84 0.73 Ameren Illinois credit risk(%) (b)(c) 0.40 (d) Fundamental energy production model Estimated future gas prices($/mmbtu) (e) 3 - 4 4 Escalation rate(%) (e)(h) 2 - 3 2 Contract price allocation Estimated renewable energy credit costs($/credit) (e) 5 - 7 6 Uranium — (1 ) Discounted cash flow Average forward uranium pricing($/pound) (e) 37 - 39 37 Ameren Missouri Fuel oils $ — $ (1 ) Option model Volatilities(%) (e) 63 (d) Discounted cash flow Ameren Missouri credit risk(%) (b)(c) 0.40 (d) Natural gas — (1 ) Option model Volatilities(%) (e) 20 - 44 33 Nodal basis($/mmbtu) (b) (0.30) - (0.10) (0.20) Discounted cash flow Nodal basis($/mmbtu) (e) (1.40) - 0.10 (0.20) Counterparty credit risk(%) (b)(c) 0.40 - 12.07 6.60 Ameren Missouri credit risk(%) (b)(c) 0.40 (d) Power (f) 23 (1 ) Discounted cash flow Average forward peak and off-peak pricing - forwards/swaps($/MWh) (b) 26 - 40 34 Estimated auction price for FTRs($/MW) (e) (480) - 2,333 193 Nodal basis($/MWh) (b) (10) - (5) (9) Counterparty credit risk(%) (b)(c) 0.46 - 0.84 0.73 Uranium — (1 ) Discounted cash flow Average forward uranium pricing($/pound) (e) 37 - 39 37 Ameren Illinois Power (f) $ — $ (170 ) Discounted cash flow Average forward peak and off-peak pricing - forwards/swaps($/MWh) (e) 27 - 33 30 Nodal basis($/MWh) (e) (6) - (1) (3) Ameren Illinois credit risk(%) (b)(c) 0.40 (d) Fundamental energy production model Estimated future gas prices($/mmbtu) (e) 3 - 4 4 Escalation rate(%) (e)(h) 2 - 3 2 Contract price allocation Estimated renewable energy credit costs($/credit) (e) 5 - 7 6 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement. (c) Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances. (d) Not applicable. (e) Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement. (f) Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2019. Valuations beyond 2019 use fundamentally modeled pricing by month for peak and off-peak demand. (g) The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes due to their opposing positions. As such, refer to the power sensitivity analysis for each company above. (h) Escalation rate applies to power prices 2026 and beyond. The following table describes the valuation techniques and unobservable inputs for the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy as of December 31, 2014 : Fair Value Weighted Assets Liabilities Valuation Technique(s) Unobservable Input Range Average Level 3 Derivative asset and liability – commodity contracts (a) : Ameren Fuel oils $ 2 $ (8 ) Option model Volatilities(%) (b) 3 - 39 32 Discounted cash flow Ameren Missouri credit risk(%) (b)(c) 0.43 (d) Escalation rate(%) (e)(f) 5 (d) Natural Gas 1 (2 ) Option model Volatilities(%) (b) 31 - 144 63 Nodal basis($/mmbtu) (e) (0.40) - 0 (0.20) Discounted cash flow Nodal basis($/mmbtu) (e) (0.40) - 0.10 (0.20) Counterparty credit risk(%) (b)(c) 0.43 - 13 3 Ameren Missouri and Ameren Illinois credit risk(%) (b)(c) 0.43 (d) Power (g) 11 (144 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps($/MWh) (h) 27 - 50 32 Estimated auction price for FTRs($/MW) (e) (1,833) - 2,743 171 Nodal basis($/MWh) (e) (6) - 0 (2) Counterparty credit risk(%) (b)(c) 0.26 (d) Ameren Missouri and Ameren Illinois credit risk(%) (b)(c) 0.43 (d) Fundamental energy production model Estimated future gas prices($/mmbtu) (e) 4 - 5 4 Escalation rate(%) (e)(i) 0 - 1 1 Contract price allocation Estimated renewable energy credit costs($/credit) (e) 5 - 7 6 Uranium — (2 ) Discounted cash flow Average forward uranium pricing($/pound) (e) 35 - 40 36 Ameren Missouri Fuel oils $ 2 $ (8 ) Option model Volatilities(%) (b) 3 - 39 32 Discounted cash flow Ameren Missouri credit risk(%) (b)(c) 0.43 (d) Escalation rate(%) (e)(f) 5 (d) Natural Gas — (1 ) Option model Volatilities(%) (b) 31 - 144 53 Nodal basis($/mmbtu) (e) (0.40) - 0 (0.30) Discounted cash flow Nodal basis($/mmbtu) (e) (0.10) (d) Counterparty credit risk(%) (b)(c) 0.57 - 13 5 Ameren Missouri credit risk(%) (b)(c) 0.43 (d) Power (g) 11 (2 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps($/MWh) (b) 27 - 50 32 Estimated auction price for FTRs($/MW) (e) (1,833) - 2,743 171 Counterparty credit risk(%) (b)(c) 0.26 (d) Ameren Missouri credit risk(%) (b)(c) 0.43 (d) Uranium — (2 ) Discounted cash flow Average forward uranium pricing($/pound) (e) 35 - 40 36 Ameren Illinois Natural Gas $ 1 $ (1 ) Option model Volatilities(%) (b) 50 - 144 94 Nodal basis($/mmbtu) (e) (0.10) - 0 (0.10) Discounted cash flow Nodal basis($/mmbtu) (e) (0.40) - 0.10 (0.20) Counterparty credit risk(%) (b)(c) 0.43 - 2 0.83 Ameren Illinois credit risk(%) (b)(c) 0.43 (d) Power (g) — (142 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps($/MWh) (e) 27 - 38 32 Nodal basis($/MWh) (e) (6) - 0 (2) Ameren Illinois credit risk(%) (b)(c) 0.43 (d) Fundamental energy production model Estimated future gas prices($/mmbtu) (e) 4 - 5 4 Escalation rate(%) (e)(i) 0 - 1 1 Contract price allocation Estimated renewable energy credit costs($/credit) (e) 5 - 7 6 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement. (c) Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances. (d) Not applicable. (e) Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement. (f) Escalation rate applies to fuel oil prices 2017 and beyond. (g) Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2018. Valuations beyond 2018 use fundamentally modeled pricing by month for peak and off-peak demand. (h) The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes due to their opposing positions. As such, refer to the power sensitivity analysis for each company above. (i) Escalation rate applies to power prices 2026 and beyond.
Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis	The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of September 30, 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 Power — — 23 23 Total derivative assets - commodity contracts $ — $ 1 $ 23 $ 24 Nuclear decommissioning trust fund: Cash and cash equivalents $ 3 $ — $ — $ 3 Equity securities: U.S. large capitalization 338 — — 338 Debt securities: Corporate bonds — 61 — 61 U.S. treasury and agency securities — 109 — 109 Other — 24 — 24 Total nuclear decommissioning trust fund $ 341 $ 194 $ — $ 535 (b) Total Ameren $ 341 $ 195 $ 23 $ 559 Ameren Derivative assets - commodity contracts (a) : Missouri Natural gas — 1 — 1 Power — — 23 23 Total derivative assets - commodity contracts $ — $ 1 $ 23 $ 24 Nuclear decommissioning trust fund: Cash and cash equivalents $ 3 $ — $ — $ 3 Equity securities: U.S. large capitalization 338 — — 338 Debt securities: Corporate bonds — 61 — 61 U.S. treasury and agency securities — 109 — 109 Other — 24 — 24 Total nuclear decommissioning trust fund $ 341 $ 194 $ — $ 535 (b) Total Ameren Missouri $ 341 $ 195 $ 23 $ 559 Liabilities: Ameren Derivative liabilities - commodity contracts (a) : Fuel oils $ 24 $ — $ 1 $ 25 Natural gas 1 56 1 58 Power — — 171 171 Uranium — — 1 1 Total Ameren $ 25 $ 56 $ 174 $ 255 Ameren Derivative liabilities - commodity contracts (a) : Missouri Fuel oils $ 24 $ — $ 1 $ 25 Natural gas 1 12 1 14 Power — — 1 1 Uranium — — 1 1 Total Ameren Missouri $ 25 $ 12 $ 4 $ 41 Ameren Derivative liabilities - commodity contracts (a) : Illinois Natural gas $ — $ 44 $ — $ 44 Power — — 170 170 Total Ameren Illinois $ — $ 44 $ 170 $ 214 (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, 2014 : 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) : Fuel oils $ — $ — $ 2 $ 2 Natural gas — 1 1 2 Power — 4 11 15 Total derivative assets - commodity contracts $ — $ 5 $ 14 $ 19 Nuclear decommissioning trust fund: Cash and cash equivalents $ 1 $ — $ — $ 1 Equity securities: U.S. large capitalization 364 — — 364 Debt securities: Corporate bonds — 63 — 63 U.S. treasury and agency securities — 102 — 102 Other — 17 — 17 Total nuclear decommissioning trust fund $ 365 $ 182 $ — $ 547 (b) Total Ameren $ 365 $ 187 $ 14 $ 566 Ameren Derivative assets - commodity contracts (a) : Missouri Fuel oils $ — $ — $ 2 $ 2 Natural gas — 1 — 1 Power — 4 11 15 Total derivative assets - commodity contracts $ — $ 5 $ 13 $ 18 Nuclear decommissioning trust fund: Cash and cash equivalents $ 1 $ — $ — $ 1 Equity securities: U.S. large capitalization 364 — — 364 Debt securities: Corporate bonds — 63 — 63 U.S. treasury and agency securities — 102 — 102 Other — 17 — 17 Total nuclear decommissioning trust fund $ 365 $ 182 $ — $ 547 (b) Total Ameren Missouri $ 365 $ 187 $ 13 $ 565 Ameren Derivative assets - commodity contracts (a) : Illinois Natural gas $ — $ — $ 1 $ 1 Liabilities: Ameren Derivative liabilities - commodity contracts (a) : Fuel oils $ 21 $ — $ 8 $ 29 Natural gas 1 53 2 56 Power — 1 144 145 Uranium — — 2 2 Total Ameren $ 22 $ 54 $ 156 $ 232 Ameren Derivative liabilities - commodity contracts (a) : Missouri Fuel oils $ 21 $ — $ 8 $ 29 Natural gas 1 10 1 12 Power — 1 2 3 Uranium — — 2 2 Total Ameren Missouri $ 22 $ 11 $ 13 $ 46 Ameren Derivative liabilities - commodity contracts (a) : Illinois Natural gas $ — $ 43 $ 1 $ 44 Power — — 142 142 Total Ameren Illinois $ — $ 43 $ 143 $ 186 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Balance excludes $2 million of receivables, payables, and accrued income, net.
Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level Three In The Fair Value Hierarchy	The following table summarizes the changes in the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy for the three months ended September 30, 2015 : Net derivative commodity contracts Ameren Missouri Ameren Illinois Ameren Fuel oils: Beginning balance at July 1, 2015 $ (1 ) $ (a) $ (1 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (1 ) (a) (1 ) Settlements 1 (a) 1 Ending balance at September 30, 2015 $ (1 ) $ (a) $ (1 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2015 $ — $ (a) $ — Natural gas: Beginning balance at July 1, 2015 $ — $ (1 ) $ (1 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (1 ) 1 — Ending balance at September 30, 2015 $ (1 ) $ — $ (1 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2015 $ — $ — $ — Power: Beginning balance at July 1, 2015 $ 27 $ (165 ) $ (138 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities 2 (8 ) (6 ) Settlements (7 ) 3 (4 ) Ending balance at September 30, 2015 $ 22 $ (170 ) $ (148 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2015 $ 1 $ (7 ) $ (6 ) Uranium: Beginning balance at July 1, 2015 $ (2 ) $ (a) $ (2 ) Settlements 1 (a) 1 Ending balance at September 30, 2015 $ (1 ) $ (a) $ (1 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2015 $ — $ (a) $ — (a) Not applicable. The following table summarizes the changes in the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy for the three months ended September 30, 2014 : Net derivative commodity contracts Ameren Missouri Ameren Illinois Ameren Fuel oils: Beginning balance at July 1, 2014 $ 2 $ (a) $ 2 Realized and unrealized gains (losses) included in regulatory assets/liabilities (2 ) (a) (2 ) Ending balance at September 30, 2014 $ — $ (a) $ — Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014 $ (2 ) $ (a) $ (2 ) Natural gas: Beginning balance at July 1, 2014 $ — $ — $ — Realized and unrealized gains (losses) included in regulatory assets/liabilities — 1 1 Ending balance at September 30, 2014 $ — $ 1 $ 1 Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014 $ — $ — $ — Power: Beginning balance at July 1, 2014 $ 15 $ (103 ) $ (88 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (5 ) (23 ) (28 ) Settlements (5 ) 2 (3 ) Ending balance at September 30, 2014 $ 5 $ (124 ) $ (119 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014 $ (6 ) $ (22 ) $ (28 ) Uranium: Beginning balance at July 1, 2014 $ (7 ) $ (a) $ (7 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities 3 (a) 3 Settlements 1 (a) 1 Ending balance at September 30, 2014 $ (3 ) $ (a) $ (3 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014 $ 3 $ (a) $ 3 (a) Not applicable. The following table summarizes the changes in the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy for the nine months ended September 30, 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 4 (a) 4 Transfers out of Level 3 2 (a) 2 Ending balance at September 30, 2015 $ (1 ) $ (a) $ (1 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2015 $ — $ (a) $ — Natural gas: Beginning balance at January 1, 2015 $ (1 ) $ — $ (1 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (1 ) 1 — Settlements 1 (1 ) — Ending balance at September 30, 2015 $ (1 ) $ — $ (1 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2015 $ — $ — $ — Power: Beginning balance at January 1, 2015 $ 9 $ (142 ) $ (133 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities — (37 ) (37 ) Purchases 29 — 29 Settlements (16 ) 9 (7 ) Ending balance at September 30, 2015 $ 22 $ (170 ) $ (148 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2015 $ 1 $ (35 ) $ (34 ) Uranium: Beginning balance at January 1, 2015 $ (2 ) $ (a) $ (2 ) Settlements 1 (a) 1 Ending balance at September 30, 2015 $ (1 ) $ (a) $ (1 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2015 $ — $ (a) $ — (a) Not applicable. The following table summarizes the changes in the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy for the nine months ended September 30, 2014 : Net derivative commodity contracts Ameren Missouri Ameren Illinois Ameren Fuel oils: Beginning balance at January 1, 2014 $ 5 $ (a) $ 5 Realized and unrealized gains (losses) included in regulatory assets/liabilities (3 ) (a) (3 ) Settlements (2 ) (a) (2 ) Ending balance at September 30, 2014 $ — $ (a) $ — Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014 $ (2 ) $ (a) $ (2 ) Natural gas: Beginning balance at January 1, 2014 $ — $ — $ — Realized and unrealized gains (losses) included in regulatory assets/liabilities — 1 1 Purchases — (1 ) (1 ) Settlements — 1 1 Ending balance at September 30, 2014 $ — $ 1 $ 1 Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014 $ — $ — $ — Power: Beginning balance at January 1, 2014 $ 19 $ (108 ) $ (89 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (23 ) (19 ) (42 ) Purchases 34 — 34 Settlements (25 ) 3 (22 ) Ending balance at September 30, 2014 $ 5 $ (124 ) $ (119 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014 $ (3 ) $ (21 ) $ (24 ) Uranium: Beginning balance at January 1, 2014 $ (6 ) $ (a) $ (6 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (1 ) (a) (1 ) Settlements 4 (a) 4 Ending balance at September 30, 2014 $ (3 ) $ (a) $ (3 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2014 $ — $ (a) $ — (a) Not applicable.
Schedule Of Carrying Amounts And Estimated Fair Values Of Long-Term Debt, Capital Lease Obligations And Preferred Stock	The following table presents the carrying amounts and estimated fair values of our long-term debt, capital lease obligations and preferred stock at September 30, 2015 , and December 31, 2014 : September 30, 2015 December 31, 2014 Carrying Amount Fair Value Carrying Amount Fair Value Ameren: (a) Long-term debt and capital lease obligations (including current portion) $ 6,376 $ 6,916 $ 6,240 $ 7,135 Preferred stock 142 124 142 122 Ameren Missouri: Long-term debt and capital lease obligations (including current portion) $ 4,135 $ 4,488 $ 3,999 $ 4,518 Preferred stock 80 75 80 73 Ameren Illinois: Long-term debt $ 2,241 $ 2,428 $ 2,241 $ 2,517 Preferred stock 62 49 62 49 (a) Preferred stock is recorded in “Noncontrolling Interests” on the consolidated balance sheet.

Related Party Transactions (Tab

Related Party Transactions (Tables)	9 Months Ended
	Sep. 30, 2015
Related Party Transactions [Abstract]
Schedule of Related Party Transactions	The following table presents the impact on Ameren Missouri and Ameren Illinois of related party transactions for the three and nine months ended September 30, 2015 and 2014 . Three Months Nine Months Agreement Income Statement Line Item Ameren Missouri Ameren Illinois Ameren Missouri Ameren Illinois Ameren Missouri power supply Operating Revenues 2015 $ 4 $ (a) $ 9 $ (a) agreements with Ameren Illinois 2014 2 (a) 5 (a) Ameren Missouri and Ameren Illinois Operating Revenues 2015 6 1 19 3 rent and facility services 2014 6 (b) 15 1 Ameren Missouri and Ameren Illinois Operating Revenues 2015 1 (b) 2 (b) miscellaneous support services 2014 (b) (b) 1 (b) Total Operating Revenues 2015 $ 11 $ 1 $ 30 $ 3 2014 8 (b) 21 1 Ameren Illinois power supply Purchased Power 2015 $ (a) $ 4 $ (a) $ 9 agreements with Ameren Missouri 2014 (a) 2 (a) 5 Ameren Illinois transmission Purchased Power 2015 (a) 1 (a) 2 services with ATXI 2014 (a) 1 (a) 2 Total Purchased Power 2015 $ (a) $ 5 $ (a) $ 11 2014 (a) 3 (a) 7 Ameren Services support services Other Operations and Maintenance 2015 $ 30 $ 28 $ 96 $ 87 agreement 2014 25 26 90 80 Money pool borrowings (advances) Interest Charges/ Miscellaneous Income 2015 $ (b) $ (b) $ (b) $ (b) 2014 (b) (b) (b) (b) (a) Not applicable. (b) Amount less than $1 million.

Commitments And Contingencies (

Commitments And Contingencies (Tables)	9 Months Ended
	Sep. 30, 2015
Commitments and Contingencies Disclosure [Abstract]
Schedule of Insurance Coverage at Callaway Energy Center	The following table presents insurance coverage at Ameren Missouri’s Callaway energy center at September 30, 2015 . The property coverage and the nuclear liability coverage must be renewed on April 1 and January 1, respectively, of each year. Both coverages were renewed in 2015. Type and Source of Coverage Maximum Coverages Maximum Assessments for Single Incidents Public liability and nuclear worker liability: American Nuclear Insurers $ 375 $ — Pool participation 12,986 (a) 127 (b) $ 13,361 (c) $ 127 Property damage: NEIL $ 2,750 (d) $ 27 (e) European Mutual Association for Nuclear Insurance 500 (f) — $ 3,250 $ 27 Replacement power: NEIL $ 490 (g) $ 10 (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 both radiation and nonradiation events. An additional $500 million is provided for radiation events only for a total of $2.75 billion . (e) All NEIL insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL. (f) European Mutual Association for Nuclear Insurance provides $500 million in excess of the $2.75 billion and $2.25 billion property coverage for radiation and nonradiation events, respectively, provided by NEIL. (g) Provides replacement power cost insurance in the event of a prolonged accidental outage. Weekly indemnity up to $4.5 million for 52 weeks, which commences after the first twelve weeks of an outage, plus up to $3.6 million per week for a minimum of 71 weeks thereafter for a total not exceeding the policy limit of $490 million . Nonradiation events are sub-limited to $328 million .
Schedule of Asbestors-Related Litigation Pending Lawsuits	The following table presents the pending asbestos-related lawsuits filed against the Ameren Companies as of September 30, 2015 : Ameren Ameren Missouri Ameren Illinois Total (a) 1 27 40 53 (a) Total does not equal the sum of the subsidiary unit lawsuits because some of the lawsuits name multiple Ameren entities as defendants.

Retirement Benefits (Tables)

Retirement Benefits (Tables)	9 Months Ended
	Sep. 30, 2015
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]
Components Of Net Periodic Benefit Cost	The following table presents the components of the net periodic benefit cost (benefit) incurred for Ameren’s pension and postretirement benefit plans for the three and nine months ended September 30, 2015 and 2014 : Pension Benefits Postretirement Benefits Three Months Nine Months Three Months Nine Months 2015 2014 2015 2014 2015 2014 2015 2014 Service cost $ 23 $ 20 $ 69 $ 60 $ 6 $ 5 $ 17 $ 14 Interest cost 43 46 130 137 12 12 36 37 Expected return on plan assets (62 ) (58 ) (186 ) (172 ) (17 ) (16 ) (51 ) (48 ) Amortization of: Prior service benefit — (1 ) — (1 ) (1 ) (2 ) (3 ) (4 ) Actuarial loss (gain) 19 13 56 37 1 (2 ) 4 (5 ) Settlement loss — — 1 — — — — — Net periodic benefit cost (benefit) $ 23 $ 20 $ 70 $ 61 $ 1 $ (3 ) $ 3 $ (6 )
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 and nine months ended September 30, 2015 and 2014 : Pension Benefits Postretirement Benefits Three Months Nine Months Three Months Nine Months 2015 2014 2015 2014 2015 2014 2015 2014 Ameren Missouri (a) $ 14 $ 13 $ 42 $ 38 $ 2 $ — $ 6 $ 2 Ameren Illinois 9 7 28 22 — (3 ) (2 ) (7 ) Other — — — 1 (1 ) — (1 ) (1 ) Ameren (a)(b) $ 23 $ 20 $ 70 $ 61 $ 1 $ (3 ) $ 3 $ (6 ) (a) The amounts above do 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 built into rates. (b) Includes amounts for Ameren registrants and nonregistrant subsidiaries.

Divestiture Transactions and 34

Divestiture Transactions and Discontinued Operations Divestiture Transactions and Discontinued Operations (Tables)	9 Months Ended
	Sep. 30, 2015
Discontinued Operations and Disposal Groups [Abstract]
Schedule of Disposal Groups, Including Discontinued Operations, Income Statement, Balance Sheet and Additional Disclosures	The following table presents the components of discontinued operations in Ameren's consolidated statement of income for the three and nine months ended September 30, 2015 and 2014 : Three Months Nine Months 2015 2014 2015 2014 Operating revenues $ — $ — $ — $ 1 Operating benefits (expenses) (1 ) (1 ) 2 (4 ) Operating income (loss) before income tax (1 ) (1 ) 2 (3 ) Income tax benefit 1 — 50 — Income (loss) from discontinued operations, net of taxes $ — $ (1 ) $ 52 $ (3 ) The following table presents the carrying amounts of the components of assets and liabilities of Ameren’s discontinued operations, which consist primarily of AROs and related deferred income tax assets associated with the abandoned Meredosia and Hutsonville energy centers, at September 30, 2015 , and December 31, 2014: September 30, 2015 December 31, 2014 Assets of discontinued operations Accumulated deferred income taxes, net $ 17 $ 15 Total assets of discontinued operations $ 17 $ 15 Liabilities of discontinued operations Accounts payable and other current obligations $ 1 $ 1 Asset retirement obligations 29 32 Total liabilities of discontinued operations $ 30 $ 33

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2015
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 stockholders from continuing operations for the three and nine months ended September 30, 2015 and 2014 , and total assets of continuing operations as of September 30, 2015 , and December 31, 2014 : Three Months Ameren Missouri Ameren Illinois Other Intersegment Eliminations Ameren 2015 External revenues $ 1,160 $ 654 $ 19 $ — $ 1,833 Intersegment revenues 11 1 1 (13 ) — Net income attributable to Ameren common stockholders from continuing operations 239 98 6 — 343 2014 External revenues $ 1,089 $ 572 $ 9 $ — $ 1,670 Intersegment revenues 8 — 2 (10 ) — Net income (loss) attributable to Ameren common stockholders from continuing operations 222 75 (3 ) — 294 Nine Months 2015 External revenues $ 2,825 $ 1,910 $ 55 $ — $ 4,790 Intersegment revenues 30 3 2 (35 ) — Net income attributable to Ameren common stockholders from continuing operations 341 182 26 — 549 2014 External revenues $ 2,793 $ 1,864 $ 26 $ — $ 4,683 Intersegment revenues 21 1 3 (25 ) — Net income (loss) attributable to Ameren common stockholders from continuing operations 395 156 (10 ) — 541 As of September 30, 2015: Total assets $ 13,941 $ 8,656 $ 1,358 $ (513 ) $ 23,442 (a) As of December 31, 2014: Total assets $ 13,541 $ 8,381 $ 942 $ (203 ) $ 22,661 (a) (a) Excludes total assets from discontinued operations of $17 million and $15 million as of September 30, 2015 and December 31, 2014, respectively.

Summary Of Significant Accoun36

Summary Of Significant Accounting Policies (Narrative) (Detail) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Basis Of Presentation And Significant Accounting Policies [Line Items]
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	1.3	1.7	1.2	1.7
Debt Issuance Cost	$ 35		$ 35
Union Electric Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Debt Issuance Cost	15		15
Ameren Illinois Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Debt Issuance Cost	$ 19		$ 19

Summary Of Significant Accoun37

Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Schedule of Asset Retirement Obligations) (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2015	Dec. 31, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation		$ 597	$ 396
Asset Retirement Obligation, Liabilities Incurred	[1]	3
Asset Retirement Obligation, Liabilities Settled		(1)
Asset Retirement Obligation, Accretion Expense	[2]	17
Asset Retirement Obligation, Revision of Estimate	[3]	182
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	[4]	7	7
Asset Retirement Obligation, Liabilities Incurred		0
Asset Retirement Obligation, Liabilities Settled	[5]	1
Asset Retirement Obligation, Accretion Expense	[2],[5]	1
Asset Retirement Obligation, Revision of Estimate	[5]	1
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation		590	$ 389
Asset Retirement Obligation, Liabilities Incurred	[1]	3
Asset Retirement Obligation, Liabilities Settled		(1)
Asset Retirement Obligation, Accretion Expense	[2]	17
Asset Retirement Obligation, Revision of Estimate	[3]	182
Callaway energy center decommissioning study
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Revision of Estimate		99
Callaway energy center decommissioning study | Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Revision of Estimate		99
New CCR Rules Estimate
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Revision of Estimate		79
New CCR Rules Estimate | Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Revision of Estimate		79
Other Estimate Changes
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Revision of Estimate		4
Other Estimate Changes | Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation, Revision of Estimate		$ 4
[1]	Ameren and Ameren Missouri recorded a new ARO of $3 million related to the Callaway energy center’s dry spent fuel storage facility. See Note 10 - Callaway Energy Center for additional information.
[2]	Accretion expense was recorded as an increase to regulatory assets.
[3]	The ARO increase resulted in a corresponding increase recorded to “Property and Plant, Net.” During 2015, Ameren and Ameren Missouri increased their AROs related to the decommissioning of the Callaway energy center by $99 million to reflect the 2015 cost study and funding analysis filed with the MoPSC, extension of the estimated operating life until 2044, and a reduction in the discount rate assumption. See Note 10 - Callaway Energy Center for additional information. In addition, as a result of new federal regulations, Ameren and Ameren Missouri recorded an increase of $79 million to their AROs associated with CCR storage facilities. See Note 9 - Commitments and Contingencies for additional information. Ameren and Ameren Missouri also increased their AROs by $4 million due to a change in the estimated retirement dates of the Meramec and Rush Island energy centers as a result of the MoPSC’s April 2015 electric rate order.
[4]	Included in “Other deferred credits and liabilities” on the balance sheet.
[5]	Less than $1 million.

Summary Of Significant Accoun38

Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Summary of Nonvested Shares Related To Long-Term Incentive Plan) (Details) - $ / shares		9 Months Ended
		Sep. 30, 2015	Dec. 31, 2014
Weighted-average Fair Value per Unit at Grant Date
Closing common share price			$ 46.13
Performance Shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Three-year risk-free rate		1.10%
Volatility rate, minimum		12.00%
Volatility rate, maximum		18.00%
Share Units
Share Units, Nonvested as of beginning of year		1,162,377
Share Units, Granted	[1]	569,892
Share Units, Forfeitures		(1,944)
Share Units, Vested	[2]	(92,892)
Share Units, Nonvested as of Current period end		1,637,433
Weighted-average Fair Value per Unit at Grant Date
Weighted-average Fair Value per Unit, Nonvested as of Beginning of year		$ 35.35
Weighted-average Fair Value per Unit, Grants	[1]	52.88
Weighted-average Fair Value per Unit, Forfeitures		34.75
Weighted-average Fair Value per Unit, Vested	[2]	45.97
Weighted-average Fair Value per Unit, Nonvested as of Current period end		$ 40.85
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 in 2015 under the 2014 Incentive Plan.
[2]	(b)Performance share units vested due to the attainment of retirement eligibility by certain employees. Actual shares issued for retirement-eligible employees will vary depending on actual performance over the three-year measurement period.

Summary Of Significant Accoun39

Summary Of Significant Accounting Policies (Schedule Of Excise Taxes) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Accounting Policies [Line Items]
Excise tax expense	$ 61	$ 56	$ 169	$ 166
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	52	47	127	120
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$ 9	$ 9	$ 42	$ 46

Summary Of Significant Accoun40

Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Schedule of Unrecognized Tax Benefits (Detriments)) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Schedule of Unrecognized Tax Benefits (Detriments) [Line Items]
Unrecognized tax benefits (detriments)	$ 0	$ 54
2013 Tax Year
Schedule of Unrecognized Tax Benefits (Detriments) [Line Items]
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	53
Union Electric Company
Schedule of Unrecognized Tax Benefits (Detriments) [Line Items]
Unrecognized tax benefits (detriments)	0	0
Ameren Illinois Company
Schedule of Unrecognized Tax Benefits (Detriments) [Line Items]
Unrecognized tax benefits (detriments)	$ 0	$ (1)

Summary Of Significant Accoun41

Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Schedule of Unrecognized Tax Benefits (Detriments) That Would Impact the Effective Tax Rate) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Schedule of Unrecognized Tax Benefits (Detriments) That Would Impact Effective Tax Rate [Line Items]
Unrecognized tax benefits (detriments) that would impact effective tax rate		$ 52
Union Electric Company
Schedule of Unrecognized Tax Benefits (Detriments) That Would Impact Effective Tax Rate [Line Items]
Unrecognized tax benefits (detriments) that would impact effective tax rate		0
Ameren Illinois Company
Schedule of Unrecognized Tax Benefits (Detriments) That Would Impact Effective Tax Rate [Line Items]
Unrecognized tax benefits (detriments) that would impact effective tax rate		$ (1)

Rate And Regulatory Matters (Na

Rate And Regulatory Matters (Narrative-Missouri) (Detail) $ in Millions	1 Months Ended	9 Months Ended	12 Months Ended	33 Months Ended
	Apr. 30, 2015USD ($)	Sep. 30, 2015mi	Dec. 31, 2012USD ($)	Sep. 30, 2015USD ($)
Union Electric Company
Rate And Regulatory Matters [Line Items]
Percentage of Customer's Sales Volume		5.00%
Electric Distribution | Union Electric Company | MEEIA
Rate And Regulatory Matters [Line Items]
Energy Efficiency Program Spending			$ 147	$ 110
Net Shared Benefits			100	$ 134
Incentive Award if Energy Efficiency Goals Are Achieved			$ 19
Achieved Percentage of Energy Efficiency Earnings For Incentive Award			100.00%
Incentive Award if Energy Efficiency Goals Are Achieved, Period of Recognition		3 years
Electric Distribution | Union Electric Company | Final Rate Order
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 122
Public Utilities, Approved Return on Equity, Percentage	9.53%
Public Utilities, Approved Equity Capital Structure, Percentage	51.80%
Rate Base	$ 7,000
Illinois Rivers Project | Ameren Transmission Company of Illinois
Rate And Regulatory Matters [Line Items]
Transmission Line Miles | mi		7
Mark Twain Project | Ameren Transmission Company of Illinois
Rate And Regulatory Matters [Line Items]
Transmission Line Miles | mi		100
Electric Distribution - Net Energy Costs | Union Electric Company | Final Rate Order
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 109

Rate And Regulatory Matters (43

Rate And Regulatory Matters (Narrative-Illinois) (Detail) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015USD ($)mi	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
Rate And Regulatory Matters [Line Items]
Regulatory assets	$ 1,578		$ 1,582
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	922		$ 883
Ameren Illinois Company | Electric Distribution | Final Rate Order
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	15
Ameren Illinois Company | Gas Distribution | Pending Rate Case
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 45
Public Utilities, Requested Equity Capital Structure, Percentage	50.00%
Rate Base	$ 1,200
Public Utilities, Requested Return on Equity, Percentage	9.60%
Ameren Transmission Company of Illinois | Spoon River Project
Rate And Regulatory Matters [Line Items]
Transmission Line Miles | mi	46
IEIMA | Ameren Illinois Company | Electric Distribution | 2015 IEMA Revenue Requirement Reconciliation
Rate And Regulatory Matters [Line Items]
Regulatory assets	$ 52
Public Utilities, Requested Rate Increase (Decrease), Amount	109
IEIMA | Ameren Illinois Company | Electric Distribution | 2014 IEMA Revenue Requirement Reconciliation
Rate And Regulatory Matters [Line Items]
Regulatory assets	103
IEIMA | Ameren Illinois Company | Electric Distribution | 2013 IEMA Revenue Requirement Reconciliation
Rate And Regulatory Matters [Line Items]
Regulatory assets	$ 14
Icc Staff Recommendation | IEIMA | Ameren Illinois Company | Electric Distribution | 2015 IEMA Revenue Requirement Reconciliation | Subsequent Event
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 107

Rate And Regulatory Matters (44

Rate And Regulatory Matters (Narrative-Federal) (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Feb. 28, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		$ 89		$ 89		$ 106
Provision for Callaway construction and operating license		0	$ 0	69	$ 0
New Nuclear Energy Center COL
Rate And Regulatory Matters [Line Items]
Provision for Callaway construction and operating license				69
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		37		37		84
Ameren Illinois Company | Final Rate Order
Rate And Regulatory Matters [Line Items]
Payments for Legal Settlements				8
Union Electric Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		41		41		$ 18
Provision for Callaway construction and operating license		0	$ 0	69	$ 0
Union Electric Company | New Nuclear Energy Center COL
Rate And Regulatory Matters [Line Items]
Investments in Power and Distribution Projects		69		69
Provision for Callaway construction and operating license				$ 69
Midwest Independent Transmission System Operator, Inc | Pending Ferc Case
Rate And Regulatory Matters [Line Items]
Customer Requested Rate on Equity	8.67%			9.15%
Public Utilities, Approved Return on Equity, Percentage				12.38%
Current regulatory liabilities		36		$ 36
Basis point reduction in FERC allowed base return on common equity				0.50%
Earnings reduction in FERC allowed base return on common equity				$ 4
Incentive adder to FERC allowed base return on common equity				0.50%
Midwest Independent Transmission System Operator, Inc | Pending Ferc Case | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities		$ 25		$ 25
Earnings reduction in FERC allowed base return on common equity				$ 2

Short-Term Debt And Liquidity (

Short-Term Debt And Liquidity (Narrative) (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Utilities
Line of Credit Facility [Line Items]
Short Term Debt, Weighted Average Interest Rate During Period	0.10%	0.10%	0.09%	0.23%
Credit Agreements 2012
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,300,000,000		$ 1,300,000,000
Actual debt-to-capital ratio	0.5		0.5
Line of Credit Facility, Covenant Terms, Default Provision, Maximum Indebtedness	$ 75,000,000		$ 75,000,000
Credit Agreements 2012 | Maximum
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.65		0.65
Interest Coverage Requirement	200.00%		200.00%
Credit Agreements 2012 | Union Electric Company | Maximum
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.65		0.65
Credit Agreements 2012 | Ameren Illinois Company | Maximum
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.65		0.65
Missouri Credit Agreement 2012 | Union Electric Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.48		0.48
Illinois Credit Agreement 2012 | Ameren Illinois Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.46		0.46

Short-Term Debt And Liquidity46

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper outstanding) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Line of Credit Facility [Line Items]
Commercial paper outstanding	$ 783	$ 714
Ameren (parent)
Line of Credit Facility [Line Items]
Commercial paper outstanding	783	585
Union Electric Company
Line of Credit Facility [Line Items]
Commercial paper outstanding	0	97
Ameren Illinois Company
Line of Credit Facility [Line Items]
Commercial paper outstanding	$ 0	$ 32

Short-Term Debt And Liquidity47

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 832	$ 609
Weighted Average Interest Rate		0.56%	0.35%
Peak Short Term Borrowings	[1]	$ 1,108	$ 907
Peak Short Term Borrowings Interest Rate		0.70%	0.75%
Ameren (parent)
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 770	$ 386
Weighted Average Interest Rate		0.56%	0.36%
Peak Short Term Borrowings	[1]	$ 874	$ 531
Peak Short Term Borrowings Interest Rate		0.70%	0.75%
Union Electric Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 56	$ 141
Weighted Average Interest Rate		0.50%	0.38%
Peak Short Term Borrowings	[1]	$ 294	$ 495
Peak Short Term Borrowings Interest Rate		0.60%	0.70%
Ameren Illinois Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 6	$ 157
Weighted Average Interest Rate		0.44%	0.31%
Peak Short Term Borrowings	[1]	$ 48	$ 300
Peak Short Term Borrowings Interest Rate		0.60%	0.34%
[1]	(a)The timing of peak commercial paper issuances varies by company, and therefore the peak amounts presented by company might not equal the Ameren Consolidated peak commercial paper issuances for the period.

Long-Term Debt And Equity Fin48

Long-Term Debt And Equity Financings (Narrative) (Detail) - USD ($)	1 Months Ended	9 Months Ended
	Apr. 30, 2015	Sep. 30, 2015	Sep. 30, 2014
Long-Term Debt And Equity Financings [Line Items]
Redemptions of Long-term Debt		$ 114,000,000	$ 692,000,000
Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Capital contribution from parent		224,000,000	0
Redemptions of Long-term Debt		$ 114,000,000	104,000,000
Ameren Missouri And Ameren Illinois
Long-Term Debt And Equity Financings [Line Items]
Assumed interest rate		5.00%
Dividend rate		6.00%
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Redemptions of Long-term Debt		$ 0	$ 163,000,000
Ameren Illinois Company | Federal Energy Regulatory Commission Restriction
Long-Term Debt And Equity Financings [Line Items]
Common stock equity to total capitalization		52.00%
Ameren Illinois Company | Minimum | Federal Energy Regulatory Commission Restriction
Long-Term Debt And Equity Financings [Line Items]
Common stock equity to total capitalization		30.00%
Secured Debt | Senior Secured Notes, 3.65%, Due 2045 | Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount	$ 250,000,000
Debt Instrument, Interest Rate, Stated Percentage	3.65%
Proceeds from Issuance of Secured Debt	$ 247,000,000
Secured Debt | Senior Secured Notes 4.75% Due 2015 | Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Redemptions of Long-term Debt	$ 114,000,000
Debt Instrument, Interest Rate, Stated Percentage	4.75%

Long-Term Debt And Equity Fin49

Long-Term Debt And Equity Financings (Schedule Of Covered Ratio) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2015USD ($)
Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Bonds Issuable	$ 3,338	[1]
Preferred Stock Issuable	2,206
Retired bond capacity	$ 946
Union Electric Company | Actual Ratio [Member]
Long-Term Debt And Equity Financings [Line Items]
Restricted payment interest coverage ratio, Actual	3.7
Dividend Coverage Ratio	99.3
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Bonds Issuable	$ 3,659	[1],[2]
Preferred Stock Issuable	203	[3]
Retired bond capacity	$ 204
Ameren Illinois Company | Actual Ratio [Member]
Long-Term Debt And Equity Financings [Line Items]
Restricted payment interest coverage ratio, Actual	7
Dividend Coverage Ratio	3
Minimum | Union Electric Company | Minimum Required Ratio [Member]
Long-Term Debt And Equity Financings [Line Items]
Restricted payment interest coverage ratio, Actual	2	[4]
Dividend Coverage Ratio	2.5	[5]
Minimum | Ameren Illinois Company | Minimum Required Ratio [Member]
Long-Term Debt And Equity Financings [Line Items]
Restricted payment interest coverage ratio, Actual	2	[4]
Dividend Coverage Ratio	1.5	[5]
[1]	Amount of bonds issuable based either on required coverage ratios or unfunded property additions, whichever is more restrictive. The amounts shown also include bonds issuable based on retired bond capacity of $946 million and $204 million at Ameren Missouri and Ameren Illinois, respectively.
[2]	Amount of bonds issuable by Ameren Illinois based on unfunded property additions and retired bonds solely under the former IP mortgage indenture. The amount of bonds issuable by Ameren Illinois is also subject to the lien restrictions contained in the 2012 Illinois Credit Agreement.
[3]	Preferred stock issuable is restricted by the amount of preferred stock that is currently authorized by Ameren 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		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	[1]	$ 8	$ 10	$ 19	$ 26
Interest income on industrial development revenue bonds	[1]	7	6	20	20
Interest and dividend income	[1]	4	3	12	8
Other	[1]	0	2	3	6
Total miscellaneous income	[1]	19	21	54	60
Donations	[1]	0	3	10	9
Other	[1]	5	4	12	11
Total miscellaneous expense	[1]	5	7	22	20
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		7	9	16	24
Interest income on industrial development revenue bonds		7	6	20	20
Interest and dividend income		0	0	1	1
Total miscellaneous income		14	15	37	45
Donations		0	2	3	5
Other		3	2	5	5
Total miscellaneous expense		3	4	8	10
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		1	1	3	2
Interest and dividend income		3	2	10	5
Other		0	1	2	5
Total miscellaneous income		4	4	15	12
Donations		0	0	4	3
Other		3	2	6	4
Total miscellaneous expense		$ 3	$ 2	$ 10	$ 7
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Derivative Financial Instrume51

Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) lb in Thousands, gal in Millions, MWh in Millions, MMBTU in Millions		Sep. 30, 2015galMWhMMBTUlb	Dec. 31, 2014galMWhMMBTUlb
Fuel Oils
Derivative [Line Items]
Quantity | gal	[1]	33	50
Natural Gas
Derivative [Line Items]
Quantity		182	136
Power
Derivative [Line Items]
Quantity | MWh		11	12
Uranium
Derivative [Line Items]
Quantity | lb		299	332
Union Electric Company | Fuel Oils
Derivative [Line Items]
Quantity | gal	[1]	33	50
Union Electric Company | Natural Gas
Derivative [Line Items]
Quantity		30	28
Union Electric Company | Power
Derivative [Line Items]
Quantity | MWh		1	1
Union Electric Company | Uranium
Derivative [Line Items]
Quantity | lb		299	332
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Quantity		152	108
Ameren Illinois Company | Power
Derivative [Line Items]
Quantity | MWh		10	11
[1]	Fuel oils consist of heating oil and ultra-low-sulfur diesel.

Derivative Financial Instrume52

Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Derivative [Line Items]
Derivative assets	$ 24	$ 19
Derivative liabilities	255	232
Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets		2
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	2
Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	31	37
Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	27	19
Power | Other Current Assets
Derivative [Line Items]
Derivative assets	22	15
Power | Other Assets
Derivative [Line Items]
Derivative assets	1
Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	13	14
Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	158	131
Uranium | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	1	2
Union Electric Company
Derivative [Line Items]
Derivative assets	24	18
Derivative liabilities	41	46
Union Electric Company | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets		2
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 Current Assets
Derivative [Line Items]
Derivative assets	1	1
Union Electric Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	5	6
Union Electric Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	9	6
Union Electric Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets	22	15
Union Electric Company | Power | Other Assets
Derivative [Line Items]
Derivative assets	1
Union Electric Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	1	3
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	1	2
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	0	1
Derivative liabilities	214	186
Ameren Illinois Company | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets		0
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	0	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	18	13
Ameren Illinois Company | Natural Gas | Mark To Market Derivative Liabilities
Derivative [Line Items]
Derivative liabilities	26	31
Ameren Illinois Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Power | Other Assets
Derivative [Line Items]
Derivative assets	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	158	131
Ameren Illinois Company | Power | Mark To Market Derivative Liabilities
Derivative [Line Items]
Derivative liabilities	12	11
Ameren Illinois Company | Uranium | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	$ 0	$ 0

Derivative Financial Instrume53

Derivative Financial Instruments (Cumulative Amount Of Pretax Net Gains (Losses) On All Derivative Instruments In OCI) (Detail) - USD ($) $ in Millions	Sep. 30, 2015		Dec. 31, 2014
Derivative [Line Items]
Current gains deferred as regulatory liabilities	$ 89		$ 106
Current losses deferred as regulatory assets	163		295
Fuel Oils
Derivative [Line Items]
Current gains deferred as regulatory liabilities	0
Current losses deferred as regulatory assets	19
Fuel Oils | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(25)	[1]	(29)
Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current losses deferred as regulatory assets	31
Natural Gas | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(57)	[2]	(54)
Power
Derivative [Line Items]
Current gains deferred as regulatory liabilities	22
Current losses deferred as regulatory assets	13
Power | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(148)	[3]	(130)
Uranium
Derivative [Line Items]
Current losses deferred as regulatory assets	1
Uranium | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(1)	[4]	(2)
Union Electric Company
Derivative [Line Items]
Current gains deferred as regulatory liabilities	41		18
Current losses deferred as regulatory assets	97		163
Union Electric Company | Fuel Oils
Derivative [Line Items]
Current gains deferred as regulatory liabilities	0
Current losses deferred as regulatory assets	19
Union Electric Company | Fuel Oils | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(25)	[1]	(29)
Union Electric Company | Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current losses deferred as regulatory assets	5
Union Electric Company | Natural Gas | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(13)	[2]	(11)
Union Electric Company | Power
Derivative [Line Items]
Current gains deferred as regulatory liabilities	22
Current losses deferred as regulatory assets	1
Union Electric Company | Power | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	22	[3]	12
Union Electric Company | Uranium
Derivative [Line Items]
Current losses deferred as regulatory assets	1
Union Electric Company | Uranium | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(1)	[4]	(2)
Ameren Illinois Company
Derivative [Line Items]
Current gains deferred as regulatory liabilities	37		84
Current losses deferred as regulatory assets	65		129
Ameren Illinois Company | Fuel Oils | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	0	[1]	0
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Current losses deferred as regulatory assets	26
Ameren Illinois Company | Natural Gas | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(44)	[2]	(43)
Ameren Illinois Company | Power
Derivative [Line Items]
Current losses deferred as regulatory assets	12
Ameren Illinois Company | Power | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(170)	[3]	(142)
Ameren Illinois Company | Uranium | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	$ 0	[4]	$ 0
[1]	Represents net losses associated with fuel oils derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s rail transportation surcharges for coal through December 2017. Losses deferred as current regulatory assets include $19 million at Ameren and Ameren Missouri.
[2]	Represents net losses associated with natural gas derivative contracts. These contracts are a partial hedge of natural gas requirements through March 2021 at Ameren and Ameren Missouri and through October 2019 at Ameren Illinois. Gains deferred as current regulatory liabilities include $1 million at Ameren and Ameren Missouri. Losses deferred as current regulatory assets include $31 million, $5 million, and $26 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively.
[3]	Represents net gains (losses) associated with power derivative contracts. These contracts are a partial hedge of power price requirements through May 2032 at Ameren and Ameren Illinois and through December 2016 at Ameren Missouri. Gains deferred as current regulatory liabilities include $22 million at Ameren and Ameren Missouri. Losses deferred as current regulatory assets include $13 million, $1 million, and $12 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively.
[4]	Represents net losses on uranium derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s uranium requirements through January 2017. Losses deferred as current regulatory assets include $1 million at Ameren and Ameren Missouri.

Derivative Financial Instrume54

Derivative Financial Instruments (Offsetting Derivative Assets and Liabilities) (Details) - Commodity Contract - USD ($) $ in Millions		Sep. 30, 2015		Dec. 31, 2014
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet		$ 24	[1]	$ 19	[2]
Derivative Instruments		2		5
Derivative, Collateral, Obligation to Return Cash	[3]	0		0
Net Amount		22		14
Gross Amounts Recognized in the Balance Sheet		255	[1]	232	[2]
Derivative Instruments		2		5
Cash Collateral Received/Posted	[3]	7		5
Net Amount		246		222
Union Electric Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet		24	[1]	18	[2]
Derivative Instruments		2		5
Derivative, Collateral, Obligation to Return Cash	[3]	0		0
Net Amount		22		13
Gross Amounts Recognized in the Balance Sheet		41	[1]	46	[2]
Derivative Instruments		2		5
Cash Collateral Received/Posted	[3]	6		5
Net Amount		33		36
Ameren Illinois Company
Offsetting Assets and Liabilities [Line Items]
Gross Amounts Recognized in the Balance Sheet		0		1
Derivative Instruments		0		0
Derivative, Collateral, Obligation to Return Cash	[3]	0		0
Net Amount		0		1
Gross Amounts Recognized in the Balance Sheet		214	[1]	186	[2]
Derivative Instruments		0		0
Cash Collateral Received/Posted	[3]	1		0
Net Amount		$ 213		$ 186
[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 Instrume55

Derivative Financial Instruments (Maximum Exposure If Counterparties Fail To Perform On Contracts) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2015USD ($)
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	$ 12
Union Electric Company
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	12
Ameren Illinois Company
Derivative [Line Items]
Maximum exposure to counterparties related to derivative contracts	$ 0

Derivative Financial Instrume56

Derivative Financial Instruments (Potential Loss On Counterparty Exposures) (Detail) $ in Millions	Sep. 30, 2015USD ($)
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$ 11
Ameren Illinois Company
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	0
Union Electric Company
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$ 11

Derivative Financial Instrume57

Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Detail) $ in Millions	Sep. 30, 2015USD ($)
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	$ 170	[1]
Cash Collateral Posted	8
Potential Aggregate Amount of Additional Collateral Required	150	[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	75	[2]
Ameren Illinois Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	82	[1]
Cash Collateral Posted	1
Potential Aggregate Amount of Additional Collateral Required	$ 75	[2]
[1]	Prior to 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		9 Months Ended		12 Months Ended
		Sep. 30, 2015USD ($)$ / credit$ / MMBTU$ / lb$ / MWh$ / MW		Dec. 31, 2014USD ($)$ / MMBTU$ / credit$ / MW$ / MWh$ / lb
Natural Gas | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk		0.40%	[1],[2]	0.43%	[3],[4]
Nodal basis		(1.40)	[5]	(0.40)	[6]
Credit risk		0.40%	[1],[2]	0.43%	[3],[4]
Natural Gas | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk		12.07%	[1],[2]	13.00%	[3],[4]
Nodal basis		0.10	[5]	0.10	[6]
Credit risk		0.40%	[1],[2]	0.43%	[3],[4]
Natural Gas | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk		6.60%	[1],[2]	3.00%	[3],[4]
Nodal basis		(0.20)	[5]	(0.20)	[6]
Credit risk		0.40%	[1],[2]	0.43%	[3],[4]
Natural Gas | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities		20.00%	[5]	31.00%	[3]
Nodal basis		(0.30)	[1]	(0.40)	[6]
Natural Gas | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities		44.00%	[5]	144.00%	[3]
Nodal basis		(0.10)	[1]	0	[6]
Natural Gas | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities		33.00%	[5]	63.00%	[3]
Nodal basis		(0.20)	[1]	(0.20)	[6]
Natural Gas | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		$ 0	[7]	$ 1	[8]
Natural Gas | Union Electric Company | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk		0.40%	[1],[2]	0.57%	[3],[4]
Nodal basis		(1.40)	[5]	(0.10)	[6]
Credit risk		0.40%	[1],[2]	0.43%	[3],[4]
Natural Gas | Union Electric Company | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk		12.07%	[1],[2]	13.00%	[3],[4]
Nodal basis		0.1	[5]	(0.10)	[6]
Credit risk		0.40%	[1],[2]	0.43%	[3],[4]
Natural Gas | Union Electric Company | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk		6.60%	[1],[2]	5.00%	[3],[4]
Nodal basis		(0.20)	[5]	(0.10)	[6]
Credit risk		0.40%	[1],[2]	0.43%	[3],[4]
Natural Gas | Union Electric Company | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities		20.00%	[5]	31.00%	[3]
Nodal basis		(0.30)	[1]	(0.40)	[6]
Natural Gas | Union Electric Company | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities		44.00%	[5]	144.00%	[3]
Nodal basis		(0.10)	[1]	0	[6]
Natural Gas | Union Electric Company | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities		33.00%	[5]	53.00%	[3]
Nodal basis		(0.20)	[1]	(0.30)	[6]
Natural Gas | Union Electric Company | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		$ 0	[7]	$ 0	[8]
Natural Gas | Ameren Illinois Company | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	[3],[4]			0.43%
Nodal basis | $ / MWh	[6]			(0.40)
Credit risk	[3],[4]			0.43%
Natural Gas | Ameren Illinois Company | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	[3],[4]			2.00%
Nodal basis | $ / MWh	[6]			0.10
Credit risk	[3],[4]			0.43%
Natural Gas | Ameren Illinois Company | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	[3],[4]			0.83%
Nodal basis | $ / MWh	[6]			(0.20)
Credit risk	[3],[4]			0.43%
Natural Gas | Ameren Illinois Company | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities	[3]			50.00%
Nodal basis | $ / MWh	[6]			(0.10)
Natural Gas | Ameren Illinois Company | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities	[3]			144.00%
Nodal basis | $ / MWh	[6]			0
Natural Gas | Ameren Illinois Company | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities	[3]			94.00%
Nodal basis | $ / MWh	[6]			(0.10)
Natural Gas | Ameren Illinois Company | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	[8]			$ 1
Natural Gas | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities		(1)	[7]	(2)	[8]
Natural Gas | Derivative Liabilities | Union Electric Company
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities		$ (1)	[7]	(1)	[8]
Natural Gas | Derivative Liabilities | Ameren Illinois Company
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	[8]			$ (1)
Fuel Oils | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Escalation rate	[6],[9]			5.00%
Credit risk		0.40%	[1],[2]	0.43%	[3],[4]
Fuel Oils | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Escalation rate	[6],[9]			5.00%
Credit risk		0.40%	[1],[2]	0.43%	[3],[4]
Fuel Oils | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Escalation rate	[6],[9]			5.00%
Credit risk		0.40%	[1],[2]	0.43%	[3],[4]
Fuel Oils | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities		63.00%	[5]	3.00%	[3]
Fuel Oils | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities		63.00%	[1]	39.00%	[3]
Fuel Oils | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities		63.00%	[1]	32.00%	[3]
Fuel Oils | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		$ 0	[7]	$ 2	[8]
Fuel Oils | Union Electric Company | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Escalation rate	[6],[9]			5.00%
Credit risk		0.40%	[1],[2]	0.43%	[3],[4]
Fuel Oils | Union Electric Company | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Escalation rate	[6],[9]			5.00%
Credit risk		0.40%	[1],[2]	0.43%	[3],[4]
Fuel Oils | Union Electric Company | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Escalation rate	[6],[9]			5.00%
Credit risk		0.40%	[1],[2]	0.43%	[3],[4]
Fuel Oils | Union Electric Company | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities		63.00%	[5]	3.00%	[3]
Fuel Oils | Union Electric Company | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities		63.00%	[5]	39.00%	[3]
Fuel Oils | Union Electric Company | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities		63.00%	[5]	32.00%	[3]
Fuel Oils | Union Electric Company | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		$ 0	[7]	$ 2	[8]
Fuel Oils | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities		(1)	[7]	(8)	[8]
Fuel Oils | Derivative Liabilities | Union Electric Company
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities		$ (1)	[7]	$ (8)	[8]
Power | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk		0.46%	[1],[2]	0.26%	[3],[4]
Nodal basis		(10)	[5]	(6)	[6]
Credit risk		0.40%	[1],[2]	0.43%	[3],[4]
Average forward peak and off-peak pricing		26	[10]	27	[11]
Estimated auction price		(480)	[5]	(1,833)	[6]
Power | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk		0.84%	[1],[2]	0.26%	[3],[4]
Nodal basis		(1)	[5]	0	[6]
Credit risk		0.40%	[1],[2]	0.43%	[3],[4]
Average forward peak and off-peak pricing		40	[10]	50	[11]
Estimated auction price		2,333	[5]	2,743	[6]
Power | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk		0.73%	[1],[2]	0.26%	[3],[4]
Nodal basis		(3)	[5]	(2)	[6]
Credit risk		0.40%	[1],[2]	0.43%	[3],[4]
Average forward peak and off-peak pricing		29	[10]	32	[11]
Estimated auction price		193	[5]	171	[6]
Power | Fundamental Energy Production Model | Minimum
Fair Value Inputs [Abstract]
Escalation rate		2.00%	[5],[12]	0.00%	[6],[13]
Estimated future gas prices		3	[5]	4	[6]
Power | Fundamental Energy Production Model | Maximum
Fair Value Inputs [Abstract]
Escalation rate		3.00%	[5],[12]	1.00%	[6],[13]
Estimated future gas prices		4	[5]	5	[6]
Power | Fundamental Energy Production Model | Weighted Average
Fair Value Inputs [Abstract]
Escalation rate		2.00%	[5],[12]	1.00%	[6],[13]
Estimated future gas prices		4	[5]	4	[6]
Power | Contract Price Allocation | Minimum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit		5	[5]	5	[6]
Power | Contract Price Allocation | Maximum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit		7	[5]	7	[6]
Power | Contract Price Allocation | Weighted Average
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit		6	[5]	6	[6]
Power | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		$ 23	[7],[14]	$ 11	[8],[15]
Power | Union Electric Company | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk		0.46%	[1],[2]	0.26%	[3],[4]
Nodal basis | $ / MWh	[1]	(10)
Credit risk	[3],[4]			0.43%
Average forward peak and off-peak pricing		26	[1]	27	[3]
Estimated auction price		(480)	[5]	(1,833)	[6]
Power | Union Electric Company | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk		0.84%	[1],[2]	0.26%	[3],[4]
Nodal basis | $ / MWh	[1]	(5)
Credit risk	[3],[4]			0.43%
Average forward peak and off-peak pricing		40	[1]	50	[3]
Estimated auction price		2,333	[5]	2,743	[6]
Power | Union Electric Company | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk		0.73%	[1],[2]	0.26%	[3],[4]
Nodal basis | $ / MWh	[1]	(9)
Credit risk	[3],[4]			0.43%
Average forward peak and off-peak pricing		34	[1]	32	[3]
Estimated auction price		193	[5]	171	[6]
Power | Union Electric Company | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		$ 23	[7],[14]	$ 11	[8],[15]
Power | Ameren Illinois Company | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Nodal basis		(6)	[5]	(6)	[6]
Credit risk		0.40%	[1],[2]	0.43%	[3],[4]
Average forward peak and off-peak pricing | $ / MWh	[5]	27
Average forward pricing | $ / lb	[6]			27
Power | Ameren Illinois Company | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Nodal basis		(1)	[5]	0	[6]
Credit risk		0.40%	[1],[2]	0.43%	[3],[4]
Average forward peak and off-peak pricing | $ / MWh	[5]	33
Average forward pricing | $ / lb	[6]			38
Power | Ameren Illinois Company | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Nodal basis		(3)	[5]	(2)	[6]
Credit risk		0.40%	[1],[2]	0.43%	[3],[4]
Average forward peak and off-peak pricing | $ / MWh	[5]	30
Average forward pricing | $ / lb	[6]			32
Power | Ameren Illinois Company | Fundamental Energy Production Model | Minimum
Fair Value Inputs [Abstract]
Escalation rate		2.00%	[5],[12]	0.00%	[6],[13]
Estimated future gas prices		3	[5]	4	[6]
Power | Ameren Illinois Company | Fundamental Energy Production Model | Maximum
Fair Value Inputs [Abstract]
Escalation rate		3.00%	[5],[12]	1.00%	[6],[13]
Estimated future gas prices		4	[5]	5	[6]
Power | Ameren Illinois Company | Fundamental Energy Production Model | Weighted Average
Fair Value Inputs [Abstract]
Escalation rate		2.00%	[5],[12]	1.00%	[6],[13]
Estimated future gas prices		4	[5]	4	[6]
Power | Ameren Illinois Company | Contract Price Allocation | Minimum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit		5	[5]	5	[6]
Power | Ameren Illinois Company | Contract Price Allocation | Maximum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit		7	[5]	7	[6]
Power | Ameren Illinois Company | Contract Price Allocation | Weighted Average
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit		6	[5]	6	[6]
Power | Ameren Illinois Company | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		$ 0	[7],[14]	$ 0	[8],[15]
Power | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities		(171)	[7],[14]	(144)	[8],[15]
Power | Derivative Liabilities | Union Electric Company
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities		(1)	[7],[14]	(2)	[8],[15]
Power | Derivative Liabilities | Ameren Illinois Company
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities		$ (170)	[7],[14]	$ (142)	[8],[15]
Uranium | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Average forward pricing		37	[5]	35	[6]
Uranium | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Average forward pricing		39	[5]	40	[6]
Uranium | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Average forward pricing		37	[5]	36	[6]
Uranium | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		$ 0	[7]	$ 0	[8]
Uranium | Union Electric Company | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Average forward pricing		37	[5]	35	[6]
Uranium | Union Electric Company | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Average forward pricing		39	[5]	40	[6]
Uranium | Union Electric Company | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Average forward pricing		37	[5]	36	[6]
Uranium | Union Electric Company | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		$ 0	[7]	$ 0	[8]
Uranium | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities		(1)	[7]	(2)	[8]
Uranium | Derivative Liabilities | Union Electric Company
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities		$ (1)	[7]	$ (2)	[8]
[1]	(b)Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.
[2]	(c)Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances.
[3]	(b)Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.
[4]	(c)Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances.
[5]	(e)Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.
[6]	(e)Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.
[7]	(a)The derivative asset and liability balances are presented net of counterparty credit considerations.
[8]	(a)The derivative asset and liability balances are presented net of counterparty credit considerations.
[9]	(f)Escalation rate applies to fuel oil prices 2017 and beyond.
[10]	(g)The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes due to their opposing positions. As such, refer to the power sensitivity analysis for each company above.
[11]	(h)The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes due to their opposing positions. As such, refer to the power sensitivity analysis for each company above.
[12]	(h)Escalation rate applies to power prices 2026 and beyond.
[13]	(i)Escalation rate applies to power prices 2026 and beyond.
[14]	(f)Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2019. Valuations beyond 2019 use fundamentally modeled pricing by month for peak and off-peak demand.
[15]	(g)Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2018. Valuations beyond 2018 use fundamentally modeled pricing by month for peak and off-peak demand.

Fair Value Measurements (Narrat

Fair Value Measurements (Narrative) (Detail) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Valuation adjustments related to net derivative contracts, liabilities	$ 1	$ 1
Union Electric Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Valuation adjustments related to net derivative contracts, liabilities	1	1
Ameren Illinois Company
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Valuation adjustments related to net derivative contracts, liabilities	$ 1	$ 1

Fair Value Measurements (Sche60

Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions		Sep. 30, 2015		Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		$ 535	[1]	$ 547	[2]
Assets fair value		559	[3]	566	[4]
Excluded receivables, payables, and accrued income, net		(1)		2
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		535	[1]	547	[2]
Assets fair value		559	[3]	565	[4]
Excluded receivables, payables, and accrued income, net		(1)		2
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		24	[3]	19	[4]
Derivative liabilities		255	[3]	232	[4]
Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		24	[3]	18	[4]
Derivative liabilities		41	[3]	46	[4]
Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0		1
Derivative liabilities		214	[3]	186	[4]
Commodity Contract | Fuel Oils
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			2
Derivative liabilities		25	[3]	29	[4]
Commodity Contract | Fuel Oils | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			2
Derivative liabilities		25	[3]	29	[4]
Commodity Contract | Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	2	[4]
Derivative liabilities		58	[3]	56	[4]
Commodity Contract | Natural Gas | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	1	[4]
Derivative liabilities		14	[3]	12	[4]
Commodity Contract | Natural Gas | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			1
Derivative liabilities		44	[3]	44	[4]
Commodity Contract | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		23	[3]	15	[4]
Derivative liabilities		171	[3]	145	[4]
Commodity Contract | Power | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		23	[3]	15	[4]
Derivative liabilities		1	[3]	3	[4]
Commodity Contract | Power | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		170	[3]	142	[4]
Commodity Contract | Uranium
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		1	[3]	2	[4]
Commodity Contract | Uranium | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		1	[3]	2	[4]
Cash and cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		3		1
Cash and cash equivalents | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		3		1
Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		338		364
Equity Securities | U.S. large capitalization | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		338		364
Debt Securities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		61		63
Debt Securities | Corporate bonds | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		61		63
Debt Securities | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		109		102
Debt Securities | US treasury and government securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		109		102
Debt Securities | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		24		17
Debt Securities | Other Debt Securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		24		17
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		341		365
Assets fair value		341	[3]	365	[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		341		365
Assets fair value		341	[3]	365	[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		25	[3]	22	[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		0	[3]	0	[4]
Derivative liabilities		25	[3]	22	[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]	0
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Fuel Oils
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			0
Derivative liabilities		24	[3]	21	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Fuel Oils | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			0
Derivative liabilities		24	[3]	21	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Derivative liabilities		1	[3]	1	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Natural Gas | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Derivative liabilities		1	[3]	1	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Natural Gas | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			0
Derivative liabilities		0	[3]	0	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Derivative liabilities		0	[3]	0	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Power | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Derivative liabilities		0	[3]	0	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Power | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0	[3]	0	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Uranium
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0	[3]	0	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Uranium | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0	[3]	0	[4]
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		3		1
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Cash and cash equivalents | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		3		1
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		338		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		338		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		194		182
Assets fair value		195	[3]	187	[4]
Significant Other Observable Inputs (Level 2) | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		194		182
Assets fair value		195	[3]	187	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	5	[4]
Derivative liabilities		56	[3]	54	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	5	[4]
Derivative liabilities		12	[3]	11	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		44	[3]	43	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Fuel Oils
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			0
Derivative liabilities		0	[3]	0	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Fuel Oils | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			0
Derivative liabilities		0	[3]	0	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	1	[4]
Derivative liabilities		56	[3]	53	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Natural Gas | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	1	[4]
Derivative liabilities		12	[3]	10	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Natural Gas | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			0
Derivative liabilities		44	[3]	43	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	4	[4]
Derivative liabilities		0	[3]	1	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Power | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	4	[4]
Derivative liabilities		0	[3]	1	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Power | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0	[3]	0
Significant Other Observable Inputs (Level 2) | Commodity Contract | Uranium
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0	[3]	0	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Uranium | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0	[3]	0	[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		61		63
Significant Other Observable Inputs (Level 2) | Debt Securities | Corporate bonds | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		61		63
Significant Other Observable Inputs (Level 2) | Debt Securities | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		109		102
Significant Other Observable Inputs (Level 2) | Debt Securities | US treasury and government securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		109		102
Significant Other Observable Inputs (Level 2) | Debt Securities | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		24		17
Significant Other Observable Inputs (Level 2) | Debt Securities | Other Debt Securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		24		17
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		23	[3]	14	[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		23	[3]	13	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		23	[3]	14	[4]
Derivative liabilities		174	[3]	156	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		23	[3]	13	[4]
Derivative liabilities		4	[3]	13	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		170	[3]	143	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Fuel Oils
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			2
Derivative liabilities		1	[3]	8	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Fuel Oils | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			2
Derivative liabilities		1	[3]	8	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	1	[4]
Derivative liabilities		1	[3]	2	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Natural Gas | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Derivative liabilities		1	[3]	1	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Natural Gas | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			1
Derivative liabilities		0	[3]	1	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		23	[3]	11	[4]
Derivative liabilities		171	[3]	144	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Power | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		23	[3]	11	[4]
Derivative liabilities		1	[3]	2	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Power | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		170	[3]	142	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Uranium
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		1	[3]	2	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Uranium | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		1	[3]	2	[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
[1]	Balance excludes $(1) million of receivables, payables, and accrued income, net.
[2]	Balance excludes $2 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 (Sche61

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		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Fuel Oils
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ (1)	$ 2	$ (6)	$ 5
Included in regulatory assets/liabilities	(1)	(2)	(1)	(3)
Settlements	1		4	(2)
Transfer out of Level 3			2
Ending balance	(1)	0	(1)	0
Change in unrealized gains (losses) related to assets/liabilities held at period end	0	(2)	0	(2)
Uranium
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(2)	(7)	(2)	(6)
Included in regulatory assets/liabilities		3		(1)
Settlements	1	1	1	4
Ending balance	(1)	(3)	(1)	(3)
Change in unrealized gains (losses) related to assets/liabilities held at period end	0	3	0	0
Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(1)	0	(1)	0
Included in regulatory assets/liabilities	0	1	0	1
Purchases				(1)
Settlements			0	1
Ending balance	(1)	1	(1)	1
Change in unrealized gains (losses) related to assets/liabilities held at period end	0	0	0	0
Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(138)	(88)	(133)	(89)
Included in regulatory assets/liabilities	(6)	(28)	(37)	(42)
Purchases			29	34
Settlements	(4)	(3)	(7)	(22)
Ending balance	(148)	(119)	(148)	(119)
Change in unrealized gains (losses) related to assets/liabilities held at period end	(6)	(28)	(34)	(24)
Union Electric Company | Fuel Oils
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(1)	2	(6)	5
Included in regulatory assets/liabilities	(1)	(2)	(1)	(3)
Settlements	1		4	(2)
Transfer out of Level 3			2
Ending balance	(1)	0	(1)	0
Change in unrealized gains (losses) related to assets/liabilities held at period end	0	(2)	0	(2)
Union Electric Company | Uranium
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(2)	(7)	(2)	(6)
Included in regulatory assets/liabilities		3		(1)
Settlements	1	1	1	4
Ending balance	(1)	(3)	(1)	(3)
Change in unrealized gains (losses) related to assets/liabilities held at period end	0	3	0	0
Union Electric Company | Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	0	(1)	0
Included in regulatory assets/liabilities	(1)	0	(1)	0
Purchases				0
Settlements			1	0
Ending balance	(1)	0	(1)	0
Change in unrealized gains (losses) related to assets/liabilities held at period end	0	0	0	0
Union Electric Company | Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	27	15	9	19
Included in regulatory assets/liabilities	2	(5)	0	(23)
Purchases			29	34
Settlements	(7)	(5)	(16)	(25)
Ending balance	22	5	22	5
Change in unrealized gains (losses) related to assets/liabilities held at period end	1	(6)	1	(3)
Ameren Illinois Company | Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(1)	0	0	0
Included in regulatory assets/liabilities	1	1	1	1
Purchases				(1)
Settlements			(1)	1
Ending balance	0	1	0	1
Change in unrealized gains (losses) related to assets/liabilities held at period end	0	0	0	0
Ameren Illinois Company | Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(165)	(103)	(142)	(108)
Included in regulatory assets/liabilities	(8)	(23)	(37)	(19)
Purchases			0	0
Settlements	3	2	9	3
Ending balance	(170)	(124)	(170)	(124)
Change in unrealized gains (losses) related to assets/liabilities held at period end	$ (7)	$ (22)	$ (35)	$ (21)

Fair Value Measurements (Sche62

Fair Value Measurements (Schedule Of Transfers Between Fair Value Hierarchy Levels) (Detail) - Fuel Oils $ in Millions	9 Months Ended
	Sep. 30, 2015USD ($)
Derivative [Line Items]
Assets Transfers out of Level 3	$ 2
Union Electric Company
Derivative [Line Items]
Assets Transfers out of Level 3	$ 2

Fair Value Measurements (Sche63

Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Long-Term Debt And Preferred Stock) (Detail) - USD ($) $ in Millions		Sep. 30, 2015	Dec. 31, 2014
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)	[1]	$ 6,916	$ 7,135
Preferred stock	[1]	124	122
Fair Value | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)		4,488	4,518
Preferred stock		75	73
Fair Value | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)		2,428	2,517
Preferred stock		49	49
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)	[1]	6,376	6,240
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)		4,135	3,999
Preferred stock		80	80
Carrying Amount | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)		2,241	2,241
Preferred stock		$ 62	$ 62
[1]	Preferred stock is recorded in “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			9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014		Sep. 30, 2015	Sep. 30, 2014
Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues		$ 11	$ 8		$ 30	$ 21
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues		1	1	[1]	3	1
Purchased Power | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses		5	3		11	7
Ameren Missouri Power Supply Agreements with Ameren Illinois | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues		4	2		9	5
Ameren Missouri and Ameren Illinois Rent and Facility Services | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues		6	6		19	15
Ameren Missouri and Ameren Illinois Rent and Facility Services | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues		1	1	[1]	3	1
Ameren Missouri and Ameren Illinois miscellaneous support services [Member] | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues		1	1	[1]	2	1
Ameren Missouri and Ameren Illinois miscellaneous support services [Member] | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	[1]	1	1		1	1
Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses		4	2		9	5
Ameren Illinois Transmission Agreements with ATXI | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses		1	1		2	2
Ameren Services Support Services Agreement | Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses		30	25		96	90
Ameren Services Support Services Agreement | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses		28	26		87	80
Related Party Money Pool Interest | Union Electric Company
Related Party Transaction [Line Items]
Interest Charges	[1]	1	1		1	1
Related Party Money Pool Interest | Ameren Illinois Company
Related Party Transaction [Line Items]
Interest Charges	[1]	$ 1	$ 1		$ 1	$ 1
[1]	Amount less than $1 million.

Related Party Transactions Narr

Related Party Transactions Narrative (Details) - Ameren Illinois Company - Ameren Illinois Power Supply Agreements with Ameren Missouri	9 Months Ended
	Sep. 30, 2015MWh$ / MWh
April 2015 Procurement [Member]
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power	667,000
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate | $ / MWh	36
September 2015 Procurement [Member]
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power	339,000
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate | $ / MWh	38

Commitments And Contingencies66

Commitments And Contingencies (Callaway Energy Center) (Detail)	9 Months Ended
	Sep. 30, 2015USD ($)
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	13,361,000,000	[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	12,986,000,000	[2]
Maximum Assessments for Single Incidents	127,000,000	[3]
Property Damage
Commitments And Contingencies [Line Items]
Maximum Coverages	3,250,000,000
Maximum Assessments for Single Incidents	27,000,000
Property Damage - Nuclear Electric Insurance Ltd
Commitments And Contingencies [Line Items]
Maximum Coverages	2,750,000,000	[4]
Maximum Assessments for Single Incidents	27,000,000	[5]
Property Damage European Mutual Association for Nuclear Insurance
Commitments And Contingencies [Line Items]
Maximum Coverages	500,000,000	[6]
Replacement Power - Nuclear Electric Insurance Ltd
Commitments And Contingencies [Line Items]
Maximum Coverages	490,000,000	[7]
Maximum Assessments for Single Incidents	10,000,000	[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	2,250,000,000
Aggregate nuclear power industry insurance policy limit for losses from terrorist attacks within twelve month period	1,830,000,000
Radiation Event
Commitments And Contingencies [Line Items]
Maximum Coverages	$ 500,000,000
[1]	Limit of liability for each incident under the Price-Anderson liability provisions of the Atomic Energy Act of 1954, as amended. A company could be assessed up to $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 both radiation and nonradiation events. An additional $500 million is provided for radiation events only for a total of $2.75 billion.
[5]	All NEIL insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL.
[6]	European Mutual Association for Nuclear Insurance provides $500 million in excess of the $2.75 billion and $2.25 billion property coverage for radiation and nonradiation events, respectively, provided by NEIL.
[7]	Provides replacement power cost insurance in the event of a prolonged accidental outage. Weekly indemnity up to $4.5 million for 52 weeks, which commences after the first twelve weeks of an outage, plus up to $3.6 million per week for a minimum of 71 weeks thereafter for a total not exceeding the policy limit of $490 million. Nonradiation events are sub-limited to $328 million.

Commitments And Contingencies67

Commitments And Contingencies (Other Obligations) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2015USD ($)MWh
Long-term Purchase Commitment [Line Items]
Total Other Obligations	$ 4,832
Union Electric Company
Long-term Purchase Commitment [Line Items]
Total Other Obligations	2,913
Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Total Other Obligations	$ 1,888
April 2015 Procurement [Member] | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Amount of Megawatthours | MWh	5,526,000
Long-term Purchase Commitment, Amount	$ 185
September 2015 Procurement [Member] | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Amount of Megawatthours | MWh	2,592,000
Long-term Purchase Commitment, Amount	$ 83

Commitments And Contingencies68

Commitments And Contingencies (Environmental Matters) (Detail) $ in Millions	Sep. 30, 2015USD ($)site
Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 350
Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	400
Manufactured Gas Plant
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 237
Manufactured Gas Plant | Ameren Illinois Company
Loss Contingencies [Line Items]
Number of remediation sites | site	44
Range of possible loss, minimum	$ 237
Range of possible loss maximum	309
Accrual for environmental loss contingencies	237
Former Coal Ash Landfill | Ameren Illinois Company
Loss Contingencies [Line Items]
Range of possible loss, minimum	0.5
Range of possible loss maximum	6
Accrual for environmental loss contingencies	0.5
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]
Range of possible loss, minimum	2
Range of possible loss maximum	5
Accrual for environmental loss contingencies	2
Sauget Area Two | Union Electric Company
Loss Contingencies [Line Items]
Range of possible loss, minimum	1
Range of possible loss maximum	2.5
Accrual for environmental loss contingencies	1
Substation in St Charles, Missouri | Union Electric Company
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 0.8

Commitments And Contingencies69

Commitments And Contingencies (Pumped-Storage Hydroelectric Facility Breach) (Detail) - Union Electric Company $ in Millions	Sep. 30, 2015USD ($)
Loss Contingencies [Line Items]
Insurance settlements receivable	$ 41
Insurance Claim	$ 54

Commitments And Contingencies70

Commitments And Contingencies (Asbestos-Related Litigation) (Detail) - Asbestos-Related $ in Millions	9 Months Ended
	Sep. 30, 2015USD ($)defendantlawsuit
Loss Contingencies [Line Items]
Average number of total defendants named | defendant	73
Asbestos-related lawsuits were pending | lawsuit	53	[1]
Range of possible loss, minimum	$ 8
Percent of allowed cash expenditures in excess of base rates to be recovered through charges assessed to customers	90.00%
Asbestos trust fund balance	$ 22
Percent of difference to be contributed to the asbestos trust fund if cash expenditures are less than amount included in base electric rates.	90.00%
Union Electric Company
Loss Contingencies [Line Items]
Asbestos-related lawsuits were pending | lawsuit	27
Range of possible loss, minimum	$ 3
Ameren Illinois Company
Loss Contingencies [Line Items]
Asbestos-related lawsuits were pending | lawsuit	40
Range of possible loss, minimum	$ 5
Ameren Corporation
Loss Contingencies [Line Items]
Asbestos-related lawsuits were pending | lawsuit	1
[1]	Total does not equal the sum of the subsidiary unit lawsuits because some of the lawsuits name multiple Ameren entities as defendants.

Callaway Energy Center (Narrati

Callaway Energy Center (Narrative) (Detail) - Nuclear Plant $ in Millions	9 Months Ended
	Sep. 30, 2015USD ($)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	Sep. 30, 2015USD ($)
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	$ 350
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	50
Maximum
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined benefit plan, estimated future employer contributions in each of the next five years	$ 110

Retirement Benefits (Components

Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		$ 23	$ 20	$ 69	$ 60
Interest cost		43	46	130	137
Expected return on plan assets		(62)	(58)	(186)	(172)
Prior service cost (benefit)			(1)		(1)
Actuarial loss		19	13	56	37
Settlement loss		0	0	1	0
Net periodic benefit cost	[1],[2]	23	20	70	61
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		6	5	17	14
Interest cost		12	12	36	37
Expected return on plan assets		(17)	(16)	(51)	(48)
Prior service cost (benefit)		(1)	(2)	(3)	(4)
Actuarial loss		1	(2)	4	(5)
Settlement loss		0	0	0	0
Net periodic benefit cost	[1],[2]	$ 1	$ (3)	$ 3	$ (6)
[1]	(a)The amounts above do 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 built into 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		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	[1],[2]	$ 23	$ 20	$ 70	$ 61
Pension Plan | Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost		14	13	42	38
Pension Plan | Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost		9	7	28	22
Pension Plan | Other Affiliated Entities and Intercompany Eliminations [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost			0	0	1
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]	1	(3)	3	(6)
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		$ 2		6	2
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			$ (3)	(2)	(7)
Other Postretirement Benefit Plan, Defined Benefit | Other Affiliated Entities and Intercompany Eliminations [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost				$ (1)	$ (1)
[1]	(a)The amounts above do 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 built into rates
[2]	(b)Includes amounts for Ameren registrants and nonregistrant subsidiaries.

Divestiture Transactions and 75

Divestiture Transactions and Discontinued Operations Divestiture Transactions and Discontinued Operations (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Jan. 31, 2014	Dec. 31, 2013	Sep. 30, 2015	Dec. 31, 2014	Mar. 14, 2013
New Ameren Energy Resources Company, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Range of possible loss maximum			$ 25
Notes, Loans and Financing Receivable, Net, Current			8	$ 12
Collateral to be posted if credit ratings are below investment grade			21
Buyer's indemnification guarantee obligation					$ 25
Loss contingency accrual			25
Guarantees Outstanding			74
Letters of Credit Outstanding, Amount			9
Elgin, Gibson City and Grand Tower Energy Centers
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Sales of Business, Affiliate and Productive Assets	$ 168	$ 137.5
Proceeds from Asset Sale Held in Escrow			14
Ameren Energy Marketing Company
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Guarantees, Maximum Exposure			$ 6

Divestiture Transactions and 76

Divestiture Transactions and Discontinued Operations Divestiture Transactions and Discontinued Operations (Components of Discontinued Operations in Consolidated Statement of Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Discontinued Operations and Disposal Groups [Abstract]
Operating revenues	$ 0	$ 0	$ 0	$ 1
Operating benefits (expenses)	(1)	(1)	2	(4)
Operating income (loss) before income tax	(1)	(1)	2	(3)
Income tax benefit	1	0	50	0
Income (loss) from discontinued operations, net of taxes	$ 0	$ (1)	$ 52	$ (3)

Divestiture Transactions and 77

Divestiture Transactions and Discontinued Operations Divestiture Transactions and Discontinued Operations (Components of Assets and Liabilities on Consolidated Balance Sheet) (Details) - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Assets of discontinued operations
Accumulated deferred income taxes, net	$ 17	$ 15
Total assets of discontinued operations	17	15
Liabilities of discontinued operations
Accounts payable and other current obligations	1	1
Asset retirement obligations	29	32
Total liabilities of discontinued operations	$ 30	$ 33

Segment Information (Schedule O

Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2015USD ($)	Sep. 30, 2014USD ($)	Sep. 30, 2015USD ($)	Sep. 30, 2014USD ($)	Dec. 31, 2014USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments				2
External revenues		$ 1,833	$ 1,670	$ 4,790	$ 4,683
Net income (loss) attributable to Ameren Common Stockholders from continuing operations		343	294	549	541
Total assets		23,459		23,459		$ 22,676
Assets of discontinued operations (Note 12)		17		$ 17		15
Union Electric Company
Segment Reporting Information [Line Items]
Number of Reportable Segments				1
External revenues		1,160	1,089	$ 2,825	2,793
Intersegment revenues		11	8	30	21
Net income (loss) attributable to Ameren Common Stockholders from continuing operations		239	222	341	395
Total assets		13,941		$ 13,941		13,541
Ameren Illinois Company
Segment Reporting Information [Line Items]
Number of Reportable Segments				1
External revenues		654	572	$ 1,910	1,864
Intersegment revenues		1		3	1
Net income (loss) attributable to Ameren Common Stockholders from continuing operations		98	75	182	156
Total assets		8,656		8,656		8,381
Other
Segment Reporting Information [Line Items]
External revenues		19	9	55	26
Intersegment revenues		1	2	2	3
Net income (loss) attributable to Ameren Common Stockholders from continuing operations		6	(3)	26	(10)
Total assets		1,358		1,358		942
Intersegment Eliminations
Segment Reporting Information [Line Items]
Intersegment revenues		(13)	(10)	(35)	(25)
Total assets		(513)		(513)		(203)
Consolidated, Continuing Operations
Segment Reporting Information [Line Items]
External revenues		1,833	1,670	4,790	4,683
Net income (loss) attributable to Ameren Common Stockholders from continuing operations		343	$ 294	549	$ 541
Total assets	[1]	$ 23,442		$ 23,442		$ 22,661
[1]	Excludes total assets from discontinued operations of $17 million and $15 million as of September 30, 2015 and December 31, 2014, respectively.