Union Electric (UEPEM) 10-K 28 Feb 17 2016 FY Annual report Financial data

Document And Entity Information

Document And Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2016	Jan. 31, 2017	Jun. 30, 2016
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2016
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus	FY
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
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Public Float			$ 13,000,372,477
Entity Well-known Seasoned Issuer	Yes
Union Electric Company
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2016
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus	FY
Trading Symbol	AEE
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
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Well-known Seasoned Issuer	No
Ameren Illinois Company
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2016
Document Fiscal Year Focus	2,016
Document Fiscal Period Focus	FY
Trading Symbol	AEE
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
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Well-known Seasoned Issuer	Yes

Consolidated Statement Of Incom

Consolidated Statement Of Income (Loss) - USD ($) shares in Millions, $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating Revenues:
Electric		$ 5,196	$ 5,180	$ 4,913
Gas		880	918	1,140
Total operating revenues		6,076	6,098	6,053
Operating Expenses:
Fuel		745	878	826
Purchased power		621	514	461
Gas purchased for resale		341	415	615
Other operations and maintenance		1,676	1,694	1,684
Provision for Callaway construction and operating license		0	69	0
Depreciation and amortization		845	796	745
Taxes other than income taxes		467	473	468
Total operating expenses		4,695	4,839	4,799
Operating Income		1,381	1,259	1,254
Other Income and Expenses:
Miscellaneous income	[1]	74	74	79
Miscellaneous expense	[1]	32	30	22
Total other income (expense)		42	44	57
Interest charges		382	355	341
Income Before Income Taxes		1,041	948	970
Income taxes		382	363	377
Income from Continuing Operations		659	585	593
Income (Loss) from Discontinued Operations, Net of Tax		0	51	(1)
Net Income (Loss)		659	636	592
Pension and other postretirement activity, net of income taxes (benefit)		(20)	6	(12)
Comprehensive Income (Loss)		633	636	574
Less: Net Income (Loss) Attributable to Noncontrolling Interests:
Continuing Operations		6	6	6
Net Income (Loss):
Net income attributable to Ameren Corporation - continuing operations		653	579	587
Discontinued Operations		0	51	(1)
Net income attributable to Ameren common shareholders		$ 653	$ 630	$ 586
Earnings (Loss) per Common Share – Basic:
Continuing Operations - Basic		$ 2.69	$ 2.39	$ 2.42
Discontinued Operations - Basic		0	0.21	0
Earnings (Loss) per Common Share – Basic		2.69	2.60	2.42
Earnings (Loss) per Common Share – Diluted:
Continuing Operations - Diluted		2.68	2.38	2.40
Discontinued Operations - Diluted		0	0.21	0
Earnings (Loss) per Common Share – Diluted		2.68	2.59	2.40
Dividends per Common Share		$ 1.715	$ 1.655	$ 1.61
Average Common Shares Outstanding - Basic		242.6	242.6	242.6
Average Common Shares Outstanding - Diluted		243.4	243.6	244.4
Union Electric Company
Operating Revenues:
Electric		$ 3,394	$ 3,470	$ 3,388
Gas		128	137	164
Other		1	2	1
Total operating revenues		3,523	3,609	3,553
Operating Expenses:
Fuel		745	878	826
Purchased power		252	111	126
Gas purchased for resale		49	57	82
Other operations and maintenance		893	925	939
Provision for Callaway construction and operating license		0	69	0
Depreciation and amortization		514	492	473
Taxes other than income taxes		325	335	322
Total operating expenses		2,778	2,867	2,768
Operating Income		745	742	785
Other Income and Expenses:
Miscellaneous income		52	52	60
Miscellaneous expense		10	11	12
Total other income (expense)		42	41	48
Interest charges		211	219	211
Income Before Income Taxes		576	564	622
Income taxes		216	209	229
Net Income (Loss)		360	355	393
Other Comprehensive Income		0	0	0
Comprehensive Income (Loss)		360	355	393
Net Income (Loss):
Preferred Stock Dividends		3	3	3
Net Income Available to Common Stockholder		357	352	390
Ameren Illinois Company
Operating Revenues:
Electric		1,736	1,683	1,522
Gas		754	783	976
Total operating revenues		2,490	2,466	2,498
Operating Expenses:
Purchased power		399	420	343
Gas purchased for resale		292	358	533
Other operations and maintenance		804	797	771
Depreciation and amortization		319	295	263
Taxes other than income taxes		132	130	138
Total operating expenses		1,946	2,000	2,048
Operating Income		544	466	450
Other Income and Expenses:
Miscellaneous income		21	21	17
Miscellaneous expense		12	12	8
Total other income (expense)		9	9	9
Interest charges		140	131	112
Income Before Income Taxes		413	344	347
Income taxes		158	127	143
Net Income (Loss)		255	217	204
Pension and other postretirement activity, net of income taxes (benefit)		(5)	(3)	(3)
Comprehensive Income (Loss)		250	214	201
Net Income (Loss):
Preferred Stock Dividends		3	3	3
Net Income Available to Common Stockholder		$ 252	$ 214	$ 201
[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	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension and other postretirement activity, tax (benefit)	$ (7)	$ 3	$ (7)
Ameren Illinois Company
Pension and other postretirement activity, tax (benefit)	$ (1)	$ (2)	$ (2)

Consolidated Statement Of Compr

Consolidated Statement Of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income from Continuing Operations	$ 659	$ 585	$ 593
Pension and other postretirement activity, net of income taxes (benefit)	(20)	6	(12)
Comprehensive Income (Loss) from Continuing Operations	639	591	581
Less: Comprehensive Income from Continuing Operations Attributable to Noncontrolling Interests	6	6	6
Comprehensive Income from Continuing Operations Attributable to Ameren Corporation	633	585	575
Comprehensive Income (Loss) from Discontinued Operations Attributable to Ameren Common shareholders	0	51	(1)
Comprehensive Income (Loss)	$ 633	$ 636	$ 574

Consolidated Statement Of Comp5

Consolidated Statement Of Comprehensive Income (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension and other postretirement activity, tax (benefit)	$ (7)	$ 3	$ (7)

Consolidated Balance Sheet

Consolidated Balance Sheet - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015
Current Assets:
Cash and cash equivalents		$ 9	$ 292
Accounts receivable - trade (less allowance for doubtful accounts)		437	388
Unbilled revenue		295	239
Miscellaneous accounts and notes receivable		63	98
Inventories		527	538
Current regulatory assets		149	260
Other current assets		98	88
Assets of discontinued operations (Note 1)		15	14
Total current assets		1,593	1,917
Property, Plant and Equipment, Net	[1]	20,113	18,799
Investments and Other Assets:
Nuclear decommissioning trust fund		607	556
Goodwill		411	411
Regulatory assets		1,437	1,382
Other assets		538	575
Total investments and other assets		2,993	2,924
TOTAL ASSETS		24,699	23,640
Current Liabilities:
Current maturities of long-term debt		681	395
Short-term debt		558	301
Accounts and wages payable		805	777
Taxes accrued		46	43
Interest accrued		93	89
Customer deposits		107	100
Current regulatory liabilities		110	80
Other current liabilities		248	279
Liabilities of discontinued operations (Note 1)		26	29
Total current liabilities		2,674	2,093
Long-term Debt, Net		6,595	6,880
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net		4,264	3,885
Accumulated deferred investment tax credits		55	60
Regulatory liabilities		1,985	1,905
Asset retirement obligations		635	618
Pension and other postretirement benefits		769	580
Other deferred credits and liabilities		477	531
Total deferred credits and other liabilities		8,185	7,579
Commitments and Contingencies (Notes 2, 10, 14 and 15)
Stockholders' Equity:
Common stock		2	2
Other paid-in capital, principally premium on common stock		5,556	5,616
Retained earnings		1,568	1,331
Accumulated other comprehensive income (loss)		(23)	(3)
Total stockholders' equity		7,103	6,946
Noncontrolling Interests		142	142
Total equity		7,245	7,088
TOTAL LIABILITIES AND EQUITY		24,699	23,640
Union Electric Company
Current Assets:
Cash and cash equivalents		0	199
Advances to money pool		161	36
Accounts receivable - trade (less allowance for doubtful accounts)		187	174
Accounts receivable - affiliates		12	54
Unbilled revenue		154	128
Miscellaneous accounts and notes receivable		14	78
Inventories		392	387
Current regulatory assets		35	89
Other current assets		49	41
Total current assets		1,004	1,186
Property, Plant and Equipment, Net	[1]	11,478	11,183
Investments and Other Assets:
Nuclear decommissioning trust fund		607	556
Regulatory assets		619	605
Other assets		327	321
Total investments and other assets		1,553	1,482
TOTAL ASSETS		14,035	13,851
Current Liabilities:
Current maturities of long-term debt		431	266
Accounts and wages payable		444	417
Accounts payable - affiliates		68	56
Taxes accrued		30	31
Interest accrued		54	59
Current regulatory liabilities		12	28
Other current liabilities		123	120
Total current liabilities		1,162	977
Long-term Debt, Net		3,563	3,844
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net		3,013	2,844
Accumulated deferred investment tax credits		53	58
Regulatory liabilities		1,215	1,172
Asset retirement obligations		629	612
Pension and other postretirement benefits		291	234
Other deferred credits and liabilities		19	28
Total deferred credits and other liabilities		5,220	4,948
Commitments and Contingencies (Notes 2, 10, 14 and 15)
Stockholders' Equity:
Common stock		511	511
Other paid-in capital, principally premium on common stock		1,828	1,822
Preferred stock not subject to mandatory redemption		80	80
Retained earnings		1,671	1,669
Total stockholders' equity		4,090	4,082
TOTAL LIABILITIES AND EQUITY		14,035	13,851
Ameren Illinois Company
Current Assets:
Cash and cash equivalents		0	71
Accounts receivable - trade (less allowance for doubtful accounts)		242	204
Accounts receivable - affiliates		10	22
Unbilled revenue		141	111
Miscellaneous accounts and notes receivable		22	19
Inventories		135	151
Current regulatory assets		108	167
Other current assets		25	15
Total current assets		683	760
Property, Plant and Equipment, Net		7,469	6,848
Investments and Other Assets:
Goodwill		411	411
Regulatory assets		816	771
Other assets		95	113
Total investments and other assets		1,322	1,295
TOTAL ASSETS		9,474	8,903
Current Liabilities:
Current maturities of long-term debt		250	129
Short-term debt		51	0
Accounts and wages payable		264	249
Accounts payable - affiliates		63	66
Taxes accrued		16	13
Interest accrued		33	28
Customer deposits		69	69
Mark-to-market derivative liabilities		15	45
Environmental remediation		38	28
Current regulatory liabilities		78	39
Other current liabilities		94	86
Total current liabilities		971	752
Long-term Debt, Net		2,338	2,342
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net		1,631	1,480
Accumulated deferred investment tax credits		2	2
Regulatory liabilities		768	732
Pension and other postretirement benefits		346	271
Accrued Environmental Loss Contingencies, Noncurrent		162	205
Other deferred credits and liabilities		222	222
Total deferred credits and other liabilities		3,131	2,912
Commitments and Contingencies (Notes 2, 10, 14 and 15)
Stockholders' Equity:
Common stock		0	0
Other paid-in capital, principally premium on common stock		2,005	2,005
Preferred stock not subject to mandatory redemption		62	62
Retained earnings		967	825
Accumulated other comprehensive income (loss)		0	5
Total stockholders' equity		3,034	2,897
TOTAL LIABILITIES AND EQUITY		$ 9,474	$ 8,903
[1]	(a)Amounts in Ameren and Ameren Missouri include two CTs under separate capital lease agreements. The gross cumulative asset value of those agreements was $232 million and $233 million at December 31, 2016 and 2015, respectively. The total accumulated depreciation associated with the two CTs was $77 million and $72 million at December 31, 2016 and 2015, respectively. In addition, Ameren Missouri has investments in debt securities, classified as held-to-maturity and recorded in "Other assets" that are related to the two CTs from the city of Bowling Green and Audrain County. As of December 31, 2016 and 2015, the carrying value of these debt securities was $282 million and $288 million, respectively.

Consolidated Balance Sheet (Par

Consolidated Balance Sheet (Parenthetical) - USD ($) shares in Millions, $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Accounts receivable - trade, allowance for doubtful accounts	$ 19	$ 19
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	400	400
Common stock, shares outstanding	242.6	242.6
Union Electric Company
Accounts receivable - trade, allowance for doubtful accounts	$ 7	$ 7
Common stock, par value	$ 5	$ 5
Common stock, shares authorized	150	150
Common stock, shares outstanding	102.1	102.1
Ameren Illinois Company
Accounts receivable - trade, allowance for doubtful accounts	$ 12	$ 12
Common Stock, No Par Value	$ 0	$ 0
Common stock, shares authorized	45	45
Common stock, shares outstanding	25.5	25.5

Consolidated Statement Of Cash

Consolidated Statement Of Cash Flows - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash Flows From Operating Activities:
Net income (loss)		$ 659	$ 636	$ 592
(Income) Loss from discontinued operations, net of tax		0	(51)	1
Adjustments to reconcile net income to net cash provided by operating activities:
Provision for Callaway construction and operating license		0	69	0
Depreciation and amortization		835	777	710
Amortization of nuclear fuel		88	97	81
Amortization of debt issuance costs and premium/discounts		22	22	22
Deferred income taxes and investment tax credits, net		386	369	451
Allowance for equity funds used during construction	[1]	(27)	(30)	(34)
Share-based Compensation		17	24	25
Other		4	(10)	(24)
Changes in assets and liabilities:
Receivables		(71)	83	31
Inventories		11	(14)	3
Accounts and wages payable		19	(2)	10
Taxes accrued		13	(22)	(44)
Regulatory assets and liabilities		215	94	(281)
Assets, other		(24)	53	30
Liabilities, other		(10)	(44)	(14)
Pension and other postretirement benefits		(16)	(9)	(10)
Counterparty collateral, net		3	(7)	22
Net cash provided by operating activities - continuing operations		2,124	2,035	1,571
Net cash provided by operating activities - discontinued operations		(1)	(4)	(6)
Net cash provided by operating activities		2,123	2,031	1,565
Cash Flows From Investing Activities:
Capital expenditures		(2,076)	(1,917)	(1,785)
Nuclear fuel expenditures		(55)	(52)	(74)
Purchases of securities - nuclear decommissioning trust fund		(392)	(363)	(405)
Sales and maturities of securities - nuclear decommissioning trust fund		377	349	391
Proceeds from Collection of Notes Receivable		0	20	95
Contributions to Note Receivable		0	(8)	(89)
Other		5	20	11
Net cash used in investing activities - continuing operations		(2,141)	(1,951)	(1,856)
Net cash used in investing activities - discontinued operations		0	(25)	139
Net cash used in investing activities		(2,141)	(1,976)	(1,717)
Cash Flows From Financing Activities:
Dividends on common stock		(416)	(402)	(390)
Dividends paid to noncontrolling interest holders		(6)	(6)	(6)
Short-term debt, net		257	(413)	346
Maturities, redemptions, and repurchases of long-term debt		(395)	(120)	(697)
Issuances of Long-term debt		389	1,197	898
Capital issuance costs		(9)	(12)	(11)
Share-based payments		(83)	(12)	(14)
Other		(2)	0	1
Net cash provided by (used in) financing activities		(265)	232	127
Net change in cash and cash equivalents		(283)	287	(25)
Cash and cash equivalents at beginning of year		292	5	30
Cash and cash equivalents at end of year		9	292	5
Cash Paid (Refunded) During the Year:
Interest net of capitalized		358	335	333
Income taxes, net		(12)	(15)	(27)
Union Electric Company
Cash Flows From Operating Activities:
Net income (loss)		360	355	393
Adjustments to reconcile net income to net cash provided by operating activities:
Provision for Callaway construction and operating license		0	69	0
Depreciation and amortization		506	476	442
Amortization of nuclear fuel		88	97	81
Amortization of debt issuance costs and premium/discounts		6	6	7
Deferred income taxes and investment tax credits, net		179	82	245
Allowance for equity funds used during construction		(23)	(22)	(32)
Other		5	2	3
Changes in assets and liabilities:
Receivables		5	72	(10)
Inventories		(4)	(39)	8
Accounts and wages payable		(18)	3	25
Taxes accrued		11	1	(197)
Regulatory assets and liabilities		84	117	(68)
Assets, other		(25)	26	52
Liabilities, other		(1)	4	0
Pension and other postretirement benefits		(4)	(2)	1
Net cash provided by operating activities		1,169	1,247	950
Cash Flows From Investing Activities:
Capital expenditures		(738)	(622)	(747)
Nuclear fuel expenditures		(55)	(52)	(74)
Purchases of securities - nuclear decommissioning trust fund		(392)	(363)	(405)
Sales and maturities of securities - nuclear decommissioning trust fund		377	349	391
Money pool advances, net		125	36	0
Other		(1)	0	(2)
Net cash used in investing activities		(934)	(724)	(837)
Cash Flows From Financing Activities:
Dividends on common stock		(355)	(575)	(340)
Return of capital to parent		0	0	(215)
Dividends on preferred stock		(3)	(3)	(3)
Short-term debt, net		0	(97)	97
Money pool borrowings, net		0	0	(105)
Maturities, redemptions, and repurchases of long-term debt		(266)	(120)	(109)
Issuances of Long-term debt		149	249	350
Capital issuance costs		(3)	(3)	(3)
Capital contribution from parent		44	224	215
Net cash provided by (used in) financing activities		(434)	(325)	(113)
Net change in cash and cash equivalents		(199)	198	0
Cash and cash equivalents at beginning of year		199	1	1
Cash and cash equivalents at end of year		0	199	1
Noncash Or Part Noncash Capital Contribution From Parent		0	38	9
Cash Paid (Refunded) During the Year:
Interest net of capitalized		209	212	203
Income taxes, net		27	72	215
Ameren Illinois Company
Cash Flows From Operating Activities:
Net income (loss)		255	217	204
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization		318	292	259
Amortization of debt issuance costs and premium/discounts		14	14	13
Deferred income taxes and investment tax credits, net		154	221	196
Allowance for equity funds used during construction		(4)	(8)	(2)
Other		(1)	(14)	(19)
Changes in assets and liabilities:
Receivables		(72)	16	(13)
Inventories		15	25	(4)
Accounts and wages payable		12	37	7
Taxes accrued		1	(2)	(7)
Regulatory assets and liabilities		120	(26)	(215)
Assets, other		(3)	17	15
Liabilities, other		(5)	(27)	1
Pension and other postretirement benefits		(8)	(4)	(6)
Counterparty collateral, net		3	(3)	14
Net cash provided by operating activities		803	763	445
Cash Flows From Investing Activities:
Capital expenditures		(924)	(918)	(835)
Other		6	5	7
Net cash used in investing activities		(918)	(913)	(828)
Cash Flows From Financing Activities:
Dividends on common stock		(110)	0	0
Dividends on preferred stock		(3)	(3)	(3)
Short-term debt, net		51	(32)	32
Money pool borrowings, net		0	(15)	(41)
Maturities, redemptions, and repurchases of long-term debt		(129)	0	(163)
Issuances of Long-term debt		240	248	548
Capital issuance costs		(4)	(3)	(6)
Capital contribution from parent		0	25	15
Other		(1)	0	1
Net cash provided by (used in) financing activities		44	220	383
Net change in cash and cash equivalents		(71)	70	0
Cash and cash equivalents at beginning of year		71	1	1
Cash and cash equivalents at end of year		0	71	1
Cash Paid (Refunded) During the Year:
Interest net of capitalized		127	120	110
Income taxes, net		$ 8	$ (113)	$ (44)
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Consolidated Statement Of Cash9

Consolidated Statement Of Cash Flows (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Interest Paid, Capitalized	$ 15	$ 17	$ 18
Union Electric Company
Interest Paid, Capitalized	12	12	16
Ameren Illinois Company
Interest Paid, Capitalized	$ 3	$ 5	$ 2

Consolidated Statement Of Stock

Consolidated Statement Of Stockholders' Equity - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Other Paid-In Capital	Retained Earnings	Deferred Retirement Benefit Costs	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interest	Total Ameren Corporation Stockholders' Equity	Union Electric Company	Union Electric CompanyCommon Stock	Union Electric CompanyOther Paid-In Capital	Union Electric CompanyPreferred Stock Not Subject To Mandatory Redemption	Union Electric CompanyRetained Earnings	Ameren Illinois Company	Ameren Illinois CompanyCommon Stock	Ameren Illinois CompanyOther Paid-In Capital	Ameren Illinois CompanyPreferred Stock Not Subject To Mandatory Redemption	Ameren Illinois CompanyRetained Earnings	Ameren Illinois CompanyDeferred Retirement Benefit Costs	Ameren Illinois CompanyAccumulated Other Comprehensive Income (Loss)
Beginning of year at Dec. 31, 2013		$ 2	$ 5,632	$ 907	$ 3		$ 142				$ 1,560		$ 1,842			$ 1,965		$ 410	$ 11
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation activity			(15)
Net income attributable to Ameren common shareholders	$ 586			586
Net income (loss)	592								$ 393				393	$ 204				204
Common stock dividends				(390)									(340)					0
Capital contribution from parent									215		224			15		15
Return of capital to parent									215		(215)
Preferred stock dividends													(3)					(3)
Change in deferred retirement benefit costs	$ 12				(12)									3						$ (3)
Net income attributable to noncontrolling interest holder							6
Dividends paid to noncontrolling interest holders							(6)
End of year (shares) at Dec. 31, 2014	242.6
End of year at Dec. 31, 2014	$ 6,855	2	5,617	1,103	(9)	$ (9)	142			$ 511	1,569	$ 80	1,892		$ 0	1,980	$ 62	611	8	8
Stockholders' equity, end of year at Dec. 31, 2014								$ 6,713	4,052					2,661
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation activity			(1)
Net income attributable to Ameren common shareholders	630			630
Net income (loss)	636								355				355	217				217
Common stock dividends				(402)									(575)					0
Capital contribution from parent									224		253			25		25
Return of capital to parent									$ 0		0
Preferred stock dividends													(3)					(3)
Change in deferred retirement benefit costs	$ (6)				6									$ 3						(3)
Net income attributable to noncontrolling interest holder							6
Dividends paid to noncontrolling interest holders							(6)
End of year (shares) at Dec. 31, 2015	242.6								102.1					25.5
End of year at Dec. 31, 2015	$ 7,088	2	5,616	1,331	(3)	(3)	142			511	1,822	80	1,669		0	2,005	62	825	5	5
Stockholders' equity, end of year at Dec. 31, 2015	6,946							6,946	$ 4,082					$ 2,897
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation activity			(60)
Net income attributable to Ameren common shareholders	653			653
Net income (loss)	659								360				360	255				255
Common stock dividends				(416)									(355)					(110)
Capital contribution from parent									44		6			0
Return of capital to parent									$ 0		0
Preferred stock dividends													(3)					(3)
Change in deferred retirement benefit costs	$ 20				(20)									$ 5						(5)
Net income attributable to noncontrolling interest holder							6
Dividends paid to noncontrolling interest holders							(6)
End of year (shares) at Dec. 31, 2016	242.6								102.1					25.5
End of year at Dec. 31, 2016	$ 7,245	$ 2	$ 5,556	$ 1,568	$ (23)	$ (23)	$ 142			$ 511	$ 1,828	$ 80	$ 1,671		$ 0	$ 2,005	$ 62	$ 967	$ 0	$ 0
Stockholders' equity, end of year at Dec. 31, 2016	$ 7,103							$ 7,103	$ 4,090					$ 3,034

Summary Of Significant Accounti

Summary Of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES General Ameren, headquartered in St. Louis, Missouri, is a public utility holding company under PUHCA 2005. Ameren’s primary assets are its equity interests in its subsidiaries, including Ameren Missouri, Ameren Illinois, and ATXI. 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. Ameren also has various other subsidiaries that conduct other activities, such as the provision of shared services. • Union Electric Company, doing business as Ameren Missouri, operates a rate-regulated electric generation, transmission, and distribution business and a rate-regulated natural gas distribution business in Missouri. Ameren Missouri was incorporated in Missouri in 1922 and is successor to a number of companies, the oldest of which was organized in 1881. It is the largest electric utility in the state of Missouri. It supplies electric and natural gas service to a 24,000 -square-mile area in central and eastern Missouri. This area has an estimated population of 2.8 million and includes the Greater St. Louis area. Ameren Missouri supplies electric service to 1.2 million customers and natural gas service to 0.1 million customers. • Ameren Illinois Company, doing business as Ameren Illinois, operates rate-regulated electric transmission, electric distribution, and natural gas distribution businesses in Illinois. Ameren Illinois was incorporated in Illinois in 1923 and is the successor to a number of companies, the oldest of which was organized in 1902. Ameren Illinois supplies electric and natural gas utility service to portions of central and southern Illinois with an estimated population of 3.1 million in an area of 40,000 square miles. Ameren Illinois supplies electric service to 1.2 million customers and natural gas service to 0.8 million customers. • ATXI 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. ATXI is also evaluating competitive electric transmission investment opportunities outside of MISO as they arise. 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. All tabular dollar amounts are in millions, unless otherwise indicated. Unless otherwise stated, these notes to the financial statements exclude discontinued operations for all periods presented. 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. Regulation We are regulated by the MoPSC, the ICC, and the FERC. We defer certain costs as assets pursuant to actions of rate regulators or because of expectations that we will be able to recover such costs in future rates charged to customers. We also defer certain amounts as liabilities pursuant to actions of rate regulators or based on the expectation that such amounts will be returned to customers in future rates. Regulatory assets and liabilities are amortized consistent with the period of expected regulatory treatment. In addition to the cost recovery mechanisms discussed in the Purchased Gas, Power, and Fuel Rate-adjustment Mechanisms section below, Ameren Missouri and Ameren Illinois have approvals from rate regulators to use other cost recovery mechanisms. Ameren Missouri has a pension and postretirement benefit cost tracker, an uncertain tax positions tracker, a renewable energy standards cost tracker, a solar rebate program tracker, and the MEEIA energy efficiency rider. Ameren Illinois' and ATXI's electric transmission rates are determined pursuant to formula ratemaking. Additionally, Ameren Illinois' electric distribution business participates in the performance-based formula ratemaking process established pursuant to the IEIMA. Ameren Illinois also has environmental cost riders, an asbestos-related litigation rider, an energy efficiency rider, a QIP rider, a VBA rider, and a bad debt rider. See Note 2 – Rate and Regulatory Matters for additional information on regulatory assets and liabilities. The Ameren Illinois asbestos-related litigation rider includes a trust fund that was established when Ameren acquired IP. At December 31, 2016 and 2015, the trust fund balance of $22 million was reflected in "Other assets" on Ameren's and Ameren Illinois' balance sheet. This balance is restricted only for the use of funding certain asbestos-related claims. The rider is subject to the following terms: 90% of the cash expenditures in excess of the amount included in base electric rates is to be recovered from the trust fund. If cash expenditures are less than the amount in base rates, Ameren Illinois will contribute 90% of the difference to the trust fund. Cash and Cash Equivalents Cash and cash equivalents include cash on hand and temporary investments purchased with an original maturity of three months or less. Allowance for Doubtful Accounts Receivable The allowance for doubtful accounts represents our estimate of existing accounts receivable that will ultimately be uncollectible. The allowance is calculated by applying estimated loss factors to various classes of outstanding receivables, including unbilled revenue. The loss factors used to estimate uncollectible accounts are based upon both historical collections experience and management’s estimate of future collections success given the existing and anticipated future collections environment. Ameren Illinois has a bad debt rider that adjusts rates for net write-offs of customer accounts receivable above or below those being collected in rates. Inventories Inventories are recorded at the lower of cost or market. Cost is determined by the average-cost method. Inventories are capitalized when purchased and then expensed as consumed or capitalized as plant assets when installed, as appropriate. The following table presents a breakdown of inventories for each of the Ameren Companies at December 31, 2016 and 2015 : Ameren Missouri Ameren Illinois Ameren 2016 Fuel (a) $ 172 $ — $ 172 Natural gas stored underground 9 73 82 Other inventories 211 62 273 Total inventories $ 392 $ 135 $ 527 2015 Fuel (a) $ 173 $ — $ 173 Natural gas stored underground 10 87 97 Other inventories 204 64 268 Total inventories $ 387 $ 151 $ 538 (a) Consists of coal, oil, and propane. Purchased Gas, Power and Fuel Rate-adjustment Mechanisms Ameren Missouri and Ameren Illinois have various rate-adjustment mechanisms in place that provide for the recovery of purchased natural gas and electric fuel and purchased power costs without a traditional rate case proceeding. See Note 2 – Rate and Regulatory Matters for the regulatory assets and liabilities recorded at December 31, 2016 and 2015 , related to the rate-adjustment mechanisms discussed below. In Ameren Missouri’s and Ameren Illinois’ natural gas businesses, changes in natural gas costs are reflected in billings to their customers through PGA clauses. The difference between actual natural gas costs and costs billed to customers in a given period is deferred as a regulatory asset or liability. The deferred amount is either billed or refunded to customers in a subsequent period. In Ameren Illinois’ electric distribution business, changes in purchased power and transmission service costs are reflected in billings to its customers through pass-through rate-adjustment clauses. The difference between actual purchased power and transmission service costs and costs billed to customers in a given period is deferred as a regulatory asset or liability. The deferred amount is either billed or refunded to customers in a subsequent period. Ameren Missouri has a FAC that allows an adjustment of electric rates three times per year for a pass-through to customers of 95% of changes in fuel and purchased power costs, including transportation charges and revenues, net of off-system sales, greater or less than the amount set in base rates, subject to MoPSC prudence review. The difference between the actual amounts incurred for these items and the amounts recovered from Ameren Missouri customers' base rates is deferred as a regulatory asset or liability. The deferred amounts are either billed or refunded to electric customers in a subsequent period. Since May 30, 2015, transmission revenues and substantially all transmission charges are excluded from net energy costs as a result of the April 2015 MoPSC electric rate order. Property, Plant, and Equipment, Net We capitalize the cost of additions to and betterments of units of property, plant and equipment. The cost includes labor, material, applicable taxes, and overhead. An allowance for funds used during construction, as discussed below, is also capitalized as a cost of our rate-regulated assets. Maintenance expenditures, including nuclear refueling and maintenance outages, are expensed as incurred. When units of depreciable property are retired, the original costs, less salvage values, are charged to accumulated depreciation. If environmental expenditures are related to assets currently in use, as in the case of the installation of pollution control equipment, the cost is capitalized and depreciated over the expected life of the asset. See Asset Retirement Obligations below and Note 3 – Property, Plant, and Equipment, Net for additional information. Depreciation Depreciation is provided over the estimated lives of the various classes of depreciable property by applying composite rates on a straight-line basis to the cost basis of such property. The provision for depreciation for the Ameren Companies in 2016 , 2015 , and 2014 ranged from 3% to 4% of the average depreciable cost. Allowance for Funds Used During Construction We capitalize allowance for funds used during construction, or the cost of borrowed funds and the cost of equity funds (preferred and common shareholders’ equity) applicable to rate-regulated construction expenditures, in accordance with the utility industry's accounting practice. Allowance for funds used during construction does not represent a current source of cash funds. This accounting practice offsets the effect on earnings of the cost of financing during construction, and it treats such financing costs in the same manner as construction charges for labor and materials. Under accepted ratemaking practice, cash recovery of allowance for funds used during construction and other construction costs occurs when completed projects are placed in service and reflected in customer rates. The following table presents the annual allowance for funds used during construction debt and equity blended rates that were applied to construction projects in 2016 , 2015 , and 2014 : 2016 2015 2014 Ameren Missouri 7 % 7 % 7 % Ameren Illinois 5 % 6 % 2 % Goodwill Goodwill represents the excess of the purchase price of an acquisition over the fair value of the net assets acquired. Ameren and Ameren Illinois evaluate goodwill for impairment in each of their reporting units as of October 31 each year, or more frequently if events and circumstances change that would more likely than not reduce the fair value of their reporting units below their carrying amounts. Ameren and Ameren Illinois had goodwill of $411 million at October 31, 2016 and October 31, 2015. To determine whether the fair value of a reporting unit is more likely than not greater than its carrying amount, Ameren and Ameren Illinois elect to perform either a qualitative assessment or to bypass the qualitative assessment and perform a two-step quantitative test, on an annual basis. On October 31, 2015, Ameren and Ameren Illinois performed a quantitative test and determined that the estimated fair value of the Ameren Illinois reporting unit significantly exceeded its carrying value as of that date. Based on these results, Ameren and Ameren Illinois elected to perform a qualitative assessment for their annual goodwill impairment test conducted as of October 31, 2016. The results of Ameren’s and Ameren Illinois’ qualitative assessment indicated that it was more likely than not that the fair value of the Ameren Illinois reporting unit exceeded its carrying value as of October 31, 2016, resulting in no impairment of Ameren’s or Ameren Illinois’ goodwill. The following factors, among others, were considered by Ameren and Ameren Illinois when assessing whether it was more likely than not that the fair value of the Ameren Illinois reporting unit exceeded its carrying value for the October 31, 2016, test: • macroeconomic conditions, including those conditions within Ameren Illinois’ service territory; • pending rate case outcomes and projections of future rate case outcomes; • changes in laws and potential law changes; • observable industry market multiples; • achievement of IEIMA performance metrics and the yield of 30-year United States Treasury bonds; • an unexpected further reduction in the FERC-allowed return on equity related to transmission services; and • projected operating results and cash flows. As of December 31, 2016, the Ameren Companies changed the manner in which they assess performance and allocate resources, driven by increasing investment in FERC rate-regulated electric transmission and Ameren Illinois electric distribution and natural gas distribution businesses as well as the unique regulatory environment for each jurisdiction. Ameren now has four reporting units: Ameren Missouri, Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Transmission. Ameren Illinois now has three reporting units: Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Illinois Transmission. See Note 16 – Segment Information for additional information related to the change in Ameren’s and Ameren Illinois' segments. As of the date of the segment change, December 31, 2016, Ameren and Ameren Illinois reassigned goodwill to the new reporting units using a relative fair value allocation approach. The Level 3 fair value hierarchy valuation approach used to reassign goodwill was based on a market participant view and used a weighted combination of a discounted cash flow analysis and a market multiples analysis. Key assumptions used in estimating the fair value of the reporting units included discount and growth rates, utility sector market performance and transactions, and projected operating results and cash flows. As a result of the goodwill reassignment, Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Illinois Transmission had goodwill of $238 million , $80 million , and $93 million , respectively, at December 31, 2016. The Ameren Transmission reporting unit was reassigned the same $93 million of goodwill as the Ameren Illinois Transmission reporting unit. In conjunction with the goodwill reassignment, Ameren and Ameren Illinois completed the first step of the quantitative test to determine whether the fair values of the new reporting units exceeded their carrying values as of December 31, 2016. Ameren and Ameren Illinois determined that the estimated fair values of the Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, Ameren Illinois Transmission, and Ameren Transmission reporting units each exceeded their respective carrying values by at least 40% , indicating no impairment of Ameren’s or Ameren Illinois’ goodwill. The Ameren and Ameren Illinois goodwill that was reassigned to the new reporting units on December 31, 2016, had no accumulated goodwill impairment losses. Impairment of Long-lived Assets We evaluate long-lived assets classified as held and used for impairment when events or changes in circumstances indicate that the carrying value of such assets may not be recoverable. Whether an impairment has occurred is determined by comparing the estimated undiscounted cash flows attributable to the assets to the carrying value of the assets. If the carrying value exceeds the undiscounted cash flows, we recognize an impairment charge equal to the amount by which the carrying value exceeds the estimated fair value of the assets. In the period in which we determine an asset meets held for sale criteria, we record an impairment charge to the extent the book value exceeds its estimated fair value less cost to sell. We did not identify any events or changes in circumstances that indicated that the carrying value of long-lived assets may not be recoverable in 2016 and 2015 . Environmental Costs Liabilities for environmental costs are recorded on an undiscounted basis when it is probable that a liability has been incurred and the amount of the liability can be reasonably estimated. Costs are expensed or deferred as a regulatory asset when it is expected that the costs will be recovered from customers in future rates. Asset Retirement Obligations We record the estimated fair value of legal obligations associated with the retirement of tangible long-lived assets in the period in which the liabilities are incurred and capitalize a corresponding amount as part of the book value of the related long-lived asset. In subsequent periods, we adjust AROs based on changes in the estimated fair values of the obligations with a corresponding increase or decrease in the asset book value. Asset book values, reflected within "Property, Plant, and Equipment, Net" on the balance sheet, are depreciated over the remaining useful life of the related asset. Due to regulatory recovery, that depreciation is recorded within a regulatory asset or liability balance related to AROs. Ameren and Ameren Missouri have a nuclear decommissioning trust fund for the decommissioning of the Callaway energy center. Net realized and unrealized gains and losses within the nuclear decommissioning trust fund are deferred and are currently recorded as a regulatory liability, along with the depreciation of the asset book values, discussed above, and the accretion of the AROs. The depreciation of the asset book values at Ameren Missouri was $31 million , $13 million , and $1 million for the years ended December 31, 2016, 2015, and 2014, respectively, which was recorded as a reduction to the regulatory liability. The depreciation recorded to the regulatory asset at Ameren Illinois was immaterial in each respective period. Uncertainties as to the probability, timing, or amount of cash expenditures associated with AROs affect our estimates of fair value. Ameren and Ameren Missouri have recorded AROs for retirement costs associated with Ameren Missouri’s Callaway energy center decommissioning, CCR facilities, and river structures. Also, Ameren, Ameren Missouri, and Ameren Illinois have recorded AROs for retirement costs associated with asbestos removal and the disposal of certain transformers. Asset removal costs that do not constitute legal obligations are classified as regulatory liabilities. See Note 2 – Rate and Regulatory Matters. The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the years ended December 31, 2016 and 2015 : Ameren Missouri Ameren Illinois Ameren Balance at December 31, 2014 $ 389 $ 7 $ 396 Liabilities incurred 3 — 3 Liabilities settled (1 ) (1 ) (2 ) Accretion in 2015 (a) 23 (b) 23 Change in estimates (c) 203 (b) 203 Balance at December 31, 2015 $ 617 (e) $ 6 (d) $ 623 (e) Liabilities incurred 3 — 3 Liabilities settled (2 ) (b) (2 ) Accretion in 2016 (a) 25 (b) 25 Change in estimates 1 — 1 Balance at December 31, 2016 $ 644 (e) $ 6 (d) $ 650 (e) (a) Accretion expense was recorded as a decrease to regulatory liabilities. (b) Less than $1 million. (c) The ARO increase resulted in a corresponding increase recorded to "Property, Plant, and Equipment, Net." 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, the 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 $100 million to their AROs associated with CCR storage facilities. See Note 15 – 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. (d) Included in “Other deferred credits and liabilities” on the balance sheet. (e) Balance included $5 million and $15 million in "Other current liabilities" on the balance sheet as of December 31, 2015 and 2016, respectively. See the Divestiture Transactions and Discontinued Operations section below for additional information on the AROs related to the abandoned Meredosia and Hutsonville energy centers, which are presented as discontinued operations and therefore not included in the table above. Noncontrolling Interests As of December 31, 2016 and 2015 , Ameren’s noncontrolling interests included the preferred stock of Ameren Missouri and Ameren Illinois. Operating Revenue The Ameren Companies record operating revenue for electric or natural gas service when it is delivered to customers. We accrue an estimate of electric and natural gas revenues for service rendered but unbilled at the end of each accounting period. Ameren Illinois participates in the performance-based formula ratemaking framework pursuant to the IEIMA. In addition, Ameren Illinois' and ATXI's electric transmission service operating revenues are regulated by the FERC. The provisions of the IEIMA and the FERC's electric transmission formula rate framework provide for annual reconciliations of the electric distribution and electric transmission service revenue requirements necessary to reflect the actual recoverable costs incurred in a given year with the revenue requirements in customer rates for that year, including an allowed return on equity. In each of those electric jurisdictions, if the current year's revenue requirement is greater than the revenue requirement reflected in that year's customer rates, an increase to electric operating revenues with an offset to a regulatory asset is recorded to reflect the expected recovery of those additional amounts from customers within two years. In each jurisdiction, if the current year's revenue requirement is less than the revenue requirement reflected in that year's customer rates, a reduction to electric operating revenues with an offset to a regulatory liability is recorded to reflect the expected refund to customers within two years. See Note 2 – Rate and Regulatory Matters for information regarding Ameren Illinois' revenue requirement reconciliation pursuant to the IEIMA. Accounting for MISO Transactions MISO-related purchase and sale transactions are recorded by Ameren, Ameren Missouri, and Ameren Illinois using settlement information provided by MISO. Ameren Missouri records these purchase and sale transactions on a net hourly position. Ameren Missouri records net purchases in a single hour in “Operating Expenses – Purchased power” and net sales in a single hour in “Operating Revenues – Electric” in its statement of income. Ameren Illinois records net purchases in “Operating Expenses – Purchased power” in its statement of income to reflect all of its MISO transactions relating to the procurement of power for its customers. On occasion, Ameren Missouri's and Ameren Illinois' prior-period transactions will be resettled outside the routine settlement process because of a change in MISO’s tariff or a material interpretation thereof. In these cases, Ameren Missouri and Ameren Illinois recognize expenses associated with resettlements once the resettlement is probable and the resettlement amount can be estimated. Revenues are recognized once the resettlement amount is received. There were no material MISO resettlements in 2016, 2015, or 2014. Nuclear Fuel Ameren Missouri’s cost of nuclear fuel is capitalized and then amortized to fuel expense on a unit-of-production basis. The cost is charged to "Operating Expenses – Fuel" in the statement of income. Stock-based Compensation Stock-based compensation cost is measured at the grant date based on the fair value of the award, net of an assumed forfeiture rate. Ameren recognizes as compensation expense the estimated fair value of stock-based compensation on a straight-line basis over the requisite service period. See Note 12 – Stock-based Compensation for additional information. Excise Taxes Ameren Missouri and Ameren Illinois collect from their customers certain excise taxes 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. They are recorded gross in “Operating Revenues – Electric,” “Operating Revenues – Natural 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 customers and are therefore not included in Ameren Illinois' revenues and expenses. The following table presents the excise taxes recorded in “Operating Revenues – Electric,” “Operating Revenues – Natural gas,” and “Operating Expenses – Taxes other than income taxes” for the years ended December 31, 2016 , 2015 , and 2014 : 2016 2015 2014 Ameren Missouri $ 151 $ 156 $ 151 Ameren Illinois 57 57 64 Ameren $ 208 $ 213 $ 215 Unamortized Debt Discounts, Premiums, and Issuance Costs Long-term debt discounts, premiums, and issuance costs are amortized over the lives of the related issuances. Credit agreement fees are amortized over the term of that agreement. Income Taxes Ameren uses an asset and liability approach for its financial accounting and reporting of income taxes. Deferred tax assets and liabilities are recognized for transactions that are treated differently for financial reporting and income tax return purposes. These deferred tax assets and liabilities are based on statutory tax rates. We recognize that regulators will probably reduce future revenues for deferred tax liabilities that were initially recorded at rates in excess of the current statutory rate. Therefore, reductions in deferred tax liabilities that were recorded because of decreases in the statutory rate have been credited to a regulatory liability. A regulatory asset has been established to recognize the probable recovery through future customer rates of tax benefits related to the equity component of allowance for funds used during construction, as well as the effects of tax rate increases. Investment tax credits used on tax returns for prior years have been deferred as a noncurrent liability. The credits are being amortized over the useful lives of the related investment. Deferred income taxes were recorded on the temporary difference represented by the deferred investment tax credits and a corresponding regulatory liability. This recognizes the expected reduction in rates for future lower income taxes associated with the amortization of the investment tax credits. See Note 13 – Income Taxes. Ameren Missouri, Ameren Illinois, and all the other Ameren subsidiary companies are parties to a tax allocation agreement with Ameren (parent) that provides for the allocation of consolidated tax liabilities. The tax allocation agreement specifies that each party be allocated an amount of tax using a stand-alone calculation, which is similar to that which would be owed or refunded had the party been separately subject to tax considering the impact of consolidation. Any net benefit attributable to the parent is reallocated to the other parties. This reallocation is treated as a capital contribution to the party receiving the benefit. Earnings per Share Basic earnings per share is computed by dividing “Net Income Attributable to Ameren Common Shareholders” 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 Shareholders” 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 0.8 million , 1.0 million , and 1.8 million for the years ended December 31, 2016 , 2015 , and 2014 , respectively. There were no potentially dilutive securities excluded from the diluted earnings per share calculations for the years ended December 31, 2016 , 2015 , and 2014 . Capital Contributions and Return of Capital In 2016, Ameren Missouri received cash capital contributions of $44 million from Ameren (parent) as a result of the tax allocation agreement, which included the accrued capital contribution from 2015. In 2015, Ameren Missouri received cash capital contributions of $224 million from Ameren (parent) as a result of the tax allocation agreement, which included the Ameren Missouri accrued capital contribution from 2014. Additionally, as of December 31, 2015, Ameren Missouri accrued a $38 million capital contribution related to the same agreement. In 2015, Ameren Illinois received cash capital contributions of $25 million from Ameren (parent). In 2014, Ameren Missouri and Ameren Illinois received cash capital contributions of $215 million and $15 million , respectively, from Ameren (parent) as a result of the tax allocation agreement. Additionally, as of December 31, 2014, Ameren Missouri accrued a $9 million capital contribution related to the same agreement. Also in 2014, Ameren Missouri returned capital of $215 million to Ameren (parent). Divestiture Transactions and Discontinued Operations In December 2013, Ameren completed the divestiture of New AER to IPH. The transaction agreement with IPH provided that if the Elgin, Gibson City, and Grand Tower natural-gas-fired energy centers were subsequently sold by Medina Valley and 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. In January 2014, Medina Valley completed the sale of the Elgin, Gibson City, and Grand Tower natural-gas-fired energy centers to Rockland Capital for a total purchase price of $168 million . The agreement with Rockland Capital required a portion of the purchase price to be held in escrow until January 31, 2016, to fund certain indemnity obligations, if any, of Medina Valley. The Rockland Capital escrow balance of $14 million and the corresponding payable due to Genco was reflected on Ameren's December 31, 2015, consolidated balance sheet in "Other current assets" and in "Other current liabilities," respectively. In 2016, Medina Valley received the amount held in escrow from Rockland Capital and paid Genco its portion of the escrow. All matters related to the final tax basis of New AER and the related tax benefit resulting from its divestiture were resolved with the completion of the IRS audit of 2013. During 2015, based on the completion of the IRS audit of 2013, Ameren removed a reserve for unrecognized tax benefits of $53 million recorded in 2013 and recognized a tax benefit from discontinued operations. The components of the assets and liabilities of Ameren's discontinued operations at December 31, 2016 and 2015, consist primarily of AROs and the related deferred income tax assets associated with the abandoned Meredosia and Hutsonville energy centers. Accounting Changes and Other Matte

Rate And Regulatory Matters

Rate And Regulatory Matters	12 Months Ended
	Dec. 31, 2016
Public Utilities, General Disclosures [Abstract]
RATE AND REGULATORY MATTERS	RATE AND REGULATORY MATTERS Below is a summary of significant regulatory proceedings and related lawsuits. We are unable to predict the ultimate outcome of these matters, the timing of final decisions of the various agencies and courts, or the effect on our results of operations, financial position, or liquidity. Missouri February 2017 Unanimous Stipulation and Agreement In July 2016, Ameren Missouri filed a request with the MoPSC seeking approval to increase its annual revenues for electric service. Relating to that request, in February 2017, Ameren Missouri, the MoPSC staff, the MoOPC, and all intervenors filed a unanimous stipulation and agreement with the MoPSC. The stipulation and agreement, which is subject to MoPSC approval, would result in a $3.4 billion revenue requirement, which is a $92 million increase in Ameren Missouri’s annual revenue requirement for electric service compared to its prior revenue requirement established in the MoPSC's April 2015 electric rate order. The stipulation and agreement did not specify the common equity percentage, the rate base, or the allowed return on common equity. The new revenue requirement reflects the current actual sales volumes of the New Madrid Smelter, whose operations remain suspended, as well as other agreed upon sales volumes. The stipulation and agreement includes the continued use of the FAC and the regulatory tracking mechanisms for pension and postretirement benefits, uncertain income tax positions, and renewable energy standards that the MoPSC previously authorized in earlier electric rate orders. These regulatory tracking mechanisms provide for a base level of expense to be reflected in Ameren Missouri’s base electric rates with differences in the actual expenses incurred recorded as a regulatory asset or liability. Excluding cost reductions associated with reduced sales volumes, the base level of net energy costs under the stipulation and agreement would decrease by $54 million from the base level established in the MoPSC's April 2015 electric rate order. Changes in amortizations and the base level of expenses for the other regulatory tracking mechanisms, including extending the amortization period of certain regulatory assets, would reduce expenses by $26 million from the base levels established in the MoPSC's April 2015 electric rate order. The stipulation and agreement contemplates that new rates will become effective on or before March 20, 2017. Ameren Missouri cannot predict whether the MoPSC will approve the stipulation and agreement or, if approved, whether any application for rehearing or appeal will be filed or the outcome if so filed. Noranda and New Madrid Smelter In the first quarter of 2016, Noranda, which was historically Ameren Missouri's largest customer, suspended operations at the New Madrid Smelter and filed voluntary petitions for a court-supervised restructuring process under Chapter 11 of the United States Bankruptcy Code. In October 2016, Noranda sold the New Madrid Smelter to ARG International AG. Operations at the New Madrid Smelter remain suspended and Ameren Missouri is uncertain of future sales to the smelter. As a result, Ameren Missouri will not fully recover its revenue requirement until rates are adjusted prospectively by the MoPSC to accurately reflect the actual sales volumes to the New Madrid Smelter. As of December 31, 2016, Ameren Missouri has been paid in full for all previous electric service amounts, and expects to continue to be paid in full for the minimal amount of electric service it is currently providing to the New Madrid Smelter. MEEIA In November 2016, the MoPSC approved a $28 million MEEIA 2013 performance incentive based on a stipulation and agreement between Ameren Missouri, the MoPSC staff, and the MoOPC. Ameren Missouri will collect the performance incentive over a two-year period that began in February 2017. In November 2015, the MoPSC issued an order regarding the determination of an input used to calculate the performance incentive. Ameren Missouri filed an appeal of the order with the Missouri Court of Appeals, Western District. In December 2016, the Missouri Court of Appeals, Western District, upheld the November 2015 MoPSC order. Ameren Missouri has appealed the decision to uphold the MoPSC order to the Missouri Supreme Court. ATXI’s Mark Twain Project The Mark Twain project is a MISO-approved 95 -mile transmission line to be located in northeast Missouri. In April 2016, the MoPSC granted ATXI a certificate of convenience and necessity for the Mark Twain project. Before starting construction, ATXI must obtain assents for road crossings from the five counties where the line will be constructed. None of the five county commissions have approved ATXI’s requests for the assents. In October 2016, ATXI filed suit in each of the five county circuit courts to obtain the assents. A decision in each of the five lawsuits is expected in 2017. ATXI plans to complete the project in 2019; however, further delays in obtaining the assents could delay the completion date. Illinois IEIMA Under the provisions of the IEIMA's performance-based formula rate-making framework, which currently extends through 2022, Ameren Illinois’ electric distribution service rates are subject to an annual revenue requirement reconciliation to its actual recoverable costs. 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 December 31, 2016 , Ameren Illinois had recorded regulatory assets of $23 million and $68 million , including interest, to reflect its expected 2016 and the 2015 approved revenue requirement reconciliation adjustments, respectively. As of December 31, 2015 , Ameren Illinois had recorded a $103 million regulatory asset to reflect its approved 2014 revenue requirement reconciliation adjustment, which was collected, with interest, from customers during 2016. In December 2016, the ICC issued an order in Ameren Illinois’ annual update filing approving a $14 million decrease in Ameren Illinois’ electric delivery service revenue requirement beginning in January 2017. This update reflects an increase to the annual formula rate based on 2015 actual recoverable costs and expected net plant additions for 2016, an increase to include the 2015 revenue requirement reconciliation adjustment, which was initially recorded as a regulatory asset in 2015, and a decrease for the conclusion of the 2014 revenue requirement reconciliation adjustment, which was fully collected from customers in 2016. FEJA The FEJA revised certain portions of the IEIMA, including extending the IEIMA formula ratemaking process through 2022 and clarifying that a common equity ratio of up to and including 50% is prudent. Also, beginning in 2017, the FEJA decouples electric distribution revenues established in a rate proceeding from actual sales volumes by providing that any revenue changes driven by actual electric distribution sales volumes differing from sales volumes reflected in that year's rates will be collected from or refunded to customers within two years. This portion of the law extends beyond the end of the IEIMA in 2022. Through 2022, revenue differences will be included in the annual IEIMA revenue requirement reconciliation. Additionally, this law creates a customer surcharge relating to certain nuclear energy centers located in Illinois that, like the cost of power purchased by Ameren Illinois on behalf of its customers, will be passed through to electric distribution customers with no effect on Ameren Illinois' earnings. Beginning as early as June 2017, the FEJA will allow Ameren Illinois to earn a return on its electric energy efficiency program investments. Ameren Illinois electric energy efficiency investments will be deferred as a regulatory asset and will earn a return at the company’s weighted average cost of capital, with the equity return based on the monthly average yield of the 30-year United States Treasury bonds plus 580 basis points. The equity portion of Ameren Illinois’ return on electric energy efficiency investments can also be increased or decreased by 200 basis points based on the achievement of annual energy savings goals. The FEJA increased the level of electric energy efficiency saving targets through 2030. Based on a formula provided in the act, Ameren Illinois estimates it can annually invest up to $100 million from 2018 through 2021, up to $107 million annually from 2022 through 2025, and up to $114 million annually from 2026 through 2030. The ICC has the ability to lower the electric energy efficiency saving goals if there are insufficient cost effective measures available. The electric energy efficiency program investments and the return on those investments will be recovered through a rider, and will not be included in the IEIMA formula rate process. Federal FERC Complaint Cases In November 2013, a customer group filed a complaint case with the FERC seeking a reduction in the allowed base return on common equity for FERC-regulated transmission rate base under the MISO tariff from 12.38% to 9.15% . In September 2016, the FERC issued a final order in the November 2013 complaint case which lowered the allowed base return on common equity to 10.32%, or a 10.82% total return on common equity with the inclusion of the 50 basis point incentive adder for participation in an RTO. The order was consistent with the initial decision an administrative law judge issued in December 2015, and requires customer refunds, with interest, to be issued for the 15-month period ended February 2015. In addition, the new allowed return on common equity is reflected in rates prospectively from the September 2016 effective date of the order. Refunds for the November 2013 complaint case are expected to be issued in the first half of 2017. 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 transmission rate base under the MISO tariff to 8.67% . In June 2016, an administrative law judge issued an initial decision in the February 2015 complaint case, which if approved by FERC, would lower the allowed base return on common equity to 9.70%, or a 10.20% total return on equity with the inclusion of the 50 basis point incentive adder for participation in an RTO. It would also require the issuance of customer refunds, with interest, for the 15-month period ended May 2016. The FERC is expected to issue a final order in the February 2015 complaint case in the second quarter of 2017. That final order will determine the allowed return on common equity for the 15-month period ended May 2016. That final order will also establish the allowed return on common equity that will apply prospectively from its expected second quarter 2017 effective date, replacing the current 10.82% total return on common equity, which became effective in September 2016. The 12.38% allowed return on common equity was effective for the period that began at the conclusion of the 15-month period for the February 2015 complaint case in May 2016 through the September 2016 effective date of the final order in the November 2013 complaint case. Beginning with the January 2015 effective date, the RTO participation incentive adder reduces any refund to customers relating to a reduction of the allowed base return on common equity from the complaint cases discussed above and has been applied prospectively from the effective date of the September 2016 FERC order, resulting in a current allowed return on common equity of 10.82% . As of December 31, 2016 , Ameren and Ameren Illinois recorded current regulatory liabilities of $62 million and $42 million , respectively, to reflect the expected refunds, including interest, associated with the reduced allowed returns on common equity in the September 2016 FERC order and the initial decision in the February 2015 complaint case. Ameren Missouri does not expect that a reduction in the FERC-allowed base return on common equity would be material to its results of operations, financial position, or liquidity. 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. Primarily because of 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 2015, Ameren and Ameren Missouri recognized a $69 million noncash pretax provision for all of the previously capitalized COL costs. Ameren Missouri has withdrawn its COL application with the NRC. Regulatory Assets and Liabilities In accordance with authoritative accounting guidance regarding accounting for the effects of certain types of regulation, we defer certain costs as regulatory assets pursuant to actions of regulators or because we expect to recover such costs in rates charged to customers. We may also defer certain amounts as regulatory liabilities because of actions of regulators or because we expect that such amounts will be returned to customers in future rates. The following table presents our regulatory assets and regulatory liabilities at December 31, 2016 and 2015 : 2016 2015 Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Current regulatory assets: Under-recovered FAC (a)(b) $ 21 $ — $ 21 $ 37 $ — $ 37 Under-recovered Illinois electric power costs (c) — 3 3 — 3 3 Under-recovered PGA (c) — 4 4 — 8 8 MTM derivative losses (d) 9 15 24 29 45 74 Energy efficiency riders (e) 5 — 5 23 — 23 IEIMA revenue requirement reconciliation adjustment (a)(f) — 68 68 — 103 103 FERC revenue requirement reconciliation adjustment (a)(g) — 7 13 — 8 12 VBA rider (a)(h) — 11 11 — — — Total current regulatory assets $ 35 $ 108 $ 149 $ 89 $ 167 $ 260 Noncurrent regulatory assets: Pension and postretirement benefit costs (i) $ 175 $ 319 $ 494 $ 95 $ 202 $ 297 Income taxes (j) 229 1 230 247 4 251 Uncertain tax positions tracker (a)(k) 7 — 7 7 — 7 ARO (l) — 3 3 — 4 4 Callaway costs (a)(m) 29 — 29 32 — 32 Unamortized loss on reacquired debt (a)(n) 65 59 124 69 69 138 Environmental cost riders (o) — 196 196 — 230 230 MTM derivative losses (d) 9 178 187 15 175 190 Storm costs (a)(p) — 15 15 — 9 9 Demand-side costs before the MEEIA implementation (a)(q) 18 — 18 31 — 31 Workers’ compensation claims (r) 6 7 13 6 7 13 Credit facilities fees (s) 4 — 4 4 — 4 Construction accounting for pollution control equipment (a)(t) 19 — 19 20 — 20 Solar rebate program (a)(u) 49 — 49 74 — 74 IEIMA revenue requirement reconciliation adjustment (a)(f) — 23 23 — 62 62 FERC revenue requirement reconciliation adjustment (a)(g) — 8 10 — 5 11 Other 9 7 16 5 4 9 Total noncurrent regulatory assets $ 619 $ 816 $ 1,437 $ 605 $ 771 $ 1,382 Current regulatory liabilities: Over-recovered FAC (b) $ — $ — $ — $ 9 $ — $ 9 Over-recovered Illinois electric power costs (c) — 25 25 — 6 6 Over-recovered PGA (c) — — — 3 — 3 MTM derivative gains (d) 12 11 23 16 1 17 Estimated refund for FERC complaint cases (v) — 42 62 — 32 45 Total current regulatory liabilities $ 12 $ 78 $ 110 $ 28 $ 39 $ 80 Noncurrent regulatory liabilities: Income taxes (w) $ 33 $ 4 $ 37 $ 36 $ 6 $ 42 Uncertain tax positions tracker (k) 3 — 3 6 — 6 Asset removal costs (x) 970 697 1,669 933 671 1,605 ARO (l) 162 — 162 167 — 167 Bad debt rider (y) — 3 3 — 6 6 Pension and postretirement benefit costs tracker (z) 35 — 35 19 — 19 Energy efficiency riders (e) — 45 45 — 36 36 Renewable energy credits (aa) — 15 15 — 12 12 Storm tracker (ab) 7 — 7 9 — 9 Other 5 4 9 2 1 3 Total noncurrent regulatory liabilities $ 1,215 $ 768 $ 1,985 $ 1,172 $ 732 $ 1,905 (a) These assets earn a return. (b) Under-recovered or over-recovered fuel costs to be recovered or refunded through the FAC. Specific accumulation periods aggregate the under-recovered or over-recovered costs over four months, any related adjustments that occur over the following four months, and the recovery from or refund to customers that occurs over the next eight months. (c) Under-recovered or over-recovered costs from utility customers. Amounts will be recovered from or refunded to customers within one year of the deferral. (d) Deferral of commodity-related derivative MTM losses or gains. See Note 7 – Derivative Financial Instruments for additional information. (e) The Ameren Missouri balance relates to the MEEIA. The MEEIA rider allows Ameren Missouri to collect from or refund to customers any annual difference in the actual amounts incurred and the amounts collected from customers for the MEEIA program costs, net shared benefits, and the throughput disincentive. Under the MEEIA rider, collections from or refunds to customers occur one year after the program costs, net shared benefits, and the throughput disincentive are incurred. The Ameren Illinois balance relates to a regulatory tracking mechanism to recover its electric and natural gas costs associated with developing, implementing, and evaluating customer energy efficiency and demand response programs. Any under-recovery or over-recovery will be collected from or refunded to customers over the year following the plan year. (f) The difference between Ameren Illinois' annual revenue requirement calculated under the IEIMA's performance-based formula ratemaking framework and the revenue requirement included in customer rates for that year. The under-recovery will be recovered from or refunded to customers with interest within two years. (g) Ameren Illinois' and ATXI's annual revenue requirement reconciliation calculated pursuant to the FERC's electric transmission formula ratemaking framework. The under-recovery or over-recovery will be recovered from or refunded to customers within two years. (h) Under-recovered natural gas sales volumes, including deviations from normal weather conditions. Each year's amount will be recovered from or refunded to customers from April through December of the following year. (i) These costs are being amortized in proportion to the recognition of prior service costs (credits) and actuarial losses (gains) attributable to Ameren’s pension plan and postretirement benefit plans. See Note 11 – Retirement Benefits for additional information. (j) Tax benefits related to the equity component of allowance for funds used during construction, as well as the effects of tax rate changes. This amount will be recovered over the expected life of the related assets. (k) The tracker is amortized over three years, beginning from the date the amounts are included in rates. See Note 13 – Income Taxes for additional information. (l) Recoverable or refundable removal costs for AROs, including net realized and unrealized gains and losses related to the nuclear decommissioning trust fund investments. See Note 1 – Summary of Significant Accounting Policies – Asset Retirement Obligations. (m) Ameren Missouri’s Callaway energy center operations and maintenance expenses, property taxes, and carrying costs incurred between the plant in-service date and the date the plant was reflected in rates. These costs are being amortized over the remaining life of the energy center's original operating license through 2024. (n) Losses related to reacquired debt. These amounts are being amortized over the lives of the related new debt issuances or the original lives of the old debt issuances if no new debt was issued. (o) The recoverable portion of accrued environmental site liabilities that will be collected from electric and natural gas customers through ICC-approved cost recovery riders. The period of recovery will depend on the timing of remediation expenditures. See Note 15 – Commitments and Contingencies for additional information. (p) Storm costs from 2013, 2015, and 2016 deferred in accordance with the IEIMA. These costs are being amortized over five-year periods beginning in the year the storm occurred. (q) Demand-side costs incurred prior to implementation of the MEEIA in 2013, including the costs of developing, implementing, and evaluating customer energy efficiency and demand response programs. Costs incurred from May 2008 through September 2008 are being amortized over a 10-year period that began in March 2009. Costs incurred from October 2008 through December 2009 are being amortized until May 2017. Costs incurred from January 2010 through February 2011 are being amortized over a six-year period that began in August 2011. Costs incurred from March 2011 through July 2012 are being amortized over a six-year period that began in January 2013. Costs incurred from August 2012 through December 2012 are being amortized over a six-year period that began in June 2015. The February 2017 stipulation and agreement, if approved, would modify these amortization periods. (r) The period of recovery will depend on the timing of actual expenditures. (s) Ameren Missouri’s costs incurred to enter into and maintain the Missouri Credit Agreement. These costs are being amortized over the life of the credit facility to construction work in progress, which will be depreciated when assets are placed into service. Additional costs were incurred in December 2016 to amend and restate the Missouri Credit Agreement. (t) The MoPSC’s May 2010 electric rate order allowed Ameren Missouri to record an allowance for funds used during construction for pollution control equipment at its Sioux energy center until the cost of that equipment was included in customer rates beginning in 2011. These costs are being amortized over the expected life of the Sioux energy center, currently through 2033. (u) Costs associated with Ameren Missouri's solar rebate program to fulfill its renewable energy portfolio requirement. These costs are being amortized over a three-year period that began in June 2015. The February 2017 stipulation and agreement, if approved, would modify this amortization period. (v) Estimated refunds to transmission customers related to FERC orders. See FERC Complaint Cases above. (w) Unamortized portion of investment tax credits and reductions to deferred tax liabilities recorded at rates in excess of current statutory rates. The unamortized portion of investment tax credits and the reduction to deferred tax liabilities are being amortized over the expected life of the underlying assets. (x) Estimated funds collected for the eventual dismantling and removal of plant retired from service, net of salvage value. (y) A regulatory tracking mechanism for the difference between the level of bad debt incurred by Ameren Illinois under GAAP and the level of such costs included in electric and natural gas rates. The over-recovery relating to 2014 was refunded to customers from June 2015 through May 2016. The over-recovery relating to 2015 is being refunded to customers from June 2016 through May 2017. The over-recovery relating to 2016 will be refunded to customers from June 2017 through May 2018. (z) A regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. For periods prior to December 2014, the MoPSC's April 2015 electric rate order directed the amortization to occur over three to five years, beginning in June 2015. For periods after December 2014, the amortization period will be determined in the July 2016 electric rate case. The February 2017 stipulation and agreement, if approved, would modify these amortization periods. (aa) Funds collected from customers for the purchase of renewable energy credits through IPA procurements for distributed generation. The balance will be amortized as renewable energy credits are purchased. (ab) A regulatory tracking mechanism at Ameren Missouri for the difference between the level of storm costs incurred in a particular year and the level of such costs included in rates. For periods prior to December 2014, the MoPSC's April 2015 electric rate order directed the amortization to occur over a five-year period that began in June 2015. For periods after December 2014, the amortization period will be determined in the July 2016 electric rate case. The April 2015 MoPSC order did not approve the continued use of the storm cost regulatory tracking mechanism. The February 2017 stipulation and agreement, if approved, would modify these amortization periods. Ameren, Ameren Missouri, and Ameren Illinois continually assess the recoverability of their regulatory assets. Regulatory assets are charged to earnings when it is no longer probable that such amounts will be recovered through future revenues. To the extent that payments of regulatory liabilities are no longer probable, the amounts are credited to earnings.

Property And Plant, Net

Property And Plant, Net	12 Months Ended
	Dec. 31, 2016
Property, Plant and Equipment [Abstract]
PROPERTY AND PLANT, NET	PROPERTY, PLANT, AND EQUIPMENT, NET The following table presents property, plant, and equipment, net, for each of the Ameren Companies at December 31, 2016 and 2015 : Ameren Missouri (a) Ameren Illinois Other Ameren (a) 2016 Property, plant, and equipment at original cost: (b) Electric generation $ 10,911 $ — $ — $ 10,911 Electric distribution 5,563 5,287 — 10,850 Electric transmission 1,151 2,016 712 3,879 Natural gas 455 2,186 — 2,641 Other (c) 879 719 239 1,837 18,959 10,208 951 30,118 Less: Accumulated depreciation and amortization 7,880 2,850 231 10,961 11,079 7,358 720 19,157 Construction work in progress: Nuclear fuel in process 206 — — 206 Other 193 111 446 750 Property and plant, net $ 11,478 $ 7,469 $ 1,166 $ 20,113 2015 Property, plant, and equipment at original cost: (b) Electric generation $ 10,431 $ — $ — $ 10,431 Electric distribution 5,303 4,952 — 10,255 Electric transmission 979 1,674 121 2,774 Natural gas 445 1,997 — 2,442 Other (c) 808 627 266 1,701 17,966 9,250 387 27,603 Less: Accumulated depreciation and amortization 7,460 2,632 255 10,347 10,506 6,618 132 17,256 Construction work in progress: Nuclear fuel in process 275 — — 275 Other 402 230 636 1,268 Property, plant, and equipment, net $ 11,183 $ 6,848 $ 768 $ 18,799 (a) Amounts in Ameren and Ameren Missouri include two CTs under separate capital lease agreements. The gross cumulative asset value of those agreements was $232 million and $233 million at December 31, 2016 and 2015, respectively. The total accumulated depreciation associated with the two CTs was $77 million and $72 million at December 31, 2016 and 2015 , respectively. In addition, Ameren Missouri has investments in debt securities, classified as held-to-maturity and recorded in "Other assets" that are related to the two CTs from the city of Bowling Green and Audrain County. As of December 31, 2016 and 2015, the carrying value of these debt securities was $282 million and $288 million , respectively. (b) The estimated lives for each asset group are as follows: 5 to 100 years for electric generation, excluding Ameren Missouri's hydro generating assets which have useful lives of up to 150 years, 18 to 75 years for electric distribution, 50 to 75 years for electric transmission, 20 to 80 years for natural gas, and 5 to 55 years for other. (c) Other property, plant, and equipment includes assets used to support multiple utility services. The following table provides accrued capital and nuclear fuel expenditures at December 31, 2016 , 2015 , and 2014 , which represent noncash investing activity excluded from the accompanying statements of cash flows: Ameren (a) Ameren Missouri Ameren Illinois Accrued capital expenditures: 2016 $ 251 $ 116 $ 87 2015 235 85 92 2014 181 72 59 Accrued nuclear fuel expenditures: 2016 20 20 (b) 2015 16 16 (b) 2014 13 13 (b) (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries. (b) Not applicable.

Short-Term Debt And Liquidity

Short-Term Debt And Liquidity	12 Months Ended
	Dec. 31, 2016
Line of Credit Facility [Abstract]
SHORT-TERM DEBT AND LIQUIDITY	SHORT-TERM DEBT AND LIQUIDITY The liquidity needs of the Ameren Companies are typically supported through the use of available cash, drawings under committed credit agreements, commercial paper issuances, or in the case of Ameren Missouri and Ameren Illinois, short-term intercompany borrowings. Credit Agreements In December 2016, the Credit Agreements were amended and restated. The amended and restated agreements, among other things, extended the maturity dates of the Credit Agreements and provide $2.1 billion of credit through the extended maturity date. The Credit Agreements, which were previously scheduled to mature in December 2019, are now scheduled to mature in December 2021. The maturity date may be extended for two additional one-year periods upon mutual consent of the borrowers and lenders. Credit available under the agreements is provided by 22 international, national, and regional lenders, with no single lender providing more than $118 million of credit in aggregate. The obligations of each borrower under the respective Credit Agreements to which it is a party are several and not joint. Except under limited circumstances relating to expenses and indemnities, the obligations of Ameren Missouri and Ameren Illinois under the respective Credit Agreements are not guaranteed by Ameren or any other subsidiary of Ameren. The following table presents the maximum aggregate amount available to each borrower under each facility (the amount being each borrower's "Borrowing Sublimit"): Missouri Credit Agreement Illinois Credit Agreement Ameren $ 700 $ 500 Ameren Missouri 800 (a) Ameren Illinois (a) 800 (a) Not applicable. The borrowers have the option to seek additional commitments from existing or new lenders to increase the total facility size of the Credit Agreements to a maximum of $1.2 billion for the Missouri Credit Agreement and $1.3 billion for the Illinois Credit Agreement. Ameren borrowings are due and payable no later than the maturity date of the Credit Agreement. Ameren Missouri and Ameren Illinois borrowings under the applicable Credit Agreement are due and payable no later than the earlier of the maturity date or 364 days after the originating date of the borrowing. The obligations of the borrowers under the Credit Agreements are unsecured. Loans are available on a revolving basis under each of the Credit Agreements. Funds borrowed may be repaid and, subject to satisfaction of the conditions to borrowing, reborrowed from time to time. At the election of each borrower, the interest rates on such loans will be the alternate base rate plus the margin applicable to the particular borrower and/or the eurodollar rate plus the margin applicable to the particular borrower. The applicable margins will be determined by the borrower's long-term unsecured credit ratings or, if no such ratings are in effect, the borrower's corporate/issuer ratings then in effect. The borrowers have received commitments from the lenders to issue letters of credit up to $ 100 million under each of the Credit Agreements. In addition, the issuance of letters of credit is subject to the $2.1 billion overall combined facility borrowing limitations of the Credit Agreements. The borrowers will use the proceeds from any borrowings under the Credit Agreements for general corporate purposes, including working capital, commercial paper liquidity support, issuance of letters of credit, loan funding under the Ameren money pool arrangements, and other short-term intercompany loan arrangements. Both of the Credit Agreements are available to Ameren to support issuances under Ameren's commercial paper program, subject to borrowing sublimits. The Missouri Credit Agreement and the Illinois Credit Agreement are available to support issuances under Ameren (parent)'s, Ameren Missouri's and Ameren Illinois' commercial paper programs, respectively. As of December 31, 2016 , based on commercial paper outstanding and letters of credit issued under the Credit Agreements, the aggregate amount of credit capacity available to Ameren (parent), Ameren Missouri, and Ameren Illinois, collectively, was $1.5 billion . Ameren, Ameren Missouri, and Ameren Illinois did not borrow under the Credit Agreements for the years ended December 31, 2016 and 2015 . Commercial Paper The following table summarizes the borrowing activity and relevant interest rates under Ameren (parent)'s, Ameren Missouri's and Ameren Illinois' commercial paper programs for the years ended December 31, 2016 and 2015 : Ameren (parent) Ameren Missouri Ameren Illinois Ameren Consolidated 2016 Average daily commercial paper outstanding $ 440 $ 60 $ 52 $ 552 Outstanding borrowings at period-end 507 — 51 558 Weighted-average interest rate 0.82 % 0.74 % 0.69 % 0.80 % Peak outstanding commercial paper during period (a) $ 574 $ 208 $ 195 $ 839 Peak interest rate 1.05 % 0.85 % 0.90 % 1.05 % 2015 Average daily commercial paper outstanding $ 721 $ 42 $ 4 $ 767 Outstanding borrowings at period-end 301 — — 301 Weighted-average interest rate 0.57 % 0.50 % 0.44 % 0.55 % Peak outstanding commercial paper during period (a) $ 874 $ 294 $ 48 $ 1,108 Peak interest rate 0.91 % 0.60 % 0.60 % 0.91 % (a) The timing of peak outstanding commercial paper issuances varies by company. Therefore, the sum of the peak amounts presented by company might not equal the Ameren Consolidated peak amount for the period. Indebtedness Provisions and Other Covenants The information below is a summary of the Ameren Companies’ compliance with indebtedness provisions and other covenants. The Credit Agreements contain conditions for borrowings and issuances of letters of credit. These conditions include the absence of default or unmatured default, material accuracy of representations and warranties (excluding any representation after the closing date as to the absence of material adverse change and material litigation, and the absence of any notice of violation, liability, or requirement under any environmental laws that could have a material adverse effect), and obtainment of required regulatory authorizations. In addition, it is a condition for any Ameren Illinois borrowing that, at the time of and after giving effect to such borrowing, Ameren Illinois not be in violation of any limitation on its ability to incur unsecured indebtedness contained in its articles of incorporation. The Credit Agreements also contain nonfinancial covenants, including restrictions on the ability to incur certain liens, to transact with affiliates, to dispose of assets, to make investments in or transfer assets to its affiliates, and to merge with other entities. The Credit Agreements require 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 December 31, 2016 , the ratios of consolidated indebtedness to total consolidated capitalization, calculated in accordance with the provisions of the Credit Agreements, were 51% , 48% , and 47% , for Ameren, Ameren Missouri, and Ameren Illinois, respectively. The Credit Agreements contain default provisions that apply separately to each borrower. However, a default of Ameren Missouri or Ameren Illinois under the applicable Credit Agreement is also deemed to constitute a default of Ameren under such agreement. Defaults include a cross-default resulting from a default of such borrower under any other agreement covering outstanding indebtedness of such borrower and certain subsidiaries (other than project finance subsidiaries and nonmaterial subsidiaries) in excess of $100 million in the aggregate (including under the other Credit Agreement). However, under the default provisions of the Credit Agreements, any default of Ameren under any Credit Agreement that results solely from a default of Ameren Missouri or Ameren Illinois does not result in a cross-default of Ameren under the other Credit Agreement. Further, the Credit Agreement default provisions provide that an Ameren default under any of the Credit Agreements does not constitute a default by Ameren Missouri or Ameren Illinois. None of the Ameren Companies' credit agreements or financing agreements contain credit rating triggers that would cause a default or acceleration of repayment of outstanding balances. The Ameren Companies were in compliance with the provisions and covenants of their credit agreements at December 31, 2016 . Money Pools Ameren has money pool agreements with and among its subsidiaries to coordinate and provide for certain short-term cash and working capital requirements. Ameren Missouri, Ameren Illinois, and ATXI may participate in the utility money pool as both lenders and borrowers. Ameren and Ameren Services may participate in the utility money pool only as lenders. Surplus internal funds are contributed to the money pool from participants. The primary sources of external funds for the utility money pool are the Credit Agreements and the commercial paper programs. The total amount available to the pool participants from the utility money pool at any given time is reduced by the amount of borrowings made by participants, but is increased to the extent that the pool participants advance surplus funds to the utility money pool or remit funds from other external sources. The availability of funds is also determined by funding requirement limits established by regulatory authorizations. Participants receiving a loan under the money pool agreement must repay the principal amount of such loan, together with accrued interest. The rate of interest depends on the composition of internal and external funds in the utility money pool. The average interest rate for borrowing under the money pool for the year ended December 31, 2016 was 0.52% ( 2015 – 0.11% ). See Note 14 – Related Party Transactions for the amount of interest income and expense from the money pool arrangements recorded by the Ameren Companies for the years ended December 31, 2016 , 2015 , and 2014 .

Long-Term Debt And Equity Finan

Long-Term Debt And Equity Financings	12 Months Ended
	Dec. 31, 2016
Long-Term Debt And Equity Financings [Abstract]
LONG-TERM DEBT AND EQUITY FINANCINGS	2.0 4.6 $ 4,077 > 2.5 105.3 $ 2,344 Ameren Illinois > 2.0 6.9 3,819 (d) > 1.5 2.8 203 (e) (a) Coverage required on the annual interest charges on first mortgage bonds outstanding and to be issued. Coverage is not required in certain cases when additional first mortgage bonds are issued on the basis of retired bonds. (b) Amount of bonds issuable based either on required coverage ratios or unfunded property additions, whichever is more restrictive. The amounts shown also include bonds issuable based on retired bond capacity of $1,206 million and $279 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 its 1992 mortgage indenture. (e) Preferred stock issuable is restricted by the amount of preferred stock that is currently authorized by Ameren Illinois' articles of incorporation. Ameren's indenture does not require Ameren to comply with any quantitative financial covenants. The indenture does, however, include certain cross-default provisions. Specifically, either (1) the failure by Ameren to pay when due and upon expiration of any applicable grace period any portion of any Ameren indebtedness in excess of $25 million , or (2) the acceleration upon default of the maturity of any Ameren indebtedness in excess of $25 million under any indebtedness agreement, including borrowings under the Credit Agreements or the Ameren commercial paper program, constitutes a default under the indenture, unless such past due or accelerated debt is discharged or the acceleration is rescinded or annulled within a specified period. Ameren Missouri and Ameren Illinois and certain other nonregistrant 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 net income and retained earnings. In addition, under Illinois law, Ameren Illinois may not pay any dividend on its stock unless, among other things, its earnings and earned surplus are sufficient to declare and pay a dividend after provision is made for reasonable and proper reserves, or unless Ameren Illinois has specific authorization from the ICC. Ameren Illinois’ articles of incorporation require dividend payments on its common stock to be based on ratios of common stock to total capitalization and other provisions related to certain operating expenses and accumulations of earned surplus. Ameren Illinois has made a commitment to the FERC to maintain a minimum 30% ratio of common stock equity to total capitalization. As of December 31, 2016 , using the FERC-agreed upon calculation method, Ameren Illinois’ ratio of common stock equity to total capitalization was 51% . In order for the Ameren Companies to issue securities in the future, they will have to comply with all applicable requirements in effect at the time of any such issuances. Off-Balance-Sheet Arrangements At December 31, 2016 , none of the Ameren Companies had off-balance-sheet financing arrangements, other than operating leases entered into in the ordinary course of business, letters of credit, and Ameren parent guarantee arrangements on behalf of its subsidiaries. None of the Ameren Companies expect to engage in any significant off-balance-sheet financing arrangements in the near future." id="sjs-B4">LONG-TERM DEBT AND EQUITY FINANCINGS The following table presents long-term debt outstanding, including maturities due within one year, for the Ameren Companies as of December 31, 2016 and 2015 : 2016 2015 Ameren (Parent): 2.70% Senior unsecured notes due 2020 $ 350 $ 350 3.65% Senior unsecured notes due 2026 350 350 Total long-term debt, gross 700 700 Less: Unamortized debt issuance costs (6 ) (6 ) Long-term debt, net $ 694 $ 694 Ameren Missouri: Senior secured notes: (a) 5.40% Senior secured notes due 2016 — 260 6.40% Senior secured notes due 2017 425 425 6.00% Senior secured notes due 2018 (b) 179 179 5.10% Senior secured notes due 2018 199 199 6.70% Senior secured notes due 2019 (b) 329 329 5.10% Senior secured notes due 2019 244 244 5.00% Senior secured notes due 2020 85 85 3.50% Senior secured notes due 2024 350 350 5.50% Senior secured notes due 2034 184 184 5.30% Senior secured notes due 2037 300 300 8.45% Senior secured notes due 2039 (b) 350 350 3.90% Senior secured notes due 2042 (b) 485 485 3.65% Senior secured notes due 2045 400 250 Environmental improvement and pollution control revenue bonds: 1992 Series due 2022 (c)(d) 47 47 1993 5.45% Series due 2028 (e) (e) (e) 1998 Series A due 2033 (c)(d) 60 60 1998 Series B due 2033 (c)(d) 50 50 1998 Series C due 2033 (c)(d) 50 50 Capital lease obligations: City of Bowling Green capital lease (Peno Creek CT) due 2022 42 48 Audrain County capital lease (Audrain County CT) due 2023 240 240 Total long-term debt, gross 4,019 4,135 Less: Unamortized discount and premium (6 ) (6 ) Less: Unamortized debt issuance costs (19 ) (19 ) Less: Maturities due within one year (431 ) (266 ) Long-term debt, net $ 3,563 $ 3,844 2016 2015 Ameren Illinois: Senior secured notes: 6.20% Senior secured notes due 2016 $ — $ 54 6.25% Senior secured notes due 2016 — 75 6.125% Senior secured notes due 2017 (g)(h) 250 250 6.25% Senior secured notes due 2018 (g)(h) 144 144 9.75% Senior secured notes due 2018 (g)(h) 313 313 2.70% Senior secured notes due 2022 (g)(h) 400 400 3.25% Senior secured notes due 2025 (g) 300 300 6.125% Senior secured notes due 2028 (g) 60 60 6.70% Senior secured notes due 2036 (g) 61 61 6.70% Senior secured notes due 2036 (f) 42 42 4.80% Senior secured notes due 2043 (g) 280 280 4.30% Senior secured notes due 2044 (g) 250 250 4.15% Senior secured notes due 2046 (g) 490 250 Environmental improvement and pollution control revenue bonds: 5.90% Series 1993 due 2023 (i) (i) (i) 5.70% 1994A Series due 2024 (j) (j) (j) 1993 Series B-1 due 2028 (d)(k) 17 17 Total long-term debt, gross 2,607 2,496 Less: Unamortized discount and premium — (7 ) Less: Unamortized debt issuance costs (19 ) (18 ) Less: Maturities due within one year (250 ) (129 ) Long-term debt, net $ 2,338 $ 2,342 Ameren consolidated long-term debt, net $ 6,595 $ 6,880 (a) These notes are collaterally secured by first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage indenture. The notes have a fall-away lien provision and will remain secured only as long as any first mortgage bonds issued under the Ameren Missouri mortgage indenture remain outstanding. Redemption, purchase, or maturity of all first mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the first mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the Ameren Missouri senior secured notes currently outstanding, we do not expect the first mortgage bond lien protection associated with these notes to fall away before 2042. (b) Ameren Missouri has agreed that so long as any of the 3.90% senior secured notes due 2042 are outstanding, Ameren Missouri will not permit a release date to occur, and so long as any of the 6.70% senior secured notes due 2019, 6.00% senior secured notes due 2018, and 8.45% senior secured notes due 2039 are outstanding, Ameren Missouri will not optionally redeem, purchase, or otherwise retire in full the outstanding first mortgage bonds not subject to release provisions. (c) These bonds are collaterally secured by first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage indenture and have a fall-away lien provision similar to that of Ameren Missouri's senior secured notes. The bonds are also backed by an insurance guarantee policy. (d) The interest rates and the periods during which such rates apply vary depending on our selection of defined rate modes. Maximum interest rates could reach 18% , depending on the series of bonds. The bonds are callable at 100% of par value. The average interest rates for 2016 and 2015 were as follows: 2016 2015 Ameren Missouri 1992 Series due 2022 0.66% 0.06% Ameren Missouri 1998 Series A due 2033 0.91% 0.24% Ameren Missouri 1998 Series B due 2033 0.92% 0.24% Ameren Missouri 1998 Series C due 2033 0.97% 0.24% Ameren Illinois 1993 Series B-1 due 2028 0.70% 0.49% (e) These bonds are first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage bond indenture and are secured by substantially all Ameren Missouri property and franchises. The bonds are callable at 100% of par value. Less than $1 million principal amount of the bonds remain outstanding. (f) These notes are collaterally secured by first mortgage bonds issued by Ameren Illinois under its 1933 mortgage indenture. The notes have a fall-away lien provision, and Ameren Illinois could cause these notes to become unsecured at any time by redeeming the pollution control bonds 5.90% Series 1993 due 2023 (of which less than $1 million remains outstanding). (g) These notes are collaterally secured by mortgage bonds issued by Ameren Illinois under its 1992 mortgage indenture. They are secured by substantially all property of the former IP and CIPS. The notes have a fall-away lien provision and will remain secured only as long as any series of first mortgage bonds issued under its 1992 mortgage indenture remain outstanding. Redemption, purchase, or maturity of all mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the Ameren Illinois senior secured notes currently outstanding, we do not expect the mortgage bond lien protection associated with these notes to fall away before 2022. (h) Ameren Illinois has agreed that so long as any of the 2.70% senior secured notes due 2022 are outstanding, Ameren Illinois will not permit a release date to occur, and so long as any of the 9.75% senior secured notes due 2018, 6.25% senior secured notes due 2018, and 6.125% senior secured notes due 2017 are outstanding, Ameren Illinois will not optionally redeem, purchase or otherwise retire in full the outstanding first mortgage bonds not subject to release provisions; therefore, a release date will not occur so long as any of these notes remain outstanding. (i) These bonds are first mortgage bonds issued by Ameren Illinois under its 1933 mortgage indenture. They are secured by substantially all property of the former CILCO. The bonds are callable at 100% of par value. Less than $1 million principal amount of the bonds remain outstanding. (j) These bonds are mortgage bonds issued by Ameren Illinois under its 1992 mortgage indenture. They are secured by substantially all property of the former IP and CIPS. The bonds are callable at 100% of par value. The bonds are also backed by an insurance guarantee policy. Less than $1 million principal amount of the bonds remains outstanding. (k) The bonds are callable at 100% of par value. The following table presents the aggregate maturities of long-term debt, including current maturities, for the Ameren Companies at December 31, 2016 : Ameren (parent) (a) Ameren Missouri (a) Ameren Illinois (a) Ameren Consolidated 2017 $ — $ 431 $ 250 $ 681 2018 — 383 457 840 2019 — 581 — 581 2020 350 92 — 442 2021 — 8 — 8 Thereafter 350 2,524 1,900 4,774 Total $ 700 $ 4,019 $ 2,607 $ 7,326 (a) Excludes unamortized discount and premium and debt issuance costs of $6 million , $25 million , and $19 million at Ameren (parent), Ameren Missouri, and Ameren Illinois, respectively. All classes of Ameren Missouri’s and Ameren Illinois’ preferred stock are entitled to cumulative dividends, have voting rights, and are not subject to mandatory redemption. The preferred stock of Ameren's subsidiaries was included in "Noncontrolling Interests" on Ameren's consolidated balance sheet. The following table presents the outstanding preferred stock of Ameren Missouri and Ameren Illinois, which is redeemable, at the option of the issuer, at the prices shown below as of December 31, 2016 and 2015 : Redemption Price(per share) 2016 2015 Ameren Missouri: Without par value and stated value of $100 per share, 25 million shares authorized $3.50 Series 130,000 shares $ 110.00 $ 13 $ 13 $3.70 Series 40,000 shares 104.75 4 4 $4.00 Series 150,000 shares 105.625 15 15 $4.30 Series 40,000 shares 105.00 4 4 $4.50 Series 213,595 shares 110.00 (a) 21 21 $4.56 Series 200,000 shares 102.47 20 20 $4.75 Series 20,000 shares 102.176 2 2 $5.50 Series A 14,000 shares 110.00 1 1 Total $ 80 $ 80 Ameren Illinois: With par value of $100 per share, 2 million shares authorized 4.00% Series 144,275 shares $ 101.00 $ 14 $ 14 4.08% Series 45,224 shares 103.00 5 5 4.20% Series 23,655 shares 104.00 2 2 4.25% Series 50,000 shares 102.00 5 5 4.26% Series 16,621 shares 103.00 2 2 4.42% Series 16,190 shares 103.00 2 2 4.70% Series 18,429 shares 103.00 2 2 4.90% Series 73,825 shares 102.00 7 7 4.92% Series 49,289 shares 103.50 5 5 5.16% Series 50,000 shares 102.00 5 5 6.625% Series 124,274 shares 100.00 12 12 7.75% Series 4,542 shares 100.00 1 1 Total $ 62 $ 62 Total Ameren $ 142 $ 142 (a) In the event of voluntary liquidation, $105.50 . Ameren has 100 million shares of $0.01 par value preferred stock authorized, with no such shares outstanding. Ameren Missouri has 7.5 million shares of $1 par value preference stock authorized, with no such shares outstanding. Ameren Illinois has 2.6 million shares of no par value preferred stock authorized, with no such shares outstanding. Ameren In November 2015, Ameren (parent) issued $350 million of 2.70% senior unsecured notes due in November 2020, with interest payable semiannually in May and November of each year, beginning in May 2016. Ameren (parent) received proceeds of $348 million , which were used to repay a portion of its short-term debt. In November 2015, Ameren (parent) issued $350 million of 3.65% senior unsecured notes due in February 2026, with interest payable semiannually in February and August of each year, beginning in February 2016. Ameren (parent) received proceeds of $347 million , which were used to repay a portion of its short-term debt. In 2015, Ameren, Ameren Missouri, and Ameren Illinois filed a Form S-3 shelf registration statement registering the issuance of an indeterminate amount of certain types of securities. The registration statement became effective immediately upon filing and will expire in June 2018. Ameren filed a Form S-3 registration statement with the SEC in 2014, authorizing the offering of 8.6 million additional shares of its common stock under DRPlus, which expires in May 2017. Shares of common stock sold under DRPlus are, at Ameren’s option, newly issued shares, treasury shares, or shares purchased in the open market or in privately negotiated transactions. As of December 31, 2016 and 2015, DRPlus participant funds of $8 million were reflected on Ameren's consolidated balance sheets in "Other current assets." In 2013, Ameren filed a Form S-8 registration statement with the SEC, authorizing the offering of 4 million additional shares of its common stock under its 401(k) plan. Shares of common stock sold under the 401(k) plan are, at Ameren’s option, newly issued shares, treasury shares, or shares purchased in the open market or in privately negotiated transactions. From 2014 through 2016, Ameren shares for its DRPlus and its 401(k) plans were purchased in the open market. Ameren Missouri In February 2016, $260 million principal amount of Ameren Missouri's 5.40% senior secured notes matured and were repaid with cash on hand and commercial paper borrowings. In June 2016 and April 2015, Ameren Missouri issued $150 million and $250 million , respectively, of 3.65% senior secured notes due in April 2045, with interest payable semiannually in April and October of each year, beginning in October 2016 and 2015, respectively. Ameren Missouri received proceeds of $148 million from the June 2016 issuance and $247 million from the April 2015 issuance, which were both 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 in April 2015. For information on Ameren Missouri's capital contributions and return of capital, refer to Capital Contributions and Return of Capital in Note 1 – Summary of Significant Accounting Policies. Ameren Illinois In June 2016, Ameren Illinois’ $54 million principal amount of 6.20% senior secured notes and $75 million principal amount of 6.25% senior secured notes matured and were repaid with commercial paper borrowings. In December 2016 and 2015, Ameren Illinois issued $240 million and $250 million , respectively, of 4.15% senior secured notes due in March 2046, with interest payable semiannually in March and September, beginning in March 2017 and 2016, respectively. Ameren Illinois received proceeds of $245 million from each issuance, which were both used to repay a portion of its short-term debt. For information on Ameren Illinois' capital contributions, refer to Capital Contributions and Return of Capital in Note 1 – Summary of Significant Accounting Policies. Indenture Provisions and Other Covenants Ameren Missouri’s and Ameren Illinois’ indentures and articles of incorporation include covenants and provisions related to issuances of first mortgage bonds and preferred stock. Ameren Missouri and Ameren Illinois are required to meet certain ratios to issue additional first mortgage bonds and preferred stock. A failure to achieve these ratios would not result in a default under these covenants and provisions but would restrict the companies’ ability to issue bonds or preferred stock. The following table summarizes the required and actual interest coverage ratios for interest charges, dividend coverage ratios, and bonds and preferred stock issuable as of December 31, 2016 , at an assumed 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 4.6 $ 4,077 > 2.5 105.3 $ 2,344 Ameren Illinois > 2.0 6.9 3,819 (d) > 1.5 2.8 203 (e) (a) Coverage required on the annual interest charges on first mortgage bonds outstanding and to be issued. Coverage is not required in certain cases when additional first mortgage bonds are issued on the basis of retired bonds. (b) Amount of bonds issuable based either on required coverage ratios or unfunded property additions, whichever is more restrictive. The amounts shown also include bonds issuable based on retired bond capacity of $1,206 million and $279 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 its 1992 mortgage indenture. (e) Preferred stock issuable is restricted by the amount of preferred stock that is currently authorized by Ameren Illinois' articles of incorporation. Ameren's indenture does not require Ameren to comply with any quantitative financial covenants. The indenture does, however, include certain cross-default provisions. Specifically, either (1) the failure by Ameren to pay when due and upon expiration of any applicable grace period any portion of any Ameren indebtedness in excess of $25 million , or (2) the acceleration upon default of the maturity of any Ameren indebtedness in excess of $25 million under any indebtedness agreement, including borrowings under the Credit Agreements or the Ameren commercial paper program, constitutes a default under the indenture, unless such past due or accelerated debt is discharged or the acceleration is rescinded or annulled within a specified period. Ameren Missouri and Ameren Illinois and certain other nonregistrant 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 net income and retained earnings. In addition, under Illinois law, Ameren Illinois may not pay any dividend on its stock unless, among other things, its earnings and earned surplus are sufficient to declare and pay a dividend after provision is made for reasonable and proper reserves, or unless Ameren Illinois has specific authorization from the ICC. Ameren Illinois’ articles of incorporation require dividend payments on its common stock to be based on ratios of common stock to total capitalization and other provisions related to certain operating expenses and accumulations of earned surplus. Ameren Illinois has made a commitment to the FERC to maintain a minimum 30% ratio of common stock equity to total capitalization. As of December 31, 2016 , using the FERC-agreed upon calculation method, Ameren Illinois’ ratio of common stock equity to total capitalization was 51% . In order for the Ameren Companies to issue securities in the future, they will have to comply with all applicable requirements in effect at the time of any such issuances. Off-Balance-Sheet Arrangements At December 31, 2016 , none of the Ameren Companies had off-balance-sheet financing arrangements, other than operating leases entered into in the ordinary course of business, letters of credit, and Ameren parent guarantee arrangements on behalf of its subsidiaries. None of the Ameren Companies expect to engage in any significant off-balance-sheet financing arrangements in the near future.

Other Income And Expenses

Other Income And Expenses	12 Months Ended
	Dec. 31, 2016
Other Nonoperating Income (Expense) [Abstract]
OTHER INCOME AND EXPENSES	OTHER INCOME AND EXPENSES The following table presents the components of "Other Income and Expenses" in the Ameren Companies’ statements of income (loss) for the years ended December 31, 2016 , 2015 , and 2014 : 2016 2015 2014 Ameren: (a) Miscellaneous income: Allowance for equity funds used during construction $ 27 $ 30 $ 34 Interest income on industrial development revenue bonds 27 27 27 Interest income (b) 13 14 10 Other 7 3 8 (c) Total miscellaneous income $ 74 $ 74 $ 79 Miscellaneous expense: Donations $ 16 $ 15 $ 10 Other 16 15 12 Total miscellaneous expense $ 32 $ 30 $ 22 Ameren Missouri: Miscellaneous income: Allowance for equity funds used during construction $ 23 $ 22 $ 32 Interest income on industrial development revenue bonds 27 27 27 Interest income 1 1 1 Other 1 2 — Total miscellaneous income $ 52 $ 52 $ 60 Miscellaneous expense: Donations $ 4 $ 5 $ 6 Other 6 6 6 Total miscellaneous expense $ 10 $ 11 $ 12 Ameren Illinois: Miscellaneous income: Allowance for equity funds used during construction $ 4 $ 8 $ 2 Interest income (b) 12 12 7 Other 5 1 8 (c) Total miscellaneous income $ 21 $ 21 $ 17 Miscellaneous expense: Donations $ 6 $ 5 $ 4 Other 6 7 4 Total miscellaneous expense $ 12 $ 12 $ 8 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations. (b) Includes Ameren Illinois' interest income on the IEIMA revenue requirement reconciliation adjustment regulatory assets. (c) Includes Ameren Illinois' income earned in 2014 from customer-requested construction.

Derivative Financial Instrument

Derivative Financial Instruments	12 Months Ended
	Dec. 31, 2016
Derivative Instrument Detail [Abstract]
DERIVATIVE FINANCIAL INSTRUMENTS	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 December 31, 2016 and 2015 . As of December 31, 2016 , these contracts extended through October 2019, March 2021, May 2032, and February 2020 for fuel oils, natural gas, power, and uranium, respectively. Quantity (in millions, except as indicated) 2016 2015 Commodity Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils (in gallons) (a) 30 (b) 30 35 (b) 35 Natural gas (in mmbtu) 25 129 154 30 151 181 Power (in megawatthours) 1 9 10 1 10 11 Uranium (pounds in thousands) 345 (b) 345 494 (b) 494 (a) Consists of ultra-low-sulfur diesel products. (b) Not applicable. All contracts considered to be derivative instruments are required to be recorded on the balance sheet at their fair values, unless the NPNS exception applies. See Note 8 – Fair Value Measurements for discussion of our methods of assessing the fair value of derivative instruments. Many of our physical contracts, such as our purchased power contracts, qualify for the NPNS exception to derivative accounting rules. The revenue or expense on NPNS contracts is recognized at the contract price upon physical delivery. If we determine that a contract meets the definition of a derivative and is not eligible for the NPNS exception, we review the contract to determine whether the resulting gains or losses qualify for regulatory deferral. Derivative contracts that qualify for regulatory deferral are recorded at fair value, with changes in fair value recorded as regulatory assets or liabilities in the period in which the change occurs. We believe derivative losses and gains deferred as regulatory assets and liabilities are probable of recovery, or refund, through future rates charged to customers. Regulatory assets and 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 December 31, 2016 and 2015 , all contracts that met the definition of a derivative and were not eligible for the NPNS exception received regulatory deferral. The following table presents the carrying value and balance sheet location of all derivative commodity contracts, none of which were designated as hedging instruments, as of December 31, 2016 and 2015 : Balance Sheet Location Ameren Missouri Ameren Illinois Ameren 2016 Fuel oils Other current assets $ 2 $ — $ 2 Other assets 1 — 1 Natural gas Other current assets 1 11 12 Other assets 1 2 3 Power Other current assets 9 — 9 Total assets (a) $ 14 $ 13 $ 27 Fuel oils Other current liabilities $ 5 $ — $ 5 Natural gas MTM derivative liabilities (b) 3 (b) Other current liabilities 1 — 4 Other deferred credits and liabilities 5 5 10 Power MTM derivative liabilities (b) 12 (b) Other current liabilities 3 — 15 Other deferred credits and liabilities — 173 173 Uranium Other deferred credits and liabilities 4 — 4 Total liabilities (c) $ 18 $ 193 $ 211 2015 Natural gas Other current assets $ — 1 $ 1 Other assets 1 — 1 Power Other current assets 16 — 16 Total assets (a) $ 17 $ 1 $ 18 Fuel oils Other current liabilities $ 22 $ — $ 22 Other deferred credits and liabilities 7 — 7 Natural gas MTM derivative liabilities (b) 32 (b) Other current liabilities 6 — 38 Other deferred credits and liabilities 8 18 26 Power MTM derivative liabilities (b) 13 (b) Other current liabilities — — 13 Other deferred credits and liabilities — 157 157 Uranium Other current liabilities 1 — 1 Total liabilities (c) $ 44 $ 220 $ 264 (a) The cumulative amount of pretax net gains on all derivative instruments is deferred as a regulatory liability. (b) Balance sheet line item not applicable to registrant. (c) The cumulative amount of pretax net losses on all derivative instruments is deferred as a regulatory asset. Derivative instruments are subject to various credit-related losses in the event of nonperformance by counterparties to the transaction. Exchange-traded contracts are supported by the financial and credit quality of the clearing members of the respective exchanges; these contracts 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. As of December 31, 2016 and 2015 , Ameren Missouri's balance sheet reflected $12 million and $11 million , respectively, of cash collateral posted within "Other Assets." As of December 31, 2015 , Ameren Illinois' balance sheet reflected $3 million of cash collateral posted within "Other Assets." 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. 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 Ameren Companies elect to present the fair value amounts of derivative assets and derivative liabilities subject to an enforceable master netting arrangement or similar agreement gross on the balance sheet. However, if the gross amounts recognized on the balance sheet were netted with derivative instruments and cash collateral received or posted, the net amounts would not be materially different from the gross amounts at December 31, 2016 and 2015 . 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 December 31, 2016 , if counterparty groups were to fail completely to perform on contracts, the Ameren Companies' maximum exposure would have been immaterial with or without consideration of the application of master netting arrangements or similar agreements and collateral held. Derivative Instruments with Credit Risk-Related Contingent Features Our commodity contracts contain collateral provisions tied to the Ameren Companies’ credit ratings. If our credit ratings were downgraded, or if a counterparty with reasonable grounds for uncertainty regarding our ability to satisfy an obligation requested adequate assurance of performance, additional collateral postings might be required. The following table presents, as of December 31, 2016 , the aggregate fair value of all derivative instruments with credit risk-related contingent features in a gross liability position, the cash collateral posted, and the aggregate amount of additional collateral that counterparties could require. The additional collateral required is the net liability position allowed under the master netting arrangements or similar agreements, assuming (1) the credit risk-related contingent features underlying these arrangements were triggered on December 31, 2016 , and (2) those counterparties with rights to do so requested collateral. Aggregate Fair Value of Derivative Liabilities (a) Cash Collateral Posted Potential Aggregate Amount of Additional Collateral Required (b) 2016 Ameren Missouri $ 64 $ 3 $ 54 Ameren Illinois 33 — 26 Ameren $ 97 $ 3 $ 80 (a) Before consideration of master netting arrangements or similar agreements and including NPNS and other accrual contract exposures. (b) As collateral requirements with certain counterparties are based on master netting arrangements or similar agreements, the aggregate amount of additional collateral required to be posted is determined after consideration of the effects of such arrangements.

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2016
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	FAIR VALUE MEASUREMENTS Fair value is defined as the price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. We use various methods to determine fair value, including market, income, and cost approaches. With these approaches, we adopt certain assumptions that market participants would use in pricing the asset or liability, including assumptions about market risk or the risks inherent in the inputs to the valuation. Inputs to valuation can be readily observable, market-corroborated, or unobservable. We use valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs. Authoritative accounting guidance established a fair value hierarchy that prioritizes the inputs used to measure fair value. All financial assets and liabilities carried at fair value are classified and disclosed in one of the following three hierarchy levels: Level 1: Inputs based on quoted prices in active markets for identical assets or liabilities. Level 1 assets and liabilities are primarily exchange-traded derivatives and assets, including cash and cash equivalents and listed equity securities, such as those held in Ameren Missouri’s nuclear decommissioning trust fund. The market approach is used to measure the fair value of equity securities held in Ameren Missouri's nuclear decommissioning trust fund. Equity securities in this fund are representative of the S&P 500 index, excluding securities of Ameren Corporation, owners and/or operators of nuclear power plants, and the trustee and investment managers. The S&P 500 index comprises stocks of large-capitalization companies. Level 2 : Market-based inputs corroborated by third-party brokers or exchanges based on transacted market data. Level 2 assets and liabilities include certain assets held in Ameren Missouri’s nuclear decommissioning trust fund, including corporate bonds and other fixed-income securities, United States Treasury and agency securities, and certain over-the-counter derivative instruments, including natural gas and financial power transactions. Fixed income securities are valued by using prices from independent industry-recognized data vendors who provide values that are either exchange-based or matrix-based. The fair value measurements of fixed-income securities classified as Level 2 are based on inputs other than quoted prices that are observable for the asset or liability. Examples are matrix pricing, market corroborated pricing, and inputs such as yield curves and indices. Level 2 fixed income securities in the nuclear decommissioning trust fund are primarily corporate bonds, asset-backed securities, and United States agency bonds. Derivative instruments classified as Level 2 are valued by corroborated observable inputs, such as pricing services or prices from similar instruments that trade in liquid markets. Our development and corroboration process entails obtaining multiple quotes or prices from outside sources. To derive our forward view to price our derivative instruments at fair value, we average the bid/ask spreads to the midpoints. To validate forward prices obtained from outside parties, we compare the pricing to recently settled market transactions. Additionally, a review of all sources is performed to identify any anomalies or potential errors. Further, we consider the volume of transactions on certain trading platforms in our reasonableness assessment of the averaged midpoints. The value of natural gas derivative contracts is based upon exchange closing prices without significant unobservable adjustments. The value of power derivatives contracts is based upon the use of multiple forward prices provided by third parties. The prices are averaged and shaped to a monthly profile when needed without significant unobservable adjustments. Level 3: Unobservable inputs that are not corroborated by market data. Level 3 assets and liabilities are valued by internally developed models and assumptions or methodologies that use significant unobservable inputs. Level 3 assets and liabilities include derivative instruments that trade in less liquid markets, where pricing is largely unobservable. We value Level 3 instruments by using pricing models with inputs that are often unobservable in the market, such as certain internal assumptions, quotes or prices from outside sources not supported by a liquid market, or escalation rates. Our development and corroboration process entails reasonableness reviews and an evaluation of all sources to identify any anomalies or potential errors. We perform an analysis each quarter to determine the appropriate hierarchy level of the assets and liabilities subject to fair value measurements. Financial assets and liabilities are classified in their entirety according to the lowest level of input that is significant to the fair value measurement. All assets and liabilities whose fair value measurement is based on significant unobservable inputs are classified as Level 3. The following table describes the valuation techniques and unobservable inputs utilized by the Ameren Companies for the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy for the periods ended December 31, 2016 and 2015 : Fair Value Weighted Assets Liabilities Valuation Technique(s) Unobservable Input Range Average Level 3 Derivative asset and liability – commodity contracts (a) : 2016 Fuel oils $ 1 $ — Option model Volatilities(%) (b) 24 – 66 28 Discounted cash flow Counterparty credit risk(%) (c)(d) 0.13 – 0.22 0.15 Ameren Missouri credit risk(%) (c)(d) 0.38 (e) Escalation rate(%) (b)(f) (2) – 2 0 Natural Gas 1 (1 ) Option model Volatilities(%) (b) 31 – 66 36 Nodal basis($/mmbtu) (b) (0.40) – (0.10) (0.20) Discounted cash flow Nodal basis($/mmbtu) (b) (0.80) – 0 (0.50) Counterparty credit risk(%) (c)(d) 0.13 – 8 1 Ameren Illinois credit risk(%) (c)(d) 0.38 (e) Power (g) 9 (187 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps($/MWh) (h) 26 – 44 29 Estimated auction price for FTRs($/MW) (b) (71) – 5,270 125 Nodal basis($/MWh) (h) (6) – 0 (2) Ameren Illinois credit risk(%) (c)(d) 0.38 (e) Fundamental energy production model Estimated future natural gas prices($/mmbtu) (b) 3 – 4 3 Escalation rate(%) (b)(i) 5 (e) Contract price allocation Estimated renewable energy credit costs($/credit) (b) 5 – 7 6 Uranium — (4 ) Option model Volatilities(%) (b) 24 (e) Discounted cash flow Average forward uranium pricing($/pound) (b) 22 – 24 22 Ameren Missouri credit risk(%) (c)(d) 0.38 (e) 2015 Natural Gas $ 1 $ (1 ) Option model Volatilities(%) (b) 35 – 55 45 Nodal basis($/mmbtu) (c) (0.30) – 0 (0.20) Discounted cash flow Nodal basis($/mmbtu) (b) (0.10) – 0 (0.10) Counterparty credit risk(%) (c)(d) 0.40 – 12 7 Ameren Missouri credit risk(%) (c)(d) 0.40 (e) Power (g) 16 (170 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps($/MWh) (h) 22 – 39 29 Estimated auction price for FTRs($/MW) (b) (270) – 2,057 211 Nodal basis($/MWh) (h) (10) – (1) (3) Fair Value Weighted Assets Liabilities Valuation Technique(s) Unobservable Input Range Average Counterparty credit risk(%) (c)(d) 0.86 (e) Ameren Illinois credit risk(%) (c)(d) 0.40 (e) Fundamental energy production model Estimated future natural gas prices($/mmbtu) (b) 3 – 4 4 Escalation rate(%) (b)(i) 3 (e) Contract price allocation Estimated renewable energy credit costs($/credit) (b) 5 – 7 6 Uranium — (1 ) Option model Volatilities(%) (b) 20 (e) Discounted cash flow Average forward uranium pricing($/pound) (b) 35 – 42 37 Ameren Missouri credit risk(%) (c)(d) 0.40 (e) (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement. (c) Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement. (d) Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances. (e) Not applicable. (f) Escalation rate applies to fuel oil prices 2019 and beyond. (g) Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2020. Valuations beyond 2020 use fundamentally modeled pricing by month for peak and off-peak demand. (h) The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes because of their opposing positions. (i) Escalation rate applies to power prices in 2031 and beyond for December 31, 2016, and to power prices in 2026 and beyond for December 31, 2015. 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 2016, 2015 or 2014. At December 31, 2016 and 2015 , the counterparty default risk valuation adjustment related to derivative contracts was immaterial for Ameren, Ameren Missouri, and Ameren Illinois. The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of December 31, 2016 : Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Total Assets: Ameren Derivative assets – commodity contracts (a) : Fuel oils $ 2 $ — $ 1 $ 3 Natural gas 2 12 1 15 Power — — 9 9 Total derivative assets – commodity contracts $ 4 $ 12 $ 11 $ 27 Nuclear decommissioning trust fund: Cash and cash equivalents $ 1 $ — $ — $ 1 Equity securities: U.S. large capitalization 408 — — 408 Debt securities: U.S. Treasury and agency securities — 112 — 112 Corporate bonds — 67 — 67 Other — 17 — 17 Total nuclear decommissioning trust fund $ 409 $ 196 $ — $ 605 (b) Total Ameren $ 413 $ 208 $ 11 $ 632 Ameren Missouri Derivative assets – commodity contracts (a) : 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 Fuel oils $ 2 $ — $ 1 $ 3 Natural gas — 1 1 2 Power — — 9 9 Total derivative assets – commodity contracts $ 2 $ 1 $ 11 $ 14 Nuclear decommissioning trust fund: Cash and cash equivalents $ 1 $ — $ — $ 1 Equity securities: U.S. large capitalization 408 — — 408 Debt securities: U.S. Treasury and agency securities — 112 — 112 Corporate bonds — 67 — 67 Other — 17 — 17 Total nuclear decommissioning trust fund $ 409 $ 196 $ — $ 605 (b) Total Ameren Missouri $ 411 $ 197 $ 11 $ 619 Ameren Illinois Derivative assets – commodity contracts (a) : Natural gas $ 2 $ 11 $ — $ 13 Liabilities: Ameren Derivative liabilities – commodity contracts (a) : Fuel oils $ 5 $ — $ — $ 5 Natural gas — 13 1 14 Power — 1 187 188 Uranium — — 4 4 Total Ameren $ 5 $ 14 $ 192 $ 211 Ameren Missouri Derivative liabilities – commodity contracts (a) : Fuel oils $ 5 $ — $ — $ 5 Natural gas — 6 — 6 Power — 1 2 3 Uranium — — 4 4 Total Ameren Missouri $ 5 $ 7 $ 6 $ 18 Ameren Illinois Derivative liabilities – commodity contracts (a) : Natural gas $ — $ 7 $ 1 $ 8 Power — — 185 185 Total Ameren Illinois $ — $ 7 $ 186 $ 193 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Balance excludes $2 million of receivables, payables, and accrued income, net. The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of December 31, 2015 : Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Total Assets: Ameren Derivative assets – commodity contracts (a) : Natural gas $ — $ 1 $ 1 $ 2 Power — — 16 16 Total derivative assets – commodity contracts $ — $ 1 $ 17 $ 18 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 Nuclear decommissioning trust fund: Cash and cash equivalents $ 4 $ — $ — $ 4 Equity securities: U.S. large capitalization 364 — — 364 Debt securities: U.S. Treasury and agency securities — 109 — 109 Corporate bonds — 58 — 58 Other — 22 — 22 Total nuclear decommissioning trust fund $ 368 $ 189 $ — $ 557 (b) Total Ameren $ 368 $ 190 $ 17 $ 575 Ameren Missouri Derivative assets – commodity contracts (a) : Natural gas $ — $ — $ 1 $ 1 Power — — 16 16 Total derivative assets – commodity contracts $ — $ — $ 17 $ 17 Nuclear decommissioning trust fund: Cash and cash equivalents $ 4 $ — $ — $ 4 Equity securities: U.S. large capitalization 364 — — 364 Debt securities: U.S. Treasury and agency securities — 109 — 109 Corporate bonds — 58 — 58 Other — 22 — 22 Total nuclear decommissioning trust fund $ 368 $ 189 $ — $ 557 (b) Total Ameren Missouri $ 368 $ 189 $ 17 $ 574 Ameren Illinois Derivative assets – commodity contracts (a) : Natural gas $ — $ 1 $ — $ 1 Liabilities: Ameren Derivative liabilities – commodity contracts (a) : Fuel oils $ 29 $ — $ — $ 29 Natural gas 1 62 1 64 Power — — 170 170 Uranium — — 1 1 Total Ameren $ 30 $ 62 $ 172 $ 264 Ameren Missouri Derivative liabilities – commodity contracts (a) : Fuel oils $ 29 $ — $ — $ 29 Natural gas — 13 1 14 Uranium — — 1 1 Total Ameren Missouri $ 29 $ 13 $ 2 $ 44 Ameren Illinois Derivative liabilities – commodity contracts (a) : Natural gas $ 1 $ 49 $ — $ 50 Power — — 170 170 Total Ameren Illinois $ 1 $ 49 $ 170 $ 220 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Balance excludes $(1) million of receivables, payables, and accrued income, net. All costs related to financial assets and liabilities, including those classified as Level 3 in the fair value hierarchy are expected to be recoverable through customer rates; therefore, there is no impact to net income resulting from changes in the fair value of these instruments. For the years ended December 31, 2016 and 2015 , the balances and changes in the fair value of Level 3 financial assets and liabilities associated with fuel oils, natural gas, and uranium were immaterial. The following table summarizes the changes in the fair value of power financial assets and liabilities classified as Level 3 in the fair value hierarchy: Net Derivative Commodity Contracts Ameren Missouri Ameren Illinois Ameren For the year ended December 31, 2015 Beginning balance at January 1, 2015 $ 9 $ (142 ) $ (133 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities: 2 (41 ) (39 ) Purchases 29 — 29 Settlements (23 ) 13 (10 ) Transfers out of Level 3 (1 ) — (1 ) Ending balance at December 31, 2015 $ 16 $ (170 ) $ (154 ) Change in unrealized gains (losses) related to assets/liabilities held at December 31, 2015 $ — $ (39 ) $ (39 ) For the year ended December 31, 2016 Beginning balance at January 1, 2016 $ 16 $ (170 ) $ (154 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities: (1 ) (29 ) (30 ) Purchases 13 — 13 Settlements (21 ) 14 (7 ) Ending balance at December 31, 2016 $ 7 $ (185 ) $ (178 ) Change in unrealized gains (losses) related to assets/liabilities held at December 31, 2016 $ — $ (27 ) $ (27 ) Transfers into 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. For the years ended December 31, 2016 and 2015 , there were no material transfers between Level 1 and Level 2, Level 1 and Level 3, or Level 2 and Level 3 related to derivative commodity contracts. See Note 11 – Retirement Benefits for the fair value hierarchy tables detailing Ameren’s pension and postretirement plan assets as of December 31, 2016 , as well as a table summarizing the changes in Level 3 plan assets during 2016 . The Ameren Companies’ carrying amounts of cash and cash equivalents approximate fair value because of the short-term nature of these instruments. They are considered to be Level 1 in the fair value hierarchy. The Ameren Companies' short-term borrowings also approximate fair value because of their short-term nature. Short-term borrowings are considered to be Level 2 in the fair value hierarchy as they are valued based on market rates for similar market transactions. The estimated fair value of long-term debt and preferred stock is based on the quoted market prices for same or similar issuances for companies with similar credit profiles or on the current rates offered to the Ameren Companies for similar financial instruments, which fair value measurement is considered Level 2 in the fair value hierarchy. The following table presents the carrying amounts and estimated fair values of our long-term debt, capital lease obligations, and preferred stock at December 31, 2016 and 2015 : 2016 2015 Carrying Amount Fair Value Carrying Amount Fair Value Ameren: (a) Long-term debt and capital lease obligations (including current portion) $ 7,276 $ 7,772 $ 7,275 $ 7,814 Preferred stock 142 131 142 125 Ameren Missouri: Long-term debt and capital lease obligations (including current portion) $ 3,994 $ 4,304 $ 4,110 $ 4,449 Preferred stock 80 79 80 75 Ameren Illinois: Long-term debt (including current portion) $ 2,588 $ 2,765 $ 2,471 $ 2,665 Preferred stock 62 52 62 50 (a) Preferred stock is recorded in "Noncontrolling Interests" on the consolidated balance sheet.

Nuclear Decommissioning Trust F

Nuclear Decommissioning Trust Fund Investments	12 Months Ended
	Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]
NUCLEAR DECOMMISSIONING TRUST FUND INVESTMENTS	NUCLEAR DECOMMISSIONING TRUST FUND INVESTMENTS Ameren Missouri has investments in debt and equity securities that are held in a trust fund for the purpose of funding the decommissioning of its Callaway energy center. We have classified these investments as available for sale, and we have recorded all such investments at their fair market value at December 31, 2016 , and 2015 . See Note 10 – Callaway Energy Center for additional information. Investments in the nuclear decommissioning trust fund have a target allocation of 60 % to 70 % in equity securities, with the balance invested in debt securities. The following table presents proceeds from the sale and maturities of investments in Ameren Missouri’s nuclear decommissioning trust fund and the gross realized gains and losses resulting from those sales for the years ended December 31, 2016 , 2015 , and 2014 : 2016 2015 2014 Proceeds from sales and maturities $ 377 $ 349 $ 391 Gross realized gains 7 8 7 Gross realized losses 4 2 2 Net realized and unrealized gains and losses are deferred and are currently recorded as a regulatory liability related to AROs on Ameren’s and Ameren Missouri’s balance sheets. This reporting is consistent with the method used to account for the decommissioning costs recovered in rates. Gains or losses associated with assets in the trust fund could result in lower or higher funding requirements for decommissioning costs, which are expected to be reflected in electric rates paid by Ameren Missouri’s customers. See Note 2 – Rate and Regulatory Matters. The following table presents the costs and fair values of investments in debt and equity securities in Ameren's and Ameren Missouri’s nuclear decommissioning trust fund at December 31, 2016 and 2015 : Security Type Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value 2016 Debt securities $ 197 $ 3 $ 4 $ 196 Equity securities 161 253 6 408 Cash 1 — — 1 Other (a) 2 — — 2 Total $ 361 $ 256 $ 10 $ 607 2015 Debt securities $ 191 $ 2 $ 4 $ 189 Equity securities 147 224 7 364 Cash 4 — — 4 Other (a) (1 ) — — (1 ) Total $ 341 $ 226 $ 11 $ 556 (a) Represents net receivables and payables relating to pending security sales, interest, and security purchases. The following table presents the costs and fair values of investments in debt securities in Ameren's and Ameren Missouri’s nuclear decommissioning trust fund according to their contractual maturities at December 31, 2016 : Cost Fair Value Less than 5 years $ 105 $ 104 5 years to 10 years 47 47 Due after 10 years 45 45 Total $ 197 $ 196 We have unrealized losses relating to certain available-for-sale investments included in our nuclear decommissioning trust fund, recorded as a regulatory asset as discussed above. Decommissioning will not occur until our nuclear energy center is retired. The Callaway energy center’s current operating license expires in 2044.

Callaway Energy Center

Callaway Energy Center	12 Months Ended
	Dec. 31, 2016
Nuclear Waste Matters [Abstract]
CALLAWAY ENERGY CENTER	CALLAWAY ENERGY CENTER Under the NWPA, the DOE is responsible for disposing of spent nuclear fuel from the Callaway energy center and other commercial nuclear energy centers. Under the NWPA, Ameren and other utilities that own and operate those energy centers are responsible for paying the disposal costs. The NWPA established the fee that these utilities pay the federal government for disposing of the spent nuclear fuel at one mill, or one-tenth of one cent, for each kilowatthour generated and sold by those plants. The NWPA also requires the DOE to review the nuclear waste fee annually against the cost of the nuclear waste disposal program and to propose to the United States Congress any fee adjustment necessary to offset the costs of the program. As required by the NWPA, Ameren Missouri and other utilities have entered into standard contracts with the DOE. Consistent with the NWPA and its standard contract, which stated that the DOE would begin to dispose of spent nuclear fuel by 1998, 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 was suspended in May 2014. The DOE's delay in carrying out its obligation to dispose of spent nuclear fuel from the Callaway energy center is not expected to adversely affect the continued operations of the energy center. As a result of the DOE's failure to fulfill its contractual obligations, Ameren Missouri and other nuclear energy center owners sued the DOE to recover costs incurred for ongoing storage of their spent fuel. The lawsuit resulted in a settlement agreement that provides for annual reimbursement of additional spent fuel storage and related costs. Ameren Missouri received reimbursements from the DOE of $24 million , $14 million , and $15 million in 2016, 2015, and 2014, respectively. Ameren Missouri will continue to apply for reimbursement from the DOE for allowable costs associated with the ongoing storage of spent fuel. Electric utility rates charged to customers provide for the recovery of the Callaway energy center's decommissioning costs, which include decontamination, dismantling, and site restoration costs, over the expected life of the nuclear energy center. Amounts collected from customers are deposited into the external nuclear decommissioning trust fund to provide for the Callaway energy center’s decommissioning. It is assumed that the Callaway energy center site will be decommissioned through the immediate dismantlement method and removed from service. Ameren and Ameren Missouri have recorded an ARO for the Callaway energy center decommissioning costs at fair value, which represents the present value of estimated future cash outflows. Annual decommissioning costs of $7 million are included in the costs used to establish electric rates for Ameren Missouri's customers. Every three years, the MoPSC requires Ameren Missouri to file an updated cost study and funding analysis for decommissioning its Callaway energy center. In April 2016, the MoPSC approved no change in electric service rates for decommissioning costs. The fair value of the trust fund for Ameren Missouri's Callaway energy center is reported as "Nuclear decommissioning trust fund" in Ameren's and Ameren Missouri's balance sheets. This amount is legally restricted and may be used only to fund the costs of nuclear decommissioning. Changes in the fair value of the trust fund are recorded as an increase or decrease to the nuclear decommissioning trust fund, with an offsetting adjustment to the related regulatory liability. If the assumed return on trust assets is not earned, Ameren Missouri believes that it is probable that any such earnings deficiency will be recovered in rates. See Note 2 – Rate and Regulatory Matters and Note 9 – Nuclear Decommissioning Trust Fund Investments for additional information related to the Callaway energy center.

Retirement Benefits

Retirement Benefits	12 Months Ended
	Dec. 31, 2016
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]
RETIREMENT BENEFITS	RETIREMENT BENEFITS The primary objective of the Ameren pension and postretirement benefit plans is to provide eligible employees with pension and postretirement health care and life insurance benefits. Ameren has defined benefit pension and postretirement benefit plans covering substantially all of its union employees. Ameren has defined benefit pension plans covering substantially all of its non-union employees and postretirement benefit plans covering non-union employees hired before October 2015. Ameren uses a measurement date of December 31 for its pension and postretirement benefit plans. Ameren Missouri and Ameren Illinois each participate in Ameren’s single-employer pension and other postretirement plans. Ameren’s qualified pension plan is the Ameren Retirement Plan. Ameren also has an unfunded nonqualified pension plan, the Ameren Supplemental Retirement Plan, which is available to provide certain management employees and retirees with a supplemental benefit when their qualified pension plan benefits are capped in compliance with Internal Revenue Code limitations. Ameren’s other postretirement plan is the Ameren Retiree Welfare Benefit Plan. Effective December 31, 2016, the applicable assets and liabilities of the Ameren Group Life Insurance Plan were merged with the Ameren Retiree Welfare Benefit Plan. Only Ameren subsidiaries participate in the plans listed above. Ameren’s unfunded obligation under its pension and other postretirement benefit plans was $774 million and $567 million as of December 31, 2016 , and December 31, 2015 , respectively. These net liabilities are recorded in "Other current liabilities," "Pension and other postretirement benefits," and "Other assets" on Ameren's consolidated balance sheet. The primary factor contributing to the increase in the unfunded obligation during 2016 was a 50 basis point decrease in the pension and other postretirement benefit plan discount rates used to determine the present value of the obligation. The increase in the unfunded obligation also resulted in an increase to "Regulatory assets" on Ameren's, Ameren Missouri's, and Ameren Illinois' consolidated balance sheet. The following table presents the net benefit liability recorded on the balance sheets of each of the Ameren Companies as of December 31, 2016 and 2015 : 2016 2015 Ameren (a) $ 774 $ 567 Ameren Missouri 293 236 Ameren Illinois 315 219 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries. Ameren recognizes the underfunded status of its pension and postretirement plans as a liability on its consolidated balance sheet, with offsetting entries to accumulated OCI and regulatory assets. The following table presents the funded status of Ameren's pension and postretirement benefit plans as of December 31, 2016 and 2015 . It also provides the amounts included in regulatory assets and accumulated OCI at December 31, 2016 and 2015 , that have not been recognized in net periodic benefit costs. 2016 2015 Pension Benefits (a) Postretirement Benefits (a) Pension Benefits (a) Postretirement Benefits (a) Accumulated benefit obligation at end of year $ 4,288 $ (b) $ 3,995 $ (b) Change in benefit obligation: Net benefit obligation at beginning of year $ 4,197 $ 1,094 $ 4,410 $ 1,203 Service cost 81 19 92 24 Interest cost 185 50 174 48 Participant contributions — 8 — 8 Actuarial (gain) loss 265 52 (256 ) (133 ) Settlement — — (2 ) — Benefits paid (210 ) (54 ) (221 ) (56 ) Federal subsidy on benefits paid (b) 1 (b) — Net benefit obligation at end of year 4,518 1,170 4,197 1,094 Change in plan assets: Fair value of plan assets at beginning of year 3,653 1,071 3,794 1,109 Actual return on plan assets 313 73 (29 ) (8 ) Employer contributions 57 2 111 18 Federal subsidy on benefits paid (b) 1 (b) — Participant contributions — 8 — 8 Settlements — — (2 ) — Benefits paid (210 ) (54 ) (221 ) (56 ) Fair value of plan assets at end of year 3,813 1,101 3,653 1,071 Funded status – deficiency 705 69 544 23 Accrued benefit cost at December 31 $ 705 $ 69 $ 544 $ 23 Amounts recognized in the balance sheet consist of: Noncurrent asset (c) $ — $ — $ — $ (18 ) Current liability (d) 3 2 3 2 Noncurrent liability 702 67 541 39 Net liability recognized $ 705 $ 69 $ 544 $ 23 Amounts recognized in regulatory assets consist of: Net actuarial (gain) loss $ 535 $ (29 ) $ 395 $ (82 ) Prior service cost (credit) (4 ) (8 ) (5 ) (11 ) Amounts (pretax) recognized in accumulated OCI consist of: Net actuarial (gain) loss 43 — 17 (3 ) Prior service cost (credit) — (1 ) — — Total $ 574 $ (38 ) $ 407 $ (96 ) (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries. (b) Not applicable. (c) Included in "Other assets" on Ameren's consolidated balance sheet. (d) Included in "Other current liabilities" on Ameren's consolidated balance sheet. The following table presents the assumptions used to determine our benefit obligations at December 31, 2016 and 2015 : Pension Benefits Postretirement Benefits 2016 2015 2016 2015 Discount rate at measurement date 4.00 % 4.50 % 4.00 % 4.50 % Increase in future compensation 3.50 3.50 3.50 3.50 Medical cost trend rate (initial) (a) (a) 5.00 5.00 Medical cost trend rate (ultimate) (a) (a) 5.00 5.00 (a) Not applicable Ameren determines discount rate assumptions by identifying a theoretical settlement portfolio of high-quality corporate bonds sufficient to provide for a plan's projected benefit payments. The settlement portfolio of bonds is selected from a pool of more than 700 high-quality corporate bonds. A single discount rate is then determined; that rate results in a discounted value of the plan's benefit payments that equates to the market value of the selected bonds. In addition, during 2016, Ameren adopted the Society of Actuaries 2016 Mortality Tables Report and Mortality Improvement Scale. The updated mortality tables assume a lower rate of mortality improvement as compared to the 2015 Mortality Tables Report and Mortality Improvement Scale that Ameren adopted in 2015. The 2016 tables lowered projected improvements in life expectancies for our employees and retirees, resulting in a decrease to our pension and other postretirement benefit obligations. Funding Pension benefits are based on the employees’ years of service, age, and compensation. Ameren’s pension plans are funded in compliance with income tax regulations, federal funding, and other regulatory requirements. As a result, Ameren expects to fund its pension plan at a level equal to the greater of the pension cost or the legally required minimum contribution. Considering its assumptions at December 31, 2016 , its investment performance in 2016, and its pension funding policy, Ameren expects to make annual contributions of $50 million to $70 million in each of the next five years, with aggregate estimated contributions of $290 million . We expect Ameren Missouri’s and Ameren Illinois’ portion of the future funding requirements to be 35% and 55% , respectively. These amounts are estimates. They may change based on actual investment performance, changes in interest rates, changes in our assumptions, changes in government regulations, and any voluntary contributions. Our funding policy for postretirement benefits is primarily to fund the Voluntary Employee Beneficiary Association (VEBA) trusts to match the annual postretirement expense. The following table presents the cash contributions made to our defined benefit retirement plan and to our postretirement plans during 2016 , 2015 , and 2014 : Pension Benefits Postretirement Benefits 2016 2015 2014 2016 2015 2014 Ameren Missouri $ 21 $ 47 $ 41 $ 1 $ 8 $ 3 Ameren Illinois 30 45 39 1 8 2 Other 6 19 19 — 2 1 Ameren 57 111 99 2 18 6 Investment Strategy and Policies Ameren manages plan assets in accordance with the “prudent investor” guidelines contained in ERISA. The investment committee, which includes members of senior management, approves and implements investment strategy and asset allocation guidelines for the plan assets. The investment committee’s goals are twofold: first, to ensure that sufficient funds are available to provide the benefits at the time they are payable; and second, to maximize total return on plan assets and to minimize expense volatility consistent with its tolerance for risk. Ameren delegates the task of investment management to specialists in each asset class. As appropriate, Ameren provides each investment manager with guidelines that specify allowable and prohibited investment types. The investment committee regularly monitors manager performance and compliance with investment guidelines. The expected return on plan assets assumption is based on historical and projected rates of return for current and planned asset classes in the investment portfolio. Projected rates of return for each asset class were estimated after an analysis of historical experience, future expectations, and the volatility of the various asset classes. After considering the target asset allocation for each asset class, we adjusted the overall expected rate of return for the portfolio for historical and expected experience of active portfolio management results compared with benchmark returns and for the effect of expenses paid from plan assets. Ameren will use an expected return on plan assets for its pension and postretirement plan assets of 7.00% in 2017. No plan assets are expected to be returned to Ameren during 2017. Ameren’s investment committee strives to assemble a portfolio of diversified assets that does not create a significant concentration of risks. The investment committee develops asset allocation guidelines between asset classes, and it creates diversification through investments in assets that differ by type (equity, debt, real estate, private equity), duration, market capitalization, country, style (growth or value), and industry, among other factors. The diversification of assets is displayed in the target allocation table below. The investment committee also routinely rebalances the plan assets to adhere to the diversification goals. The investment committee’s strategy reduces the concentration of investment risk; however, Ameren is still subject to overall market risk. The following table presents our target allocations for 2017 and our pension and postretirement plans’ asset categories as of December 31, 2016 and 2015 : Asset Category Target Allocation 2017 Percentage of Plan Assets at December 31, 2016 2015 Pension Plan: Cash and cash equivalents 0% – 5% 1 % 1 % Equity securities: U.S. large-capitalization 29% – 39% 34 % 34 % U.S. small- and mid-capitalization 3% – 13% 9 % 7 % International and emerging markets 9% – 19% 14 % 13 % Total equity 51% – 61% 57 % 54 % Debt securities 35% – 45% 37 % 40 % Real estate 0% – 9% 5 % 5 % Private equity 0% – 5% (a) (a) Total 100 % 100 % Postretirement Plans: Cash and cash equivalents 0% – 7% 3 % 4 % Equity securities: U.S. large-capitalization 34% – 44% 40 % 39 % U.S. small- and mid-capitalization 2% – 12% 7 % 7 % International 9% – 19% 14 % 13 % Total equity 55% – 65% 61 % 59 % Debt securities 33% – 43% 36 % 37 % Total 100 % 100 % (a) Less than 1% of plan assets. In general, the United States large-capitalization equity investments are passively managed or indexed, whereas the international, emerging markets, United States small-capitalization, and United States mid-capitalization equity investments are actively managed by investment managers. Debt securities include a broad range of fixed-income vehicles. Debt security investments in high-yield securities, emerging market securities, and non-United-States-dollar-denominated securities are owned by the plans, but in limited quantities to reduce risk. Most of the debt security investments are under active management by investment managers. Real estate investments include private real estate vehicles; however, Ameren does not, by policy, hold direct investments in real estate property. Additionally, Ameren’s investment committee allows investment managers to use derivatives, such as index futures, exchange traded funds, foreign exchange futures, and options, in certain situations, to increase or to reduce market exposure in an efficient and timely manner. Fair Value Measurements of Plan Assets Investments in the pension and postretirement benefit plans were stated at fair value as of December 31, 2016 . The fair value of an asset is the amount that would be received upon its sale in an orderly transaction between market participants at the measurement date. Cash and cash equivalents have initial maturities of three months or less and are recorded at cost plus accrued interest. The carrying amounts of cash and cash equivalents approximate fair value because of the short-term nature of these instruments. Investments traded in active markets on national or international securities exchanges are valued at closing prices on the last business day on or before the measurement date. Securities traded in over-the-counter markets are valued based on quoted market prices, broker or dealer quotations, or alternative pricing sources with reasonable levels of price transparency. Investments measured under NAV as a practical expedient are based on the fair values of the underlying assets provided by the funds and their administrators. The fair value of real estate investments are based on NAV determined by annual appraisal reports prepared by an independent real estate appraiser. Investments measured at NAV often provide for daily, monthly, or quarterly redemptions with 60 or less days of notice depending on the fund. For some funds, redemption may also require approval from the fund's board of directors. Derivative contracts are valued at fair value, as determined by the investment managers (or independent third parties on behalf of the investment managers), who use proprietary models and take into consideration exchange quotations on underlying instruments, dealer quotations, and other market information. The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the pension plan assets measured at fair value as of December 31, 2016 : Quoted Prices in Active Markets for Identified Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Measured at NAV (a) Total Cash and cash equivalents $ — $ — $ — $ 33 $ 33 Equity securities: U.S. large-capitalization — — — 1,352 1,352 U.S. small- and mid-capitalization 361 — — — 361 International and emerging markets 133 — — 389 522 Debt securities: Corporate bonds — 617 — 13 630 Municipal bonds — 95 — — 95 U.S. Treasury and agency securities — 701 — — 701 Other — 21 — — 21 Real estate — — — 202 202 Private equity — — — 6 6 Total $ 494 $ 1,434 $ — $ 1,995 $ 3,923 Less: Medical benefit assets at December 31 (b) (132 ) Plus: Net receivables at December 31 (c) 22 Fair value of pension plans assets at year end $ 3,813 (a) Reflects the adoption of the new authoritative accounting guidance related to investments measured at the NAV practical expedient. See Note 1 - Summary of Significant Accounting Policies for additional information. (b) Medical benefit (health and welfare) component for accounts maintained in accordance with Section 401(h) of the Internal Revenue Code to fund a portion of the postretirement obligation. (c) Receivables related to pending security sales, offset by payables related to pending security purchases. The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the pension plan assets measured at fair value as of December 31, 2015 : Quoted Prices in Active Markets for Identified Assets or Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Measured at NAV (a) Total Cash and cash equivalents $ — $ — $ — $ 20 $ 20 Equity securities: U.S. large-capitalization — — — 1,296 1,296 U.S. small- and mid-capitalization 268 — — — 268 International and emerging markets 122 126 — 243 491 Debt securities: Corporate bonds — 617 — 14 631 Municipal bonds — 104 — — 104 U.S. Treasury and agency securities 6 751 — — 757 Other — 5 — — 5 Real estate — — — 168 168 Private equity — — — 8 8 Total $ 396 $ 1,603 $ — $ 1,749 $ 3,748 Less: Medical benefit assets at December 31 (b) (123 ) Plus: Net receivables at December 31 (c) 28 Fair value of pension plans assets at year end $ 3,653 (a) Reflects the adoption of the new authoritative accounting guidance related to investments measured at the NAV practical expedient. See Note 1 - Summary of Significant Accounting Policies for additional information. (b) Medical benefit (health and welfare) component for accounts maintained in accordance with Section 401(h) of the Internal Revenue Code to fund a portion of the postretirement obligation. (c) Receivables related to pending security sales, offset by payables related to pending security purchases. The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the postretirement benefit plans assets measured at fair value as of December 31, 2016 : Quoted Prices in Active Markets for Identified Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Measured at NAV (a) Total Cash and cash equivalents $ 53 $ — $ — $ — $ 53 Equity securities: U.S. large-capitalization 291 — — 101 392 U.S. small- and mid-capitalization 72 — — — 72 International 40 — — 92 132 Other — 7 — — 7 Debt securities: Corporate bonds — 141 — — 141 Municipal bonds — 110 — — 110 U.S. Treasury and agency securities — 68 — — 68 Other — — — 19 19 Total $ 456 $ 326 $ — $ 212 $ 994 Plus: Medical benefit assets at December 31 (b) 132 Less: Net payables at December 31 (c) (25 ) Fair value of postretirement benefit plans assets at year end $ 1,101 (a) Reflects the adoption of the new authoritative accounting guidance related to investments measured at the NAV practical expedient. See Note 1 - Summary of Significant Accounting Policies for additional information. (b) Medical benefit (health and welfare) component for 401(h) accounts to fund a portion of the postretirement obligation. These 401(h) assets are included in the pension plan assets shown above. (c) Payables related to pending security purchases, offset by interest receivables and receivables related to pending security sales. The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the postretirement benefit plans assets measured at fair value as of December 31, 2015 : Quoted Prices in Active Markets for Identified Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Measured at NAV (a) Total Cash and cash equivalents $ 61 $ — $ — $ — $ 61 Equity securities: U.S. large-capitalization 272 — — 98 370 U.S. small- and mid-capitalization 65 — — — 65 International 33 38 — 55 126 Other — 7 — — 7 Debt securities: Corporate bonds — 138 — — 138 Municipal bonds — 114 — — 114 U.S. Treasury and agency securities — 55 — — 55 Other — 4 — 36 40 Total $ 431 $ 356 $ — $ 189 $ 976 Plus: Medical benefit assets at December 31 (a) 123 Less: Net payables at December 31 (b) (28 ) Fair value of postretirement benefit plans assets at year end $ 1,071 (a) Reflects the adoption of the new authoritative accounting guidance related to investments measured at the NAV practical expedient. See Note 1 - Summary of Significant Accounting Policies for additional information. (b) Medical benefit (health and welfare) component for 401(h) accounts to fund a portion of the postretirement obligation. These 401(h) assets are included in the pension plan assets shown above. (c) Payables related to pending security purchases, offset by Medicare, interest receivables, and receivables related to pending security sales. Net Periodic Benefit Cost The following table presents the components of the net periodic benefit cost of Ameren's pension and postretirement benefit plans during 2016 , 2015 , and 2014 : Pension Benefits Postretirement Benefits 2016 Service cost $ 81 $ 19 Interest cost 185 50 Expected return on plan assets (253 ) (72 ) Amortization of: Prior service credit (1 ) (5 ) Actuarial (gain) loss 32 (11 ) Net periodic benefit cost (benefit) $ 44 $ (19 ) 2015 Service cost $ 92 $ 24 Interest cost 174 48 Expected return on plan assets (248 ) (68 ) Amortization of: Prior service credit (1 ) (5 ) Actuarial loss 74 5 Settlement loss 1 — Net periodic benefit cost (benefit) $ 92 $ 4 2014 Service cost $ 79 $ 19 Interest cost 183 50 Expected return on plan assets (229 ) (65 ) Amortization of: Prior service credit (1 ) (5 ) Actuarial (gain) loss 49 (7 ) Net periodic benefit cost (benefit) $ 81 $ (8 ) The estimated amounts that will be amortized from regulatory assets and accumulated OCI into Ameren's net periodic benefit cost in 2017 are as follows: Pension Benefits (a) Postretirement Benefits (a) Regulatory assets: Prior service credit $ (1 ) $ (5 ) Net actuarial (gain) loss 50 (7 ) Accumulated OCI: Net actuarial loss 4 — Total $ 53 $ (12 ) (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries. Prior service cost is amortized on a straight-line basis over the average future service of active participants benefiting under the plan amendment. Net actuarial gains or losses subject to amortization are amortized on a straight-line basis over 10 years. The Ameren Companies are responsible for their share of the pension and postretirement benefit costs. The following table presents the pension costs and the postretirement benefit costs incurred and included in continuing operations for the years ended December 31, 2016 , 2015 , and 2014 : Pension Costs Postretirement Costs 2016 2015 2014 2016 2015 2014 Ameren Missouri (a) $ 26 $ 54 $ 50 $ (5 ) $ 8 $ 3 Ameren Illinois 22 38 30 (13 ) (3 ) (9 ) Other (4 ) — 1 (1 ) (1 ) (2 ) Ameren 44 92 81 (19 ) 4 (8 ) (a) Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. The expected pension and postretirement benefit payments from qualified trust and company funds, which reflect expected future service, as of December 31, 2016 , are as follows: Pension Benefits Postretirement Benefits Paid from Qualified Trust Funds Paid from Company Funds Paid from Qualified Trust Funds Paid from Company Funds 2017 $ 248 $ 3 $ 54 $ 2 2018 254 3 57 2 2019 261 3 59 2 2020 265 3 61 2 2021 273 3 63 2 2022 – 2026 1,405 13 331 12 The following table presents the assumptions used to determine net periodic benefit cost for our pension and postretirement benefit plans for the years ended December 31, 2016 , 2015 , and 2014 : Pension Benefits Postretirement Benefits 2016 2015 2014 2016 2015 2014 Discount rate at measurement date 4.50 % 4.00 % 4.75 % 4.50 % 4.00 % 4.75 % Expected return on plan assets 7.00 7.25 7.25 7.00 7.00 7.00 Increase in future compensation 3.50 3.50 3.50 3.50 3.50 3.50 Medical cost trend rate (initial) (a) (a) (a) 5.00 5.00 5.00 Medical cost trend rate (ultimate) (a) (a) (a) 5.00 5.00 5.00 (a) Not applicable The table below reflects the sensitivity of Ameren’s plans to potential changes in key assumptions: Pension Benefits Postretirement Benefits Service Cost and Interest Cost Projected Benefit Obligation Service Cost and Interest Cost Postretirement Benefit Obligation 0.25% decrease in discount rate $ (1 ) $ 142 $ — $ 38 0.25% increase in salary scale 2 16 — — 1.00% increase in annual medical trend — — 3 54 1.00% decrease in annual medical trend — — (3 ) (54 ) Other Ameren sponsors a 401(k) plan for eligible employees. The Ameren 401(k) plan covered all eligible employees at December 31, 2016 . The plan allows employees to contribute a portion of their compensation in accordance with specific guidelines. Ameren matches a percentage of the employee contributions up to certain limits. The following table presents the portion of the matching contribution to the Ameren 401(k) plan attributable to the continuing operations for each of the Ameren Companies for the years ended December 31, 2016 , 2015 , and 2014 : 2016 2015 2014 Ameren Missouri $ 16 $ 16 $ 16 Ameren Illinois 12 12 11 Other 1 1 1 Ameren 29 29 28

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
STOCK-BASED COMPENSATION	STOCK-BASED COMPENSATION The 2014 Incentive Plan is Ameren’s long-term stock compensation plan for eligible employees and directors. The 2006 Incentive Plan was replaced prospectively for new grants beginning in April 2014. The 2014 Incentive Plan provides for a maximum of 8 million common shares to be available for grant to eligible employees and directors. At December 31, 2016, there were 5.8 million common shares remaining for grant under the 2014 Incentive Plan. The 2014 Incentive Plan awards may be stock options, stock appreciation rights, restricted stock, restricted stock units, performance shares, performance share units, cash-based awards, and other stock-based awards. Performance Share Units A share unit vests and entitles an employee to receive shares of Ameren common stock (plus accumulated dividends) if, at the end of the three -year performance period, certain specified performance or market conditions have been met and if the individual remains employed by Ameren through the required vesting period. The exact number of shares issued pursuant to a share unit varies from 0% to 200% of the target award, depending on actual company performance relative to the performance goals. The vesting period for share units awarded in 2015 and 2016 extended beyond the three-year performance period to the payout date, while the vesting period for share units awarded in 2014 matched the three -year performance period and vested on December 31, 2016. A summary of nonvested performance share units at December 31, 2016 , and changes during the year ended December 31, 2016 , under the 2006 Incentive Plan and the 2014 Incentive Plan are presented below: Performance Share Units Share Units Weighted-average Fair Value per Share Unit Nonvested at January 1, 2016 1,024,870 $ 46.08 Granted (a) 588,615 44.13 Forfeitures (15,949 ) 45.07 Earned and vested (b) (537,897 ) 40.12 Nonvested at December 31, 2016 1,059,639 $ 48.04 (a) Includes performance share units (share units) granted to certain executive and nonexecutive officers and other eligible employees in 2016 under the 2014 Incentive Plan. (b) Includes share units granted in 2014 that vested as of December 31, 2016 and were earned pursuant to the terms of the award grants. Also includes share units that vested due to 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 following table presents the stock-based compensation expense for the years ended December 31, 2016, 2015 and 2014: 2016 2015 2014 Ameren Missouri $ 4 $ 5 $ 5 Ameren Illinois 2 3 2 Other (a) 11 11 12 Ameren 17 19 19 Less income tax benefit 6 7 7 Stock-based compensation expense, net $ 11 $ 12 $ 12 (a) Represents compensation expense of employees of Ameren Services. These amounts are not included in the Ameren Missouri and Ameren Illinois amounts above. Ameren settled performance share units of $83 million , $27 million , and $33 million for the years ended December 31, 2016 , 2015 , and 2014 . There were no significant compensation costs capitalized related to the performance share units during the years ended December 31, 2016 , 2015 , and 2014 . As of December 31, 2016 , total compensation cost of $25 million related to nonvested awards not yet recognized is expected to be recognized over a weighted-average period of 22 months . The fair value of each share unit awarded in 2016 under the 2014 Incentive Plan was determined to be $44.13 , which was based on Ameren's closing common share price of $43.23 at December 31, 2015 , and lattice simulations. Lattice simulations are used to estimate expected share payout based on Ameren's total shareholder return for a three -year performance period relative to the designated peer group beginning January 1, 2016 . The simulations can produce a greater fair value for the share unit than the applicable closing common share price because they include the weighted payout scenarios in which an increase in the share price has occurred. The significant assumptions used to calculate fair value also included a three-year risk-free rate of 1.31% , volatility of 15% to 20% for the peer group, and Ameren's attainment of a three-year average earnings per share threshold during the performance period. The fair value of each 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. The lattice simulations reflected the three-year performance period relative to the designated peer group beginning January 1, 2015. 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. The fair value of each share unit awarded in 2014, excluding the grants issued in April 2014 for certain executive officers, under the 2006 Incentive Plan and the 2014 Incentive Plan was determined to be $38.90 , which was based on Ameren’s closing common share price of $36.16 at December 31, 2013, and lattice simulations. The lattice simulations reflected the three-year performance period relative to the designated peer group beginning January 1, 2014. The significant assumptions used to calculate fair value also included a three-year risk-free rate of 0.78% , volatility of 12% to 18% for the peer group, and Ameren’s attainment of a three-year average earnings per share threshold during the performance period.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2016
Income Tax Disclosure [Abstract]
INCOME TAXES	INCOME TAXES The following table presents the principal reasons for the difference between the effective income tax rate and the statutory federal income tax rate for the years ended December 31, 2016 , 2015 , and 2014 : Ameren Missouri Ameren Illinois Ameren 2016 Statutory federal income tax rate: 35 % 35 % 35 % Increases (decreases) from: Depreciation differences 1 — — Amortization of deferred investment tax credit (1 ) — — State tax 3 5 4 Stock-based compensation (a) — — (2 ) Valuation allowance — — 1 Other permanent items — (2 ) (1 ) Effective income tax rate 38 % 38 % 37 % 2015 Statutory federal income tax rate: 35 % 35 % 35 % Increases (decreases) from: Depreciation differences — (2 ) (1 ) Amortization of deferred investment tax credit (1 ) — (1 ) State tax 3 5 5 Other permanent items — (1 ) — Effective income tax rate 37 % 37 % 38 % 2014 Statutory federal income tax rate: 35 % 35 % 35 % Increases (decreases) from: Amortization of deferred investment tax credit (1 ) — (1 ) State tax 3 6 4 Other permanent items — — 1 Effective income tax rate 37 % 41 % 39 % (a) Reflects the adoption of new authoritative accounting guidance related to share-based compensation. See Note 1 – Summary of Significant Accounting Policies for more information. The following table presents the components of income tax expense (benefit) for the years ended December 31, 2016 , 2015 , and 2014 : Ameren Missouri Ameren Illinois Other Ameren 2016 Current taxes: Federal $ 31 $ (8 ) $ (24 ) $ (1 ) State 6 12 (21 ) (3 ) Deferred taxes: Federal 161 117 21 299 State 23 37 32 92 Amortization of deferred investment tax credits (5 ) — — (5 ) Total income tax expense $ 216 $ 158 $ 8 $ 382 2015 Current taxes: Federal $ 110 $ (83 ) $ (29 ) $ (2 ) State 17 (11 ) (10 ) (4 ) Deferred taxes: Federal 71 193 35 299 State 16 29 31 76 Amortization of deferred investment tax credits (5 ) (1 ) — (6 ) Total income tax expense $ 209 $ 127 $ 27 $ 363 2014 Current taxes: Federal $ (13 ) $ (51 ) $ 27 $ (37 ) State (3 ) (2 ) (32 ) (37 ) Deferred taxes: Federal 222 159 (12 ) 369 State 28 38 22 88 Amortization of deferred investment tax credits (5 ) (1 ) — (6 ) Total income tax expense (benefit) $ 229 $ 143 $ 5 $ 377 The Illinois corporate income tax rate was 9.5% in 2014. The tax rate decreased to 7.75% on January 1, 2015, and is scheduled to decrease to 7.3% on January 1, 2025. The following table presents the deferred tax assets and deferred tax liabilities recorded as a result of temporary differences at December 31, 2016 and 2015 : Ameren Missouri Ameren Illinois Other Ameren 2016 Accumulated deferred income taxes, net liability (asset): Plant related $ 3,103 $ 1,769 $ 147 $ 5,019 Regulatory assets, net 75 (1 ) — 74 Deferred employee benefit costs (76 ) (38 ) (97 ) (211 ) Revenue requirement reconciliation adjustments — 34 — 34 Tax carryforwards (66 ) (138 ) (472 ) (676 ) Other (23 ) 5 42 24 Total net accumulated deferred income tax liabilities (assets) $ 3,013 $ 1,631 $ (380 ) $ 4,264 2015 Accumulated deferred income taxes, net liability (asset): Plant related $ 2,931 $ 1,587 $ 37 $ 4,555 Regulatory assets, net 81 (1 ) — 80 Deferred employee benefit costs (76 ) (40 ) (91 ) (207 ) Revenue requirement reconciliation adjustments — 66 — 66 Tax carryforwards (65 ) (133 ) (405 ) (603 ) Other (27 ) 1 20 (6 ) Total net accumulated deferred income tax liabilities (assets) $ 2,844 $ 1,480 $ (439 ) $ 3,885 The following table presents the components of deferred tax assets relating to net operating loss carryforwards, tax credit carryforwards, and charitable contribution carryforwards at December 31, 2016 and 2015 : Ameren Missouri Ameren Illinois Other Ameren 2016 Net operating loss carryforwards: Federal (a) $ 33 $ 137 $ 324 $ 494 State (a) 4 — 41 45 Total net operating loss carryforwards $ 37 $ 137 $ 365 $ 539 Tax credit carryforwards: Federal (a) $ 29 $ 1 $ 79 $ 109 State (b) — — 21 21 Total tax credit carryforwards $ 29 $ 1 $ 100 $ 130 Charitable contribution carryforwards (b) $ — $ — $ 18 $ 18 Valuation allowance (c) — — (11 ) (11 ) Total charitable contribution carryforwards $ — $ — $ 7 $ 7 2015 Net operating loss carryforwards: Federal $ 35 $ 127 $ 245 $ 407 State 4 4 38 46 Total net operating loss carryforwards $ 39 $ 131 $ 283 $ 453 Tax credit carryforwards: Federal $ 26 $ 1 $ 78 $ 105 State — 1 40 41 State valuation allowance — — (2 ) (2 ) Total tax credit carryforwards $ 26 $ 2 $ 116 $ 144 Charitable contribution carryforwards $ — $ — $ 10 $ 10 Valuation allowance — — (4 ) (4 ) Total charitable contribution carryforwards $ — $ — $ 6 $ 6 (a) Will expire between 2029 and 2036 . (b) Will expire between 2017 and 2021 . (c) See Schedule II under Part IV, Item 15, in this report for information on changes in the valuation allowance. Uncertain Tax Positions As of December 31, 2016 and 2015, the Ameren Companies did not record any uncertain tax positions. The settlements discussed below resolved previously recorded uncertain tax positions. In 2015, final settlements for tax years 2012 and 2013 were reached with the IRS. The 2015 settlement of the 2013 tax year impacted discontinued operations. See Note 1 – Summary of Significant Accounting Policies for additional information. In 2014, final settlements for tax years 2007 through 2011 were reached with the IRS. These settlements, which resolved the uncertain tax positions associated with the timing of research tax deductions for these years, resulted in a decrease in Ameren’s and Ameren Missouri’s unrecognized tax benefits of $20 million , and $13 million , respectively. In addition, the settlement for tax years 2007 through 2011 provided certainty for the previously uncertain tax positions associated with the timing of research tax deductions for the remaining open tax years of 2012, 2013, and 2014. The certainty provided from the settlement resulted in an $18 million decrease in both Ameren’s and Ameren Missouri’s unrecognized tax benefits. The settlement also resulted in a $2 million increase to Ameren’s state unrecognized tax benefits. The net reduction in unrecognized tax benefits in 2014 did not materially affect income tax expense for the Ameren Companies. State income tax returns are generally subject to examination for a period of three years after filing. 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. The Ameren Companies currently do not have material state income tax issues under examination, administrative appeals, or litigation. Ameren Missouri has an uncertain tax position tracker. Under Missouri's regulatory framework, uncertain tax positions do not reduce Ameren Missouri's electric rate base. When an uncertain income tax position liability is resolved, the MoPSC requires, through the uncertain tax position tracker, the creation of a regulatory asset or regulatory liability to reflect the time value, using the weighted-average cost of capital included in each of the electric rate orders in effect before the tax position was resolved, of the difference between the uncertain tax position liability that was excluded from rate base and the final tax liability. The resulting regulatory asset or liability will affect earnings in the year it is created and then will be amortized over three years, beginning on the effective date of new rates established in the next electric rate case.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2016
Related Party Transactions [Abstract]
RELATED PARTY TRANSACTIONS	In the normal course of business, the Ameren Companies have engaged in, and may in the future engage in, affiliate transactions. These transactions primarily consist of natural gas and power purchases and sales, services received or rendered, and borrowings and lendings. Transactions between affiliates are reported as intercompany transactions on their financial statements, but are eliminated in consolidation for Ameren’s financial statements. Below are the material related party agreements. Electric Power Supply Agreements Ameren Illinois must acquire capacity and energy sufficient to meet its obligations to customers. Ameren Illinois uses periodic RFP processes, administered by the IPA and approved by the ICC, to contract capacity and energy on behalf of its customers. Ameren Missouri participates in the RFP process and has been a winning supplier for certain periods. Capacity Supply Agreements In a procurement event in 2012, Ameren Missouri contracted to supply a portion of Ameren Illinois' capacity requirements for $1 million and $3 million for the 12 months ended May 31, 2014, and 2015, respectively. In a procurement event in 2015, Ameren Missouri contracted to supply a portion of Ameren Illinois' capacity requirements for $15 million for the 12 months ending May 31, 2017. Energy Swaps and Energy Products Based on the outcome of IPA administered procurement events, Ameren Missouri and Ameren Illinois have entered into energy product agreements by which Ameren Missouri agreed to sell, and Ameren Illinois agreed to purchase, a set amount of megawatthours at a predetermined price over a specified period of time. The following table presents the agreements the companies have entered into, as well as the specified time period, price, and amount of megawatthours included in each agreement: IPA Procurement Event Time Period MWh Average Price per MWh May 2014 January 2015 – February 2017 168,400 $ 51 April 2015 June 2015 – June 2017 667,000 36 September 2015 November 2015 – May 2018 339,000 38 April 2016 June 2017 – September 2018 375,200 35 September 2016 May 2017 – September 2018 82,800 34 Collateral Postings Under the terms of the Illinois energy product agreements entered into through RFP processes administered by the IPA, suppliers must post collateral under certain market conditions to protect Ameren Illinois in the event of nonperformance. The collateral postings are unilateral, which means that only the suppliers can be required to post collateral. Therefore, Ameren Missouri, as a winning supplier in the RFP process, may be required to post collateral. As of December 31, 2016 and 2015 , there were no collateral postings required of Ameren Missouri related to the Illinois energy product agreements. Interconnection and Transmission Agreements Ameren Missouri and Ameren Illinois are parties to an interconnection agreement for the use of their respective transmission lines and other facilities for the distribution of power. These agreements have no contractual expiration date, but may be terminated by either party with three years’ notice. Support Services Agreements Ameren Services provides support services to its affiliates. The costs of support services, including wages, employee benefits, professional services, and other expenses, are based on, or are an allocation of, actual costs incurred. The support services agreement can be terminated at any time by the mutual agreement of Ameren Services and that affiliate or by either party with 60 days' notice before the end of a calendar year. In addition, Ameren Missouri and Ameren Illinois provide affiliates, primarily Ameren Services, with access to their facilities for administrative purposes. The costs of the rent and facility services are based on, or are an allocation of, actual costs incurred. Separately, Ameren Missouri and Ameren Illinois provide storm-related and miscellaneous support services to each other on an as-needed basis. Transmission Services Ameren Illinois receives transmission services from ATXI for its retail load in the AMIL pricing zone. Money Pool See Note 4 – Short-term Debt and Liquidity and Note 5 – Long-term Debt and Equity Financings for a discussion of affiliate borrowing arrangements. Tax Allocation Agreement See Note 1 – Summary of Significant Accounting Policies for a discussion of the tax allocation agreement and the related capital contributions and return of capital. The following table presents the impact on Ameren Missouri and Ameren Illinois of related party transactions for the years ended December 31, 2016 , 2015 , and 2014 . It is based primarily on the agreements discussed above and the money pool arrangements discussed in Note 4 – Short-term Debt and Liquidity. Agreement Income Statement Line Item Ameren Missouri Ameren Illinois Ameren Missouri power supply agreements Operating Revenues 2016 $ 28 $ (a) with Ameren Illinois 2015 15 (a) 2014 5 (a) Ameren Missouri and Ameren Illinois Operating Revenues 2016 25 5 rent and facility services 2015 25 4 2014 21 2 Ameren Missouri and Ameren Illinois Operating Revenues 2016 1 (b) miscellaneous support services 2015 2 (b) 2014 1 (b) Total Operating Revenues 2016 $ 54 $ 5 2015 42 4 2014 27 2 Ameren Illinois power supply Purchased Power 2016 $ (a) $ 28 agreements with Ameren Missouri 2015 (a) 15 2014 (a) 5 Ameren Illinois transmission Purchased Power 2016 (a) 2 services from ATXI 2015 (a) 2 2014 (a) 2 Total Purchased Power 2016 $ (a) $ 30 2015 (a) 17 2014 (a) 7 Ameren Services support services Other Operations and 2016 $ 129 $ 123 agreement Maintenance 2015 131 119 2014 124 109 Money pool borrowings (advances) Interest (Charges) 2016 $ (b) $ (b) Income 2015 (b) (b) 2014 (b) (b) (a) Not applicable. (b) Amount less than $1 million.

Commitments And Contingencies

Commitments And Contingencies	12 Months Ended
	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	COMMITMENTS AND CONTINGENCIES We are involved in legal, tax, and regulatory proceedings before various courts, regulatory commissions, authorities, and governmental agencies with respect to matters that arise in the ordinary course of business, some of which involve substantial amounts of money. We believe that the final disposition of these proceedings, except as otherwise disclosed in these notes to our financial statements, will not have a material adverse effect on our results of operations, financial position, or liquidity. See also Note 1 – Summary of Significant Accounting Policies, Note 2 – Rate and Regulatory Matters, Note 10 – Callaway Energy Center, and Note 14 – Related Party Transactions in this report. Callaway Energy Center The following table presents insurance coverage at Ameren Missouri’s Callaway energy center at December 31, 2016 . The property coverage and the nuclear liability coverage renewal dates are April 1 and January 1, respectively, of each year. Type and Source of Coverage Maximum Coverages Maximum Assessments Public liability and nuclear worker liability: American Nuclear Insurers $ 375 (a) $ — Pool participation 12,986 (b) 127 (c) $ 13,361 (d) $ 127 Property damage: Nuclear Electric Insurance Limited $ 2,710 (e) $ 30 (f) European Mutual Association for Nuclear Insurance 450 (g) — $ 3,160 $ 30 Replacement power: Nuclear Electric Insurance Limited $ 490 (h) $ 7 (f) (a) Effective January 1, 2017, limit was increased to $450 million. (b) Provided through mandatory participation in an industrywide retrospective premium assessment program. (c) 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. (d) 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. (e) NEIL provides $2.71 billion in property damage, decontamination, and premature decommissioning insurance for radiation events. NEIL provides $2.3 billion in property damage for nonradiation events. (f) All NEIL-insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL. (g) European Mutual Association for Nuclear Insurance provides $450 million in excess of the $2.71 billion and $2.3 billion property coverage for radiation and nonradiation events, respectively, provided by NEIL. (h) Provides replacement power cost insurance in the event of a prolonged accidental outage. Weekly indemnity is up to $4.5 million for 52 weeks, which commences after the first 12 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 insurance policies, subject to an industrywide aggregate policy coverage 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 insurance 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, and liquidity. Leases We lease various facilities, office equipment, plant equipment, and rail cars under capital and operating leases. The following table presents our lease obligations at December 31, 2016 : 2017 2018 2019 2020 2021 After 5 Years Total Ameren: (a) Minimum capital lease payments (b) $ 33 $ 32 $ 32 $ 32 $ 32 $ 297 $ 458 Less amount representing interest 27 26 25 25 25 48 176 Present value of minimum capital lease payments $ 6 $ 6 $ 7 $ 7 $ 7 $ 249 $ 282 Operating leases (c) 13 12 12 11 10 23 81 Total lease obligations $ 19 $ 18 $ 19 $ 18 $ 17 $ 272 $ 363 Ameren Missouri: Minimum capital lease payments (b) $ 33 $ 32 $ 32 $ 32 $ 32 $ 297 $ 458 Less amount representing interest 27 26 25 25 25 48 176 Present value of minimum capital lease payments $ 6 $ 6 $ 7 $ 7 $ 7 $ 249 $ 282 Operating leases (c) 11 11 11 10 9 21 73 Total lease obligations $ 17 $ 17 $ 18 $ 17 $ 16 $ 270 $ 355 Ameren Illinois: Operating leases (c) $ 1 $ 1 $ 1 $ 1 $ 1 $ 1 $ 6 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations. (b) See Properties under Part I, Item 2, and Note 3 – Property, Plant, and Equipment, Net, of this report for additional information. (c) Amounts related to certain land-related leases have indefinite payment periods. The annual obligations of $3 million , $2 million , and $1 million for Ameren, Ameren Missouri, and Ameren Illinois for these items are included in the 2017 through 2021 columns, respectively. The following table presents total rental expense included in operating expenses for the years ended December 31, 2016 , 2015 , and 2014 : 2016 2015 2014 Ameren (a) $ 38 $ 36 $ 37 Ameren Missouri 34 34 32 Ameren Illinois 30 28 25 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations. Other Obligations To supply a portion of the fuel requirements of our energy centers, we have entered into various long-term commitments for the procurement of coal, natural gas, nuclear fuel, and methane gas. We also have entered into various long-term commitments for purchased power and natural gas for distribution. The table below presents our estimated fuel, purchased power, and other commitments for fuel at December 31, 2016 . Ameren’s and Ameren Missouri’s purchased power commitments include a 102-megawatt power purchase agreement with a wind farm operator, which expires in 2024. Ameren’s and Ameren Illinois’ purchased power commitments include the Ameren Illinois power purchase agreements entered into as part of the IPA-administered power procurement process. Included in the Other column are minimum purchase commitments under contracts for equipment, design and construction, and meter reading services at December 31, 2016 . Coal Natural Gas (a) Nuclear Fuel Purchased Power (b) Methane Gas Other Total Ameren: (c) 2017 $ 599 $ 238 $ 45 $ 255 $ 3 $ 118 $ 1,258 2018 371 167 70 156 4 60 828 2019 311 99 27 79 4 60 580 2020 27 45 38 58 5 56 229 2021 — 12 44 58 5 29 148 Thereafter — 43 45 478 65 198 829 Total $ 1,308 $ 604 $ 269 $ 1,084 $ 86 $ 521 $ 3,872 Ameren Missouri: 2017 $ 599 $ 43 $ 45 $ 22 $ 3 $ 39 $ 751 2018 371 29 70 22 4 29 525 2019 311 15 27 22 4 29 408 2020 27 10 38 22 5 29 131 2021 — 5 44 22 5 28 104 Thereafter — 18 45 59 65 183 370 Total $ 1,308 $ 120 $ 269 $ 169 $ 86 $ 337 $ 2,289 Ameren Illinois: 2017 $ — $ 195 $ — $ 233 $ — $ 36 $ 464 2018 — 138 — 134 — 24 296 2019 — 83 — 57 — 27 167 2020 — 35 — 36 — 27 98 2021 — 8 — 36 — — 44 Thereafter — 25 — 419 — — 444 Total $ — $ 484 $ — $ 915 $ — $ 114 $ 1,513 (a) Includes amounts for generation and for distribution. (b) The purchased power amounts for Ameren and Ameren Illinois include agreements through 2032 for renewable energy credits with various renewable energy suppliers. The agreements contain a provision that allows Ameren Illinois to reduce the quantity purchased in the event that Ameren Illinois would not be able to recover the costs associated with the renewable energy credits. (c) Includes amounts for Ameren registrant and nonregistrant subsidiaries. Environmental Matters We are subject to various environmental laws and regulations enforced by federal, state, and local authorities. The development and operation of electric generation, transmission, and distribution facilities and natural gas storage, transmission, and distribution facilities, can trigger 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, and 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 has promulgated environmental regulations that have a significant impact on the electric utility industry. Over time, compliance with these regulations could be costly for Ameren Missouri, which operates coal-fired power plants. As of December 31, 2016, Ameren Missouri’s fossil-fueled energy centers represented 18% and 34% of Ameren’s and Ameren Missouri’s rate base, respectively. Regulations impacting the electric utility industry include the regulation of CO 2 emissions from existing power plants through the Clean Power Plan and from new power plants through the revised NSPS; the CSAPR, which requires further reductions of SO 2 emissions and NO x emissions from power plants; a regulation governing management and storage of CCR; the MATS, which requires 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; effluent standards applicable to wastewater discharges from power plants; and regulations under the Clean Water Act that could require significant capital expenditures, such as modifications to water intake structures at Ameren Missouri’s energy centers. The EPA also periodically reviews and revises national ambient air quality standards, including those standards associated with emissions from power plants, such as particulate matter, ozone, SO 2 and NO x . Certain of these regulations are being or are likely to be challenged through litigation, so their ultimate implementation, as well as the timing of any such implementation, is uncertain. Although many details of future regulations are unknown, the individual or combined effects of recent environmental regulations could result in significant capital expenditures and increased operating costs for Ameren and Ameren Missouri. Compliance with these environmental laws and regulations could be prohibitively expensive, result in the closure or alteration of the operation of some of Ameren Missouri’s energy centers, or require further capital investment. Ameren and Ameren Missouri expect that these costs would be recoverable through rates, subject to MoPSC prudence review, but the nature and timing of costs and their recovery could result in regulatory lag. Ameren Missouri's current plan for compliance with existing environmental regulations for air emissions includes burning ultra-low-sulfur coal and installing new or optimizing existing pollution control equipment. Ameren and Ameren Missouri estimate that they will need to make capital expenditures of $425 million to $525 million in the aggregate from 2017 through 2021 in order to comply with existing environmental regulations. Ameren Missouri may be required to install additional air emissions controls beyond 2021. This estimate of capital expenditures includes expenditures required for the CCR regulations, the Clean Water Act rule applicable to cooling water intake structures at existing power plants, and the Clean Water Act effluent limitation guidelines applicable to steam electric generating units, all of which are discussed below. This estimate does not include the potential impacts of the Clean Power Plan discussed below. The actual amount of capital expenditures required to comply with existing environmental regulations may vary substantially from the above estimate because of 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 recent 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 first phase of the CSAPR emission reduction requirements became effective in 2015. The second phase of emission reduction requirements, which were revised by the EPA in 2016, will become effective in 2017; additional emission reduction requirements may apply in subsequent years. To achieve compliance with the CSAPR, Ameren Missouri burns ultra-low-sulfur coal, operates two scrubbers at its Sioux energy center, and optimizes other existing pollution control equipment. Ameren Missouri does not expect to make additional capital investments to comply with the 2017 CSAPR requirements. However, Ameren Missouri expects to incur additional costs to lower its emissions at one or more of its energy centers to comply with the CSAPR in future years. These higher costs are expected to be recovered from customers through the FAC or higher base rates. CO 2 Emissions Standards In 2015, the EPA issued final regulations that set CO 2 emissions standards for new power plants. These new standards establish separate emissions limits for new natural-gas-fired combined cycle plants and new coal-fired plants. The Clean Power Plan sets forth CO 2 emissions standards applicable to existing power plants. The rule was stayed by the United States Supreme Court in February 2016, pending the outcome of various legal challenges. If upheld and implemented, the Clean Power Plan would require Missouri and Illinois to reduce CO 2 emissions from power plants within their states significantly below 2005 levels by 2030. The rule contains interim compliance periods commencing in 2022 that would require each state to demonstrate progress in achieving its CO 2 emissions reduction target. Ameren continues to evaluate the Clean Power Plan's potential impacts to its operations, including those related to electric system reliability, and to its level of investment in customer energy efficiency programs, renewable energy, and other forms of generation. Significant uncertainty exists regarding the impact of the Clean Power Plan as its implementation will depend upon plans to be developed by the states. Numerous legal challenges are pending, which could result in the rule being declared invalid or the nature and timing of CO 2 emissions reductions being revised. All implementation requirements are deferred until such time as these legal challenges are concluded. A decision by the District of Columbia Circuit Court of Appeals is expected to be issued in 2017, and subsequent appeals to the United States Supreme Court are likely. We cannot predict the outcome of such legal challenges or their impact on our results of operations, financial position, or liquidity. If the rule is ultimately upheld and not rescinded or altered significantly by the new federal administration, 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 in turn result in increased operating costs and require Ameren Missouri to make unplanned or accelerated capital expenditures. Ameren Missouri expects substantially all of these increased costs to be recoverable, subject to MoPSC prudence review, through higher rates to customers, which could be significant. Federal and state legislation or regulations that mandate limits on the emission of CO 2 may result in significant increases in capital expenditures and operating costs, which could lead to increased liquidity needs and higher financing costs. Mandatory limits on the emission of CO 2 could increase costs for Ameren Missouri’s customers or have a material adverse effect on Ameren's and Ameren Missouri's results of operations, financial position, and liquidity if regulators delay or deny recovery in rates of these compliance costs. The cost of Ameren Illinois’ purchased power and natural gas purchased for resale could increase. However, Ameren Illinois expects these costs would be recovered from customers with no material adverse effect on its results of operations, financial position, or liquidity. Ameren's and Ameren Missouri's earnings might benefit from increased investment to comply with CO 2 emission limitations to the extent that the investments are reflected and recovered on a timely basis in rates charged to customers. NSR and Clean Air Litigation In January 2011, the Department of Justice, on behalf of the EPA, filed a complaint against Ameren Missouri in the United States District Court for the Eastern District of Missouri. The complaint, as amended in October 2013, alleged 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. The litigation has been divided into two phases: liability and remedy. In January 2017, the district court issued a liability ruling that the projects violated provisions of the Clean Air Act and Missouri law. The case will now proceed to the second phase to determine the actions required to remedy the violations found in the liability phase of the litigation. The EPA previously withdrew all claims for penalties and fines. At the conclusion of both phases of the litigation, Ameren Missouri intends to appeal the liability ruling to the United States Circuit Court of Appeals for the Eighth Circuit. A decision by the district court regarding the remedy phase of the litigation could occur in 2018. The ultimate resolution of this matter could have a material adverse effect on the results of operations, financial position, and liquidity of Ameren and Ameren Missouri. Among other things and subject to economic and regulatory considerations, resolution of this matter could result in increased capital expenditures for the installation of pollution control equipment, as well as increased operations and maintenance expenses. We are unable to predict the ultimate resolution of this matter or the costs that might be incurred. Clean Water Act In 2014, the EPA issued its final rule applicable to cooling water intake structures at existing power plants. The rule requires a case-by-case evaluation and plan for reducing aquatic organisms impinged on the facility’s intake screens or entrained through the plant's cooling water system. Additionally, in 2015, the EPA issued its final rule to revise the effluent limitation guidelines applicable to steam electric generating units. Effluent limitation guidelines are national standards for water discharges that are based on the effectiveness of available control technology. The EPA's 2015 rule prohibits effluent discharges of certain waste streams and imposes more stringent limitations on certain components in water discharges from power plants. All of Ameren Missouri’s coal-fired and nuclear energy centers are subject to the cooling water intake structures rule. All of Ameren Missouri’s coal-fired energy centers are subject to the effluent limitations rule. Implementation of both rules will occur during the renewal process of each energy center’s water discharge permit, which will occur between 2018 and 2023. The rules could have an adverse effect on Ameren’s and Ameren Missouri’s results of operations, financial position, and liquidity if their implementation requires extensive modifications to the cooling water systems and water discharge systems at Ameren Missouri’s energy centers and if those investments are not recovered on a timely basis in electric rates charged to Ameren Missouri’s customers. Ash Management In 2015, the EPA issued regulations regarding the management and disposal of CCR from coal fired energy centers. These regulations affect CCR disposal and handling costs at Ameren Missouri's energy centers. They require closure of impoundments if performance criteria relating to groundwater impacts and location restrictions are not achieved. During 2015, Ameren and Ameren Missouri recorded an increase to their AROs associated with CCR storage facilities and accelerated the closure of certain CCR storage facilities at its energy centers as a result of the new regulations. Ameren plans to close these CCR storage facilities between 2018 and 2023. 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. Remediation The Ameren Companies are involved in a number of remediation actions to clean up sites affected by the use or disposal of materials containing hazardous substances. Federal and state laws can require responsible parties to fund remediation actions regardless of their degree of fault, the legality of original disposal, or the ownership of a disposal site. Ameren Missouri and Ameren Illinois have each been identified by federal or state governments as a potentially responsible party at several contaminated sites. As of December 31, 2016 , Ameren Illinois owned or was otherwise responsible for 44 former MGP sites in Illinois, which are in various stages of investigation, evaluation, remediation, and closure. Ameren Illinois estimates it could substantially conclude remediation efforts by 2023. 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 cost riders. Costs are subject to annual prudence review by the ICC. As of December 31, 2016 , Ameren Illinois estimated the obligation related to these former MGP sites at $200 million to $268 million . Ameren and Ameren Illinois recorded a liability of $200 million to represent the estimated minimum obligation for these sites, as no other amount within the range was a better estimate. The scope of the remediation activities at these former MGP sites may increase as remediation efforts continue. Considerable uncertainty remains in these estimates because many site-specific factors can influence the ultimate actual costs, including unanticipated underground structures, the degree to which groundwater is encountered, regulatory changes, local ordinances, and site accessibility. The actual costs may vary substantially from these estimates. Ameren Missouri participated in the investigation of various sites known as Sauget Area 2 located in Sauget, Illinois. In 2000, the EPA notified Ameren Missouri and numerous other companies that former landfills and lagoons at those sites may contain soil and groundwater contamination. From about 1926 until 1976, Ameren Missouri operated an energy center adjacent to Sauget Area 2. Ameren Missouri currently owns a parcel of property at Sauget Area 2 that was once used by others as a landfill. In December 2013, the EPA issued its record of decision for Sauget Area 2 approving the investigation and the remediation actions 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 December 31, 2016 , and December 31, 2015, Ameren Missouri estimated its obligation related to Sauget Area 2 at $1 million to $2.5 million . Ameren Missouri recorded a liability of $1 million to represent its estimated minimum obligation, as no other amount within the range was a better estimate. Our operations or those of our predecessor companies involve the use of, disposal of, and in appropriate circumstances, the cleanup of substances regulated under environmental laws. We are unable to determine whether such practices will result in future environmental commitments or will affect our results of operations, financial position, or liquidity. Ameren Missouri Municipal Taxes The cities of Creve Coeur and Winchester, Missouri, on behalf of themselves and other municipalities in Ameren Missouri’s service area, filed a class action lawsuit in November 2011 against Ameren Missouri in the Circuit Court of St. Louis County, Missouri. The lawsuit alleges that Ameren Missouri failed to collect and pay gross receipts taxes or license fees on certain revenues, including revenues from wholesale power and interchange sales. Ameren and Ameren Missouri recorded immaterial liabilities on their respective balance sheets as of December 31, 2016 , and December 31, 2015, representing their estimate of the probable loss due as a result of this lawsuit. Ameren and Ameren Missouri believe there is a remote possibility that a liability relating to this lawsuit could be material to Ameren's and Ameren Missouri’s results of operations, financial position, and liquidity. Ameren Missouri believes its defenses are meritorious and is defending itself vigorously. However, there can be no assurances that Ameren Missouri will be successful in its efforts.

Segment Information

Segment Information	12 Months Ended
	Dec. 31, 2016
Segment Reporting Information, Profit (Loss) [Abstract]
SEGMENT INFORMATION	SEGMENT INFORMATION During the fourth quarter of 2016, the Ameren Companies changed the manner in which performance is assessed and resources are allocated, driven by increasing investment in FERC-regulated electric transmission and Ameren Illinois electric distribution and natural gas distribution businesses, as well as the unique regulatory environment for each jurisdiction. Ameren now has four segments: Ameren Missouri, Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Transmission. The Ameren Missouri segment includes all of the operations of Ameren Missouri. Ameren Illinois Electric Distribution consists of the electric distribution business of Ameren Illinois. Ameren Illinois Natural Gas consists of the natural gas business of Ameren Illinois. Ameren Transmission is primarily composed of the aggregated electric transmission businesses of Ameren Illinois and ATXI and associated Ameren (parent) interest charges . The category called Other primarily includes Ameren parent company activities and Ameren Services. Ameren Missouri has one segment. Ameren Illinois has three segments: Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Illinois Transmission . See Note 1 – Summary of Significant Accounting Policies for additional information regarding the operations of Ameren Missouri and Ameren Illinois. Segment operating revenue and a majority of operating expenses are directly assigned by Ameren Illinois to each Ameren Illinois segment. Common operating expenses, miscellaneous income and expense, interest charges, and income tax expense are allocated by Ameren Illinois to each Ameren Illinois segment based on certain factors, which primarily relate to the nature of the cost. Additionally, Ameren Illinois Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. The transmission expense for Illinois customers who have elected to purchase their power from Ameren Illinois is recovered through a cost recovery mechanism with no net effect on Ameren Illinois Electric Distribution earnings, as costs are offset by corresponding revenues. Transmission revenues from these transactions are reflected at Ameren Transmission and Ameren Illinois Transmission. An intersegment elimination at Ameren and Ameren Illinois occurs to eliminate these transmission revenues and expenses. Prior to the fourth quarter of 2016, Ameren had two segments: Ameren Missouri and Ameren Illinois, which comprised the operations of the respective subsidiaries. The category called Other primarily included Ameren parent company activities, Ameren Services, and ATXI. Prior-period presentation has been adjusted for comparative purposes to reflect the 2016 change in segments. The following tables present information about the reported revenues and specified items reflected in net income attributable to common shareholders from continuing operations and capital expenditures at Ameren and Ameren Illinois for the years ended December 31, 2016 , 2015 , and 2014 . Ameren, Ameren Missouri, and Ameren Illinois management review segment capital expenditure information rather than any individual or total asset amount. Ameren Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Other Intersegment Eliminations Consolidated 2016 External revenues $ 3,469 $ 1,545 $ 753 $ 309 $ — $ — $ 6,076 Intersegment revenues 54 4 1 46 (a) — (105 ) — Depreciation and amortization 514 226 55 43 7 — 845 Interest income 28 11 — 1 11 (11 ) 40 Interest charges 211 72 34 58 18 (11 ) 382 Income taxes 216 78 39 74 (25 ) — 382 Net income (loss) attributable to Ameren common shareholders from continuing operations 357 126 59 117 (6 ) — 653 Capital expenditures 738 470 181 689 4 (b) (6 ) 2,076 2015 External revenues $ 3,566 $ 1,529 $ 782 $ 219 $ 2 $ — $ 6,098 Intersegment revenues 43 3 1 40 (a) — (87 ) — Depreciation and amortization 492 212 52 33 7 — 796 Interest income 28 12 — — 7 (6 ) 41 Interest charges 219 71 35 35 1 (6 ) 355 Income taxes 209 71 24 51 8 — 363 Net income (loss) attributable to Ameren common shareholders from continuing operations 352 123 37 83 (16 ) — 579 Capital expenditures 622 491 133 669 2 (b) — 1,917 2014 External revenues $ 3,526 $ 1,401 $ 976 $ 150 $ — $ — $ 6,053 Intersegment revenues 27 2 — 37 (a) — (66 ) — Depreciation and amortization 473 197 41 26 8 — 745 Interest income 28 7 — — 5 (3 ) 37 Interest charges 211 63 28 26 16 (3 ) 341 Income taxes 229 75 39 38 (4 ) — 377 Net income (loss) attributable to Ameren common shareholders from continuing operations 390 113 50 51 (17 ) — 587 Capital expenditures 747 403 137 491 7 (b) — 1,785 (a) Ameren Illinois Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above. (b) Includes the elimination of intercompany transfers. Ameren Illinois Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Consolidated 2016 External revenues $ 1,549 $ 754 $ 187 $ — $ 2,490 Intersegment revenues — — 45 (a) (45 ) — Depreciation and amortization 226 55 38 — 319 Interest income 11 — 1 — 12 Interest charges 72 34 34 — 140 Income taxes 78 39 41 — 158 Net income available to common shareholder 126 59 67 — 252 Capital expenditures 470 181 273 — 924 2015 External revenues $ 1,532 $ 783 $ 151 $ — $ 2,466 Intersegment revenues — — 38 (a) (38 ) — Depreciation and amortization 212 52 31 — 295 Interest income 12 — — — 12 Interest charges 71 35 25 — 131 Income taxes 71 24 32 — 127 Net income available to common shareholder 123 37 54 — 214 Capital expenditures 491 133 294 — 918 2014 External revenues $ 1,403 $ 976 $ 119 $ — $ 2,498 Intersegment revenues — — 35 (a) (35 ) — Depreciation and amortization 197 41 25 — 263 Interest income 7 — — — 7 Interest charges 63 28 21 — 112 Income taxes 75 39 29 — 143 Net income available to common shareholder 113 50 38 — 201 Capital expenditures 403 137 295 — 835 (a) Ameren Illinois Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above.

Selected Quarterly Information

Selected Quarterly Information	12 Months Ended
	Dec. 31, 2016
Selected Quarterly Financial Information [Abstract]
SELECTED QUARTERLY INFORMATION	Ameren 2016 2015 Quarter ended March 31 June 30 September 30 December 31 March 31 June 30 September 30 December 31 Operating revenues $ 1,434 $ 1,427 $ 1,859 $ 1,356 $ 1,556 $ 1,401 $ 1,833 $ 1,308 Operating income 220 325 691 145 256 237 626 140 Net income 107 148 371 33 110 151 345 30 Net income attributable to Ameren common shareholders – continuing operations $ 105 $ 147 $ 369 $ 32 $ 108 $ 98 $ 343 $ 30 Net income (loss) attributable to Ameren common shareholders – discontinued operations — — — — — 52 — (1 ) Net income attributable to Ameren common shareholders $ 105 $ 147 $ 369 $ 32 $ 108 $ 150 $ 343 $ 29 Earnings per common share – basic – continuing operations $ 0.43 $ 0.61 $ 1.52 $ 0.13 $ 0.45 $ 0.40 $ 1.42 $ 0.12 Earnings per common share – basic – discontinued operations — — — — — 0.21 — — Earnings per common share – basic $ 0.43 $ 0.61 $ 1.52 $ 0.13 $ 0.45 $ 0.61 $ 1.42 $ 0.12 Earnings per common share – diluted – continuing operations (a) $ 0.43 $ 0.61 $ 1.52 $ 0.13 $ 0.45 $ 0.40 $ 1.41 $ 0.12 Earnings per common share – diluted – discontinued operations — — — — — 0.21 — — Earnings per common share – diluted (a) $ 0.43 $ 0.61 $ 1.52 $ 0.13 $ 0.45 $ 0.61 $ 1.41 $ 0.12 (a) The sum of quarterly amounts, including per share amounts, may not equal amounts reported for year-to-date periods. This is because of the effects of rounding and the changes in the number of weighted-average diluted shares outstanding each period. Ameren Missouri Quarter ended Operating Revenues Operating Income Net Income Net Income Available to Common Shareholder March 31, 2016 $ 741 $ 63 $ 15 $ 14 March 31, 2015 800 115 42 41 June 30, 2016 867 197 93 92 June 30, 2015 884 146 62 61 September 30, 2016 1,165 431 242 241 September 30, 2015 1,171 423 240 239 December 31, 2016 750 54 10 10 December 31, 2015 754 58 11 11 Ameren Illinois Quarter ended Operating Revenues Operating Income Net Income Net Income Available to Common Shareholder March 31, 2016 $ 677 $ 133 $ 60 $ 59 March 31, 2015 745 120 54 53 June 30, 2016 542 107 46 45 June 30, 2015 513 83 32 31 September 30, 2016 676 230 119 119 September 30, 2015 655 189 98 98 December 31, 2016 595 74 30 29 December 31, 2015 553 74 33 32

Schedule I - Condensed Financia

Schedule I - Condensed Financial Information Of Parent	12 Months Ended
	Dec. 31, 2016
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Condensed Financial Information Of Parent	SCHEDULE I – CONDENSED FINANCIAL INFORMATION OF PARENT (In millions) 2016 2015 2014 Operating revenues $ — $ — $ — Operating expenses 14 14 11 Operating loss (14 ) (14 ) (11 ) Equity in earnings of subsidiaries 663 600 607 Interest income from affiliates 10 6 3 Total other income (expense), net (5 ) (5 ) 2 Interest charges 28 3 16 Income tax (benefit) (27 ) 5 (2 ) Net Income Attributable to Ameren Common Shareholders – Continuing Operations 653 579 587 Net Income (Loss) Attributable to Ameren Common Shareholders – Discontinued Operations — 51 (1 ) Net Income Attributable to Ameren Common Shareholders $ 653 $ 630 $ 586 Net Income Attributable to Ameren Common Shareholders – Continuing Operations $ 653 $ 579 $ 587 Other Comprehensive Income, Net of Taxes: Pension and other postretirement benefit plan activity, net of income taxes (benefit) of $(7), $3, and $(7), respectively (20 ) 6 (12 ) Comprehensive Income from Continuing Operations Attributable to Ameren Common Shareholders 633 585 575 Comprehensive Income (Loss) from Discontinued Operations Attributable to Ameren Common Shareholders — 51 (1 ) Comprehensive Income Attributable to Ameren Common Shareholders $ 633 $ 636 $ 574 SCHEDULE I – CONDENSED FINANCIAL INFORMATION OF PARENT (In millions) December 31, 2016 December 31, 2015 Assets: Cash and cash equivalents $ 1 $ — Advances to money pool 27 — Accounts receivable – affiliates 31 53 Miscellaneous accounts and notes receivable 26 3 Other current assets 8 9 Total current assets 93 65 Investments in subsidiaries 7,498 7,227 Note receivable – ATXI 350 290 Accumulated deferred income taxes, net 419 426 Other assets 135 158 Total assets $ 8,495 $ 8,166 Liabilities and Shareholders’ Equity: Short-term debt 507 301 Borrowings from money pool 33 14 Accounts payable – affiliates 13 75 Other current liabilities 17 22 Total current liabilities 570 412 Long-term debt 694 694 Pension and other postretirement benefits 45 33 Other deferred credits and liabilities 83 81 Total liabilities 1,392 1,220 Commitments and Contingencies (Notes 4 and 5) Shareholders’ Equity: Common stock, $.01 par value, 400.0 shares authorized – shares outstanding of 242.6 2 2 Other paid-in capital, principally premium on common stock 5,556 5,616 Retained earnings 1,568 1,331 Accumulated other comprehensive loss (23 ) (3 ) Total shareholders’ equity 7,103 6,946 Total liabilities and shareholders’ equity $ 8,495 $ 8,166 SCHEDULE I – CONDENSED FINANCIAL INFORMATION OF PARENT (In millions) 2016 2015 2014 Net cash flows provided by operating activities $ 483 $ 551 $ 528 Cash flows from investing activities: Money pool advances, net (27 ) 55 279 Notes receivable – affiliates, net (60 ) (96 ) (134 ) Investments in subsidiaries (123 ) (509 ) (280 ) Distributions from subsidiaries — — 215 Proceeds from note receivable – Marketing Company — 20 95 Contributions to note receivable – Marketing Company — (8 ) (89 ) Other 2 (24 ) (12 ) Net cash flows provided by (used in) investing activities (208 ) (562 ) 74 Cash flows from financing activities: Dividends on common stock (416 ) (402 ) (390 ) Short-term debt, net 206 (284 ) 217 Money pool borrowings, net 19 14 — Maturities of long-term debt — — (425 ) Issuances of long-term debt — 700 — Capital issuance costs — (6 ) — Share-based payments (83 ) (12 ) (14 ) Net cash flows provided by (used in) financing activities (274 ) 10 (612 ) Net change in cash and cash equivalents $ 1 $ (1 ) $ (10 ) Cash and cash equivalents at beginning of year — 1 11 Cash and cash equivalents at end of year $ 1 $ — $ 1 Cash dividends received from consolidated subsidiaries $ 465 $ 575 $ 340 Noncash investing activity – investments in subsidiaries — (38 ) (19 ) AMEREN CORPORATION (parent company only) NOTES TO CONDENSED FINANCIAL STATEMENTS December 31, 2016 NOTE 1 – BASIS OF PRESENTATION Ameren Corporation (parent company only) is a public utility holding company that conducts substantially all of its business operations through its subsidiaries. Ameren Corporation (parent company only) has accounted for its subsidiaries using the equity method. These financial statements are presented on a condensed basis. See Note 1 – Summary of Significant Accounting Policies under Part II, Item 8, of this report for additional information. See Note 14 – Related Party Transactions under Part II, Item 8, of this report for information on the tax allocation agreement between Ameren Corporation (parent company only) and its subsidiaries. NOTE 2 – SHORT-TERM DEBT AND LIQUIDITY Ameren, Ameren Services, and other non-state-regulated Ameren subsidiaries have the ability, subject to Ameren parent company and applicable regulatory short-term borrowing authorizations, to access funding from the Credit Agreements and the commercial paper programs through a non-state-regulated subsidiary money pool agreement. All participants may borrow from or lend to the non-state-regulated money pool. The total amount available to pool participants from the non-state-regulated subsidiary 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 non-state-regulated subsidiary money pool or remit funds from other external sources. The non-state-regulated subsidiary money pool was established to coordinate and to provide short-term cash and working capital for the participants. Participants receiving a loan under the non-state-regulated subsidiary money pool agreement must repay the principal amount of such loan, together with accrued interest. The rate of interest depends on the composition of internal and external funds in the non-state-regulated subsidiary money pool. Interest revenues and interest charges related to non-state-regulated money pool advances and borrowings were immaterial in 2014, 2015 and 2016. Ameren Corporation (parent company only) had a total of $51 million in guarantees outstanding, primarily for ATXI, that were not recorded on its December 31, 2016 balance sheet. The ATXI guarantees were issued to local governments as assurance for potential remediation of damage caused by ATXI construction. See Note 4 – Short-term Debt and Liquidity under Part II, Item 8, of this report for a description and details of short-term debt and liquidity needs of Ameren Corporation (parent company only). NOTE 3 – LONG-TERM OBLIGATIONS See Note 5 – Long-term Debt and Equity Financings under Part II, Item 8, of this report for additional information on Ameren Corporation's (parent company only) long-term debt, indenture provisions, and restricted cash balance. NOTE 4 – COMMITMENTS AND CONTINGENCIES See Note 15 – Commitments and Contingencies under Part II, Item 8, of this report for a description of all material contingencies of Ameren Corporation (parent company only). NOTE 5 – DIVESTITURE TRANSACTIONS AND DISCONTINUED OPERATIONS See Note 1 – Summary of Significant Accounting Policies under Part II, Item 8, of this report for information regarding the divestiture transactions and discontinued operations.

Schedule II - Valuation And Qua

Schedule II - Valuation And Qualifying Accounts	12 Months Ended
	Dec. 31, 2016
Valuation and Qualifying Accounts [Abstract]
Valuation And Qualifying Accounts	SCHEDULE II – VALUATION AND QUALIFYING ACCOUNTS (in millions) Column A Column B Column C Column D Column E Description Balance at Beginning of Period (1) Charged to Costs and Expenses (2) Charged to Other Accounts (a) Deductions (b) Balance at End of Period Ameren: Deducted from assets – allowance for doubtful accounts: 2016 $ 19 $ 32 $ 3 $ 35 $ 19 2015 21 33 5 40 19 2014 18 36 4 37 21 Deferred tax valuation allowance: 2016 $ 6 $ 7 $ (2 ) $ — $ 11 2015 10 4 (8 ) — 6 2014 7 3 — — 10 Ameren Missouri: Deducted from assets – allowance for doubtful accounts: 2016 $ 7 $ 10 $ — $ 10 $ 7 2015 8 13 — 14 7 2014 5 16 — 13 8 Deferred tax valuation allowance: 2016 $ — $ — $ — $ — $ — 2015 1 — (1 ) — — 2014 1 — — — 1 Ameren Illinois: Deducted from assets – allowance for doubtful accounts: 2016 $ 12 $ 22 $ 3 $ 25 $ 12 2015 13 20 5 26 12 2014 13 20 4 24 13 Deferred tax valuation allowance: 2016 $ — $ — $ — $ — $ — 2015 1 — (1 ) — — 2014 1 — — — 1 (a) Amounts associated with the allowance for doubtful accounts relate to the uncollectible account reserve associated with receivables purchased by Ameren Illinois from alternative retail electric suppliers, as required by the Illinois Public Utilities Act. The amounts relating to the deferred tax valuation allowance are for items that have expired and were removed from both the underlying accumulated deferred income tax account as well as the offsetting valuation account. (b) Uncollectible accounts charged off, less recoveries.

Summary Of Significant Accoun30

Summary Of Significant Accounting Policies (Policy)	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
General	Ameren’s primary assets are its equity interests in its subsidiaries, including Ameren Missouri, Ameren Illinois, and ATXI. 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. Ameren also has various other subsidiaries that conduct other activities, such as the provision of shared services. • Union Electric Company, doing business as Ameren Missouri, operates a rate-regulated electric generation, transmission, and distribution business and a rate-regulated natural gas distribution business in Missouri. Ameren Missouri was incorporated in Missouri in 1922 and is successor to a number of companies, the oldest of which was organized in 1881. It is the largest electric utility in the state of Missouri. It supplies electric and natural gas service to a 24,000 -square-mile area in central and eastern Missouri. This area has an estimated population of 2.8 million and includes the Greater St. Louis area. Ameren Missouri supplies electric service to 1.2 million customers and natural gas service to 0.1 million customers. • Ameren Illinois Company, doing business as Ameren Illinois, operates rate-regulated electric transmission, electric distribution, and natural gas distribution businesses in Illinois. Ameren Illinois was incorporated in Illinois in 1923 and is the successor to a number of companies, the oldest of which was organized in 1902. Ameren Illinois supplies electric and natural gas utility service to portions of central and southern Illinois with an estimated population of 3.1 million in an area of 40,000 square miles. Ameren Illinois supplies electric service to 1.2 million customers and natural gas service to 0.8 million customers. • ATXI 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. ATXI is also evaluating competitive electric transmission investment opportunities outside of MISO as they arise. 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. All tabular dollar amounts are in millions, unless otherwise indicated.
Consolidation	Unless otherwise stated, these notes to the financial statements exclude discontinued operations for all periods presented. 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.
Public Utilities	Purchased Gas, Power and Fuel Rate-adjustment Mechanisms Ameren Missouri and Ameren Illinois have various rate-adjustment mechanisms in place that provide for the recovery of purchased natural gas and electric fuel and purchased power costs without a traditional rate case proceeding. See Note 2 – Rate and Regulatory Matters for the regulatory assets and liabilities recorded at December 31, 2016 and 2015 , related to the rate-adjustment mechanisms discussed below. In Ameren Missouri’s and Ameren Illinois’ natural gas businesses, changes in natural gas costs are reflected in billings to their customers through PGA clauses. The difference between actual natural gas costs and costs billed to customers in a given period is deferred as a regulatory asset or liability. The deferred amount is either billed or refunded to customers in a subsequent period. In Ameren Illinois’ electric distribution business, changes in purchased power and transmission service costs are reflected in billings to its customers through pass-through rate-adjustment clauses. The difference between actual purchased power and transmission service costs and costs billed to customers in a given period is deferred as a regulatory asset or liability. The deferred amount is either billed or refunded to customers in a subsequent period. Ameren Missouri has a FAC that allows an adjustment of electric rates three times per year for a pass-through to customers of 95% of changes in fuel and purchased power costs, including transportation charges and revenues, net of off-system sales, greater or less than the amount set in base rates, subject to MoPSC prudence review. The difference between the actual amounts incurred for these items and the amounts recovered from Ameren Missouri customers' base rates is deferred as a regulatory asset or liability. The deferred amounts are either billed or refunded to electric customers in a subsequent period. Since May 30, 2015, transmission revenues and substantially all transmission charges are excluded from net energy costs as a result of the April 2015 MoPSC electric rate order. Environmental Costs Liabilities for environmental costs are recorded on an undiscounted basis when it is probable that a liability has been incurred and the amount of the liability can be reasonably estimated. Costs are expensed or deferred as a regulatory asset when it is expected that the costs will be recovered from customers in future rates. Regulation We are regulated by the MoPSC, the ICC, and the FERC. We defer certain costs as assets pursuant to actions of rate regulators or because of expectations that we will be able to recover such costs in future rates charged to customers. We also defer certain amounts as liabilities pursuant to actions of rate regulators or based on the expectation that such amounts will be returned to customers in future rates. Regulatory assets and liabilities are amortized consistent with the period of expected regulatory treatment. In addition to the cost recovery mechanisms discussed in the Purchased Gas, Power, and Fuel Rate-adjustment Mechanisms section below, Ameren Missouri and Ameren Illinois have approvals from rate regulators to use other cost recovery mechanisms. Ameren Missouri has a pension and postretirement benefit cost tracker, an uncertain tax positions tracker, a renewable energy standards cost tracker, a solar rebate program tracker, and the MEEIA energy efficiency rider. Ameren Illinois' and ATXI's electric transmission rates are determined pursuant to formula ratemaking. Additionally, Ameren Illinois' electric distribution business participates in the performance-based formula ratemaking process established pursuant to the IEIMA. Ameren Illinois also has environmental cost riders, an asbestos-related litigation rider, an energy efficiency rider, a QIP rider, a VBA rider, and a bad debt rider. See Note 2 – Rate and Regulatory Matters for additional information on regulatory assets and liabilities.
Cash and Cash Equivalents	Cash and Cash Equivalents Cash and cash equivalents include cash on hand and temporary investments purchased with an original maturity of three months or less.
Allowance for Doubtful Accounts Receivable	Allowance for Doubtful Accounts Receivable The allowance for doubtful accounts represents our estimate of existing accounts receivable that will ultimately be uncollectible. The allowance is calculated by applying estimated loss factors to various classes of outstanding receivables, including unbilled revenue. The loss factors used to estimate uncollectible accounts are based upon both historical collections experience and management’s estimate of future collections success given the existing and anticipated future collections environment. Ameren Illinois has a bad debt rider that adjusts rates for net write-offs of customer accounts receivable above or below those being collected in rates.
Materials and Supplies	recorded at the lower of cost or market. Cost is determined by the average-cost method. Inventories are capitalized when purchased and then expensed as consumed or capitalized as plant assets when installed, as appropriate.
Property and Plant	Property, Plant, and Equipment, Net We capitalize the cost of additions to and betterments of units of property, plant and equipment. The cost includes labor, material, applicable taxes, and overhead. An allowance for funds used during construction, as discussed below, is also capitalized as a cost of our rate-regulated assets. Maintenance expenditures, including nuclear refueling and maintenance outages, are expensed as incurred. When units of depreciable property are retired, the original costs, less salvage values, are charged to accumulated depreciation. If environmental expenditures are related to assets currently in use, as in the case of the installation of pollution control equipment, the cost is capitalized and depreciated over the expected life of the asset. See Asset Retirement Obligations below and Note 3 – Property, Plant, and Equipment, Net for additional information. Depreciation Depreciation is provided over the estimated lives of the various classes of depreciable property by applying composite rates on a straight-line basis to the cost basis of such property. The provision for depreciation for the Ameren Companies in 2016 , 2015 , and 2014 ranged from 3% to 4% of the average depreciable cost.
Allowance for Funds Used During Construction	Allowance for Funds Used During Construction We capitalize allowance for funds used during construction, or the cost of borrowed funds and the cost of equity funds (preferred and common shareholders’ equity) applicable to rate-regulated construction expenditures, in accordance with the utility industry's accounting practice. Allowance for funds used during construction does not represent a current source of cash funds. This accounting practice offsets the effect on earnings of the cost of financing during construction, and it treats such financing costs in the same manner as construction charges for labor and materials. Under accepted ratemaking practice, cash recovery of allowance for funds used during construction and other construction costs occurs when completed projects are placed in service and reflected in customer rates.
Goodwill	$411 million at October 31, 2016 and October 31, 2015. To determine whether the fair value of a reporting unit is more likely than not greater than its carrying amount, Ameren and Ameren Illinois elect to perform either a qualitative assessment or to bypass the qualitative assessment and perform a two-step quantitative test, on an annual basis. On October 31, 2015, Ameren and Ameren Illinois performed a quantitative test and determined that the estimated fair value of the Ameren Illinois reporting unit significantly exceeded its carrying value as of that date. Based on these results, Ameren and Ameren Illinois elected to perform a qualitative assessment for their annual goodwill impairment test conducted as of October 31, 2016. The results of Ameren’s and Ameren Illinois’ qualitative assessment indicated that it was more likely than not that the fair value of the Ameren Illinois reporting unit exceeded its carrying value as of October 31, 2016, resulting in no impairment of Ameren’s or Ameren Illinois’ goodwill. The following factors, among others, were considered by Ameren and Ameren Illinois when assessing whether it was more likely than not that the fair value of the Ameren Illinois reporting unit exceeded its carrying value for the October 31, 2016, test: • macroeconomic conditions, including those conditions within Ameren Illinois’ service territory; • pending rate case outcomes and projections of future rate case outcomes; • changes in laws and potential law changes; • observable industry market multiples; • achievement of IEIMA performance metrics and the yield of 30-year United States Treasury bonds; • an unexpected further reduction in the FERC-allowed return on equity related to transmission services; and • projected operating results and cash flows. As of December 31, 2016, the Ameren Companies changed the manner in which they assess performance and allocate resources, driven by increasing investment in FERC rate-regulated electric transmission and Ameren Illinois electric distribution and natural gas distribution businesses as well as the unique regulatory environment for each jurisdiction. Ameren now has four reporting units: Ameren Missouri, Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Transmission. Ameren Illinois now has three reporting units: Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Illinois Transmission. See Note 16 – Segment Information for additional information related to the change in Ameren’s and Ameren Illinois' segments. As of the date of the segment change, December 31, 2016, Ameren and Ameren Illinois reassigned goodwill to the new reporting units using a relative fair value allocation approach. The Level 3 fair value hierarchy valuation approach used to reassign goodwill was based on a market participant view and used a weighted combination of a discounted cash flow analysis and a market multiples analysis. Key assumptions used in estimating the fair value of the reporting units included discount and growth rates, utility sector market performance and transactions, and projected operating results and cash flows. As a result of the goodwill reassignment, Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Illinois Transmission had goodwill of $238 million , $80 million , and $93 million , respectively, at December 31, 2016. The Ameren Transmission reporting unit was reassigned the same $93 million of goodwill as the Ameren Illinois Transmission reporting unit. In conjunction with the goodwill reassignment, Ameren and Ameren Illinois completed the first step of the quantitative test to determine whether the fair values of the new reporting units exceeded their carrying values as of December 31, 2016. Ameren and Ameren Illinois determined that the estimated fair values of the Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, Ameren Illinois Transmission, and Ameren Transmission reporting units each exceeded their respective carrying values by at least 40% , indicating no impairment of Ameren’s or Ameren Illinois’ goodwill. The Ameren and Ameren Illinois goodwill that was reassigned to the new reporting units on December 31, 2016, had no accumulated goodwill impairment losses.
Impairment of Long-lived Assets	We evaluate long-lived assets classified as held and used for impairment when events or changes in circumstances indicate that the carrying value of such assets may not be recoverable. Whether an impairment has occurred is determined by comparing the estimated undiscounted cash flows attributable to the assets to the carrying value of the assets. If the carrying value exceeds the undiscounted cash flows, we recognize an impairment charge equal to the amount by which the carrying value exceeds the estimated fair value of the assets. In the period in which we determine an asset meets held for sale criteria, we record an impairment charge to the extent the book value exceeds its estimated fair value less cost to sell. We did not identify any events or changes in circumstances that indicated that the carrying value of long-lived assets may not be recoverable in 2016 and 2015 .
Asset Retirement Obligations	Asset Retirement Obligations We record the estimated fair value of legal obligations associated with the retirement of tangible long-lived assets in the period in which the liabilities are incurred and capitalize a corresponding amount as part of the book value of the related long-lived asset. In subsequent periods, we adjust AROs based on changes in the estimated fair values of the obligations with a corresponding increase or decrease in the asset book value. Asset book values, reflected within "Property, Plant, and Equipment, Net" on the balance sheet, are depreciated over the remaining useful life of the related asset. Due to regulatory recovery, that depreciation is recorded within a regulatory asset or liability balance related to AROs. Ameren and Ameren Missouri have a nuclear decommissioning trust fund for the decommissioning of the Callaway energy center. Net realized and unrealized gains and losses within the nuclear decommissioning trust fund are deferred and are currently recorded as a regulatory liability, along with the depreciation of the asset book values, discussed above, and the accretion of the AROs. The depreciation of the asset book values at Ameren Missouri was $31 million , $13 million , and $1 million for the years ended December 31, 2016, 2015, and 2014, respectively, which was recorded as a reduction to the regulatory liability. The depreciation recorded to the regulatory asset at Ameren Illinois was immaterial in each respective period. Uncertainties as to the probability, timing, or amount of cash expenditures associated with AROs affect our estimates of fair value. Ameren and Ameren Missouri have recorded AROs for retirement costs associated with Ameren Missouri’s Callaway energy center decommissioning, CCR facilities, and river structures. Also, Ameren, Ameren Missouri, and Ameren Illinois have recorded AROs for retirement costs associated with asbestos removal and the disposal of certain transformers. Asset removal costs that do not constitute legal obligations are classified as regulatory liabilities. See Note 2 – Rate and Regulatory Matters.
Noncontrolling Interest	Noncontrolling Interests As of December 31, 2016 and 2015 , Ameren’s noncontrolling interests included the preferred stock of Ameren Missouri and Ameren Illinois.
Revenue	Operating Revenue The Ameren Companies record operating revenue for electric or natural gas service when it is delivered to customers. We accrue an estimate of electric and natural gas revenues for service rendered but unbilled at the end of each accounting period. Ameren Illinois participates in the performance-based formula ratemaking framework pursuant to the IEIMA. In addition, Ameren Illinois' and ATXI's electric transmission service operating revenues are regulated by the FERC. The provisions of the IEIMA and the FERC's electric transmission formula rate framework provide for annual reconciliations of the electric distribution and electric transmission service revenue requirements necessary to reflect the actual recoverable costs incurred in a given year with the revenue requirements in customer rates for that year, including an allowed return on equity. In each of those electric jurisdictions, if the current year's revenue requirement is greater than the revenue requirement reflected in that year's customer rates, an increase to electric operating revenues with an offset to a regulatory asset is recorded to reflect the expected recovery of those additional amounts from customers within two years. In each jurisdiction, if the current year's revenue requirement is less than the revenue requirement reflected in that year's customer rates, a reduction to electric operating revenues with an offset to a regulatory liability is recorded to reflect the expected refund to customers within two years. See Note 2 – Rate and Regulatory Matters for information regarding Ameren Illinois' revenue requirement reconciliation pursuant to the IEIMA.
Cost Of Sales	Nuclear Fuel Ameren Missouri’s cost of nuclear fuel is capitalized and then amortized to fuel expense on a unit-of-production basis. The cost is charged to "Operating Expenses – Fuel" in the statement of income. Accounting for MISO Transactions MISO-related purchase and sale transactions are recorded by Ameren, Ameren Missouri, and Ameren Illinois using settlement information provided by MISO. Ameren Missouri records these purchase and sale transactions on a net hourly position. Ameren Missouri records net purchases in a single hour in “Operating Expenses – Purchased power” and net sales in a single hour in “Operating Revenues – Electric” in its statement of income. Ameren Illinois records net purchases in “Operating Expenses – Purchased power” in its statement of income to reflect all of its MISO transactions relating to the procurement of power for its customers. On occasion, Ameren Missouri's and Ameren Illinois' prior-period transactions will be resettled outside the routine settlement process because of a change in MISO’s tariff or a material interpretation thereof. In these cases, Ameren Missouri and Ameren Illinois recognize expenses associated with resettlements once the resettlement is probable and the resettlement amount can be estimated. Revenues are recognized once the resettlement amount is received.
Stock-Based Compensation	Stock-based Compensation Stock-based compensation cost is measured at the grant date based on the fair value of the award, net of an assumed forfeiture rate. Ameren recognizes as compensation expense the estimated fair value of stock-based compensation on a straight-line basis over the requisite service period.
Excise Taxes	Excise Taxes Ameren Missouri and Ameren Illinois collect from their customers certain excise taxes 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. They are recorded gross in “Operating Revenues – Electric,” “Operating Revenues – Natural 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 customers and are therefore not included in Ameren Illinois' revenues and expenses.
Unamortized Debt Discount, Premium, And Expense	Unamortized Debt Discounts, Premiums, and Issuance Costs Long-term debt discounts, premiums, and issuance costs are amortized over the lives of the related issuances. Credit agreement fees are amortized over the term of that agreement.
Income Taxes	Income Taxes Ameren uses an asset and liability approach for its financial accounting and reporting of income taxes. Deferred tax assets and liabilities are recognized for transactions that are treated differently for financial reporting and income tax return purposes. These deferred tax assets and liabilities are based on statutory tax rates. We recognize that regulators will probably reduce future revenues for deferred tax liabilities that were initially recorded at rates in excess of the current statutory rate. Therefore, reductions in deferred tax liabilities that were recorded because of decreases in the statutory rate have been credited to a regulatory liability. A regulatory asset has been established to recognize the probable recovery through future customer rates of tax benefits related to the equity component of allowance for funds used during construction, as well as the effects of tax rate increases. Investment tax credits used on tax returns for prior years have been deferred as a noncurrent liability. The credits are being amortized over the useful lives of the related investment. Deferred income taxes were recorded on the temporary difference represented by the deferred investment tax credits and a corresponding regulatory liability. This recognizes the expected reduction in rates for future lower income taxes associated with the amortization of the investment tax credits. See Note 13 – Income Taxes. Ameren Missouri, Ameren Illinois, and all the other Ameren subsidiary companies are parties to a tax allocation agreement with Ameren (parent) that provides for the allocation of consolidated tax liabilities. The tax allocation agreement specifies that each party be allocated an amount of tax using a stand-alone calculation, which is similar to that which would be owed or refunded had the party been separately subject to tax considering the impact of consolidation. Any net benefit attributable to the parent is reallocated to the other parties. This reallocation is treated as a capital contribution to the party receiving the benefit.
Earnings Per Share	Earnings per Share Basic earnings per share is computed by dividing “Net Income Attributable to Ameren Common Shareholders” 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 Shareholders” 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 0.8 million , 1.0 million , and 1.8 million for the years ended December 31, 2016 , 2015 , and 2014 , respectively. There were no potentially dilutive securities excluded from the diluted earnings per share calculations for the years ended December 31, 2016 , 2015 , and 2014 .
Accounting Changes and Other Matters	Accounting Changes and Other Matters The following is a summary of recently adopted authoritative accounting guidance, as well as guidance issued but not yet adopted, that could affect the Ameren Companies. Revenue from Contracts with Customers In May 2014, the FASB issued authoritative guidance that changes the criteria for recognizing revenue from a contract with a customer. The underlying principle of the guidance is that an entity will recognize revenue for the transfer of promised goods or services to customers at an amount that the entity expects to be entitled to in exchange for those goods or services. The guidance also requires additional disclosures to enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers. Entities can apply the guidance retrospectively to each reporting period presented, the full retrospective method, or retrospectively by recording a cumulative effect adjustment to retained earnings in the period of initial adoption, the modified retrospective method. The utility industry continues to assess the impacts on accounting for contributions in aid of construction and similar arrangements, and collectibility, among other issues. The outcome of these assessments could have a significant impact on our results of operations and financial position. We plan to complete our assessment of the impacts of this guidance on our results of operations, financial position, presentation and disclosures, and transition method, in the next several months prior to our adoption in the first quarter of 2018. Amendments to the Consolidation Analysis In February 2015, the FASB issued authoritative guidance that amends the consolidation analysis for variable interest entities and voting interest entities. The new guidance affects (1) limited partnerships, similar legal entities, and certain investment funds, (2) the evaluation of fees paid to a decision maker or service provider as a variable interest, (3) how fee arrangements impact the primary beneficiary determination, and (4) the evaluation of related party relationships on the primary beneficiary determination. The adoption of this guidance in 2016 did not impact the Ameren Companies' results of operations, financial position, cash flows, or disclosures. Disclosures for Investments in Certain Entities That Calculate Net Asset Value per Share In May 2015, to address diversity in practice, the FASB issued authoritative guidance that removes the requirement to categorize within the fair value hierarchy all investments for which fair value is measured using the NAV practical expedient. The Ameren Companies have investments measured using the NAV practical expedient within the pension plan and postretirement benefit plan assets. We adopted this guidance on January 1, 2016 and retrospectively updated the presentation of these assets in the fair value hierarchy tables included in Note 11 - Retirement Benefits. The adoption of this guidance did not impact our results of operations, financial position or cash flows. Financial Instruments - Recognition and Measurement, and Credit Losses In January 2016, the FASB issued authoritative guidance that addressed certain aspects of recognition, measurement, presentation and disclosure of financial instruments. This guidance requires an entity to measure equity investments, other than those accounted for under the equity method of accounting, at fair value with changes in fair value recognized in net income. The recognition and measurement guidance will be effective for the Ameren Companies in the first quarter of 2018, and requires changes to be applied retrospectively with a cumulative effect adjustment to retained earnings as of the adoption date. Also, in June 2016, the FASB issued authoritative guidance that requires an entity to recognize an allowance for financial instruments that reflects its current estimate of credit losses expected to be incurred over the life of the financial instruments. The guidance requires an entity to measure expected credit losses based on relevant information about past events, current conditions, and reasonable and supportable forecasts that affect the collectibility of the reported amount. The credit loss guidance will be effective for the Ameren Companies in the first quarter of 2020, and requires changes to be applied retrospectively with a cumulative effect adjustment to retained earnings as of the adoption date. We are currently assessing the impacts of the new financial instruments guidance on our results of operations, financial position, and disclosures. Leases In February 2016, the FASB issued authoritative guidance that requires an entity to recognize assets and liabilities arising from all leases with a term greater than one year. Consistent with current GAAP, the recognition, measurement, and presentation of expenses and cash flows arising from a lease will depend on its classification as a finance or operating lease. The guidance also requires additional disclosures to enable users of financial statements to understand the amount, timing, and uncertainty of cash flows arising from leases. This guidance will affect the Ameren Companies' financial position by increasing the assets and liabilities recorded relating to their operating leases, which will be recognized and measured at the beginning of the earliest period presented. We are currently assessing the impacts of this guidance for other effects on our results of operations, cash flows and disclosures. We expect to adopt this guidance in the first quarter of 2019. See Note 15 – Commitments and Contingencies for additional information on our leases. Improvements to Employee Share-Based Payment Accounting In March 2016, the FASB issued authoritative guidance that simplifies the accounting for share-based payment transactions, including the income tax consequences, the calculation of diluted earnings per share, the treatment of forfeitures, the classification of awards as either equity or liabilities, and the classification on the statement of cash flows. Ameren determines for each performance share unit award whether the difference between the deduction for tax purposes and the compensation cost recognized for financial reporting purposes results in either an excess tax benefit or an excess tax deficit. Previously, excess tax benefits were recognized in "Other paid-in capital" on Ameren’s consolidated balance sheet, and in certain cases, excess tax deficits were recognized in “Income taxes” on Ameren’s consolidated income statement. The new guidance increases income statement volatility by requiring all excess tax benefits and deficits to be recognized in “Income taxes,” and treated as discrete items in the period in which they occur. Ameren adopted this guidance in 2016 and prospectively applied the amendment in this guidance requiring recognition of excess tax benefits and deficits in the income statement, which resulted in recognition of a $21 million income tax benefit and a corresponding $21 million increase in income from continuing operations and net income ( 9 cents per diluted share) during the period. Also as a result of the adoption of this guidance, Ameren made an accounting policy election to continue to estimate the number of forfeitures expected to occur. The amendments in the guidance that require application using a modified retrospective transition method did not impact Ameren. Therefore, there was no cumulative-effect adjustment to retained earnings recognized as of January 1, 2016. Ameren applied the amendments in this guidance relating to classification on the statement of cash flows retrospectively. For the year ended December 31, 2015, Ameren reclassified $2 million of excess tax benefits on the statement of cash flows from financing to operating activity. Also, for the years ended December 31, 2015 and December 31, 2014, Ameren reclassified $12 million and $14 million , respectively, of employee payroll taxes related to share-based payments from operating to financing activity. Restricted Cash In November 2016, the FASB issued authoritative guidance that requires restricted cash and restricted cash equivalents to be included with cash and cash equivalents when reconciling the beginning-of-period and end-of-period total amounts shown on the statement of cash flows. We are currently assessing the impacts of this guidance on our statements of cash flows and disclosures. The guidance will be effective for the Ameren Companies in the first quarter of 2018, and requires changes to be applied retrospectively to each period presented.

Derivative Financial Instrume31

Derivative Financial Instruments Derivative Financial Instruments (Policies)	12 Months Ended
	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivatives, Policy [Policy Text Block]	The Ameren Companies elect to present the fair value amounts of derivative assets and derivative liabilities subject to an enforceable master netting arrangement or similar agreement gross on the balance sheet. 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.

Summary Of Significant Accoun32

Summary Of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2016
Accounting Policies [Abstract]
Schedule Of Inventories	The following table presents a breakdown of inventories for each of the Ameren Companies at December 31, 2016 and 2015 : Ameren Missouri Ameren Illinois Ameren 2016 Fuel (a) $ 172 $ — $ 172 Natural gas stored underground 9 73 82 Other inventories 211 62 273 Total inventories $ 392 $ 135 $ 527 2015 Fuel (a) $ 173 $ — $ 173 Natural gas stored underground 10 87 97 Other inventories 204 64 268 Total inventories $ 387 $ 151 $ 538 (a) Consists of coal, oil, and propane.
Schedule Of Rates Used For Allowance For Funds Used During Construction	The following table presents the annual allowance for funds used during construction debt and equity blended rates that were applied to construction projects in 2016 , 2015 , and 2014 : 2016 2015 2014 Ameren Missouri 7 % 7 % 7 % Ameren Illinois 5 % 6 % 2 %
Schedule Of Asset Retirement Obligations	The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the years ended December 31, 2016 and 2015 : Ameren Missouri Ameren Illinois Ameren Balance at December 31, 2014 $ 389 $ 7 $ 396 Liabilities incurred 3 — 3 Liabilities settled (1 ) (1 ) (2 ) Accretion in 2015 (a) 23 (b) 23 Change in estimates (c) 203 (b) 203 Balance at December 31, 2015 $ 617 (e) $ 6 (d) $ 623 (e) Liabilities incurred 3 — 3 Liabilities settled (2 ) (b) (2 ) Accretion in 2016 (a) 25 (b) 25 Change in estimates 1 — 1 Balance at December 31, 2016 $ 644 (e) $ 6 (d) $ 650 (e) (a) Accretion expense was recorded as a decrease to regulatory liabilities. (b) Less than $1 million. (c) The ARO increase resulted in a corresponding increase recorded to "Property, Plant, and Equipment, Net." 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, the 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 $100 million to their AROs associated with CCR storage facilities. See Note 15 – 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. (d) Included in “Other deferred credits and liabilities” on the balance sheet. (e) Balance included $5 million and $15 million in "Other current liabilities" on the balance sheet as of December 31, 2015 and 2016, respectively.
Schedule Of Excise Taxes	The following table presents the excise taxes recorded in “Operating Revenues – Electric,” “Operating Revenues – Natural gas,” and “Operating Expenses – Taxes other than income taxes” for the years ended December 31, 2016 , 2015 , and 2014 : 2016 2015 2014 Ameren Missouri $ 151 $ 156 $ 151 Ameren Illinois 57 57 64 Ameren $ 208 $ 213 $ 215

Rate And Regulatory Matters (Ta

Rate And Regulatory Matters (Tables)	12 Months Ended
	Dec. 31, 2016
Public Utilities, General Disclosures [Abstract]
Schedule Of Regulatory Assets And Liabilities	The following table presents our regulatory assets and regulatory liabilities at December 31, 2016 and 2015 : 2016 2015 Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Current regulatory assets: Under-recovered FAC (a)(b) $ 21 $ — $ 21 $ 37 $ — $ 37 Under-recovered Illinois electric power costs (c) — 3 3 — 3 3 Under-recovered PGA (c) — 4 4 — 8 8 MTM derivative losses (d) 9 15 24 29 45 74 Energy efficiency riders (e) 5 — 5 23 — 23 IEIMA revenue requirement reconciliation adjustment (a)(f) — 68 68 — 103 103 FERC revenue requirement reconciliation adjustment (a)(g) — 7 13 — 8 12 VBA rider (a)(h) — 11 11 — — — Total current regulatory assets $ 35 $ 108 $ 149 $ 89 $ 167 $ 260 Noncurrent regulatory assets: Pension and postretirement benefit costs (i) $ 175 $ 319 $ 494 $ 95 $ 202 $ 297 Income taxes (j) 229 1 230 247 4 251 Uncertain tax positions tracker (a)(k) 7 — 7 7 — 7 ARO (l) — 3 3 — 4 4 Callaway costs (a)(m) 29 — 29 32 — 32 Unamortized loss on reacquired debt (a)(n) 65 59 124 69 69 138 Environmental cost riders (o) — 196 196 — 230 230 MTM derivative losses (d) 9 178 187 15 175 190 Storm costs (a)(p) — 15 15 — 9 9 Demand-side costs before the MEEIA implementation (a)(q) 18 — 18 31 — 31 Workers’ compensation claims (r) 6 7 13 6 7 13 Credit facilities fees (s) 4 — 4 4 — 4 Construction accounting for pollution control equipment (a)(t) 19 — 19 20 — 20 Solar rebate program (a)(u) 49 — 49 74 — 74 IEIMA revenue requirement reconciliation adjustment (a)(f) — 23 23 — 62 62 FERC revenue requirement reconciliation adjustment (a)(g) — 8 10 — 5 11 Other 9 7 16 5 4 9 Total noncurrent regulatory assets $ 619 $ 816 $ 1,437 $ 605 $ 771 $ 1,382 Current regulatory liabilities: Over-recovered FAC (b) $ — $ — $ — $ 9 $ — $ 9 Over-recovered Illinois electric power costs (c) — 25 25 — 6 6 Over-recovered PGA (c) — — — 3 — 3 MTM derivative gains (d) 12 11 23 16 1 17 Estimated refund for FERC complaint cases (v) — 42 62 — 32 45 Total current regulatory liabilities $ 12 $ 78 $ 110 $ 28 $ 39 $ 80 Noncurrent regulatory liabilities: Income taxes (w) $ 33 $ 4 $ 37 $ 36 $ 6 $ 42 Uncertain tax positions tracker (k) 3 — 3 6 — 6 Asset removal costs (x) 970 697 1,669 933 671 1,605 ARO (l) 162 — 162 167 — 167 Bad debt rider (y) — 3 3 — 6 6 Pension and postretirement benefit costs tracker (z) 35 — 35 19 — 19 Energy efficiency riders (e) — 45 45 — 36 36 Renewable energy credits (aa) — 15 15 — 12 12 Storm tracker (ab) 7 — 7 9 — 9 Other 5 4 9 2 1 3 Total noncurrent regulatory liabilities $ 1,215 $ 768 $ 1,985 $ 1,172 $ 732 $ 1,905 (a) These assets earn a return. (b) Under-recovered or over-recovered fuel costs to be recovered or refunded through the FAC. Specific accumulation periods aggregate the under-recovered or over-recovered costs over four months, any related adjustments that occur over the following four months, and the recovery from or refund to customers that occurs over the next eight months. (c) Under-recovered or over-recovered costs from utility customers. Amounts will be recovered from or refunded to customers within one year of the deferral. (d) Deferral of commodity-related derivative MTM losses or gains. See Note 7 – Derivative Financial Instruments for additional information. (e) The Ameren Missouri balance relates to the MEEIA. The MEEIA rider allows Ameren Missouri to collect from or refund to customers any annual difference in the actual amounts incurred and the amounts collected from customers for the MEEIA program costs, net shared benefits, and the throughput disincentive. Under the MEEIA rider, collections from or refunds to customers occur one year after the program costs, net shared benefits, and the throughput disincentive are incurred. The Ameren Illinois balance relates to a regulatory tracking mechanism to recover its electric and natural gas costs associated with developing, implementing, and evaluating customer energy efficiency and demand response programs. Any under-recovery or over-recovery will be collected from or refunded to customers over the year following the plan year. (f) The difference between Ameren Illinois' annual revenue requirement calculated under the IEIMA's performance-based formula ratemaking framework and the revenue requirement included in customer rates for that year. The under-recovery will be recovered from or refunded to customers with interest within two years. (g) Ameren Illinois' and ATXI's annual revenue requirement reconciliation calculated pursuant to the FERC's electric transmission formula ratemaking framework. The under-recovery or over-recovery will be recovered from or refunded to customers within two years. (h) Under-recovered natural gas sales volumes, including deviations from normal weather conditions. Each year's amount will be recovered from or refunded to customers from April through December of the following year. (i) These costs are being amortized in proportion to the recognition of prior service costs (credits) and actuarial losses (gains) attributable to Ameren’s pension plan and postretirement benefit plans. See Note 11 – Retirement Benefits for additional information. (j) Tax benefits related to the equity component of allowance for funds used during construction, as well as the effects of tax rate changes. This amount will be recovered over the expected life of the related assets. (k) The tracker is amortized over three years, beginning from the date the amounts are included in rates. See Note 13 – Income Taxes for additional information. (l) Recoverable or refundable removal costs for AROs, including net realized and unrealized gains and losses related to the nuclear decommissioning trust fund investments. See Note 1 – Summary of Significant Accounting Policies – Asset Retirement Obligations. (m) Ameren Missouri’s Callaway energy center operations and maintenance expenses, property taxes, and carrying costs incurred between the plant in-service date and the date the plant was reflected in rates. These costs are being amortized over the remaining life of the energy center's original operating license through 2024. (n) Losses related to reacquired debt. These amounts are being amortized over the lives of the related new debt issuances or the original lives of the old debt issuances if no new debt was issued. (o) The recoverable portion of accrued environmental site liabilities that will be collected from electric and natural gas customers through ICC-approved cost recovery riders. The period of recovery will depend on the timing of remediation expenditures. See Note 15 – Commitments and Contingencies for additional information. (p) Storm costs from 2013, 2015, and 2016 deferred in accordance with the IEIMA. These costs are being amortized over five-year periods beginning in the year the storm occurred. (q) Demand-side costs incurred prior to implementation of the MEEIA in 2013, including the costs of developing, implementing, and evaluating customer energy efficiency and demand response programs. Costs incurred from May 2008 through September 2008 are being amortized over a 10-year period that began in March 2009. Costs incurred from October 2008 through December 2009 are being amortized until May 2017. Costs incurred from January 2010 through February 2011 are being amortized over a six-year period that began in August 2011. Costs incurred from March 2011 through July 2012 are being amortized over a six-year period that began in January 2013. Costs incurred from August 2012 through December 2012 are being amortized over a six-year period that began in June 2015. The February 2017 stipulation and agreement, if approved, would modify these amortization periods. (r) The period of recovery will depend on the timing of actual expenditures. (s) Ameren Missouri’s costs incurred to enter into and maintain the Missouri Credit Agreement. These costs are being amortized over the life of the credit facility to construction work in progress, which will be depreciated when assets are placed into service. Additional costs were incurred in December 2016 to amend and restate the Missouri Credit Agreement. (t) The MoPSC’s May 2010 electric rate order allowed Ameren Missouri to record an allowance for funds used during construction for pollution control equipment at its Sioux energy center until the cost of that equipment was included in customer rates beginning in 2011. These costs are being amortized over the expected life of the Sioux energy center, currently through 2033. (u) Costs associated with Ameren Missouri's solar rebate program to fulfill its renewable energy portfolio requirement. These costs are being amortized over a three-year period that began in June 2015. The February 2017 stipulation and agreement, if approved, would modify this amortization period. (v) Estimated refunds to transmission customers related to FERC orders. See FERC Complaint Cases above. (w) Unamortized portion of investment tax credits and reductions to deferred tax liabilities recorded at rates in excess of current statutory rates. The unamortized portion of investment tax credits and the reduction to deferred tax liabilities are being amortized over the expected life of the underlying assets. (x) Estimated funds collected for the eventual dismantling and removal of plant retired from service, net of salvage value. (y) A regulatory tracking mechanism for the difference between the level of bad debt incurred by Ameren Illinois under GAAP and the level of such costs included in electric and natural gas rates. The over-recovery relating to 2014 was refunded to customers from June 2015 through May 2016. The over-recovery relating to 2015 is being refunded to customers from June 2016 through May 2017. The over-recovery relating to 2016 will be refunded to customers from June 2017 through May 2018. (z) A regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. For periods prior to December 2014, the MoPSC's April 2015 electric rate order directed the amortization to occur over three to five years, beginning in June 2015. For periods after December 2014, the amortization period will be determined in the July 2016 electric rate case. The February 2017 stipulation and agreement, if approved, would modify these amortization periods. (aa) Funds collected from customers for the purchase of renewable energy credits through IPA procurements for distributed generation. The balance will be amortized as renewable energy credits are purchased. (ab) A regulatory tracking mechanism at Ameren Missouri for the difference between the level of storm costs incurred in a particular year and the level of such costs included in rates. For periods prior to December 2014, the MoPSC's April 2015 electric rate order directed the amortization to occur over a five-year period that began in June 2015. For periods after December 2014, the amortization period will be determined in the July 2016 electric rate case. The April 2015 MoPSC order did not approve the continued use of the storm cost regulatory tracking mechanism.

Property And Plant, Net (Tables

Property And Plant, Net (Tables)	12 Months Ended
	Dec. 31, 2016
Property, Plant and Equipment [Abstract]
Schedule Of Property And Plant, Net	The following table presents property, plant, and equipment, net, for each of the Ameren Companies at December 31, 2016 and 2015 : Ameren Missouri (a) Ameren Illinois Other Ameren (a) 2016 Property, plant, and equipment at original cost: (b) Electric generation $ 10,911 $ — $ — $ 10,911 Electric distribution 5,563 5,287 — 10,850 Electric transmission 1,151 2,016 712 3,879 Natural gas 455 2,186 — 2,641 Other (c) 879 719 239 1,837 18,959 10,208 951 30,118 Less: Accumulated depreciation and amortization 7,880 2,850 231 10,961 11,079 7,358 720 19,157 Construction work in progress: Nuclear fuel in process 206 — — 206 Other 193 111 446 750 Property and plant, net $ 11,478 $ 7,469 $ 1,166 $ 20,113 2015 Property, plant, and equipment at original cost: (b) Electric generation $ 10,431 $ — $ — $ 10,431 Electric distribution 5,303 4,952 — 10,255 Electric transmission 979 1,674 121 2,774 Natural gas 445 1,997 — 2,442 Other (c) 808 627 266 1,701 17,966 9,250 387 27,603 Less: Accumulated depreciation and amortization 7,460 2,632 255 10,347 10,506 6,618 132 17,256 Construction work in progress: Nuclear fuel in process 275 — — 275 Other 402 230 636 1,268 Property, plant, and equipment, net $ 11,183 $ 6,848 $ 768 $ 18,799 (a) Amounts in Ameren and Ameren Missouri include two CTs under separate capital lease agreements. The gross cumulative asset value of those agreements was $232 million and $233 million at December 31, 2016 and 2015, respectively. The total accumulated depreciation associated with the two CTs was $77 million and $72 million at December 31, 2016 and 2015 , respectively. In addition, Ameren Missouri has investments in debt securities, classified as held-to-maturity and recorded in "Other assets" that are related to the two CTs from the city of Bowling Green and Audrain County. As of December 31, 2016 and 2015, the carrying value of these debt securities was $282 million and $288 million , respectively. (b) The estimated lives for each asset group are as follows: 5 to 100 years for electric generation, excluding Ameren Missouri's hydro generating assets which have useful lives of up to 150 years, 18 to 75 years for electric distribution, 50 to 75 years for electric transmission, 20 to 80 years for natural gas, and 5 to 55 years for other. (c) Other property, plant, and equipment includes assets used to support multiple utility services.
Accrued Capital Expenditures	The following table provides accrued capital and nuclear fuel expenditures at December 31, 2016 , 2015 , and 2014 , which represent noncash investing activity excluded from the accompanying statements of cash flows: Ameren (a) Ameren Missouri Ameren Illinois Accrued capital expenditures: 2016 $ 251 $ 116 $ 87 2015 235 85 92 2014 181 72 59 Accrued nuclear fuel expenditures: 2016 20 20 (b) 2015 16 16 (b) 2014 13 13 (b) (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries. (b) Not applicable.

Short-Term Debt And Liquidity (

Short-Term Debt And Liquidity (Tables)	12 Months Ended
	Dec. 31, 2016
Short-term Debt [Line Items]
Schedule Of Maximum Aggregate Amount Available On Credit Agreements	The following table presents the maximum aggregate amount available to each borrower under each facility (the amount being each borrower's "Borrowing Sublimit"): Missouri Credit Agreement Illinois Credit Agreement Ameren $ 700 $ 500 Ameren Missouri 800 (a) Ameren Illinois (a) 800 (a) Not applicable.
Schedule of Commercial Paper	The following table summarizes the borrowing activity and relevant interest rates under Ameren (parent)'s, Ameren Missouri's and Ameren Illinois' commercial paper programs for the years ended December 31, 2016 and 2015 : Ameren (parent) Ameren Missouri Ameren Illinois Ameren Consolidated 2016 Average daily commercial paper outstanding $ 440 $ 60 $ 52 $ 552 Outstanding borrowings at period-end 507 — 51 558 Weighted-average interest rate 0.82 % 0.74 % 0.69 % 0.80 % Peak outstanding commercial paper during period (a) $ 574 $ 208 $ 195 $ 839 Peak interest rate 1.05 % 0.85 % 0.90 % 1.05 % 2015 Average daily commercial paper outstanding $ 721 $ 42 $ 4 $ 767 Outstanding borrowings at period-end 301 — — 301 Weighted-average interest rate 0.57 % 0.50 % 0.44 % 0.55 % Peak outstanding commercial paper during period (a) $ 874 $ 294 $ 48 $ 1,108 Peak interest rate 0.91 % 0.60 % 0.60 % 0.91 % (a) The timing of peak outstanding commercial paper issuances varies by company. Therefore, the sum of the peak amounts presented by company might not equal the Ameren Consolidated peak amount for the period.

Long-Term Debt And Equity Fin36

Long-Term Debt And Equity Financings (Tables)	12 Months Ended
	Dec. 31, 2016
Debt Instrument [Line Items]
Schedule of Long-term Debt Instruments	The following table presents long-term debt outstanding, including maturities due within one year, for the Ameren Companies as of December 31, 2016 and 2015 : 2016 2015 Ameren (Parent): 2.70% Senior unsecured notes due 2020 $ 350 $ 350 3.65% Senior unsecured notes due 2026 350 350 Total long-term debt, gross 700 700 Less: Unamortized debt issuance costs (6 ) (6 ) Long-term debt, net $ 694 $ 694 Ameren Missouri: Senior secured notes: (a) 5.40% Senior secured notes due 2016 — 260 6.40% Senior secured notes due 2017 425 425 6.00% Senior secured notes due 2018 (b) 179 179 5.10% Senior secured notes due 2018 199 199 6.70% Senior secured notes due 2019 (b) 329 329 5.10% Senior secured notes due 2019 244 244 5.00% Senior secured notes due 2020 85 85 3.50% Senior secured notes due 2024 350 350 5.50% Senior secured notes due 2034 184 184 5.30% Senior secured notes due 2037 300 300 8.45% Senior secured notes due 2039 (b) 350 350 3.90% Senior secured notes due 2042 (b) 485 485 3.65% Senior secured notes due 2045 400 250 Environmental improvement and pollution control revenue bonds: 1992 Series due 2022 (c)(d) 47 47 1993 5.45% Series due 2028 (e) (e) (e) 1998 Series A due 2033 (c)(d) 60 60 1998 Series B due 2033 (c)(d) 50 50 1998 Series C due 2033 (c)(d) 50 50 Capital lease obligations: City of Bowling Green capital lease (Peno Creek CT) due 2022 42 48 Audrain County capital lease (Audrain County CT) due 2023 240 240 Total long-term debt, gross 4,019 4,135 Less: Unamortized discount and premium (6 ) (6 ) Less: Unamortized debt issuance costs (19 ) (19 ) Less: Maturities due within one year (431 ) (266 ) Long-term debt, net $ 3,563 $ 3,844 2016 2015 Ameren Illinois: Senior secured notes: 6.20% Senior secured notes due 2016 $ — $ 54 6.25% Senior secured notes due 2016 — 75 6.125% Senior secured notes due 2017 (g)(h) 250 250 6.25% Senior secured notes due 2018 (g)(h) 144 144 9.75% Senior secured notes due 2018 (g)(h) 313 313 2.70% Senior secured notes due 2022 (g)(h) 400 400 3.25% Senior secured notes due 2025 (g) 300 300 6.125% Senior secured notes due 2028 (g) 60 60 6.70% Senior secured notes due 2036 (g) 61 61 6.70% Senior secured notes due 2036 (f) 42 42 4.80% Senior secured notes due 2043 (g) 280 280 4.30% Senior secured notes due 2044 (g) 250 250 4.15% Senior secured notes due 2046 (g) 490 250 Environmental improvement and pollution control revenue bonds: 5.90% Series 1993 due 2023 (i) (i) (i) 5.70% 1994A Series due 2024 (j) (j) (j) 1993 Series B-1 due 2028 (d)(k) 17 17 Total long-term debt, gross 2,607 2,496 Less: Unamortized discount and premium — (7 ) Less: Unamortized debt issuance costs (19 ) (18 ) Less: Maturities due within one year (250 ) (129 ) Long-term debt, net $ 2,338 $ 2,342 Ameren consolidated long-term debt, net $ 6,595 $ 6,880 (a) These notes are collaterally secured by first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage indenture. The notes have a fall-away lien provision and will remain secured only as long as any first mortgage bonds issued under the Ameren Missouri mortgage indenture remain outstanding. Redemption, purchase, or maturity of all first mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the first mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the Ameren Missouri senior secured notes currently outstanding, we do not expect the first mortgage bond lien protection associated with these notes to fall away before 2042. (b) Ameren Missouri has agreed that so long as any of the 3.90% senior secured notes due 2042 are outstanding, Ameren Missouri will not permit a release date to occur, and so long as any of the 6.70% senior secured notes due 2019, 6.00% senior secured notes due 2018, and 8.45% senior secured notes due 2039 are outstanding, Ameren Missouri will not optionally redeem, purchase, or otherwise retire in full the outstanding first mortgage bonds not subject to release provisions. (c) These bonds are collaterally secured by first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage indenture and have a fall-away lien provision similar to that of Ameren Missouri's senior secured notes. The bonds are also backed by an insurance guarantee policy. (d) The interest rates and the periods during which such rates apply vary depending on our selection of defined rate modes. Maximum interest rates could reach 18% , depending on the series of bonds. The bonds are callable at 100% of par value. The average interest rates for 2016 and 2015 were as follows: 2016 2015 Ameren Missouri 1992 Series due 2022 0.66% 0.06% Ameren Missouri 1998 Series A due 2033 0.91% 0.24% Ameren Missouri 1998 Series B due 2033 0.92% 0.24% Ameren Missouri 1998 Series C due 2033 0.97% 0.24% Ameren Illinois 1993 Series B-1 due 2028 0.70% 0.49% (e) These bonds are first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage bond indenture and are secured by substantially all Ameren Missouri property and franchises. The bonds are callable at 100% of par value. Less than $1 million principal amount of the bonds remain outstanding. (f) These notes are collaterally secured by first mortgage bonds issued by Ameren Illinois under its 1933 mortgage indenture. The notes have a fall-away lien provision, and Ameren Illinois could cause these notes to become unsecured at any time by redeeming the pollution control bonds 5.90% Series 1993 due 2023 (of which less than $1 million remains outstanding). (g) These notes are collaterally secured by mortgage bonds issued by Ameren Illinois under its 1992 mortgage indenture. They are secured by substantially all property of the former IP and CIPS. The notes have a fall-away lien provision and will remain secured only as long as any series of first mortgage bonds issued under its 1992 mortgage indenture remain outstanding. Redemption, purchase, or maturity of all mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the Ameren Illinois senior secured notes currently outstanding, we do not expect the mortgage bond lien protection associated with these notes to fall away before 2022. (h) Ameren Illinois has agreed that so long as any of the 2.70% senior secured notes due 2022 are outstanding, Ameren Illinois will not permit a release date to occur, and so long as any of the 9.75% senior secured notes due 2018, 6.25% senior secured notes due 2018, and 6.125% senior secured notes due 2017 are outstanding, Ameren Illinois will not optionally redeem, purchase or otherwise retire in full the outstanding first mortgage bonds not subject to release provisions; therefore, a release date will not occur so long as any of these notes remain outstanding. (i) These bonds are first mortgage bonds issued by Ameren Illinois under its 1933 mortgage indenture. They are secured by substantially all property of the former CILCO. The bonds are callable at 100% of par value. Less than $1 million principal amount of the bonds remain outstanding. (j) These bonds are mortgage bonds issued by Ameren Illinois under its 1992 mortgage indenture. They are secured by substantially all property of the former IP and CIPS. The bonds are callable at 100% of par value. The bonds are also backed by an insurance guarantee policy. Less than $1 million principal amount of the bonds remains outstanding. (k) The bonds are callable at 100% of par value.
Schedule Of Maturities Of Long-Term Debt	The following table presents the aggregate maturities of long-term debt, including current maturities, for the Ameren Companies at December 31, 2016 : Ameren (parent) (a) Ameren Missouri (a) Ameren Illinois (a) Ameren Consolidated 2017 $ — $ 431 $ 250 $ 681 2018 — 383 457 840 2019 — 581 — 581 2020 350 92 — 442 2021 — 8 — 8 Thereafter 350 2,524 1,900 4,774 Total $ 700 $ 4,019 $ 2,607 $ 7,326 (a) Excludes unamortized discount and premium and debt issuance costs of $6 million , $25 million , and $19 million at Ameren (parent), Ameren Missouri, and Ameren Illinois, respectively.
Schedule Of Outstanding Preferred Stock	The following table presents the outstanding preferred stock of Ameren Missouri and Ameren Illinois, which is redeemable, at the option of the issuer, at the prices shown below as of December 31, 2016 and 2015 : Redemption Price(per share) 2016 2015 Ameren Missouri: Without par value and stated value of $100 per share, 25 million shares authorized $3.50 Series 130,000 shares $ 110.00 $ 13 $ 13 $3.70 Series 40,000 shares 104.75 4 4 $4.00 Series 150,000 shares 105.625 15 15 $4.30 Series 40,000 shares 105.00 4 4 $4.50 Series 213,595 shares 110.00 (a) 21 21 $4.56 Series 200,000 shares 102.47 20 20 $4.75 Series 20,000 shares 102.176 2 2 $5.50 Series A 14,000 shares 110.00 1 1 Total $ 80 $ 80 Ameren Illinois: With par value of $100 per share, 2 million shares authorized 4.00% Series 144,275 shares $ 101.00 $ 14 $ 14 4.08% Series 45,224 shares 103.00 5 5 4.20% Series 23,655 shares 104.00 2 2 4.25% Series 50,000 shares 102.00 5 5 4.26% Series 16,621 shares 103.00 2 2 4.42% Series 16,190 shares 103.00 2 2 4.70% Series 18,429 shares 103.00 2 2 4.90% Series 73,825 shares 102.00 7 7 4.92% Series 49,289 shares 103.50 5 5 5.16% Series 50,000 shares 102.00 5 5 6.625% Series 124,274 shares 100.00 12 12 7.75% Series 4,542 shares 100.00 1 1 Total $ 62 $ 62 Total Ameren $ 142 $ 142 (a) In the event of voluntary liquidation, $105.50 .
Schedule of Required and Actual Debt Ratios	The following table summarizes the required and actual interest coverage ratios for interest charges, dividend coverage ratios, and bonds and preferred stock issuable as of December 31, 2016 , at an assumed 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 4.6 $ 4,077 > 2.5 105.3 $ 2,344 Ameren Illinois > 2.0 6.9 3,819 (d) > 1.5 2.8 203 (e) (a) Coverage required on the annual interest charges on first mortgage bonds outstanding and to be issued. Coverage is not required in certain cases when additional first mortgage bonds are issued on the basis of retired bonds. (b) Amount of bonds issuable based either on required coverage ratios or unfunded property additions, whichever is more restrictive. The amounts shown also include bonds issuable based on retired bond capacity of $1,206 million and $279 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 its 1992 mortgage indenture. (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)	12 Months Ended
	Dec. 31, 2016
Other Nonoperating Income (Expense) [Abstract]
Other Income And Expenses	The following table presents the components of "Other Income and Expenses" in the Ameren Companies’ statements of income (loss) for the years ended December 31, 2016 , 2015 , and 2014 : 2016 2015 2014 Ameren: (a) Miscellaneous income: Allowance for equity funds used during construction $ 27 $ 30 $ 34 Interest income on industrial development revenue bonds 27 27 27 Interest income (b) 13 14 10 Other 7 3 8 (c) Total miscellaneous income $ 74 $ 74 $ 79 Miscellaneous expense: Donations $ 16 $ 15 $ 10 Other 16 15 12 Total miscellaneous expense $ 32 $ 30 $ 22 Ameren Missouri: Miscellaneous income: Allowance for equity funds used during construction $ 23 $ 22 $ 32 Interest income on industrial development revenue bonds 27 27 27 Interest income 1 1 1 Other 1 2 — Total miscellaneous income $ 52 $ 52 $ 60 Miscellaneous expense: Donations $ 4 $ 5 $ 6 Other 6 6 6 Total miscellaneous expense $ 10 $ 11 $ 12 Ameren Illinois: Miscellaneous income: Allowance for equity funds used during construction $ 4 $ 8 $ 2 Interest income (b) 12 12 7 Other 5 1 8 (c) Total miscellaneous income $ 21 $ 21 $ 17 Miscellaneous expense: Donations $ 6 $ 5 $ 4 Other 6 7 4 Total miscellaneous expense $ 12 $ 12 $ 8 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations. (b) Includes Ameren Illinois' interest income on the IEIMA revenue requirement reconciliation adjustment regulatory assets. (c) Includes Ameren Illinois' income earned in 2014 from customer-requested construction.

Derivative Financial Instrume38

Derivative Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2016
Derivative Instrument Detail [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 December 31, 2016 and 2015 . As of December 31, 2016 , these contracts extended through October 2019, March 2021, May 2032, and February 2020 for fuel oils, natural gas, power, and uranium, respectively. Quantity (in millions, except as indicated) 2016 2015 Commodity Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils (in gallons) (a) 30 (b) 30 35 (b) 35 Natural gas (in mmbtu) 25 129 154 30 151 181 Power (in megawatthours) 1 9 10 1 10 11 Uranium (pounds in thousands) 345 (b) 345 494 (b) 494 (a) Consists of ultra-low-sulfur diesel products. (b) Not applicable.
Derivative Instruments Carrying Value	The following table presents the carrying value and balance sheet location of all derivative commodity contracts, none of which were designated as hedging instruments, as of December 31, 2016 and 2015 : Balance Sheet Location Ameren Missouri Ameren Illinois Ameren 2016 Fuel oils Other current assets $ 2 $ — $ 2 Other assets 1 — 1 Natural gas Other current assets 1 11 12 Other assets 1 2 3 Power Other current assets 9 — 9 Total assets (a) $ 14 $ 13 $ 27 Fuel oils Other current liabilities $ 5 $ — $ 5 Natural gas MTM derivative liabilities (b) 3 (b) Other current liabilities 1 — 4 Other deferred credits and liabilities 5 5 10 Power MTM derivative liabilities (b) 12 (b) Other current liabilities 3 — 15 Other deferred credits and liabilities — 173 173 Uranium Other deferred credits and liabilities 4 — 4 Total liabilities (c) $ 18 $ 193 $ 211 2015 Natural gas Other current assets $ — 1 $ 1 Other assets 1 — 1 Power Other current assets 16 — 16 Total assets (a) $ 17 $ 1 $ 18 Fuel oils Other current liabilities $ 22 $ — $ 22 Other deferred credits and liabilities 7 — 7 Natural gas MTM derivative liabilities (b) 32 (b) Other current liabilities 6 — 38 Other deferred credits and liabilities 8 18 26 Power MTM derivative liabilities (b) 13 (b) Other current liabilities — — 13 Other deferred credits and liabilities — 157 157 Uranium Other current liabilities 1 — 1 Total liabilities (c) $ 44 $ 220 $ 264 (a) The cumulative amount of pretax net gains on all derivative instruments is deferred as a regulatory liability. (b) Balance sheet line item not applicable to registrant. (c) The cumulative amount of pretax net losses on all derivative instruments is deferred as a regulatory asset.
Derivative Instruments With Credit Risk-Related Contingent Features	The following table presents, as of December 31, 2016 , the aggregate fair value of all derivative instruments with credit risk-related contingent features in a gross liability position, the cash collateral posted, and the aggregate amount of additional collateral that counterparties could require. The additional collateral required is the net liability position allowed under the master netting arrangements or similar agreements, assuming (1) the credit risk-related contingent features underlying these arrangements were triggered on December 31, 2016 , and (2) those counterparties with rights to do so requested collateral. Aggregate Fair Value of Derivative Liabilities (a) Cash Collateral Posted Potential Aggregate Amount of Additional Collateral Required (b) 2016 Ameren Missouri $ 64 $ 3 $ 54 Ameren Illinois 33 — 26 Ameren $ 97 $ 3 $ 80 (a) Before consideration of master netting arrangements or similar agreements and including NPNS and other accrual contract exposures. (b) As collateral requirements with certain counterparties are based on master netting arrangements or similar agreements, the aggregate amount of additional collateral required to be posted is determined after consideration of the effects of such arrangements.

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2016
Fair Value Disclosures [Abstract]
Fair Value Inputs, Assets and Liabilities, Quantitative Information	The following table describes the valuation techniques and unobservable inputs utilized by the Ameren Companies for the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy for the periods ended December 31, 2016 and 2015 : Fair Value Weighted Assets Liabilities Valuation Technique(s) Unobservable Input Range Average Level 3 Derivative asset and liability – commodity contracts (a) : 2016 Fuel oils $ 1 $ — Option model Volatilities(%) (b) 24 – 66 28 Discounted cash flow Counterparty credit risk(%) (c)(d) 0.13 – 0.22 0.15 Ameren Missouri credit risk(%) (c)(d) 0.38 (e) Escalation rate(%) (b)(f) (2) – 2 0 Natural Gas 1 (1 ) Option model Volatilities(%) (b) 31 – 66 36 Nodal basis($/mmbtu) (b) (0.40) – (0.10) (0.20) Discounted cash flow Nodal basis($/mmbtu) (b) (0.80) – 0 (0.50) Counterparty credit risk(%) (c)(d) 0.13 – 8 1 Ameren Illinois credit risk(%) (c)(d) 0.38 (e) Power (g) 9 (187 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps($/MWh) (h) 26 – 44 29 Estimated auction price for FTRs($/MW) (b) (71) – 5,270 125 Nodal basis($/MWh) (h) (6) – 0 (2) Ameren Illinois credit risk(%) (c)(d) 0.38 (e) Fundamental energy production model Estimated future natural gas prices($/mmbtu) (b) 3 – 4 3 Escalation rate(%) (b)(i) 5 (e) Contract price allocation Estimated renewable energy credit costs($/credit) (b) 5 – 7 6 Uranium — (4 ) Option model Volatilities(%) (b) 24 (e) Discounted cash flow Average forward uranium pricing($/pound) (b) 22 – 24 22 Ameren Missouri credit risk(%) (c)(d) 0.38 (e) 2015 Natural Gas $ 1 $ (1 ) Option model Volatilities(%) (b) 35 – 55 45 Nodal basis($/mmbtu) (c) (0.30) – 0 (0.20) Discounted cash flow Nodal basis($/mmbtu) (b) (0.10) – 0 (0.10) Counterparty credit risk(%) (c)(d) 0.40 – 12 7 Ameren Missouri credit risk(%) (c)(d) 0.40 (e) Power (g) 16 (170 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps($/MWh) (h) 22 – 39 29 Estimated auction price for FTRs($/MW) (b) (270) – 2,057 211 Nodal basis($/MWh) (h) (10) – (1) (3) Fair Value Weighted Assets Liabilities Valuation Technique(s) Unobservable Input Range Average Counterparty credit risk(%) (c)(d) 0.86 (e) Ameren Illinois credit risk(%) (c)(d) 0.40 (e) Fundamental energy production model Estimated future natural gas prices($/mmbtu) (b) 3 – 4 4 Escalation rate(%) (b)(i) 3 (e) Contract price allocation Estimated renewable energy credit costs($/credit) (b) 5 – 7 6 Uranium — (1 ) Option model Volatilities(%) (b) 20 (e) Discounted cash flow Average forward uranium pricing($/pound) (b) 35 – 42 37 Ameren Missouri credit risk(%) (c)(d) 0.40 (e) (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement. (c) Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement. (d) Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances. (e) Not applicable. (f) Escalation rate applies to fuel oil prices 2019 and beyond. (g) Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2020. Valuations beyond 2020 use fundamentally modeled pricing by month for peak and off-peak demand. (h) The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes because of their opposing positions. (i) Escalation rate applies to power prices in 2031 and beyond for December 31, 2016, and to power prices in 2026 and beyond for December 31, 2015.
Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis	The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of December 31, 2016 : Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Total Assets: Ameren Derivative assets – commodity contracts (a) : Fuel oils $ 2 $ — $ 1 $ 3 Natural gas 2 12 1 15 Power — — 9 9 Total derivative assets – commodity contracts $ 4 $ 12 $ 11 $ 27 Nuclear decommissioning trust fund: Cash and cash equivalents $ 1 $ — $ — $ 1 Equity securities: U.S. large capitalization 408 — — 408 Debt securities: U.S. Treasury and agency securities — 112 — 112 Corporate bonds — 67 — 67 Other — 17 — 17 Total nuclear decommissioning trust fund $ 409 $ 196 $ — $ 605 (b) Total Ameren $ 413 $ 208 $ 11 $ 632 Ameren Missouri Derivative assets – commodity contracts (a) : 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 Fuel oils $ 2 $ — $ 1 $ 3 Natural gas — 1 1 2 Power — — 9 9 Total derivative assets – commodity contracts $ 2 $ 1 $ 11 $ 14 Nuclear decommissioning trust fund: Cash and cash equivalents $ 1 $ — $ — $ 1 Equity securities: U.S. large capitalization 408 — — 408 Debt securities: U.S. Treasury and agency securities — 112 — 112 Corporate bonds — 67 — 67 Other — 17 — 17 Total nuclear decommissioning trust fund $ 409 $ 196 $ — $ 605 (b) Total Ameren Missouri $ 411 $ 197 $ 11 $ 619 Ameren Illinois Derivative assets – commodity contracts (a) : Natural gas $ 2 $ 11 $ — $ 13 Liabilities: Ameren Derivative liabilities – commodity contracts (a) : Fuel oils $ 5 $ — $ — $ 5 Natural gas — 13 1 14 Power — 1 187 188 Uranium — — 4 4 Total Ameren $ 5 $ 14 $ 192 $ 211 Ameren Missouri Derivative liabilities – commodity contracts (a) : Fuel oils $ 5 $ — $ — $ 5 Natural gas — 6 — 6 Power — 1 2 3 Uranium — — 4 4 Total Ameren Missouri $ 5 $ 7 $ 6 $ 18 Ameren Illinois Derivative liabilities – commodity contracts (a) : Natural gas $ — $ 7 $ 1 $ 8 Power — — 185 185 Total Ameren Illinois $ — $ 7 $ 186 $ 193 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Balance excludes $2 million of receivables, payables, and accrued income, net. The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of December 31, 2015 : Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Total Assets: Ameren Derivative assets – commodity contracts (a) : Natural gas $ — $ 1 $ 1 $ 2 Power — — 16 16 Total derivative assets – commodity contracts $ — $ 1 $ 17 $ 18 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 Nuclear decommissioning trust fund: Cash and cash equivalents $ 4 $ — $ — $ 4 Equity securities: U.S. large capitalization 364 — — 364 Debt securities: U.S. Treasury and agency securities — 109 — 109 Corporate bonds — 58 — 58 Other — 22 — 22 Total nuclear decommissioning trust fund $ 368 $ 189 $ — $ 557 (b) Total Ameren $ 368 $ 190 $ 17 $ 575 Ameren Missouri Derivative assets – commodity contracts (a) : Natural gas $ — $ — $ 1 $ 1 Power — — 16 16 Total derivative assets – commodity contracts $ — $ — $ 17 $ 17 Nuclear decommissioning trust fund: Cash and cash equivalents $ 4 $ — $ — $ 4 Equity securities: U.S. large capitalization 364 — — 364 Debt securities: U.S. Treasury and agency securities — 109 — 109 Corporate bonds — 58 — 58 Other — 22 — 22 Total nuclear decommissioning trust fund $ 368 $ 189 $ — $ 557 (b) Total Ameren Missouri $ 368 $ 189 $ 17 $ 574 Ameren Illinois Derivative assets – commodity contracts (a) : Natural gas $ — $ 1 $ — $ 1 Liabilities: Ameren Derivative liabilities – commodity contracts (a) : Fuel oils $ 29 $ — $ — $ 29 Natural gas 1 62 1 64 Power — — 170 170 Uranium — — 1 1 Total Ameren $ 30 $ 62 $ 172 $ 264 Ameren Missouri Derivative liabilities – commodity contracts (a) : Fuel oils $ 29 $ — $ — $ 29 Natural gas — 13 1 14 Uranium — — 1 1 Total Ameren Missouri $ 29 $ 13 $ 2 $ 44 Ameren Illinois Derivative liabilities – commodity contracts (a) : Natural gas $ 1 $ 49 $ — $ 50 Power — — 170 170 Total Ameren Illinois $ 1 $ 49 $ 170 $ 220 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Balance excludes $(1) million of receivables, payables, and accrued income, net.
Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level 3 In The Fair Value Hierarchy	The following table summarizes the changes in the fair value of power financial assets and liabilities classified as Level 3 in the fair value hierarchy: Net Derivative Commodity Contracts Ameren Missouri Ameren Illinois Ameren For the year ended December 31, 2015 Beginning balance at January 1, 2015 $ 9 $ (142 ) $ (133 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities: 2 (41 ) (39 ) Purchases 29 — 29 Settlements (23 ) 13 (10 ) Transfers out of Level 3 (1 ) — (1 ) Ending balance at December 31, 2015 $ 16 $ (170 ) $ (154 ) Change in unrealized gains (losses) related to assets/liabilities held at December 31, 2015 $ — $ (39 ) $ (39 ) For the year ended December 31, 2016 Beginning balance at January 1, 2016 $ 16 $ (170 ) $ (154 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities: (1 ) (29 ) (30 ) Purchases 13 — 13 Settlements (21 ) 14 (7 ) Ending balance at December 31, 2016 $ 7 $ (185 ) $ (178 ) Change in unrealized gains (losses) related to assets/liabilities held at December 31, 2016 $ — $ (27 ) $ (27 )
Schedule Of Carrying Amounts And Estimated Fair Values Of Long-Term Debt 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 December 31, 2016 and 2015 : 2016 2015 Carrying Amount Fair Value Carrying Amount Fair Value Ameren: (a) Long-term debt and capital lease obligations (including current portion) $ 7,276 $ 7,772 $ 7,275 $ 7,814 Preferred stock 142 131 142 125 Ameren Missouri: Long-term debt and capital lease obligations (including current portion) $ 3,994 $ 4,304 $ 4,110 $ 4,449 Preferred stock 80 79 80 75 Ameren Illinois: Long-term debt (including current portion) $ 2,588 $ 2,765 $ 2,471 $ 2,665 Preferred stock 62 52 62 50 (a) Preferred stock is recorded in "Noncontrolling Interests" on the consolidated balance sheet.

Nuclear Decommissioning Trust40

Nuclear Decommissioning Trust Fund Investments (Tables)	12 Months Ended
	Dec. 31, 2016
Nuclear Decommissioning Trust Fund Investments [Line Items]
Fair Value Of Securities In Nuclear Decommissioning Trust Fund	The following table presents the costs and fair values of investments in debt and equity securities in Ameren's and Ameren Missouri’s nuclear decommissioning trust fund at December 31, 2016 and 2015 : Security Type Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value 2016 Debt securities $ 197 $ 3 $ 4 $ 196 Equity securities 161 253 6 408 Cash 1 — — 1 Other (a) 2 — — 2 Total $ 361 $ 256 $ 10 $ 607 2015 Debt securities $ 191 $ 2 $ 4 $ 189 Equity securities 147 224 7 364 Cash 4 — — 4 Other (a) (1 ) — — (1 ) Total $ 341 $ 226 $ 11 $ 556 (a) Represents net receivables and payables relating to pending security sales, interest, and security purchases.
Regulatory Asset
Nuclear Decommissioning Trust Fund Investments [Line Items]
Proceeds From Sale Of Investments In Nuclear Decommissioning Trust Fund And Gross Realized Gains And Losses	The following table presents proceeds from the sale and maturities of investments in Ameren Missouri’s nuclear decommissioning trust fund and the gross realized gains and losses resulting from those sales for the years ended December 31, 2016 , 2015 , and 2014 : 2016 2015 2014 Proceeds from sales and maturities $ 377 $ 349 $ 391 Gross realized gains 7 8 7 Gross realized losses 4 2 2
Fair Value Of Securities In Nuclear Decommissioning Trust Fund According To Their Contractual Maturities	The following table presents the costs and fair values of investments in debt securities in Ameren's and Ameren Missouri’s nuclear decommissioning trust fund according to their contractual maturities at December 31, 2016 : Cost Fair Value Less than 5 years $ 105 $ 104 5 years to 10 years 47 47 Due after 10 years 45 45 Total $ 197 $ 196

Retirement Benefits (Tables)

Retirement Benefits (Tables)	12 Months Ended
	Dec. 31, 2016
Summary Of Benefit Liability Recorded	The following table presents the net benefit liability recorded on the balance sheets of each of the Ameren Companies as of December 31, 2016 and 2015 : 2016 2015 Ameren (a) $ 774 $ 567 Ameren Missouri 293 236 Ameren Illinois 315 219 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries.
Funded Status Of Benefit Plans And Amounts Included In Regulatory Assets And OCI	The following table presents the funded status of Ameren's pension and postretirement benefit plans as of December 31, 2016 and 2015 . It also provides the amounts included in regulatory assets and accumulated OCI at December 31, 2016 and 2015 , that have not been recognized in net periodic benefit costs. 2016 2015 Pension Benefits (a) Postretirement Benefits (a) Pension Benefits (a) Postretirement Benefits (a) Accumulated benefit obligation at end of year $ 4,288 $ (b) $ 3,995 $ (b) Change in benefit obligation: Net benefit obligation at beginning of year $ 4,197 $ 1,094 $ 4,410 $ 1,203 Service cost 81 19 92 24 Interest cost 185 50 174 48 Participant contributions — 8 — 8 Actuarial (gain) loss 265 52 (256 ) (133 ) Settlement — — (2 ) — Benefits paid (210 ) (54 ) (221 ) (56 ) Federal subsidy on benefits paid (b) 1 (b) — Net benefit obligation at end of year 4,518 1,170 4,197 1,094 Change in plan assets: Fair value of plan assets at beginning of year 3,653 1,071 3,794 1,109 Actual return on plan assets 313 73 (29 ) (8 ) Employer contributions 57 2 111 18 Federal subsidy on benefits paid (b) 1 (b) — Participant contributions — 8 — 8 Settlements — — (2 ) — Benefits paid (210 ) (54 ) (221 ) (56 ) Fair value of plan assets at end of year 3,813 1,101 3,653 1,071 Funded status – deficiency 705 69 544 23 Accrued benefit cost at December 31 $ 705 $ 69 $ 544 $ 23 Amounts recognized in the balance sheet consist of: Noncurrent asset (c) $ — $ — $ — $ (18 ) Current liability (d) 3 2 3 2 Noncurrent liability 702 67 541 39 Net liability recognized $ 705 $ 69 $ 544 $ 23 Amounts recognized in regulatory assets consist of: Net actuarial (gain) loss $ 535 $ (29 ) $ 395 $ (82 ) Prior service cost (credit) (4 ) (8 ) (5 ) (11 ) Amounts (pretax) recognized in accumulated OCI consist of: Net actuarial (gain) loss 43 — 17 (3 ) Prior service cost (credit) — (1 ) — — Total $ 574 $ (38 ) $ 407 $ (96 ) (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries. (b) Not applicable. (c) Included in "Other assets" on Ameren's consolidated balance sheet. (d) Included in "Other current liabilities" on Ameren's consolidated balance sheet.
Assumptions Used To Determine Benefit Obligations	The following table presents the assumptions used to determine our benefit obligations at December 31, 2016 and 2015 : Pension Benefits Postretirement Benefits 2016 2015 2016 2015 Discount rate at measurement date 4.00 % 4.50 % 4.00 % 4.50 % Increase in future compensation 3.50 3.50 3.50 3.50 Medical cost trend rate (initial) (a) (a) 5.00 5.00 Medical cost trend rate (ultimate) (a) (a) 5.00 5.00 (a) Not applicable
Schedule Of Cash Contributions Made To Benefit Plans	The following table presents the cash contributions made to our defined benefit retirement plan and to our postretirement plans during 2016 , 2015 , and 2014 : Pension Benefits Postretirement Benefits 2016 2015 2014 2016 2015 2014 Ameren Missouri $ 21 $ 47 $ 41 $ 1 $ 8 $ 3 Ameren Illinois 30 45 39 1 8 2 Other 6 19 19 — 2 1 Ameren 57 111 99 2 18 6
Target Allocation Of The Plans' Asset Categories	The following table presents our target allocations for 2017 and our pension and postretirement plans’ asset categories as of December 31, 2016 and 2015 : Asset Category Target Allocation 2017 Percentage of Plan Assets at December 31, 2016 2015 Pension Plan: Cash and cash equivalents 0% – 5% 1 % 1 % Equity securities: U.S. large-capitalization 29% – 39% 34 % 34 % U.S. small- and mid-capitalization 3% – 13% 9 % 7 % International and emerging markets 9% – 19% 14 % 13 % Total equity 51% – 61% 57 % 54 % Debt securities 35% – 45% 37 % 40 % Real estate 0% – 9% 5 % 5 % Private equity 0% – 5% (a) (a) Total 100 % 100 % Postretirement Plans: Cash and cash equivalents 0% – 7% 3 % 4 % Equity securities: U.S. large-capitalization 34% – 44% 40 % 39 % U.S. small- and mid-capitalization 2% – 12% 7 % 7 % International 9% – 19% 14 % 13 % Total equity 55% – 65% 61 % 59 % Debt securities 33% – 43% 36 % 37 % Total 100 % 100 % (a) Less than 1% of plan assets.
Components Of Net Periodic Benefit Cost	The following table presents the components of the net periodic benefit cost of Ameren's pension and postretirement benefit plans during 2016 , 2015 , and 2014 : Pension Benefits Postretirement Benefits 2016 Service cost $ 81 $ 19 Interest cost 185 50 Expected return on plan assets (253 ) (72 ) Amortization of: Prior service credit (1 ) (5 ) Actuarial (gain) loss 32 (11 ) Net periodic benefit cost (benefit) $ 44 $ (19 ) 2015 Service cost $ 92 $ 24 Interest cost 174 48 Expected return on plan assets (248 ) (68 ) Amortization of: Prior service credit (1 ) (5 ) Actuarial loss 74 5 Settlement loss 1 — Net periodic benefit cost (benefit) $ 92 $ 4 2014 Service cost $ 79 $ 19 Interest cost 183 50 Expected return on plan assets (229 ) (65 ) Amortization of: Prior service credit (1 ) (5 ) Actuarial (gain) loss 49 (7 ) Net periodic benefit cost (benefit) $ 81 $ (8 )
Summary Of Estimated Amortizable Amounts From Regulatory Assets and Accumulated OCI Into Net Periodic Benefit Cost	The estimated amounts that will be amortized from regulatory assets and accumulated OCI into Ameren's net periodic benefit cost in 2017 are as follows: Pension Benefits (a) Postretirement Benefits (a) Regulatory assets: Prior service credit $ (1 ) $ (5 ) Net actuarial (gain) loss 50 (7 ) Accumulated OCI: Net actuarial loss 4 — Total $ 53 $ (12 ) (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries.
Summary Of Benefit Plan Costs Incurred	The Ameren Companies are responsible for their share of the pension and postretirement benefit costs. The following table presents the pension costs and the postretirement benefit costs incurred and included in continuing operations for the years ended December 31, 2016 , 2015 , and 2014 : Pension Costs Postretirement Costs 2016 2015 2014 2016 2015 2014 Ameren Missouri (a) $ 26 $ 54 $ 50 $ (5 ) $ 8 $ 3 Ameren Illinois 22 38 30 (13 ) (3 ) (9 ) Other (4 ) — 1 (1 ) (1 ) (2 ) Ameren 44 92 81 (19 ) 4 (8 ) (a) Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates.
Schedule Of Expected Payments From Qualified Trust And Company Funds	The expected pension and postretirement benefit payments from qualified trust and company funds, which reflect expected future service, as of December 31, 2016 , are as follows: Pension Benefits Postretirement Benefits Paid from Qualified Trust Funds Paid from Company Funds Paid from Qualified Trust Funds Paid from Company Funds 2017 $ 248 $ 3 $ 54 $ 2 2018 254 3 57 2 2019 261 3 59 2 2020 265 3 61 2 2021 273 3 63 2 2022 – 2026 1,405 13 331 12
Assumptions Used To Determine Net Periodic Benefit Cost	The following table presents the assumptions used to determine net periodic benefit cost for our pension and postretirement benefit plans for the years ended December 31, 2016 , 2015 , and 2014 : Pension Benefits Postretirement Benefits 2016 2015 2014 2016 2015 2014 Discount rate at measurement date 4.50 % 4.00 % 4.75 % 4.50 % 4.00 % 4.75 % Expected return on plan assets 7.00 7.25 7.25 7.00 7.00 7.00 Increase in future compensation 3.50 3.50 3.50 3.50 3.50 3.50 Medical cost trend rate (initial) (a) (a) (a) 5.00 5.00 5.00 Medical cost trend rate (ultimate) (a) (a) (a) 5.00 5.00 5.00 (a) Not applicable
Schedule Of Potential Changes In Key Assumptions	The table below reflects the sensitivity of Ameren’s plans to potential changes in key assumptions: Pension Benefits Postretirement Benefits Service Cost and Interest Cost Projected Benefit Obligation Service Cost and Interest Cost Postretirement Benefit Obligation 0.25% decrease in discount rate $ (1 ) $ 142 $ — $ 38 0.25% increase in salary scale 2 16 — — 1.00% increase in annual medical trend — — 3 54 1.00% decrease in annual medical trend — — (3 ) (54 )
Schedule Of Matching Contributions	The following table presents the portion of the matching contribution to the Ameren 401(k) plan attributable to the continuing operations for each of the Ameren Companies for the years ended December 31, 2016 , 2015 , and 2014 : 2016 2015 2014 Ameren Missouri $ 16 $ 16 $ 16 Ameren Illinois 12 12 11 Other 1 1 1 Ameren 29 29 28
Pension Benefits
Target Allocation Of The Plans' Asset Categories	The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the pension plan assets measured at fair value as of December 31, 2016 : Quoted Prices in Active Markets for Identified Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Measured at NAV (a) Total Cash and cash equivalents $ — $ — $ — $ 33 $ 33 Equity securities: U.S. large-capitalization — — — 1,352 1,352 U.S. small- and mid-capitalization 361 — — — 361 International and emerging markets 133 — — 389 522 Debt securities: Corporate bonds — 617 — 13 630 Municipal bonds — 95 — — 95 U.S. Treasury and agency securities — 701 — — 701 Other — 21 — — 21 Real estate — — — 202 202 Private equity — — — 6 6 Total $ 494 $ 1,434 $ — $ 1,995 $ 3,923 Less: Medical benefit assets at December 31 (b) (132 ) Plus: Net receivables at December 31 (c) 22 Fair value of pension plans assets at year end $ 3,813 (a) Reflects the adoption of the new authoritative accounting guidance related to investments measured at the NAV practical expedient. See Note 1 - Summary of Significant Accounting Policies for additional information. (b) Medical benefit (health and welfare) component for accounts maintained in accordance with Section 401(h) of the Internal Revenue Code to fund a portion of the postretirement obligation. (c) Receivables related to pending security sales, offset by payables related to pending security purchases. The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the pension plan assets measured at fair value as of December 31, 2015 : Quoted Prices in Active Markets for Identified Assets or Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Measured at NAV (a) Total Cash and cash equivalents $ — $ — $ — $ 20 $ 20 Equity securities: U.S. large-capitalization — — — 1,296 1,296 U.S. small- and mid-capitalization 268 — — — 268 International and emerging markets 122 126 — 243 491 Debt securities: Corporate bonds — 617 — 14 631 Municipal bonds — 104 — — 104 U.S. Treasury and agency securities 6 751 — — 757 Other — 5 — — 5 Real estate — — — 168 168 Private equity — — — 8 8 Total $ 396 $ 1,603 $ — $ 1,749 $ 3,748 Less: Medical benefit assets at December 31 (b) (123 ) Plus: Net receivables at December 31 (c) 28 Fair value of pension plans assets at year end $ 3,653 (a) Reflects the adoption of the new authoritative accounting guidance related to investments measured at the NAV practical expedient. See Note 1 - Summary of Significant Accounting Policies for additional information. (b) Medical benefit (health and welfare) component for accounts maintained in accordance with Section 401(h) of the Internal Revenue Code to fund a portion of the postretirement obligation. (c) Receivables related to pending security sales, offset by payables related to pending security purchases.
Postretirement Benefits
Target Allocation Of The Plans' Asset Categories	The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the postretirement benefit plans assets measured at fair value as of December 31, 2016 : Quoted Prices in Active Markets for Identified Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Measured at NAV (a) Total Cash and cash equivalents $ 53 $ — $ — $ — $ 53 Equity securities: U.S. large-capitalization 291 — — 101 392 U.S. small- and mid-capitalization 72 — — — 72 International 40 — — 92 132 Other — 7 — — 7 Debt securities: Corporate bonds — 141 — — 141 Municipal bonds — 110 — — 110 U.S. Treasury and agency securities — 68 — — 68 Other — — — 19 19 Total $ 456 $ 326 $ — $ 212 $ 994 Plus: Medical benefit assets at December 31 (b) 132 Less: Net payables at December 31 (c) (25 ) Fair value of postretirement benefit plans assets at year end $ 1,101 (a) Reflects the adoption of the new authoritative accounting guidance related to investments measured at the NAV practical expedient. See Note 1 - Summary of Significant Accounting Policies for additional information. (b) Medical benefit (health and welfare) component for 401(h) accounts to fund a portion of the postretirement obligation. These 401(h) assets are included in the pension plan assets shown above. (c) Payables related to pending security purchases, offset by interest receivables and receivables related to pending security sales. The following table sets forth, by level within the fair value hierarchy discussed in Note 8 – Fair Value Measurements, the postretirement benefit plans assets measured at fair value as of December 31, 2015 : Quoted Prices in Active Markets for Identified Assets (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Measured at NAV (a) Total Cash and cash equivalents $ 61 $ — $ — $ — $ 61 Equity securities: U.S. large-capitalization 272 — — 98 370 U.S. small- and mid-capitalization 65 — — — 65 International 33 38 — 55 126 Other — 7 — — 7 Debt securities: Corporate bonds — 138 — — 138 Municipal bonds — 114 — — 114 U.S. Treasury and agency securities — 55 — — 55 Other — 4 — 36 40 Total $ 431 $ 356 $ — $ 189 $ 976 Plus: Medical benefit assets at December 31 (a) 123 Less: Net payables at December 31 (b) (28 ) Fair value of postretirement benefit plans assets at year end $ 1,071 (a) Reflects the adoption of the new authoritative accounting guidance related to investments measured at the NAV practical expedient. See Note 1 - Summary of Significant Accounting Policies for additional information. (b) Medical benefit (health and welfare) component for 401(h) accounts to fund a portion of the postretirement obligation. These 401(h) assets are included in the pension plan assets shown above. (c) Payables related to pending security purchases, offset by Medicare, interest receivables, and receivables related to pending security sales.

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary Of Nonvested Shares Related To Long-Term Incentive Plan	A summary of nonvested performance share units at December 31, 2016 , and changes during the year ended December 31, 2016 , under the 2006 Incentive Plan and the 2014 Incentive Plan are presented below: Performance Share Units Share Units Weighted-average Fair Value per Share Unit Nonvested at January 1, 2016 1,024,870 $ 46.08 Granted (a) 588,615 44.13 Forfeitures (15,949 ) 45.07 Earned and vested (b) (537,897 ) 40.12 Nonvested at December 31, 2016 1,059,639 $ 48.04 (a) Includes performance share units (share units) granted to certain executive and nonexecutive officers and other eligible employees in 2016 under the 2014 Incentive Plan. (b) Includes share units granted in 2014 that vested as of December 31, 2016 and were earned pursuant to the terms of the award grants. Also includes share units that vested due to 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.
Disclosure of Share-based Compensation Arrangements by Share-based Payment Award [Table Text Block]	The following table presents the stock-based compensation expense for the years ended December 31, 2016, 2015 and 2014: 2016 2015 2014 Ameren Missouri $ 4 $ 5 $ 5 Ameren Illinois 2 3 2 Other (a) 11 11 12 Ameren 17 19 19 Less income tax benefit 6 7 7 Stock-based compensation expense, net $ 11 $ 12 $ 12 (a) Represents compensation expense of employees of Ameren Services. These amounts are not included in the Ameren Missouri and Ameren Illinois amounts above.

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Schedule Of Effective Income Tax Rate Reconciliation	The following table presents the principal reasons for the difference between the effective income tax rate and the statutory federal income tax rate for the years ended December 31, 2016 , 2015 , and 2014 : Ameren Missouri Ameren Illinois Ameren 2016 Statutory federal income tax rate: 35 % 35 % 35 % Increases (decreases) from: Depreciation differences 1 — — Amortization of deferred investment tax credit (1 ) — — State tax 3 5 4 Stock-based compensation (a) — — (2 ) Valuation allowance — — 1 Other permanent items — (2 ) (1 ) Effective income tax rate 38 % 38 % 37 % 2015 Statutory federal income tax rate: 35 % 35 % 35 % Increases (decreases) from: Depreciation differences — (2 ) (1 ) Amortization of deferred investment tax credit (1 ) — (1 ) State tax 3 5 5 Other permanent items — (1 ) — Effective income tax rate 37 % 37 % 38 % 2014 Statutory federal income tax rate: 35 % 35 % 35 % Increases (decreases) from: Amortization of deferred investment tax credit (1 ) — (1 ) State tax 3 6 4 Other permanent items — — 1 Effective income tax rate 37 % 41 % 39 % (a) Reflects the adoption of new authoritative accounting guidance related to share-based compensation. See Note 1 – Summary of Significant Accounting Policies for more information.
Schedule Of Components Of Income Tax Expense (Benefit)	The following table presents the components of income tax expense (benefit) for the years ended December 31, 2016 , 2015 , and 2014 : Ameren Missouri Ameren Illinois Other Ameren 2016 Current taxes: Federal $ 31 $ (8 ) $ (24 ) $ (1 ) State 6 12 (21 ) (3 ) Deferred taxes: Federal 161 117 21 299 State 23 37 32 92 Amortization of deferred investment tax credits (5 ) — — (5 ) Total income tax expense $ 216 $ 158 $ 8 $ 382 2015 Current taxes: Federal $ 110 $ (83 ) $ (29 ) $ (2 ) State 17 (11 ) (10 ) (4 ) Deferred taxes: Federal 71 193 35 299 State 16 29 31 76 Amortization of deferred investment tax credits (5 ) (1 ) — (6 ) Total income tax expense $ 209 $ 127 $ 27 $ 363 2014 Current taxes: Federal $ (13 ) $ (51 ) $ 27 $ (37 ) State (3 ) (2 ) (32 ) (37 ) Deferred taxes: Federal 222 159 (12 ) 369 State 28 38 22 88 Amortization of deferred investment tax credits (5 ) (1 ) — (6 ) Total income tax expense (benefit) $ 229 $ 143 $ 5 $ 377
Schedule Of Deferred Tax Assets And Liabilities Resulting From Temporary Differences	The following table presents the deferred tax assets and deferred tax liabilities recorded as a result of temporary differences at December 31, 2016 and 2015 : Ameren Missouri Ameren Illinois Other Ameren 2016 Accumulated deferred income taxes, net liability (asset): Plant related $ 3,103 $ 1,769 $ 147 $ 5,019 Regulatory assets, net 75 (1 ) — 74 Deferred employee benefit costs (76 ) (38 ) (97 ) (211 ) Revenue requirement reconciliation adjustments — 34 — 34 Tax carryforwards (66 ) (138 ) (472 ) (676 ) Other (23 ) 5 42 24 Total net accumulated deferred income tax liabilities (assets) $ 3,013 $ 1,631 $ (380 ) $ 4,264 2015 Accumulated deferred income taxes, net liability (asset): Plant related $ 2,931 $ 1,587 $ 37 $ 4,555 Regulatory assets, net 81 (1 ) — 80 Deferred employee benefit costs (76 ) (40 ) (91 ) (207 ) Revenue requirement reconciliation adjustments — 66 — 66 Tax carryforwards (65 ) (133 ) (405 ) (603 ) Other (27 ) 1 20 (6 ) Total net accumulated deferred income tax liabilities (assets) $ 2,844 $ 1,480 $ (439 ) $ 3,885
Schedule Of Net Operating Loss Carryforwards And Tax Credit Carryforwards	The following table presents the components of deferred tax assets relating to net operating loss carryforwards, tax credit carryforwards, and charitable contribution carryforwards at December 31, 2016 and 2015 : Ameren Missouri Ameren Illinois Other Ameren 2016 Net operating loss carryforwards: Federal (a) $ 33 $ 137 $ 324 $ 494 State (a) 4 — 41 45 Total net operating loss carryforwards $ 37 $ 137 $ 365 $ 539 Tax credit carryforwards: Federal (a) $ 29 $ 1 $ 79 $ 109 State (b) — — 21 21 Total tax credit carryforwards $ 29 $ 1 $ 100 $ 130 Charitable contribution carryforwards (b) $ — $ — $ 18 $ 18 Valuation allowance (c) — — (11 ) (11 ) Total charitable contribution carryforwards $ — $ — $ 7 $ 7 2015 Net operating loss carryforwards: Federal $ 35 $ 127 $ 245 $ 407 State 4 4 38 46 Total net operating loss carryforwards $ 39 $ 131 $ 283 $ 453 Tax credit carryforwards: Federal $ 26 $ 1 $ 78 $ 105 State — 1 40 41 State valuation allowance — — (2 ) (2 ) Total tax credit carryforwards $ 26 $ 2 $ 116 $ 144 Charitable contribution carryforwards $ — $ — $ 10 $ 10 Valuation allowance — — (4 ) (4 ) Total charitable contribution carryforwards $ — $ — $ 6 $ 6 (a) Will expire between 2029 and 2036 . (b) Will expire between 2017 and 2021 . (c) See Schedule II under Part IV, Item 15, in this report for information on changes in the valuation allowance.

Related Party Transactions (Tab

Related Party Transactions (Tables)	12 Months Ended
	Dec. 31, 2016
Related Party Transactions [Abstract]
Schedule of Related Party Transactions	The following table presents the agreements the companies have entered into, as well as the specified time period, price, and amount of megawatthours included in each agreement: IPA Procurement Event Time Period MWh Average Price per MWh May 2014 January 2015 – February 2017 168,400 $ 51 April 2015 June 2015 – June 2017 667,000 36 September 2015 November 2015 – May 2018 339,000 38 April 2016 June 2017 – September 2018 375,200 35 September 2016 May 2017 – September 2018 82,800 34 The following table presents the impact on Ameren Missouri and Ameren Illinois of related party transactions for the years ended December 31, 2016 , 2015 , and 2014 . It is based primarily on the agreements discussed above and the money pool arrangements discussed in Note 4 – Short-term Debt and Liquidity. Agreement Income Statement Line Item Ameren Missouri Ameren Illinois Ameren Missouri power supply agreements Operating Revenues 2016 $ 28 $ (a) with Ameren Illinois 2015 15 (a) 2014 5 (a) Ameren Missouri and Ameren Illinois Operating Revenues 2016 25 5 rent and facility services 2015 25 4 2014 21 2 Ameren Missouri and Ameren Illinois Operating Revenues 2016 1 (b) miscellaneous support services 2015 2 (b) 2014 1 (b) Total Operating Revenues 2016 $ 54 $ 5 2015 42 4 2014 27 2 Ameren Illinois power supply Purchased Power 2016 $ (a) $ 28 agreements with Ameren Missouri 2015 (a) 15 2014 (a) 5 Ameren Illinois transmission Purchased Power 2016 (a) 2 services from ATXI 2015 (a) 2 2014 (a) 2 Total Purchased Power 2016 $ (a) $ 30 2015 (a) 17 2014 (a) 7 Ameren Services support services Other Operations and 2016 $ 129 $ 123 agreement Maintenance 2015 131 119 2014 124 109 Money pool borrowings (advances) Interest (Charges) 2016 $ (b) $ (b) Income 2015 (b) (b) 2014 (b) (b) (a) Not applicable. (b) Amount less than $1 million.

Commitments And Contingencies (

Commitments And Contingencies (Tables)	12 Months Ended
	Dec. 31, 2016
Commitments and Contingencies Disclosure [Abstract]
Schedule Of Insurance Coverage At Callaway Energy Center	The following table presents insurance coverage at Ameren Missouri’s Callaway energy center at December 31, 2016 . The property coverage and the nuclear liability coverage renewal dates are April 1 and January 1, respectively, of each year. Type and Source of Coverage Maximum Coverages Maximum Assessments Public liability and nuclear worker liability: American Nuclear Insurers $ 375 (a) $ — Pool participation 12,986 (b) 127 (c) $ 13,361 (d) $ 127 Property damage: Nuclear Electric Insurance Limited $ 2,710 (e) $ 30 (f) European Mutual Association for Nuclear Insurance 450 (g) — $ 3,160 $ 30 Replacement power: Nuclear Electric Insurance Limited $ 490 (h) $ 7 (f) (a) Effective January 1, 2017, limit was increased to $450 million. (b) Provided through mandatory participation in an industrywide retrospective premium assessment program. (c) 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. (d) 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. (e) NEIL provides $2.71 billion in property damage, decontamination, and premature decommissioning insurance for radiation events. NEIL provides $2.3 billion in property damage for nonradiation events. (f) All NEIL-insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL. (g) European Mutual Association for Nuclear Insurance provides $450 million in excess of the $2.71 billion and $2.3 billion property coverage for radiation and nonradiation events, respectively, provided by NEIL. (h) Provides replacement power cost insurance in the event of a prolonged accidental outage. Weekly indemnity is up to $4.5 million for 52 weeks, which commences after the first 12 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 Lease Obligations	We lease various facilities, office equipment, plant equipment, and rail cars under capital and operating leases. The following table presents our lease obligations at December 31, 2016 : 2017 2018 2019 2020 2021 After 5 Years Total Ameren: (a) Minimum capital lease payments (b) $ 33 $ 32 $ 32 $ 32 $ 32 $ 297 $ 458 Less amount representing interest 27 26 25 25 25 48 176 Present value of minimum capital lease payments $ 6 $ 6 $ 7 $ 7 $ 7 $ 249 $ 282 Operating leases (c) 13 12 12 11 10 23 81 Total lease obligations $ 19 $ 18 $ 19 $ 18 $ 17 $ 272 $ 363 Ameren Missouri: Minimum capital lease payments (b) $ 33 $ 32 $ 32 $ 32 $ 32 $ 297 $ 458 Less amount representing interest 27 26 25 25 25 48 176 Present value of minimum capital lease payments $ 6 $ 6 $ 7 $ 7 $ 7 $ 249 $ 282 Operating leases (c) 11 11 11 10 9 21 73 Total lease obligations $ 17 $ 17 $ 18 $ 17 $ 16 $ 270 $ 355 Ameren Illinois: Operating leases (c) $ 1 $ 1 $ 1 $ 1 $ 1 $ 1 $ 6 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations. (b) See Properties under Part I, Item 2, and Note 3 – Property, Plant, and Equipment, Net, of this report for additional information. (c) Amounts related to certain land-related leases have indefinite payment periods. The annual obligations of $3 million , $2 million , and $1 million for Ameren, Ameren Missouri, and Ameren Illinois for these items are included in the 2017 through 2021 columns, respectively.
Schedule Of Rental Expense	The following table presents total rental expense included in operating expenses for the years ended December 31, 2016 , 2015 , and 2014 : 2016 2015 2014 Ameren (a) $ 38 $ 36 $ 37 Ameren Missouri 34 34 32 Ameren Illinois 30 28 25 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.
Schedule Of Estimated Purchased Commitments	December 31, 2016 . Ameren’s and Ameren Missouri’s purchased power commitments include a 102-megawatt power purchase agreement with a wind farm operator, which expires in 2024. Ameren’s and Ameren Illinois’ purchased power commitments include the Ameren Illinois power purchase agreements entered into as part of the IPA-administered power procurement process. Included in the Other column are minimum purchase commitments under contracts for equipment, design and construction, and meter reading services at December 31, 2016 . Coal Natural Gas (a) Nuclear Fuel Purchased Power (b) Methane Gas Other Total Ameren: (c) 2017 $ 599 $ 238 $ 45 $ 255 $ 3 $ 118 $ 1,258 2018 371 167 70 156 4 60 828 2019 311 99 27 79 4 60 580 2020 27 45 38 58 5 56 229 2021 — 12 44 58 5 29 148 Thereafter — 43 45 478 65 198 829 Total $ 1,308 $ 604 $ 269 $ 1,084 $ 86 $ 521 $ 3,872 Ameren Missouri: 2017 $ 599 $ 43 $ 45 $ 22 $ 3 $ 39 $ 751 2018 371 29 70 22 4 29 525 2019 311 15 27 22 4 29 408 2020 27 10 38 22 5 29 131 2021 — 5 44 22 5 28 104 Thereafter — 18 45 59 65 183 370 Total $ 1,308 $ 120 $ 269 $ 169 $ 86 $ 337 $ 2,289 Ameren Illinois: 2017 $ — $ 195 $ — $ 233 $ — $ 36 $ 464 2018 — 138 — 134 — 24 296 2019 — 83 — 57 — 27 167 2020 — 35 — 36 — 27 98 2021 — 8 — 36 — — 44 Thereafter — 25 — 419 — — 444 Total $ — $ 484 $ — $ 915 $ — $ 114 $ 1,513 (a) Includes amounts for generation and for distribution. (b) The purchased power amounts for Ameren and Ameren Illinois include agreements through 2032 for renewable energy credits with various renewable energy suppliers. The agreements contain a provision that allows Ameren Illinois to reduce the quantity purchased in the event that Ameren Illinois would not be able to recover the costs associated with the renewable energy credits. (c) Includes amounts for Ameren registrant and nonregistrant subsidiaries.

Segment Information (Tables)

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2016
Segment Reporting Information, Profit (Loss) [Abstract]
Schedule Of Segment Reporting Information, By Segment	The following tables present information about the reported revenues and specified items reflected in net income attributable to common shareholders from continuing operations and capital expenditures at Ameren and Ameren Illinois for the years ended December 31, 2016 , 2015 , and 2014 . Ameren, Ameren Missouri, and Ameren Illinois management review segment capital expenditure information rather than any individual or total asset amount. Ameren Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Other Intersegment Eliminations Consolidated 2016 External revenues $ 3,469 $ 1,545 $ 753 $ 309 $ — $ — $ 6,076 Intersegment revenues 54 4 1 46 (a) — (105 ) — Depreciation and amortization 514 226 55 43 7 — 845 Interest income 28 11 — 1 11 (11 ) 40 Interest charges 211 72 34 58 18 (11 ) 382 Income taxes 216 78 39 74 (25 ) — 382 Net income (loss) attributable to Ameren common shareholders from continuing operations 357 126 59 117 (6 ) — 653 Capital expenditures 738 470 181 689 4 (b) (6 ) 2,076 2015 External revenues $ 3,566 $ 1,529 $ 782 $ 219 $ 2 $ — $ 6,098 Intersegment revenues 43 3 1 40 (a) — (87 ) — Depreciation and amortization 492 212 52 33 7 — 796 Interest income 28 12 — — 7 (6 ) 41 Interest charges 219 71 35 35 1 (6 ) 355 Income taxes 209 71 24 51 8 — 363 Net income (loss) attributable to Ameren common shareholders from continuing operations 352 123 37 83 (16 ) — 579 Capital expenditures 622 491 133 669 2 (b) — 1,917 2014 External revenues $ 3,526 $ 1,401 $ 976 $ 150 $ — $ — $ 6,053 Intersegment revenues 27 2 — 37 (a) — (66 ) — Depreciation and amortization 473 197 41 26 8 — 745 Interest income 28 7 — — 5 (3 ) 37 Interest charges 211 63 28 26 16 (3 ) 341 Income taxes 229 75 39 38 (4 ) — 377 Net income (loss) attributable to Ameren common shareholders from continuing operations 390 113 50 51 (17 ) — 587 Capital expenditures 747 403 137 491 7 (b) — 1,785 (a) Ameren Illinois Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above. (b) Includes the elimination of intercompany transfers. Ameren Illinois Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Consolidated 2016 External revenues $ 1,549 $ 754 $ 187 $ — $ 2,490 Intersegment revenues — — 45 (a) (45 ) — Depreciation and amortization 226 55 38 — 319 Interest income 11 — 1 — 12 Interest charges 72 34 34 — 140 Income taxes 78 39 41 — 158 Net income available to common shareholder 126 59 67 — 252 Capital expenditures 470 181 273 — 924 2015 External revenues $ 1,532 $ 783 $ 151 $ — $ 2,466 Intersegment revenues — — 38 (a) (38 ) — Depreciation and amortization 212 52 31 — 295 Interest income 12 — — — 12 Interest charges 71 35 25 — 131 Income taxes 71 24 32 — 127 Net income available to common shareholder 123 37 54 — 214 Capital expenditures 491 133 294 — 918 2014 External revenues $ 1,403 $ 976 $ 119 $ — $ 2,498 Intersegment revenues — — 35 (a) (35 ) — Depreciation and amortization 197 41 25 — 263 Interest income 7 — — — 7 Interest charges 63 28 21 — 112 Income taxes 75 39 29 — 143 Net income available to common shareholder 113 50 38 — 201 Capital expenditures 403 137 295 — 835 (a) Ameren Illinois Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above.

Selected Quarterly Information

Selected Quarterly Information (Tables)	12 Months Ended
	Dec. 31, 2016
Selected Quarterly Financial Information [Abstract]
Summary Of Selected Quarterly Information	SELECTED QUARTERLY INFORMATION (Unaudited) (In millions, except per share amounts) Ameren 2016 2015 Quarter ended March 31 June 30 September 30 December 31 March 31 June 30 September 30 December 31 Operating revenues $ 1,434 $ 1,427 $ 1,859 $ 1,356 $ 1,556 $ 1,401 $ 1,833 $ 1,308 Operating income 220 325 691 145 256 237 626 140 Net income 107 148 371 33 110 151 345 30 Net income attributable to Ameren common shareholders – continuing operations $ 105 $ 147 $ 369 $ 32 $ 108 $ 98 $ 343 $ 30 Net income (loss) attributable to Ameren common shareholders – discontinued operations — — — — — 52 — (1 ) Net income attributable to Ameren common shareholders $ 105 $ 147 $ 369 $ 32 $ 108 $ 150 $ 343 $ 29 Earnings per common share – basic – continuing operations $ 0.43 $ 0.61 $ 1.52 $ 0.13 $ 0.45 $ 0.40 $ 1.42 $ 0.12 Earnings per common share – basic – discontinued operations — — — — — 0.21 — — Earnings per common share – basic $ 0.43 $ 0.61 $ 1.52 $ 0.13 $ 0.45 $ 0.61 $ 1.42 $ 0.12 Earnings per common share – diluted – continuing operations (a) $ 0.43 $ 0.61 $ 1.52 $ 0.13 $ 0.45 $ 0.40 $ 1.41 $ 0.12 Earnings per common share – diluted – discontinued operations — — — — — 0.21 — — Earnings per common share – diluted (a) $ 0.43 $ 0.61 $ 1.52 $ 0.13 $ 0.45 $ 0.61 $ 1.41 $ 0.12 (a) The sum of quarterly amounts, including per share amounts, may not equal amounts reported for year-to-date periods. This is because of the effects of rounding and the changes in the number of weighted-average diluted shares outstanding each period. Ameren Missouri Quarter ended Operating Revenues Operating Income Net Income Net Income Available to Common Shareholder March 31, 2016 $ 741 $ 63 $ 15 $ 14 March 31, 2015 800 115 42 41 June 30, 2016 867 197 93 92 June 30, 2015 884 146 62 61 September 30, 2016 1,165 431 242 241 September 30, 2015 1,171 423 240 239 December 31, 2016 750 54 10 10 December 31, 2015 754 58 11 11 Ameren Illinois Quarter ended Operating Revenues Operating Income Net Income Net Income Available to Common Shareholder March 31, 2016 $ 677 $ 133 $ 60 $ 59 March 31, 2015 745 120 54 53 June 30, 2016 542 107 46 45 June 30, 2015 513 83 32 31 September 30, 2016 676 230 119 119 September 30, 2015 655 189 98 98 December 31, 2016 595 74 30 29 December 31, 2015 553 74 33 32

Summary Of Significant Accoun48

Summary Of Significant Accounting Policies (Narrative) (Details) $ / shares in Units, people in Millions, customer in Millions, $ in Millions	1 Months Ended	3 Months Ended	12 Months Ended
	Jan. 31, 2014USD ($)	Dec. 31, 2013USD ($)	Dec. 31, 2016USD ($)mi²customerpeople$ / shares	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
Accounting Policies [Line Items]
Goodwill			$ 411	$ 411
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities					$ 18
Adjustments related to Income Tax Benefit from Share-Based Payment			$ 21
Adjustments related to Income Tax Benefit from Share-Based payment per diluted share | $ / shares			$ 0.09
Excess Tax Benefit from Share-Based Compensation				2
Payments Related to Tax Withholding for Share-based Compensation			$ 83	12	14
Percentage reporting unit exceeded carrying value			40.00%
Ameren Illinois Company
Accounting Policies [Line Items]
Public Utilities, Area Serviced | mi²			40,000
Public Utilities, Estimated Population of Service Territory | people			3.1
Goodwill			$ 411	411
Capital contribution from parent			$ 0	25	15
Union Electric Company
Accounting Policies [Line Items]
Public Utilities, Area Serviced | mi²			24,000
Public Utilities, Estimated Population of Service Territory | people			2.8
Capital contribution from parent			$ 44	224	215
Noncash Or Part Noncash Capital Contribution From Parent			0	38	9
Return of capital to parent			$ 0	$ 0	(215)
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities					$ 18
Minimum
Accounting Policies [Line Items]
Percent of average depreciable cost			3.00%	3.00%	3.00%
Maximum
Accounting Policies [Line Items]
Percent of average depreciable cost			4.00%	4.00%	4.00%
Power | Ameren Illinois Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers | customer			1.2
Power | Union Electric Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers | customer			1.2
Natural Gas | Ameren Illinois Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers | customer			0.8
Natural Gas | Union Electric Company
Accounting Policies [Line Items]
Public Utilities, Number of Customers | customer			0.1
FAC | Union Electric Company
Accounting Policies [Line Items]
Sharing Level For Fac			95.00%
State
Accounting Policies [Line Items]
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities				$ 53	$ 2
Elgin, Gibson City and Grand Tower Energy Centers [Member]
Accounting Policies [Line Items]
Proceeds from Sales of Business, Affiliate and Productive Assets	$ 168	$ 137.5
Proceeds from Asset Sale Held in Escrow				$ 14
Asbestos-Related
Accounting Policies [Line Items]
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%
Asbestos-Related | Ameren Illinois Company
Accounting Policies [Line Items]
Asbestos trust fund balance			$ 22
Ameren Illinois Electric Distribution [Member]
Accounting Policies [Line Items]
Goodwill			238
Ameren Illinois Gas [Member]
Accounting Policies [Line Items]
Goodwill			80
Ameren Illinois Transmission [Member]
Accounting Policies [Line Items]
Goodwill			93
Ameren Transmission [Member]
Accounting Policies [Line Items]
Goodwill			$ 93

Summary Of Significant Accoun49

Summary Of Significant Accounting Policies (Schedule Of Inventories) (Details) - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Line Items]
Fuel	[1]	$ 172	$ 173
Gas stored underground		82	97
Other materials and supplies		273	268
Total Inventories		527	538
Union Electric Company
Accounting Policies [Line Items]
Fuel	[1]	172	173
Gas stored underground		9	10
Other materials and supplies		211	204
Total Inventories		392	387
Ameren Illinois Company
Accounting Policies [Line Items]
Fuel	[1]	0	0
Gas stored underground		73	87
Other materials and supplies		62	64
Total Inventories		$ 135	$ 151
[1]	(a)Consists of coal, oil, and propane.

Summary Of Significant Accoun50

Summary Of Significant Accounting Policies (Schedule Of Rates Used For Allowance For Funds Used During Construction) (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Union Electric Company
Accounting Policies [Line Items]
Allowance for funds used during construction, rate	7.00%	7.00%	7.00%
Ameren Illinois Company
Accounting Policies [Line Items]
Allowance for funds used during construction, rate	5.00%	6.00%	2.00%

Summary Of Significant Accoun51

Summary Of Significant Accounting Policies (Schedule Of Asset Retirement Obligations) (Details) - USD ($)		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance		$ 623,000,000	[1]	$ 396,000,000
Liabilities incurred		3,000,000		3,000,000
Liabilities settled		(2,000,000)		(2,000,000)
Accretion in period	[2]	25,000,000		23,000,000
Change in estimates		1,000,000		203,000,000	[3]
Balance		650,000,000	[1]	623,000,000	[1]	$ 396,000,000
Other current liabilities		248,000,000		279,000,000
Callaway energy center decommissioning study [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Change in estimates		99,000,000
New CCR Rules Estimate [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Change in estimates		100,000,000
Other Estimate Changes [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Change in estimates		4,000,000
Asset Retirement Obligation Balance [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Other current liabilities		15,000,000		5,000,000
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance		617,000,000	[1]	389,000,000
Liabilities incurred		3,000,000		3,000,000
Liabilities settled		(2,000,000)		(1,000,000)
Accretion in period	[2]	25,000,000		23,000,000
Change in estimates		1,000,000		203,000,000	[3]
Balance		644,000,000	[1]	617,000,000	[1]	389,000,000
Other current liabilities		123,000,000		120,000,000
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance		6,000,000	[4]	7,000,000
Liabilities incurred		0		0
Liabilities settled		(1,000,000)	[5]	(1,000,000)
Accretion in period	[2],[5]	1,000,000		1,000,000
Change in estimates		0		1,000,000	[3],[5]
Balance		6,000,000	[4]	6,000,000	[4]	7,000,000
Other current liabilities		94,000,000		86,000,000
Asset Retirement Obligation | Union Electric Company
Asset Retirement Obligation [Line Items]
Noncash Depreciation related to ARO		$ 31,000,000		$ 13,000,000		$ 1,000,000
[1]	(e)Balance included $5 million and $15 million in "Other current liabilities" on the balance sheet as of December 31, 2015 and 2016, respectively.
[2]	(a)Accretion expense was recorded as a decrease to regulatory liabilities.
[3]	(c)The ARO increase resulted in a corresponding increase recorded to "Property, Plant, and Equipment, Net." 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, the 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 $100 million to their AROs associated with CCR storage facilities. See Note 15 – 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]	(d)Included in “Other deferred credits and liabilities” on the balance sheet.
[5]	(b)Less than $1 million.

Summary Of Significant Accoun52

Summary Of Significant Accounting Policies (Schedule Of Excise Taxes) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Line Items]
Excise tax expense	$ 208	$ 213	$ 215
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	151	156	151
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$ 57	$ 57	$ 64

Summary Of Significant Accoun53

Summary Of Significant Accounting Policies (Basic and Diluted Earnings Per Share Calculations) (Details) - shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Assumed Settlement of Performance Share Units	800,000	1,000,000	1,800,000
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	0	0	0

Rate And Regulatory Matters Rat

Rate And Regulatory Matters Rate and Regulatory Matters (Narrative-Missouri) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016USD ($)mi
Ameren Transmission Company of Illinois [Member] | Mark Twain Project [Member]
Public Utilities, General Disclosures [Line Items]
Transmission Line Miles | mi	95
Pending Rate Case [Member] | Union Electric Company | Electric Distribution
Public Utilities, General Disclosures [Line Items]
Revenue Requirement	$ 3,400
Public Utilities, Requested Rate Increase (Decrease), Amount	92
Components of Rate Increase - Net Energy Costs [Member] | Union Electric Company | Electric Distribution
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	54
Components of Rate Increase - Regulatory Amortizations [Member] | Union Electric Company | Electric Distribution
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	26
Final Rate Order | Union Electric Company | MEEIA | Electric Distribution
Public Utilities, General Disclosures [Line Items]
Incentive Award if Energy Efficiency Goals Are Achieved	$ 28

Rate And Regulatory Matters (Na

Rate And Regulatory Matters (Narrative-Illinois) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Public Utilities, General Disclosures [Line Items]
Regulatory assets	$ 1,437	$ 1,382
Current regulatory assets	149	260
Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	816	771
Current regulatory assets	$ 108	167
FEJA [Member] | Electric Distribution | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Equity Capital Structure, Percentage	50.00%
Public Utilities, Approved Return on Equity, Percentage	5.80%
Increase Decrease in Return on Equity for Energy Savings Goals	2.00%
FEJA 2018 through 2021 [Member] | Electric Distribution | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
FEJA Energy Efficiency Investments	$ 100
FEJA 2022 through 2025 [Member] | Electric Distribution | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
FEJA Energy Efficiency Investments	107
FEJA 2026 through 2030 [Member] | Electric Distribution | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
FEJA Energy Efficiency Investments	114
IEIMA | 2016 IEIMA Revenue Requirement Reconciliation [Member] | Electric Distribution | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	23
IEIMA | 2015 IEMA Revenue Requirement Reconciliation [Member] | Electric Distribution | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	68
IEIMA | 2014 IEMA Revenue Requirement Reconciliation [Member] | Electric Distribution | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets		$ 103
IEIMA | Final Rate Order | Electric Distribution | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 14

Rate And Regulatory Matters R56

Rate And Regulatory Matters Rate and Regulatory Matters (Narrative-Federal) (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Feb. 28, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities		$ 110	$ 80
Provision for Callaway construction and operating license		0	69	$ 0
New Nuclear Energy Center COL [Member]
Public Utilities, General Disclosures [Line Items]
Provision for Callaway construction and operating license		69
Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities		78	39
Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities		12	28
Provision for Callaway construction and operating license		0	$ 69	$ 0
Union Electric Company | New Nuclear Energy Center COL [Member]
Public Utilities, General Disclosures [Line Items]
Investments in Power and Distribution Projects		69
Provision for Callaway construction and operating license		$ 69
Administrative Law Judge [Member] | Midwest Independent Transmission System Operator, Inc [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Return on Equity, Percentage		9.70%
Incentive adder to FERC allowed base return on common equity		0.50%
Administrative Law Judge [Member] | Midwest Independent Transmission System Operator, Inc [Member] | Maximum
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Return on Equity, Percentage		10.20%
Pending Ferc Case [Member] | Midwest Independent Transmission System Operator, Inc [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Return on Equity, Percentage		12.38%
Customer Requested Rate on Equity	8.67%	9.15%
Current regulatory liabilities		$ 62
Pending Ferc Case [Member] | Midwest Independent Transmission System Operator, Inc [Member] | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities		$ 42
Final Rate Order | Midwest Independent Transmission System Operator, Inc [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Return on Equity, Percentage		10.32%
Incentive adder to FERC allowed base return on common equity		0.50%
Final Rate Order | Midwest Independent Transmission System Operator, Inc [Member] | Maximum
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Return on Equity, Percentage		10.82%

Rate And Regulatory Matters (Sc

Rate And Regulatory Matters (Schedule Of Regulatory Assets And Liabilities) (Details) - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015
Public Utilities, General Disclosures [Line Items]
Current regulatory assets		$ 149	$ 260
Regulatory assets		1,437	1,382
Current regulatory liabilities		110	80
Regulatory liabilities		1,985	1,905
Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets		35	89
Regulatory assets		619	605
Current regulatory liabilities		12	28
Regulatory liabilities		1,215	1,172
Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets		108	167
Regulatory assets		816	771
Current regulatory liabilities		78	39
Regulatory liabilities		768	732
Under-Recovered FAC
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[1],[2]	21	37
Under-Recovered FAC | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[1],[2]	21	37
Under-Recovered FAC | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[1],[2]	0	0
Under-Recovered Illinois Electric Power Costs
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[3]	3	3
Under-Recovered Illinois Electric Power Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[3]	0	0
Under-Recovered Illinois Electric Power Costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[3]	3	3
Under-Recovered PGA
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[3]	4	8
Under-Recovered PGA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[3]	0	0
Under-Recovered PGA | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[3]	4	8
MTM Derivative Losses
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[4]	24	74
Regulatory assets	[4]	187	190
MTM Derivative Losses | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[4]	9	29
Regulatory assets	[4]	9	15
MTM Derivative Losses | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[4]	15	45
Regulatory assets	[4]	178	175
Energy Efficiency Rider
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[5]	5	23
Regulatory liabilities	[5]	45	36
Energy Efficiency Rider | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[5]	5	23
Regulatory liabilities	[5]	0	0
Energy Efficiency Rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[5]	0	0
Regulatory liabilities	[5]	45	36
IEIMA
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[1],[6]	68	103
Regulatory assets	[1],[6]	23	62
IEIMA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[1],[6]	0	0
Regulatory assets	[1],[6]	0	0
IEIMA | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[1],[6]	68	103
Regulatory assets	[1],[6]	23	62
FERC Revenue Requirement Reconciliation
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[1],[7]	13	12
Regulatory assets	[1],[7]	10	11
FERC Revenue Requirement Reconciliation | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[1],[7]	0	0
Regulatory assets	[1],[7]	0	0
FERC Revenue Requirement Reconciliation | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[1],[7]	7	8
Regulatory assets	[1],[7]	8	5
VBA Rider
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[1],[8]	11	0
VBA Rider | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[1],[8]	0	0
VBA Rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	[1],[8]	11	0
Pension And Postretirement Benefit Costs
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[9]	494	297
Pension And Postretirement Benefit Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[9]	175	95
Pension And Postretirement Benefit Costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[9]	319	202
Income Taxes
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[10]	230	251
Regulatory liabilities	[11]	37	42
Income Taxes | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[10]	229	247
Regulatory liabilities	[11]	33	36
Income Taxes | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[10]	1	4
Regulatory liabilities	[11]	4	6
Uncertain tax positions tracker
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[1],[12]	7	7
Regulatory liabilities	[12]	3	6
Uncertain tax positions tracker | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[1],[12]	7	7
Regulatory liabilities	[12]	3	6
Uncertain tax positions tracker | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[1],[12]	0	0
Asset Retirement Obligation
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[13]	3	4
Regulatory liabilities	[13]	162	167
Asset Retirement Obligation | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[13]	0	0
Regulatory liabilities	[13]	162	167
Asset Retirement Obligation | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[13]	3	4
Regulatory liabilities	[13]	0	0
Callaway Costs
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[1],[14]	29	32
Callaway Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[1],[14]	29	32
Unamortized Loss On Reacquired Debt
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[1],[15]	124	138
Unamortized Loss On Reacquired Debt | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[1],[15]	65	69
Unamortized Loss On Reacquired Debt | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[1],[15]	59	69
Recoverable Costs Contaminated Facilities
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[16]	196	230
Recoverable Costs Contaminated Facilities | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[16]	0	0
Recoverable Costs Contaminated Facilities | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[16]	196	230
Storm Costs
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[1],[17]	15	9
Storm Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[1],[17]	0	0
Storm Costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[1],[17]	15	9
Demand-Side Costs
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[1],[18]	18	31
Demand-Side Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[1],[18]	18	31
Reserve For Workers' Compensation Liabilities
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[19]	13	13
Reserve For Workers' Compensation Liabilities | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[19]	6	6
Reserve For Workers' Compensation Liabilities | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[19]	7	7
Credit Facilities Fees
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[20]	4	4
Credit Facilities Fees | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[20]	4	4
Construction Accounting For Pollution Control Equipment
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[1],[21]	19	20
Construction Accounting For Pollution Control Equipment | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[1],[21]	19	20
Solar Rebates
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[1],[22]	49	74
Solar Rebates | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	[1],[22]	49	74
Other
Public Utilities, General Disclosures [Line Items]
Regulatory assets		16	9
Regulatory liabilities		9	3
Other | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets		9	5
Regulatory liabilities		5	2
Other | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets		7	4
Regulatory liabilities		4	1
Over-Recovered FAC
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	[2]	0	9
Over-Recovered FAC | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	[2]	0	9
Over-Recovered FAC | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	[2]	0	0
Over-Recovered Illinois Electric Power Costs
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	[3]	25	6
Over-Recovered Illinois Electric Power Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	[3]	0	0
Over-Recovered Illinois Electric Power Costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	[3]	25	6
Over-Recovered PGA
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	[3]	0	3
Over-Recovered PGA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	[3]	0	3
Over-Recovered PGA | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	[3]	0	0
MTM Derivative Gains
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	[4]	23	17
MTM Derivative Gains | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	[4]	12	16
MTM Derivative Gains | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	[4]	11	1
Refund Reserves for FERC Orders and Audit Findings
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	[23]	62	45
Refund Reserves for FERC Orders and Audit Findings | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	[23]	0	0
Refund Reserves for FERC Orders and Audit Findings | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	[23]	42	32
Removal Costs
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	[24]	1,669	1,605
Removal Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	[24]	970	933
Removal Costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	[24]	697	671
Bad Debt Rider
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	[25]	3	6
Bad Debt Rider | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	[25]	0	0
Bad Debt Rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	[25]	3	6
Pension And Postretirement Benefit Costs Tracker
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	[26]	35	19
Pension And Postretirement Benefit Costs Tracker | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	[26]	35	19
Renewable Energy Credits
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	[27]	15	12
Renewable Energy Credits | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	[27]	0	0
Renewable Energy Credits | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	[27]	15	12
Storm Tracker
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	[28]	7	9
Storm Tracker | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	[28]	7	9
Storm Tracker | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	[28]	$ 0	$ 0
[1]	(a)These assets earn a return.
[2]	(b)Under-recovered or over-recovered fuel costs to be recovered or refunded through the FAC. Specific accumulation periods aggregate the under-recovered or over-recovered costs over four months, any related adjustments that occur over the following four months, and the recovery from or refund to customers that occurs over the next eight months.
[3]	(c)Under-recovered or over-recovered costs from utility customers. Amounts will be recovered from or refunded to customers within one year of the deferral.
[4]	(d)Deferral of commodity-related derivative MTM losses or gains. See Note 7 – Derivative Financial Instruments for additional information.
[5]	(e)The Ameren Missouri balance relates to the MEEIA. The MEEIA rider allows Ameren Missouri to collect from or refund to customers any annual difference in the actual amounts incurred and the amounts collected from customers for the MEEIA program costs, net shared benefits, and the throughput disincentive. Under the MEEIA rider, collections from or refunds to customers occur one year after the program costs, net shared benefits, and the throughput disincentive are incurred. The Ameren Illinois balance relates to a regulatory tracking mechanism to recover its electric and natural gas costs associated with developing, implementing, and evaluating customer energy efficiency and demand response programs. Any under-recovery or over-recovery will be collected from or refunded to customers over the year following the plan year.
[6]	(f)The difference between Ameren Illinois' annual revenue requirement calculated under the IEIMA's performance-based formula ratemaking framework and the revenue requirement included in customer rates for that year. The under-recovery will be recovered from or refunded to customers with interest within two years.
[7]	(g)Ameren Illinois' and ATXI's annual revenue requirement reconciliation calculated pursuant to the FERC's electric transmission formula ratemaking framework. The under-recovery or over-recovery will be recovered from or refunded to customers within two years.
[8]	(h)Under-recovered natural gas sales volumes, including deviations from normal weather conditions. Each year's amount will be recovered from or refunded to customers from April through December of the following year.
[9]	(i)These costs are being amortized in proportion to the recognition of prior service costs (credits) and actuarial losses (gains) attributable to Ameren’s pension plan and postretirement benefit plans. See Note 11 – Retirement Benefits for additional information.
[10]	(j)Tax benefits related to the equity component of allowance for funds used during construction, as well as the effects of tax rate changes. This amount will be recovered over the expected life of the related assets.
[11]	(w)Unamortized portion of investment tax credits and reductions to deferred tax liabilities recorded at rates in excess of current statutory rates. The unamortized portion of investment tax credits and the reduction to deferred tax liabilities are being amortized over the expected life of the underlying assets.
[12]	(k)The tracker is amortized over three years, beginning from the date the amounts are included in rates. See Note 13 – Income Taxes for additional information.
[13]	(l)Recoverable or refundable removal costs for AROs, including net realized and unrealized gains and losses related to the nuclear decommissioning trust fund investments. See Note 1 – Summary of Significant Accounting Policies – Asset Retirement Obligations.
[14]	(m)Ameren Missouri’s Callaway energy center operations and maintenance expenses, property taxes, and carrying costs incurred between the plant in-service date and the date the plant was reflected in rates. These costs are being amortized over the remaining life of the energy center's original operating license through 2024.
[15]	(n)Losses related to reacquired debt. These amounts are being amortized over the lives of the related new debt issuances or the original lives of the old debt issuances if no new debt was issued.
[16]	(o)The recoverable portion of accrued environmental site liabilities that will be collected from electric and natural gas customers through ICC-approved cost recovery riders. The period of recovery will depend on the timing of remediation expenditures. See Note 15 – Commitments and Contingencies for additional information.
[17]	(p)Storm costs from 2013, 2015, and 2016 deferred in accordance with the IEIMA. These costs are being amortized over five-year periods beginning in the year the storm occurred.
[18]	(q)Demand-side costs incurred prior to implementation of the MEEIA in 2013, including the costs of developing, implementing, and evaluating customer energy efficiency and demand response programs. Costs incurred from May 2008 through September 2008 are being amortized over a 10-year period that began in March 2009. Costs incurred from October 2008 through December 2009 are being amortized until May 2017. Costs incurred from January 2010 through February 2011 are being amortized over a six-year period that began in August 2011. Costs incurred from March 2011 through July 2012 are being amortized over a six-year period that began in January 2013. Costs incurred from August 2012 through December 2012 are being amortized over a six-year period that began in June 2015. The February 2017 stipulation and agreement, if approved, would modify these amortization periods.
[19]	(r)The period of recovery will depend on the timing of actual expenditures.
[20]	(s)Ameren Missouri’s costs incurred to enter into and maintain the Missouri Credit Agreement. These costs are being amortized over the life of the credit facility to construction work in progress, which will be depreciated when assets are placed into service. Additional costs were incurred in December 2016 to amend and restate the Missouri Credit Agreement.
[21]	(t)The MoPSC’s May 2010 electric rate order allowed Ameren Missouri to record an allowance for funds used during construction for pollution control equipment at its Sioux energy center until the cost of that equipment was included in customer rates beginning in 2011. These costs are being amortized over the expected life of the Sioux energy center, currently through 2033.
[22]	(u)Costs associated with Ameren Missouri's solar rebate program to fulfill its renewable energy portfolio requirement. These costs are being amortized over a three-year period that began in June 2015. The February 2017 stipulation and agreement, if approved, would modify this amortization period.
[23]	(v)Estimated refunds to transmission customers related to FERC orders. See FERC Complaint Cases above.
[24]	(x)Estimated funds collected for the eventual dismantling and removal of plant retired from service, net of salvage value.
[25]	(y)A regulatory tracking mechanism for the difference between the level of bad debt incurred by Ameren Illinois under GAAP and the level of such costs included in electric and natural gas rates. The over-recovery relating to 2014 was refunded to customers from June 2015 through May 2016. The over-recovery relating to 2015 is being refunded to customers from June 2016 through May 2017. The over-recovery relating to 2016 will be refunded to customers from June 2017 through May 2018.
[26]	(z)A regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. For periods prior to December 2014, the MoPSC's April 2015 electric rate order directed the amortization to occur over three to five years, beginning in June 2015. For periods after December 2014, the amortization period will be determined in the July 2016 electric rate case. The February 2017 stipulation and agreement, if approved, would modify these amortization periods.
[27]	(aa)Funds collected from customers for the purchase of renewable energy credits through IPA procurements for distributed generation. The balance will be amortized as renewable energy credits are purchased.
[28]	(ab)A regulatory tracking mechanism at Ameren Missouri for the difference between the level of storm costs incurred in a particular year and the level of such costs included in rates. For periods prior to December 2014, the MoPSC's April 2015 electric rate order directed the amortization to occur over a five-year period that began in June 2015. For periods after December 2014, the amortization period will be determined in the July 2016 electric rate case. The April 2015 MoPSC order did not approve the continued use of the storm cost regulatory tracking mechanism. The February 2017 stipulation and agreement, if approved, would modify these amortization periods.

Property And Plant, Net (Schedu

Property And Plant, Net (Schedule Of Property And Plant, Net) (Details) $ in Millions		12 Months Ended
		Dec. 31, 2016USD ($)equipment	Dec. 31, 2015USD ($)
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[1],[2]	$ 30,118	$ 27,603
Accumulated depreciation and amortization	[1]	10,961	10,347
Property and plant, before construction work in progress	[1]	19,157	17,256
Property, Plant and Equipment, Net	[1]	$ 20,113	18,799
Number of combustion turbine electric generation equipment under capital lease agreements | equipment		2
Number of capital lease agreements | equipment		2
Capital lease agreements, gross asset value		$ 232	233
Total accumulated depreciation, capital lease agreements		77	72
Held-to-maturity Securities		282	288
Electric Generation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[1],[2]	10,911	10,431
Electric distribution equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[1],[2]	10,850	10,255
Electric transmission equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[1],[2]	3,879	2,774
Gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[1],[2]	2,641	2,442
Property, Plant and Equipment, Other Types [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[1],[2],[3]	1,837	1,701
Construction work in progress	[1]	750	1,268
Nuclear Fuel
Property, Plant and Equipment [Line Items]
Construction work in progress	[1]	206	275
Union Electric Company
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[1],[2]	18,959	17,966
Accumulated depreciation and amortization	[1]	7,880	7,460
Property and plant, before construction work in progress	[1]	11,079	10,506
Property, Plant and Equipment, Net	[1]	11,478	11,183
Union Electric Company | Electric Generation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[1],[2]	10,911	10,431
Union Electric Company | Electric distribution equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[1],[2]	5,563	5,303
Union Electric Company | Electric transmission equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[1],[2]	1,151	979
Union Electric Company | Gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[1],[2]	455	445
Union Electric Company | Property, Plant and Equipment, Other Types [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[1],[2],[3]	879	808
Construction work in progress	[1]	193	402
Union Electric Company | Nuclear Fuel
Property, Plant and Equipment [Line Items]
Construction work in progress	[1]	206	275
Ameren Illinois Company
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[2]	10,208	9,250
Accumulated depreciation and amortization		2,850	2,632
Property and plant, before construction work in progress		7,358	6,618
Property, Plant and Equipment, Net		7,469	6,848
Ameren Illinois Company | Electric Generation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[2]	0	0
Ameren Illinois Company | Electric distribution equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[2]	5,287	4,952
Ameren Illinois Company | Electric transmission equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[2]	2,016	1,674
Ameren Illinois Company | Gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[2]	2,186	1,997
Ameren Illinois Company | Property, Plant and Equipment, Other Types [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[2],[3]	719	627
Construction work in progress		111	230
Ameren Illinois Company | Nuclear Fuel
Property, Plant and Equipment [Line Items]
Construction work in progress		0	0
Other
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[2]	951	387
Accumulated depreciation and amortization		231	255
Property and plant, before construction work in progress		720	132
Property, Plant and Equipment, Net		1,166	768
Other | Electric Generation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[2]	0	0
Other | Electric distribution equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[2]	0	0
Other | Electric transmission equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[2]	712	121
Other | Gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[2]	0	0
Other | Property, Plant and Equipment, Other Types [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	[2],[3]	239	266
Construction work in progress		446	636
Other | Nuclear Fuel
Property, Plant and Equipment [Line Items]
Construction work in progress		$ 0	$ 0
Minimum | Electric Generation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		5 years
Minimum | Electric distribution equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		18 years
Minimum | Electric transmission equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		50 years
Minimum | Gas
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		20 years
Minimum | Property, Plant and Equipment, Other Types [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		5 years
Maximum | Electric Generation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		100 years
Maximum | Electric distribution equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		75 years
Maximum | Electric transmission equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		75 years
Maximum | Gas
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		80 years
Maximum | Property, Plant and Equipment, Other Types [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		55 years
Maximum | Union Electric Company | Electric Generation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life		150 years
[1]	(a)Amounts in Ameren and Ameren Missouri include two CTs under separate capital lease agreements. The gross cumulative asset value of those agreements was $232 million and $233 million at December 31, 2016 and 2015, respectively. The total accumulated depreciation associated with the two CTs was $77 million and $72 million at December 31, 2016 and 2015, respectively. In addition, Ameren Missouri has investments in debt securities, classified as held-to-maturity and recorded in "Other assets" that are related to the two CTs from the city of Bowling Green and Audrain County. As of December 31, 2016 and 2015, the carrying value of these debt securities was $282 million and $288 million, respectively.
[2]	(b)The estimated lives for each asset group are as follows: 5 to 100 years for electric generation, excluding Ameren Missouri's hydro generating assets which have useful lives of up to 150 years, 18 to 75 years for electric distribution, 50 to 75 years for electric transmission, 20 to 80 years for natural gas, and 5 to 55 years for other.
[3]	(c)Other property, plant, and equipment includes assets used to support multiple utility services.

Property And Plant, Net (Accrue

Property And Plant, Net (Accrued Capital Expenditures) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	[1]	$ 251	$ 235	$ 181
Union Electric Company
Property, Plant and Equipment [Line Items]
Accrued capital expenditures		116	85	72
Ameren Illinois Company
Property, Plant and Equipment [Line Items]
Accrued capital expenditures		87	92	59
Nuclear Fuel
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	[1]	20	16	13
Nuclear Fuel | Union Electric Company
Property, Plant and Equipment [Line Items]
Accrued capital expenditures		$ 20	$ 16	$ 13
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries.

Short-Term Debt And Liquidity60

Short-Term Debt And Liquidity (Narrative) (Details)	12 Months Ended
	Dec. 31, 2016USD ($)lender	Dec. 31, 2015
Illinois Credit Agreement 2012 | Maximum
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,300,000,000
Illinois Credit Agreement 2012 | Ameren Illinois Company
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 800,000,000
Actual debt-to-capital ratio	0.47
Missouri Credit Agreement 2012 | Maximum
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,200,000,000
Missouri Credit Agreement 2012 | Union Electric Company
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 800,000,000
Actual debt-to-capital ratio	0.48
Credit Agreements 2012
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,500,000,000
Covenant terms, default provisions, maximum indebtedness	100,000,000
Multiyear Credit Facility
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 2,100,000,000
Number of lenders | lender	22
Line of credit facility, maximum borrowing capacity, per lender	$ 118,000,000
Actual debt-to-capital ratio	0.51
Line of Credit Facility, Commitment Fee Amount	$ 100,000,000
Multiyear Credit Facility | Maximum
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.65
Utilities
Line of Credit Facility [Line Items]
Short Term Debt, Weighted Average Interest Rate During Period	0.52%	0.11%

Short-Term Debt And Liquidity61

Short-Term Debt And Liquidity (Schedule Of Maximum Aggregate Amount Available On Credit Agreements) (Details) $ in Millions	Dec. 31, 2016USD ($)
Illinois Credit Agreement 2012 | Parent Company
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 500
Illinois Credit Agreement 2012 | Ameren Illinois Company
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	800
Missouri Credit Agreement 2012 | Parent Company
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	700
Missouri Credit Agreement 2012 | Union Electric Company
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 800

Short-Term Debt And Liquidity S

Short-Term Debt And Liquidity Short-Term Debt And Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Commercial Paper
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 552	$ 767
Commercial paper outstanding		$ 558	$ 301
Weighted average interest rate		0.80%	0.55%
Peak short-term borrowings	[1]	$ 839	$ 1,108
Peak short-term borrowings interest rate		1.05%	0.91%
Parent Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 440	$ 721
Weighted average interest rate		0.82%	0.57%
Peak short-term borrowings	[1]	$ 574	$ 874
Peak short-term borrowings interest rate		1.05%	0.91%
Parent Company | Commercial Paper
Line of Credit Facility [Line Items]
Commercial paper outstanding		$ 507	$ 301
Union Electric Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 60	$ 42
Weighted average interest rate		0.74%	0.50%
Peak short-term borrowings	[1]	$ 208	$ 294
Peak short-term borrowings interest rate		0.85%	0.60%
Ameren Illinois Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 52	$ 4
Weighted average interest rate		0.69%	0.44%
Peak short-term borrowings	[1]	$ 195	$ 48
Peak short-term borrowings interest rate		0.90%	0.60%
Ameren Illinois Company | Commercial Paper
Line of Credit Facility [Line Items]
Commercial paper outstanding		$ 51	$ 0
Ameren Missouri [Member] | Commercial Paper
Line of Credit Facility [Line Items]
Commercial paper outstanding		$ 0	$ 0
[1]	(a)The timing of peak outstanding commercial paper issuances varies by company. Therefore, the sum of the peak amounts presented by company might not equal the Ameren Consolidated peak amount for the period.

Long-Term Debt And Equity Fin63

Long-Term Debt And Equity Financings (Narrative) (Details) - USD ($)		3 Months Ended		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014	Mar. 31, 2016	May 31, 2014	Oct. 31, 2013
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized		100,000,000		100,000,000
Preferred stock, par value		$ 0.01		$ 0.01
Preferred stock, shares outstanding		0		0
Common stock, shares authorized		400,000,000		400,000,000		400,000,000
Restricted Cash and Cash Equivalents		$ 8,000,000		$ 8,000,000
Repayments of Other Long-term Debt				395,000,000		$ 120,000,000		$ 697,000,000
Debt Default Provision Excess				$ 25,000,000
Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized		7,500,000		7,500,000
Preferred stock, par value		$ 1		$ 1
Common stock, shares authorized		150,000,000		150,000,000		150,000,000
Repayments of Other Long-term Debt				$ 266,000,000		$ 120,000,000		109,000,000
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized		2,600,000		2,600,000
Common stock, shares authorized		45,000,000		45,000,000		45,000,000
Repayments of Other Long-term Debt				$ 129,000,000		$ 0		163,000,000
Common stock equity to capitalization ratio		51.00%		51.00%
Parent Company
Long-Term Debt And Equity Financings [Line Items]
Common stock, shares authorized										8,600,000	4,000,000
Repayments of Other Long-term Debt				$ 0		0		$ 425,000,000
Ameren Missouri and Ameren Illinois
Long-Term Debt And Equity Financings [Line Items]
Bonds interest rate assumption		5.00%		5.00%
Dividend rate on preferred shares, percentage				6.00%
Senior Unsecured Notes270 due 2020 [Member] | Parent Company
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate		2.70%		2.70%
Senior Unsecured Notes270 due 2020 [Member] | Parent Company | Unsecured Debt [Member]
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 350,000,000		$ 350,000,000		350,000,000
Long-term debt interest rate		2.70%		2.70%
Proceeds from issuance of secured debt		$ 348,000,000
Senior Unsecured Notes365 due 2026 [Member] | Parent Company
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate		3.65%		3.65%
Senior Unsecured Notes365 due 2026 [Member] | Parent Company | Unsecured Debt [Member]
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 350,000,000		$ 350,000,000		350,000,000
Long-term debt interest rate		3.65%		3.65%
Proceeds from issuance of secured debt				$ 347,000,000
5.40% Senior secured notes due 2016 | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 0	[1]	$ 0	[1]	260,000,000	[1]		$ 260,000,000
Long-term debt interest rate		5.40%		5.40%
Senior Secured Notes, 3.65%, Due 2045 - $150 Issuance [Member] | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 150,000,000		$ 150,000,000
Proceeds from issuance of secured debt				148,000,000
Senior Secured Notes, 3.65%, Due 2045 - $250 Issuance [Member] | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		250,000,000		250,000,000
Proceeds from issuance of secured debt				247,000,000
Senior Secured Notes, 3.65%, Due 2045 [Member] | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	[1]	$ 400,000,000		$ 400,000,000		250,000,000
Long-term debt interest rate		3.65%		3.65%
Senior Secured Notes 4.75% Due2015 [Member] | Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate		4.75%		4.75%
Repayments of Other Long-term Debt				$ 114,000,000
Senior Secured Notes 6.20% Due 2016 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 0		$ 0		54,000,000
Long-term debt interest rate		6.20%		6.20%
Repayments of Other Long-term Debt				$ 54,000,000
Senior Secured Notes 6.25% Due 2016 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 0		$ 0		75,000,000
Long-term debt interest rate		6.25%		6.25%
Repayments of Other Long-term Debt				$ 75,000,000
Senior Secured Notes, 4.15%, Due 2046 - $240 Issuance [Member] | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 240,000,000		240,000,000
Proceeds from issuance of secured debt		245,000,000
Senior Secured Notes, 4.15%, Due 2046 - $250 Issuance [Member] | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		250,000,000		250,000,000
Proceeds from issuance of secured debt		245,000,000
Senior Secured Notes, 4.15%, Due 2046 [Member] | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	[2]	$ 490,000,000		$ 490,000,000		$ 250,000,000
Long-term debt interest rate		4.15%		4.15%
Minimum | Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Common stock equity to capitalization ratio		30.00%		30.00%
[1]	(a)These notes are collaterally secured by first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage indenture. The notes have a fall-away lien provision and will remain secured only as long as any first mortgage bonds issued under the Ameren Missouri mortgage indenture remain outstanding. Redemption, purchase, or maturity of all first mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the first mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the Ameren Missouri senior secured notes currently outstanding, we do not expect the first mortgage bond lien protection associated with these notes to fall away before 2042.
[2]	These notes are collaterally secured by mortgage bonds issued by Ameren Illinois under its 1992 mortgage indenture. They are secured by substantially all property of the former IP and CIPS. The notes have a fall-away lien provision and will remain secured only as long as any series of first mortgage bonds issued under its 1992 mortgage indenture remain outstanding. Redemption, purchase, or maturity of all mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the Ameren Illinois senior secured notes currently outstanding, we do not expect the mortgage bond lien protection associated with these notes to fall away before 2022.

Long-Term Debt And Equity Fin64

Long-Term Debt And Equity Financings (Schedule Of Long-Term Debt Outstanding) (Details) - USD ($)		12 Months Ended
		Dec. 31, 2016		Mar. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Less: Maturities due within one year		$ (681,000,000)			$ (395,000,000)
Long-term Debt, Net		6,595,000,000			6,880,000,000
Parent Company
Debt Instrument [Line Items]
Long-term debt, gross		700,000,000			700,000,000
Debt Issuance Costs, Net		(6,000,000)			(6,000,000)
Long-term Debt, Net		$ 694,000,000			694,000,000
Parent Company | Senior Unsecured Notes270 due 2020 [Member]
Debt Instrument [Line Items]
Long-term debt interest rate		2.70%
Long-term debt maturity date		2,020
Parent Company | Senior Unsecured Notes365 due 2026 [Member]
Debt Instrument [Line Items]
Long-term debt interest rate		3.65%
Long-term debt maturity date		2,026
Ameren Illinois Company
Debt Instrument [Line Items]
Long-term debt, gross		$ 2,607,000,000			2,496,000,000
Less: Unamortized discount and premium		0			(7,000,000)
Debt Issuance Costs, Net		(19,000,000)			(18,000,000)
Less: Maturities due within one year		(250,000,000)			(129,000,000)
Long-term Debt, Net		2,338,000,000			2,342,000,000
Union Electric Company
Debt Instrument [Line Items]
Long-term debt, gross		4,019,000,000			4,135,000,000
Less: Unamortized discount and premium		(6,000,000)			(6,000,000)
Debt Issuance Costs, Net		(19,000,000)			(19,000,000)
Less: Maturities due within one year		(431,000,000)			(266,000,000)
Long-term Debt, Net		3,563,000,000			3,844,000,000
Union Electric Company | City Of Bowling Green Capital Lease Peno Creek Ct
Debt Instrument [Line Items]
Capital lease obligations		$ 42,000,000			48,000,000
Long-term debt maturity date		2,022
Union Electric Company | Audrain County Capital Lease Audrain County Ct
Debt Instrument [Line Items]
Capital lease obligations		$ 240,000,000			240,000,000
Long-term debt maturity date		2,023
Unsecured Debt [Member] | Parent Company | Senior Unsecured Notes270 due 2020 [Member]
Debt Instrument [Line Items]
Debt instrument face amount		$ 350,000,000			350,000,000
Long-term debt interest rate		2.70%
Unsecured Debt [Member] | Parent Company | Senior Unsecured Notes365 due 2026 [Member]
Debt Instrument [Line Items]
Debt instrument face amount		$ 350,000,000			350,000,000
Long-term debt interest rate		3.65%
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.20% Due 2016
Debt Instrument [Line Items]
Debt instrument face amount		$ 0			54,000,000
Long-term debt interest rate		6.20%
Long-term debt maturity date		2,016
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.25% Due 2016
Debt Instrument [Line Items]
Debt instrument face amount		$ 0			75,000,000
Long-term debt interest rate		6.25%
Long-term debt maturity date		2,016
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.125% Due 2017
Debt Instrument [Line Items]
Debt instrument face amount	[1]	$ 250,000,000	[2]		250,000,000	[3]
Long-term debt interest rate		6.125%
Long-term debt maturity date		2,017
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.25% Due 2018
Debt Instrument [Line Items]
Debt instrument face amount	[1],[2]	$ 144,000,000			144,000,000
Long-term debt interest rate		6.25%
Long-term debt maturity date		2,018
Secured Debt | Ameren Illinois Company | Senior Secured Notes 9.75% Due 2018
Debt Instrument [Line Items]
Debt instrument face amount	[1],[2]	$ 313,000,000			313,000,000
Long-term debt interest rate		9.75%
Long-term debt maturity date		2,018
Secured Debt | Ameren Illinois Company | Senior Secured Notes, 2.70%, Due 2022
Debt Instrument [Line Items]
Debt instrument face amount	[1],[2]	$ 400,000,000			400,000,000
Long-term debt interest rate		2.70%
Long-term debt maturity date		2,022
Secured Debt | Ameren Illinois Company | Senior Secured Notes, 3.25%, Due 2025
Debt Instrument [Line Items]
Debt instrument face amount	[2]	$ 300,000,000			300,000,000
Long-term debt interest rate		3.25%
Long-term debt maturity date		2,025
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.125% Due 2028
Debt Instrument [Line Items]
Debt instrument face amount	[2]	$ 60,000,000			60,000,000
Long-term debt interest rate		6.125%
Long-term debt maturity date		2,028
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.70% Due 2036
Debt Instrument [Line Items]
Debt instrument face amount	[2]	$ 61,000,000			61,000,000
Long-term debt interest rate		6.70%
Long-term debt maturity date		2,036
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.70% Due 2036
Debt Instrument [Line Items]
Debt instrument face amount	[4]	$ 42,000,000			42,000,000
Long-term debt interest rate		6.70%
Long-term debt maturity date		2,036
Secured Debt | Ameren Illinois Company | Senior Secured Notes 4.80% Due 2043
Debt Instrument [Line Items]
Debt instrument face amount	[2]	$ 280,000,000			280,000,000
Long-term debt interest rate		4.80%
Long-term debt maturity date		2,043
Secured Debt | Ameren Illinois Company | Senior Secured Notes 4.30% Due 2044
Debt Instrument [Line Items]
Debt instrument face amount	[2]	$ 250,000,000			250,000,000
Long-term debt interest rate		4.30%
Long-term debt maturity date		2,044
Secured Debt | Ameren Illinois Company | Senior Secured Notes, 4.15%, Due 2046 [Member]
Debt Instrument [Line Items]
Debt instrument face amount	[2]	$ 490,000,000			250,000,000
Long-term debt interest rate		4.15%
Long-term debt maturity date		2,046
Secured Debt | Ameren Illinois Company | Series 1993 5.90% Due 2023
Debt Instrument [Line Items]
Debt instrument face amount	[4]	$ 1,000,000
Long-term debt interest rate		5.90%
Secured Debt | Union Electric Company | 5.40% Senior secured notes due 2016
Debt Instrument [Line Items]
Debt instrument face amount		$ 0	[5]	$ 260,000,000	260,000,000	[5]
Long-term debt interest rate		5.40%
Long-term debt maturity date		2,016
Secured Debt | Union Electric Company | 6.40% Senior secured notes due 2017
Debt Instrument [Line Items]
Debt instrument face amount	[5]	$ 425,000,000			425,000,000
Long-term debt interest rate		6.40%
Long-term debt maturity date		2,017
Secured Debt | Union Electric Company | 6.00% Senior secured notes due 2018
Debt Instrument [Line Items]
Debt instrument face amount	[5],[6]	$ 179,000,000			179,000,000
Long-term debt interest rate		6.00%
Long-term debt maturity date		2,018
Secured Debt | Union Electric Company | 5.10% Senior secured notes due 2018
Debt Instrument [Line Items]
Debt instrument face amount	[5]	$ 199,000,000			199,000,000
Long-term debt interest rate		5.10%
Long-term debt maturity date		2,018
Secured Debt | Union Electric Company | 6.70% Senior secured notes due 2019
Debt Instrument [Line Items]
Debt instrument face amount	[5],[6]	$ 329,000,000			329,000,000
Long-term debt interest rate		6.70%
Long-term debt maturity date		2,019
Secured Debt | Union Electric Company | 5.10% Senior secured notes due 2019
Debt Instrument [Line Items]
Debt instrument face amount	[5]	$ 244,000,000			244,000,000
Long-term debt interest rate		5.10%
Long-term debt maturity date		2,019
Secured Debt | Union Electric Company | 5.00% Senior secured notes due 2020
Debt Instrument [Line Items]
Debt instrument face amount	[5]	$ 85,000,000			85,000,000
Long-term debt interest rate		5.00%
Long-term debt maturity date		2,020
Secured Debt | Union Electric Company | Senior Secured Notes350 Due2024 [Member]
Debt Instrument [Line Items]
Debt instrument face amount	[5]	$ 350,000,000			350,000,000
Long-term debt interest rate		3.50%
Long-term debt maturity date		2,024
Secured Debt | Union Electric Company | 5.50% Senior secured notes due 2034
Debt Instrument [Line Items]
Debt instrument face amount	[5]	$ 184,000,000			184,000,000
Long-term debt interest rate		5.50%
Long-term debt maturity date		2,034
Secured Debt | Union Electric Company | 5.30% Senior secured notes due 2037
Debt Instrument [Line Items]
Debt instrument face amount	[5]	$ 300,000,000			300,000,000
Long-term debt interest rate		5.30%
Long-term debt maturity date		2,037
Secured Debt | Union Electric Company | 8.45% Senior secured notes due 2039
Debt Instrument [Line Items]
Debt instrument face amount	[5],[6]	$ 350,000,000			350,000,000
Long-term debt interest rate		8.45%
Long-term debt maturity date		2,039
Secured Debt | Union Electric Company | 3.90% Senior secured notes due 2042
Debt Instrument [Line Items]
Debt instrument face amount	[5],[6]	$ 485,000,000			485,000,000
Long-term debt interest rate		3.90%
Long-term debt maturity date		2,042
Secured Debt | Union Electric Company | Senior Secured Notes, 3.65%, Due 2045 [Member]
Debt Instrument [Line Items]
Debt instrument face amount	[5]	$ 400,000,000			250,000,000
Long-term debt interest rate		3.65%
Long-term debt maturity date		2,045
Environmental Improvement And Pollution Control Revenue Bonds | Ameren Illinois Company | Series 1993 5.90% Due 2023
Debt Instrument [Line Items]
Debt instrument face amount	[3]	$ 1,000,000	[4]		1,000,000
Long-term debt interest rate		5.90%
Long-term debt maturity date		2,023
Redemption price, percentage		100.00%
Environmental Improvement And Pollution Control Revenue Bonds | Ameren Illinois Company | Series 1994 A 5.70% Due 2024
Debt Instrument [Line Items]
Debt instrument face amount	[7]	$ 1,000,000	[4]		1,000,000
Long-term debt interest rate		5.70%
Long-term debt maturity date		2,024
Redemption price, percentage		100.00%
Environmental Improvement And Pollution Control Revenue Bonds | Ameren Illinois Company | Series B-1 1993 Due 2028
Debt Instrument [Line Items]
Debt instrument face amount	[8],[9]	$ 17,000,000			17,000,000
Long-term debt maturity date		2,028
Redemption price, percentage		100.00%
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1992 Series due 2022
Debt Instrument [Line Items]
Debt instrument face amount	[8],[10]	$ 47,000,000			47,000,000
Long-term debt maturity date		2,022
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1993 5.45% Series due 2028
Debt Instrument [Line Items]
Debt instrument face amount	[11]	$ 1,000,000
Long-term debt interest rate		5.45%
Long-term debt maturity date		2,028
Redemption price, percentage		100.00%
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1998 Series A due 2033
Debt Instrument [Line Items]
Debt instrument face amount	[8],[10]	$ 60,000,000			60,000,000
Long-term debt maturity date		2,033
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1998 Series B due 2033
Debt Instrument [Line Items]
Debt instrument face amount	[8],[10]	$ 50,000,000			50,000,000
Long-term debt maturity date		2,033
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1998 Series C due 2033
Debt Instrument [Line Items]
Debt instrument face amount	[8],[10]	$ 50,000,000			$ 50,000,000
Long-term debt maturity date		2,033
Maximum | Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1992 Series due 2022
Debt Instrument [Line Items]
Long-term debt interest rate		18.00%
Maximum | Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1998 Series A due 2033
Debt Instrument [Line Items]
Long-term debt interest rate		0.00%
Maximum | Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1998 Series B due 2033
Debt Instrument [Line Items]
Long-term debt interest rate		0.00%
Maximum | Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1998 Series C due 2033
Debt Instrument [Line Items]
Long-term debt interest rate		0.00%
[1]	(h)Ameren Illinois has agreed that so long as any of the 2.70% senior secured notes due 2022 are outstanding, Ameren Illinois will not permit a release date to occur, and so long as any of the 9.75% senior secured notes due 2018, 6.25% senior secured notes due 2018, and 6.125% senior secured notes due 2017 are outstanding, Ameren Illinois will not optionally redeem, purchase or otherwise retire in full the outstanding first mortgage bonds not subject to release provisions; therefore, a release date will not occur so long as any of these notes remain outstanding.
[2]	These notes are collaterally secured by mortgage bonds issued by Ameren Illinois under its 1992 mortgage indenture. They are secured by substantially all property of the former IP and CIPS. The notes have a fall-away lien provision and will remain secured only as long as any series of first mortgage bonds issued under its 1992 mortgage indenture remain outstanding. Redemption, purchase, or maturity of all mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the Ameren Illinois senior secured notes currently outstanding, we do not expect the mortgage bond lien protection associated with these notes to fall away before 2022.
[3]	(i)These bonds are first mortgage bonds issued by Ameren Illinois under its 1933 mortgage indenture. They are secured by substantially all property of the former CILCO. The bonds are callable at 100% of par value. Less than $1 million principal amount of the bonds remain outstanding.
[4]	(f)These notes are collaterally secured by first mortgage bonds issued by Ameren Illinois under its 1933 mortgage indenture. The notes have a fall-away lien provision, and Ameren Illinois could cause these notes to become unsecured at any time by redeeming the pollution control bonds 5.90% Series 1993 due 2023 (of which less than $1 million remains outstanding).
[5]	(a)These notes are collaterally secured by first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage indenture. The notes have a fall-away lien provision and will remain secured only as long as any first mortgage bonds issued under the Ameren Missouri mortgage indenture remain outstanding. Redemption, purchase, or maturity of all first mortgage bonds, including first mortgage bonds currently outstanding and any that may be issued in the future, would result in a release of the first mortgage bonds currently securing these notes, at which time these notes would become unsecured obligations. Considering the Ameren Missouri senior secured notes currently outstanding, we do not expect the first mortgage bond lien protection associated with these notes to fall away before 2042.
[6]	(b)Ameren Missouri has agreed that so long as any of the 3.90% senior secured notes due 2042 are outstanding, Ameren Missouri will not permit a release date to occur, and so long as any of the 6.70% senior secured notes due 2019, 6.00% senior secured notes due 2018, and 8.45% senior secured notes due 2039 are outstanding, Ameren Missouri will not optionally redeem, purchase, or otherwise retire in full the outstanding first mortgage bonds not subject to release provisions.
[7]	(j)These bonds are mortgage bonds issued by Ameren Illinois under its 1992 mortgage indenture. They are secured by substantially all property of the former IP and CIPS. The bonds are callable at 100% of par value. The bonds are also backed by an insurance guarantee policy. Less than $1 million principal amount of the bonds remains outstanding.
[8]	(d)The interest rates and the periods during which such rates apply vary depending on our selection of defined rate modes. Maximum interest rates could reach 18%, depending on the series of bonds. The bonds are callable at 100% of par value. The average interest rates for 2016 and 2015 were as follows: 2016 2015Ameren Missouri 1992 Series due 20220.66% 0.06%Ameren Missouri 1998 Series A due 20330.91% 0.24%Ameren Missouri 1998 Series B due 20330.92% 0.24%Ameren Missouri 1998 Series C due 20330.97% 0.24%Ameren Illinois 1993 Series B-1 due 20280.70% 0.49%
[9]	(k)The bonds are callable at 100% of par value.
[10]	c)These bonds are collaterally secured by first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage indenture and have a fall-away lien provision similar to that of Ameren Missouri's senior secured notes. The bonds are also backed by an insurance guarantee policy.
[11]	(e)These bonds are first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage bond indenture and are secured by substantially all Ameren Missouri property and franchises. The bonds are callable at 100% of par value. Less than $1 million principal amount of the bonds remain outstanding.

Long-Term Debt And Equity Fin65

Long-Term Debt And Equity Financings (Schedule Of Average Interest Rates) (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
1992 Series due 2022 | Union Electric Company
Debt Instrument [Line Items]
Debt instrument, interest rate during period	0.66%	0.06%
1998 Series A due 2033 | Union Electric Company
Debt Instrument [Line Items]
Debt instrument, interest rate during period	0.91%	0.24%
1998 Series B due 2033 | Union Electric Company
Debt Instrument [Line Items]
Debt instrument, interest rate during period	0.92%	0.24%
1998 Series C due 2033 | Union Electric Company
Debt Instrument [Line Items]
Debt instrument, interest rate during period	0.97%	0.24%
Series B-1 1993 Due 2028 | Ameren Illinois Company
Debt Instrument [Line Items]
Debt instrument, interest rate during period	0.70%	0.49%

Long-Term Debt And Equity Fin66

Long-Term Debt And Equity Financings (Schedule Of Maturities Of Long-Term Debt) (Details) $ in Millions	Dec. 31, 2016USD ($)
Debt Instrument [Line Items]
2,017	$ 681
2,018	840
2,019	581
2,020	442
2,021	8
Thereafter	4,774
Total	7,326
Parent Company
Debt Instrument [Line Items]
2,020	350	[1]
Thereafter	350	[1]
Total	700	[1]
Debt Instrument, Unamortized Discount or Premium and Debt Issuance Costs	(6)
Union Electric Company
Debt Instrument [Line Items]
2,017	431	[1]
2,018	383	[1]
2,019	581	[1]
2,020	92	[1]
2,021	8	[1]
Thereafter	2,524	[1]
Total	4,019	[1]
Debt Instrument, Unamortized Discount or Premium and Debt Issuance Costs	(25)
Ameren Illinois Company
Debt Instrument [Line Items]
2,017	250	[1]
2,018	457	[1]
Thereafter	1,900	[1]
Total	2,607	[1]
Debt Instrument, Unamortized Discount or Premium and Debt Issuance Costs	$ (19)
[1]	Excludes unamortized discount and premium and debt issuance costs of $6 million, $25 million, and $19 million at Ameren (parent), Ameren Missouri, and Ameren Illinois, respectively.

Long-Term Debt And Equity Fin67

Long-Term Debt And Equity Financings (Schedule Of Outstanding Preferred Stock) (Details) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, par value		$ 0.01
Preferred stock, authorized		100,000,000
Preferred stock, shares outstanding		0
Preferred stock, voluntary liquidation		$ 105.50
Union Electric Company and Ameren Illinois [Member]
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, issued		$ 142	$ 142
Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, par value		$ 1
Preferred stock, authorized		7,500,000
Preferred stock, issued		$ 80	80
Union Electric Company | Par Value $100 [Member]
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, par value		$ 100
Preferred stock, authorized		25,000,000
Union Electric Company | $3.50 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount		$ 3.50
Preferred stock, shares outstanding		130,000
Preferred stock, redemption price per share		$ 110
Preferred stock, issued		$ 13	13
Union Electric Company | $3.70 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount		$ 3.70
Preferred stock, shares outstanding		40,000
Preferred stock, redemption price per share		$ 104.75
Preferred stock, issued		$ 4	4
Union Electric Company | $4.00 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount		$ 4
Preferred stock, shares outstanding		150,000
Preferred stock, redemption price per share		$ 105.625
Preferred stock, issued		$ 15	15
Union Electric Company | $4.30 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount		$ 4.30
Preferred stock, shares outstanding		40,000
Preferred stock, redemption price per share		$ 105
Preferred stock, issued		$ 4	4
Union Electric Company | $4.50 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount		$ 4.50
Preferred stock, shares outstanding		213,595
Preferred stock, redemption price per share	[1]	$ 110
Preferred stock, issued		$ 21	21
Union Electric Company | $4.56 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount		$ 4.56
Preferred stock, shares outstanding		200,000
Preferred stock, redemption price per share		$ 102.47
Preferred stock, issued		$ 20	20
Union Electric Company | $4.75 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount		$ 4.75
Preferred stock, shares outstanding		20,000
Preferred stock, redemption price per share		$ 102.176
Preferred stock, issued		$ 2	2
Union Electric Company | $5.50 Series A
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount		$ 5.50
Preferred stock, shares outstanding		14,000
Preferred stock, redemption price per share		$ 110
Preferred stock, issued		$ 1	1
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized		2,600,000
Preferred stock, issued		$ 62	62
Ameren Illinois Company | Par Value $100 [Member]
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, par value		$ 100
Preferred stock, authorized		2,000,000
Ameren Illinois Company | 4.00% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		4.00%
Preferred stock, shares outstanding		144,275
Preferred stock, redemption price per share		$ 101
Preferred stock, issued		$ 14	14
Ameren Illinois Company | 4.08% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		4.08%
Preferred stock, shares outstanding		45,224
Preferred stock, redemption price per share		$ 103
Preferred stock, issued		$ 5	5
Ameren Illinois Company | 4.20% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		4.20%
Preferred stock, shares outstanding		23,655
Preferred stock, redemption price per share		$ 104
Preferred stock, issued		$ 2	2
Ameren Illinois Company | 4.25% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		4.25%
Preferred stock, shares outstanding		50,000
Preferred stock, redemption price per share		$ 102
Preferred stock, issued		$ 5	5
Ameren Illinois Company | 4.26% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		4.26%
Preferred stock, shares outstanding		16,621
Preferred stock, redemption price per share		$ 103
Preferred stock, issued		$ 2	2
Ameren Illinois Company | 4.42% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		4.42%
Preferred stock, shares outstanding		16,190
Preferred stock, redemption price per share		$ 103
Preferred stock, issued		$ 2	2
Ameren Illinois Company | 4.70% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		4.70%
Preferred stock, shares outstanding		18,429
Preferred stock, redemption price per share		$ 103
Preferred stock, issued		$ 2	2
Ameren Illinois Company | 4.90% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		4.90%
Preferred stock, shares outstanding		73,825
Preferred stock, redemption price per share		$ 102
Preferred stock, issued		$ 7	7
Ameren Illinois Company | 4.92% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		4.92%
Preferred stock, shares outstanding		49,289
Preferred stock, redemption price per share		$ 103.50
Preferred stock, issued		$ 5	5
Ameren Illinois Company | 5.16% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		5.16%
Preferred stock, shares outstanding		50,000
Preferred stock, redemption price per share		$ 102
Preferred stock, issued		$ 5	5
Ameren Illinois Company | 6.625% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		6.625%
Preferred stock, shares outstanding		124,274
Preferred stock, redemption price per share		$ 100
Preferred stock, issued		$ 12	12
Ameren Illinois Company | 7.75% Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, percentage		7.75%
Preferred stock, shares outstanding		4,542
Preferred stock, redemption price per share		$ 100
Preferred stock, issued		$ 1	$ 1
[1]	In the event of voluntary liquidation, $105.50

Long-Term Debt and Equity Fin68

Long-Term Debt and Equity Financings (Schedule of Required and Actual Debt Ratios) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016USD ($)
Union Electric Company
Debt Instrument [Line Items]
Bonds Issuable Based On Coverage Ratio	$ 4,077	[1]
Preferred Stock Issuable Based On Coverage Ratio	2,344
Retired Bond Capacity	$ 1,206
Union Electric Company | Actual Interest Coverage Ratio
Debt Instrument [Line Items]
Interest Coverage Ratio	4.6
Dividend Coverage Ratio	105.3
Ameren Illinois Company
Debt Instrument [Line Items]
Bonds Issuable Based On Coverage Ratio	$ 3,819	[1],[2]
Preferred Stock Issuable Based On Coverage Ratio	203	[3]
Retired Bond Capacity	$ 279
Ameren Illinois Company | Actual Interest Coverage Ratio
Debt Instrument [Line Items]
Interest Coverage Ratio	6.9
Dividend Coverage Ratio	2.8
Minimum | Union Electric Company | Required Dividend Coverage Ratio
Debt Instrument [Line Items]
Interest Coverage Ratio	2	[4]
Dividend Coverage Ratio	2.5	[5]
Minimum | Ameren Illinois Company | Required Dividend Coverage Ratio
Debt Instrument [Line Items]
Interest Coverage Ratio	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 $1,206 million and $279 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 its 1992 mortgage indenture.
[3]	Preferred stock issuable is restricted by the amount of preferred stock that is currently authorized by Ameren Illinois' articles of incorporation.
[4]	Coverage required on the annual interest charges on first mortgage bonds outstanding and to be issued. Coverage is not required in certain cases when additional first mortgage bonds are issued on the basis of retired bonds.
[5]	Coverage required on the annual dividend on preferred stock outstanding and to be issued, as required in the respective company’s articles of incorporation.

Other Income And Expenses (Othe

Other Income And Expenses (Other Income And Expenses) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	[1]	$ 27	$ 30	$ 34
Interest income on industrial development revenue bonds	[1]	27	27	27
Interest and dividend income	[1],[2]	13	14	10
Other	[1]	7	3	8	[3]
Total miscellaneous income	[1]	74	74	79
Donations	[1]	16	15	10
Other	[1]	16	15	12
Total miscellaneous expense	[1]	32	30	22
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		23	22	32
Interest income on industrial development revenue bonds		27	27	27
Interest and dividend income		1	1	1
Other		1	2	0
Total miscellaneous income		52	52	60
Donations		4	5	6
Other		6	6	6
Total miscellaneous expense		10	11	12
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		4	8	2
Interest and dividend income	[2]	12	12	7
Other		5	1	8	[3]
Total miscellaneous income		21	21	17
Donations		6	5	4
Other		6	7	4
Total miscellaneous expense		$ 12	$ 12	$ 8
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.
[2]	Includes Ameren Illinois' interest income on the IEIMA revenue requirement reconciliation adjustment regulatory assets.
[3]	Includes Ameren Illinois' income earned in 2014 from customer-requested construction.

Derivative Financial Instrume70

Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Details) lb in Thousands, gal in Millions, MWh in Millions, MMBTU in Millions		Dec. 31, 2016MWhgalMMBTUlb	Dec. 31, 2015MWhgalMMBTUlb
Fuel Oils
Derivative [Line Items]
Quantity | gal	[1]	30	35
Natural Gas
Derivative [Line Items]
Quantity | MMBTU		154	181
Power
Derivative [Line Items]
Quantity | MWh		10	11
Uranium
Derivative [Line Items]
Quantity | lb		345	494
Union Electric Company | Fuel Oils
Derivative [Line Items]
Quantity | gal	[1]	30	35
Union Electric Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU		25	30
Union Electric Company | Power
Derivative [Line Items]
Quantity | MWh		1	1
Union Electric Company | Uranium
Derivative [Line Items]
Quantity | lb		345	494
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU		129	151
Ameren Illinois Company | Power
Derivative [Line Items]
Quantity | MWh		9	10
[1]	Consists of ultra-low-sulfur diesel products.

Derivative Financial Instrume71

Derivative Financial Instruments (Derivative Instruments Carrying Value) (Details) - Not Designated As Hedging Instrument - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Derivative assets	[1]	$ 27	$ 18
Derivative liabilities	[2]	211	264
Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets		2
Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities		5	22
Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities			7
Natural Gas | Other current assets
Derivative [Line Items]
Derivative assets		12	1
Natural Gas | Other assets
Derivative [Line Items]
Derivative assets		3
Natural Gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities		4	38
Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities		10	26
Power | Other current assets
Derivative [Line Items]
Derivative assets		9	16
Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities		15	13
Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities		173	157
Uranium | Other current liabilities
Derivative [Line Items]
Derivative liabilities			1
Union Electric Company
Derivative [Line Items]
Derivative assets	[1]	14	17
Derivative liabilities	[2]	18	44
Union Electric Company | Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets		2
Union Electric Company | Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets	[1]	1
Union Electric Company | Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities		5	22
Union Electric Company | Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities			7
Union Electric Company | Natural Gas | Other current assets
Derivative [Line Items]
Derivative assets		1
Union Electric Company | Natural Gas | Other assets
Derivative [Line Items]
Derivative assets		1	1	[1]
Union Electric Company | Natural Gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities		1	6
Union Electric Company | Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities		5	8
Union Electric Company | Power | Other current assets
Derivative [Line Items]
Derivative assets		9	16
Union Electric Company | Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities		3
Union Electric Company | Uranium | Other current liabilities
Derivative [Line Items]
Derivative liabilities			1
Union Electric Company | Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	[1]	4
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	[1]	13	1
Derivative liabilities	[2]	193	220
Ameren Illinois Company | Natural Gas | Other current assets
Derivative [Line Items]
Derivative assets		11	1
Ameren Illinois Company | Natural Gas | Other assets
Derivative [Line Items]
Derivative assets		2
Ameren Illinois Company | Natural Gas | MTM derivative liabilities
Derivative [Line Items]
Derivative liabilities		3	32
Ameren Illinois Company | Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities		5	18
Ameren Illinois Company | Power | MTM derivative liabilities
Derivative [Line Items]
Derivative liabilities		12	13
Ameren Illinois Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities		$ 173	$ 157
[1]	The cumulative amount of pretax net gains on all derivative instruments is deferred as a regulatory liability.
[2]	The cumulative amount of pretax net losses on all derivative instruments is deferred as a regulatory asset.

Derivative Financial Instrume72

Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Details) - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Restricted Cash and Cash Equivalents		$ 8
Aggregate Fair Value of Derivative Liabilities	[1]	97
Cash Collateral Posted		3
Potential Aggregate Amount of Additional Collateral Required	[2]	80
Union Electric Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	[1]	64
Cash Collateral Posted		3
Potential Aggregate Amount of Additional Collateral Required	[2]	54
Ameren Illinois Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	[1]	33
Cash Collateral Posted		0
Potential Aggregate Amount of Additional Collateral Required	[2]	26
Cash | Union Electric Company
Derivative [Line Items]
Restricted Cash and Cash Equivalents		$ 12	$ 11
Cash | Ameren Illinois Company
Derivative [Line Items]
Restricted Cash and Cash Equivalents			$ 3
[1]	Before consideration of master netting arrangements or similar agreements and including NPNS and other accrual contract exposures.
[2]	As collateral requirements with certain counterparties are based on master netting arrangements or similar agreements, the aggregate amount of additional collateral required to be posted is determined after consideration of the effects of such arrangements.

Fair Value Measurements (Schedu

Fair Value Measurements (Schedule of Valuation Process and Unobservable Inputs) (Details) $ in Millions		12 Months Ended
		Dec. 31, 2016USD ($)$ / MMBTU$ / credit$ / MW$ / MWh$ / lb		Dec. 31, 2015USD ($)$ / MMBTU$ / credit$ / MW$ / MWh$ / lb
Fuel Oils
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	[1]	$ 3
Derivative liabilities		(5)	[1]	$ (29)	[2]
Natural Gas
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		15	[1]	2	[2]
Derivative liabilities		(14)	[1]	(64)	[2]
Power
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		9	[1]	16	[2]
Derivative liabilities		(188)	[1]	(170)	[2]
Uranium
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities		(4)	[1]	(1)	[2]
Union Electric Company | Fuel Oils
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	[1]	3
Derivative liabilities		(5)	[1]	(29)	[2]
Union Electric Company | Natural Gas
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		2	[1]	1	[2]
Derivative liabilities		(6)	[1]	(14)	[2]
Union Electric Company | Power
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		9	[1]	16	[2]
Derivative liabilities	[1]	(3)
Union Electric Company | Uranium
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities		(4)	[1]	(1)	[2]
Ameren Illinois Company | Natural Gas
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		13	[1]	1	[2]
Derivative liabilities		(8)	[1]	(50)	[2]
Ameren Illinois Company | Power
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities		$ (185)	[1]	$ (170)	[2]
Discounted Cash Flow | Minimum | Fuel Oils
Fair Value Inputs [Abstract]
Escalation rate	[3],[4]	(2.00%)
Counterparty credit risk	[5],[6]	0.13%
Credit risk	[5],[6]	0.38%
Discounted Cash Flow | Minimum | Natural Gas
Fair Value Inputs [Abstract]
Counterparty credit risk	[5],[6]	0.13%		0.40%
Credit risk	[5],[6]	0.38%		0.40%
Nodal basis | $ / MWh	[4]	(0.80)		(0.10)
Discounted Cash Flow | Minimum | Power
Fair Value Inputs [Abstract]
Counterparty credit risk	[5],[6]			0.86%
Credit risk	[5],[6]	0.38%		0.40%
Average bid/ask consensus peak and off-peak pricing | $ / MWh	[7]	26		22
Estimated auction price for FTRs | $ / MW	[4]	(71)		(270)
Nodal basis | $ / MWh	[7]	(6)		(10)
Discounted Cash Flow | Minimum | Uranium
Fair Value Inputs [Abstract]
Credit risk	[5],[6]	0.38%		0.40%
Average bid/ask consensus pricing | $ / lb	[4]	22		35
Discounted Cash Flow | Maximum | Fuel Oils
Fair Value Inputs [Abstract]
Escalation rate	[3],[4]	2.00%
Counterparty credit risk	[5],[6]	0.22%
Credit risk	[5],[6]	0.38%
Discounted Cash Flow | Maximum | Natural Gas
Fair Value Inputs [Abstract]
Counterparty credit risk	[5],[6]	8.00%		12.00%
Credit risk	[5],[6]	0.38%		0.40%
Nodal basis | $ / MWh	[4]	0		0
Discounted Cash Flow | Maximum | Power
Fair Value Inputs [Abstract]
Counterparty credit risk	[5],[6]			0.86%
Credit risk	[5],[6]	0.38%		0.40%
Average bid/ask consensus peak and off-peak pricing | $ / MWh	[7]	44		39
Estimated auction price for FTRs | $ / MW	[4]	5,270		2,057
Nodal basis | $ / MWh	[7]	0		(1)
Discounted Cash Flow | Maximum | Uranium
Fair Value Inputs [Abstract]
Credit risk	[5],[6]	0.38%		0.40%
Average bid/ask consensus pricing | $ / lb	[4]	24		42
Discounted Cash Flow | Weighted Average | Fuel Oils
Fair Value Inputs [Abstract]
Escalation rate	[3],[4]	0.00%
Counterparty credit risk	[5],[6]	0.15%
Credit risk	[5],[6]	0.38%
Discounted Cash Flow | Weighted Average | Natural Gas
Fair Value Inputs [Abstract]
Counterparty credit risk	[5],[6]	1.00%		7.00%
Credit risk	[5],[6]	0.38%		0.40%
Nodal basis | $ / MWh	[4]	(0.50)		(0.10)
Discounted Cash Flow | Weighted Average | Power
Fair Value Inputs [Abstract]
Counterparty credit risk	[5],[6]			0.86%
Credit risk	[5],[6]	0.38%		0.40%
Average bid/ask consensus peak and off-peak pricing | $ / MWh	[7]	29		29
Estimated auction price for FTRs | $ / MW	[4]	125		211
Nodal basis | $ / MWh	[7]	(2)		(3)
Discounted Cash Flow | Weighted Average | Uranium
Fair Value Inputs [Abstract]
Credit risk	[5],[6]	0.38%		0.40%
Average bid/ask consensus pricing | $ / lb	[4]	22		37
Option Model | Minimum | Fuel Oils
Fair Value Inputs [Abstract]
Volatilities	[4]	24.00%
Option Model | Minimum | Natural Gas
Fair Value Inputs [Abstract]
Volatilities	[4]	31.00%		35.00%
Nodal basis | $ / MWh		(0.40)	[4]	(0.30)	[6]
Option Model | Minimum | Uranium
Fair Value Inputs [Abstract]
Volatilities	[4]	24.00%		20.00%
Option Model | Maximum | Fuel Oils
Fair Value Inputs [Abstract]
Volatilities	[4]	66.00%
Option Model | Maximum | Natural Gas
Fair Value Inputs [Abstract]
Volatilities	[4]	66.00%		55.00%
Nodal basis | $ / MWh		(0.10)	[4]	0	[6]
Option Model | Maximum | Uranium
Fair Value Inputs [Abstract]
Volatilities	[4]	24.00%		20.00%
Option Model | Weighted Average | Fuel Oils
Fair Value Inputs [Abstract]
Volatilities	[4]	28.00%
Option Model | Weighted Average | Natural Gas
Fair Value Inputs [Abstract]
Volatilities	[4]	36.00%		45.00%
Nodal basis | $ / MWh		(0.20)	[4]	(0.20)	[6]
Option Model | Weighted Average | Uranium
Fair Value Inputs [Abstract]
Volatilities	[4]	24.00%		20.00%
Fundamental Energy Production Model | Minimum | Power
Fair Value Inputs [Abstract]
Escalation rate	[4],[8]	5.00%		3.00%
Estimated future gas prices | $ / MMBTU	[4]	3		3
Fundamental Energy Production Model | Maximum | Power
Fair Value Inputs [Abstract]
Escalation rate	[4],[8]	5.00%		3.00%
Estimated future gas prices | $ / MMBTU	[4]	4		4
Fundamental Energy Production Model | Weighted Average | Power
Fair Value Inputs [Abstract]
Escalation rate	[4],[8]	5.00%		3.00%
Estimated future gas prices | $ / MMBTU	[4]	3		4
Contract Price Allocation | Minimum | Power
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit	[4]	5		5
Contract Price Allocation | Maximum | Power
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit	[4]	7		7
Contract Price Allocation | Weighted Average | Power
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit	[4]	6		6
Derivative liabilities | Fuel Oils
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	[9]	$ 0
Derivative liabilities | Natural Gas
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	[9]	(1)		$ (1)
Derivative liabilities | Power
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	[9],[10]	(187)		(170)
Derivative liabilities | Uranium
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	[9]	(4)		(1)
Derivative assets | Fuel Oils
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	[9]	1
Derivative assets | Natural Gas
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	[9]	1		1
Derivative assets | Power
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	[9],[10]	$ 9		$ 16
[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]	Escalation rate applies to fuel oil prices 2019 and beyond.
[4]	Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.
[5]	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.
[6]	Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.
[7]	The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes because of their opposing positions.
[8]	Escalation rate applies to power prices in 2031 and beyond for December 31, 2016, and to power prices in 2026 and beyond for December 31, 2015.
[9]	The derivative asset and liability balances are presented net of counterparty credit considerations.
[10]	Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2020. Valuations beyond 2020 use fundamentally modeled pricing by month for peak and off-peak demand.

Fair Value Measurements (Sche74

Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Details) - USD ($) $ in Millions		Dec. 31, 2016		Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		$ 605	[1]	$ 557	[2]
Assets		632	[3]	575	[4]
Excluded receivables, payables, and accrued income, net		2		(1)
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		409		368
Assets		413	[3]	368	[4]
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		196		189
Assets		208	[3]	190	[4]
Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets		11	[3]	17	[4]
Cash And Cash Equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		1		4
Cash And Cash Equivalents | 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		1		4
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		27	[3]	18	[4]
Derivative liabilities		211	[3]	264	[4]
Commodity Contract | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	4
Derivative liabilities		5	[3]	30	[4]
Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		12	[3]	1	[4]
Derivative liabilities		14	[3]	62	[4]
Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		11	[3]	17	[4]
Derivative liabilities		192	[3]	172	[4]
Fuel Oils
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	3
Derivative liabilities		5	[3]	29	[4]
Fuel Oils | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	2
Derivative liabilities		5	[3]	29	[4]
Fuel Oils | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	1
Derivative liabilities	[3]
Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		15	[3]	2	[4]
Derivative liabilities		14	[3]	64	[4]
Natural Gas | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	2
Derivative liabilities		0	[3]	1	[4]
Natural Gas | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		12	[3]	1	[4]
Derivative liabilities		13	[3]	62	[4]
Natural Gas | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	1
Derivative liabilities		1	[3]	1	[4]
Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		9	[3]	16	[4]
Derivative liabilities		188	[3]	170	[4]
Power | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			0
Derivative liabilities		1	[3]	0	[4]
Power | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	9
Derivative liabilities		187	[3]	170	[4]
Uranium
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		4	[3]	1	[4]
Uranium | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		4	[3]	1	[4]
Equity Securities | U.S. Large Capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		408		364
Equity Securities | U.S. Large Capitalization | 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		408		364
Debt Securities | Corporate Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		67		58
Debt Securities | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		67		58
Debt Securities | U.S. treasury and agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		112		109
Debt Securities | U.S. treasury and agency securities | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		112		109
Debt Securities | Other debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		17		22
Debt Securities | Other debt securities | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		17		22
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		605	[1]	557	[2]
Assets		619	[3]	574	[4]
Union Electric Company | 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		409		368
Assets		411	[3]	368	[4]
Union Electric Company | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		196		189
Assets		197	[3]	189	[4]
Union Electric Company | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets		11	[3]	17	[4]
Union Electric Company | Cash And Cash Equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		1		4
Union Electric Company | Cash And Cash Equivalents | 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		1		4
Union Electric Company | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		14	[3]	17	[4]
Derivative liabilities		18	[3]	44	[4]
Union Electric Company | Commodity Contract | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	2
Derivative liabilities		5	[3]	29	[4]
Union Electric Company | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	0	[4]
Derivative liabilities		7	[3]	13	[4]
Union Electric Company | Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		11	[3]	17	[4]
Derivative liabilities		6	[3]	2	[4]
Union Electric Company | Fuel Oils
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	3
Derivative liabilities		5	[3]	29	[4]
Union Electric Company | Fuel Oils | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	2
Derivative liabilities		5	[3]	29	[4]
Union Electric Company | Fuel Oils | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	1
Union Electric Company | Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		2	[3]	1	[4]
Derivative liabilities		6	[3]	14	[4]
Union Electric Company | Natural Gas | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	[4]			0
Union Electric Company | Natural Gas | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	1
Derivative liabilities		6	[3]	13	[4]
Union Electric Company | Natural Gas | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	1	[4]
Derivative liabilities	[4]			1
Union Electric Company | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		9	[3]	16	[4]
Derivative liabilities	[3]	3
Union Electric Company | Power | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			0
Derivative liabilities	[3]	1
Union Electric Company | Power | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		9	[3]	16	[4]
Derivative liabilities	[3]	2
Union Electric Company | Uranium
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		4	[3]	1	[4]
Union Electric Company | Uranium | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		4	[3]	1	[4]
Union Electric Company | Equity Securities | U.S. Large Capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		408		364
Union Electric Company | Equity Securities | U.S. Large Capitalization | 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		408		364
Union Electric Company | Debt Securities | Corporate Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		67		58
Union Electric Company | Debt Securities | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		67		58
Union Electric Company | Debt Securities | U.S. treasury and agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		112		109
Union Electric Company | Debt Securities | U.S. treasury and agency securities | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		112		109
Union Electric Company | Debt Securities | Other debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		17		22
Union Electric Company | Debt Securities | Other debt securities | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		17		22
Ameren Illinois Company | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		193	[3]	220	[4]
Ameren Illinois Company | Commodity Contract | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	[4]			1
Ameren Illinois Company | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		7	[3]	49	[4]
Ameren Illinois Company | Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		186	[3]	170	[4]
Ameren Illinois Company | Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		13	[3]	1	[4]
Derivative liabilities		8	[3]	50	[4]
Ameren Illinois Company | Natural Gas | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	2
Derivative liabilities	[4]			1
Ameren Illinois Company | Natural Gas | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		11	[3]	1	[4]
Derivative liabilities		7	[3]	49	[4]
Ameren Illinois Company | Natural Gas | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	[3]	1
Ameren Illinois Company | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		185	[3]	170	[4]
Ameren Illinois Company | Power | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		$ 185	[3]	$ 170	[4]
[1]	Balance excludes $2 million of receivables, payables, and accrued income, net.
[2]	Balance excludes $(1) million of receivables, payables, and accrued income, net.
[3]	The derivative asset and liability balances are presented net of counterparty credit considerations.
[4]	The derivative asset and liability balances are presented net of counterparty credit considerations.

Fair Value Measurements (Sche75

Fair Value Measurements (Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level 3 In The Fair Value Hierarchy) (Details) - Power - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	$ 178	$ (154)	$ (133)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Included in regulatory assets/liabilities	(30)	(39)
Purchases	13	29
Settlements	(7)	(10)
Transfers out of Level 3		(1)
Change in unrealized gains (losses) related to assets/liabilities still held	(27)	(39)
Union Electric Company
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	16	9
Included in regulatory assets/liabilities	(1)	2
Purchases	13	29
Settlements	(21)	(23)
Transfers out of Level 3		(1)
Ending balance	7	16
Change in unrealized gains (losses) related to assets/liabilities still held	0	0
Ameren Illinois Company
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	185	(170)	$ (142)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Included in regulatory assets/liabilities	(29)	(41)
Settlements	14	13
Change in unrealized gains (losses) related to assets/liabilities still held	$ (27)	$ (39)

Fair Value Measurements (Sche76

Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Long-Term Debt And Preferred Stock) (Details) - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt and Capital Lease Obligations	[1]	$ 7,276	$ 7,275
Preferred stock	[1]	142	142
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt and capital lease obligations (including current portion)	[1]	7,772	7,814
Preferred stock	[1]	131	125
Union Electric Company | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt and Capital Lease Obligations		3,994	4,110
Preferred stock		80	80
Union Electric Company | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt and capital lease obligations (including current portion)		4,304	4,449
Preferred stock		79	75
Ameren Illinois Company | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt and Capital Lease Obligations		2,588	2,471
Preferred stock		62	62
Ameren Illinois Company | Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt and capital lease obligations (including current portion)		2,765	2,665
Preferred stock		$ 52	$ 50
[1]	Preferred stock is recorded in "Noncontrolling Interests" on the consolidated balance sheet.

Nuclear Decommissioning Trust77

Nuclear Decommissioning Trust Fund Investments (Proceeds From The Sale Of Investments And Related Gross Realized Gains And Losses In Nuclear Decommissioning Trust Fund) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Nuclear Decommissioning Trust Fund Investments [Line Items]
Sales and maturities of securities - nuclear decommissioning trust fund	$ 377	$ 349	$ 391
Gross realized gains	7	8	7
Gross realized losses	$ 4	2	2
Minimum | Nuclear Decommissioning Trust Fund
Nuclear Decommissioning Trust Fund Investments [Line Items]
Trust fund investments, target allocation percentage	60.00%
Maximum | Nuclear Decommissioning Trust Fund
Nuclear Decommissioning Trust Fund Investments [Line Items]
Trust fund investments, target allocation percentage	70.00%
Union Electric Company
Nuclear Decommissioning Trust Fund Investments [Line Items]
Sales and maturities of securities - nuclear decommissioning trust fund	$ 377	$ 349	$ 391

Nuclear Decommissioning Trust78

Nuclear Decommissioning Trust Fund Investments (Fair Values Of Investments In Debt And Equity Securities In Nuclear Decommissioning Trust Fund) (Details) - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015
Nuclear Decommissioning Trust Fund Investments [Line Items]
Cost		$ 361	$ 341
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		256	226
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax		10	11
Fair Value		607	556
Debt Securities
Nuclear Decommissioning Trust Fund Investments [Line Items]
Cost		197	191
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		3	2
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax		4	4
Fair Value		196	189
Equity Securities
Nuclear Decommissioning Trust Fund Investments [Line Items]
Cost		161	147
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		253	224
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax		6	7
Fair Value		408	364
Cash
Nuclear Decommissioning Trust Fund Investments [Line Items]
Cost		1	4
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		0	0
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax		0	0
Fair Value		1	4
Other Debt And Equity Securities
Nuclear Decommissioning Trust Fund Investments [Line Items]
Cost	[1]	2	1
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax		0	0
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax		0	0
Fair Value	[1]	$ 2	$ 1
[1]	Represents net receivables and payables relating to pending security sales, interest, and security purchases.

Nuclear Decommissioning Trust79

Nuclear Decommissioning Trust Fund Investments (Costs And Fair Values Of Investments In Debt Securities In Nuclear Decommissioning Trust Fund According To Contractual Maturities) (Details) $ in Millions	Dec. 31, 2016USD ($)
Investments, Debt and Equity Securities [Abstract]
Cost, Less than 5 years	$ 105
Cost, 5 years to 10 years	47
Cost, Due after 10 years	45
Cost, Total	197
Fair Value, Less than 5 years	104
Fair Value, 5 years to 10 years	47
Fair Value, Due after 10 years	45
Fair Value, Total	$ 196

Callaway Energy Center (Details

Callaway Energy Center (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016USD ($)mill	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
Loss Contingencies [Line Items]
Nwf Fee Number Of Mills | mill	1
Former Gain Contingency, Recognized in Current Period	$ 24	$ 14	$ 15
Nuclear Plant
Loss Contingencies [Line Items]
Annual decommissioning costs included in costs of service	$ 7

Retirement Benefits (Narrative)

Retirement Benefits (Narrative) (Details) $ in Millions		12 Months Ended
		Dec. 31, 2016USD ($)bond	Dec. 31, 2015USD ($)	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded Status of Plan	[1]	$ 774	$ 567
Defined Benefit Plan Assumptions Used Calculating Benefit Obligation, Change in Discount Rate		0.50%
Number of high-quality corporate bonds | bond		700
Defined benefit plan, estimated future employer contributions during the next five years		$ 290
Amortization basis, straight line, in years		10 years
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded Status of Plan	[1]	$ (705)	$ (544)
Expected return on plan assets		7.00%	7.25%	7.25%
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded Status of Plan	[1]	$ (69)	$ (23)
Expected return on plan assets		7.00%	7.00%	7.00%
Minimum
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, estimated future employer contributions during the next five years		$ 50
Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, estimated future employer contributions during the next five years		$ 70
Union Electric Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Future funding requirement, percentage		35.00%
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Future funding requirement, percentage		55.00%
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries.

Retirement Benefits (Summary Of

Retirement Benefits (Summary Of Benefit Liability Recorded) (Details) - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	[1]	$ 774	$ 567
Union Electric Company
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet		293	236
Ameren Illinois Company
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet		$ 315	$ 219
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries.

Retirement Benefits (Funded Sta

Retirement Benefits (Funded Status Of Benefit Plans And Amounts Included In Regulatory Assets And OCI) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Change in plan assets:
Funded status – deficiency	[1]	$ (774)		$ (567)
Amounts recognized in the balance sheet consist of:
Noncurrent liability		769		580
Net liability recognized	[2]	774		567
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation at end of year	[1]	4,288		3,995
Change in benefit obligation:
Net benefit obligation at beginning of year	[1]	4,197		4,410
Service cost		81	[1]	92	[1]	$ 79
Interest cost		185	[1]	174	[1]	183
Actuarial (gain) loss	[1]	265		(256)
Settlement	[1]			(2)
Benefits paid	[1]	(210)		(221)
Net benefit obligation at end of year	[1]	4,518		4,197		4,410
Change in plan assets:
Fair value of plan assets at beginning of year	[1]	3,653		3,794
Actual return on plan assets	[1]	313		(29)
Employer contributions		57	[1]	111	[1]	99
Settlement	[1]			(2)
Benefits paid	[1]	(210)		(221)
Fair value of plan assets at end of year	[1]	3,813		3,653		3,794
Funded status – deficiency	[1]	705		544
Accrued benefit cost at December 31	[1]	705		544
Amounts recognized in the balance sheet consist of:
Current liability(d)	[1],[3]	3		3
Noncurrent liability	[1]	702		541
Net liability recognized	[1]	705		544
Amounts recognized in regulatory assets consist of:
Net actuarial (gain) loss	[1]	535		395
Prior service cost (credit)	[1]	(4)		(5)
Amounts (pretax) recognized in accumulated OCI consist of:
Net actuarial (gain) loss	[1]	43		17
Prior service cost (credit)	[1]			0
Total	[1]	574		407
Postretirement Benefits
Change in benefit obligation:
Net benefit obligation at beginning of year	[1]	1,094		1,203
Service cost		19	[1]	24	[1]	19
Interest cost		50	[1]	48	[1]	50
Participant contributions	[1]	8		8
Actuarial (gain) loss	[1]	52		(133)
Benefits paid	[1]	(54)		(56)
Federal subsidy on benefits paid	[1]	1		0
Net benefit obligation at end of year	[1]	1,170		1,094		1,203
Change in plan assets:
Fair value of plan assets at beginning of year	[1]	1,071		1,109
Actual return on plan assets	[1]	73		(8)
Employer contributions		2	[1]	18	[1]	6
Federal subsidy on benefits paid	[1]	1		0
Participant contributions	[1]	8		8
Benefits paid	[1]	(54)		(56)
Fair value of plan assets at end of year	[1]	1,101		1,071		$ 1,109
Funded status – deficiency	[1]	69		23
Accrued benefit cost at December 31	[1]	69		23
Amounts recognized in the balance sheet consist of:
Noncurrent asset	[1],[4]	0		(18)
Current liability(d)	[1],[3]	2		2
Noncurrent liability	[1]	67		39
Net liability recognized	[1]	69		23
Amounts recognized in regulatory assets consist of:
Net actuarial (gain) loss	[1]	(29)		(82)
Prior service cost (credit)	[1]	(8)		(11)
Amounts (pretax) recognized in accumulated OCI consist of:
Net actuarial (gain) loss	[1]	0		(3)
Prior service cost (credit)	[1]	(1)		0
Total	[1]	$ (38)		$ (96)
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
[2]	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
[3]	Included in "Other current liabilities" on Ameren's consolidated balance sheet.
[4]	Included in "Other assets" on Ameren's consolidated balance sheet.

Retirement Benefits (Assumption

Retirement Benefits (Assumptions Used To Determine Benefit Obligations) (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate at measurement date	4.00%	4.50%
Increase in future compensation	3.50%	3.50%
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate at measurement date	4.00%	4.50%
Increase in future compensation	3.50%	3.50%
Medical cost trend rate (initial)	5.00%	5.00%	5.00%
Medical cost trend rate (ultimate)	5.00%	5.00%	5.00%

Retirement Benefits (Cash Contr

Retirement Benefits (Cash Contributions Made To Benefit Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	$ 57	[1]	$ 111	[1]	$ 99
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	2	[1]	18	[1]	6
Union Electric Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	21		47		41
Union Electric Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	1		8		3
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	30		45		39
Ameren Illinois Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	1		8		2
Other | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	6		19		19
Other | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	$ 0		$ 2		$ 1
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries.

Retirement Benefits (Target All

Retirement Benefits (Target Allocation Of The Plans' Asset Categories) (Details)		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Actual Plan Asset Allocations		100.00%	100.00%
Pension Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation		0.00%
Maximum Target Allocation		5.00%
Defined Benefit Plan, Actual Plan Asset Allocations		1.00%	1.00%
Pension Benefits | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation		51.00%
Maximum Target Allocation		61.00%
Defined Benefit Plan, Actual Plan Asset Allocations		57.00%	54.00%
Pension Benefits | U.S. large capitalization
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation		29.00%
Maximum Target Allocation		39.00%
Defined Benefit Plan, Actual Plan Asset Allocations		34.00%	34.00%
Pension Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation		3.00%
Maximum Target Allocation		13.00%
Defined Benefit Plan, Actual Plan Asset Allocations		9.00%	7.00%
Pension Benefits | International and emerging markets
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation		9.00%
Maximum Target Allocation		19.00%
Defined Benefit Plan, Actual Plan Asset Allocations		14.00%	13.00%
Pension Benefits | Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation		35.00%
Maximum Target Allocation		45.00%
Defined Benefit Plan, Actual Plan Asset Allocations		37.00%	40.00%
Pension Benefits | Real estate
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation		0.00%
Maximum Target Allocation		9.00%
Defined Benefit Plan, Actual Plan Asset Allocations		5.00%	5.00%
Pension Benefits | Private equity
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation		0.00%
Maximum Target Allocation		5.00%
Defined Benefit Plan, Actual Plan Asset Allocations	[1]	1.00%	1.00%
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Actual Plan Asset Allocations		100.00%	100.00%
Postretirement Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation		0.00%
Maximum Target Allocation		7.00%
Defined Benefit Plan, Actual Plan Asset Allocations		3.00%	4.00%
Postretirement Benefits | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation		55.00%
Maximum Target Allocation		65.00%
Defined Benefit Plan, Actual Plan Asset Allocations		61.00%	59.00%
Postretirement Benefits | U.S. large capitalization
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation		34.00%
Maximum Target Allocation		44.00%
Defined Benefit Plan, Actual Plan Asset Allocations		40.00%	39.00%
Postretirement Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation		2.00%
Maximum Target Allocation		12.00%
Defined Benefit Plan, Actual Plan Asset Allocations		7.00%	7.00%
Postretirement Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation		9.00%
Maximum Target Allocation		19.00%
Defined Benefit Plan, Actual Plan Asset Allocations		14.00%	13.00%
Postretirement Benefits | Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Minimum Target Allocation		33.00%
Maximum Target Allocation		43.00%
Defined Benefit Plan, Actual Plan Asset Allocations		36.00%	37.00%
[1]	(a)Less than 1% of plan assets.

Retirement Benefits (Fair Value

Retirement Benefits (Fair Value Of Plan Assets Utilizing Fair Value Hierarchy) (Details) - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	$ 3,813	$ 3,653	$ 3,794
Alternative Investments, Fair Value Disclosure	[2]	1,995	1,749
Pension Benefits | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		494	396
Pension Benefits | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1,434	1,603
Pension Benefits | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		33	20
Alternative Investments, Fair Value Disclosure	[2]	33	20
Pension Benefits | Cash And Cash Equivalents | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Cash And Cash Equivalents | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Cash And Cash Equivalents | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | U.S. large capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1,352	1,296
Alternative Investments, Fair Value Disclosure	[2]	1,352	1,296
Pension Benefits | U.S. large capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | U.S. large capitalization | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | U.S. large capitalization | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		361	268
Alternative Investments, Fair Value Disclosure	[2]	0	0
Pension Benefits | U.S. small and mid-capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		361	268
Pension Benefits | U.S. small and mid-capitalization | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | U.S. small and mid-capitalization | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | International and emerging markets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		522	491
Alternative Investments, Fair Value Disclosure	[2]	389	243
Pension Benefits | International and emerging markets | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		133	122
Pension Benefits | International and emerging markets | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	126
Pension Benefits | International and emerging markets | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		630	631
Alternative Investments, Fair Value Disclosure	[2]	13	14
Pension Benefits | Corporate Bonds | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		617	617
Pension Benefits | Corporate Bonds | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		95	104
Alternative Investments, Fair Value Disclosure	[2]	0	0
Pension Benefits | Municipal Bonds | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Municipal Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		95	104
Pension Benefits | Municipal Bonds | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | U.S. treasury and agency securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		701	757
Alternative Investments, Fair Value Disclosure	[2]	0	0
Pension Benefits | U.S. treasury and agency securities | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	6
Pension Benefits | U.S. treasury and agency securities | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		701	751
Pension Benefits | U.S. treasury and agency securities | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		21	5
Alternative Investments, Fair Value Disclosure	[2]	0	0
Pension Benefits | Other | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Other | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		21	5
Pension Benefits | Other | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Real estate
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		202	168
Alternative Investments, Fair Value Disclosure	[2]	202	168
Pension Benefits | Real estate | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Real estate | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Real estate | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Private equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		6	8
Alternative Investments, Fair Value Disclosure	[2]	6	8
Pension Benefits | Private equity | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Private equity | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Private equity | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Medical benefit assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[3]	(132)	(123)
Pension Benefits | Net receivables
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[4]	22	28
Pension Benefits | Includes Medical Benefit Component Under Section401 H And Excludes Receivables Related To Pending Security Sales [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		3,923	3,748
Pension Benefits | Excludes Medical Benefit Component Under Section401 H And Includes Receivables Related To Pending Security Sales [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		3,813	3,653
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	1,101	1,071	$ 1,109
Alternative Investments, Fair Value Disclosure	[2]	212	189
Postretirement Benefits | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		456	431
Postretirement Benefits | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		326	356
Postretirement Benefits | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		53	61
Alternative Investments, Fair Value Disclosure	[2]	0	0
Postretirement Benefits | Cash And Cash Equivalents | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		53	61
Postretirement Benefits | Cash And Cash Equivalents | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | Cash And Cash Equivalents | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | U.S. large capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		392	370
Alternative Investments, Fair Value Disclosure	[2]	101	98
Postretirement Benefits | U.S. large capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		291	272
Postretirement Benefits | U.S. large capitalization | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | U.S. large capitalization | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		72	65
Alternative Investments, Fair Value Disclosure	[2]	0	0
Postretirement Benefits | U.S. small and mid-capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		72	65
Postretirement Benefits | U.S. small and mid-capitalization | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | U.S. small and mid-capitalization | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		132	126
Alternative Investments, Fair Value Disclosure	[2]	92	55
Postretirement Benefits | International | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		40	33
Postretirement Benefits | International | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	38
Postretirement Benefits | International | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | Other Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		7	7
Alternative Investments, Fair Value Disclosure	[2]	0	0
Postretirement Benefits | Other Equity | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | Other Equity | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		7	7
Postretirement Benefits | Other Equity | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		141	138
Alternative Investments, Fair Value Disclosure	[2]	0	0
Postretirement Benefits | Corporate Bonds | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		141	138
Postretirement Benefits | Corporate Bonds | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		110	114
Alternative Investments, Fair Value Disclosure	[2]	0	0
Postretirement Benefits | Municipal Bonds | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | Municipal Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		110	114
Postretirement Benefits | Municipal Bonds | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | U.S. treasury and agency securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		68	55
Alternative Investments, Fair Value Disclosure	[2]	0	0
Postretirement Benefits | U.S. treasury and agency securities | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | U.S. treasury and agency securities | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		68	55
Postretirement Benefits | U.S. treasury and agency securities | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		19	40
Alternative Investments, Fair Value Disclosure	[2]	19	36
Postretirement Benefits | Other | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | Other | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	4
Postretirement Benefits | Other | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | Medical benefit assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[3]	132	123
Postretirement Benefits | Net payables
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[5]	(25)	(28)
Postretirement Benefits | Excludes Medical Benefit Component Under Section401 H And Excludes Payables Related To Pending Security Sales [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		994	976
Postretirement Benefits | Includes Medical Benefit Component Under Section401 H And Excludes Payables Related To Pending Security Sales [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 1,101	$ 1,071
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
[2]	Reflects the adoption of the new authoritative accounting guidance related to investments measured at the NAV practical expedient. See Note 1 - Summary of Significant Accounting Policies for additional information.
[3]	Medical benefit (health and welfare) component for accounts maintained in accordance with Section 401(h) of the Internal Revenue Code to fund a portion of the postretirement obligation.
[4]	Receivables related to pending security sales, offset by payables related to pending security purchases.
[5]	Payables related to pending security purchases, offset by interest receivables and receivables related to pending security sales.

Retirement Benefits (Components

Retirement Benefits (Components Of Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 81	[1]	$ 92	[1]	$ 79
Interest cost	185	[1]	174	[1]	183
Expected return on plan assets	(253)		(248)		(229)
Prior service credit	(1)		(1)		(1)
Actuarial (gain) loss	32		74		49
Settlement Loss			1
Net periodic benefit cost (benefit)	44		92		81
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	19	[1]	24	[1]	19
Interest cost	50	[1]	48	[1]	50
Expected return on plan assets	(72)		(68)		(65)
Prior service credit	(5)		(5)		(5)
Actuarial (gain) loss	(11)		5		(7)
Settlement Loss			0
Net periodic benefit cost (benefit)	$ (19)		$ 4		$ (8)
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries.

Retirement Benefits (Summary 89

Retirement Benefits (Summary Of Estimated Amortizable Amounts From Regulatory Assets and Accumulated OCI Into Net Periodic Benefit Cost) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016USD ($)	[1]
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	$ (1)
Net actuarial (gain) loss	50
Net actuarial loss	4
Net periodic benefit cost	53
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	(5)
Net actuarial (gain) loss	(7)
Net actuarial loss	0
Net periodic benefit cost	$ (12)
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries.

Retirement Benefits (Summary 90

Retirement Benefits (Summary Of Benefit Plan Costs Incurred) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		$ 44	$ 92	$ 81
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		(19)	4	(8)
Union Electric Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	[1]	26	54	50
Union Electric Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	[1]	(5)	8	3
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		22	38	30
Ameren Illinois Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		(13)	(3)	(9)
Other | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		(4)	0	1
Other | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		(1)	(1)	(2)
Parent Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		44	92	81
Parent Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost		$ (19)	$ 4	$ (8)
[1]	(a)Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates.

Retirement Benefits (Schedule O

Retirement Benefits (Schedule Of Expected Payments From Qualified Trust And Company Funds) (Details) $ in Millions	Dec. 31, 2016USD ($)
Pension Benefits | Paid From Qualified Trust
Defined Benefit Plan Disclosure [Line Items]
2,017	$ 248
2,018	254
2,019	261
2,020	265
2,021	273
2022 - 2026	1,405
Pension Benefits | Paid From Company Funds
Defined Benefit Plan Disclosure [Line Items]
2,017	3
2,018	3
2,019	3
2,020	3
2,021	3
2022 - 2026	13
Postretirement Benefits | Paid From Qualified Trust
Defined Benefit Plan Disclosure [Line Items]
2,017	54
2,018	57
2,019	59
2,020	61
2,021	63
2022 - 2026	331
Postretirement Benefits | Paid From Company Funds
Defined Benefit Plan Disclosure [Line Items]
2,017	2
2,018	2
2,019	2
2,020	2
2,021	2
2022 - 2026	$ 12

Retirement Benefits (Assumpti92

Retirement Benefits (Assumptions Used To Determine Net Periodic Benefit Cost) (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate at measurement date	4.50%	4.00%	4.75%
Expected return on plan assets	7.00%	7.25%	7.25%
Increase in future compensation	3.50%	3.50%	3.50%
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate at measurement date	4.50%	4.00%	4.75%
Expected return on plan assets	7.00%	7.00%	7.00%
Increase in future compensation	3.50%	3.50%	3.50%
Medical cost trend rate (initial)	5.00%	5.00%	5.00%
Medical cost trend rate (ultimate)	5.00%	5.00%	5.00%

Retirement Benefits (Schedule93

Retirement Benefits (Schedule Of Potential Changes In Key Assumptions) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016USD ($)
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service Cost and Interest Cost, .25% decrease in discount rate	$ (1)
Benefit Obligation, .25% decrease in discount rate	142
Service Cost and Interest Cost, .25% increase in salary rate	2
Benefit Obligation, .25% increase in salary rate	16
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service Cost and Interest Cost, .25% decrease in discount rate	0
Benefit Obligation, .25% decrease in discount rate	38
Service Cost and Interest Cost, 1.00% increase in annual medical trend	3
Benefit Obligation, 1.00% increase in annual medical trend	54
Service Cost and Interest Cost, 1.00% decrease in annual medical trend	(3)
Benefit Obligation, 1.00% decrease in annual medical trend	$ (54)

Retirement Benefits (Schedule94

Retirement Benefits (Schedule Of Matching Contributions) (Details) - 401 (K) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 29	$ 29	$ 28
Union Electric Company
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	16	16	16
Ameren Illinois Company
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	12	12	11
Other
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 1	$ 1	$ 1

Stock-Based Compensation (Narra

Stock-Based Compensation (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2016		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized	8,000,000
Maximum shares available for grants	5,800,000
Performance Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Performance period	3 years
Percentage of shares issued per share unit, minimum	0.00%
Percentage of shares issued per share unit, maximum	200.00%
Amounts paid to settle share units	$ 83		$ 27	$ 33
Compensation cost not yet recognized	$ 25
Expected weighted average recognition period for share-based compensation expense, in months	22 months
Fair value of each share unit, per share	$ 44.13	[1]	$ 52.88	$ 38.90
Closing common share price			$ 43.23	$ 46.13	$ 36.16
Three-year risk-free rate	1.31%		1.10%	0.78%
Volatility rate, minimum	15.00%		12.00%	12.00%
Volatility rate, maximum	20.00%		18.00%	18.00%
[1]	Includes performance share units (share units) granted to certain executive and nonexecutive officers and other eligible employees in 2016 under the 2014 Incentive Plan.

Stock-Based Compensation (Summa

Stock-Based Compensation (Summary Of Nonvested Shares) (Details) - Performance Share Units - $ / shares		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Share Units, Nonvested at beginning of year		1,024,870
Share Units, Granted	[1]	588,615
Share Units, Unearned or forfeited		(15,949)
Share Units, Earned and vested	[2]	(537,897)
Share Units, Nonvested at end of year		1,059,639		1,024,870
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Weighted-average Fair Value per Unit, Nonvested at beginning of year		$ 46.08
Weighted-averageFair Value per Unit, granted		44.13	[1]	$ 52.88	$ 38.90
Weighted-average Fair Value per Unit, Unearned or forfeited		45.07
Weighted-average Fair Value per Unit, Earned and vested	[2]	40.12
Weighted-average Fair Value per Unit, Nonvested at end of year		$ 48.04		$ 46.08
[1]	Includes performance share units (share units) granted to certain executive and nonexecutive officers and other eligible employees in 2016 under the 2014 Incentive Plan.
[2]	Includes share units granted in 2014 that vested as of December 31, 2016 and were earned pursuant to the terms of the award grants. Also includes share units that vested due to 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.

Stock-Based Compensation Stock-

Stock-Based Compensation Stock-Based Compensation (Summary of Expense) (Details) - Performance Share Units - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense		$ 17	$ 19	$ 19
Employee service share-based compensation, tax benefit from compensation expense		6	7	7
Share-based Compensation Expense, Net of Tax		11	12	12
Ameren Missouri [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense		4	5	5
Ameren Illinois Company
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense		2	3	2
Other
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	[1]	$ 11	$ 11	$ 12
[1]	(a)Represents compensation expense of employees of Ameren Services. These amounts are not included in the Ameren Missouri and Ameren Illinois amounts above.

Income Taxes (Schedule Of Effec

Income Taxes (Schedule Of Effective Income Tax Rate Reconciliation) (Details)		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Taxes [Line Items]
Statutory federal income tax rate:		35.00%	35.00%	35.00%
Depreciation differences		0.00%	(1.00%)
Amortization of deferred investment tax credit		(0.00%)	(1.00%)	(1.00%)
State tax		4.00%	5.00%	4.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Percent	[1]	(2.00%)
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent		1.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent		(1.00%)		1.00%
Effective income tax rate		37.00%	38.00%	39.00%
Union Electric Company
Income Taxes [Line Items]
Statutory federal income tax rate:		35.00%	35.00%	35.00%
Depreciation differences		1.00%	0.00%
Amortization of deferred investment tax credit		(1.00%)	(1.00%)	(1.00%)
State tax		3.00%	3.00%	3.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Percent	[1]	0.00%
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent		0.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent		0.00%	0.00%	0.00%
Effective income tax rate		38.00%	37.00%	37.00%
Ameren Illinois Company
Income Taxes [Line Items]
Statutory federal income tax rate:		35.00%	35.00%	35.00%
Depreciation differences		0.00%	(2.00%)
Amortization of deferred investment tax credit		(0.00%)	(0.00%)	(0.00%)
State tax		5.00%	5.00%	6.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Compensation Cost, Percent	[1]	0.00%
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent		0.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent		(2.00%)	(1.00%)
Effective income tax rate		38.00%	37.00%	41.00%
[1]	(a)Reflects the adoption of new authoritative accounting guidance related to share-based compensation. See Note 1 – Summary of Significant Accounting Policies for more information.

Income Taxes (Schedule Of Compo

Income Taxes (Schedule Of Components Of Income Tax Expense (Benefit)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Taxes [Line Items]
Current Federal taxes	$ (1)	$ (2)	$ (37)
Current State taxes	(3)	(4)	(37)
Deferred Federal taxes	299	299	369
Deferred State taxes	92	76	88
Deferred investment tax credits, amortization	(5)	(6)	(6)
Total income tax expense	382	363	377
Union Electric Company
Income Taxes [Line Items]
Current Federal taxes	31	110	(13)
Current State taxes	6	17	(3)
Deferred Federal taxes	161	71	222
Deferred State taxes	23	16	28
Deferred investment tax credits, amortization	(5)	(5)	(5)
Total income tax expense	216	209	229
Ameren Illinois Company
Income Taxes [Line Items]
Current Federal taxes	(8)	(83)	(51)
Current State taxes	12	(11)	(2)
Deferred Federal taxes	117	193	159
Deferred State taxes	37	29	38
Deferred investment tax credits, amortization	0	(1)	(1)
Total income tax expense	158	127	143
Other
Income Taxes [Line Items]
Current Federal taxes	(24)	(29)	27
Current State taxes	(21)	(10)	(32)
Deferred Federal taxes	21	35	(12)
Deferred State taxes	32	31	22
Deferred investment tax credits, amortization	0	0	0
Total income tax expense	$ 8	$ 27	$ 5

Income Taxes (Schedule Of Defer

Income Taxes (Schedule Of Deferred Tax Assets And Liabilities Resulting From Temporary Differences) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Plant related	$ 5,019	$ 4,555
Regulatory assets, net	74	80
Deferred benefit costs	(211)	(207)
Revenue Requirement Reconciliation Adjustment	34	66
Tax carryforwards	(676)	(603)
Other	24
Other		(6)
Total net accumulated deferred income tax liabilities	4,264	3,885
Union Electric Company
Income Taxes [Line Items]
Plant related	3,103	2,931
Regulatory assets, net	75	81
Deferred benefit costs	(76)	(76)
Revenue Requirement Reconciliation Adjustment	0	0
Tax carryforwards	(66)	(65)
Other	(23)	(27)
Total net accumulated deferred income tax liabilities	3,013	2,844
Ameren Illinois Company
Income Taxes [Line Items]
Plant related	1,769	1,587
Regulatory assets, net	(1)	(1)
Deferred benefit costs	(38)	(40)
Revenue Requirement Reconciliation Adjustment	34	66
Tax carryforwards	(138)	(133)
Other	5	1
Total net accumulated deferred income tax liabilities	1,631	1,480
Other
Income Taxes [Line Items]
Plant related	147	37
Regulatory assets, net	0	0
Deferred benefit costs	(97)	(91)
Revenue Requirement Reconciliation Adjustment	0	0
Tax carryforwards	(472)	(405)
Other	42	20
Total net accumulated deferred income tax liabilities	$ (380)	$ (439)

Income Taxes (Schedule Of Net O

Income Taxes (Schedule Of Net Operating Loss Carryforwards And Tax Credit Carryforwards) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016		Dec. 31, 2015
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 539		$ 453
Tax credit carryforwards	130		144
Deferred Tax Assets, Charitable Contribution Carryforwards	18	[1]	10
Deferred Tax Assets, Valuation Allowance, Noncurrent	(11)	[2]	(4)
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	7		6
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	494	[3]	407
Tax credit carryforwards	109	[3]	105
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	45	[3]	46
Tax credit carryforwards	21	[1]	41
Tax Credit Carryforward, Valuation Allowance			(2)
Union Electric Company
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	37		39
Tax credit carryforwards	29		26
Deferred Tax Assets, Charitable Contribution Carryforwards	0	[1]	0
Deferred Tax Assets, Valuation Allowance, Noncurrent	0	[2]	0
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	0		0
Union Electric Company | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	33	[3]	35
Tax credit carryforwards	29	[3]	26
Union Electric Company | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	4	[3]	4
Tax credit carryforwards	0	[1]	0
Tax Credit Carryforward, Valuation Allowance			0
Ameren Illinois Company
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	137		131
Tax credit carryforwards	1		2
Deferred Tax Assets, Charitable Contribution Carryforwards	0	[1]	0
Deferred Tax Assets, Valuation Allowance, Noncurrent	0	[2]	0
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	0		0
Ameren Illinois Company | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	137	[3]	127
Tax credit carryforwards	1	[3]	1
Ameren Illinois Company | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	0	[3]	4
Tax credit carryforwards	0	[1]	1
Tax Credit Carryforward, Valuation Allowance			0
Other
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	365		283
Tax credit carryforwards	100		116
Deferred Tax Assets, Charitable Contribution Carryforwards	18	[1]	10
Deferred Tax Assets, Valuation Allowance, Noncurrent	(11)	[2]	(4)
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	7		6
Other | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	324	[3]	245
Tax credit carryforwards	79	[3]	78
Other | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	41	[3]	38
Tax credit carryforwards	$ 21	[1]	40
Tax Credit Carryforward, Valuation Allowance			$ (2)
Minimum | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward, expiration period start	Jan. 1, 2029
Tax credit carryforward, expiration period start	Jan. 1, 2029
Minimum | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward, expiration period start	Jan. 1, 2029
Tax credit carryforward, expiration period start	Jan. 1, 2017
Maximum | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward, expiration period start	Jan. 1, 2036
Tax credit carryforward, expiration period start	Jan. 1, 2036
Maximum | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward, expiration period start	Jan. 1, 2036
Tax credit carryforward, expiration period start	Jan. 1, 2021
[1]	Will expire between 2017
[2]	See Schedule II under Part IV, Item 15, in this report for information on changes in the valuation allowance.
[3]	Will expire between 2029 and 2036.

Income Taxes (Narrative) (Detai

Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Taxes [Line Items]
State corporate income tax rate	4.00%	5.00%	4.00%
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities			$ 18
Union Electric Company
Income Taxes [Line Items]
State corporate income tax rate	3.00%	3.00%	3.00%
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities			$ 18
State
Income Taxes [Line Items]
State corporate income tax rate		7.75%
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities		$ 53	$ 2
State | Minimum
Income Taxes [Line Items]
State corporate income tax rate	7.30%
State | Maximum
Income Taxes [Line Items]
State corporate income tax rate			9.50%
2007 through 2011 tax year [Member]
Income Taxes [Line Items]
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities			$ 20
2007 through 2011 tax year [Member] | Union Electric Company
Income Taxes [Line Items]
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities			$ 13

Related Party Transactions (Det

Related Party Transactions (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015		Dec. 31, 2014
Union Electric Company | Ameren Missouri Power Supply Agreements with Ameren Illinois
Related Party Transaction [Line Items]
Operating Revenues		$ 28	$ 15		$ 5
Union Electric Company | Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues		25	25		21
Union Electric Company | Ameren Missouri and Ameren Illinois miscellaneous support services
Related Party Transaction [Line Items]
Operating Revenues		1	2		1
Union Electric Company | Total Related Party Operating Revenues [Member]
Related Party Transaction [Line Items]
Operating Revenues		54	42		27
Union Electric Company | Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses		129	131		124
Union Electric Company | Money Pool
Related Party Transaction [Line Items]
Interest (Charges) Income	[1]	1	1		1
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues		0	0		0
Ameren Illinois Company | Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues		5	4		2
Ameren Illinois Company | Ameren Missouri and Ameren Illinois miscellaneous support services
Related Party Transaction [Line Items]
Operating Revenues		1	1	[1]	1	[1]
Ameren Illinois Company | Total Related Party Operating Revenues [Member]
Related Party Transaction [Line Items]
Operating Revenues		5	4		2
Ameren Illinois Company | Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Operating Expenses		28	15		5
Ameren Illinois Company | Ameren Illinois transmission agreements with ATXI
Related Party Transaction [Line Items]
Operating Expenses		2	2		2
Ameren Illinois Company | Purchased Power
Related Party Transaction [Line Items]
Operating Expenses		30	17		7
Ameren Illinois Company | Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses		123	119		109
Ameren Illinois Company | Money Pool
Related Party Transaction [Line Items]
Interest (Charges) Income	[1]	$ 1	$ 1		$ 1
[1]	Amount less than $1 million.

Related Party Transactions (Nar

Related Party Transactions (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016USD ($)MWh$ / MWh
May 31, 2014 | Union Electric Company
Related Party Transaction [Line Items]
Energy Supply Agreements Amount | $	$ 1
May 31, 2015 | Union Electric Company
Related Party Transaction [Line Items]
Energy Supply Agreements Amount | $	3
May 31, 2017 | Union Electric Company
Related Party Transaction [Line Items]
Energy Supply Agreements Amount | $	$ 15
May 2014 Procurement [Member] | Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power | MWh	168,400,000,000
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate | $ / MWh	51,000,000
April 2015 Procurement [Member] | Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power | MWh	667,000,000,000
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate | $ / MWh	36,000,000
September 2015 Procurement [Member] | Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power | MWh	339,000,000,000
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate | $ / MWh	38,000,000
April 2016 Procurement [Member] | Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power | MWh	375,200,000,000
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate | $ / MWh	35,000,000
September 2016 Procurement [Member] | Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power | MWh	82,800,000,000
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate | $ / MWh	34,000,000

Commitments And Contingencie105

Commitments And Contingencies (Callaway Nuclear Energy Center) (Details)	12 Months Ended
	Dec. 31, 2016USD ($)
Commitments and Contingencies [Line Items]
Insurance aggregate maximum coverage	$ 13,361,000,000	[1]
Insurance maximum coverage per incident	127,000,000
Threshold Amount For Retrospective Insurance Assessment For Covered Loss Under Public Liability And Nuclear Worker Liability Insurance Policy	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 to be paid in a calendar year per reactor incident under liability provisions of Atomic Energy Act	19,000,000
Amount of coverage in excess of primary property liability coverage	2,300,000,000
Amount of weekly indemnity coverage commencing eight weeks after power outage	$ 4,500,000
Number of weeks of coverage after the first eight weeks of an outage	1 year
Amount of additional weekly indemnity coverage commencing after initial indemnity coverage	$ 3,600,000
Number of additional weeks after initial indemnity coverage for power outage, minimum	1 year 4 months 10 days
Amount of weekly indemnity coverage thereafter not exceeding policy limit	$ 490,000,000
Number Of Years The Limit Of Liability And The Maximum Potential Annual Payments Are Adjusted	5 years
Aggregate nuclear power industry insurance policy limit for losses from terrorist attacks within twelve month period	$ 3,240,000,000
Public Liability And Nuclear Worker Liability - American Nuclear Insurers
Commitments and Contingencies [Line Items]
Insurance aggregate maximum coverage	375,000,000	[2]
Insurance maximum coverage per incident	0
Public Liability And Nuclear Worker Liability - Pool Participation
Commitments and Contingencies [Line Items]
Insurance aggregate maximum coverage	12,986,000,000	[3]
Insurance maximum coverage per incident	127,000,000	[4]
Property Damage - Nuclear Electric Insurance Ltd
Commitments and Contingencies [Line Items]
Insurance aggregate maximum coverage	2,710,000,000	[5]
Insurance maximum coverage per incident	30,000,000	[6]
Replacement Power - Nuclear Electric Insurance Ltd
Commitments and Contingencies [Line Items]
Insurance aggregate maximum coverage	490,000,000	[7]
Insurance maximum coverage per incident	7,000,000	[6]
Sub-limit of Amount of Weekly indemnity Coverage Thereafter Not Exceeding Policy Limit for Non-Nuclear Events	328,000,000
Property Damage European Mutual Association for Nuclear Insurance
Commitments and Contingencies [Line Items]
Insurance aggregate maximum coverage	450,000,000	[7]
Amount of primary property liability coverage	450,000,000
Property Damage
Commitments and Contingencies [Line Items]
Insurance aggregate maximum coverage	3,160,000,000
Insurance maximum coverage per incident	30,000,000
Radiation Event
Commitments and Contingencies [Line Items]
Insurance aggregate maximum coverage	2,710,000,000
Non-radiation event
Commitments and Contingencies [Line Items]
Insurance aggregate maximum coverage	2,300,000,000
Aggregate nuclear power industry insurance policy limit for losses from terrorist attacks within twelve month period	$ 1,830,000,000
[1]	Limit of liability for each incident under the Price-Anderson liability provisions of the Atomic Energy Act of 1954, as amended. A company could be assessed up to $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]	Effective January 1, 2017, limit was increased to $450 million
[3]	Provided through mandatory participation in an industrywide retrospective premium assessment program.
[4]	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.
[5]	NEIL provides $2.71 billion in property damage, decontamination, and premature decommissioning insurance for radiation events. NEIL provides $2.3 billion in property damage for nonradiation events.
[6]	All NEIL-insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL
[7]	European Mutual Association for Nuclear Insurance provides $450 million in excess of the $2.71 billion and $2.3 billion property coverage for radiation and nonradiation events, respectively, provided by NEIL.

Commitments And Contingencie106

Commitments And Contingencies (Schedule Of Lease Obligations) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Commitments and Contingencies [Line Items]
Capital lease payments, due in one year	[1],[2]	$ 33
Capital lease payments, due in two years	[1],[2]	32
Capital lease payments, due in three years	[1],[2]	32
Capital lease payments, due in four years	[1],[2]	32
Capital lease payments, due in five years	[1],[2]	32
Capital lease payments, After 5 Years	[1],[2]	297
Capital lease payments, Total	[1],[2]	458
Less Amount representing interest, due in one year	[2]	27
Less Amount representing interest, due in two years	[2]	26
Less Amount representing interest, due in three years	[2]	25
Less Amount representing interest, due in four years	[2]	25
Less Amount representing interest, due in 5 years	[2]	25
Less Amount representing interest, After 5 Years	[2]	48
Less Amount representing interest, Total	[2]	176
Present value of minimum capital lease payments, due in one year	[2]	6
Present value of minimum capital lease payments, due in two years	[2]	6
Present value of minimum capital lease payments, due in three years	[2]	7
Present value of minimum capital lease payments, due in four years	[2]	7
Present value of minimum capital lease payments, due in five years	[2]	7
Present value of minimum capital lease payments, After 5 Years	[2]	249
Present value of minimum capital lease payments, Total	[2]	282
Operating leases, due in one year	[2],[3]	13
Operating leases, due in two years	[2],[3]	12
Operating leases, due in three years	[2],[3]	12
Operating leases, due in four years	[2],[3]	11
Operating leases, due in five years	[2],[3]	10
Operating leases, After 5 Years	[2],[3]	23
Operating leases, Total	[2],[3]	81
Total lease obligations, due in one year	[2]	19
Total lease obligations, due in two years	[2]	18
Total lease obligations, due in three years	[2]	19
Total lease obligations, due in four years	[2]	18
Total lease obligations, due in five years	[2]	17
Total lease obligations, After 5 Years	[2]	272
Total lease obligations, Total	[2]	363
Annual obligation for real estate leases and railroad licenses		3
Total rental expense	[4]	38	$ 36	$ 37
Union Electric Company
Commitments and Contingencies [Line Items]
Capital lease payments, due in one year	[1]	33
Capital lease payments, due in two years	[1]	32
Capital lease payments, due in three years	[1]	32
Capital lease payments, due in four years	[1]	32
Capital lease payments, due in five years	[1]	32
Capital lease payments, After 5 Years	[1]	297
Capital lease payments, Total	[1]	458
Less Amount representing interest, due in one year		27
Less Amount representing interest, due in two years		26
Less Amount representing interest, due in three years		25
Less Amount representing interest, due in four years		25
Less Amount representing interest, due in 5 years		25
Less Amount representing interest, After 5 Years		48
Less Amount representing interest, Total		176
Present value of minimum capital lease payments, due in one year		6
Present value of minimum capital lease payments, due in two years		6
Present value of minimum capital lease payments, due in three years		7
Present value of minimum capital lease payments, due in four years		7
Present value of minimum capital lease payments, due in five years		7
Present value of minimum capital lease payments, After 5 Years		249
Present value of minimum capital lease payments, Total		282
Operating leases, due in one year	[3]	11
Operating leases, due in two years	[3]	11
Operating leases, due in three years	[3]	11
Operating leases, due in four years	[3]	10
Operating leases, due in five years	[3]	9
Operating leases, After 5 Years	[3]	21
Operating leases, Total	[3]	73
Total lease obligations, due in one year		17
Total lease obligations, due in two years		17
Total lease obligations, due in three years		18
Total lease obligations, due in four years		17
Total lease obligations, due in five years		16
Total lease obligations, After 5 Years		270
Total lease obligations, Total		355
Annual obligation for real estate leases and railroad licenses		2
Total rental expense		34	34	32
Ameren Illinois Company
Commitments and Contingencies [Line Items]
Operating leases, due in one year	[3]	1
Operating leases, due in two years	[3]	1
Operating leases, due in three years	[3]	1
Operating leases, due in four years	[3]	1
Operating leases, due in five years	[3]	1
Operating leases, After 5 Years	[3]	1
Operating leases, Total	[3]	6
Annual obligation for real estate leases and railroad licenses		1
Total rental expense		$ 30	$ 28	$ 25
[1]	See Properties under Part I, Item 2, and Note 3 – Property, Plant, and Equipment, Net, of this report for additional information.
[2]	ncludes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.
[3]	Amounts related to certain land-related leases have indefinite payment periods. The annual obligations of $3 million, $2 million, and $1 million for Ameren, Ameren Missouri, and Ameren Illinois for these items are included in the 2017 through 2021 columns, respectively.
[4]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Commitments And Contingencie107

Commitments And Contingencies (Schedule Of Estimated Purchased Power Commitments) (Details) $ in Millions	Dec. 31, 2016USD ($)
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	$ 1,258	[1]
2,018	828	[1]
2,019	580	[1]
2,020	229	[1]
2,021	148	[1]
Thereafter	829	[1]
Total	3,872	[1]
Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	599	[1]
2,018	371	[1]
2,019	311	[1]
2,020	27	[1]
2,021	0	[1]
Thereafter	0	[1]
Total	1,308	[1]
Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	238	[1],[2]
2,018	167	[1],[2]
2,019	99	[1],[2]
2,020	45	[1],[2]
2,021	12	[1],[2]
Thereafter	43	[1],[2]
Total	604	[1],[2]
Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	45	[1]
2,018	70	[1]
2,019	27	[1]
2,020	38	[1]
2,021	44	[1]
Thereafter	45	[1]
Total	269	[1]
Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	255	[1],[3]
2,018	156	[1],[3]
2,019	79	[1],[3]
2,020	58	[1],[3]
2,021	58	[1],[3]
Thereafter	478	[1],[3]
Total	1,084	[1],[3]
Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	3	[1]
2,018	4	[1]
2,019	4	[1]
2,020	5	[1]
2,021	5	[1]
Thereafter	65	[1]
Total	86	[1]
Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	118	[1]
2,018	60	[1]
2,019	60	[1]
2,020	56	[1]
2,021	29	[1]
Thereafter	198	[1]
Total	521	[1]
Union Electric Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	751
2,018	525
2,019	408
2,020	131
2,021	104
Thereafter	370
Total	2,289
Union Electric Company | Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	599
2,018	371
2,019	311
2,020	27
2,021	0
Thereafter	0
Total	1,308
Union Electric Company | Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	43	[2]
2,018	29	[2]
2,019	15	[2]
2,020	10	[2]
2,021	5	[2]
Thereafter	18	[2]
Total	120	[2]
Union Electric Company | Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	45
2,018	70
2,019	27
2,020	38
2,021	44
Thereafter	45
Total	269
Union Electric Company | Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	22
2,018	22
2,019	22
2,020	22
2,021	22
Thereafter	59
Total	169
Union Electric Company | Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	3
2,018	4
2,019	4
2,020	5
2,021	5
Thereafter	65
Total	86
Union Electric Company | Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	39
2,018	29
2,019	29
2,020	29
2,021	28
Thereafter	183
Total	337
Ameren Illinois Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	464
2,018	296
2,019	167
2,020	98
2,021	44
Thereafter	444
Total	1,513
Ameren Illinois Company | Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	195	[2]
2,018	138	[2]
2,019	83	[2]
2,020	35	[2]
2,021	8	[2]
Thereafter	25	[2]
Total	484	[2]
Ameren Illinois Company | Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	233	[3]
2,018	134	[3]
2,019	57	[3]
2,020	36	[3]
2,021	36	[3]
Thereafter	419	[3]
Total	915	[3]
Ameren Illinois Company | Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	36
2,018	24
2,019	27
2,020	27
2,021	0
Thereafter	0
Total	$ 114
[1]	(c)Includes amounts for Ameren registrant and nonregistrant subsidiaries.
[2]	(a)Includes amounts for generation and for distribution.
[3]	(b)The purchased power amounts for Ameren and Ameren Illinois include agreements through 2032 for renewable energy credits with various renewable energy suppliers. The agreements contain a provision that allows Ameren Illinois to reduce the quantity purchased in the event that Ameren Illinois would not be able to recover the costs associated with the renewable energy credits.

Commitments And Contingencie108

Commitments And Contingencies (Environmental Matters) (Details) $ in Millions	Dec. 31, 2016USD ($)site	Sep. 30, 2016scrubber
Percentage of Rate Base Related to Carbon Dioxide Energy Centers	18.00%
Union Electric Company
Percentage of Rate Base Related to Carbon Dioxide Energy Centers	34.00%
Number of Energy Center Scrubbers | scrubber		2
Manufactured Gas Plant
Accrual for environmental loss contingencies	$ 200
Manufactured Gas Plant | Ameren Illinois Company
Number of remediation sites | site	44
Accrual for environmental loss contingencies	$ 200
Sauget Area 2 | Union Electric Company
Accrual for environmental loss contingencies	1
Minimum
Estimated capital costs to comply with existing and known federal and state air emissions regulations	425
Minimum | Manufactured Gas Plant | Ameren Illinois Company
Loss contingency, estimate of possible loss	200
Minimum | Sauget Area 2 | Union Electric Company
Loss contingency, estimate of possible loss	1
Maximum
Estimated capital costs to comply with existing and known federal and state air emissions regulations	525
Maximum | Manufactured Gas Plant | Ameren Illinois Company
Loss contingency, estimate of possible loss	268
Maximum | Sauget Area 2 | Union Electric Company
Loss contingency, estimate of possible loss	$ 2.5

Segment Information (Details)

Segment Information (Details) $ in Millions		3 Months Ended								9 Months Ended	12 Months Ended
		Dec. 31, 2016USD ($)	Sep. 30, 2016USD ($)	Jun. 30, 2016USD ($)	Mar. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Sep. 30, 2015USD ($)	Jun. 30, 2015USD ($)	Mar. 31, 2015USD ($)	Sep. 30, 2016segment	Dec. 31, 2016USD ($)segment	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment										2	4
External revenues		$ 1,356	$ 1,859	$ 1,427	$ 1,434	$ 1,308	$ 1,833	$ 1,401	$ 1,556		$ 6,076	$ 6,098	$ 6,053
Depreciation and amortization											845	796	745
Interest charges											382	355	341
Income taxes (benefit)											382	363	377
Net income (loss) attributable to Ameren common shareholders from continuing operations		32	369	147	105	30	343	98	108		653	579	587
Capital expenditures											$ 2,076	1,917	1,785
Union Electric Company
Segment Reporting Information [Line Items]
Number of reportable segments | segment											1
Segment, Continuing Operations
Segment Reporting Information [Line Items]
External revenues											$ 6,076	6,098	6,053
Intersegment revenues											0	0	0
Depreciation and amortization											845	796	745
Interest income											40	41	37
Interest charges											382	355	341
Income taxes (benefit)											382	363	377
Net income (loss) attributable to Ameren common shareholders from continuing operations											653	579	587
Capital expenditures											$ 2,076	1,917	1,785
Ameren Illinois Company
Segment Reporting Information [Line Items]
Number of reportable segments | segment											3
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues		595	676	542	677	553	655	513	745		$ 2,490	2,466	2,498
Intersegment revenues											0	0	0
Depreciation and amortization											319	295	263
Interest income											12	12	7
Interest charges											140	131	112
Income taxes (benefit)											158	127	143
Net Income (Loss) Available to Common Shareholder		$ 29	$ 119	$ 45	$ 59	$ 32	$ 98	$ 31	$ 53		252	214	201
Capital expenditures											924	918	835
Intersegment Elimination
Segment Reporting Information [Line Items]
External revenues											0	0	0
Intersegment revenues											(105)	(87)	(66)
Depreciation and amortization											0	0	0
Interest income											(11)	(6)	(3)
Interest charges											(11)	(6)	(3)
Income taxes (benefit)											0	0	0
Net Income (Loss) Available to Common Shareholder											0	0	0
Capital expenditures											(6)	0	0
Intersegment Elimination | Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues											0	0	0
Intersegment revenues											(45)	(38)	(35)
Depreciation and amortization											0	0	0
Interest income											0	0	0
Interest charges											0	0	0
Income taxes (benefit)											0	0	0
Net Income (Loss) Available to Common Shareholder											0	0	0
Capital expenditures											0	0	0
Operating Segments [Member] | Union Electric Company
Segment Reporting Information [Line Items]
External revenues											3,469	3,566	3,526
Intersegment revenues											54	43	27
Depreciation and amortization											514	492	473
Interest income											28	28	28
Interest charges											211	219	211
Income taxes (benefit)											216	209	229
Net Income (Loss) Available to Common Shareholder											357	352	390
Capital expenditures											738	622	747
Operating Segments [Member] | Ameren Illinois Electric Distribution [Member]
Segment Reporting Information [Line Items]
External revenues											1,545	1,529	1,401
Intersegment revenues											4	3	2
Depreciation and amortization											226	212	197
Interest income											11	12	7
Interest charges											72	71	63
Income taxes (benefit)											78	71	75
Net Income (Loss) Available to Common Shareholder											126	123	113
Capital expenditures											470	491	403
Operating Segments [Member] | Ameren Illinois Gas [Member]
Segment Reporting Information [Line Items]
External revenues											753	782	976
Intersegment revenues											1	1	0
Depreciation and amortization											55	52	41
Interest income											0	0	0
Interest charges											34	35	28
Income taxes (benefit)											39	24	39
Net Income (Loss) Available to Common Shareholder											59	37	50
Capital expenditures											181	133	137
Operating Segments [Member] | Ameren Transmission [Member]
Segment Reporting Information [Line Items]
External revenues											309	219	150
Intersegment revenues	[1]										46	40	37
Depreciation and amortization											43	33	26
Interest income											1	0	0
Interest charges											58	35	26
Income taxes (benefit)											74	51	38
Net Income (Loss) Available to Common Shareholder											117	83	51
Capital expenditures											689	669	491
Operating Segments [Member] | Other Segment
Segment Reporting Information [Line Items]
External revenues											0	2	0
Intersegment revenues											0	0	0
Depreciation and amortization											7	7	8
Interest income											11	7	5
Interest charges											18	1	16
Income taxes (benefit)											(25)	8	(4)
Net Income (Loss) Available to Common Shareholder											(6)	(16)	(17)
Capital expenditures	[2]										4	2	7
Operating Segments [Member] | Ameren Illinois Company | Ameren Illinois Electric Distribution [Member]
Segment Reporting Information [Line Items]
External revenues											1,549	1,532	1,403
Intersegment revenues											0	0	0
Depreciation and amortization											226	212	197
Interest income											11	12	7
Interest charges											72	71	63
Income taxes (benefit)											78	71	75
Net Income (Loss) Available to Common Shareholder											126	123	113
Capital expenditures											470	491	403
Operating Segments [Member] | Ameren Illinois Company | Ameren Illinois Gas [Member]
Segment Reporting Information [Line Items]
External revenues											754	783	976
Intersegment revenues											0	0	0
Depreciation and amortization											55	52	41
Interest income											0	0	0
Interest charges											34	35	28
Income taxes (benefit)											39	24	39
Net Income (Loss) Available to Common Shareholder											59	37	50
Capital expenditures											181	133	137
Operating Segments [Member] | Ameren Illinois Company | Ameren Illinois Transmission [Member]
Segment Reporting Information [Line Items]
External revenues											187	151	119
Intersegment revenues	[3]										45	38	35
Depreciation and amortization											38	31	25
Interest income											1	0	0
Interest charges											34	25	21
Income taxes (benefit)											41	32	29
Net Income (Loss) Available to Common Shareholder											67	54	38
Capital expenditures											$ 273	$ 294	$ 295
[1]	(a)Ameren Illinois Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above.
[2]	b)Includes the elimination of intercompany transfers.
[3]	(a)Ameren Illinois Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above.

Selected Quarterly Informati110

Selected Quarterly Information (Summary Of Selected Quarterly Information) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Selected Quarterly Financial Information [Line Items]
Operating revenues	$ 1,356	$ 1,859	$ 1,427	$ 1,434	$ 1,308	$ 1,833	$ 1,401	$ 1,556	$ 6,076	$ 6,098	$ 6,053
Operating Income	145	691	325	220	140	626	237	256	1,381	1,259	1,254
Net income	33	371	148	107	30	345	151	110	659	636	592
Net income attributable to Ameren common shareholders - continuing operations	32	369	147	105	30	343	98	108	653	579	587
Net Income (Loss) Attributable to Ameren common shareholders- Discontinued Operations	0	0	0	0	(1)	0	52	0	0	51	(1)
Net income attributable to Ameren common shareholders	$ 32	$ 369	$ 147	$ 105	$ 29	$ 343	$ 150	$ 108	$ 653	$ 630	$ 586
Earnings per common share - basic - continuing operations	$ 0.13	$ 1.52	$ 0.61	$ 0.43	$ 0.12	$ 1.42	$ 0.40	$ 0.45	$ 2.69	$ 2.39	$ 2.42
Earnings per common share - basic - discontinued operations	0	0	0	0	0	0	0.21	0	0	0.21	0
Earnings per common share - basic	0.13	1.52	0.61	0.43	0.12	1.42	0.61	0.45	2.69	2.60	2.42
Earnings per common share - diluted - continuing operations	0.13	1.52	0.61	0.43	0.12	1.41	0.40	0.45	2.68	2.38	2.40
Earnings per common share - diluted - discontinued operations	0	0	0	0	0	0	0.21	0	0	0.21	0
Earnings per common share - diluted	$ 0.13	$ 1.52	$ 0.61	$ 0.43	$ 0.12	$ 1.41	$ 0.61	$ 0.45	$ 2.68	$ 2.59	$ 2.40
Union Electric Company
Selected Quarterly Financial Information [Line Items]
Operating revenues	$ 750	$ 1,165	$ 867	$ 741	$ 754	$ 1,171	$ 884	$ 800	$ 3,523	$ 3,609	$ 3,553
Operating Income	54	431	197	63	58	423	146	115	745	742	785
Net income	10	242	93	15	11	240	62	42	360	355	393
Net Income Available to Common Shareholder	10	241	92	14	11	239	61	41	357	352	390
Ameren Illinois Company
Selected Quarterly Financial Information [Line Items]
Operating revenues	595	676	542	677	553	655	513	745	2,490	2,466	2,498
Operating Income	74	230	107	133	74	189	83	120	544	466	450
Net income	30	119	46	60	33	98	32	54	255	217	204
Net Income Available to Common Shareholder	$ 29	$ 119	$ 45	$ 59	$ 32	$ 98	$ 31	$ 53	$ 252	$ 214	$ 201

Schedule I - Condensed Finan111

Schedule I - Condensed Financial Information Of Parent (Statement of Income) (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Operating revenues		$ 1,356	$ 1,859	$ 1,427	$ 1,434	$ 1,308	$ 1,833	$ 1,401	$ 1,556	$ 6,076	$ 6,098	$ 6,053
Operating expenses										4,695	4,839	4,799
Operating loss		145	691	325	220	140	626	237	256	1,381	1,259	1,254
Interest income from affiliates	[1],[2]									13	14	10
Other Nonoperating Income (Expense)										42	44	57
Interest charges										382	355	341
Income taxes (benefit)										382	363	377
Net income attributable to Ameren common shareholders - continuing operations		32	369	147	105	30	343	98	108	653	579	587
Net Income (Loss) Attributable to Ameren common shareholders- Discontinued Operations		0	0	0	0	(1)	0	52	0	0	51	(1)
Net income attributable to Ameren common shareholders		$ 32	$ 369	$ 147	$ 105	$ 29	$ 343	$ 150	$ 108	653	630	586
Comprehensive Income from Continuing Operations
Pension and other postretirement activity, net of income taxes (benefit)										(20)	6	(12)
Comprehensive Income from Continuing Operations Attributable to Ameren Common Shareholders										633	585	575
Comprehensive Income (Loss) from Discontinued Operations Attributable to Ameren Common shareholders										0	51	(1)
Comprehensive Income (Loss) Attributable to Ameren Common Shareholders										633	636	574
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement activity, tax (benefit)										(7)	3	(7)
Parent Company
Condensed Financial Statements, Captions [Line Items]
Operating expenses										14	14	11
Operating loss										(14)	(14)	(11)
Equity in earnings of subsidiaries										663	600	607
Interest income from affiliates										10	6	3
Other Nonoperating Income (Expense)										(5)	(5)	2
Interest charges										28	3	16
Income taxes (benefit)										(27)	5	(2)
Net income attributable to Ameren common shareholders - continuing operations										653	579	587
Net Income (Loss) Attributable to Ameren common shareholders- Discontinued Operations										0	51	(1)
Net income attributable to Ameren common shareholders										653	630	586
Comprehensive Income from Continuing Operations
Pension and other postretirement activity, net of income taxes (benefit)										(20)	6	(12)
Comprehensive Income from Continuing Operations Attributable to Ameren Common Shareholders										633	585	575
Comprehensive Income (Loss) from Discontinued Operations Attributable to Ameren Common shareholders										0	51	(1)
Comprehensive Income (Loss) Attributable to Ameren Common Shareholders										633	636	574
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement activity, tax (benefit)										$ (7)	$ 3	$ (7)
[1]	Includes Ameren Illinois' interest income on the IEIMA revenue requirement reconciliation adjustment regulatory assets.
[2]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Schedule I - Condensed Finan112

Schedule I - Condensed Financial Information Of Parent (Balance Sheet) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
ASSETS
Cash and cash equivalents	$ 9	$ 292	$ 5	$ 30
Miscellaneous accounts and notes receivable	63	98
Other current assets	98	88
Total current assets	1,593	1,917
Accumulated deferred income taxes, net	7	6
Other assets	538	575
TOTAL ASSETS	24,699	23,640
LIABILITIES AND EQUITY
Short-term debt	558	301
Other current liabilities	248	279
Total current liabilities	2,674	2,093
Long-term Debt, Net	6,595	6,880
Pension and other postretirement benefits	769	580
Other deferred credits and liabilities	477	531
Commitments and Contingencies
Retained earnings	1,568	1,331
Accumulated other comprehensive loss	(23)	(3)
Total equity	7,245	7,088	6,855
TOTAL LIABILITIES AND EQUITY	24,699	23,640
Parent Company
ASSETS
Cash and cash equivalents	1	0	$ 1	$ 11
Advances to money pool	27	0
Accounts receivable - affiliates	31	53
Miscellaneous accounts and notes receivable	26	3
Other current assets	8	9
Total current assets	93	65
Investments in subsidiaries	7,498	7,227
Note receivable - affiliates	350	290
Accumulated deferred income taxes, net	419	426
Other assets	135	158
TOTAL ASSETS	8,495	8,166
LIABILITIES AND EQUITY
Short-term debt	507	301
Borrowings from money pool	33	14
Accounts payable – affiliates	13	75
Other current liabilities	17	22
Total current liabilities	570	412
Long-term Debt, Net	694	694
Pension and other postretirement benefits	45	33
Other deferred credits and liabilities	83	81
Total liabilities	1,392	1,220
Common stock, $.01 par value, 400.0 shares authorized – shares outstanding of 242.6	2	2
Other paid-in capital, principally premium on common stock	5,556	5,616
Retained earnings	1,568	1,331
Accumulated other comprehensive loss	(23)	(3)
Total equity	7,103	6,946
TOTAL LIABILITIES AND EQUITY	$ 8,495	$ 8,166

Schedule I - Condensed Finan113

Schedule I - Condensed Financial Information Of Parent (Statement of Cash Flows) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	$ 2,123	$ 2,031	$ 1,565
Cash Flows From Investing Activities:
Proceeds from Collection of Notes Receivable	0	20	95
Contributions to Note Receivable	0	(8)	(89)
Other	5	20	11
Net Cash Provided by (Used in) Investing Activities	(2,141)	(1,976)	(1,717)
Cash flows from financing activities:
Dividends on common stock	(416)	(402)	(390)
Short-term debt, net	257	(413)	346
Repayments of Other Long-term Debt	(395)	(120)	(697)
Issuances of Long-term debt	389	1,197	898
Capital issuance costs	(9)	(12)	(11)
Share-based payments	(83)	(12)	(14)
Net cash provided by (used in) financing activities	(265)	232	127
Net change in cash and cash equivalents	(283)	287	(25)
Cash and cash equivalents at beginning of year	292	5	30
Cash and cash equivalents at end of year	9	292	5
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	483	551	528
Cash Flows From Investing Activities:
Money pool advances, net	(27)	55	279
Intercompany notes receivable, net	(60)	(96)	(134)
Investments in subsidiaries	(123)	(509)	(280)
Return of investments	0	0	215
Proceeds from Collection of Notes Receivable	0	20	95
Contributions to Note Receivable	0	(8)	(89)
Other	2	(24)	(12)
Net Cash Provided by (Used in) Investing Activities	(208)	(562)	74
Cash flows from financing activities:
Dividends on common stock	(416)	(402)	(390)
Short-term debt, net	206	(284)	217
Money pool borrowings, net	19	14	0
Repayments of Other Long-term Debt	0	0	(425)
Issuances of Long-term debt	0	700	0
Capital issuance costs	0	(6)	0
Share-based payments	(83)	(12)	(14)
Net cash provided by (used in) financing activities	(274)	10	(612)
Net change in cash and cash equivalents	1	(1)	(10)
Cash and cash equivalents at beginning of year	0	1	11
Cash and cash equivalents at end of year	1	0	1
Cash dividends received from consolidated subsidiaries	465	575	340
Noncash Investing activity- investment in subsidiaries	$ 0	$ (38)	$ (19)

Schedule I - Condensed Finan114

Schedule I - Condensed Financial Information Of Parent Outstanding gurarantee (Details) $ in Millions	Dec. 31, 2016USD ($)
Parent Company
Other Commitments [Line Items]
Guarantees Outstanding	$ 51

Schedule II - Valuation And 115

Schedule II - Valuation And Qualifying Accounts (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Allowance For Doubtful Accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		$ 19	$ 21	$ 18
Charged to Costs and Expenses		32	33	36
Charged to Other Accounts	[1]	3	5	4
Deductions	[2]	35	40	37
Balance at End of Period		19	19	21
Valuation Allowance of Deferred Tax Assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		6	10	7
Charged to Costs and Expenses		7	4	3
Charged to Other Accounts	[1]	(2)	(8)	0
Deductions		0	0	0
Balance at End of Period		11	6	10
Union Electric Company | Allowance For Doubtful Accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		7	8	5
Charged to Costs and Expenses		10	13	16
Charged to Other Accounts	[1]	0	0	0
Deductions	[2]	10	14	13
Balance at End of Period		7	7	8
Union Electric Company | Valuation Allowance of Deferred Tax Assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		0	1	1
Charged to Costs and Expenses		0	0	0
Charged to Other Accounts	[1]	0	(1)	0
Deductions		0	0	0
Balance at End of Period		0	0	1
Ameren Illinois Company | Allowance For Doubtful Accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		12	13	13
Charged to Costs and Expenses		22	20	20
Charged to Other Accounts	[1]	3	5	4
Deductions	[2]	25	26	24
Balance at End of Period		12	12	13
Ameren Illinois Company | Valuation Allowance of Deferred Tax Assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		0	1	1
Charged to Costs and Expenses		0	0	0
Charged to Other Accounts	[1]	0	(1)	0
Deductions		0	0	0
Balance at End of Period		$ 0	$ 0	$ 1
[1]	Amounts associated with the allowance for doubtful accounts relate to the uncollectible account reserve associated with receivables purchased by Ameren Illinois from alternative retail electric suppliers, as required by the Illinois Public Utilities Act. The amounts relating to the deferred tax valuation allowance are for items that have expired and were removed from both the underlying accumulated deferred income tax account as well as the offsetting valuation account.
[2]	Uncollectible accounts charged off, less recoveries.