Ameren (AEE) 10-K 28 Feb 18 2017 FY Annual report Financial data

Document And Entity Information

Document And Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Jan. 31, 2018	Jun. 30, 2017
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
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			$ 13,230,607,078
Entity Well-known Seasoned Issuer	Yes
Union Electric Company
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
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, 2017
Document Fiscal Year Focus	2,017
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	No

Consolidated Statement Of Incom

Consolidated Statement Of Income (Loss) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Revenues:
Electric	$ 5,310	$ 5,196	$ 5,180
Natural Gas	867	880	918
Total operating revenues	6,177	6,076	6,098
Operating Expenses:
Fuel	737	745	878
Purchased power	638	621	514
Natural gas purchased for resale	311	341	415
Other operations and maintenance	1,660	1,676	1,694
Provision for Callaway construction and operating license	0	0	69
Depreciation and amortization	896	845	796
Taxes other than income taxes	477	467	473
Total operating expenses	4,719	4,695	4,839
Operating Income	1,458	1,381	1,259
Other Income and Expenses:
Miscellaneous income	59	74	74
Miscellaneous expense	21	32	30
Total other income (expense)	38	42	44
Interest charges	391	382	355
Income Before Income Taxes	1,105	1,041	948
Income taxes	576	382	363
Income from Continuing Operations	529	659	585
Income from Discontinued Operations, Net of Tax	0	0	51
Net Income	529	659	636
Pension and other postretirement activity, net of income taxes (benefit)	5	(20)	6
Comprehensive Income	528	633	636
Less: Net Income Attributable to Noncontrolling Interests:
Continuing Operations	6	6	6
Net Income:
Net income attributable to Ameren Corporation - continuing operations	523	653	579
Discontinued Operations	0	0	51
Net income attributable to Ameren common shareholders	$ 523	$ 653	$ 630
Earnings per Common Share – Basic:
Continuing Operations - Basic (in dollars per share)	$ 2.16	$ 2.69	$ 2.39
Discontinued Operations - Basic (in dollars per share)	0	0	0.21
Earnings per Common Share – Basic (in dollars per share)	2.16	2.69	2.60
Earnings per Common Share – Diluted:
Continuing Operations - Diluted (in dollars per share)	2.14	2.68	2.38
Discontinued Operations - Diluted (in dollars per share)	0	0	0.21
Earnings per Common Share – Diluted (in dollars per share)	2.14	2.68	2.59
Dividends per Common Share (in dollars per share)	$ 1.778	$ 1.715	$ 1.655
Average Common Shares Outstanding - Basic (in shares)	242.6	242.6	242.6
Average Common Shares Outstanding - Diluted (in shares)	244.2	243.4	243.6
Union Electric Company
Operating Revenues:
Electric	$ 3,413	$ 3,394	$ 3,470
Natural Gas	126	128	137
Other	0	1	2
Total operating revenues	3,539	3,523	3,609
Operating Expenses:
Fuel	737	745	878
Purchased power	245	252	111
Natural gas purchased for resale	47	49	57
Other operations and maintenance	902	893	925
Provision for Callaway construction and operating license	0	0	69
Depreciation and amortization	533	514	492
Taxes other than income taxes	328	325	335
Total operating expenses	2,792	2,778	2,867
Operating Income	747	745	742
Other Income and Expenses:
Miscellaneous income	48	52	52
Miscellaneous expense	8	10	11
Total other income (expense)	40	42	41
Interest charges	207	211	219
Income Before Income Taxes	580	576	564
Income taxes	254	216	209
Net Income	326	360	355
Other Comprehensive Income	0	0	0
Comprehensive Income	326	360	355
Net Income:
Preferred Stock Dividends	3	3	3
Net Income Available to Common Stockholder	323	357	352
Ameren Illinois Company
Operating Revenues:
Electric	1,784	1,736	1,683
Natural Gas	743	754	783
Other	1	0	0
Total operating revenues	2,528	2,490	2,466
Operating Expenses:
Purchased power	417	399	420
Natural gas purchased for resale	264	292	358
Other operations and maintenance	789	804	797
Depreciation and amortization	341	319	295
Taxes other than income taxes	137	132	130
Total operating expenses	1,948	1,946	2,000
Operating Income	580	544	466
Other Income and Expenses:
Miscellaneous income	11	21	21
Miscellaneous expense	10	12	12
Total other income (expense)	1	9	9
Interest charges	144	140	131
Income Before Income Taxes	437	413	344
Income taxes	166	158	127
Net Income	271	255	217
Pension and other postretirement activity, net of income taxes (benefit)	0	(5)	(3)
Comprehensive Income	271	250	214
Net Income:
Preferred Stock Dividends	3	3	3
Net Income Available to Common Stockholder	$ 268	$ 252	$ 214

Consolidated Statement Of Inco3

Consolidated Statement Of Income (Loss) (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension and other postretirement activity, tax (benefit)	$ 3	$ (7)	$ 3
Ameren Illinois Company
Pension and other postretirement activity, tax (benefit)	$ 0	$ (1)	$ (2)

Consolidated Statement Of Compr

Consolidated Statement Of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income from Continuing Operations	$ 529	$ 659	$ 585
Pension and other postretirement activity, net of income taxes (benefit)	5	(20)	6
Comprehensive Income from Continuing Operations	534	639	591
Less: Comprehensive Income from Continuing Operations Attributable to Noncontrolling Interests	6	6	6
Comprehensive Income from Continuing Operations Attributable to Ameren Corporation	528	633	585
Comprehensive Income from Discontinued Operations Attributable to Ameren Common shareholders	0	0	51
Comprehensive Income	$ 528	$ 633	$ 636

Consolidated Statement Of Comp5

Consolidated Statement Of Comprehensive Income (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension and other postretirement activity, tax (benefit)	$ 3	$ (7)	$ 3

Consolidated Balance Sheet

Consolidated Balance Sheet - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Current Assets:
Cash and cash equivalents	$ 10	$ 9
Accounts receivable - trade (less allowance for doubtful accounts)	445	437
Unbilled revenue	323	295
Miscellaneous accounts and notes receivable	70	63
Inventories	522	527
Current regulatory assets	144	149
Other current assets	98	113
Total current assets	1,612	1,593
Property, Plant and Equipment, Net	21,466	20,113
Investments and Other Assets:
Nuclear decommissioning trust fund	704	607
Goodwill	411	411
Regulatory assets	1,230	1,437
Other assets	522	538
Total investments and other assets	2,867	2,993
TOTAL ASSETS	25,945	24,699
Current Liabilities:
Current maturities of long-term debt	841	681
Short-term debt	484	558
Accounts and wages payable	902	805
Taxes accrued	52	46
Interest accrued	99	93
Customer deposits	108	107
Current regulatory liabilities	128	110
Other current liabilities	326	274
Total current liabilities	2,940	2,674
Long-term Debt, Net	7,094	6,595
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	2,506	4,264
Accumulated deferred investment tax credits	49	55
Regulatory liabilities	4,387	1,985
Asset retirement obligations	638	635
Pension and other postretirement benefits	545	769
Other deferred credits and liabilities	460	477
Total deferred credits and other liabilities	8,585	8,185
Commitments and Contingencies (Notes 2, 9, and 14)
Stockholders' Equity:
Common stock	2	2
Other paid-in capital, principally premium on common stock	5,540	5,556
Retained earnings	1,660	1,568
Accumulated other comprehensive loss	(18)	(23)
Total stockholders' equity	7,184	7,103
Noncontrolling Interests	142	142
Total equity	7,326	7,245
TOTAL LIABILITIES AND EQUITY	25,945	24,699
Union Electric Company
Current Assets:
Cash and cash equivalents	0	0
Advances to money pool	0	161
Accounts receivable - trade (less allowance for doubtful accounts)	200	187
Accounts receivable - affiliates	11	12
Unbilled revenue	165	154
Miscellaneous accounts and notes receivable	35	14
Inventories	388	392
Current regulatory assets	56	35
Other current assets	50	49
Total current assets	905	1,004
Property, Plant and Equipment, Net	11,751	11,478
Investments and Other Assets:
Nuclear decommissioning trust fund	704	607
Regulatory assets	395	619
Other assets	288	327
Total investments and other assets	1,387	1,553
TOTAL ASSETS	14,043	14,035
Current Liabilities:
Current maturities of long-term debt	384	431
Short-term debt	39	0
Accounts and wages payable	475	444
Accounts payable - affiliates	60	68
Taxes accrued	30	30
Interest accrued	54	54
Current regulatory liabilities	19	12
Other current liabilities	103	123
Total current liabilities	1,164	1,162
Long-term Debt, Net	3,577	3,563
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,650	3,013
Accumulated deferred investment tax credits	48	53
Regulatory liabilities	2,664	1,215
Asset retirement obligations	634	629
Pension and other postretirement benefits	213	291
Other deferred credits and liabilities	12	19
Total deferred credits and other liabilities	5,221	5,220
Commitments and Contingencies (Notes 2, 9, and 14)
Stockholders' Equity:
Common stock	511	511
Other paid-in capital, principally premium on common stock	1,858	1,828
Preferred stock not subject to mandatory redemption	80	80
Retained earnings	1,632	1,671
Total stockholders' equity	4,081	4,090
TOTAL LIABILITIES AND EQUITY	14,043	14,035
Ameren Illinois Company
Current Assets:
Cash and cash equivalents	0	0
Accounts receivable - trade (less allowance for doubtful accounts)	234	242
Accounts receivable - affiliates	9	10
Unbilled revenue	158	141
Miscellaneous accounts and notes receivable	35	22
Inventories	134	135
Current regulatory assets	87	108
Other current assets	15	25
Total current assets	672	683
Property, Plant and Equipment, Net	8,293	7,469
Investments and Other Assets:
Goodwill	411	411
Regulatory assets	822	816
Other assets	147	95
Total investments and other assets	1,380	1,322
TOTAL ASSETS	10,345	9,474
Current Liabilities:
Current maturities of long-term debt	457	250
Short-term debt	62	51
Accounts and wages payable	337	264
Accounts payable - affiliates	70	63
Taxes accrued	19	16
Interest accrued	33	33
Customer deposits	69	69
Environmental remediation	42	38
Current regulatory liabilities	92	78
Other current liabilities	177	109
Total current liabilities	1,358	971
Long-term Debt, Net	2,373	2,338
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,021	1,631
Accumulated deferred investment tax credits	1	2
Regulatory liabilities	1,629	768
Pension and other postretirement benefits	285	346
Accrued Environmental Loss Contingencies, Noncurrent	134	162
Other deferred credits and liabilities	234	222
Total deferred credits and other liabilities	3,304	3,131
Commitments and Contingencies (Notes 2, 9, and 14)
Stockholders' Equity:
Common stock	0	0
Other paid-in capital, principally premium on common stock	2,013	2,005
Preferred stock not subject to mandatory redemption	62	62
Retained earnings	1,235	967
Total stockholders' equity	3,310	3,034
TOTAL LIABILITIES AND EQUITY	$ 10,345	$ 9,474

Consolidated Balance Sheet (Par

Consolidated Balance Sheet (Parenthetical) - USD ($) shares in Millions, $ in Millions	Dec. 31, 2017	Dec. 31, 2016
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, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flows From Operating Activities:
Net income	$ 529	$ 659	$ 636
(Income) Loss from discontinued operations, net of tax	0	0	(51)
Adjustments to reconcile net income to net cash provided by operating activities:
Provision for Callaway construction and operating license	0	0	69
Depreciation and amortization	876	835	777
Amortization of nuclear fuel	76	88	97
Amortization of debt issuance costs and premium/discounts	22	22	22
Deferred income taxes and investment tax credits, net	539	386	369
Allowance for equity funds used during construction	(24)	(27)	(30)
Share-based Compensation	17	17	24
Other	(10)	4	(10)
Changes in assets and liabilities:
Receivables	(53)	(71)	83
Inventories	17	11	(14)
Accounts and wages payable	32	19	(2)
Taxes accrued	55	13	(22)
Regulatory assets and liabilities	36	215	94
Assets, other	20	(22)	46
Liabilities, other	(7)	(9)	(44)
Pension and other postretirement benefits	(21)	(16)	(9)
Net cash provided by operating activities - continuing operations	2,104	2,124	2,035
Net cash provided by operating activities - discontinued operations	0	(1)	(4)
Net cash provided by operating activities	2,104	2,123	2,031
Cash Flows From Investing Activities:
Capital expenditures	(2,132)	(2,076)	(1,917)
Nuclear fuel expenditures	(63)	(55)	(52)
Purchases of securities - nuclear decommissioning trust fund	(413)	(392)	(363)
Sales and maturities of securities - nuclear decommissioning trust fund	396	377	349
Other	7	5	32
Net cash used in investing activities - continuing operations	(2,205)	(2,141)	(1,951)
Net cash used in investing activities - discontinued operations	0	0	(25)
Net cash used in investing activities	(2,205)	(2,141)	(1,976)
Cash Flows From Financing Activities:
Dividends on common stock	(431)	(416)	(402)
Dividends paid to noncontrolling interest holders	(6)	(6)	(6)
Short-term debt, net	(74)	257	(413)
Maturities, redemptions, and repurchases of long-term debt	(681)	(395)	(120)
Issuances of Long-term debt	1,345	389	1,197
Capital issuance costs	(11)	(9)	(12)
Share-based payments	(39)	(83)	(12)
Other	(1)	(2)	0
Net cash provided by (used in) financing activities	102	(265)	232
Net change in cash and cash equivalents	1	(283)	287
Cash and cash equivalents at beginning of year	9	292	5
Cash and cash equivalents at end of year	10	9	292
Cash Paid (Refunded) During the Year:
Interest net of capitalized	370	358	335
Income taxes, net	(19)	(12)	(15)
Union Electric Company
Cash Flows From Operating Activities:
Net income	326	360	355
Adjustments to reconcile net income to net cash provided by operating activities:
Provision for Callaway construction and operating license	0	0	69
Depreciation and amortization	514	506	476
Amortization of nuclear fuel	76	88	97
Amortization of debt issuance costs and premium/discounts	6	6	6
Deferred income taxes and investment tax credits, net	82	179	82
Allowance for equity funds used during construction	(21)	(23)	(22)
Other	4	5	2
Changes in assets and liabilities:
Receivables	(46)	5	72
Inventories	18	(4)	(39)
Accounts and wages payable	27	(18)	3
Taxes accrued	(1)	11	1
Regulatory assets and liabilities	26	84	117
Assets, other	30	(25)	26
Liabilities, other	(23)	(1)	4
Pension and other postretirement benefits	(2)	(4)	(2)
Net cash provided by operating activities	1,016	1,169	1,247
Cash Flows From Investing Activities:
Capital expenditures	(773)	(738)	(622)
Nuclear fuel expenditures	(63)	(55)	(52)
Purchases of securities - nuclear decommissioning trust fund	(413)	(392)	(363)
Sales and maturities of securities - nuclear decommissioning trust fund	396	377	349
Money pool advances, net	161	(125)	(36)
Other	7	(1)	0
Net cash used in investing activities	(685)	(934)	(724)
Cash Flows From Financing Activities:
Dividends on common stock	(362)	(355)	(575)
Dividends on preferred stock	(3)	(3)	(3)
Short-term debt, net	39	0	(97)
Maturities, redemptions, and repurchases of long-term debt	(431)	(266)	(120)
Issuances of Long-term debt	399	149	249
Capital issuance costs	(3)	(3)	(3)
Capital contribution from parent	30	44	224
Net cash provided by (used in) financing activities	(331)	(434)	(325)
Net change in cash and cash equivalents	0	(199)	198
Cash and cash equivalents at beginning of year	0	199	1
Cash and cash equivalents at end of year	0	0	199
Noncash Or Part Noncash Capital Contribution From Parent	0	0	38
Cash Paid (Refunded) During the Year:
Interest net of capitalized	202	209	212
Income taxes, net	178	27	72
Ameren Illinois Company
Cash Flows From Operating Activities:
Net income	271	255	217
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	341	318	292
Amortization of debt issuance costs and premium/discounts	13	14	14
Deferred income taxes and investment tax credits, net	171	154	221
Allowance for equity funds used during construction	(3)	(4)	(8)
Other	0	(1)	(14)
Changes in assets and liabilities:
Receivables	(7)	(72)	16
Inventories	(1)	15	25
Accounts and wages payable	19	12	37
Taxes accrued	18	1	(2)
Regulatory assets and liabilities	16	120	(26)
Assets, other	(15)	(3)	17
Liabilities, other	3	(5)	(27)
Pension and other postretirement benefits	(14)	(8)	(4)
Counterparty collateral, net	0	3	(3)
Net cash provided by operating activities	815	803	763
Cash Flows From Investing Activities:
Capital expenditures	(1,076)	(924)	(918)
Other	6	6	5
Net cash used in investing activities	(1,070)	(918)	(913)
Cash Flows From Financing Activities:
Dividends on common stock	0	(110)	0
Dividends on preferred stock	(3)	(3)	(3)
Short-term debt, net	11	51	(32)
Money pool borrowings, net	0	0	(15)
Maturities, redemptions, and repurchases of long-term debt	(250)	(129)	0
Issuances of Long-term debt	496	240	248
Capital issuance costs	(6)	(4)	(3)
Capital contribution from parent	8	0	25
Other	(1)	(1)	0
Net cash provided by (used in) financing activities	255	44	220
Net change in cash and cash equivalents	0	(71)	70
Cash and cash equivalents at beginning of year	0	71	1
Cash and cash equivalents at end of year	0	0	71
Cash Paid (Refunded) During the Year:
Interest net of capitalized	139	127	120
Income taxes, net	$ (22)	$ 8	$ (113)

Consolidated Statement Of Cash9

Consolidated Statement Of Cash Flows (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest Paid, Capitalized	$ 14	$ 15	$ 17
Union Electric Company
Interest Paid, Capitalized	10	12	12
Ameren Illinois Company
Interest Paid, Capitalized	$ 4	$ 3	$ 5

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, 2014			$ 5,617	$ 1,103	$ (9)		$ 142				$ 1,569		$ 1,892			$ 1,980		$ 611	$ 8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation activity			(1)
Net income attributable to Ameren common shareholders	$ 630			630
Net income	636								$ 355				355	$ 217				217
Common stock dividends				(402)									(575)					0
Capital contribution from parent									224		253			25		25
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
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	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	659								360				360	255				255
Common stock dividends				(416)									(355)					(110)
Capital contribution from parent									$ 44		6			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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock-based compensation activity			(16)
Net income attributable to Ameren common shareholders	523			523
Net income	529								326				326	271				271
Common stock dividends				(431)									(362)					0
Capital contribution from parent									$ 30		30			8		8
Preferred stock dividends													(3)					(3)
Change in deferred retirement benefit costs	$ (5)				5									$ 0						0
Net income attributable to noncontrolling interest holder							6
Dividends paid to noncontrolling interest holders							(6)
End of year (shares) at Dec. 31, 2017	242.6								102.1					25.5
End of year at Dec. 31, 2017	$ 7,326	$ 2	$ 5,540	$ 1,660	$ (18)	$ (18)	$ 142			$ 511	$ 1,858	$ 80	$ 1,632		$ 0	$ 2,013	$ 62	$ 1,235	$ 0	$ 0
Stockholders' equity, end of year at Dec. 31, 2017	$ 7,184							$ 7,184	$ 4,081					$ 3,310

Summary Of Significant Accounti

Summary Of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
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 whose primary assets are its equity interests in its subsidiaries. Ameren’s subsidiaries are separate, independent legal entities with separate businesses, assets, and liabilities. Dividends on Ameren’s common stock and the payment of expenses by Ameren depend on distributions made to it by its subsidiaries. Ameren’s principal subsidiaries are listed below, including Ameren Missouri, Ameren Illinois, and ATXI. Ameren also has other subsidiaries that conduct other activities, such as the provision of shared services. Ameren evaluates competitive electric transmission investment opportunities as they arise. • 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, which 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 a 40,000 square mile area in central and southern Illinois. 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 and Mark Twain projects, and placed the Spoon River project in service in February 2018. 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. As of December 31, 2017 and December 31, 2016, Ameren had unconsolidated variable interests as a limited partner in various equity method investments totaling $17 million and $9 million , respectively, included in “Other assets” on Ameren’s consolidated balance sheet. Ameren is not the primary beneficiary of these investments because it does not have the power to direct matters that most significantly impact the activities of these variable interest entities. As of December 31, 2017, the maximum exposure to loss related to these variable interests is limited to the investment in these partnerships of $17 million plus associated outstanding funding commitments of $20 million . 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. 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 regulatory rate review. In Ameren Missouri’s and Ameren Illinois’ natural gas businesses, changes in natural gas costs are reflected in billings to their respective 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. Ameren Missouri has a FAC that allows an adjustment of electric rates three times per year, without a traditional rate proceeding, for a pass-through to customers of 95% of the variance in net energy costs from 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. 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. In addition to the rate-adjustment mechanisms discussed above, 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 participates in performance-based formula ratemaking frameworks established pursuant to the IEIMA and the FEJA for its electric distribution business and its electric energy-efficiency investments. Ameren Illinois also has environmental cost riders, an asbestos-related litigation rider, natural gas 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 the regulatory assets and liabilities recorded at December 31, 2017 and 2016. The Ameren Illinois asbestos-related litigation rider includes a trust fund. At December 31, 2017 and 2016 , the trust fund balance of $23 million and $22 million , respectively, was reflected in “Other assets” on Ameren’s and Ameren Illinois’ balance sheets. 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 weighted-average cost or net realizable value. Inventories are capitalized when purchased and then expensed as consumed or capitalized as property, plant, and equipment when installed, as appropriate. The following table presents a breakdown of inventories for each of the Ameren Companies at December 31, 2017 and 2016 : Ameren Missouri Ameren Illinois Ameren 2017 Fuel (a) $ 154 $ — $ 154 Natural gas stored underground 8 74 82 Materials, supplies, and other 226 60 286 Total inventories $ 388 $ 134 $ 522 2016 Fuel (a) $ 172 $ — $ 172 Natural gas stored underground 9 73 82 Materials, supplies, and other 211 62 273 Total inventories $ 392 $ 135 $ 527 (a) Consists of coal, oil, and propane. 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 section 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 2017 , 2016 , and 2015 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 2017 , 2016 , and 2015 : 2017 2016 2015 Ameren Missouri 7 % 7 % 7 % Ameren Illinois 4 % 5 % 6 % 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 had goodwill of $411 million at December 31, 2017 and 2016. Ameren has four reporting units: Ameren Missouri, Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Transmission. Ameren Illinois has three reporting units: Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Illinois Transmission. 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, 2017 and 2016. The Ameren Transmission reporting unit had the same $93 million of goodwill as the Ameren Illinois Transmission reporting unit at December 31, 2017 and 2016. 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. 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 quantitative test, on an annual basis. On December 31, 2016, due to a change in reporting units, Ameren and Ameren Illinois performed a quantitative test and determined that the estimated fair value of each reporting unit significantly exceeded its respective 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, 2017. The results of Ameren’s and Ameren Illinois’ qualitative assessment indicated that it was more likely than not that the fair value of each reporting unit significantly exceeded its carrying value as of October 31, 2017, resulting in no impairment of Ameren’s or Ameren Illinois’ goodwill. The following factors, among others, were considered by Ameren and Ameren Illinois when they assessed whether it was more likely than not that the fair value of each of their reporting units exceeded its carrying value as of October 31, 2017: • macroeconomic conditions, including those conditions within Ameren Illinois’ service territory; • pending regulatory rate review outcomes and projections of future regulatory rate review outcomes; • changes in laws and potential law changes; • observable industry market multiples; • achievement of IEIMA and FEJA performance metrics and the yield of 30-year United States Treasury bonds; • an unexpected further reduction in the FERC-allowed return on equity with respect to transmission services; and • projected operating results and cash flows. 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 2017 and 2016 . 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 deferred as a regulatory balance. The depreciation of the asset book values at Ameren Missouri was $26 million , $31 million , and $13 million for the years ended December 31, 2017, 2016, and 2015, respectively, which was deferred as a reduction to the net regulatory liability. The depreciation deferred to the regulatory asset at Ameren Illinois was immaterial in each respective period. 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 as a regulatory liability. 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, 2017 and 2016 : Ameren Missouri Ameren Illinois Ameren Balance at December 31, 2015 $ 617 $ 6 $ 623 Liabilities incurred 3 — 3 Liabilities settled (2 ) (a) (2 ) Accretion in 2016 (b) 25 (a) 25 Change in estimates 1 — 1 Balance at December 31, 2016 $ 644 (c) $ 6 (d) $ 650 (c) Liabilities incurred — — — Liabilities settled (12 ) (1 ) (13 ) Accretion in 2017 (b) 26 (a) 26 Change in estimates (e) (18 ) (1 ) (19 ) Balance at December 31, 2017 $ 640 (c) $ 4 (d) $ 644 (c) (a) Less than $1 million. (b) Ameren Missouri’s accretion expense was deferred as a decrease to regulatory liabilities. (c) Balance included $6 million and $15 million in “Other current liabilities” on the balance sheet as of December 31, 2017 and 2016, respectively. (d) Included in “Other deferred credits and liabilities” on the balance sheet. (e) Ameren Missouri changed its fair value estimate primarily because of an extension of the remediation period of certain CCR storage facilities, an update to the decommissioning of the Callaway energy center to reflect the cost study and funding analysis filed with the MoPSC in 2017, and an increase in the assumed discount rate. Noncontrolling Interests As of December 31, 2017 and 2016 , 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 and the FEJA. 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 varies from the amount collected from customers, an adjustment is made to electric operating revenues with an offset to a regulatory asset or liability to reflect that year’s actual revenue requirement. The regulatory balance is then collected from, or refunded 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 2017, 2016, or 2015. 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 vesting period. See Note 11 – 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, 2017 , 2016 , and 2015 : 2017 2016 2015 Ameren Missouri $ 153 $ 151 $ 156 Ameren Illinois 57 57 57 Ameren $ 210 $ 208 $ 213 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 the 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 expect that regulators will reduce future revenues for deferred tax liabilities that were initially recorded at rates in excess of the current statutory rate. Therefore, reductions in certain 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. To the extent deferred tax balances are included in rate base, the revaluation of deferred taxes is recorded as a regulatory asset or liability on the balance sheet and will be collected from or refunded to customers. For deferred tax balances not included in rate base, the revaluation of deferred taxes is recorded as an adjustment to income tax expense on the income statement. See Note 12 – Income Taxes for further information regarding both the revaluation of deferred taxes related to the TCJA. 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 Ameren (parent) is reallocated to the other parties. This reallocation is treated as a capital contribution to the party receiving the benefit. See Note 13 – Related-party Transactions for information regarding capital contributions under the tax allocation agreement. 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 1.6 million , 0.8 million , and 1.0 million for the years ended December 31, 2017 , 2016 , and 2015 , respectively. There were no potentially dilutive securities excluded from the diluted earnings per share calculations for the years ended December 31, 2017 , 2016 , and 2015 . Divestiture Transactions and Discontinued Operations In December 2013 and January 2014, Ameren completed the divestiture of New AER and certain other assets. 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. Ameren also paid $25 million and concluded its obligations with New AER. 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 receive in exchange for those goods or services. The guidance 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, as well as separate presentation of alternative revenue programs on the income statement. Entities can apply the guidance to each reporting period presented (the full retrospective method), or they can record a cumulative effect adjustment to retained earnings in the period of initial adoption (the modified retrospective method). We have completed the evaluation of our contracts. Adoption of this guidance will not result in material changes to the amount or timing of revenue recognition. We will apply the guidance using the full retrospective method. We will include disaggregated revenue disclosures by segment and customer class in the combined notes to the financial statements. This guidance will be effective for the Ameren Companies for the first quarter of 2018. Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost In March 2017, the FASB issued authoritative guidance that requires an entity to report, including on a retrospective basis, the non-service cost or income components of net benefit cost separately from the service cost component and outside of operating income. Our adoption of this guidance will result in the reclassification of 2017 net benefit income of $44 million , $22 million , and $10 million , currently presented as a reduction of "Other operations and maintenance expense," on Ameren's, Ameren Missouri's, and Ameren Illinois' respective statements of income. These amounts will be presented outside of operating income. Similarly, 2016 net benefit income of $55 million , $18 million , and $24 million , currently presented as a reduction of "Other operations and maintenance expense" on Ameren's, Ameren Missouri's, and Ameren Illinois' respective statements of income, will also be reclassified and presented outside of operating income. The guidance also permits an entity to capitalize only the service cost component as part of an asset, such as inventory or property, plant, and equipment, on a prospective basis. Previously, all of the net benefit cost components were eligible for capitalization. This change in the capitalization of net benefit costs is not expected to affect our ability to recover total net benefit cost through customer rates. This guidance will be effective for the Ameren Companies in the first quarter of 2018. See Note 10 – Retirement Benefits for the components of net benefit cost. 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. Classification of Certain Cash Receipts and Cash Payments In August 2016, the FASB issued authoritative guidance that specifies the classification and presentation of certain cash flow items to reduce diversity in practice. This guidance will be effective for the Ameren Companies in the first quarter of 2018, and requires changes to be applied retrospectively. For Ameren and Ameren Illinois, the adoption of this guidance will result in the retrospective reclassification from operating activities to financing activities of $7 million of bond premiums received in 2016. 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 and to recognize changes in fair value in net income. The adoption of this guidance will not have a material impact on our results of operations or financial position. The recognition, measurement, and disclosure guidance will be effective for the Ameren Companies in the first quarter of 2018. The guidance requires changes to be applied retrospectively with a cumulative effect adjustment to retained earnings as of the a

Rate And Regulatory Matters

Rate And Regulatory Matters	12 Months Ended
	Dec. 31, 2017
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 March 2017 Electric Rate Order In March 2017, the MoPSC issued an order approving a unanimous stipulation and agreement in Ameren Missouri’s July 2016 regulatory rate review. The order resulted in a $3.4 billion revenue requirement, which was a $92 million increase in Ameren Missouri’s annual revenue requirement for electric service, compared with the prior revenue requirement established in the MoPSC’s April 2015 electric rate order. The new rates, base level of expenses, and amortizations became effective on April 1, 2017. The order authorized 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 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 between the base amount and the actual expenses incurred deferred as a regulatory asset or liability . Excluding cost reductions associated with reduced sales volumes, the base level of net energy costs decreased 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, reduced expenses by $26 million from the base levels established in the MoPSC’s April 2015 electric rate order. MEEIA In November 2016, the MoPSC approved a $28 million MEEIA 2013 performance incentive based on a stipulation and agreement among 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 a certain 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 then appealed that decision to the Missouri Supreme Court. If the decision is overturned, Ameren Missouri would recognize an additional $9 million MEEIA 2013 performance incentive. The MEEIA 2016 program provided Ameren Missouri with a performance incentive to earn additional revenues by achieving certain customer energy-efficiency goals, including $27 million if 100% of the goals were achieved during the three-year period, with the potential to earn more if Ameren Missouri’s energy savings exceeded those goals. In September 2017, Ameren Missouri received an order from the MoPSC approving Ameren Missouri’s energy savings results for the first year of the MEEIA 2016 programs. As a result of this order and in accordance with revenue recognition guidance, Ameren Missouri will recognize $5 million of additional revenues in the first quarter of 2018 relating to the MEEIA 2016 performance incentive. MoPSC Federal Income Tax Proceeding In February 2018, the MoPSC initiated proceedings to investigate how the effect of the reduction in the federal statutory corporate income tax rate enacted under the TCJA should be reflected in rates paid by customers of Missouri’s regulated utilities, including rates paid by electric and natural gas customers of Ameren Missouri. At this time, Ameren Missouri is unable to predict the timing or the magnitude of any impact on its electric and natural gas rates that may result from the ultimate resolution of this matter. ATXI’s Mark Twain Project The Mark Twain project is a MISO-approved transmission line to be located in northeast Missouri with an expected investment of $250 million . In the third quarter of 2017, ATXI finalized an alternative project route and reached agreements with Ameren Missouri and an electric cooperative in northeast Missouri to locate almost all of the Mark Twain project on existing line corridors. It also received assents for road crossings from the five affected counties in northeast Missouri. In January 2018, the MoPSC granted ATXI a certificate of convenience and necessity for the Mark Twain project. ATXI plans to begin construction in the second quarter of 2018 and to complete the project by the end of 2019. Illinois IEIMA & FEJA Under a formula ratemaking framework effective through 2022, Ameren Illinois’ electric distribution service rates are subject to an annual revenue requirement reconciliation to its actual recoverable costs and allowed return on equity. The formula ratemaking framework qualifies as an alternative revenue program under GAAP. 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. As of December 31, 2017 , Ameren Illinois had recorded regulatory assets of $54 million and $24 million , including interest, to reflect its expected 2017 and its approved 2016 revenue requirement reconciliation adjustments, respectively. As of December 31, 2016 , Ameren Illinois had recorded a $68 million regulatory asset to reflect its approved 2015 revenue requirement reconciliation adjustment, which was collected, with interest, from customers during 2017. In December 2017, the ICC issued an order in Ameren Illinois’ annual update filing that approved a $17 million decrease in Ameren Illinois’ electric delivery service revenue requirement beginning in January 2018. This update reflected an increase to the annual formula rate based on 2016 actual costs and expected net plant additions for 2017, as well as an increase to include the 2016 revenue requirement reconciliation adjustment. The increases in the update filing were more than offset by a decrease for the conclusion of the 2015 revenue requirement reconciliation adjustment, which was fully collected from customers in 2017, consistent with the ICC’s December 2016 annual update filing order. The FEJA revised certain portions of the IEIMA, including extending the IEIMA formula ratemaking framework through 2022 , and clarifying that a common equity ratio up to and including 50% is prudent. Beginning in 2017, the FEJA permitted Ameren Illinois to recover, within the following two years, its electric distribution revenue requirement for a given year, independent of actual sales volumes. Prior to the FEJA, Ameren Illinois’ interim period revenue recognition was volume-based, as revenues were affected by the timing of sales volumes due to seasonal rates and changes in volumes resulting from, among other things, weather and energy efficiency. This previous revenue recognition method resulted in more revenue during the third quarter and less revenue during the other quarters of each year. Beginning in 2017, in connection with the decoupling provisions of the FEJA, Ameren Illinois changed the method it uses to recognize interim-period revenue. Ameren Illinois now recognizes revenue consistent with the timing of actual incurred electric distribution recoverable costs, and it recognizes revenue associated with the expected return on its rate base ratably over the year. The decoupling provisions of the FEJA do not expire at the end of 2022. The FEJA allows Ameren Illinois to earn a return on its electric energy-efficiency program investments. Ameren Illinois’ electric energy-efficiency investments are deferred as a regulatory asset and 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 be increased or decreased by up to 200 basis points, depending on the achievement of annual energy savings goals. The FEJA increased the level of electric energy-efficiency saving targets through 2030. In June 2017, pursuant to the FEJA, Ameren Illinois filed with the ICC an energy-efficiency plan for 2018 through 2021. In September 2017, the ICC issued an order approving Ameren Illinois’ implementation of the FEJA electric energy-efficiency savings targets and investments. Ameren Illinois plans to invest up to $99 million per year in electric energy-efficiency programs from 2018 through 2021. Ameren Illinois plans to make similar yearly investments in electric energy-efficiency programs from 2022 through 2030. The ICC has the ability to reduce electric energy-efficiency savings goals if there are insufficient cost-effective programs available or if the savings goals would require investment levels that exceed amounts allowed by legislation. The electric energy-efficiency program investments and the return on those investments will be collected from customers through a rider; they will not be included in the IEIMA formula ratemaking framework. Income Tax Regulatory Mechanisms In February 2018, the ICC granted Ameren Illinois’ request, filed in January 2018, to establish a rider to pass through to Ameren Illinois’ electric distribution customers the reduction in the federal statutory corporate income tax rate enacted under the TCJA and the return of excess deferred taxes, net of the increase in state income taxes enacted in July 2017. Ameren Illinois' electric distribution customers will receive up to an estimated $50 million per year through the rider beginning in the first quarter of 2018 and continuing through 2019. Absent this rider, Ameren Illinois' electric distribution customers would not benefit from Ameren Illinois' reduced income tax liability until 2020, at which time the net reduction in income taxes would have been reflected in customer rates through the revenue reconciliation process. In January 2018, the ICC initiated a proceeding to require that Ameren Illinois record a regulatory liability, beginning January 25, 2018, for the net amount of the difference between revenues billed under natural gas rates in effect, pursuant to Ameren Illinois’ most recent natural gas rate order, and the revenues that would have been billed had the state and federal tax rate changes been in effect. In February 2018, Ameren Illinois filed a response to the ICC seeking approval of a rider that calculates such differences, specifically by evaluating the return of excess deferred taxes and income taxes included in the revenue requirement prior to the reduction in the federal statutory corporate income tax rate enacted under the TCJA and the increase in state income taxes enacted in July 2017. Ameren Illinois’ natural gas customers may receive up to an estimated $16 million through the proposed rider, or through some other tariff approved by the ICC, over a one-year period beginning in May 2018. 2018 Natural Gas Delivery Service Regulatory Rate Review In January 2018, Ameren Illinois filed a request with the ICC seeking approval to increase its annual revenues for natural gas delivery service by $49 million, which included an estimated $42 million of annual revenues that would otherwise be recovered under a QIP rider . The request was based on a 10.3% return on common equity, a capital structure composed of 50% common equity, and a rate base of $1.6 billion. The request reflects the reduction in the federal corporate income tax rate as a result of the TCJA, as well as the increase in the Illinois corporate income tax rate that became effective in July 2017. In an attempt to reduce regulatory lag, Ameren Illinois used a 2019 future test year in this proceeding. A decision by the ICC in this proceeding is required by December 2018, with new rates expected to be effective in January 2019. Ameren Illinois cannot predict the level of any delivery service rate changes the ICC may approve, nor whether any rate changes that may eventually be approved will be sufficient to enable Ameren Illinois to recover its costs and to earn a reasonable return on investments when the rate changes go into effect. ATXI’s Illinois Rivers Project In August 2017, the Illinois Circuit Court for Edgar County dismissed several of ATXI’s condemnation cases related to one line segment in the Illinois Rivers project. The estimated line segment capital expenditure investment is approximately $85 million , of which $36 million was invested as of December 31, 2017 . These cases had been filed to obtain easements and rights of way necessary to complete the line segment. The court found that required notice was not given to the relevant landowners during the underlying ICC proceeding. In November 2017, ATXI appealed this decision to the Illinois Supreme Court. ATXI plans to complete the project by the end of 2019; however, delays associated with the condemnation proceedings or an appeal arising from the order dismissing the Edgar County cases could delay the completion date. The other eight line segments of the Illinois Rivers project are not affected by these proceedings. 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 for the 15-month period of November 2013 to February 2015 to 10.32%, or a 10.82% total allowed return on common equity with the inclusion of a 50 basis point incentive adder for participation in an RTO. The order required customer refunds, with interest, to be issued for that 15-month period. In 2017, Ameren and Ameren Illinois refunded $21 million and $17 million , respectively, related to the November 2013 complaint case. The 10.82% total allowed return on common equity has been reflected in rates since September 2016. The 10.82% allowed return on common equity may be replaced prospectively after the FERC issues a final order in the February 2015 complaint case, discussed below. Since 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. MISO transmission owners subsequently filed a motion to dismiss the February 2015 complaint, as discussed below. The February 2015 complaint case seeks a further reduction in the allowed base return on common equity for FERC-regulated transmission rate base under the MISO tariff. In June 2016, an administrative law judge issued an initial decision in the February 2015 complaint case. If approved by the FERC, it would lower the allowed base return on common equity for the 15-month period of February 2015 to May 2016 to 9.70%, or a 10.20% total allowed return on equity with the inclusion of a 50 basis point incentive adder for participation in an RTO. It would also require customer refunds, with interest, for that 15-month period. A final FERC order would also establish the allowed return on common equity that will apply prospectively from the effective date of such order, replacing the current 10.82% total return on common equity. The timing of the issuance of the final order in the February 2015 complaint case is uncertain for two reasons. First, while the FERC reestablished a quorum of commissioners in August 2017 after six months without a quorum, the FERC is under no deadline to issue a final order. Second, in the second quarter of 2017, the United States Court of Appeals for the District of Columbia Circuit vacated and remanded to the FERC an order in a separate case in which the FERC established the allowed base return on common equity methodology used in the two MISO complaint cases described above. Ameren is unable to predict the impact of the outcome of the United States Court of Appeals for the District of Columbia Circuit’s remand on the MISO FERC complaint cases at this time. In September 2017, MISO transmission owners, including Ameren Missouri, Ameren Illinois, and ATXI, filed a motion to dismiss the February 2015 complaint case with the FERC. The MISO transmission owners maintain that the February 2015 complaint was predicated on the premise that the now superseded 12.38% allowed base return on common equity was an unjust and unreasonable return and is therefore inapplicable given the current 10.32% allowed base return on common equity. The MISO transmission owners further maintain that the current 10.32% allowed base return on common equity has not been proven to be unjust and unreasonable based on information provided, including the base return on common equity methodology ranges set forth in the February 2015 complaint case and in the initial decision issued by an administrative law judge in June 2016. Additionally, the MISO transmission owners maintain that the February 2015 complaint should be dismissed because the approach utilized in the case to assert that a return on common equity was unjust and unreasonable was insufficient. That same approach was rejected by the United States Court of Appeals for the District of Columbia Circuit, as discussed above. FERC is under no deadline to issue an order on this motion. As of December 31, 2017 , Ameren and Ameren Illinois recorded current regulatory liabilities of $42 million and $25 million , respectively, to reflect the expected refunds, including interest, associated with the reduced allowed returns on common equity in 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. MISO Federal Income Tax Proceeding In February 2018, MISO transmission owners with forward-looking rate formulas, including Ameren Illinois and ATXI, filed a request with the FERC to allow revisions to their 2018 electric transmission rates to reflect the impact of the reduction in federal income taxes enacted under the TCJA. If approved, Ameren Illinois and ATXI’s 2018 electric transmission rates would be reduced by $27 million and $23 million , respectively. Absent this revision, the reduction in federal income taxes enacted under the TCJA would not be reflected in Ameren Illinois' and ATXI's electric transmission rates until 2020 through the revenue reconciliation process. 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, 2017 and 2016 : 2017 2016 Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Current regulatory assets: Under-recovered FAC (a)(b) $ 47 $ — $ 47 $ 21 $ — $ 21 Under-recovered Illinois electric power costs (c) — — — — 3 3 Under-recovered PGA (c) 1 13 14 — 4 4 MTM derivative losses (d) 8 25 33 9 15 24 Energy-efficiency riders (e) — — — 5 — 5 IEIMA revenue requirement reconciliation adjustment (a)(f) — 24 24 — 68 68 FERC revenue requirement reconciliation adjustment (a)(g) — 9 10 — 7 13 VBA rider (a)(h) — 15 15 — 11 11 2017 2016 Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Other — 1 1 — — — Total current regulatory assets $ 56 $ 87 $ 144 $ 35 $ 108 $ 149 Noncurrent regulatory assets: Pension and postretirement benefit costs (i) $ 84 $ 215 $ 299 $ 175 $ 319 $ 494 Income taxes (j) 139 56 197 229 1 230 Uncertain tax positions tracker (a)(k) 5 — 5 7 — 7 ARO (l) — 1 1 — 3 3 Callaway costs (a)(m) 25 — 25 29 — 29 Unamortized loss on reacquired debt (a)(n) 61 49 110 65 59 124 Environmental cost riders (o) — 173 173 — 196 196 MTM derivative losses (d) 4 192 196 9 178 187 Storm costs (a)(p) — 10 10 — 15 15 Demand-side costs before the MEEIA implementation (a)(q) 11 — 11 18 — 18 Workers’ compensation claims (r) 5 7 12 6 7 13 Credit facilities fees (s) 3 — 3 4 — 4 Construction accounting for pollution control equipment (a)(t) 18 — 18 19 — 19 Solar rebate program (a)(u) 31 — 31 49 — 49 IEIMA revenue requirement reconciliation adjustment (a)(f) — 54 54 — 23 23 FERC revenue requirement reconciliation adjustment (a)(g) — 16 27 — 8 10 FEJA energy-efficiency riders (a)(v) — 41 41 — — — Other 9 8 17 9 7 16 Total noncurrent regulatory assets $ 395 $ 822 $ 1,230 $ 619 $ 816 $ 1,437 Current regulatory liabilities: Over-recovered FAC (b) $ 4 $ — $ 4 $ — $ — $ — Over-recovered Illinois electric power costs (c) — 16 16 — 25 25 Over-recovered PGA (c) — 1 1 — — — MTM derivative gains (d) 13 — 13 12 11 23 Energy-efficiency riders (e) 2 40 42 — — — Estimated refund for FERC complaint case (w) — 25 42 — 42 62 Other — 10 10 — — — Total current regulatory liabilities $ 19 $ 92 $ 128 $ 12 $ 78 $ 110 Noncurrent regulatory liabilities: Income taxes (j) $ 1,392 $ 842 $ 2,323 $ 33 $ 4 $ 37 Uncertain tax positions tracker (k) 2 — 2 3 — 3 Asset removal costs (x) 995 725 1,725 970 697 1,669 ARO (l) 223 — 223 162 — 162 Bad debt rider (y) — 2 2 — 3 3 Pension and postretirement benefit costs tracker (z) 35 — 35 35 — 35 Energy-efficiency riders (e) — — — — 45 45 Renewable energy credits and zero-emission credits (aa) — 58 58 — 15 15 Storm tracker (ab) 6 — 6 7 — 7 Other 11 2 13 5 4 9 Total noncurrent regulatory liabilities $ 2,664 $ 1,629 $ 4,387 $ 1,215 $ 768 $ 1,985 (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’ electric distribution service annual revenue requirement calculated under the performance-based formula ratemaking framework and the revenue requirement included in customer rates for that year. Any under-recovery or over-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. Any 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 10 – Retirement Benefits for additional information. (j) The regulatory assets represent deferred income taxes that will be recovered from customers related to the equity component of allowance for funds used during construction and the effects of tax rate changes from the TCJA and the increased income tax rate in Illinois. The regulatory liabilities represent deferred income taxes that will be refunded to customers related to depreciation differences, other tax liabilities, and the unamortized portion of investment tax credits recorded at rates in excess of current statutory rates. Amounts associated with the equity component of allowance for funds used during construction, depreciation differences, and the unamortized portion of investment tax credits will be amortized over the expected life of the related assets. The amortization period for the effects of tax rate changes from the TCJA and the increased income tax rate in Illinois and the other tax liabilities will be determined in future rate orders by the applicable regulators. See Note 12 – Income Taxes for amounts related to the revaluation of deferred income taxes under the TCJA. (k) The tracker is amortized over three years, beginning from the date the amounts are included in rates. See Note 12 – 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 14 – 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. The MoPSC March 2017 electric rate order modified certain amortization periods for these costs. Costs incurred from May 2008 through September 2008, and from January 2010 through July 2012, are being amortized over a two-year period that began in April 2017. Costs incurred from October 2008 through December 2009 are no longer being amortized as of April 2017, and a new amortization period for these costs will be determined in a future regulatory rate review. Costs incurred from August 2012 through December 2012 are being amortized over a six-year period that began in June 2015. (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 in 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. Costs incurred from 2010 to 2014 are being amortized over a two-year period that began in April 2017 as modified per the MoPSC March 2017 electric rate order. Costs incurred from 2015 to 2016 are being amortized over a three-year period that began in April 2017. (v) Electric energy-efficiency program investments deferred under the FEJA. These investments will earn a return at Ameren Illinois’ 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 investments are being amortized over their weighted-average useful lives beginning in the period in which they were made. (w) Estimated refunds to transmission customers related to the February 2015 FERC Complaint Case discussed above. (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 2015 was refunded to customers from June 2016 through May 2017. The over-recovery relating to 2016 is being refunded to customers from June 2017 through May 2018. The over-recovery relating to 2017 will be refunded to customers from June 2018 through May 2019. (z) A regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri and the level of such costs included in customer rates. For costs incurred prior to August 2012, the amounts are being amortized over a two-year period that began in April 2017 as modified per the MoPSC’s March 2017 electric rate order. For costs incurred between August 2012 and December 2014, the MoPSC’s May 2015 electric rate order directed the amortization period to occur over a five-year period that began in June 2015. For costs incurred between January 2012 and December 2016, the MoPSC’s March 2017 electric rate order directed the amortization period to occur over a five-year period that began in April 2017. For costs incurred after December 2016, the amortization period will be determined in a future electric regulatory rate review. (aa) Funds collected from customers and alternative retail electric suppliers for the purchase of renewable energy credits and zero-emission credits through IPA procurements. The balance will be amortized as the credits are purchased. (ab) A regulatory trackin

Property And Plant, Net

Property And Plant, Net	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 : Ameren Missouri (a) Ameren Illinois Other Ameren (a) 2017 Property, plant, and equipment at original cost: (b) Electric generation $ 11,132 $ — $ — $ 11,132 Electric distribution 5,766 5,649 — 11,415 Electric transmission 1,201 2,298 1,167 4,666 Natural gas 474 2,419 — 2,893 Other (c) 922 757 242 1,921 19,495 11,123 1,409 32,027 Less: Accumulated depreciation and amortization 8,305 3,082 246 11,633 11,190 8,041 1,163 20,394 Construction work in progress: Nuclear fuel in process 148 — — 148 Other 413 252 259 924 Property, plant, and equipment, net $ 11,751 $ 8,293 $ 1,422 $ 21,466 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, plant, and equipment, net $ 11,478 $ 7,469 $ 1,166 $ 20,113 (a) Amounts in Ameren and Ameren Missouri include two CTs under separate capital lease agreements. The gross cumulative asset value of those agreements was $233 million and $232 million at December 31, 2017 and 2016, respectively. The total accumulated depreciation associated with the two CTs was $83 million and $77 million at December 31, 2017 and 2016 , respectively. See Note 5 – Long-term Debt and Equity Financings for additional information on these capital lease agreements. (b) The estimated lives for each asset group are as follows: 5 to 72 years for electric generation, excluding Ameren Missouri’s hydro generating assets which have useful lives of up to 150 years, 20 to 80 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 electric and natural gas services. Capitalized software costs are classified within “Property, Plant, and Equipment, Net” on the balance sheet and are amortized on a straight-line basis over the expected period of benefit, ranging from 5 to 10 years. The following table presents the gross carrying value of capitalized software, the related accumulated amortization, and the amortization expense of capitalized software by year: Amortization Expense (a) Gross Carrying Value Accumulated Amortization 2017 2016 2015 2017 2016 2017 2016 Ameren $ 58 $ 52 $ 47 $ 655 $ 622 $ (466 ) $ (408 ) Ameren Missouri 20 17 16 191 178 (107 ) (87 ) Ameren Illinois 36 33 27 241 225 (146 ) (110 ) (a) As of December 31, 2017, the estimated amortization expense of capitalized software for each of the five succeeding years is not expected to differ materially from the current year expense. The following table provides accrued capital and nuclear fuel expenditures at December 31, 2017 , 2016 , and 2015 , which represent noncash investing activity excluded from the accompanying statements of cash flows: Ameren (a) Ameren Missouri Ameren Illinois Accrued capital expenditures: 2017 $ 361 $ 159 $ 175 2016 251 116 87 2015 235 85 92 Accrued nuclear fuel expenditures: 2017 10 10 (b) 2016 20 20 (b) 2015 16 16 (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, 2017
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 affiliate borrowings. Credit Agreements The Credit Agreements provide $2.1 billion of credit cumulatively through maturity 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 a group of 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 (parent) or any other subsidiary of Ameren. The following table presents the maximum aggregate amount available to each borrower under each facility: Missouri Credit Agreement Illinois Credit Agreement Ameren (parent) $ 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 (parent) borrowings are due and payable no later than the maturity date of the Credit Agreements. 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 affiliate loan arrangements. 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, subject to borrowing sublimits. As of December 31, 2017 , 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 $1.6 billion . Ameren, Ameren Missouri, and Ameren Illinois did not borrow under the Credit Agreements for the years ended December 31, 2017 and 2016 . 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, 2017 and 2016 : Ameren (parent) Ameren Missouri Ameren Illinois Ameren Consolidated 2017 Average daily commercial paper outstanding $ 573 $ 5 $ 90 $ 668 Outstanding borrowings at period-end 383 39 62 484 Weighted-average interest rate 1.30 % 1.24 % 1.35 % 1.31 % Peak outstanding commercial paper during period (a) $ 841 $ 64 $ 469 $ 948 Peak interest rate 1.90 % 1.78 % 2.00 % 2.00 % 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 % (a) The timing of peak outstanding commercial paper issuances varies by company. Therefore, the sum of the peak amounts presented by the companies may 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 obtaining 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, 2017 , the ratios of consolidated indebtedness to total consolidated capitalization, calculated in accordance with the provisions of the Credit Agreements, were 53% , 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 (parent) 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 (parent) under either credit agreement that results solely from a default of Ameren Missouri or Ameren Illinois does not result in a cross-default of Ameren (parent) under the other credit agreement. Further, the Credit Agreements default provisions provide that an Ameren (parent) default under either 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, 2017 . 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 (parent) 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 it 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, 2017 , was 1.19% ( 2016 – 0.52% ). See Note 13 – 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, 2017 , 2016 , and 2015 .

Long-Term Debt And Equity Finan

Long-Term Debt And Equity Financings	12 Months Ended
	Dec. 31, 2017
Long-Term Debt And Equity Financings [Abstract]
LONG-TERM DEBT AND EQUITY FINANCINGS	2.0 4.8 $ 4,222 > 2.5 95.4 $ 2,118 Ameren Illinois > 2.0 7.1 4,119 (d) > 1.5 2.9 203 (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 $1,629 million and $529 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, 2017 , using the FERC-agreed upon calculation method, Ameren Illinois’ ratio of common stock equity to total capitalization was 51% . ATXI’s note purchase agreement includes financial covenants that require ATXI not to permit at any time (1) debt to exceed 70% of total capitalization or (2) secured debt to exceed 10% of total assets. The note purchase agreement also contains restrictive covenants that, among other things, restrict the ability of ATXI to (1) enter into certain transactions with affiliates; (2) consolidate, merge, transfer or lease all or substantially all of its assets; and (3) create liens. At December 31, 2017, the Ameren Companies were in compliance with the provisions and covenants contained in their indentures and articles of incorporation, as applicable, and ATXI was in compliance with the provisions and covenants contained in its note purchase agreement. 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, 2017 , none of the Ameren Companies had any significant 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." 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, 2017 and 2016 : 2017 2016 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 (4 ) (6 ) Long-term debt, net $ 696 $ 694 Ameren Missouri: Bonds and notes: 6.40% Senior secured notes due 2017 (a) $ — $ 425 6.00% Senior secured notes due 2018 (a)(b) 179 179 5.10% Senior secured notes due 2018 (a) 199 199 6.70% Senior secured notes due 2019 (a)(b) 329 329 5.10% Senior secured notes due 2019 (a) 244 244 5.00% Senior secured notes due 2020 (a) 85 85 1992 Series bonds due 2022 (c)(d) 47 47 3.50% Senior secured notes due 2024 (a) 350 350 2.95% Senior secured notes due 2027 (a) 400 — 5.45% First mortgage bonds due 2028 (e) (e) (e) 1998 Series A bonds due 2033 (c)(d) 60 60 1998 Series B bonds due 2033 (c)(d) 50 50 1998 Series C bonds due 2033 (c)(d) 50 50 5.50% Senior secured notes due 2034 (a) 184 184 5.30% Senior secured notes due 2037 (a) 300 300 8.45% Senior secured notes due 2039 (a)(b) 350 350 3.90% Senior secured notes due 2042 (a)(b) 485 485 3.65% Senior secured notes due 2045 (a) 400 400 Capital lease obligations: City of Bowling Green capital lease (Peno Creek CT) due 2022 (f) 36 42 Audrain County capital lease (Audrain County CT) due 2023 (f) 240 240 Total long-term debt, gross 3,988 4,019 Less: Unamortized discount and premium (7 ) (6 ) Less: Unamortized debt issuance costs (20 ) (19 ) Less: Maturities due within one year (384 ) (431 ) Long-term debt, net $ 3,577 $ 3,563 2017 2016 Ameren Illinois: Bonds and notes: 6.125% Senior secured notes due 2017 (g)(h) $ — $ 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 5.90% First mortgage bonds due 2023 (i) (i) (i) 5.70% First mortgage bonds due 2024 (j) (j) (j) 3.25% Senior secured notes due 2025 (g) 300 300 6.125% Senior secured notes due 2028 (g) 60 60 1993 Series B-1 Senior unsecured notes due 2028 (d)(k) 17 17 6.70% Senior secured notes due 2036 (g) 61 61 6.70% Senior secured notes due 2036 (l) 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) 490 490 3.70% First mortgage bonds due 2047 (m) 500 — Total long-term debt, gross 2,857 2,607 Less: Unamortized discount and premium (3 ) — Less: Unamortized debt issuance costs (24 ) (19 ) Less: Maturities due within one year (457 ) (250 ) Long-term debt, net $ 2,373 $ 2,338 ATXI: 3.43% Senior notes due 2050 (n) $ 450 $ — Total long-term debt, gross 450 — Less: Unamortized debt issuance costs (2 ) — Long-term debt, net $ 448 $ — Ameren consolidated long-term debt, net $ 7,094 $ 6,595 (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 6.00% senior secured notes due 2018, 6.70% senior secured notes due 2019, 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 2017 and 2016 were as follows: 2017 2016 Ameren Missouri 1992 Series due 2022 1.43% 0.66% Ameren Missouri 1998 Series A due 2033 1.77% 0.91% Ameren Missouri 1998 Series B due 2033 1.75% 0.92% Ameren Missouri 1998 Series C due 2033 1.73% 0.97% Ameren Illinois 1993 Series B-1 due 2028 1.08% 0.70% (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) Payments due to the lessor under these capital lease obligations are paid to a trustee, which is authorized to utilize the cash only to pay equal amounts due to Ameren Missouri under related bonds issued by the lessor and held by Ameren Missouri. The timing and amounts of payments due from Ameren Missouri under the capital lease agreements are equal to the timing and amount of bond service payments due to Ameren Missouri, resulting in no net cash flow. The balance of both the capital lease obligations and the related investments in debt securities, recorded in "Other Assets," was $276 million and $282 million , respectively, as of December 31, 2017 and 2016. (g) These notes are collaterally secured by first 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 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 maturity date of these senior secured notes and the 3.70% first mortgage bonds due 2047, we do not expect the mortgage bond lien protection associated with these notes to fall away. (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 and 6.25% senior secured notes due 2018 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 first 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. (l) These notes are collaterally secured by first mortgage bonds issued by Ameren Illinois under its 1933 mortgage indenture. They are secured by substantially all property of the former CILCO. The notes have a fall-away lien provision, and Ameren Illinois could cause these notes to become unsecured at any time by redeeming the 5.90% first mortgage bonds due 2023 (of which less than $1 million principal amount remains outstanding). (m) These bonds are first mortgage bonds issued by Ameren Illinois under its 1992 mortgage indenture. They are secured by substantially all property of the former IP and CIPS. (n) The following table presents the principal maturities schedule for the 3.43% senior notes due 2050: Payment Date Principal Payment August 2022 $ 49.5 August 2024 49.5 August 2027 49.5 August 2030 49.5 August 2032 49.5 August 2038 49.5 August 2043 76.5 August 2050 76.5 Total $ 450.0 The following table presents the aggregate maturities of long-term debt, including current maturities, for the Ameren Companies at December 31, 2017 : Ameren (parent) (a) Ameren Missouri (a) Ameren Illinois (a) ATXI (a) Ameren Consolidated 2018 $ — $ 384 $ 457 $ — $ 841 2019 — 581 — — 581 2020 350 92 — — 442 2021 — 8 — — 8 2022 — 56 400 50 506 Thereafter 350 2,867 2,000 400 5,617 Total $ 700 $ 3,988 $ 2,857 $ 450 $ 7,995 (a) Excludes unamortized discount, unamortized premium, and debt issuance costs of $4 million , $27 million , $27 million and $2 million at Ameren (parent), Ameren Missouri, Ameren Illinois and ATXI, 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 is 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, 2017 and 2016 : Redemption Price (per share) 2017 2016 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 December 2017, Ameren, Ameren Missouri, and Ameren Illinois filed a Form S-3 shelf registration statement with the SEC, registering the issuance of an indeterminate amount of certain types of securities. The registration statement became effective immediately upon filing and expires in December 2020. Ameren filed a Form S-3 registration statement with the SEC in May 2017, authorizing the offering of 6 million additional shares of its common stock under DRPlus, which expires in 2020. 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, 2017 and 2016, 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 2015 through 2017, Ameren shares for its DRPlus and its 401(k) plans were purchased in the open market. Ameren Missouri In June 2017, Ameren Missouri issued $400 million of 2.95% senior secured notes due June 2027, with interest payable semiannually on June 15 and December 15 of each year, beginning December 15, 2017. Ameren Missouri received proceeds of $396 million , which were used, in conjunction with other available funds, to repay at maturity $425 million of Ameren Missouri’s 6.40% senior secured notes in June 2017. 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, Ameren Missouri issued $150 million 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. Ameren Missouri received proceeds of $148 million from the June 2016 issuance, which was 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, refer to Capital Contributions in Note 13 – Related-party Transactions. Ameren Illinois In November 2017, Ameren Illinois issued $500 million of 3.70% first mortgage bonds due December 2047, with interest payable semiannually on June 1 and December 1 of each year, beginning June 1, 2018. Ameren Illinois received proceeds of $492 million , which were used to repay outstanding short-term debt, including short-term debt that Ameren Illinois incurred in connection with the repayment of $250 million of its 6.125% senior secured notes that matured in November 2017. 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, Ameren Illinois issued $240 million of 4.15% senior secured notes due in March 2046, with interest payable semiannually in March and September, beginning in March 2017. Ameren Illinois received proceeds of $245 million from the issuance, which was used to repay a portion of its short-term debt. For information on Ameren Illinois’ capital contributions, refer to Capital Contributions in Note 13 – Related-party Transactions. ATXI In June 2017, pursuant to a note purchase agreement, ATXI agreed to issue $450 million principal amount of 3.43% senior unsecured notes, due 2050, with interest payable semiannually on the last day of February and August of each year, beginning February 28, 2018, through a private placement offering exempt from registration under the Securities Act of 1933, as amended. ATXI issued $150 million principal amount of the notes in June 2017 and the remaining $300 million principal amount of the notes in August 2017. ATXI received proceeds of $449 million from the notes, which were used by ATXI to repay existing short-term and long-term affiliate debt. ATXI may prepay at any time not less than 5% of the principal amount of notes then outstanding at 100% of the principal amount plus a make-whole premium. In the event of a change of control, as defined in the agreement, each holder of notes may require ATXI to prepay the entire unpaid principal amount of the notes held by such holder at a price equal to 100% of the principal amount of such notes together with accrued and unpaid interest thereon. 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, 2017 , 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 4.8 $ 4,222 > 2.5 95.4 $ 2,118 Ameren Illinois > 2.0 7.1 4,119 (d) > 1.5 2.9 203 (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 $1,629 million and $529 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, 2017 , using the FERC-agreed upon calculation method, Ameren Illinois’ ratio of common stock equity to total capitalization was 51% . ATXI’s note purchase agreement includes financial covenants that require ATXI not to permit at any time (1) debt to exceed 70% of total capitalization or (2) secured debt to exceed 10% of total assets. The note purchase agreement also contains restrictive covenants that, among other things, restrict the ability of ATXI to (1) enter into certain transactions with affiliates; (2) consolidate, merge, transfer or lease all or substantially all of its assets; and (3) create liens. At December 31, 2017, the Ameren Companies were in compliance with the provisions and covenants contained in their indentures and articles of incorporation, as applicable, and ATXI was in compliance with the provisions and covenants contained in its note purchase agreement. 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, 2017 , none of the Ameren Companies had any significant 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.

Other Income And Expenses

Other Income And Expenses	12 Months Ended
	Dec. 31, 2017
Other Nonoperating Income (Expense) [Abstract]
OTHER INCOME AND EXPENSES	OTHER INCOME AND EXPENSES The following table presents the components of “Other Income and Expenses” in the Ameren Companies’ statements of income for the years ended December 31, 2017 , 2016 , and 2015 : 2017 2016 2015 Ameren: (a) Miscellaneous income: Allowance for equity funds used during construction $ 24 $ 27 $ 30 Interest income on industrial development revenue bonds 26 27 27 Interest income (b) 8 13 14 Other 1 7 3 Total miscellaneous income $ 59 $ 74 $ 74 Miscellaneous expense: Donations $ 8 $ 16 $ 15 Other 13 16 15 Total miscellaneous expense $ 21 $ 32 $ 30 Ameren Missouri: Miscellaneous income: Allowance for equity funds used during construction $ 21 $ 23 $ 22 Interest income on industrial development revenue bonds 26 27 27 Interest income 1 1 1 Other — 1 2 Total miscellaneous income $ 48 $ 52 $ 52 Miscellaneous expense: Donations $ 2 $ 4 $ 5 Other 6 6 6 Total miscellaneous expense $ 8 $ 10 $ 11 Ameren Illinois: Miscellaneous income: Allowance for equity funds used during construction $ 3 $ 4 $ 8 Interest income (b) 7 12 12 Other 1 5 1 Total miscellaneous income $ 11 $ 21 $ 21 Miscellaneous expense: Donations $ 5 $ 6 $ 5 Other 5 6 7 Total miscellaneous expense $ 10 $ 12 $ 12 (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.

Derivative Financial Instrument

Derivative Financial Instruments	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 . As of December 31, 2017 , these contracts extended through October 2019, March 2023, May 2032, and September 2021 for fuel oils, natural gas, power, and uranium, respectively. Quantity (in millions, except as indicated) 2017 2016 Commodity Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils (in gallons) (a) 28 (b) 28 30 (b) 30 Natural gas (in mmbtu) 24 139 163 25 129 154 Power (in megawatthours) 3 9 12 1 9 10 Uranium (pounds in thousands) 370 (b) 370 345 (b) 345 (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, 2017 and 2016 , 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, 2017 and 2016 : Balance Sheet Location Ameren Missouri Ameren Illinois Ameren 2017 Fuel oils Other current assets $ 5 $ — $ 5 Other assets 2 — 2 Natural gas Other assets 1 — 1 Power Other current assets 9 — 9 Total assets (a) $ 17 $ — $ 17 Natural gas Other current liabilities 5 12 17 Other deferred credits and liabilities 3 10 13 Power Other current liabilities 1 13 14 Other deferred credits and liabilities — 182 182 Uranium Other deferred credits and liabilities — (b) — — (b) Total liabilities (c) $ 9 $ 217 $ 226 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 Other current liabilities 1 3 4 Other deferred credits and liabilities 5 5 10 Power Other current liabilities 3 12 15 Other deferred credits and liabilities — 173 173 Uranium Other deferred credits and liabilities 4 — 4 Total liabilities (c) $ 18 $ 193 $ 211 (a) The cumulative amount of pretax net gains on all derivative instruments is deferred as a regulatory liability. (b) Beginning in 2017, as a result of rulebook amendments at the Chicago Mercantile Exchange, the fair value of uranium derivative liabilities are offset by certain settlement payments made to the exchange previously characterized as collateral and included within “Other assets” on Ameren’s and Ameren Missouri’s balance sheet. (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. 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, 2017 and 2016 . Concentrations of Credit Risk In determining our concentrations of credit risk related to derivative instruments, we review our individual counterparties and categorize each counterparty into groupings according to the primary business in which each engages. We calculate maximum exposures based on the gross fair value of financial instruments, including NPNS and other accrual contracts. These exposures are calculated on a gross basis, which include affiliate exposure not eliminated at the consolidated Ameren level. As of December 31, 2017 , 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, 2017 , 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, 2017 , 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) 2017 Ameren Missouri $ 55 $ 3 $ 44 Ameren Illinois 43 — 38 Ameren $ 98 $ 3 $ 82 (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, 2017
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 exchange closing prices or 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, 2017 and 2016 : Fair Value Weighted Assets Liabilities Valuation Technique(s) Unobservable Input Range Average Level 3 Derivative asset and liability – commodity contracts (a) : 2017 Fuel oils $ 3 $ — Option model Volatilities(%) (b) 20 – 26 22 Discounted cash flow Counterparty credit risk(%) (c)(d) 0.12 – 0.72 0.41 Ameren Missouri credit risk(%) (c)(d) 0.37 (e) Natural Gas 1 (4 ) Option model Volatilities(%) (b) 26 – 46 37 Nodal basis($/mmbtu) (c) (0.50) – (0.30) (0.40) Discounted cash flow Nodal basis($/mmbtu) (b) (1.20) – 0.10 (1) Counterparty credit risk(%) (c)(d) 0.37 – 0.92 0.53 Ameren credit risk(%) (c)(d) 0.37 (e) Power (f) 8 (196 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps($/MWh) (g) 24 – 46 28 Estimated auction price for FTRs($/MW) (b) (65) – 1,823 251 Nodal basis($/MWh) (g) (10) – 0 (2) Counterparty credit risk(%) (c)(d) 0.28 (e) Ameren Illinois credit risk(%) (c)(d) 0.37 (e) Fundamental energy production model Estimated future natural gas prices($/mmbtu) (b) 3 – 4 3 Escalation rate(%) (b)(h) 5 (e) Contract price allocation Estimated renewable energy credit costs($/credit) (b) 5 – 7 6 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)(i) (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 (f) 9 (187 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps($/MWh) (g) 26 – 44 29 Estimated auction price for FTRs($/MW) (b) (71) – 5,270 125 Nodal basis($/MWh) (g) (6) – 0 (2) Ameren Illinois credit risk(%) (c)(d) 0.38 (e) Fundamental energy production model Estimated future natural gas prices($/mmbtu) (b) 3 – 4 3 Escalation rate(%) (b)(h) 5 (e) Contract price allocation Estimated renewable energy credit costs($/credit) (b) 5 – 7 6 Uranium — (4 ) Option model Volatilities(%) (b) 24 (e) Fair Value Weighted Assets Liabilities Valuation Technique(s) Unobservable Input Range Average Discounted cash flow Average forward uranium pricing($/pound) (b) 22 – 24 22 Ameren Missouri credit risk(%) (c)(d) 0.38 (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) Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2021. Valuations beyond 2021 use fundamentally modeled pricing by month for peak and off-peak demand. (g) Ameren Missouri and Ameren Illinois power contracts respond differently to unobservable input changes because of their opposing positions. (h) Escalation rate applies to power prices in 2031 and beyond. (i) Escalation rate applies to fuel oil prices in 2019 and beyond. 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 2017, 2016, or 2015. At December 31, 2017 and 2016 , 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, 2017 : 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 $ 4 $ — $ 3 $ 7 Natural gas — — 1 1 Power — 1 8 9 Total derivative assets – commodity contracts $ 4 $ 1 $ 12 $ 17 Nuclear decommissioning trust fund: Cash and cash equivalents $ 2 $ — $ — $ 2 Equity securities: U.S. large capitalization 468 — — 468 Debt securities: U.S. Treasury and agency securities — 125 — 125 Corporate bonds — 82 — 82 Other — 25 — 25 Total nuclear decommissioning trust fund $ 470 $ 232 $ — $ 702 (b) Total Ameren $ 474 $ 233 $ 12 $ 719 Ameren Missouri Derivative assets – commodity contracts (a) : Fuel oils $ 4 $ — $ 3 $ 7 Natural gas — — 1 1 Power — 1 8 9 Total derivative assets – commodity contracts $ 4 $ 1 $ 12 $ 17 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 $ 2 $ — $ — $ 2 Equity securities: U.S. large capitalization 468 — — 468 Debt securities: U.S. Treasury and agency securities — 125 — 125 Corporate bonds — 82 — 82 Other — 25 — 25 Total nuclear decommissioning trust fund $ 470 $ 232 $ — $ 702 (b) Total Ameren Missouri $ 474 $ 233 $ 12 $ 719 Liabilities: Ameren Derivative liabilities – commodity contracts (a) : Natural gas 1 25 4 30 Power — — 196 196 Total Ameren $ 1 $ 25 $ 200 $ 226 Ameren Missouri Derivative liabilities – commodity contracts (a) : Natural gas — 7 1 8 Power — — 1 1 Total Ameren Missouri $ — $ 7 $ 2 $ 9 Ameren Illinois Derivative liabilities – commodity contracts (a) : Natural gas $ 1 $ 18 $ 3 $ 22 Power — — 195 195 Total Ameren Illinois $ 1 $ 18 $ 198 $ 217 (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, 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 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 $ 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) : 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. All costs related to financial assets and liabilities 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, 2017 and 2016 , 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, 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 ) For the year ended December 31, 2017 Beginning balance at January 1, 2017 $ 7 $ (185 ) $ (178 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (4 ) (21 ) (25 ) Purchases 14 — 14 Sales 1 — 1 Settlements (11 ) 11 — Ending balance at December 31, 2017 $ 7 $ (195 ) $ (188 ) Change in unrealized gains (losses) related to assets/liabilities held at December 31, 2017 $ — $ (22 ) $ (22 ) 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, 2017 and 2016 , 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 10 – Retirement Benefits for the fair value hierarchy tables detailing Ameren’s pension and postretirement plan assets as of December 31, 2017 , as well as a table summarizing the changes in Level 3 plan assets during 2017 . The Ameren Companies’ carrying amounts of cash and cash equivalents, accounts receivable, unbilled revenue, accounts payable, and other current financial instruments 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. Ameren and Ameren Illinois have company-owned life insurance that is recorded in “Other Assets” on the respective balance sheet and measured at net asset value. These investments do not consider the observability of inputs; therefore, they are not included within the fair value hierarchy. As of December 31, 2017 and 2016, the net asset value of Ameren (parent)’s company-owned life insurance was $136 million and $123 million, respectively. As of December 31, 2017 and 2016, the net asset value of Ameren Illinois’ company owned life insurance was $9 million and $8 million, respectively. 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, 2017 and 2016 : 2017 2016 Carrying Amount Fair Value Carrying Amount Fair Value Ameren: Long-term debt and capital lease obligations (including current portion) (a) $ 7,935 $ 8,531 $ 7,276 $ 7,772 Preferred stock (b) 142 131 142 131 Ameren Missouri: Long-term debt and capital lease obligations (including current portion) (a) $ 3,961 $ 4,348 $ 3,994 $ 4,304 Preferred stock 80 80 80 79 Ameren Illinois: Long-term debt (including current portion) $ 2,830 $ 3,028 $ 2,588 $ 2,765 Preferred stock 62 51 62 52 (a) Ameren and Ameren Missouri have two CTs under separate capital lease agreements. The capital lease obligations as of December 31, 2017 and 2016, were $276 million and $282 million , respectively. In addition, Ameren and Ameren Missouri have investments in debt securities, classified as held-to-maturity and recorded in “Other Assets” that are related to the capital lease obligation CTs from the city of Bowling Green and Audrain County. As of December 31, 2017 and 2016, the fair value of these investments approximate carrying value of $276 million and $282 million , respectively. (b) Preferred stock is recorded in “Noncontrolling Interests” on the consolidated balance sheet.

Callaway Energy Center

Callaway Energy Center	12 Months Ended
	Dec. 31, 2017
Nuclear Waste Matters [Abstract]
CALLAWAY ENERGY CENTER	Spent Nuclear Fuel Under the NWPA, the DOE is responsible for disposing of spent nuclear fuel from the Callaway energy center and other commercial nuclear energy centers. The NWPA established the fee paid by Ameren Missouri and other utilities that own and operate those energy centers to the federal government for disposing of the spent nuclear fuel at one mill, (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 $3 million , $24 million , and $14 million in 2017, 2016, and 2015, respectively. Ameren Missouri will continue to apply for reimbursement from the DOE for allowable costs associated with the ongoing storage of spent fuel. Supplier of Fuel Assemblies The Callaway energy center uses nuclear fuel assemblies fabricated by Westinghouse, which is the only NRC-licensed supplier authorized to provide fuel assemblies to the Callaway energy center. During the first quarter of 2017, Westinghouse filed voluntary petitions for a court-supervised restructuring process under Chapter 11 of the United States Bankruptcy Code. Westinghouse could petition the bankruptcy court to reject Ameren Missouri’s contracts as part of the restructuring process. If the bankruptcy court agrees, this could result in Ameren Missouri not having access to the fuel assemblies necessary to refuel the Callaway energy center in future scheduled refueling and maintenance outages. At this time, Ameren and Ameren Missouri believe the restructuring proceeding will not affect Westinghouse’s performance under the terms of its existing contracts with Ameren Missouri, and therefore do not expect any material impact to Ameren Missouri’s operations. However, Ameren and Ameren Missouri could incur material unexpected costs as a result of the Westinghouse bankruptcy, such as the loss of fuel inventory that is stored at Westinghouse’s facility and the cost of replacement power if nuclear fuel assemblies were not available for a future scheduled refueling and maintenance outage. A change of fuel suppliers or a change in the type of fuel assembly design that is currently licensed for use at the Callaway energy center could take an estimated three years of analysis and NRC licensing efforts to implement. Decommissioning Electric 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 eventually 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. An updated cost study and funding analysis was filed with the MoPSC in September 2017 and reflected within the ARO. In January 2018, the MoPSC approved no change in electric rates for decommissioning costs based on Ameren Missouri’s updated cost study and funding analysis. 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. 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, 2017 and 2016 . 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, 2017 , 2016 , and 2015 : 2017 2016 2015 Proceeds from sales and maturities $ 396 $ 377 $ 349 Gross realized gains 13 7 8 Gross realized losses 5 4 2 Net realized and unrealized gains and losses are deferred and are currently reflected in the 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, 2017 and 2016 : Security Type Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value 2017 Debt securities $ 228 $ 5 $ 1 $ 232 Equity securities 155 318 5 468 Cash and cash equivalents 2 — — 2 Other (a) 2 — — 2 Total $ 387 $ 323 $ 6 $ 704 2016 Debt securities $ 197 $ 3 $ 4 $ 196 Equity securities 161 253 6 408 Cash and cash equivalents 1 — — 1 Other (a) 2 — — 2 Total $ 361 $ 256 $ 10 $ 607 (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, 2017 : Cost Fair Value Less than 5 years $ 120 $ 120 5 years to 10 years 54 55 Due after 10 years 54 57 Total $ 228 $ 232 There are unrealized losses relating to certain available-for-sale investments included in the nuclear decommissioning trust fund, deferred within the regulatory liability as discussed above. Decommissioning will not occur until Ameren Missouri’s nuclear energy center is retired. The Callaway energy center’s operating license expires in 2044. Insurance The following table presents insurance coverage at Ameren Missouri’s Callaway energy center at December 31, 2017 . 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 for Single Incidents Public liability and nuclear worker liability: American Nuclear Insurers $ 450 $ — Pool participation 12,986 (a) 127 (b) $ 13,436 (c) $ 127 Property damage: NEIL and EMANI $ 3,200 (d) $ 30 (e) Replacement power: NEIL $ 490 (f) $ 7 (e) (a) Provided through mandatory participation in an industrywide retrospective premium assessment program. (b) Retrospective premium under the Price-Anderson Act. This is subject to retrospective assessment with respect to a covered loss in excess of $450 million in the event of an incident at any licensed United States commercial reactor, payable at $19 million per year. (c) Limit of liability for each incident under the Price-Anderson liability provisions of the Atomic Energy Act of 1954, as amended. This limit is subject to change to account for the effects of inflation and changes in the number of licensed reactors. (d) NEIL provides $2.7 billion in property damage, stabilization, decontamination, and premature decommissioning insurance for radiation events and $2.3 billion in property damage insurance for nonradiation events. EMANI provides $490 million in property damage insurance for both radiation and nonradiation events. (e) All NEIL insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL. (f) Provides replacement power cost insurance in the event of a prolonged accidental outage. Weekly indemnity up to $4.5 million for 52 weeks, which commences after the first 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 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 subject to industrywide aggregates. Terrorist acts against one or more commercial nuclear power plants insured by NEIL or EMANI within a stated time period would be treated as a single event, and the owners of the nuclear power plants would share one full limit of liability. NEIL policies have an aggregate limit of $3.2 billion within a 12-month period for radiation events, or $1.8 billion for events not involving radiation contamination. The EMANI policies have an aggregate limit of €600 million for radiation and nonradiation events within a period of 72 hours. If losses from a nuclear incident at the Callaway energy center exceed the insurance limits, or are not covered by insurance, or if coverage is unavailable, Ameren Missouri is at risk for any uninsured losses. If a serious nuclear incident were to occur, it could have a material adverse effect on Ameren’s and Ameren Missouri’s results of operations, financial position, or liquidity.

Retirement Benefits

Retirement Benefits	12 Months Ended
	Dec. 31, 2017
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 $551 million and $774 million as of December 31, 2017 and 2016 , respectively. These net liabilities are recorded in “Other current liabilities” and “Pension and other postretirement benefits” on Ameren’s consolidated balance sheet. The decrease in the unfunded obligation during 2017 was the result of a larger-than-expected increase in the return on plan assets of the pension and postretirement trusts, offset by 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 decrease in the unfunded obligation also resulted in a decrease to “Regulatory assets” on Ameren’s, Ameren Missouri’s, and Ameren Illinois’ balance sheets. The following table presents the net benefit liability recorded on the balance sheets of each of the Ameren Companies as of December 31, 2017 and 2016 : 2017 2016 Ameren (a) $ 551 $ 774 Ameren Missouri 215 293 Ameren Illinois (b) 213 315 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries. (b) Other postretirement benefit liability is recorded in “Other assets” on the balance sheet. 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, 2017 and 2016 . It also provides the amounts included in regulatory assets and accumulated OCI at December 31, 2017 and 2016 , that have not been recognized in net periodic benefit costs. 2017 2016 Pension Benefits (a) Postretirement Benefits (a) Pension Benefits (a) Postretirement Benefits (a) Accumulated benefit obligation at end of year $ 4,577 $ (b) $ 4,288 $ (b) Change in benefit obligation: Net benefit obligation at beginning of year $ 4,518 $ 1,170 $ 4,197 $ 1,094 Service cost 93 21 81 19 Interest cost 179 47 185 50 Participant contributions — 8 — 8 Actuarial loss 255 53 265 52 Benefits paid (218 ) (59 ) (210 ) (54 ) Federal subsidy on benefits paid (b) — (b) 1 Net benefit obligation at end of year 4,827 1,240 4,518 1,170 Change in plan assets: Fair value of plan assets at beginning of year 3,813 1,101 3,653 1,071 Actual return on plan assets 634 171 313 73 Employer contributions 64 2 57 2 Federal subsidy on benefits paid (b) — (b) 1 Participant contributions — 8 — 8 Benefits paid (218 ) (59 ) (210 ) (54 ) Fair value of plan assets at end of year 4,293 1,223 3,813 1,101 Funded status – deficiency 534 17 705 69 Accrued benefit cost at December 31 $ 534 $ 17 $ 705 $ 69 Amounts recognized in the balance sheet consist of: Current liability (c) 3 3 3 2 Noncurrent liability 531 14 702 67 Net liability recognized $ 534 $ 17 $ 705 $ 69 Amounts recognized in regulatory assets consist of: Net actuarial (gain) loss $ 374 $ (69 ) $ 535 $ (29 ) Prior service credit (3 ) (3 ) (4 ) (8 ) Amounts (pretax) recognized in accumulated OCI consist of: Net actuarial loss 30 2 43 — Prior service credit — — — (1 ) Total $ 401 $ (70 ) $ 574 $ (38 ) (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries. (b) Not applicable. (c) 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, 2017 and 2016 : Pension Benefits Postretirement Benefits 2017 2016 2017 2016 Discount rate at measurement date 3.50 % 4.00 % 3.50 % 4.00 % Increase in future compensation 3.50 3.50 3.50 3.50 Medical cost trend rate (initial) (a) (b) (b) 5.00 5.00 Medical cost trend rate (ultimate) (a) (b) (b) 5.00 5.00 (a) Initial and ultimate medical cost trend rate for certain Medicare-eligible participants is 3.00% . (b) 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 600 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 2017, Ameren adopted the Society of Actuaries 2017 Mortality Improvement Scale. The updated scale assumes a lower rate of mortality improvement as compared to the 2016 Mortality Improvement Scale that Ameren used in 2016, 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, 2017 , its investment performance in 2017, and its pension funding policy, Ameren expects to make annual contributions of less than $1 million to $60 million in each of the next five years, with aggregate estimated contributions of $120 million . Ameren Missouri and Ameren Illinois expect their 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 2017 , 2016 , and 2015 : Pension Benefits Postretirement Benefits 2017 2016 2015 2017 2016 2015 Ameren Missouri $ 19 $ 21 $ 47 $ 1 $ 1 $ 8 Ameren Illinois 37 30 45 1 1 8 Other 8 6 19 — — 2 Ameren 64 57 111 2 2 18 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 2018 . No plan assets are expected to be returned to Ameren during 2018 . 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 2018 and our pension and postretirement plans’ asset categories as of December 31, 2017 and 2016 : Asset Category Target Allocation 2018 Percentage of Plan Assets at December 31, 2017 2016 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 % 9 % International and emerging markets 9% – 19% 14 % 14 % Total equity 51% – 61% 57 % 57 % Debt securities 35% – 45% 37 % 37 % Real estate 0% – 9% 5 % 5 % Private equity 0% – 5% (a) (a) Total 100 % 100 % Postretirement Plans: Cash and cash equivalents 0% – 7% 2 % 3 % Equity securities: U.S. large-capitalization 34% – 44% 41 % 40 % U.S. small- and mid-capitalization 2% – 12% 8 % 7 % International and emerging markets 9% – 19% 14 % 14 % Total equity 55% – 65% 63 % 61 % Debt securities 33% – 43% 35 % 36 % 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, 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, 2017 . 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 measurement date or, if that is not a business day, on the last business day before that date. Securities traded in over-the-counter markets are valued by 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 is based on NAV; it is 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 plans’ assets measured at fair value as of December 31, 2017 : 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 Total Cash and cash equivalents $ — $ — $ — $ 25 $ 25 Equity securities: U.S. large-capitalization — — — 1,523 1,523 U.S. small- and mid-capitalization 379 — — — 379 International and emerging markets 179 — — 450 629 Debt securities: Corporate bonds — 726 — 15 741 Municipal bonds — 91 — — 91 U.S. Treasury and agency securities 8 816 — — 824 Other — 7 — — 7 Real estate — — — 196 196 Private equity — — — 4 4 Total $ 566 $ 1,640 $ — $ 2,213 $ 4,419 Less: Medical benefit assets at December 31 (a) (153 ) Plus: Net receivables at December 31 (b) 27 Fair value of pension plans’ assets at December 31 $ 4,293 (a) 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. (b) 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 plans’ assets measured at fair value as of December 31, 2016 : 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 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 (a) (132 ) Plus: Net receivables at December 31 (b) 22 Fair value of pension plans’ assets at December 31 $ 3,813 (a) 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. (b) 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, 2017 : 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 Total Cash and cash equivalents $ 44 $ — $ — $ — $ 44 Equity securities: U.S. large-capitalization 332 — — 110 442 U.S. small- and mid-capitalization 80 — — — 80 International and emerging markets 53 — — 101 154 Other — 8 — — 8 Debt securities: Corporate bonds — 144 — — 144 Municipal bonds — 110 — — 110 U.S. Treasury and agency securities — 76 — — 76 Other — 4 — 34 38 Total $ 509 $ 342 $ — $ 245 $ 1,096 Plus: Medical benefit assets at December 31 (a) 153 Less: Net payables at December 31 (b) (26 ) Fair value of postretirement benefit plans’ assets at December 31 $ 1,223 (a) 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. (b) 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, 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 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 and emerging markets 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 (a) 132 Less: Net payables at December 31 (b) (25 ) Fair value of postretirement benefit plans’ assets at December 31 $ 1,101 (a) 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. (b) Payables related to pending security purchases, offset by 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 2017 , 2016 , and 2015 : Pension Benefits Postretirement Benefits 2017 Service cost $ 93 $ 21 Interest cost 179 47 Expected return on plan assets (262 ) (75 ) Amortization of: Prior service credit (1 ) (5 ) Actuarial (gain) loss 55 (6 ) Net periodic benefit cost (income) $ 64 $ (18 ) 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 (income) $ 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 Curtailment gain 1 — Net periodic benefit cost $ 92 $ 4 The estimated amounts that will be amortized from regulatory assets and accumulated OCI into Ameren’s net periodic benefit cost in 2018 are as follows: Pension Benefits (a) Postretirement Benefits (a) Regulatory assets: Prior service credit $ (1 ) $ (2 ) Net actuarial (gain) loss 60 (1 ) Accumulated OCI: Net actuarial loss 5 — Total $ 64 $ (3 ) (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 for the years ended December 31, 2017 , 2016 , and 2015 : Pension Costs Postretirement Costs 2017 2016 2015 2017 2016 2015 Ameren Missouri (a) $ 24 $ 26 $ 54 $ (4 ) $ (5 ) $ 8 Ameren Illinois 41 22 38 (14 ) (13 ) (3 ) Other (1 ) (4 ) — — (1 ) (1 ) Ameren 64 44 92 (18 ) (19 ) 4 (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 and the level of such costs included in customer rates. The expected pension and postretirement benefit payments from qualified trust and company funds, which reflect expected future service, as of December 31, 2017 , 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 2018 $ 255 $ 3 $ 57 $ 2 2019 261 3 59 2 2020 266 3 62 2 2021 277 3 64 2 2022 280 3 65 2 2023 – 2027 1,421 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, 2017 , 2016 , and 2015 : Pension Benefits Postretirement Benefits 2017 2016 2015 2017 2016 2015 Discount rate at measurement date 4.00 % 4.50 % 4.00 % 4.00 % 4.50 % 4.00 % Expected return on plan assets 7.00 7.00 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) (b) (b) (b) 5.00 5.00 5.00 Medical cost trend rate (ultimate) (a) (b) (b) (b) 5.00 5.00 5.00 (a) Initial and ultimate medical cost trend rate for certain Medicare-eligible participants is 3.00% . (b) 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 ) $ 157 $ — $ 44 0.25% increase in salary scale 2 15 — — 1.00% increase in annual medical trend — — 4 71 1.00% decrease in annual medical trend — — (4 ) (71 ) Other Ameren sponsors a 401(k) plan for eligible employees. The Ameren 401(k) plan covered all eligible employees at December 31, 2017 . 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, 2017 , 2016 , and 2015 : 2017 2016 2015 Ameren Missouri $ 16 $ 16 $ 16 Ameren Illinois 13 12 12 Other 1 1 1 Ameren 30 29 29

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2017
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 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, 2017, there were 4.9 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 extends beyond the three-year performance period to the payout date. The following table summarizes the nonvested performance share unit activity for the year ended December 31, 2017 : Performance Share Units Share Units Weighted-average Grant Date Fair Value per Share Unit Nonvested at January 1, 2017 (a) 780,545 $ 47.54 Granted (b) 508,161 59.16 Forfeitures (50,523 ) 52.50 Undistributed vested units (c) (342,694 ) 51.65 Nonvested at December 31, 2017 (a) 895,489 $ 52.28 (a) Excludes 369,878 and 712,572 performance share units granted to retirement-eligible employees as of January 1, 2017 and December 31, 2017, respectively, as the undistributed performance share units are fully vested. (b) Includes performance share units granted to certain executive and nonexecutive officers and other eligible employees in 2017 under the 2014 Incentive Plan. (c) Includes performance 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, 2017, 2016, and 2015: 2017 2016 2015 Ameren Missouri $ 4 $ 4 $ 5 Ameren Illinois 2 2 3 Other (a) 12 11 11 Ameren 18 17 19 Less income tax benefit 7 6 7 Stock-based compensation expense, net $ 11 $ 11 $ 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 $39 million , $83 million , and $27 million for the years ended December 31, 2017 , 2016 , and 2015 . There were no significant compensation costs capitalized related to the performance share units during the years ended December 31, 2017 , 2016 , and 2015 . As of December 31, 2017 , total compensation cost of $29 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 under the 2014 Incentive Plan is based on Ameren’s closing common share price at December 31st of the year prior to the award year and lattice simulations. Lattice simulations are used to estimate expected share payout based on Ameren’s total shareholder return for a 3-year performance period relative to the designated peer group beginning January 1st of the award year. 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 include a three-year risk-free rate, volatility for the peer group, and Ameren’s attainment of a three-year average earnings per share threshold during the performance period. The following table presents the fair value of each share unit awarded under the 2014 Incentive Plan along with the significant assumptions used to calculate the fair value of each share unit for the years ended December 31, 2017 , 2016 , and 2015 : 2017 2016 2015 Fair value of share units awarded $59.16 $44.13 $52.88 Ameren’s closing common share price at December 31 of the prior year $52.46 $43.23 $46.13 Three-year risk free rate 1.47% 1.31% 1.10% Volatility range 15% - 21% 15% - 20% 12% - 18%

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
INCOME TAXES	INCOME TAXES Federal Tax Reform The TCJA was enacted on December 22, 2017. Substantially all of the provisions of the TCJA affecting the Ameren Companies, other than certain transition depreciation rules, are effective for taxable years beginning after December 31, 2017. The TCJA includes significant changes to the Internal Revenue Code, including amendments that significantly change the taxation of business entities and specific provisions related to regulated public utilities. The most significant change that affects the Ameren Companies is the reduction in the federal corporate statutory income tax rate from 35% to 21% . Specific provisions related to regulated public utilities generally allow for the continued deductibility of interest expense, the elimination of accelerated depreciation tax benefits from certain regulated utility capital investments acquired after September 27, 2017, and the continuation of certain rate normalization requirements related to the flow back of excess deferred taxes. Ameren (parent) will be subject to provisions of the TCJA that limit the deductibility of interest expense. In accordance with GAAP, the tax effects of changes in tax laws must be recognized in the period in which the law is enacted. GAAP also requires deferred tax assets and liabilities to be measured at the tax rate that is expected to apply when temporary differences are realized or settled. Thus, in December 2017, the Ameren Companies’ deferred taxes were revalued using the new tax rate. To the extent deferred tax balances are included in rate base, the revaluation of deferred taxes was deferred as a regulatory asset or liability on the balance sheet and will be collected from or refunded to customers. For deferred tax balances not included in rate base, the revaluation of deferred taxes was recorded as income tax expense. As a result of the complexity of the TCJA, the SEC staff issued guidance to clarify the accounting for income taxes if information is not yet available or complete. This guidance provides for up to a one year period in which to complete the required analysis and update provisional estimates. The guidance provides three scenarios associated with a company’s status of accounting for income tax reform: (1) a company has completed its accounting for certain effects of tax reform, (2) a company is able to make a reasonable estimate for certain effects of tax reform and records that estimate as a provisional amount, or (3) a company is not able to make a reasonable estimate and therefore continues to apply income tax accounting that is based on the tax laws in effect immediately prior to the enactment of the TCJA. As of December 31, 2017, the Ameren Companies have made reasonable estimates for the measurement and accounting of certain effects of the TCJA, which have been reflected in their financial statements. We have recorded provisional estimates primarily related to depreciation transition rules and 2017 property, plant, and equipment, compensation, and pension-related deductions which would impact our revaluation of deferred taxes at December 31, 2017. These items may be resolved through additional analysis, which is incomplete due to the timing of the enactment of the TCJA and complexity associated with applying its provisions. Additionally, interpretations, regulations, amendments, and technical corrections of the TCJA by various regulators could also resolve provisional items. The TCJA had the following provisional effects for the year ended December 31, 2017: Ameren Missouri Ameren Illinois Other Ameren Increase (Decrease) Accumulated deferred income taxes, net $ (1,419 ) $ (871 ) $ 37 $ (2,253 ) Income tax expense (benefit) (a) 32 (5 ) 127 154 Noncurrent regulatory assets (89 ) (24 ) (1 ) (114 ) Noncurrent regulatory liabilities 1,362 842 89 2,293 For our regulated operations, reductions in accumulated deferred income tax balances due to the reduction in the federal statutory corporate income tax rate to 21% will result in amounts previously collected from utility customers for these deferred taxes being refundable to those customers, generally through reductions in future rates. The TCJA includes provisions related to the IRS normalization rules that address the time period in which certain plant-related components of the excess deferred taxes are to be reflected in customer rates. This time period for the Ameren Companies is approximately 35 to 60 years. Other components of the excess deferred taxes will be reflected in customer rates as determined by our state and federal regulators, which could be a shorter time period than that applicable to certain plant-related components. See Note 2 – Rate and Regulatory Matters for information regarding the various proceedings for the TCJA impacts with our regulators. Illinois Income Tax Rate In July 2017, Illinois enacted a law that increased the state’s corporate income tax rate from 7.75% to 9.5% as of July 1, 2017. The law made the increase in the state’s corporate income tax rate permanent. That rate was previously scheduled to go to 7.3% in 2025. In July 2017, Ameren recorded an expense of $14 million at Ameren (parent) due to the revaluation of accumulated deferred taxes and the estimated state apportionment of such taxes. Beyond this expense, Ameren does not expect this tax increase to have a material impact on its consolidated net income prospectively. The tax increase is not expected to materially impact the earnings of the Ameren Illinois Electric Distribution, the Ameren Transmission, or the Ameren Illinois Transmission segments, since these businesses operate under formula ratemaking frameworks. The tax increase unfavorably affected the 2017 net income of the Ameren Illinois Natural Gas segment by less than $1 million . In addition, in the third quarter of 2017, Ameren’s and Ameren Illinois’ accumulated deferred tax balances were revalued using the state’s new corporate income tax rate, which resulted in a net increase to the liability balances of $97 million and $79 million , respectively. These increased liabilities were offset by a regulatory asset, as well as income tax expense, as discussed above. The following table presents the principal reasons for the difference between the effective income tax rate and the federal statutory corporate income tax rate for the years ended December 31, 2017 , 2016 , and 2015 : Ameren Missouri Ameren Illinois Ameren 2017 Federal statutory corporate income tax rate: 35 % 35 % 35 % Increases (decreases) from: Depreciation differences 1 (1 ) — Amortization of deferred investment tax credit (1 ) — (1 ) State tax 4 6 6 TCJA 6 (1 ) 14 Tax credits (1 ) — — Other permanent items — (1 ) (2 ) Effective income tax rate 44 % 38 % 52 % 2016 Federal statutory corporate 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 Federal statutory corporate 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 % (a) Reflects the adoption of authoritative accounting guidance related to share-based compensation, which resulted in the recognition of a $21 million income tax benefit in 2016. The following table presents the components of income tax expense for the years ended December 31, 2017 , 2016 , and 2015 : Ameren Missouri Ameren Illinois Other Ameren 2017 Current taxes: Federal $ 149 $ (34 ) $ (110 ) $ 5 State 23 29 (20 ) 32 Deferred taxes: Federal 76 185 250 511 State 11 (13 ) 36 34 Amortization of deferred investment tax credits (5 ) (1 ) — (6 ) Total income tax expense $ 254 $ 166 $ 156 $ 576 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 The following table presents the accumulated deferred income tax assets and liabilities recorded as a result of temporary differences at December 31, 2017 and 2016 : Ameren Missouri Ameren Illinois Other Ameren 2017 Accumulated deferred income taxes, net liability (asset): Plant related $ 2,064 $ 1,264 $ 146 $ 3,474 Regulatory assets and liabilities, net (317 ) (206 ) (24 ) (547 ) Deferred employee benefit costs (53 ) (17 ) (61 ) (131 ) Revenue requirement reconciliation adjustments — 20 — 20 Tax carryforwards (31 ) (43 ) (287 ) (361 ) Other (13 ) 3 61 51 Total net accumulated deferred income tax liabilities (assets) $ 1,650 $ 1,021 $ (165 ) $ 2,506 2016 Accumulated deferred income taxes, net liability (asset): Plant related $ 3,103 $ 1,769 $ 147 $ 5,019 Regulatory assets and liabilities, 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 The following table presents the components of accumulated deferred income tax assets relating to net operating loss carryforwards, tax credit carryforwards, and charitable contribution carryforwards at December 31, 2017 and 2016 : Ameren Missouri Ameren Illinois Other Ameren 2017 Net operating loss carryforwards: Federal (a) $ — $ 41 $ 162 $ 203 State (a) — — 32 32 Total net operating loss carryforwards $ — $ 41 $ 194 $ 235 Tax credit carryforwards: Federal (b) $ 31 $ 2 $ 80 $ 113 State (c) — — 7 7 Total tax credit carryforwards $ 31 $ 2 $ 87 $ 120 Charitable contribution carryforwards (d) $ — $ — $ 11 $ 11 Valuation allowance (e) — — (5 ) (5 ) Total charitable contribution carryforwards $ — $ — $ 6 $ 6 2016 Net operating loss carryforwards: Federal $ 33 $ 137 $ 324 $ 494 State 4 — 41 45 Total net operating loss carryforwards $ 37 $ 137 $ 365 $ 539 Tax credit carryforwards: Federal $ 29 $ 1 $ 79 $ 109 State — — 21 21 Total tax credit carryforwards $ 29 $ 1 $ 100 $ 130 Charitable contribution carryforwards $ — $ — $ 18 $ 18 Valuation allowance — — (11 ) (11 ) Total charitable contribution carryforwards $ — $ — $ 7 $ 7 (a) Will expire between 2033 and 2036 . Any net operating loss carryforward generated after January 1, 2018, will not have an expiration date as a result of the TCJA. (b) Will expire between 2029 and 2037 . (c) Will expire between 2019 and 2022 . (d) Will expire between 2018 and 2021 . (e) 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, 2017 and 2016, the Ameren Companies did not record any 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 affected discontinued operations. See Note 1 – Summary of Significant Accounting Policies for additional information. 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. It will then will be amortized over three years, beginning on the effective date of new rates established in the next electric regulatory rate review.

Related Party Transactions

Related Party Transactions	12 Months Ended
	Dec. 31, 2017
Related Party Transactions [Abstract]
RELATED PARTY TRANSACTIONS	RELATED-PARTY TRANSACTIONS In the normal course of business, Ameren Missouri and Ameren Illinois 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 Ameren’s subsidiaries are reported as affiliate transactions on their individual financial statements, but those transactions are eliminated in consolidation for Ameren’s consolidated 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 $3 million for the 12 months ended May 31, 2015. 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 performance period, price, and amount of megawatthours included in each agreement: IPA Procurement Event Performance 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 April 2017 March 2019 – May 2020 85,600 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, 2017 and 2016 , 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 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. As of December 31, 2017 and 2016, Ameren Missouri had income taxes payable to Ameren (parent) of $11 million and $16 million , respectively, included in “Accounts payable - affiliates” on its balance sheet. As of December 31, 2017 and 2016, Ameren Illinois had income taxes payable to Ameren (parent) of $17 million and $3 million , respectively, included in “Accounts payable - affiliates” on its balance sheet. See below for capital contributions received related to the tax allocation agreement. Capital Contributions In 2017, Ameren Missouri received cash capital contributions of $30 million from Ameren (parent) as a result of the tax allocation agreement. In 2017, Ameren Illinois received cash capital contributions of $8 million from Ameren (parent). 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 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). The following table presents the impact on Ameren Missouri and Ameren Illinois of related-party transactions for the years ended December 31, 2017 , 2016 , and 2015 . 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 2017 $ 23 $ (a) with Ameren Illinois 2016 28 (a) 2015 15 (a) Ameren Missouri and Ameren Illinois Operating Revenues 2017 26 4 rent and facility services 2016 25 5 2015 25 4 Ameren Missouri and Ameren Illinois Operating Revenues 2017 (b) 1 miscellaneous support services 2016 1 (b) 2015 2 (b) Total Operating Revenues 2017 $ 49 $ 5 2016 54 5 2015 42 4 Ameren Illinois power supply Purchased Power 2017 $ (a) $ 23 agreements with Ameren Missouri 2016 (a) 28 2015 (a) 15 Ameren Illinois transmission Purchased Power 2017 (a) 2 services from ATXI 2016 (a) 2 2015 (a) 2 Total Purchased Power 2017 $ (a) $ 25 2016 (a) 30 2015 (a) 17 Ameren Services support services Other Operations and 2017 $ 149 $ 139 agreement Maintenance 2016 129 123 2015 131 119 Money pool borrowings (advances) (Interest Charges) 2017 $ 1 $ (b) Miscellaneous Income 2016 (b) (b) 2015 (b) (b) (a) Not applicable. (b) Amount less than $1 million.

Commitments And Contingencies

Commitments And Contingencies	12 Months Ended
	Dec. 31, 2017
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 9 – Callaway Energy Center, and Note 13 – Related-party Transactions in this report. 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, 2017 : 2018 2019 2020 2021 2022 After 5 Years Total Ameren: (a) Minimum capital lease payments (b)(c) $ 32 $ 32 $ 32 $ 33 $ 32 $ 264 $ 425 Less amount representing interest 26 25 25 25 24 24 149 Present value of minimum capital lease payments $ 6 $ 7 $ 7 $ 8 $ 8 $ 240 $ 276 Operating leases 10 9 8 6 6 14 53 Total lease obligations $ 16 $ 16 $ 15 $ 14 $ 14 $ 254 $ 329 Ameren Missouri: Minimum capital lease payments (b)(c) $ 32 $ 32 $ 32 $ 33 $ 32 $ 264 $ 425 Less amount representing interest 26 25 25 25 24 24 149 Present value of minimum capital lease payments $ 6 $ 7 $ 7 $ 8 $ 8 $ 240 $ 276 Operating leases 8 8 7 6 6 14 49 Total lease obligations $ 14 $ 15 $ 14 $ 14 $ 14 $ 254 $ 325 Ameren Illinois: Operating leases $ 1 (d) (d) (d) (d) $ 1 $ 2 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations. (b) See Note 3 – Property, Plant, and Equipment, Net for additional information. (c) See Note 5 – Long-term Debt and Equity Financings for additional information on Ameren’s and Ameren Missouri’s capital lease agreements. (d) Less than $1 million. The following table presents total operating lease expenses included in “Operating Expenses” in the statement of income for the years ended December 31, 2017 , 2016 , and 2015 : 2017 2016 2015 Ameren (a) $ 11 $ 38 $ 36 Ameren Missouri 10 34 34 Ameren Illinois 1 30 28 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations. Other Obligations To supply a portion of the fuel requirements of Ameren Missouri’s energy centers, Ameren Missouri has entered into various long-term commitments for the procurement of coal, natural gas, nuclear fuel, and methane gas. Ameren Missouri and Ameren Illinois also have entered into various long-term commitments for purchased power and natural gas for distribution. The table below presents our estimated minimum fuel, purchased power, and other commitments at December 31, 2017 . Ameren’s and Ameren Illinois’ purchased power commitments include the Ameren Illinois 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, among other agreements, at December 31, 2017 . Coal Natural Gas (a) Nuclear Fuel Purchased Power (b)(c) Methane Gas Other Total Ameren: (d) 2018 $ 463 $ 205 $ 67 $ 170 $ 3 $ 73 $ 981 2019 383 163 26 63 4 37 676 2020 85 110 39 14 4 36 288 2021 27 46 45 3 5 25 151 2022 — 11 12 2 5 25 55 Thereafter — 38 45 18 58 95 254 Total $ 958 $ 573 $ 234 $ 270 $ 79 $ 291 $ 2,405 Ameren Missouri: 2018 $ 463 $ 42 $ 67 $ — $ 3 $ 53 $ 628 2019 383 36 26 — 4 24 473 2020 85 29 39 — 4 24 181 2021 27 13 45 — 5 25 115 2022 — 6 12 — 5 25 48 Thereafter — 16 45 — 58 75 194 Total $ 958 $ 142 $ 234 $ — $ 79 $ 226 $ 1,639 Ameren Illinois: 2018 $ — $ 163 $ — $ 170 $ — $ 19 $ 352 2019 — 127 — 63 — 13 203 2020 — 81 — 14 — 12 107 2021 — 33 — 3 — — 36 2022 — 5 — 2 — — 7 Thereafter — 22 — 18 — — 40 Total $ — $ 431 $ — $ 270 $ — $ 44 $ 745 (a) Includes amounts for generation and for distribution. (b) The purchased power amounts for Ameren and Ameren Illinois exclude agreements for renewable energy credits through 2032 with various renewable energy suppliers due to the contingent nature of the payment amounts. (c) The purchased power amounts for Ameren and Ameren Missouri exclude a 102-megawatt power purchase agreement with a wind farm operator, which expires in 2024, due to the contingent nature of the payment amounts. (d) 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 obligations with respect to 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, 2017, Ameren Missouri’s fossil fuel-fired energy centers represented 17% and 33% of Ameren’s and Ameren Missouri’s rate base, respectively. Regulations that apply to air emissions from the electric utility industry include the NSPS, the CSAPR, the MATS, and the revised National Ambient Air Quality Standards, which are subject to periodic review for certain pollutants. Collectively, these regulations cover a variety of pollutants, such as SO 2 , particulate matter, NO x, mercury, toxic metals, and acid gases, and CO 2 emissions from new power plants. Water intake and discharges from power plants are regulated under the Clean Water Act. Such regulation could require modifications to water intake structures or more stringent limitations on wastewater discharges at Ameren Missouri’s energy centers, either of which could result in significant capital expenditures. The management and disposal of coal ash is regulated under the CCR rule, which will require the closure of surface impoundments and the installations of dry ash handling systems at several of Ameren Missouri’s energy centers. The individual or combined effects of existing environmental regulations could result in significant capital expenditures, increased operating costs, or the closure or alteration of operations at some of Ameren Missouri’s energy centers. Ameren and Ameren Missouri expect that such compliance costs would be recoverable through rates, subject to MoPSC prudence review, but the timing of costs and their recovery could be subject to regulatory lag. Ameren Missouri’s current plan for compliance with existing air emission regulations 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 $325 million to $425 million from 2018 through 2022 in order to comply with existing environmental regulations. Additional environmental controls beyond 2022 could be required. This estimate of capital expenditures includes expenditures required by the CCR regulations, by the Clean Water Act rule applicable to cooling water intake structures at existing power plants, and by effluent limitation guidelines applicable to steam electric generating units, all of which are 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 whether the EPA will substantially revise regulatory obligations, exactly which compliance strategies will be used and their ultimate cost, among other things. The following sections describe the more significant environmental laws and rules and environmental enforcement and remediation matters that affect or could affect our operations. The EPA has initiated an administrative review of several regulations and rulemaking activities, including the effluent limitation guidelines and the CCR rule, which could ultimately result in the revision of all or part of such rules. 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, became 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 did not 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 the Clean Power Plan, which would have established CO 2 emissions standards applicable to existing power plants. The United States Supreme Court stayed the rule in February 2016, pending various legal challenges. In October 2017, the EPA announced a proposal to repeal the Clean Power Plan. In December 2017, the EPA issued an advanced notice of proposed rulemaking to solicit input from stakeholders as to how the EPA should regulate CO 2 emissions from existing power plants under the Clean Air Act. Accordingly, we no longer expect the Clean Power Plan to take effect. However, the EPA may issue new requirements that would regulate CO 2 emissions from existing power plants. We cannot predict the outcome of the EPA’s future rulemaking or the outcome of any legal challenges relating to such future rulemakings, any of which could have an adverse effect on our results of operations, financial position, and liquidity. 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 then proceeded to the second phase to determine the actions required to remedy the violations found in the liability phase. The EPA previously withdrew all claims for penalties and fines. No date has been set by the district court for a trial on the remedy phase of the litigation. At the conclusion of both phases of the litigation, Ameren Missouri intends to appeal the liability ruling to the United States Court of Appeals for the Eighth Circuit. 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. All of Ameren Missouri’s coal-fired and nuclear energy centers are subject to the cooling water intake structures rule. The rule will be implemented during the permit renewal process of each energy center’s water discharge permit, between 2018 and 2023. Additionally, in 2015, the EPA issued a rule to revise the effluent limitation guidelines applicable to steam electric generating units. These guidelines established 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 water discharges from power plants. In September 2017, the EPA published a rule that postponed the compliance dates by two years for the limitations applicable to two specific waste streams so that it could potentially revise those standards. Both the intake and effluent rules, if implemented as enacted, could have an adverse effect on Ameren’s and Ameren Missouri’s results of operations, financial position, and liquidity should such implementation require extensive modifications to the cooling water systems and water discharge systems at Ameren Missouri’s energy centers, and if such investments are not recovered on a timely basis in electric rates charged to Ameren Missouri’s customers. CCR 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. In September 2017, the EPA granted petitions filed on behalf of coal-fired electricity generators in which the EPA agreed to reconsider certain provisions of the CCR rules. Ameren and Ameren Missouri have AROs of $150 million recorded on their respective balance sheets as of December 31, 2017, associated with CCR storage facilities that reflect the regulations issued in 2015. Ameren plans to close these CCR storage facilities between 2018 and 2024. Ameren Missouri also estimates it will need to make capital expenditures of $300 million to $350 million from 2018 through 2022 to implement its CCR management 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 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, 2017 , 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 such remediation and related litigation costs 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, 2017 , Ameren Illinois estimated the obligation related to these former MGP sites at $175 million to $249 million . Ameren and Ameren Illinois recorded a liability of $175 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 and timing of completion 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. In 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, 2017 and 2016, Ameren Missouri estimated its obligation related to Sauget Area 2 at $1 million to $2.5 million . Ameren Missouri recorded a liability of $1 million to represent its estimated minimum obligation for this site, as no other amount within the range was a better estimate. 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. In December 2017, the court issued a final order approving a settlement agreement between Ameren Missouri and the municipalities. The settlement agreement requires Ameren Missouri to make payments representing certain tax receipts to the municipalities during the first quarter of 2018, in addition to payment of certain future gross receipts taxes. The future gross receipts taxes are recoverable from customers. Ameren and Ameren Missouri recorded immaterial current liabilities on their respective balance sheets as of December 31, 2017 , to represent the payments made in February 2018 under the settlement agreement.

Segment Information

Segment Information	12 Months Ended
	Dec. 31, 2017
Segment Reporting Information, Profit (Loss) [Abstract]
SEGMENT INFORMATION	SEGMENT INFORMATION Ameren 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 . 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, Ameren Illinois, and ATXI. Segment operating revenues and a majority of operating expenses are directly recognized and incurred by Ameren Illinois to each Ameren Illinois segment. Common operating expenses, miscellaneous income and expenses, 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 and wholesale customers. 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 in Ameren Transmission’s and Ameren Illinois Transmission’s operating revenues. An intersegment elimination at Ameren and Ameren Illinois occurs to eliminate these transmission revenues and expenses. The following tables present revenues, net income attributable to common shareholders, and capital expenditures by segment at Ameren and Ameren Illinois for the years ended December 31, 2017 , 2016 , and 2015 . 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 2017 External revenues $ 3,490 $ 1,565 $ 742 $ 382 $ (2 ) $ — $ 6,177 Intersegment revenues 49 4 1 44 (a) — (98 ) — Depreciation and amortization 533 239 59 60 5 — 896 Interest income 27 7 — — 11 (11 ) 34 Interest charges 207 73 36 67 (b) 19 (11 ) 391 Income taxes 254 83 36 90 113 — 576 Net income (loss) attributable to Ameren common shareholders from continuing operations 323 131 60 140 (131 ) — 523 Capital expenditures 773 476 245 644 1 (7 ) 2,132 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 (b) 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 (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 (b) 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 — 1,917 (a) Ameren Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above. (b) Ameren Transmission interest charges include an allocation of financing costs from Ameren (parent). Ameren Illinois Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Consolidated 2017 External revenues $ 1,569 $ 743 $ 216 $ — $ 2,528 Intersegment revenues — — 42 (a) (42 ) — Depreciation and amortization 239 59 43 — 341 Interest income 7 — — — 7 Interest charges 73 36 35 — 144 Income taxes 83 36 47 — 166 Net income available to common shareholder 131 60 77 — 268 Capital expenditures 476 245 355 — 1,076 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 (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, 2017
Selected Quarterly Financial Information [Abstract]
SELECTED QUARTERLY INFORMATION	Ameren 2017 2016 Quarter ended March 31 June 30 September 30 December 31 March 31 June 30 September 30 December 31 Operating revenues $ 1,514 $ 1,538 $ 1,723 $ 1,402 $ 1,434 $ 1,427 $ 1,859 $ 1,356 Operating income 254 398 581 225 220 325 691 145 Net income (loss) 104 194 290 (59 ) (a) 107 148 371 33 Net income (loss) attributable to Ameren common shareholders $ 102 $ 193 $ 288 $ (60 ) $ 105 $ 147 $ 369 $ 32 Earnings (loss) per common share – basic $ 0.42 $ 0.79 $ 1.19 $ (0.24 ) $ 0.43 $ 0.61 $ 1.52 $ 0.13 Earnings (loss) per common share – diluted (b) $ 0.42 $ 0.79 $ 1.18 $ (0.24 ) $ 0.43 $ 0.61 $ 1.52 $ 0.13 (a) Includes an increase to income tax expense of $154 million recorded in 2017 as a result of the TCJA. (b) 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 (Loss) Net Income (Loss) Available to Common Shareholder March 31, 2017 $ 790 $ 53 $ 6 $ 5 March 31, 2016 741 63 15 14 June 30, 2017 935 237 121 120 June 30, 2016 867 197 93 92 September 30, 2017 1,115 417 235 234 September 30, 2016 1,165 431 242 241 December 31, 2017 699 40 (36 ) (a) (36 ) December 31, 2016 750 54 10 10 (a) Includes an increase to income tax expense of $32 million recorded in 2017 as a result of the TCJA. Ameren Illinois Quarter ended (a) Operating Revenues Operating Income Net Income Net Income Available to Common Shareholder March 31, 2017 $ 703 $ 172 $ 80 $ 79 March 31, 2016 677 133 60 59 June 30, 2017 576 130 58 57 June 30, 2016 542 107 46 45 September 30, 2017 575 128 55 55 September 30, 2016 676 230 119 119 December 31, 2017 674 150 78 77 December 31, 2016 595 74 30 29 (a) In 2017, in connection with the decoupling provisions of the FEJA, Ameren Illinois changed the method it used to recognize its interim-period revenue. Ameren Illinois now recognizes revenue consistent with the timing of incurred electric distribution recoverable costs, and it recognizes revenue associated with the expected return on its rate base ratably over the year. As a result of this change in recognition of the interim period revenue for the IEIMA formula rate framework, as modified by the FEJA, Ameren Illinois incurred quarterly year-over-year increases to earnings in 2017 in comparison to 2016 for the first, second, and fourth quarters and a decrease to earnings in the third quarter. The change in interim period revenue recognition did not affect 2017 annual earnings.

Schedule I - Condensed Financia

Schedule I - Condensed Financial Information Of Parent	12 Months Ended
	Dec. 31, 2017
Condensed Financial Information of Parent Company Only Disclosure [Abstract]
Condensed Financial Information Of Parent	SCHEDULE I – CONDENSED FINANCIAL INFORMATION OF PARENT (In millions) 2017 2016 2015 Operating revenues $ — $ — $ — Operating expenses 13 14 14 Operating loss (13 ) (14 ) (14 ) Equity in earnings of subsidiaries 659 663 600 Interest income from affiliates 9 10 6 Total other expense, net — (5 ) (5 ) Interest charges 31 28 3 Income tax (benefit) 101 (27 ) 5 Net Income Attributable to Ameren Common Shareholders – Continuing Operations 523 653 579 Net Income Attributable to Ameren Common Shareholders – Discontinued Operations — — 51 Net Income Attributable to Ameren Common Shareholders $ 523 $ 653 $ 630 Net Income Attributable to Ameren Common Shareholders – Continuing Operations $ 523 $ 653 $ 579 Other Comprehensive Income, Net of Taxes: Pension and other postretirement benefit plan activity, net of income taxes (benefit) of $3, $(7), and $3, respectively 5 (20 ) 6 Comprehensive Income from Continuing Operations Attributable to Ameren Common Shareholders 528 633 585 Comprehensive Income from Discontinued Operations Attributable to Ameren Common Shareholders — — 51 Comprehensive Income Attributable to Ameren Common Shareholders $ 528 $ 633 $ 636 SCHEDULE I – CONDENSED FINANCIAL INFORMATION OF PARENT (In millions) December 31, 2017 December 31, 2016 Assets: Cash and cash equivalents $ — $ 1 Advances to money pool 13 27 Accounts receivable – affiliates 46 31 Miscellaneous accounts and notes receivable — 26 Other current assets 8 8 Total current assets 67 93 Investments in subsidiaries 7,944 7,498 Note receivable – ATXI 75 350 Accumulated deferred income taxes, net 222 419 Other assets 140 135 Total assets $ 8,448 $ 8,495 Liabilities and Shareholders’ Equity: Short-term debt 383 507 Borrowings from money pool 28 33 Accounts payable – affiliates 6 13 Other current liabilities 27 17 Total current liabilities 444 570 Long-term debt 696 694 Pension and other postretirement benefits 37 45 Other deferred credits and liabilities 87 83 Total liabilities 1,264 1,392 Commitments and Contingencies (Note 4) Shareholders’ Equity: Common stock, $.01 par value, 400.0 shares authorized – 242.6 shares outstanding 2 2 Other paid-in capital, principally premium on common stock 5,540 5,556 Retained earnings 1,660 1,568 Accumulated other comprehensive loss (18 ) (23 ) Total shareholders’ equity 7,184 7,103 Total liabilities and shareholders’ equity $ 8,448 $ 8,495 SCHEDULE I – CONDENSED FINANCIAL INFORMATION OF PARENT (In millions) 2017 2016 2015 Net cash flows provided by operating activities $ 454 $ 483 $ 551 Cash flows from investing activities: Money pool advances, net 14 (27 ) 55 Notes receivable – ATXI, net 275 (60 ) (96 ) Investments in subsidiaries (151 ) (123 ) (509 ) Other 6 2 (12 ) Net cash flows provided by (used in) investing activities 144 (208 ) (562 ) Cash flows from financing activities: Dividends on common stock (431 ) (416 ) (402 ) Short-term debt, net (124 ) 206 (284 ) Money pool borrowings, net (5 ) 19 14 Issuances of long-term debt — — 700 Debt issuance costs — — (6 ) Share-based payments (39 ) (83 ) (12 ) Net cash flows provided by (used in) financing activities (599 ) (274 ) 10 Net change in cash and cash equivalents $ (1 ) $ 1 $ (1 ) Cash and cash equivalents at beginning of year 1 — 1 Cash and cash equivalents at end of year $ — $ 1 $ — Cash dividends received from consolidated subsidiaries $ 362 $ 465 $ 575 Noncash investing activity – investments in subsidiaries — — (38 ) AMEREN CORPORATION (parent company only) NOTES TO CONDENSED FINANCIAL STATEMENTS December 31, 2017 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 13 – 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 2015, 2016, and 2017. Ameren Corporation (parent company only) had a total of $46 million in guarantees outstanding, primarily for ATXI, that were not recorded on its December 31, 2017 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 14 – 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. NOTE 6 – INCOME TAXES See Note 12 – Income Taxes under Part II, Item 8, of this report for information regarding the impacts of the TCJA on Ameren Corporation (parent company only).

Schedule II - Valuation And Qua

Schedule II - Valuation And Qualifying Accounts	12 Months Ended
	Dec. 31, 2017
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: 2017 $ 19 $ 26 $ 7 $ 33 $ 19 2016 19 32 3 35 19 2015 21 33 5 40 19 Deferred tax valuation allowance: 2017 $ 11 $ (6 ) (c) $ — $ — $ 5 2016 6 7 (2 ) — 11 2015 10 4 (8 ) — 6 Ameren Missouri: Deducted from assets – allowance for doubtful accounts: 2017 $ 7 $ 9 $ — $ 9 $ 7 2016 7 10 — 10 7 2015 8 13 — 14 7 Deferred tax valuation allowance: 2017 $ — $ — $ — $ — $ — 2016 — — — — — 2015 1 — (1 ) — — Ameren Illinois: Deducted from assets – allowance for doubtful accounts: 2017 $ 12 $ 17 $ 7 $ 24 $ 12 2016 12 22 3 25 12 2015 13 20 5 26 12 Deferred tax valuation allowance: 2017 $ — $ — $ — $ — $ — 2016 — — — — — 2015 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. (c) Includes an adjustment of $3 million to Ameren (parent)’s valuation allowance for certain deferred tax assets existing at December 31, 2017, for the reduction in the income tax rate.

Summary Of Significant Accoun29

Summary Of Significant Accounting Policies (Policy)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
General	Ameren, headquartered in St. Louis, Missouri, is a public utility holding company whose primary assets are its equity interests in its subsidiaries. Ameren’s subsidiaries are separate, independent legal entities with separate businesses, assets, and liabilities. Dividends on Ameren’s common stock and the payment of expenses by Ameren depend on distributions made to it by its subsidiaries. Ameren’s principal subsidiaries are listed below, including Ameren Missouri, Ameren Illinois, and ATXI. Ameren also has other subsidiaries that conduct other activities, such as the provision of shared services. Ameren evaluates competitive electric transmission investment opportunities as they arise. • 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, which 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 a 40,000 square mile area in central and southern Illinois. Ameren Illinois supplies electric service to 1.2 million customers and natural gas service to 0.8 million customers. •
Consolidation	Ameren’s financial statements are prepared on a consolidated basis and therefore include the accounts of its majority-owned subsidiaries. All intercompany transactions have been eliminated. Ameren Missouri and Ameren Illinois have no subsidiaries. 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. As of December 31, 2017 and December 31, 2016, Ameren had unconsolidated variable interests as a limited partner in various equity method investments totaling $17 million and $9 million , respectively, included in “Other assets” on Ameren’s consolidated balance sheet. Ameren is not the primary beneficiary of these investments because it does not have the power to direct matters that most significantly impact the activities of these variable interest entities. As of December 31, 2017, the maximum exposure to loss related to these variable interests is limited to the investment in these partnerships of $17 million plus associated outstanding funding commitments of $20 million . 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	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. 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 regulatory rate review. In Ameren Missouri’s and Ameren Illinois’ natural gas businesses, changes in natural gas costs are reflected in billings to their respective 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. Ameren Missouri has a FAC that allows an adjustment of electric rates three times per year, without a traditional rate proceeding, for a pass-through to customers of 95% of the variance in net energy costs from 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. 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. In addition to the rate-adjustment mechanisms discussed above, 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 participates in performance-based formula ratemaking frameworks established pursuant to the IEIMA and the FEJA for its electric distribution business and its electric energy-efficiency investments. Ameren Illinois also has environmental cost riders, an asbestos-related litigation rider, natural gas energy-efficiency rider, a QIP rider, a VBA rider, and a bad debt rider. 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.
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	Inventories Inventories are recorded at the lower of weighted-average cost or net realizable value. Inventories are capitalized when purchased and then expensed as consumed or capitalized as property, plant, and equipment 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 section 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 2017 , 2016 , and 2015 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	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 had goodwill of $411 million at December 31, 2017 and 2016. Ameren has four reporting units: Ameren Missouri, Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Transmission. Ameren Illinois has three reporting units: Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Illinois Transmission. 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, 2017 and 2016. The Ameren Transmission reporting unit had the same $93 million of goodwill as the Ameren Illinois Transmission reporting unit at December 31, 2017 and 2016. 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. 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 quantitative test, on an annual basis. On December 31, 2016, due to a change in reporting units, Ameren and Ameren Illinois performed a quantitative test and determined that the estimated fair value of each reporting unit significantly exceeded its respective 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, 2017. The results of Ameren’s and Ameren Illinois’ qualitative assessment indicated that it was more likely than not that the fair value of each reporting unit significantly exceeded its carrying value as of October 31, 2017, resulting in no impairment of Ameren’s or Ameren Illinois’ goodwill. The following factors, among others, were considered by Ameren and Ameren Illinois when they assessed whether it was more likely than not that the fair value of each of their reporting units exceeded its carrying value as of October 31, 2017: • macroeconomic conditions, including those conditions within Ameren Illinois’ service territory; • pending regulatory rate review outcomes and projections of future regulatory rate review outcomes; • changes in laws and potential law changes; • observable industry market multiples; • achievement of IEIMA and FEJA performance metrics and the yield of 30-year United States Treasury bonds; • an unexpected further reduction in the FERC-allowed return on equity with respect to transmission services; and • projected operating results and cash flows.
Impairment of Long-lived Assets	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 2017 and 2016 .
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 deferred as a regulatory balance. The depreciation of the asset book values at Ameren Missouri was $26 million , $31 million , and $13 million for the years ended December 31, 2017, 2016, and 2015, respectively, which was deferred as a reduction to the net regulatory liability. The depreciation deferred to the regulatory asset at Ameren Illinois was immaterial in each respective period. 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 as a regulatory liability. 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, 2017 and 2016 , 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 and the FEJA. 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 varies from the amount collected from customers, an adjustment is made to electric operating revenues with an offset to a regulatory asset or liability to reflect that year’s actual revenue requirement. The regulatory balance is then collected from, or refunded 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	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. 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 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 vesting 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 the 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 expect that regulators will reduce future revenues for deferred tax liabilities that were initially recorded at rates in excess of the current statutory rate. Therefore, reductions in certain 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. To the extent deferred tax balances are included in rate base, the revaluation of deferred taxes is recorded as a regulatory asset or liability on the balance sheet and will be collected from or refunded to customers. For deferred tax balances not included in rate base, the revaluation of deferred taxes is recorded as an adjustment to income tax expense on the income statement. See Note 12 – Income Taxes for further information regarding both the revaluation of deferred taxes related to the TCJA. 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 Ameren (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 1.6 million , 0.8 million , and 1.0 million for the years ended December 31, 2017 , 2016 , and 2015 , respectively. There were no potentially dilutive securities excluded from the diluted earnings per share calculations for the years ended December 31, 2017 , 2016 , and 2015 .
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 receive in exchange for those goods or services. The guidance 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, as well as separate presentation of alternative revenue programs on the income statement. Entities can apply the guidance to each reporting period presented (the full retrospective method), or they can record a cumulative effect adjustment to retained earnings in the period of initial adoption (the modified retrospective method). We have completed the evaluation of our contracts. Adoption of this guidance will not result in material changes to the amount or timing of revenue recognition. We will apply the guidance using the full retrospective method. We will include disaggregated revenue disclosures by segment and customer class in the combined notes to the financial statements. This guidance will be effective for the Ameren Companies for the first quarter of 2018. Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost In March 2017, the FASB issued authoritative guidance that requires an entity to report, including on a retrospective basis, the non-service cost or income components of net benefit cost separately from the service cost component and outside of operating income. Our adoption of this guidance will result in the reclassification of 2017 net benefit income of $44 million , $22 million , and $10 million , currently presented as a reduction of "Other operations and maintenance expense," on Ameren's, Ameren Missouri's, and Ameren Illinois' respective statements of income. These amounts will be presented outside of operating income. Similarly, 2016 net benefit income of $55 million , $18 million , and $24 million , currently presented as a reduction of "Other operations and maintenance expense" on Ameren's, Ameren Missouri's, and Ameren Illinois' respective statements of income, will also be reclassified and presented outside of operating income. The guidance also permits an entity to capitalize only the service cost component as part of an asset, such as inventory or property, plant, and equipment, on a prospective basis. Previously, all of the net benefit cost components were eligible for capitalization. This change in the capitalization of net benefit costs is not expected to affect our ability to recover total net benefit cost through customer rates. This guidance will be effective for the Ameren Companies in the first quarter of 2018. See Note 10 – Retirement Benefits for the components of net benefit cost. 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. Classification of Certain Cash Receipts and Cash Payments In August 2016, the FASB issued authoritative guidance that specifies the classification and presentation of certain cash flow items to reduce diversity in practice. This guidance will be effective for the Ameren Companies in the first quarter of 2018, and requires changes to be applied retrospectively. For Ameren and Ameren Illinois, the adoption of this guidance will result in the retrospective reclassification from operating activities to financing activities of $7 million of bond premiums received in 2016. 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 and to recognize changes in fair value in net income. The adoption of this guidance will not have a material impact on our results of operations or financial position. The recognition, measurement, and disclosure guidance will be effective for the Ameren Companies in the first quarter of 2018. The guidance requires changes to be applied retrospectively with a cumulative effect adjustment to retained earnings as of the adoption date. 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 using relevant information about past events, current conditions, and reasonable and supportable forecasts that affect the collectibility of the reported amount. We are currently assessing the impacts of this guidance on our results of operations, financial position, and disclosures. The credit loss guidance will be effective for the Ameren Companies in the first quarter of 2020. It requires changes to be applied retrospectively with a cumulative effect adjustment to retained earnings as of the adoption date. 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 lease 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. Other arrangements not previously accounted for as leases may be required to be accounted for as leases; these arrangements would similarly result in increases to assets and liabilities recorded. We are currently assessing our arrangements to determine those that are within the scope of this guidance. We are also assessing the impacts of this guidance for effects on our results of operations, cash flows, and disclosures. This guidance will be effective for the Ameren Companies in the first quarter of 2019. See Note 14 – Commitments and Contingencies for additional information on our leases.

Derivative Financial Instrume30

Derivative Financial Instruments Derivative Financial Instruments (Policies)	12 Months Ended
	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivatives, Policy [Policy Text Block]	• 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 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.

Summary Of Significant Accoun31

Summary Of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Schedule Of Inventories	The following table presents a breakdown of inventories for each of the Ameren Companies at December 31, 2017 and 2016 : Ameren Missouri Ameren Illinois Ameren 2017 Fuel (a) $ 154 $ — $ 154 Natural gas stored underground 8 74 82 Materials, supplies, and other 226 60 286 Total inventories $ 388 $ 134 $ 522 2016 Fuel (a) $ 172 $ — $ 172 Natural gas stored underground 9 73 82 Materials, supplies, and other 211 62 273 Total inventories $ 392 $ 135 $ 527 (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 2017 , 2016 , and 2015 : 2017 2016 2015 Ameren Missouri 7 % 7 % 7 % Ameren Illinois 4 % 5 % 6 %
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, 2017 and 2016 : Ameren Missouri Ameren Illinois Ameren Balance at December 31, 2015 $ 617 $ 6 $ 623 Liabilities incurred 3 — 3 Liabilities settled (2 ) (a) (2 ) Accretion in 2016 (b) 25 (a) 25 Change in estimates 1 — 1 Balance at December 31, 2016 $ 644 (c) $ 6 (d) $ 650 (c) Liabilities incurred — — — Liabilities settled (12 ) (1 ) (13 ) Accretion in 2017 (b) 26 (a) 26 Change in estimates (e) (18 ) (1 ) (19 ) Balance at December 31, 2017 $ 640 (c) $ 4 (d) $ 644 (c) (a) Less than $1 million. (b) Ameren Missouri’s accretion expense was deferred as a decrease to regulatory liabilities. (c) Balance included $6 million and $15 million in “Other current liabilities” on the balance sheet as of December 31, 2017 and 2016, respectively. (d) Included in “Other deferred credits and liabilities” on the balance sheet. (e) Ameren Missouri changed its fair value estimate primarily because of an extension of the remediation period of certain CCR storage facilities, an update to the decommissioning of the Callaway energy center to reflect the cost study and funding analysis filed with the MoPSC in 2017, and an increase in the assumed discount rate.
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, 2017 , 2016 , and 2015 : 2017 2016 2015 Ameren Missouri $ 153 $ 151 $ 156 Ameren Illinois 57 57 57 Ameren $ 210 $ 208 $ 213

Rate And Regulatory Matters (Ta

Rate And Regulatory Matters (Tables)	12 Months Ended
	Dec. 31, 2017
Public Utilities, General Disclosures [Abstract]
Schedule Of Regulatory Assets And Liabilities	The following table presents our regulatory assets and regulatory liabilities at December 31, 2017 and 2016 : 2017 2016 Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Current regulatory assets: Under-recovered FAC (a)(b) $ 47 $ — $ 47 $ 21 $ — $ 21 Under-recovered Illinois electric power costs (c) — — — — 3 3 Under-recovered PGA (c) 1 13 14 — 4 4 MTM derivative losses (d) 8 25 33 9 15 24 Energy-efficiency riders (e) — — — 5 — 5 IEIMA revenue requirement reconciliation adjustment (a)(f) — 24 24 — 68 68 FERC revenue requirement reconciliation adjustment (a)(g) — 9 10 — 7 13 VBA rider (a)(h) — 15 15 — 11 11 2017 2016 Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Other — 1 1 — — — Total current regulatory assets $ 56 $ 87 $ 144 $ 35 $ 108 $ 149 Noncurrent regulatory assets: Pension and postretirement benefit costs (i) $ 84 $ 215 $ 299 $ 175 $ 319 $ 494 Income taxes (j) 139 56 197 229 1 230 Uncertain tax positions tracker (a)(k) 5 — 5 7 — 7 ARO (l) — 1 1 — 3 3 Callaway costs (a)(m) 25 — 25 29 — 29 Unamortized loss on reacquired debt (a)(n) 61 49 110 65 59 124 Environmental cost riders (o) — 173 173 — 196 196 MTM derivative losses (d) 4 192 196 9 178 187 Storm costs (a)(p) — 10 10 — 15 15 Demand-side costs before the MEEIA implementation (a)(q) 11 — 11 18 — 18 Workers’ compensation claims (r) 5 7 12 6 7 13 Credit facilities fees (s) 3 — 3 4 — 4 Construction accounting for pollution control equipment (a)(t) 18 — 18 19 — 19 Solar rebate program (a)(u) 31 — 31 49 — 49 IEIMA revenue requirement reconciliation adjustment (a)(f) — 54 54 — 23 23 FERC revenue requirement reconciliation adjustment (a)(g) — 16 27 — 8 10 FEJA energy-efficiency riders (a)(v) — 41 41 — — — Other 9 8 17 9 7 16 Total noncurrent regulatory assets $ 395 $ 822 $ 1,230 $ 619 $ 816 $ 1,437 Current regulatory liabilities: Over-recovered FAC (b) $ 4 $ — $ 4 $ — $ — $ — Over-recovered Illinois electric power costs (c) — 16 16 — 25 25 Over-recovered PGA (c) — 1 1 — — — MTM derivative gains (d) 13 — 13 12 11 23 Energy-efficiency riders (e) 2 40 42 — — — Estimated refund for FERC complaint case (w) — 25 42 — 42 62 Other — 10 10 — — — Total current regulatory liabilities $ 19 $ 92 $ 128 $ 12 $ 78 $ 110 Noncurrent regulatory liabilities: Income taxes (j) $ 1,392 $ 842 $ 2,323 $ 33 $ 4 $ 37 Uncertain tax positions tracker (k) 2 — 2 3 — 3 Asset removal costs (x) 995 725 1,725 970 697 1,669 ARO (l) 223 — 223 162 — 162 Bad debt rider (y) — 2 2 — 3 3 Pension and postretirement benefit costs tracker (z) 35 — 35 35 — 35 Energy-efficiency riders (e) — — — — 45 45 Renewable energy credits and zero-emission credits (aa) — 58 58 — 15 15 Storm tracker (ab) 6 — 6 7 — 7 Other 11 2 13 5 4 9 Total noncurrent regulatory liabilities $ 2,664 $ 1,629 $ 4,387 $ 1,215 $ 768 $ 1,985 (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’ electric distribution service annual revenue requirement calculated under the performance-based formula ratemaking framework and the revenue requirement included in customer rates for that year. Any under-recovery or over-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. Any 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 10 – Retirement Benefits for additional information. (j) The regulatory assets represent deferred income taxes that will be recovered from customers related to the equity component of allowance for funds used during construction and the effects of tax rate changes from the TCJA and the increased income tax rate in Illinois. The regulatory liabilities represent deferred income taxes that will be refunded to customers related to depreciation differences, other tax liabilities, and the unamortized portion of investment tax credits recorded at rates in excess of current statutory rates. Amounts associated with the equity component of allowance for funds used during construction, depreciation differences, and the unamortized portion of investment tax credits will be amortized over the expected life of the related assets. The amortization period for the effects of tax rate changes from the TCJA and the increased income tax rate in Illinois and the other tax liabilities will be determined in future rate orders by the applicable regulators. See Note 12 – Income Taxes for amounts related to the revaluation of deferred income taxes under the TCJA. (k) The tracker is amortized over three years, beginning from the date the amounts are included in rates. See Note 12 – 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 14 – 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. The MoPSC March 2017 electric rate order modified certain amortization periods for these costs. Costs incurred from May 2008 through September 2008, and from January 2010 through July 2012, are being amortized over a two-year period that began in April 2017. Costs incurred from October 2008 through December 2009 are no longer being amortized as of April 2017, and a new amortization period for these costs will be determined in a future regulatory rate review. Costs incurred from August 2012 through December 2012 are being amortized over a six-year period that began in June 2015. (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 in 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. Costs incurred from 2010 to 2014 are being amortized over a two-year period that began in April 2017 as modified per the MoPSC March 2017 electric rate order. Costs incurred from 2015 to 2016 are being amortized over a three-year period that began in April 2017. (v) Electric energy-efficiency program investments deferred under the FEJA. These investments will earn a return at Ameren Illinois’ 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 investments are being amortized over their weighted-average useful lives beginning in the period in which they were made. (w) Estimated refunds to transmission customers related to the February 2015 FERC Complaint Case discussed above. (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 2015 was refunded to customers from June 2016 through May 2017. The over-recovery relating to 2016 is being refunded to customers from June 2017 through May 2018. The over-recovery relating to 2017 will be refunded to customers from June 2018 through May 2019. (z) A regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri and the level of such costs included in customer rates. For costs incurred prior to August 2012, the amounts are being amortized over a two-year period that began in April 2017 as modified per the MoPSC’s March 2017 electric rate order. For costs incurred between August 2012 and December 2014, the MoPSC’s May 2015 electric rate order directed the amortization period to occur over a five-year period that began in June 2015. For costs incurred between January 2012 and December 2016, the MoPSC’s March 2017 electric rate order directed the amortization period to occur over a five-year period that began in April 2017. For costs incurred after December 2016, the amortization period will be determined in a future electric regulatory rate review. (aa) Funds collected from customers and alternative retail electric suppliers for the purchase of renewable energy credits and zero-emission credits through IPA procurements. The balance will be amortized as the 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 MoPSC’s March 2017 electric rate order directed the amortization to occur over a five-year period that began in April 2017. The April 2015 MoPSC order did not approve the continued use of the storm cost regulatory tracking mechanism. A

Property And Plant, Net (Tables

Property And Plant, Net (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 : Ameren Missouri (a) Ameren Illinois Other Ameren (a) 2017 Property, plant, and equipment at original cost: (b) Electric generation $ 11,132 $ — $ — $ 11,132 Electric distribution 5,766 5,649 — 11,415 Electric transmission 1,201 2,298 1,167 4,666 Natural gas 474 2,419 — 2,893 Other (c) 922 757 242 1,921 19,495 11,123 1,409 32,027 Less: Accumulated depreciation and amortization 8,305 3,082 246 11,633 11,190 8,041 1,163 20,394 Construction work in progress: Nuclear fuel in process 148 — — 148 Other 413 252 259 924 Property, plant, and equipment, net $ 11,751 $ 8,293 $ 1,422 $ 21,466 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, plant, and equipment, net $ 11,478 $ 7,469 $ 1,166 $ 20,113 (a) Amounts in Ameren and Ameren Missouri include two CTs under separate capital lease agreements. The gross cumulative asset value of those agreements was $233 million and $232 million at December 31, 2017 and 2016, respectively. The total accumulated depreciation associated with the two CTs was $83 million and $77 million at December 31, 2017 and 2016 , respectively. See Note 5 – Long-term Debt and Equity Financings for additional information on these capital lease agreements. (b) The estimated lives for each asset group are as follows: 5 to 72 years for electric generation, excluding Ameren Missouri’s hydro generating assets which have useful lives of up to 150 years, 20 to 80 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 electric and natural gas services.
Accrued Capital Expenditures	The following table provides accrued capital and nuclear fuel expenditures at December 31, 2017 , 2016 , and 2015 , which represent noncash investing activity excluded from the accompanying statements of cash flows: Ameren (a) Ameren Missouri Ameren Illinois Accrued capital expenditures: 2017 $ 361 $ 159 $ 175 2016 251 116 87 2015 235 85 92 Accrued nuclear fuel expenditures: 2017 10 10 (b) 2016 20 20 (b) 2015 16 16 (b) (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries. (b) Not applicable.
Schedule of Capitalized Software [Table Text Block]	Capitalized software costs are classified within “Property, Plant, and Equipment, Net” on the balance sheet and are amortized on a straight-line basis over the expected period of benefit, ranging from 5 to 10 years. The following table presents the gross carrying value of capitalized software, the related accumulated amortization, and the amortization expense of capitalized software by year: Amortization Expense (a) Gross Carrying Value Accumulated Amortization 2017 2016 2015 2017 2016 2017 2016 Ameren $ 58 $ 52 $ 47 $ 655 $ 622 $ (466 ) $ (408 ) Ameren Missouri 20 17 16 191 178 (107 ) (87 ) Ameren Illinois 36 33 27 241 225 (146 ) (110 ) (a) As of December 31, 2017, the estimated amortization expense of capitalized software for each of the five succeeding years is not expected to differ materially from the current year expense.

Short-Term Debt And Liquidity (

Short-Term Debt And Liquidity (Tables)	12 Months Ended
	Dec. 31, 2017
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: Missouri Credit Agreement Illinois Credit Agreement Ameren (parent) $ 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, 2017 and 2016 : Ameren (parent) Ameren Missouri Ameren Illinois Ameren Consolidated 2017 Average daily commercial paper outstanding $ 573 $ 5 $ 90 $ 668 Outstanding borrowings at period-end 383 39 62 484 Weighted-average interest rate 1.30 % 1.24 % 1.35 % 1.31 % Peak outstanding commercial paper during period (a) $ 841 $ 64 $ 469 $ 948 Peak interest rate 1.90 % 1.78 % 2.00 % 2.00 % 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 % (a) The timing of peak outstanding commercial paper issuances varies by company. Therefore, the sum of the peak amounts presented by the companies may not equal the Ameren consolidated peak amount for the period.

Long-Term Debt And Equity Fin35

Long-Term Debt And Equity Financings (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 : 2017 2016 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 (4 ) (6 ) Long-term debt, net $ 696 $ 694 Ameren Missouri: Bonds and notes: 6.40% Senior secured notes due 2017 (a) $ — $ 425 6.00% Senior secured notes due 2018 (a)(b) 179 179 5.10% Senior secured notes due 2018 (a) 199 199 6.70% Senior secured notes due 2019 (a)(b) 329 329 5.10% Senior secured notes due 2019 (a) 244 244 5.00% Senior secured notes due 2020 (a) 85 85 1992 Series bonds due 2022 (c)(d) 47 47 3.50% Senior secured notes due 2024 (a) 350 350 2.95% Senior secured notes due 2027 (a) 400 — 5.45% First mortgage bonds due 2028 (e) (e) (e) 1998 Series A bonds due 2033 (c)(d) 60 60 1998 Series B bonds due 2033 (c)(d) 50 50 1998 Series C bonds due 2033 (c)(d) 50 50 5.50% Senior secured notes due 2034 (a) 184 184 5.30% Senior secured notes due 2037 (a) 300 300 8.45% Senior secured notes due 2039 (a)(b) 350 350 3.90% Senior secured notes due 2042 (a)(b) 485 485 3.65% Senior secured notes due 2045 (a) 400 400 Capital lease obligations: City of Bowling Green capital lease (Peno Creek CT) due 2022 (f) 36 42 Audrain County capital lease (Audrain County CT) due 2023 (f) 240 240 Total long-term debt, gross 3,988 4,019 Less: Unamortized discount and premium (7 ) (6 ) Less: Unamortized debt issuance costs (20 ) (19 ) Less: Maturities due within one year (384 ) (431 ) Long-term debt, net $ 3,577 $ 3,563 2017 2016 Ameren Illinois: Bonds and notes: 6.125% Senior secured notes due 2017 (g)(h) $ — $ 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 5.90% First mortgage bonds due 2023 (i) (i) (i) 5.70% First mortgage bonds due 2024 (j) (j) (j) 3.25% Senior secured notes due 2025 (g) 300 300 6.125% Senior secured notes due 2028 (g) 60 60 1993 Series B-1 Senior unsecured notes due 2028 (d)(k) 17 17 6.70% Senior secured notes due 2036 (g) 61 61 6.70% Senior secured notes due 2036 (l) 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) 490 490 3.70% First mortgage bonds due 2047 (m) 500 — Total long-term debt, gross 2,857 2,607 Less: Unamortized discount and premium (3 ) — Less: Unamortized debt issuance costs (24 ) (19 ) Less: Maturities due within one year (457 ) (250 ) Long-term debt, net $ 2,373 $ 2,338 ATXI: 3.43% Senior notes due 2050 (n) $ 450 $ — Total long-term debt, gross 450 — Less: Unamortized debt issuance costs (2 ) — Long-term debt, net $ 448 $ — Ameren consolidated long-term debt, net $ 7,094 $ 6,595 (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 6.00% senior secured notes due 2018, 6.70% senior secured notes due 2019, 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 2017 and 2016 were as follows: 2017 2016 Ameren Missouri 1992 Series due 2022 1.43% 0.66% Ameren Missouri 1998 Series A due 2033 1.77% 0.91% Ameren Missouri 1998 Series B due 2033 1.75% 0.92% Ameren Missouri 1998 Series C due 2033 1.73% 0.97% Ameren Illinois 1993 Series B-1 due 2028 1.08% 0.70% (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) Payments due to the lessor under these capital lease obligations are paid to a trustee, which is authorized to utilize the cash only to pay equal amounts due to Ameren Missouri under related bonds issued by the lessor and held by Ameren Missouri. The timing and amounts of payments due from Ameren Missouri under the capital lease agreements are equal to the timing and amount of bond service payments due to Ameren Missouri, resulting in no net cash flow. The balance of both the capital lease obligations and the related investments in debt securities, recorded in "Other Assets," was $276 million and $282 million , respectively, as of December 31, 2017 and 2016. (g) These notes are collaterally secured by first 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 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 maturity date of these senior secured notes and the 3.70% first mortgage bonds due 2047, we do not expect the mortgage bond lien protection associated with these notes to fall away. (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 and 6.25% senior secured notes due 2018 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 first 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. (l) These notes are collaterally secured by first mortgage bonds issued by Ameren Illinois under its 1933 mortgage indenture. They are secured by substantially all property of the former CILCO. The notes have a fall-away lien provision, and Ameren Illinois could cause these notes to become unsecured at any time by redeeming the 5.90% first mortgage bonds due 2023 (of which less than $1 million principal amount remains outstanding). (m) These bonds are first mortgage bonds issued by Ameren Illinois under its 1992 mortgage indenture. They are secured by substantially all property of the former IP and CIPS. (n) The following table presents the principal maturities schedule for the 3.43% senior notes due 2050: Payment Date Principal Payment August 2022 $ 49.5 August 2024 49.5 August 2027 49.5 August 2030 49.5 August 2032 49.5 August 2038 49.5 August 2043 76.5 August 2050 76.5 Total $ 450.0
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, 2017 : Ameren (parent) (a) Ameren Missouri (a) Ameren Illinois (a) ATXI (a) Ameren Consolidated 2018 $ — $ 384 $ 457 $ — $ 841 2019 — 581 — — 581 2020 350 92 — — 442 2021 — 8 — — 8 2022 — 56 400 50 506 Thereafter 350 2,867 2,000 400 5,617 Total $ 700 $ 3,988 $ 2,857 $ 450 $ 7,995 (a) Excludes unamortized discount, unamortized premium, and debt issuance costs of $4 million , $27 million , $27 million and $2 million at Ameren (parent), Ameren Missouri, Ameren Illinois and ATXI, 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, 2017 and 2016 : Redemption Price (per share) 2017 2016 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, 2017 , 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 4.8 $ 4,222 > 2.5 95.4 $ 2,118 Ameren Illinois > 2.0 7.1 4,119 (d) > 1.5 2.9 203 (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 $1,629 million and $529 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, 2017
Other Nonoperating Income (Expense) [Abstract]
Other Income And Expenses	The following table presents the components of “Other Income and Expenses” in the Ameren Companies’ statements of income for the years ended December 31, 2017 , 2016 , and 2015 : 2017 2016 2015 Ameren: (a) Miscellaneous income: Allowance for equity funds used during construction $ 24 $ 27 $ 30 Interest income on industrial development revenue bonds 26 27 27 Interest income (b) 8 13 14 Other 1 7 3 Total miscellaneous income $ 59 $ 74 $ 74 Miscellaneous expense: Donations $ 8 $ 16 $ 15 Other 13 16 15 Total miscellaneous expense $ 21 $ 32 $ 30 Ameren Missouri: Miscellaneous income: Allowance for equity funds used during construction $ 21 $ 23 $ 22 Interest income on industrial development revenue bonds 26 27 27 Interest income 1 1 1 Other — 1 2 Total miscellaneous income $ 48 $ 52 $ 52 Miscellaneous expense: Donations $ 2 $ 4 $ 5 Other 6 6 6 Total miscellaneous expense $ 8 $ 10 $ 11 Ameren Illinois: Miscellaneous income: Allowance for equity funds used during construction $ 3 $ 4 $ 8 Interest income (b) 7 12 12 Other 1 5 1 Total miscellaneous income $ 11 $ 21 $ 21 Miscellaneous expense: Donations $ 5 $ 6 $ 5 Other 5 6 7 Total miscellaneous expense $ 10 $ 12 $ 12 (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.

Derivative Financial Instrume37

Derivative Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 . As of December 31, 2017 , these contracts extended through October 2019, March 2023, May 2032, and September 2021 for fuel oils, natural gas, power, and uranium, respectively. Quantity (in millions, except as indicated) 2017 2016 Commodity Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils (in gallons) (a) 28 (b) 28 30 (b) 30 Natural gas (in mmbtu) 24 139 163 25 129 154 Power (in megawatthours) 3 9 12 1 9 10 Uranium (pounds in thousands) 370 (b) 370 345 (b) 345 (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, 2017 and 2016 : Balance Sheet Location Ameren Missouri Ameren Illinois Ameren 2017 Fuel oils Other current assets $ 5 $ — $ 5 Other assets 2 — 2 Natural gas Other assets 1 — 1 Power Other current assets 9 — 9 Total assets (a) $ 17 $ — $ 17 Natural gas Other current liabilities 5 12 17 Other deferred credits and liabilities 3 10 13 Power Other current liabilities 1 13 14 Other deferred credits and liabilities — 182 182 Uranium Other deferred credits and liabilities — (b) — — (b) Total liabilities (c) $ 9 $ 217 $ 226 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 Other current liabilities 1 3 4 Other deferred credits and liabilities 5 5 10 Power Other current liabilities 3 12 15 Other deferred credits and liabilities — 173 173 Uranium Other deferred credits and liabilities 4 — 4 Total liabilities (c) $ 18 $ 193 $ 211 (a) The cumulative amount of pretax net gains on all derivative instruments is deferred as a regulatory liability. (b) Beginning in 2017, as a result of rulebook amendments at the Chicago Mercantile Exchange, the fair value of uranium derivative liabilities are offset by certain settlement payments made to the exchange previously characterized as collateral and included within “Other assets” on Ameren’s and Ameren Missouri’s balance sheet. (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, 2017 , 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, 2017 , 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) 2017 Ameren Missouri $ 55 $ 3 $ 44 Ameren Illinois 43 — 38 Ameren $ 98 $ 3 $ 82 (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, 2017
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, 2017 and 2016 : Fair Value Weighted Assets Liabilities Valuation Technique(s) Unobservable Input Range Average Level 3 Derivative asset and liability – commodity contracts (a) : 2017 Fuel oils $ 3 $ — Option model Volatilities(%) (b) 20 – 26 22 Discounted cash flow Counterparty credit risk(%) (c)(d) 0.12 – 0.72 0.41 Ameren Missouri credit risk(%) (c)(d) 0.37 (e) Natural Gas 1 (4 ) Option model Volatilities(%) (b) 26 – 46 37 Nodal basis($/mmbtu) (c) (0.50) – (0.30) (0.40) Discounted cash flow Nodal basis($/mmbtu) (b) (1.20) – 0.10 (1) Counterparty credit risk(%) (c)(d) 0.37 – 0.92 0.53 Ameren credit risk(%) (c)(d) 0.37 (e) Power (f) 8 (196 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps($/MWh) (g) 24 – 46 28 Estimated auction price for FTRs($/MW) (b) (65) – 1,823 251 Nodal basis($/MWh) (g) (10) – 0 (2) Counterparty credit risk(%) (c)(d) 0.28 (e) Ameren Illinois credit risk(%) (c)(d) 0.37 (e) Fundamental energy production model Estimated future natural gas prices($/mmbtu) (b) 3 – 4 3 Escalation rate(%) (b)(h) 5 (e) Contract price allocation Estimated renewable energy credit costs($/credit) (b) 5 – 7 6 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)(i) (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 (f) 9 (187 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps($/MWh) (g) 26 – 44 29 Estimated auction price for FTRs($/MW) (b) (71) – 5,270 125 Nodal basis($/MWh) (g) (6) – 0 (2) Ameren Illinois credit risk(%) (c)(d) 0.38 (e) Fundamental energy production model Estimated future natural gas prices($/mmbtu) (b) 3 – 4 3 Escalation rate(%) (b)(h) 5 (e) Contract price allocation Estimated renewable energy credit costs($/credit) (b) 5 – 7 6 Uranium — (4 ) Option model Volatilities(%) (b) 24 (e) Fair Value Weighted Assets Liabilities Valuation Technique(s) Unobservable Input Range Average Discounted cash flow Average forward uranium pricing($/pound) (b) 22 – 24 22 Ameren Missouri credit risk(%) (c)(d) 0.38 (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) Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2021. Valuations beyond 2021 use fundamentally modeled pricing by month for peak and off-peak demand. (g) Ameren Missouri and Ameren Illinois power contracts respond differently to unobservable input changes because of their opposing positions. (h) Escalation rate applies to power prices in 2031 and beyond. (i) Escalation rate applies to fuel oil prices in 2019 and beyond.
Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis	The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of December 31, 2017 : 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 $ 4 $ — $ 3 $ 7 Natural gas — — 1 1 Power — 1 8 9 Total derivative assets – commodity contracts $ 4 $ 1 $ 12 $ 17 Nuclear decommissioning trust fund: Cash and cash equivalents $ 2 $ — $ — $ 2 Equity securities: U.S. large capitalization 468 — — 468 Debt securities: U.S. Treasury and agency securities — 125 — 125 Corporate bonds — 82 — 82 Other — 25 — 25 Total nuclear decommissioning trust fund $ 470 $ 232 $ — $ 702 (b) Total Ameren $ 474 $ 233 $ 12 $ 719 Ameren Missouri Derivative assets – commodity contracts (a) : Fuel oils $ 4 $ — $ 3 $ 7 Natural gas — — 1 1 Power — 1 8 9 Total derivative assets – commodity contracts $ 4 $ 1 $ 12 $ 17 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 $ 2 $ — $ — $ 2 Equity securities: U.S. large capitalization 468 — — 468 Debt securities: U.S. Treasury and agency securities — 125 — 125 Corporate bonds — 82 — 82 Other — 25 — 25 Total nuclear decommissioning trust fund $ 470 $ 232 $ — $ 702 (b) Total Ameren Missouri $ 474 $ 233 $ 12 $ 719 Liabilities: Ameren Derivative liabilities – commodity contracts (a) : Natural gas 1 25 4 30 Power — — 196 196 Total Ameren $ 1 $ 25 $ 200 $ 226 Ameren Missouri Derivative liabilities – commodity contracts (a) : Natural gas — 7 1 8 Power — — 1 1 Total Ameren Missouri $ — $ 7 $ 2 $ 9 Ameren Illinois Derivative liabilities – commodity contracts (a) : Natural gas $ 1 $ 18 $ 3 $ 22 Power — — 195 195 Total Ameren Illinois $ 1 $ 18 $ 198 $ 217 (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, 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 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 $ 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) : 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.
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, 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 ) For the year ended December 31, 2017 Beginning balance at January 1, 2017 $ 7 $ (185 ) $ (178 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (4 ) (21 ) (25 ) Purchases 14 — 14 Sales 1 — 1 Settlements (11 ) 11 — Ending balance at December 31, 2017 $ 7 $ (195 ) $ (188 ) Change in unrealized gains (losses) related to assets/liabilities held at December 31, 2017 $ — $ (22 ) $ (22 )
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, 2017 and 2016 : 2017 2016 Carrying Amount Fair Value Carrying Amount Fair Value Ameren: Long-term debt and capital lease obligations (including current portion) (a) $ 7,935 $ 8,531 $ 7,276 $ 7,772 Preferred stock (b) 142 131 142 131 Ameren Missouri: Long-term debt and capital lease obligations (including current portion) (a) $ 3,961 $ 4,348 $ 3,994 $ 4,304 Preferred stock 80 80 80 79 Ameren Illinois: Long-term debt (including current portion) $ 2,830 $ 3,028 $ 2,588 $ 2,765 Preferred stock 62 51 62 52 (a) Ameren and Ameren Missouri have two CTs under separate capital lease agreements. The capital lease obligations as of December 31, 2017 and 2016, were $276 million and $282 million , respectively. In addition, Ameren and Ameren Missouri have investments in debt securities, classified as held-to-maturity and recorded in “Other Assets” that are related to the capital lease obligation CTs from the city of Bowling Green and Audrain County. As of December 31, 2017 and 2016, the fair value of these investments approximate carrying value of $276 million and $282 million , respectively. (b) Preferred stock is recorded in “Noncontrolling Interests” on the consolidated balance sheet.

Callaway Energy Center Callaway

Callaway Energy Center Callaway Energy Center (Tables)	12 Months Ended
	Dec. 31, 2017
Nuclear Waste Matters [Abstract]
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, 2017 , 2016 , and 2015 : 2017 2016 2015 Proceeds from sales and maturities $ 396 $ 377 $ 349 Gross realized gains 13 7 8 Gross realized losses 5 4 2
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, 2017 and 2016 : Security Type Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value 2017 Debt securities $ 228 $ 5 $ 1 $ 232 Equity securities 155 318 5 468 Cash and cash equivalents 2 — — 2 Other (a) 2 — — 2 Total $ 387 $ 323 $ 6 $ 704 2016 Debt securities $ 197 $ 3 $ 4 $ 196 Equity securities 161 253 6 408 Cash and cash equivalents 1 — — 1 Other (a) 2 — — 2 Total $ 361 $ 256 $ 10 $ 607 (a) Represents net receivables and payables relating to pending security sales, interest, and security purchases.
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, 2017 : Cost Fair Value Less than 5 years $ 120 $ 120 5 years to 10 years 54 55 Due after 10 years 54 57 Total $ 228 $ 232
Schedule Of Insurance Coverage At Callaway Energy Center	The following table presents insurance coverage at Ameren Missouri’s Callaway energy center at December 31, 2017 . 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 for Single Incidents Public liability and nuclear worker liability: American Nuclear Insurers $ 450 $ — Pool participation 12,986 (a) 127 (b) $ 13,436 (c) $ 127 Property damage: NEIL and EMANI $ 3,200 (d) $ 30 (e) Replacement power: NEIL $ 490 (f) $ 7 (e) (a) Provided through mandatory participation in an industrywide retrospective premium assessment program. (b) Retrospective premium under the Price-Anderson Act. This is subject to retrospective assessment with respect to a covered loss in excess of $450 million in the event of an incident at any licensed United States commercial reactor, payable at $19 million per year. (c) Limit of liability for each incident under the Price-Anderson liability provisions of the Atomic Energy Act of 1954, as amended. This limit is subject to change to account for the effects of inflation and changes in the number of licensed reactors. (d) NEIL provides $2.7 billion in property damage, stabilization, decontamination, and premature decommissioning insurance for radiation events and $2.3 billion in property damage insurance for nonradiation events. EMANI provides $490 million in property damage insurance for both radiation and nonradiation events. (e) All NEIL insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL. (f) Provides replacement power cost insurance in the event of a prolonged accidental outage. Weekly indemnity up to $4.5 million for 52 weeks, which commences after the first 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 limited to $328 million .

Retirement Benefits (Tables)

Retirement Benefits (Tables)	12 Months Ended
	Dec. 31, 2017
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, 2017 and 2016 : 2017 2016 Ameren (a) $ 551 $ 774 Ameren Missouri 215 293 Ameren Illinois (b) 213 315 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries. (b) Other postretirement benefit liability is recorded in “Other assets” on the balance sheet.
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, 2017 and 2016 . It also provides the amounts included in regulatory assets and accumulated OCI at December 31, 2017 and 2016 , that have not been recognized in net periodic benefit costs. 2017 2016 Pension Benefits (a) Postretirement Benefits (a) Pension Benefits (a) Postretirement Benefits (a) Accumulated benefit obligation at end of year $ 4,577 $ (b) $ 4,288 $ (b) Change in benefit obligation: Net benefit obligation at beginning of year $ 4,518 $ 1,170 $ 4,197 $ 1,094 Service cost 93 21 81 19 Interest cost 179 47 185 50 Participant contributions — 8 — 8 Actuarial loss 255 53 265 52 Benefits paid (218 ) (59 ) (210 ) (54 ) Federal subsidy on benefits paid (b) — (b) 1 Net benefit obligation at end of year 4,827 1,240 4,518 1,170 Change in plan assets: Fair value of plan assets at beginning of year 3,813 1,101 3,653 1,071 Actual return on plan assets 634 171 313 73 Employer contributions 64 2 57 2 Federal subsidy on benefits paid (b) — (b) 1 Participant contributions — 8 — 8 Benefits paid (218 ) (59 ) (210 ) (54 ) Fair value of plan assets at end of year 4,293 1,223 3,813 1,101 Funded status – deficiency 534 17 705 69 Accrued benefit cost at December 31 $ 534 $ 17 $ 705 $ 69 Amounts recognized in the balance sheet consist of: Current liability (c) 3 3 3 2 Noncurrent liability 531 14 702 67 Net liability recognized $ 534 $ 17 $ 705 $ 69 Amounts recognized in regulatory assets consist of: Net actuarial (gain) loss $ 374 $ (69 ) $ 535 $ (29 ) Prior service credit (3 ) (3 ) (4 ) (8 ) Amounts (pretax) recognized in accumulated OCI consist of: Net actuarial loss 30 2 43 — Prior service credit — — — (1 ) Total $ 401 $ (70 ) $ 574 $ (38 ) (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries. (b) Not applicable. (c) 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, 2017 and 2016 : Pension Benefits Postretirement Benefits 2017 2016 2017 2016 Discount rate at measurement date 3.50 % 4.00 % 3.50 % 4.00 % Increase in future compensation 3.50 3.50 3.50 3.50 Medical cost trend rate (initial) (a) (b) (b) 5.00 5.00 Medical cost trend rate (ultimate) (a) (b) (b) 5.00 5.00 (a) Initial and ultimate medical cost trend rate for certain Medicare-eligible participants is 3.00% . (b) 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 2017 , 2016 , and 2015 : Pension Benefits Postretirement Benefits 2017 2016 2015 2017 2016 2015 Ameren Missouri $ 19 $ 21 $ 47 $ 1 $ 1 $ 8 Ameren Illinois 37 30 45 1 1 8 Other 8 6 19 — — 2 Ameren 64 57 111 2 2 18
Target Allocation Of The Plans' Asset Categories	The following table presents our target allocations for 2018 and our pension and postretirement plans’ asset categories as of December 31, 2017 and 2016 : Asset Category Target Allocation 2018 Percentage of Plan Assets at December 31, 2017 2016 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 % 9 % International and emerging markets 9% – 19% 14 % 14 % Total equity 51% – 61% 57 % 57 % Debt securities 35% – 45% 37 % 37 % Real estate 0% – 9% 5 % 5 % Private equity 0% – 5% (a) (a) Total 100 % 100 % Postretirement Plans: Cash and cash equivalents 0% – 7% 2 % 3 % Equity securities: U.S. large-capitalization 34% – 44% 41 % 40 % U.S. small- and mid-capitalization 2% – 12% 8 % 7 % International and emerging markets 9% – 19% 14 % 14 % Total equity 55% – 65% 63 % 61 % Debt securities 33% – 43% 35 % 36 % 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 2017 , 2016 , and 2015 : Pension Benefits Postretirement Benefits 2017 Service cost $ 93 $ 21 Interest cost 179 47 Expected return on plan assets (262 ) (75 ) Amortization of: Prior service credit (1 ) (5 ) Actuarial (gain) loss 55 (6 ) Net periodic benefit cost (income) $ 64 $ (18 ) 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 (income) $ 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 Curtailment gain 1 — Net periodic benefit cost $ 92 $ 4
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 2018 are as follows: Pension Benefits (a) Postretirement Benefits (a) Regulatory assets: Prior service credit $ (1 ) $ (2 ) Net actuarial (gain) loss 60 (1 ) Accumulated OCI: Net actuarial loss 5 — Total $ 64 $ (3 ) (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 for the years ended December 31, 2017 , 2016 , and 2015 : Pension Costs Postretirement Costs 2017 2016 2015 2017 2016 2015 Ameren Missouri (a) $ 24 $ 26 $ 54 $ (4 ) $ (5 ) $ 8 Ameren Illinois 41 22 38 (14 ) (13 ) (3 ) Other (1 ) (4 ) — — (1 ) (1 ) Ameren 64 44 92 (18 ) (19 ) 4 (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 and the level of such costs included in customer 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, 2017 , 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 2018 $ 255 $ 3 $ 57 $ 2 2019 261 3 59 2 2020 266 3 62 2 2021 277 3 64 2 2022 280 3 65 2 2023 – 2027 1,421 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, 2017 , 2016 , and 2015 : Pension Benefits Postretirement Benefits 2017 2016 2015 2017 2016 2015 Discount rate at measurement date 4.00 % 4.50 % 4.00 % 4.00 % 4.50 % 4.00 % Expected return on plan assets 7.00 7.00 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) (b) (b) (b) 5.00 5.00 5.00 Medical cost trend rate (ultimate) (a) (b) (b) (b) 5.00 5.00 5.00 (a) Initial and ultimate medical cost trend rate for certain Medicare-eligible participants is 3.00% . (b) 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 ) $ 157 $ — $ 44 0.25% increase in salary scale 2 15 — — 1.00% increase in annual medical trend — — 4 71 1.00% decrease in annual medical trend — — (4 ) (71 )
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, 2017 , 2016 , and 2015 : 2017 2016 2015 Ameren Missouri $ 16 $ 16 $ 16 Ameren Illinois 13 12 12 Other 1 1 1 Ameren 30 29 29
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 plans’ assets measured at fair value as of December 31, 2017 : 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 Total Cash and cash equivalents $ — $ — $ — $ 25 $ 25 Equity securities: U.S. large-capitalization — — — 1,523 1,523 U.S. small- and mid-capitalization 379 — — — 379 International and emerging markets 179 — — 450 629 Debt securities: Corporate bonds — 726 — 15 741 Municipal bonds — 91 — — 91 U.S. Treasury and agency securities 8 816 — — 824 Other — 7 — — 7 Real estate — — — 196 196 Private equity — — — 4 4 Total $ 566 $ 1,640 $ — $ 2,213 $ 4,419 Less: Medical benefit assets at December 31 (a) (153 ) Plus: Net receivables at December 31 (b) 27 Fair value of pension plans’ assets at December 31 $ 4,293 (a) 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. (b) 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 plans’ assets measured at fair value as of December 31, 2016 : 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 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 (a) (132 ) Plus: Net receivables at December 31 (b) 22 Fair value of pension plans’ assets at December 31 $ 3,813 (a) 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. (b) 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, 2017 : 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 Total Cash and cash equivalents $ 44 $ — $ — $ — $ 44 Equity securities: U.S. large-capitalization 332 — — 110 442 U.S. small- and mid-capitalization 80 — — — 80 International and emerging markets 53 — — 101 154 Other — 8 — — 8 Debt securities: Corporate bonds — 144 — — 144 Municipal bonds — 110 — — 110 U.S. Treasury and agency securities — 76 — — 76 Other — 4 — 34 38 Total $ 509 $ 342 $ — $ 245 $ 1,096 Plus: Medical benefit assets at December 31 (a) 153 Less: Net payables at December 31 (b) (26 ) Fair value of postretirement benefit plans’ assets at December 31 $ 1,223 (a) 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. (b) 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, 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 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 and emerging markets 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 (a) 132 Less: Net payables at December 31 (b) (25 ) Fair value of postretirement benefit plans’ assets at December 31 $ 1,101 (a) 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. (b) Payables related to pending security purchases, offset by interest receivables and receivables related to pending security sales.

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary Of Nonvested Shares Related To Long-Term Incentive Plan	nonvested performance share unit activity for the year ended December 31, 2017 : Performance Share Units Share Units Weighted-average Grant Date Fair Value per Share Unit Nonvested at January 1, 2017 (a) 780,545 $ 47.54 Granted (b) 508,161 59.16 Forfeitures (50,523 ) 52.50 Undistributed vested units (c) (342,694 ) 51.65 Nonvested at December 31, 2017 (a) 895,489 $ 52.28 (a) Excludes 369,878 and 712,572 performance share units granted to retirement-eligible employees as of January 1, 2017 and December 31, 2017, respectively, as the undistributed performance share units are fully vested. (b) Includes performance share units granted to certain executive and nonexecutive officers and other eligible employees in 2017 under the 2014 Incentive Plan. (c) Includes performance 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 fair value of each share unit awarded under the 2014 Incentive Plan along with the significant assumptions used to calculate the fair value of each share unit for the years ended December 31, 2017 , 2016 , and 2015 : 2017 2016 2015 Fair value of share units awarded $59.16 $44.13 $52.88 Ameren’s closing common share price at December 31 of the prior year $52.46 $43.23 $46.13 Three-year risk free rate 1.47% 1.31% 1.10% Volatility range 15% - 21% 15% - 20% 12% - 18% The following table presents the stock-based compensation expense for the years ended December 31, 2017, 2016, and 2015: 2017 2016 2015 Ameren Missouri $ 4 $ 4 $ 5 Ameren Illinois 2 2 3 Other (a) 12 11 11 Ameren 18 17 19 Less income tax benefit 7 6 7 Stock-based compensation expense, net $ 11 $ 11 $ 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, 2017
Income Tax Disclosure [Abstract]
Schedule of Remeasurement of Deferred Incomes Taxes due to the Tax Cuts and Jobs Act [Table Text Block]	The TCJA had the following provisional effects for the year ended December 31, 2017: Ameren Missouri Ameren Illinois Other Ameren Increase (Decrease) Accumulated deferred income taxes, net $ (1,419 ) $ (871 ) $ 37 $ (2,253 ) Income tax expense (benefit) (a) 32 (5 ) 127 154 Noncurrent regulatory assets (89 ) (24 ) (1 ) (114 ) Noncurrent regulatory liabilities 1,362 842 89 2,293
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 federal statutory corporate income tax rate for the years ended December 31, 2017 , 2016 , and 2015 : Ameren Missouri Ameren Illinois Ameren 2017 Federal statutory corporate income tax rate: 35 % 35 % 35 % Increases (decreases) from: Depreciation differences 1 (1 ) — Amortization of deferred investment tax credit (1 ) — (1 ) State tax 4 6 6 TCJA 6 (1 ) 14 Tax credits (1 ) — — Other permanent items — (1 ) (2 ) Effective income tax rate 44 % 38 % 52 % 2016 Federal statutory corporate 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 Federal statutory corporate 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 % (a) Reflects the adoption of authoritative accounting guidance related to share-based compensation, which resulted in the recognition of a $21 million income tax benefit in 2016.
Schedule Of Components Of Income Tax Expense (Benefit)	The following table presents the components of income tax expense for the years ended December 31, 2017 , 2016 , and 2015 : Ameren Missouri Ameren Illinois Other Ameren 2017 Current taxes: Federal $ 149 $ (34 ) $ (110 ) $ 5 State 23 29 (20 ) 32 Deferred taxes: Federal 76 185 250 511 State 11 (13 ) 36 34 Amortization of deferred investment tax credits (5 ) (1 ) — (6 ) Total income tax expense $ 254 $ 166 $ 156 $ 576 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
Schedule Of Deferred Tax Assets And Liabilities Resulting From Temporary Differences	The following table presents the accumulated deferred income tax assets and liabilities recorded as a result of temporary differences at December 31, 2017 and 2016 : Ameren Missouri Ameren Illinois Other Ameren 2017 Accumulated deferred income taxes, net liability (asset): Plant related $ 2,064 $ 1,264 $ 146 $ 3,474 Regulatory assets and liabilities, net (317 ) (206 ) (24 ) (547 ) Deferred employee benefit costs (53 ) (17 ) (61 ) (131 ) Revenue requirement reconciliation adjustments — 20 — 20 Tax carryforwards (31 ) (43 ) (287 ) (361 ) Other (13 ) 3 61 51 Total net accumulated deferred income tax liabilities (assets) $ 1,650 $ 1,021 $ (165 ) $ 2,506 2016 Accumulated deferred income taxes, net liability (asset): Plant related $ 3,103 $ 1,769 $ 147 $ 5,019 Regulatory assets and liabilities, 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
Schedule Of Net Operating Loss Carryforwards And Tax Credit Carryforwards	The following table presents the components of accumulated deferred income tax assets relating to net operating loss carryforwards, tax credit carryforwards, and charitable contribution carryforwards at December 31, 2017 and 2016 : Ameren Missouri Ameren Illinois Other Ameren 2017 Net operating loss carryforwards: Federal (a) $ — $ 41 $ 162 $ 203 State (a) — — 32 32 Total net operating loss carryforwards $ — $ 41 $ 194 $ 235 Tax credit carryforwards: Federal (b) $ 31 $ 2 $ 80 $ 113 State (c) — — 7 7 Total tax credit carryforwards $ 31 $ 2 $ 87 $ 120 Charitable contribution carryforwards (d) $ — $ — $ 11 $ 11 Valuation allowance (e) — — (5 ) (5 ) Total charitable contribution carryforwards $ — $ — $ 6 $ 6 2016 Net operating loss carryforwards: Federal $ 33 $ 137 $ 324 $ 494 State 4 — 41 45 Total net operating loss carryforwards $ 37 $ 137 $ 365 $ 539 Tax credit carryforwards: Federal $ 29 $ 1 $ 79 $ 109 State — — 21 21 Total tax credit carryforwards $ 29 $ 1 $ 100 $ 130 Charitable contribution carryforwards $ — $ — $ 18 $ 18 Valuation allowance — — (11 ) (11 ) Total charitable contribution carryforwards $ — $ — $ 7 $ 7 (a) Will expire between 2033 and 2036 . Any net operating loss carryforward generated after January 1, 2018, will not have an expiration date as a result of the TCJA. (b) Will expire between 2029 and 2037 . (c) Will expire between 2019 and 2022 . (d) Will expire between 2018 and 2021 . (e) 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, 2017
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 performance period, price, and amount of megawatthours included in each agreement: IPA Procurement Event Performance 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 April 2017 March 2019 – May 2020 85,600 34 The following table presents the impact on Ameren Missouri and Ameren Illinois of related-party transactions for the years ended December 31, 2017 , 2016 , and 2015 . 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 2017 $ 23 $ (a) with Ameren Illinois 2016 28 (a) 2015 15 (a) Ameren Missouri and Ameren Illinois Operating Revenues 2017 26 4 rent and facility services 2016 25 5 2015 25 4 Ameren Missouri and Ameren Illinois Operating Revenues 2017 (b) 1 miscellaneous support services 2016 1 (b) 2015 2 (b) Total Operating Revenues 2017 $ 49 $ 5 2016 54 5 2015 42 4 Ameren Illinois power supply Purchased Power 2017 $ (a) $ 23 agreements with Ameren Missouri 2016 (a) 28 2015 (a) 15 Ameren Illinois transmission Purchased Power 2017 (a) 2 services from ATXI 2016 (a) 2 2015 (a) 2 Total Purchased Power 2017 $ (a) $ 25 2016 (a) 30 2015 (a) 17 Ameren Services support services Other Operations and 2017 $ 149 $ 139 agreement Maintenance 2016 129 123 2015 131 119 Money pool borrowings (advances) (Interest Charges) 2017 $ 1 $ (b) Miscellaneous Income 2016 (b) (b) 2015 (b) (b) (a) Not applicable. (b) Amount less than $1 million.

Commitments And Contingencies (

Commitments And Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
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, 2017 : 2018 2019 2020 2021 2022 After 5 Years Total Ameren: (a) Minimum capital lease payments (b)(c) $ 32 $ 32 $ 32 $ 33 $ 32 $ 264 $ 425 Less amount representing interest 26 25 25 25 24 24 149 Present value of minimum capital lease payments $ 6 $ 7 $ 7 $ 8 $ 8 $ 240 $ 276 Operating leases 10 9 8 6 6 14 53 Total lease obligations $ 16 $ 16 $ 15 $ 14 $ 14 $ 254 $ 329 Ameren Missouri: Minimum capital lease payments (b)(c) $ 32 $ 32 $ 32 $ 33 $ 32 $ 264 $ 425 Less amount representing interest 26 25 25 25 24 24 149 Present value of minimum capital lease payments $ 6 $ 7 $ 7 $ 8 $ 8 $ 240 $ 276 Operating leases 8 8 7 6 6 14 49 Total lease obligations $ 14 $ 15 $ 14 $ 14 $ 14 $ 254 $ 325 Ameren Illinois: Operating leases $ 1 (d) (d) (d) (d) $ 1 $ 2 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations. (b) See Note 3 – Property, Plant, and Equipment, Net for additional information. (c) See Note 5 – Long-term Debt and Equity Financings for additional information on Ameren’s and Ameren Missouri’s capital lease agreements. (d) Less than $1 million.
Schedule Of Rental Expense	The following table presents total operating lease expenses included in “Operating Expenses” in the statement of income for the years ended December 31, 2017 , 2016 , and 2015 : 2017 2016 2015 Ameren (a) $ 11 $ 38 $ 36 Ameren Missouri 10 34 34 Ameren Illinois 1 30 28 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.
Schedule Of Estimated Purchased Commitments	The table below presents our estimated minimum fuel, purchased power, and other commitments at December 31, 2017 . Ameren’s and Ameren Illinois’ purchased power commitments include the Ameren Illinois 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, among other agreements, at December 31, 2017 . Coal Natural Gas (a) Nuclear Fuel Purchased Power (b)(c) Methane Gas Other Total Ameren: (d) 2018 $ 463 $ 205 $ 67 $ 170 $ 3 $ 73 $ 981 2019 383 163 26 63 4 37 676 2020 85 110 39 14 4 36 288 2021 27 46 45 3 5 25 151 2022 — 11 12 2 5 25 55 Thereafter — 38 45 18 58 95 254 Total $ 958 $ 573 $ 234 $ 270 $ 79 $ 291 $ 2,405 Ameren Missouri: 2018 $ 463 $ 42 $ 67 $ — $ 3 $ 53 $ 628 2019 383 36 26 — 4 24 473 2020 85 29 39 — 4 24 181 2021 27 13 45 — 5 25 115 2022 — 6 12 — 5 25 48 Thereafter — 16 45 — 58 75 194 Total $ 958 $ 142 $ 234 $ — $ 79 $ 226 $ 1,639 Ameren Illinois: 2018 $ — $ 163 $ — $ 170 $ — $ 19 $ 352 2019 — 127 — 63 — 13 203 2020 — 81 — 14 — 12 107 2021 — 33 — 3 — — 36 2022 — 5 — 2 — — 7 Thereafter — 22 — 18 — — 40 Total $ — $ 431 $ — $ 270 $ — $ 44 $ 745 (a) Includes amounts for generation and for distribution. (b) The purchased power amounts for Ameren and Ameren Illinois exclude agreements for renewable energy credits through 2032 with various renewable energy suppliers due to the contingent nature of the payment amounts. (c) The purchased power amounts for Ameren and Ameren Missouri exclude a 102-megawatt power purchase agreement with a wind farm operator, which expires in 2024, due to the contingent nature of the payment amounts. (d) Includes amounts for Ameren registrant and nonregistrant subsidiaries.

Segment Information (Tables)

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2017
Segment Reporting Information, Profit (Loss) [Abstract]
Schedule Of Segment Reporting Information, By Segment	The following tables present revenues, net income attributable to common shareholders, and capital expenditures by segment at Ameren and Ameren Illinois for the years ended December 31, 2017 , 2016 , and 2015 . 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 2017 External revenues $ 3,490 $ 1,565 $ 742 $ 382 $ (2 ) $ — $ 6,177 Intersegment revenues 49 4 1 44 (a) — (98 ) — Depreciation and amortization 533 239 59 60 5 — 896 Interest income 27 7 — — 11 (11 ) 34 Interest charges 207 73 36 67 (b) 19 (11 ) 391 Income taxes 254 83 36 90 113 — 576 Net income (loss) attributable to Ameren common shareholders from continuing operations 323 131 60 140 (131 ) — 523 Capital expenditures 773 476 245 644 1 (7 ) 2,132 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 (b) 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 (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 (b) 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 — 1,917 (a) Ameren Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above. (b) Ameren Transmission interest charges include an allocation of financing costs from Ameren (parent). Ameren Illinois Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Consolidated 2017 External revenues $ 1,569 $ 743 $ 216 $ — $ 2,528 Intersegment revenues — — 42 (a) (42 ) — Depreciation and amortization 239 59 43 — 341 Interest income 7 — — — 7 Interest charges 73 36 35 — 144 Income taxes 83 36 47 — 166 Net income available to common shareholder 131 60 77 — 268 Capital expenditures 476 245 355 — 1,076 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 (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, 2017
Selected Quarterly Financial Information [Abstract]
Summary Of Selected Quarterly Information	SELECTED QUARTERLY INFORMATION (Unaudited) (In millions, except per share amounts) Ameren 2017 2016 Quarter ended March 31 June 30 September 30 December 31 March 31 June 30 September 30 December 31 Operating revenues $ 1,514 $ 1,538 $ 1,723 $ 1,402 $ 1,434 $ 1,427 $ 1,859 $ 1,356 Operating income 254 398 581 225 220 325 691 145 Net income (loss) 104 194 290 (59 ) (a) 107 148 371 33 Net income (loss) attributable to Ameren common shareholders $ 102 $ 193 $ 288 $ (60 ) $ 105 $ 147 $ 369 $ 32 Earnings (loss) per common share – basic $ 0.42 $ 0.79 $ 1.19 $ (0.24 ) $ 0.43 $ 0.61 $ 1.52 $ 0.13 Earnings (loss) per common share – diluted (b) $ 0.42 $ 0.79 $ 1.18 $ (0.24 ) $ 0.43 $ 0.61 $ 1.52 $ 0.13 (a) Includes an increase to income tax expense of $154 million recorded in 2017 as a result of the TCJA. (b) 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 (Loss) Net Income (Loss) Available to Common Shareholder March 31, 2017 $ 790 $ 53 $ 6 $ 5 March 31, 2016 741 63 15 14 June 30, 2017 935 237 121 120 June 30, 2016 867 197 93 92 September 30, 2017 1,115 417 235 234 September 30, 2016 1,165 431 242 241 December 31, 2017 699 40 (36 ) (a) (36 ) December 31, 2016 750 54 10 10 (a) Includes an increase to income tax expense of $32 million recorded in 2017 as a result of the TCJA. Ameren Illinois Quarter ended (a) Operating Revenues Operating Income Net Income Net Income Available to Common Shareholder March 31, 2017 $ 703 $ 172 $ 80 $ 79 March 31, 2016 677 133 60 59 June 30, 2017 576 130 58 57 June 30, 2016 542 107 46 45 September 30, 2017 575 128 55 55 September 30, 2016 676 230 119 119 December 31, 2017 674 150 78 77 December 31, 2016 595 74 30 29 (a) In 2017, in connection with the decoupling provisions of the FEJA, Ameren Illinois changed the method it used to recognize its interim-period revenue. Ameren Illinois now recognizes revenue consistent with the timing of incurred electric distribution recoverable costs, and it recognizes revenue associated with the expected return on its rate base ratably over the year. As a result of this change in recognition of the interim period revenue for the IEIMA formula rate framework, as modified by the FEJA, Ameren Illinois incurred quarterly year-over-year increases to earnings in 2017 in comparison to 2016 for the first, second, and fourth quarters and a decrease to earnings in the third quarter. The change in interim period revenue recognition did not affect 2017 annual earnings.

Summary Of Significant Accoun47

Summary Of Significant Accounting Policies (Narrative) (Details) customer in Millions, $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2016USD ($)	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)mi²customer	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Accounting Policies [Line Items]
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets and Liabilities, Net	$ 9		$ 17	$ 9
Goodwill	411		411	411
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount			20
Loss Contingency Accrual, Payments					$ 25
Defined Benefit Plan, Non-service Cost or Income Components			$ 44	55
Ameren Illinois Company
Accounting Policies [Line Items]
Public Utilities, Area Serviced | mi²			40,000
Goodwill	411		$ 411	411
Defined Benefit Plan, Non-service Cost or Income Components			$ 10	24
Union Electric Company
Accounting Policies [Line Items]
Public Utilities, Area Serviced | mi²			24,000
Defined Benefit Plan, Non-service Cost or Income Components			$ 22	$ 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
Asbestos-Related
Accounting Policies [Line Items]
Asbestos trust fund balance	$ 22		$ 23
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
Subsequent Event [Member]
Accounting Policies [Line Items]
Premiums from Issuance of Debt		$ 7

Summary Of Significant Accoun48

Summary Of Significant Accounting Policies (Schedule Of Inventories) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Line Items]
Fuel	$ 154	$ 172
Gas stored underground	82	82
Other materials and supplies	286	273
Total Inventories	522	527
Union Electric Company
Accounting Policies [Line Items]
Fuel	154	172
Gas stored underground	8	9
Other materials and supplies	226	211
Total Inventories	388	392
Ameren Illinois Company
Accounting Policies [Line Items]
Fuel	0	0
Gas stored underground	74	73
Other materials and supplies	60	62
Total Inventories	$ 134	$ 135

Summary Of Significant Accoun49

Summary Of Significant Accounting Policies (Schedule Of Rates Used For Allowance For Funds Used During Construction) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	4.00%	5.00%	6.00%

Summary Of Significant Accoun50

Summary Of Significant Accounting Policies (Schedule Of Asset Retirement Obligations) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance	$ 650,000,000	$ 623,000,000
Liabilities incurred	0	3,000,000
Liabilities settled	(13,000,000)	(2,000,000)
Accretion in period	26,000,000	25,000,000
Change in estimates	(19,000,000)	1,000,000
Balance	644,000,000	650,000,000	$ 623,000,000
Other current liabilities	326,000,000	274,000,000
Asset Retirement Obligation Balance [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Other current liabilities	6,000,000	15,000,000
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance	644,000,000	617,000,000
Liabilities incurred		3,000,000
Liabilities settled	(12,000,000)	(2,000,000)
Accretion in period	26,000,000	25,000,000
Change in estimates	(18,000,000)	1,000,000
Balance	640,000,000	644,000,000	617,000,000
Other current liabilities	103,000,000	123,000,000
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance	6,000,000	6,000,000
Liabilities incurred	0	0
Liabilities settled	(1,000,000)	(1,000,000)
Accretion in period	1,000,000	1,000,000
Change in estimates	(1,000,000)
Balance	4,000,000	6,000,000	6,000,000
Other current liabilities	177,000,000	109,000,000
Asset Retirement Obligation | Union Electric Company
Asset Retirement Obligation [Line Items]
Noncash Depreciation related to ARO	$ 26,000,000	$ 31,000,000	$ 13,000,000

Summary Of Significant Accoun51

Summary Of Significant Accounting Policies (Schedule Of Excise Taxes) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Line Items]
Excise tax expense	$ 210	$ 208	$ 213
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	153	151	156
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$ 57	$ 57	$ 57

Summary Of Significant Accoun52

Summary Of Significant Accounting Policies (Basic and Diluted Earnings Per Share Calculations) (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Assumed Settlement of Performance Share Units	1,600,000	800,000	1,000,000
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) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Ameren Transmission Company of Illinois
Public Utilities, General Disclosures [Line Items]
Estimated Capital Project Costs		$ 85
Ameren Transmission Company of Illinois | Mark Twain Project
Public Utilities, General Disclosures [Line Items]
Estimated Capital Project Costs		250
Final Rate Order | Union Electric Company | Electric Distribution
Public Utilities, General Disclosures [Line Items]
Revenue Requirement		3,400
Public Utilities, Requested Rate Increase (Decrease), Amount		92
Final Rate Order | Union Electric Company | MEEIA 2013 | Electric Distribution
Public Utilities, General Disclosures [Line Items]
Incentive Award if Energy Efficiency Goals Are Achieved		28
Final Rate Order | Union Electric Company | MEEIA 2016 | Electric Distribution
Public Utilities, General Disclosures [Line Items]
Incentive Award if Energy Efficiency Goals Are Achieved		$ 27
Achieved Percentage of Energy Efficiency Earnings For Incentive Award		100.00%
Pending Rate Case | Union Electric Company | MEEIA 2013 | Electric Distribution
Public Utilities, General Disclosures [Line Items]
Incentive Award if Energy Efficiency Goals Are Achieved		$ 9
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
Subsequent Event [Member] | Final Rate Order | Union Electric Company | MEEIA 2016 | Electric Distribution
Public Utilities, General Disclosures [Line Items]
Incentive Award if Energy Efficiency Goals Are Achieved	$ 5

Rate And Regulatory Matters (Na

Rate And Regulatory Matters (Narrative-Illinois) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Public Utilities, General Disclosures [Line Items]
Regulatory assets		$ 1,230	$ 1,437
Current regulatory assets		144	149
Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets		822	816
Current regulatory assets		87	108
Ameren Transmission Company of Illinois
Public Utilities, General Disclosures [Line Items]
Estimated Capital Project Costs		85
Property, Plant and Equipment, Additions		$ 36
FEJA | 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		$ 99
IEIMA | 2017 IEIMA Revenue Requirement Reconciliation [Member] | Electric Distribution | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets		54
IEIMA | 2016 IEIMA Revenue Requirement Reconciliation [Member] | Electric Distribution | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets		24
IEIMA | 2015 IEMA Revenue Requirement Reconciliation [Member] | Electric Distribution | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets			$ 68
IEIMA | Final Rate Order | Electric Distribution | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 17
Subsequent Event [Member] | Electric Distribution | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Tax Reform Refunds From the Tax Cuts and Jobs Act	$ 50
Subsequent Event [Member] | Natural Gas | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Tax Reform Refunds From the Tax Cuts and Jobs Act	$ 16
Subsequent Event [Member] | Pending Rate Case | Natural Gas | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Equity Capital Structure, Percentage	50.00%
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 49
Public Utilities, Requested Return on Equity, Percentage	10.30%
Rate Base	$ 1,600
Subsequent Event [Member] | Components of Rate Increase | Natural Gas | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 42

Rate And Regulatory Matters R55

Rate And Regulatory Matters Rate and Regulatory Matters (Narrative-Federal) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities		$ 128	$ 110
Provision for Callaway construction and operating license		0	0	$ 69
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		92	78
Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities		19	12
Provision for Callaway construction and operating license		0	$ 0	$ 69
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
Midwest Independent Transmission System Operator, Inc [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Return on Equity, Percentage		12.38%
Payments for Legal Settlements		$ 21
Midwest Independent Transmission System Operator, Inc [Member] | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Payments for Legal Settlements		$ 17
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 | Midwest Independent Transmission System Operator, Inc [Member]
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities		$ 42
Pending Ferc Case | Midwest Independent Transmission System Operator, Inc [Member] | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities		$ 25
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%
Customer Requested Rate on Equity		9.15%
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%
Subsequent Event [Member] | Midwest Independent Transmission System Operator, Inc [Member] | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Tax Reform Refunds From the Tax Cuts and Jobs Act	$ 27
Subsequent Event [Member] | Midwest Independent Transmission System Operator, Inc [Member] | Ameren Transmission Company of Illinois
Public Utilities, General Disclosures [Line Items]
Tax Reform Refunds From the Tax Cuts and Jobs Act	$ 23

Rate And Regulatory Matters (Sc

Rate And Regulatory Matters (Schedule Of Regulatory Assets And Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	$ 144	$ 149
Regulatory assets	1,230	1,437
Current regulatory liabilities	128	110
Regulatory liabilities	4,387	1,985
Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	56	35
Regulatory assets	395	619
Current regulatory liabilities	19	12
Regulatory liabilities	2,664	1,215
Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	87	108
Regulatory assets	822	816
Current regulatory liabilities	92	78
Regulatory liabilities	1,629	768
Under-Recovered FAC
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	47	21
Under-Recovered FAC | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	47	21
Under-Recovered FAC | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	0	0
Under-Recovered Illinois Electric Power Costs
Public Utilities, General Disclosures [Line Items]
Current regulatory assets		3
Under-Recovered Illinois Electric Power Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	0	0
Under-Recovered Illinois Electric Power Costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets		3
Under-Recovered PGA
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	14	4
Under-Recovered PGA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	1	0
Under-Recovered PGA | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	13	4
MTM Derivative Losses
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	33	24
Regulatory assets	196	187
MTM Derivative Losses | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	8	9
Regulatory assets	4	9
MTM Derivative Losses | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	25	15
Regulatory assets	192	178
Energy Efficiency Rider
Public Utilities, General Disclosures [Line Items]
Current regulatory assets		5
Current regulatory liabilities	42	0
Regulatory liabilities		45
Energy Efficiency Rider | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets		5
Current regulatory liabilities	2	0
Regulatory liabilities	0	0
Energy Efficiency Rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	0	0
Current regulatory liabilities	40	0
Regulatory liabilities		45
IEIMA
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	24	68
Regulatory assets	54	23
IEIMA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	0	0
Regulatory assets	0	0
IEIMA | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	24	68
Regulatory assets	54	23
FERC Revenue Requirement Reconciliation
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	10	13
Regulatory assets	27	10
FERC Revenue Requirement Reconciliation | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	0	0
Regulatory assets	0	0
FERC Revenue Requirement Reconciliation | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	9	7
Regulatory assets	16	8
VBA Rider
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	15	11
VBA Rider | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	0	0
VBA Rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	15	11
Other Regulatory Assets
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	1	0
Regulatory assets	17	16
Other Regulatory Assets | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	0	0
Regulatory assets	9	9
Other Regulatory Assets | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	1	0
Regulatory assets	8	7
Pension And Postretirement Benefit Costs
Public Utilities, General Disclosures [Line Items]
Regulatory assets	299	494
Pension And Postretirement Benefit Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	84	175
Pension And Postretirement Benefit Costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	215	319
Income Taxes
Public Utilities, General Disclosures [Line Items]
Regulatory assets	197	230
Regulatory liabilities	2,323	37
Income Taxes | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	139	229
Regulatory liabilities	1,392	33
Income Taxes | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	56	1
Regulatory liabilities	842	4
Uncertain tax positions tracker
Public Utilities, General Disclosures [Line Items]
Regulatory assets	5	7
Regulatory liabilities	2	3
Uncertain tax positions tracker | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	5	7
Regulatory liabilities	2	3
Uncertain tax positions tracker | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	0	0
Asset Retirement Obligation
Public Utilities, General Disclosures [Line Items]
Regulatory assets	1	3
Regulatory liabilities	223	162
Asset Retirement Obligation | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	0	0
Regulatory liabilities	223	162
Asset Retirement Obligation | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	1	3
Regulatory liabilities	0	0
Callaway Costs
Public Utilities, General Disclosures [Line Items]
Regulatory assets	25	29
Callaway Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	25	29
Unamortized Loss On Reacquired Debt
Public Utilities, General Disclosures [Line Items]
Regulatory assets	110	124
Unamortized Loss On Reacquired Debt | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	61	65
Unamortized Loss On Reacquired Debt | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	49	59
Environmental cost riders
Public Utilities, General Disclosures [Line Items]
Regulatory assets	173	196
Environmental cost riders | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	0	0
Environmental cost riders | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	173	196
Storm Costs
Public Utilities, General Disclosures [Line Items]
Regulatory assets	10	15
Storm Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	0	0
Storm Costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	10	15
Demand-Side Costs
Public Utilities, General Disclosures [Line Items]
Regulatory assets	11	18
Demand-Side Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	11	18
Reserve For Workers' Compensation Liabilities
Public Utilities, General Disclosures [Line Items]
Regulatory assets	12	13
Reserve For Workers' Compensation Liabilities | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	5	6
Reserve For Workers' Compensation Liabilities | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	7	7
Credit Facilities Fees
Public Utilities, General Disclosures [Line Items]
Regulatory assets	3	4
Credit Facilities Fees | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	3	4
Construction Accounting For Pollution Control Equipment
Public Utilities, General Disclosures [Line Items]
Regulatory assets	18	19
Construction Accounting For Pollution Control Equipment | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	18	19
Solar Rebates
Public Utilities, General Disclosures [Line Items]
Regulatory assets	31	49
Solar Rebates | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	31	49
FEJA energy-efficiency rider
Public Utilities, General Disclosures [Line Items]
Regulatory assets	41	0
FEJA energy-efficiency rider | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	0	0
FEJA energy-efficiency rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory assets	41	0
Over-Recovered FAC
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	4	0
Over-Recovered FAC | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	4	0
Over-Recovered FAC | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	0	0
Over-Recovered Illinois Electric Power Costs
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	16	25
Over-Recovered Illinois Electric Power Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	0	0
Over-Recovered Illinois Electric Power Costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	16	25
Over-Recovered PGA
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	1	0
Over-Recovered PGA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	0	0
Over-Recovered PGA | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	1	0
MTM Derivative Gains
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	13	23
MTM Derivative Gains | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	13	12
MTM Derivative Gains | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	0	11
Refund Reserves for FERC Orders and Audit Findings
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	42	62
Refund Reserves for FERC Orders and Audit Findings | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	25	42
Other Regulatory Liabilities
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	10	0
Regulatory liabilities	13	9
Other Regulatory Liabilities | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	0	0
Regulatory liabilities	11	5
Other Regulatory Liabilities | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities	10	0
Regulatory liabilities	2	4
Removal Costs
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	1,725	1,669
Removal Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	995	970
Removal Costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	725	697
Bad Debt Rider
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	2	3
Bad Debt Rider | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	0	0
Bad Debt Rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	2	3
Pension And Postretirement Benefit Costs Tracker
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	35	35
Pension And Postretirement Benefit Costs Tracker | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	35	35
Renewable Energy Credits
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	58	15
Renewable Energy Credits | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	0	0
Renewable Energy Credits | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	58	15
Storm Tracker
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	6	7
Storm Tracker | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	6	7
Storm Tracker | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory liabilities	$ 0	$ 0

Property And Plant, Net (Schedu

Property And Plant, Net (Schedule Of Property And Plant, Net) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)equipment	Dec. 31, 2016USD ($)
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	$ 32,027	$ 30,118
Accumulated depreciation and amortization	11,633	10,961
Property and plant, before construction work in progress	20,394	19,157
Property, Plant and Equipment, Net	$ 21,466	20,113
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	$ 233	232
Total accumulated depreciation, capital lease agreements	83	77
Electric Generation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	11,132	10,911
Electric distribution equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	11,415	10,850
Electric transmission equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	4,666	3,879
Gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	2,893	2,641
Property, Plant and Equipment, Other Types [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	1,921	1,837
Construction work in progress	924	750
Nuclear Fuel
Property, Plant and Equipment [Line Items]
Construction work in progress	148	206
Union Electric Company
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	19,495	18,959
Accumulated depreciation and amortization	8,305	7,880
Property and plant, before construction work in progress	11,190	11,079
Property, Plant and Equipment, Net	11,751	11,478
Union Electric Company | Electric Generation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	11,132	10,911
Union Electric Company | Electric distribution equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	5,766	5,563
Union Electric Company | Electric transmission equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	1,201	1,151
Union Electric Company | Gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	474	455
Union Electric Company | Property, Plant and Equipment, Other Types [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	922	879
Construction work in progress	413	193
Union Electric Company | Nuclear Fuel
Property, Plant and Equipment [Line Items]
Construction work in progress	148	206
Ameren Illinois Company
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	11,123	10,208
Accumulated depreciation and amortization	3,082	2,850
Property and plant, before construction work in progress	8,041	7,358
Property, Plant and Equipment, Net	8,293	7,469
Ameren Illinois Company | Electric Generation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	0	0
Ameren Illinois Company | Electric distribution equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	5,649	5,287
Ameren Illinois Company | Electric transmission equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	2,298	2,016
Ameren Illinois Company | Gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	2,419	2,186
Ameren Illinois Company | Property, Plant and Equipment, Other Types [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	757	719
Construction work in progress	252	111
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	1,409	951
Accumulated depreciation and amortization	246	231
Property and plant, before construction work in progress	1,163	720
Property, Plant and Equipment, Net	1,422	1,166
Other | Electric Generation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	0	0
Other | Electric distribution equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	0	0
Other | Electric transmission equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	1,167	712
Other | Gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	0	0
Other | Property, Plant and Equipment, Other Types [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	242	239
Construction work in progress	259	446
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	20 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	72 years
Maximum | Electric distribution equipment [Member]
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	80 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

Property And Plant, Net (Accrue

Property And Plant, Net (Accrued Capital Expenditures) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	$ 361	$ 251	$ 235
Union Electric Company
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	159	116	85
Ameren Illinois Company
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	175	87	92
Nuclear Fuel
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	10	20	16
Nuclear Fuel | Union Electric Company
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	$ 10	$ 20	$ 16

Property And Plant, Net Propert

Property And Plant, Net Property and Plant, Net (Schedule of Capitalized Software) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Capitalized Computer Software, Amortization	$ 58	$ 52	$ 47
Capitalized Computer Software, Gross	655	622
Capitalized Computer Software, Accumulated Amortization	$ (466)	(408)
Software and Software Development Costs [Member] | Maximum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	10 years
Software and Software Development Costs [Member] | Minimum
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Union Electric Company
Property, Plant and Equipment [Line Items]
Capitalized Computer Software, Amortization	$ 20	17	16
Capitalized Computer Software, Gross	191	178
Capitalized Computer Software, Accumulated Amortization	(107)	(87)
Ameren Illinois Company
Property, Plant and Equipment [Line Items]
Capitalized Computer Software, Amortization	36	33	$ 27
Capitalized Computer Software, Gross	241	225
Capitalized Computer Software, Accumulated Amortization	$ (146)	$ (110)

Short-Term Debt And Liquidity60

Short-Term Debt And Liquidity (Narrative) (Details)	12 Months Ended
	Dec. 31, 2017USD ($)lender	Dec. 31, 2016
Credit Agreements
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,600,000,000
Covenant terms, default provisions, maximum indebtedness	100,000,000
Multiyear Credit Facility
Short-term Debt [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	0.53
Line of Credit Facility, Commitment Fee Amount	$ 100,000,000
Multiyear Credit Facility | Maximum
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.65
Illinois Credit Agreement 2012 | Maximum
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,300,000,000
Missouri Credit Agreement 2012 | Maximum
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	1,200,000,000
Parent Company | Illinois Credit Agreement 2012
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	500,000,000
Parent Company | Missouri Credit Agreement 2012
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	700,000,000
Union Electric Company | Missouri Credit Agreement 2012
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 800,000,000
Actual debt-to-capital ratio	0.48
Ameren Illinois Company | Illinois Credit Agreement 2012
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 800,000,000
Actual debt-to-capital ratio	0.47
Utilities [Member]
Short-term Debt [Line Items]
Short Term Debt, Weighted Average Interest Rate During Period	1.19%	0.52%

Short-Term Debt And Liquidity61

Short-Term Debt And Liquidity (Schedule Of Maximum Aggregate Amount Available On Credit Agreements) (Details) $ in Millions	Dec. 31, 2017USD ($)
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, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 668	$ 552
Short-term debt	$ 484	$ 558
Short-term Debt, Weighted Average Interest Rate	1.31%	0.80%
Peak short-term borrowings	$ 948	$ 839
Peak short-term borrowings interest rate	2.00%	1.05%
Parent Company
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 573	$ 440
Short-term debt	$ 383	$ 507
Short-term Debt, Weighted Average Interest Rate	1.30%	0.82%
Peak short-term borrowings	$ 841	$ 574
Peak short-term borrowings interest rate	1.90%	1.05%
Union Electric Company
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 5	$ 60
Short-term debt	$ 39	$ 0
Short-term Debt, Weighted Average Interest Rate	1.24%	0.74%
Peak short-term borrowings	$ 64	$ 208
Peak short-term borrowings interest rate	1.78%	0.85%
Ameren Illinois Company
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 90	$ 52
Short-term debt	$ 62	$ 51
Short-term Debt, Weighted Average Interest Rate	1.35%	0.69%
Peak short-term borrowings	$ 469	$ 195
Peak short-term borrowings interest rate	2.00%	0.90%

Long-Term Debt And Equity Fin63

Long-Term Debt And Equity Financings (Narrative) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Mar. 31, 2017	Oct. 31, 2013
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized (in shares)	100,000,000	100,000,000
Preferred stock, par value (in dollars per share)	$ 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		681,000,000	$ 395,000,000	$ 120,000,000
Debt Default Provision Excess		$ 25,000,000
Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized (in shares)	7,500,000	7,500,000
Preferred stock, par value (in dollars per share)	$ 1	$ 1
Common stock, shares authorized	150,000,000	150,000,000	150,000,000
Repayments of Other Long-term Debt		$ 431,000,000	$ 266,000,000	120,000,000
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized (in shares)	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		$ 250,000,000	$ 129,000,000	$ 0
Common stock equity to capitalization ratio	51.00%	51.00%
Ameren Transmission Company of Illinois
Long-Term Debt And Equity Financings [Line Items]
Redemption price, percentage		100.00%
Parent Company
Long-Term Debt And Equity Financings [Line Items]
Common stock, shares authorized	6,000,000	6,000,000				4,000,000
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 Secured Notes, 2.95%, Due 2027 | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 400,000,000	$ 400,000,000
Long-term debt interest rate	2.95%	2.95%
Proceeds from issuance of secured debt		$ 396,000,000
6.40% Senior secured notes due 2017 | Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate	6.40%	6.40%
Repayments of Other Long-term Debt		$ 425,000,000
6.40% Senior secured notes due 2017 | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 0	$ 0	425,000,000
Long-term debt interest rate	6.40%	6.40%
5.40% Senior secured notes due 2016 | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount					$ 260,000,000
Long-term debt interest rate	5.40%	5.40%
Senior Secured Notes, 3.65%, Due 2045 - $150 Issuance | 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 | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 400,000,000	$ 400,000,000	400,000,000
Long-term debt interest rate	3.65%	3.65%
Senior Secured Notes 4.75% Due2015 | 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, 3.70%, Due 2047 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 500,000,000	$ 500,000,000	0
Long-term debt interest rate	3.70%	3.70%
Proceeds from issuance of secured debt		$ 492,000,000
Senior Secured Notes6125 Due2017 | Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate	6.125%	6.125%
Repayments of Other Long-term Debt		$ 250,000,000
Senior Secured Notes6125 Due2017 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 0	$ 0	250,000,000
Long-term debt interest rate	6.125%	6.125%
Senior Secured Notes 6.20% Due 2016 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
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]
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 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 240,000,000	240,000,000
Senior Secured Notes, 4.15%, Due 2046 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 490,000,000	$ 490,000,000	490,000,000
Long-term debt interest rate	4.15%	4.15%
Proceeds from issuance of secured debt	$ 245,000,000
Senior Unsecured Notes, 3.43%, Due 2050 | Ameren Transmission Company of Illinois | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate	3.43%	3.43%
Senior Unsecured Notes, 3.43%, Due 2050 | Ameren Transmission Company of Illinois | Unsecured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 300,000,000	$ 300,000,000	$ 0
Long-term debt interest rate	3.43%	3.43%
Proceeds from Issuance of Unsecured Debt		$ 449,000,000
Ratio of Indebtedness to Net Capital	0.70	0.70
Ratio of Indebtedness to Total Assets	0.10	0.10
Series 1993 5.90% Due 2023 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 1,000,000	$ 1,000,000
Long-term debt interest rate	5.90%	5.90%
Maximum | Senior Unsecured Notes, 3.43%, Due 2050 | Ameren Transmission Company of Illinois | Unsecured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 450,000,000	$ 450,000,000
Minimum | Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Common stock equity to capitalization ratio	30.00%	30.00%
Minimum | Ameren Transmission Company of Illinois
Long-Term Debt And Equity Financings [Line Items]
Redemption price, percentage		5.00%
Minimum | Senior Unsecured Notes, 3.43%, Due 2050 | Ameren Transmission Company of Illinois | Unsecured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount	$ 150,000,000	$ 150,000,000

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, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Capital Lease Obligations	$ 276,000,000	$ 282,000,000
Less: Maturities due within one year	(841,000,000)	(681,000,000)
Long-term Debt, Net	7,094,000,000	6,595,000,000
Held-to-maturity Securities	276,000,000	282,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Five	506,000,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	5,617,000,000
Parent Company
Debt Instrument [Line Items]
Long-term debt, gross	700,000,000	700,000,000
Debt Issuance Costs, Net	(4,000,000)	(6,000,000)
Long-term Debt, Net	696,000,000	694,000,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	350,000,000
Union Electric Company
Debt Instrument [Line Items]
Long-term debt, gross	3,988,000,000	4,019,000,000
Less: Unamortized discount and premium	(7,000,000)	(6,000,000)
Debt Issuance Costs, Net	(20,000,000)	(19,000,000)
Less: Maturities due within one year	(384,000,000)	(431,000,000)
Long-term Debt, Net	3,577,000,000	3,563,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Five	56,000,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	$ 2,867,000,000
Union Electric Company | 6.40% Senior secured notes due 2017
Debt Instrument [Line Items]
Long-term debt interest rate	6.40%
Union Electric Company | City Of Bowling Green Capital Lease Peno Creek Ct
Debt Instrument [Line Items]
Capital Lease Obligations	$ 36,000,000	42,000,000
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
Ameren Illinois Company
Debt Instrument [Line Items]
Long-term debt, gross	$ 2,857,000,000	2,607,000,000
Less: Unamortized discount and premium	(3,000,000)	0
Debt Issuance Costs, Net	(24,000,000)	(19,000,000)
Less: Maturities due within one year	(457,000,000)	(250,000,000)
Long-term Debt, Net	2,373,000,000	2,338,000,000
Long-term Debt, Maturities, Repayments of Principal in Year Five	400,000,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	$ 2,000,000,000
Ameren Illinois Company | Senior Secured Notes6125 Due2017
Debt Instrument [Line Items]
Long-term debt interest rate	6.125%
Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Long-term debt, gross	$ 450,000,000	0
Debt Issuance Costs, Net	(2,000,000)	0
Long-term Debt, Net	$ 448,000,000	0
Redemption price, percentage	100.00%
Long-term Debt, Maturities, Repayments of Principal in Year Five	$ 50,000,000
Long-term Debt, Maturities, Repayments of Principal after Year Five	400,000,000
Unsecured Debt | Parent Company | Senior Unsecured Notes270 due 2020
Debt Instrument [Line Items]
Debt instrument face amount	$ 350,000,000	350,000,000
Long-term debt interest rate	2.70%
Long-term debt maturity date	2,020
Unsecured Debt | Parent Company | Senior Unsecured Notes365 due 2026
Debt Instrument [Line Items]
Debt instrument face amount	$ 350,000,000	350,000,000
Long-term debt interest rate	3.65%
Long-term debt maturity date	2,026
Unsecured Debt | Ameren Transmission Company of Illinois | Senior Unsecured Notes, 3.43%, Due 2050
Debt Instrument [Line Items]
Debt instrument face amount	$ 300,000,000	0
Long-term debt interest rate	3.43%
Secured Debt | Union Electric Company | 6.40% Senior secured notes due 2017
Debt Instrument [Line Items]
Debt instrument face amount	$ 0	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	$ 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	$ 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	$ 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	$ 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	$ 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
Debt Instrument [Line Items]
Debt instrument face amount	$ 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 | Senior Secured Notes, 2.95%, Due 2027
Debt Instrument [Line Items]
Debt instrument face amount	$ 400,000,000
Long-term debt interest rate	2.95%
Long-term debt maturity date	2,027
Secured Debt | Union Electric Company | 5.50% Senior secured notes due 2034
Debt Instrument [Line Items]
Debt instrument face amount	$ 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	$ 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	$ 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	$ 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
Debt Instrument [Line Items]
Debt instrument face amount	$ 400,000,000	400,000,000
Long-term debt interest rate	3.65%
Long-term debt maturity date	2,045
Secured Debt | Ameren Illinois Company | Senior Secured Notes6125 Due2017
Debt Instrument [Line Items]
Debt instrument face amount	$ 0	250,000,000
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	$ 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	$ 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	$ 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 | Series 1993 5.90% Due 2023
Debt Instrument [Line Items]
Debt instrument face amount	$ 1,000,000
Long-term debt interest rate	5.90%
Secured Debt | Ameren Illinois Company | Senior Secured Notes, 3.25%, Due 2025
Debt Instrument [Line Items]
Debt instrument face amount	$ 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	$ 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	$ 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	$ 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	$ 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	$ 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
Debt Instrument [Line Items]
Debt instrument face amount	$ 490,000,000	490,000,000
Long-term debt interest rate	4.15%
Long-term debt maturity date	2,046
Secured Debt | Ameren Illinois Company | Senior Secured Notes, 3.70%, Due 2047
Debt Instrument [Line Items]
Debt instrument face amount	$ 500,000,000	0
Long-term debt interest rate	3.70%
Long-term debt maturity date	2,047
Secured Debt | Ameren Transmission Company of Illinois | Senior Unsecured Notes, 3.43%, Due 2050
Debt Instrument [Line Items]
Long-term debt interest rate	3.43%
Long-term debt maturity date	2,050
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | 1992 Series due 2022
Debt Instrument [Line Items]
Debt instrument face amount	$ 47,000,000	47,000,000
Long-term debt maturity date	2,022
Redemption price, percentage	100.00%
Environmental Improvement And Pollution Control Revenue Bonds | Union Electric Company | Series1993 Due2028
Debt Instrument [Line Items]
Debt instrument face amount	$ 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	$ 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	$ 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	$ 50,000,000	50,000,000
Long-term debt maturity date	2,033
Environmental Improvement And Pollution Control Revenue Bonds | Ameren Illinois Company | Series 1993 5.90% Due 2023
Debt Instrument [Line Items]
Debt instrument face amount	$ 1,000,000	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	$ 1,000,000	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	$ 17,000,000	17,000,000
Long-term debt maturity date	2,028
Redemption price, percentage	100.00%
Capital Lease Obligations [Member] | Union Electric Company
Debt Instrument [Line Items]
Held-to-maturity Securities	$ 276,000,000	$ 282,000,000
Maximum | Unsecured Debt | Ameren Transmission Company of Illinois | Senior Unsecured Notes, 3.43%, Due 2050
Debt Instrument [Line Items]
Debt instrument face amount	$ 450,000,000
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	18.00%
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	18.00%
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	18.00%
Debt Instrument, Redemption, Period One | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal in Year Five	$ 49,500,000
Debt Instrument, Redemption, Period Two | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal after Year Five	49,500,000
Debt Instrument, Redemption, Period Three | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal after Year Five	49,500,000
Debt Instrument, Redemption, Period Four | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal after Year Five	49,500,000
Debt Instrument, Redemption, Period Five | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal after Year Five	49,500,000
Debt Instrument, Redemption, Period Six | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal after Year Five	49,500,000
Debt Instrument, Redemption, Period Seven | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal after Year Five	76,500,000
Debt Instrument, Redemption, Period Eight | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Long-term Debt, Maturities, Repayments of Principal after Year Five	$ 76,500,000

Long-Term Debt And Equity Fin65

Long-Term Debt And Equity Financings (Schedule Of Average Interest Rates) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
1998 Series A due 2033 | Union Electric Company
Debt Instrument [Line Items]
Debt instrument, interest rate during period	1.77%	0.91%
1998 Series B due 2033 | Union Electric Company
Debt Instrument [Line Items]
Debt instrument, interest rate during period	1.75%	0.92%
1998 Series C due 2033 | Union Electric Company
Debt Instrument [Line Items]
Debt instrument, interest rate during period	1.73%	0.97%
Series B-1 1993 Due 2028 | Ameren Illinois Company
Debt Instrument [Line Items]
Debt instrument, interest rate during period	1.08%	0.70%
1992 Series due 2022 | Union Electric Company
Debt Instrument [Line Items]
Debt instrument, interest rate during period	1.43%	0.66%

Long-Term Debt And Equity Fin66

Long-Term Debt And Equity Financings (Schedule Of Maturities Of Long-Term Debt) (Details) $ in Millions	Dec. 31, 2017USD ($)
Debt Instrument [Line Items]
2,018	$ 841
2,019	581
2,020	442
2,021	8
2,022	506
Thereafter	5,617
Total	7,995
Parent Company
Debt Instrument [Line Items]
2,020	350
Thereafter	350
Total	700
Debt Instrument, Unamortized Discount or Premium and Debt Issuance Costs	4
Union Electric Company
Debt Instrument [Line Items]
2,018	384
2,019	581
2,020	92
2,021	8
2,022	56
Thereafter	2,867
Total	3,988
Debt Instrument, Unamortized Discount or Premium and Debt Issuance Costs	27
Ameren Illinois Company
Debt Instrument [Line Items]
2,018	457
2,022	400
Thereafter	2,000
Total	2,857
Debt Instrument, Unamortized Discount or Premium and Debt Issuance Costs	27
Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
2,022	50
Thereafter	400
Total	450
Debt Instrument, Unamortized Discount or Premium and Debt Issuance Costs	$ 2

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, 2017	Dec. 31, 2016
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, par value (in dollars per share)	$ 0.01
Preferred stock, authorized (in shares)	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 (in dollars per share)	$ 1
Preferred stock, authorized (in shares)	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 (in dollars per share)	$ 100
Preferred stock, authorized (in shares)	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	$ 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 (in shares)	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 (in dollars per share)	$ 100
Preferred stock, authorized (in shares)	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

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, 2017USD ($)
Union Electric Company
Debt Instrument [Line Items]
Bonds Issuable Based On Coverage Ratio	$ 4,222
Preferred Stock Issuable Based On Coverage Ratio	2,118
Retired Bond Capacity	$ 1,629
Union Electric Company | Actual Interest Coverage Ratio
Debt Instrument [Line Items]
Interest Coverage Ratio	4.8
Dividend Coverage Ratio	95.4
Ameren Illinois Company
Debt Instrument [Line Items]
Bonds Issuable Based On Coverage Ratio	$ 4,119
Preferred Stock Issuable Based On Coverage Ratio	203
Retired Bond Capacity	$ 529
Ameren Illinois Company | Actual Interest Coverage Ratio
Debt Instrument [Line Items]
Interest Coverage Ratio	7.1
Dividend Coverage Ratio	2.9
Minimum | Union Electric Company | Required Dividend Coverage Ratio
Debt Instrument [Line Items]
Interest Coverage Ratio	2
Dividend Coverage Ratio	2.5
Minimum | Ameren Illinois Company | Required Dividend Coverage Ratio
Debt Instrument [Line Items]
Interest Coverage Ratio	2
Dividend Coverage Ratio	1.5

Other Income And Expenses (Othe

Other Income And Expenses (Other Income And Expenses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 24	$ 27	$ 30
Interest income on industrial development revenue bonds	26	27	27
Interest and dividend income	8	13	14
Other	1	7	3
Total miscellaneous income	59	74	74
Donations	8	16	15
Other	13	16	15
Total miscellaneous expense	21	32	30
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	21	23	22
Interest income on industrial development revenue bonds	26	27	27
Interest and dividend income	1	1	1
Other		1	2
Total miscellaneous income	48	52	52
Donations	2	4	5
Other	6	6	6
Total miscellaneous expense	8	10	11
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	3	4	8
Interest and dividend income	7	12	12
Other	1	5	1
Total miscellaneous income	11	21	21
Donations	5	6	5
Other	5	6	7
Total miscellaneous expense	$ 10	$ 12	$ 12

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, 2017galMWhMMBTUlb	Dec. 31, 2016galMWhMMBTUlb
Fuel Oils
Derivative [Line Items]
Quantity | gal	28	30
Natural Gas
Derivative [Line Items]
Quantity | MMBTU	163	154
Power
Derivative [Line Items]
Quantity | MWh	12	10
Uranium
Derivative [Line Items]
Quantity | lb	370	345
Union Electric Company | Fuel Oils
Derivative [Line Items]
Quantity | gal	28	30
Union Electric Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU	24	25
Union Electric Company | Power
Derivative [Line Items]
Quantity | MWh	3	1
Union Electric Company | Uranium
Derivative [Line Items]
Quantity | lb	370	345
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU	139	129
Ameren Illinois Company | Power
Derivative [Line Items]
Quantity | MWh	9	9

Derivative Financial Instrume71

Derivative Financial Instruments (Derivative Instruments Carrying Value) (Details) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivative assets	$ 17	$ 27
Derivative liabilities	226	211
Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets	5	2
Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets	2	1
Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities		5
Natural Gas | Other current assets
Derivative [Line Items]
Derivative assets		12
Natural Gas | Other assets
Derivative [Line Items]
Derivative assets	1	3
Natural Gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	17	4
Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	13	10
Power | Other current assets
Derivative [Line Items]
Derivative assets	9	9
Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	14	15
Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	182	173
Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	0	4
Union Electric Company
Derivative [Line Items]
Derivative assets	17	14
Derivative liabilities	9	18
Union Electric Company | Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets	5	2
Union Electric Company | Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets	2	1
Union Electric Company | Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities		5
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
Union Electric Company | Natural Gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	5	1
Union Electric Company | Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	3	5
Union Electric Company | Power | Other current assets
Derivative [Line Items]
Derivative assets	9	9
Union Electric Company | Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	1	3
Union Electric Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Union Electric Company | Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	0	4
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	0	13
Derivative liabilities	217	193
Ameren Illinois Company | Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities		0
Ameren Illinois Company | Natural Gas | Other current assets
Derivative [Line Items]
Derivative assets		11
Ameren Illinois Company | Natural Gas | Other assets
Derivative [Line Items]
Derivative assets	0	2
Ameren Illinois Company | Natural Gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	12	3
Ameren Illinois Company | Natural Gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	10	5
Ameren Illinois Company | Power | Other current assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	13	12
Ameren Illinois Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	182	173
Ameren Illinois Company | Uranium | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	$ 0	$ 0

Derivative Financial Instrume72

Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Details) $ in Millions	Dec. 31, 2017USD ($)
Derivative [Line Items]
Restricted Cash and Cash Equivalents	$ 8
Aggregate Fair Value of Derivative Liabilities	98
Cash Collateral Posted	3
Potential Aggregate Amount of Additional Collateral Required	82
Union Electric Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	55
Cash Collateral Posted	3
Potential Aggregate Amount of Additional Collateral Required	44
Ameren Illinois Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	43
Cash Collateral Posted	0
Potential Aggregate Amount of Additional Collateral Required	$ 38

Fair Value Measurements (Schedu

Fair Value Measurements (Schedule of Valuation Process and Unobservable Inputs) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)$ / MMBTU$ / credit$ / MW$ / MWh	Dec. 31, 2016USD ($)$ / MMBTU$ / credit$ / MW$ / MWh$ / lb
Fuel Oils
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	$ 7	$ 3
Derivative liabilities		(5)
Natural Gas
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	1	15
Derivative liabilities	(30)	(14)
Power
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	9	9
Derivative liabilities	$ (196)	(188)
Uranium
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities		$ (4)
Discounted Cash Flow | Minimum | Fuel Oils
Fair Value Inputs [Abstract]
Escalation rate		(2.00%)
Counterparty credit risk	12.00%	0.13%
Credit risk	37.00%	0.38%
Discounted Cash Flow | Minimum | Natural Gas
Fair Value Inputs [Abstract]
Counterparty credit risk	37.00%	0.13%
Credit risk	37.00%	0.38%
Nodal basis | $ / MWh	(1.2)	(0.80)
Discounted Cash Flow | Minimum | Power
Fair Value Inputs [Abstract]
Counterparty credit risk	28.00%
Credit risk	37.00%	0.38%
Average bid/ask consensus peak and off-peak pricing | $ / MWh	24	26
Estimated auction price for FTRs | $ / MW	(65)	(71)
Nodal basis | $ / MWh	(10)	(6)
Discounted Cash Flow | Minimum | Uranium
Fair Value Inputs [Abstract]
Credit risk		0.38%
Average bid/ask consensus pricing | $ / lb		22
Discounted Cash Flow | Maximum | Fuel Oils
Fair Value Inputs [Abstract]
Escalation rate		2.00%
Counterparty credit risk	72.00%	0.22%
Credit risk	37.00%	0.38%
Discounted Cash Flow | Maximum | Natural Gas
Fair Value Inputs [Abstract]
Counterparty credit risk	92.00%	8.00%
Credit risk	37.00%	0.38%
Nodal basis | $ / MWh	0.1	0
Discounted Cash Flow | Maximum | Power
Fair Value Inputs [Abstract]
Counterparty credit risk	28.00%
Credit risk	37.00%	0.38%
Average bid/ask consensus peak and off-peak pricing | $ / MWh	46	44
Estimated auction price for FTRs | $ / MW	1,823	5,270
Nodal basis | $ / MWh	0	0
Discounted Cash Flow | Maximum | Uranium
Fair Value Inputs [Abstract]
Credit risk		0.38%
Average bid/ask consensus pricing | $ / lb		24
Discounted Cash Flow | Weighted Average | Fuel Oils
Fair Value Inputs [Abstract]
Escalation rate		0.00%
Counterparty credit risk	41.00%	0.15%
Credit risk	37.00%	0.38%
Discounted Cash Flow | Weighted Average | Natural Gas
Fair Value Inputs [Abstract]
Counterparty credit risk	53.00%	1.00%
Credit risk	37.00%	0.38%
Nodal basis | $ / MWh	(1)	(0.50)
Discounted Cash Flow | Weighted Average | Power
Fair Value Inputs [Abstract]
Counterparty credit risk	28.00%
Credit risk	37.00%	0.38%
Average bid/ask consensus peak and off-peak pricing | $ / MWh	28	29
Estimated auction price for FTRs | $ / MW	251	125
Nodal basis | $ / MWh	(2)	(2)
Discounted Cash Flow | Weighted Average | Uranium
Fair Value Inputs [Abstract]
Credit risk		0.38%
Average bid/ask consensus pricing | $ / lb		22
Option Model | Minimum | Fuel Oils
Fair Value Inputs [Abstract]
Volatilities	20.00%	24.00%
Option Model | Minimum | Natural Gas
Fair Value Inputs [Abstract]
Volatilities	26.00%	31.00%
Nodal basis | $ / MWh	(0.5)	(0.40)
Option Model | Minimum | Uranium
Fair Value Inputs [Abstract]
Volatilities		24.00%
Option Model | Maximum | Fuel Oils
Fair Value Inputs [Abstract]
Volatilities	26.00%	66.00%
Option Model | Maximum | Natural Gas
Fair Value Inputs [Abstract]
Volatilities	46.00%	66.00%
Nodal basis | $ / MWh	(0.3)	(0.10)
Option Model | Maximum | Uranium
Fair Value Inputs [Abstract]
Volatilities		24.00%
Option Model | Weighted Average | Fuel Oils
Fair Value Inputs [Abstract]
Volatilities	22.00%	28.00%
Option Model | Weighted Average | Natural Gas
Fair Value Inputs [Abstract]
Volatilities	37.00%	36.00%
Nodal basis | $ / MWh	(0.4)	(0.20)
Option Model | Weighted Average | Uranium
Fair Value Inputs [Abstract]
Volatilities		24.00%
Fundamental Energy Production Model | Minimum | Power
Fair Value Inputs [Abstract]
Escalation rate	5.00%	5.00%
Estimated future gas prices | $ / MMBTU	3	3
Fundamental Energy Production Model | Maximum | Power
Fair Value Inputs [Abstract]
Escalation rate	5.00%	5.00%
Estimated future gas prices | $ / MMBTU	4	4
Fundamental Energy Production Model | Weighted Average | Power
Fair Value Inputs [Abstract]
Escalation rate	5.00%	5.00%
Estimated future gas prices | $ / MMBTU	3	3
Contract Price Allocation | Minimum | Power
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit	5	5
Contract Price Allocation | Maximum | Power
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit	7	7
Contract Price Allocation | Weighted Average | Power
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit	6	6
Derivative liabilities | Fuel Oils
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	$ 0	$ 0
Derivative liabilities | Natural Gas
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(4)	(1)
Derivative liabilities | Power
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities	(196)	(187)
Derivative liabilities | Uranium
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities		(4)
Derivative assets | Fuel Oils
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	3
Derivative assets | Natural Gas
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	1
Derivative assets | Power
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets	$ 8
Derivative assets | Uranium
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets		$ 0

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, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	$ 702	$ 605
Assets	719	632
Excluded receivables, payables, and accrued income, net	2	2
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	470	409
Assets	474	413
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	232	196
Assets	233	208
Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Assets	12	11
Cash And Cash Equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	2	1
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	2	1
Cash And Cash Equivalents | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Cash And Cash Equivalents | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	17	27
Derivative liabilities	226	211
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	4	4
Derivative liabilities	1	5
Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	12
Derivative liabilities	25	14
Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	12	11
Derivative liabilities	200	192
Fuel Oils
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	7	3
Derivative liabilities		5
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	4	2
Derivative liabilities		5
Fuel Oils | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities		0
Fuel Oils | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	1
Derivative liabilities		0
Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	15
Derivative liabilities	30	14
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	0	2
Derivative liabilities	1	0
Natural Gas | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	12
Derivative liabilities	25	13
Natural Gas | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	1
Derivative liabilities	4	1
Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	9	9
Derivative liabilities	196	188
Power | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Power | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	0
Derivative liabilities	0	1
Power | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	8	9
Derivative liabilities	196	187
Uranium
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		4
Uranium | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0
Uranium | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0
Uranium | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		4
Equity Securities | U.S. Large Capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	468	408
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	468	408
Equity Securities | U.S. Large Capitalization | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Equity Securities | U.S. Large Capitalization | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Debt Securities | Corporate Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	82	67
Debt Securities | Corporate Bonds | 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	0	0
Debt Securities | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	82	67
Debt Securities | Corporate Bonds | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Debt Securities | U.S. treasury and agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	125	112
Debt Securities | U.S. treasury and agency securities | 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	0	0
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	125	112
Debt Securities | U.S. treasury and agency securities | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Debt Securities | Other debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	25	17
Debt Securities | Other debt securities | 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	0	0
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	25	17
Debt Securities | Other debt securities | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	702	605
Assets	719	619
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	470	409
Assets	474	411
Union Electric Company | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	232	196
Assets	233	197
Union Electric Company | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Assets	12	11
Union Electric Company | Cash And Cash Equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	2	1
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	2	1
Union Electric Company | Cash And Cash Equivalents | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Cash And Cash Equivalents | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	17	14
Derivative liabilities	9	18
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	4	2
Derivative liabilities	0	5
Union Electric Company | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	1
Derivative liabilities	7	7
Union Electric Company | Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	12	11
Derivative liabilities	2	6
Union Electric Company | Fuel Oils
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	7	3
Derivative liabilities		5
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	4	2
Derivative liabilities		5
Union Electric Company | Fuel Oils | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities		0
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
Derivative liabilities		0
Union Electric Company | Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	2
Derivative liabilities	8	6
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 assets	0	0
Derivative liabilities	0	0
Union Electric Company | Natural Gas | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	1
Derivative liabilities	7	6
Union Electric Company | Natural Gas | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	1
Derivative liabilities	1	0
Union Electric Company | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	9	9
Derivative liabilities	1	3
Union Electric Company | Power | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Union Electric Company | Power | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	0
Derivative liabilities	0	1
Union Electric Company | Power | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	8	9
Derivative liabilities	1	2
Union Electric Company | Uranium
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		4
Union Electric Company | Uranium | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0
Union Electric Company | Uranium | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0
Union Electric Company | Uranium | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		4
Union Electric Company | Equity Securities | U.S. Large Capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	468	408
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	468	408
Union Electric Company | Equity Securities | U.S. Large Capitalization | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Equity Securities | U.S. Large Capitalization | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt Securities | Corporate Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	82	67
Union Electric Company | Debt Securities | Corporate Bonds | 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	0	0
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	82	67
Union Electric Company | Debt Securities | Corporate Bonds | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
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	125	112
Union Electric Company | Debt Securities | U.S. treasury and agency securities | 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	0	0
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	125	112
Union Electric Company | Debt Securities | U.S. treasury and agency securities | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Union Electric Company | Debt Securities | Other debt securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	25	17
Union Electric Company | Debt Securities | Other debt securities | 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	0	0
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	25	17
Union Electric Company | Debt Securities | Other debt securities | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Ameren Illinois Company | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	217	193
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	1	0
Ameren Illinois Company | Commodity Contract | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	18	7
Ameren Illinois Company | Commodity Contract | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	198	186
Ameren Illinois Company | Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		13
Derivative liabilities	22	8
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		2
Derivative liabilities	1	0
Ameren Illinois Company | Natural Gas | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		11
Derivative liabilities	18	7
Ameren Illinois Company | Natural Gas | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0
Derivative liabilities	3	1
Ameren Illinois Company | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	195	185
Ameren Illinois Company | Power | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Power | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Power | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	$ 195	$ 185

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, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	$ (188)	$ (178)	$ (154)
Included in regulatory assets/liabilities	(25)	(30)
Purchases	14	13
Sales	1
Settlements	0	(7)
Change in unrealized gains (losses) related to assets/liabilities still held	(22)	(27)
Union Electric Company
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	7	7	16
Included in regulatory assets/liabilities	(4)	(1)
Purchases	14	13
Sales	1
Settlements	(11)	(21)
Change in unrealized gains (losses) related to assets/liabilities still held	0	0
Ameren Illinois Company
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	(195)	(185)	$ (170)
Included in regulatory assets/liabilities	(21)	(29)
Purchases	0	0
Sales	0
Settlements	11	14
Change in unrealized gains (losses) related to assets/liabilities still held	$ (22)	$ (27)

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, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Capital Lease Obligations	$ 276	$ 282
Held-to-maturity Securities	276	282
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt and Capital Lease Obligations (including current portion)	7,935	7,276
Preferred stock	142	142
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt and capital lease obligations (including current portion)	8,531	7,772
Preferred stock	131	131
Union Electric Company | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt and Capital Lease Obligations (including current portion)	3,961	3,994
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)	4,348	4,304
Preferred stock	80	79
Ameren Illinois Company | Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt and Capital Lease Obligations (including current portion)	2,830	2,588
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)	3,028	2,765
Preferred stock	$ 51	$ 52

Callaway Energy Center (Details

Callaway Energy Center (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)mill	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Nuclear Waste Matters [Line Items]
Litigation Settlement, Amount	$ 3	$ 24	$ 14
Nuclear Plant
Nuclear Waste Matters [Line Items]
Nwf Fee Number Of Mills | mill	1
Annual decommissioning costs included in costs of service	$ 7

Callaway Energy Center Callaw78

Callaway Energy Center Callaway Energy Center (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, 2017	Dec. 31, 2016	Dec. 31, 2015
Nuclear Waste Matters [Line Items]
Sales and maturities of securities - nuclear decommissioning trust fund	$ 396	$ 377	$ 349
Gross realized gains	13	7	8
Gross realized losses	5	4	2
Union Electric Company
Nuclear Waste Matters [Line Items]
Sales and maturities of securities - nuclear decommissioning trust fund	$ 396	$ 377	$ 349
Minimum | Nuclear Decommissioning Trust Fund [Member]
Nuclear Waste Matters [Line Items]
Trust fund investments, target allocation percentage	60.00%
Maximum | Nuclear Decommissioning Trust Fund [Member]
Nuclear Waste Matters [Line Items]
Trust fund investments, target allocation percentage	70.00%

Callaway Energy Center Callaw79

Callaway Energy Center Callaway Energy Center (Fair Values Of Investments In Debt And Equity Securities In Nuclear Decommissioning Trust Fund) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Nuclear Waste Matters [Line Items]
Cost	$ 387	$ 361
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	323	256
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	6	10
Fair Value	704	607
Debt Securities
Nuclear Waste Matters [Line Items]
Cost	228	197
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	5	3
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	1	4
Fair Value	232	196
Equity Securities
Nuclear Waste Matters [Line Items]
Cost	155	161
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	318	253
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax	5	6
Fair Value	468	408
Cash
Nuclear Waste Matters [Line Items]
Cost	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	2	1
Other Debt And Equity Securities
Nuclear Waste Matters [Line Items]
Cost	2	2
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	$ 2	$ 2

Callaway Energy Center Callaw80

Callaway Energy Center Callaway Energy Center (Cost and Fair Values of Investments In Debt Securities in Nuclear Decommissioning Trust Fund According to Contractual Maturities) (Details) $ in Millions	Dec. 31, 2017USD ($)
Nuclear Waste Matters [Abstract]
Cost, Less than 5 years	$ 120
Cost, 5 years to 10 years	54
Cost, Due after 10 years	54
Cost, Total	228
Fair Value, Less than 5 years	120
Fair Value, 5 years to 10 years	55
Fair Value, Due after 10 years	57
Fair Value, Total	$ 232

Callaway Energy Center Callaw81

Callaway Energy Center Callaway Energy Center (Insurance Disclosure) (Details) € in Millions	12 Months Ended
	Dec. 31, 2017USD ($)	Dec. 31, 2017EUR (€)
Nuclear Waste Matters [Line Items]
Number of weeks of coverage after the first eight weeks of an outage	1 year	1 year
Threshold Amount For which a Retrospective Assessment For a Covered loss is necessary	$ 450,000,000
Annual Payment in the event of an Incident at any Licensed Commercial Reactor	19,000,000
Amount of Weekly Indemnity Coverage Commencing Eight Weeks After Power Outage	4,500,000
Amount of additional weekly indemnity coverage commencing after initial indemnity coverage	$ 3,600,000
Number Of Years The Limit Of Liability And The Maximum Potential Annual Payments Are Adjusted	5 years	5 years
Aggregate nuclear power industry insurance policy limit for losses from terrorist attacks within twelve month period	$ 3,200,000,000	€ 600
Number Of Additional Weeks After Initial Indemnity Coverage For Power Outage	1 year 4 months 10 days	1 year 4 months 10 days
Public Liability And Nuclear Worker Liability - American Nuclear Insurers
Nuclear Waste Matters [Line Items]
Maximum coverages	$ 450,000,000
Maximum assessments for single incidents	0
Public Liability And Nuclear Worker Liability - Pool Participation
Nuclear Waste Matters [Line Items]
Maximum coverages	12,986,000,000
Maximum assessments for single incidents	127,000,000
Public Liability
Nuclear Waste Matters [Line Items]
Maximum coverages	13,436,000,000
Maximum assessments for single incidents	127,000,000
Property Damage - Nuclear Electric Insurance Ltd
Nuclear Waste Matters [Line Items]
Maximum coverages	3,200,000,000
Maximum assessments for single incidents	30,000,000
Replacement Power - Nuclear Electric Insurance Ltd
Nuclear Waste Matters [Line Items]
Maximum coverages	490,000,000
Maximum assessments for single incidents	7,000,000
Amount Of Weekly Indemnity Coverage Thereafter Not Exceeding Policy Limit	490,000,000
Sub-limit of for Non-Nuclear Events	328,000,000
Property Damage European Mutual Association for Nuclear Insurance
Nuclear Waste Matters [Line Items]
Maximum coverages	490,000,000
Radiation Event
Nuclear Waste Matters [Line Items]
Maximum coverages	2,700,000,000
Non-radiation event
Nuclear Waste Matters [Line Items]
Maximum coverages	2,300,000,000
Aggregate nuclear power industry insurance policy limit for losses from terrorist attacks within twelve month period	$ 1,800,000,000

Retirement Benefits (Narrative)

Retirement Benefits (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)bond	Dec. 31, 2016USD ($)	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded Status of Plan	$ 551	$ 774
Defined Benefit Plan Assumptions Used Calculating Benefit Obligation, Change in Discount Rate	0.50%
Number of high-quality corporate bonds | bond	600
Defined benefit plan, estimated future employer contributions during the next five years	$ 120
Amortization basis, straight line, in years	10 years
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded Status of Plan	$ (534)	$ (705)
Expected return on plan assets	7.00%	7.00%	7.25%
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded Status of Plan	$ (17)	$ (69)
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	$ 1
Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, estimated future employer contributions during the next five years	$ 60
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%

Retirement Benefits (Summary Of

Retirement Benefits (Summary Of Benefit Liability Recorded) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	$ 551	$ 774
Union Electric Company
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	215	293
Ameren Illinois Company
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	$ 213	$ 315

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, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in plan assets:
Funded status – deficiency	$ (551)	$ (774)
Amounts recognized in the balance sheet consist of:
Noncurrent liability	545	769
Net liability recognized	551	774
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation at end of year	4,577	4,288
Change in benefit obligation:
Net benefit obligation at beginning of year	4,518	4,197
Service cost	93	81	$ 92
Interest cost	179	185	174
Actuarial loss	255	265
Benefits paid	(218)	(210)
Net benefit obligation at end of year	4,827	4,518	4,197
Change in plan assets:
Fair value of plan assets at beginning of year	3,813	3,653
Actual return on plan assets	634	313
Employer contributions	64	57	111
Benefits paid	(218)	(210)
Fair value of plan assets at end of year	4,293	3,813	3,653
Funded status – deficiency	534	705
Accrued benefit cost at December 31	534	705
Amounts recognized in the balance sheet consist of:
Current liability(c)	3	3
Noncurrent liability	531	702
Net liability recognized	534	705
Amounts recognized in regulatory assets consist of:
Net actuarial (gain) loss	374	535
Prior service credit	(3)	(4)
Amounts (pretax) recognized in accumulated OCI consist of:
Net actuarial loss	30	43
Prior service credit	0	0
Total	401	574
Postretirement Benefits
Change in benefit obligation:
Net benefit obligation at beginning of year	1,170	1,094
Service cost	21	19	24
Interest cost	47	50	48
Participant contributions	8	8
Actuarial loss	53	52
Benefits paid	(59)	(54)
Federal subsidy on benefits paid	0	1
Net benefit obligation at end of year	1,240	1,170	1,094
Change in plan assets:
Fair value of plan assets at beginning of year	1,101	1,071
Actual return on plan assets	171	73
Employer contributions	2	2	18
Federal subsidy on benefits paid	0	1
Participant contributions	8	8
Benefits paid	(59)	(54)
Fair value of plan assets at end of year	1,223	1,101	$ 1,071
Funded status – deficiency	17	69
Accrued benefit cost at December 31	17	69
Amounts recognized in the balance sheet consist of:
Current liability(c)	3	2
Noncurrent liability	14	67
Net liability recognized	17	69
Amounts recognized in regulatory assets consist of:
Net actuarial (gain) loss	(69)	(29)
Prior service credit	(3)	(8)
Amounts (pretax) recognized in accumulated OCI consist of:
Net actuarial loss	2	0
Prior service credit	0	(1)
Total	$ (70)	$ (38)

Retirement Benefits (Assumption

Retirement Benefits (Assumptions Used To Determine Benefit Obligations) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Postretirement Health Coverage
Defined Benefit Plan Disclosure [Line Items]
Medical cost trend rate (initial)	3.00%
Medical cost trend rate (ultimate)	3.00%
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate at measurement date	3.50%	4.00%
Increase in future compensation	3.50%	3.50%
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate at measurement date	3.50%	4.00%
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, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	$ 64	$ 57	$ 111
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	2	2	18
Union Electric Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	19	21	47
Union Electric Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	1	1	8
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	37	30	45
Ameren Illinois Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	1	1	8
Other | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	8	6	19
Other | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	$ 0	$ 0	$ 2

Retirement Benefits (Target All

Retirement Benefits (Target Allocation Of The Plans' Asset Categories) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
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%	57.00%
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%	9.00%
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%	14.00%
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%	37.00%
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	1.00%	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	2.00%	3.00%
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	63.00%	61.00%
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	41.00%	40.00%
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	8.00%	7.00%
Postretirement 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%	14.00%
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	35.00%	36.00%

Retirement Benefits (Fair Value

Retirement Benefits (Fair Value Of Plan Assets Utilizing Fair Value Hierarchy) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 4,293	$ 3,813	$ 3,653
Alternative Investments, Fair Value Disclosure	2,213	1,995
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	566	494
Pension Benefits | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,640	1,434
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	25	33
Alternative Investments, Fair Value Disclosure	25	33
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	1,523	1,352
Alternative Investments, Fair Value Disclosure	1,523	1,352
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	379	361
Alternative Investments, Fair Value Disclosure	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	379	361
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	629	522
Alternative Investments, Fair Value Disclosure	450	389
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	179	133
Pension Benefits | International and emerging markets | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
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	741	630
Alternative Investments, Fair Value Disclosure	15	13
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	726	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	91	95
Alternative Investments, Fair Value Disclosure	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	91	95
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	824	701
Alternative Investments, Fair Value Disclosure	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	8	0
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	816	701
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	7	21
Alternative Investments, Fair Value Disclosure	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	7	21
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	196	202
Alternative Investments, Fair Value Disclosure	196	202
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	4	6
Alternative Investments, Fair Value Disclosure	4	6
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	(153)	(132)
Pension Benefits | Net receivables
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	27	22
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	4,419	3,923
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	4,293	3,813
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,223	1,101	$ 1,071
Alternative Investments, Fair Value Disclosure	245	212
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	509	456
Postretirement Benefits | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	342	326
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	44	53
Alternative Investments, Fair Value Disclosure	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	44	53
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	442	392
Alternative Investments, Fair Value Disclosure	110	101
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	332	291
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	80	72
Alternative Investments, Fair Value Disclosure	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	80	72
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 and emerging markets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	154	132
Alternative Investments, Fair Value Disclosure	101	92
Postretirement 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	53	40
Postretirement Benefits | International and emerging markets | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | International and emerging markets | 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	8	7
Alternative Investments, Fair Value Disclosure	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	8	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	144	141
Alternative Investments, Fair Value Disclosure	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	144	141
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	110
Alternative Investments, Fair Value Disclosure	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	110
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	76	68
Alternative Investments, Fair Value Disclosure	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	76	68
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	38	19
Alternative Investments, Fair Value Disclosure	34	19
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	4	0
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	153	132
Postretirement Benefits | Net payables
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(26)	(25)
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	1,096	994
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	$ 1,223	$ 1,101

Retirement Benefits (Components

Retirement Benefits (Components Of Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 93	$ 81	$ 92
Interest cost	179	185	174
Expected return on plan assets	(262)	(253)	(248)
Prior service credit	(1)	(1)	(1)
Actuarial (gain) loss	55	32	74
Curtailment gain			1
Net periodic benefit cost (income)	64	44	92
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	21	19	24
Interest cost	47	50	48
Expected return on plan assets	(75)	(72)	(68)
Prior service credit	(5)	(5)	(5)
Actuarial (gain) loss	(6)	(11)	5
Curtailment gain			0
Net periodic benefit cost (income)	$ (18)	$ (19)	$ 4

Retirement Benefits (Summary 90

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, 2017USD ($)
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	$ (1)
Net actuarial (gain) loss	60
Net actuarial loss	5
Net periodic benefit cost	64
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	(2)
Net actuarial (gain) loss	(1)
Net actuarial loss	0
Net periodic benefit cost	$ (3)

Retirement Benefits (Summary 91

Retirement Benefits (Summary Of Benefit Plan Costs Incurred) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 64	$ 44	$ 92
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	(18)	(19)	4
Union Electric Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	24	26	54
Union Electric Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	(4)	(5)	8
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	41	22	38
Ameren Illinois Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	(14)	(13)	(3)
Other | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	(1)	(4)	0
Other | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 0	$ (1)	$ (1)

Retirement Benefits (Schedule O

Retirement Benefits (Schedule Of Expected Payments From Qualified Trust And Company Funds) (Details) $ in Millions	Dec. 31, 2017USD ($)
Pension Benefits | Paid From Qualified Trust
Defined Benefit Plan Disclosure [Line Items]
2,018	$ 255
2,019	261
2,020	266
2,021	277
2,022	280
2023 - 2027	1,421
Pension Benefits | Paid From Company Funds
Defined Benefit Plan Disclosure [Line Items]
2,018	3
2,019	3
2,020	3
2,021	3
2,022	3
2023 - 2027	13
Postretirement Benefits | Paid From Qualified Trust
Defined Benefit Plan Disclosure [Line Items]
2,018	57
2,019	59
2,020	62
2,021	64
2,022	65
2023 - 2027	331
Postretirement Benefits | Paid From Company Funds
Defined Benefit Plan Disclosure [Line Items]
2,018	2
2,019	2
2,020	2
2,021	2
2,022	2
2023 - 2027	$ 12

Retirement Benefits (Assumpti93

Retirement Benefits (Assumptions Used To Determine Net Periodic Benefit Cost) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate at measurement date	4.00%	4.50%	4.00%
Expected return on plan assets	7.00%	7.00%	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	4.00%	4.50%	4.00%
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%
Postretirement Health Coverage
Defined Benefit Plan Disclosure [Line Items]
Medical cost trend rate (initial)	3.00%
Medical cost trend rate (ultimate)	3.00%

Retirement Benefits (Schedule94

Retirement Benefits (Schedule Of Potential Changes In Key Assumptions) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)
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	157
Service Cost and Interest Cost, .25% increase in salary rate	2
Benefit Obligation, .25% increase in salary rate	15
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	44
Service Cost and Interest Cost, 1.00% increase in annual medical trend	4
Benefit Obligation, 1.00% increase in annual medical trend	71
Service Cost and Interest Cost, 1.00% decrease in annual medical trend	(4)
Benefit Obligation, 1.00% decrease in annual medical trend	$ (71)

Retirement Benefits (Schedule95

Retirement Benefits (Schedule Of Matching Contributions) (Details) - United States Postretirement Benefit Plan of US Entity [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 30	$ 29	$ 29
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	13	12	12
Other
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 1	$ 1	$ 1

Stock-Based Compensation (Narra

Stock-Based Compensation (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized	8,000,000
Maximum shares available for grants	4,900,000
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	$ 39	$ 83	$ 27
Compensation cost not yet recognized	$ 29
Expected weighted average recognition period for share-based compensation expense, in months	22 months

Stock-Based Compensation (Summa

Stock-Based Compensation (Summary Of Nonvested Shares) (Details) - Performance Share Units - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fully Vested Undistributed Retirement-eligible units	712,572	369,878
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	780,545
Share Units, Granted	508,161
Share Units, Unearned or forfeited	(50,523)
Undistributed Vested Units	(342,694)
Share Units, Nonvested at end of year	895,489	780,545
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	$ 47.54
Fair value of share units granted (in dollars per share)	59.16	$ 44.13	$ 52.88
Weighted-average Fair Value per Unit, Unearned or forfeited	52.50
Weighted-average Fair Value per Unit, Earned and vested	51.65
Weighted-average Fair Value per Unit, Nonvested at end of year	$ 52.28	$ 47.54

Stock-Based Compensation Stock-

Stock-Based Compensation Stock-Based Compensation (Summary of Expense) (Details) - Performance Share Units - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 18	$ 17	$ 19
Employee service share-based compensation, tax benefit from compensation expense	7	6	7
Share-based Compensation Expense, Net of Tax	$ 11	$ 11	$ 12
Performance period	3 years
Fair value of share units granted (in dollars per share)	$ 59.16	$ 44.13	$ 52.88
Closing common share price (in dollars per share)		$ 52.46	$ 43.23	$ 46.13
Three-year risk-free rate	1.47%	1.31%	1.10%
Volatility rate, minimum	15.00%	15.00%	12.00%
Volatility rate, maximum	21.00%	20.00%	18.00%
Ameren Missouri [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 4	$ 4	$ 5
Ameren Illinois Company
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	2	2	3
Other
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 12	$ 11	$ 11

Income Taxes (Narrative) (Detai

Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Federal statutory corporate income tax rate:		35.00%	35.00%	35.00%
State corporate income tax rate		6.00%	4.00%	5.00%
Deferred Tax Liabilities, Net		$ 2,506	$ 4,264
Minimum
Income Taxes [Line Items]
Regulatory Liability, Amortization Period		35 years
Maximum
Income Taxes [Line Items]
Regulatory Liability, Amortization Period		60 years
Other
Income Taxes [Line Items]
Deferred Tax Liabilities, Net		$ (165)	$ (380)
Union Electric Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate:		35.00%	35.00%	35.00%
State corporate income tax rate		4.00%	3.00%	3.00%
Deferred Tax Liabilities, Net		$ 1,650	$ 3,013
Ameren Illinois Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate:		35.00%	35.00%	35.00%
State corporate income tax rate		6.00%	5.00%	5.00%
Deferred Tax Liabilities, Net		$ 1,021	$ 1,631
State
Income Taxes [Line Items]
State Income Tax Statutory Rate		7.75%
Deferred Tax Liabilities, Net		$ 97
State | Minimum
Income Taxes [Line Items]
State Income Tax Statutory Rate		7.30%
State | Maximum
Income Taxes [Line Items]
State Income Tax Statutory Rate		9.50%
State | Other
Income Taxes [Line Items]
Increase (Decrease) in Income Taxes		$ 14
State | Ameren Illinois Company
Income Taxes [Line Items]
Increase (Decrease) in Income Taxes		1
Deferred Tax Liabilities, Net		$ 79
Subsequent Event [Member]
Income Taxes [Line Items]
Federal statutory corporate income tax rate:	21.00%

Income Taxes Income Taxes (Sche

Income Taxes Income Taxes (Schedule of Remeasurement of Deferred Income Taxes due to the Tax Cuts and Jobs Act) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Deferred Federal Income Tax Expense (Benefit)	$ 511	$ 299	$ 299
Tax Cuts and Jobs Act [Member]
Income Taxes [Line Items]
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Accumulated Deferred Income Taxes	(2,253)
Deferred Federal Income Tax Expense (Benefit)	154
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Regulatory Assets	(114)
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Regulatory Liabilities	2,293
Union Electric Company
Income Taxes [Line Items]
Deferred Federal Income Tax Expense (Benefit)	76	161	71
Union Electric Company | Tax Cuts and Jobs Act [Member]
Income Taxes [Line Items]
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Accumulated Deferred Income Taxes	(1,419)
Deferred Federal Income Tax Expense (Benefit)	32
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Regulatory Assets	(89)
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Regulatory Liabilities	1,362
Ameren Illinois Company
Income Taxes [Line Items]
Deferred Federal Income Tax Expense (Benefit)	185	117	193
Ameren Illinois Company | Tax Cuts and Jobs Act [Member]
Income Taxes [Line Items]
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Accumulated Deferred Income Taxes	(871)
Deferred Federal Income Tax Expense (Benefit)	(5)
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Regulatory Assets	(24)
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Regulatory Liabilities	842
Other
Income Taxes [Line Items]
Deferred Federal Income Tax Expense (Benefit)	250	$ 21	$ 35
Other | Tax Cuts and Jobs Act [Member]
Income Taxes [Line Items]
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Accumulated Deferred Income Taxes	37
Deferred Federal Income Tax Expense (Benefit)	127
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Regulatory Assets	(1)
Tax Cuts and Jobs Act, Change in Tax Rate, Change In Regulatory Liabilities	$ 89

Income Taxes (Schedule Of Effec

Income Taxes (Schedule Of Effective Income Tax Rate Reconciliation) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Federal statutory corporate income tax rate:	35.00%	35.00%	35.00%
Depreciation differences	0.00%		(1.00%)
Amortization of deferred investment tax credit	(1.00%)	(0.00%)	(1.00%)
State tax	6.00%	4.00%	5.00%
TCJA	14.00%
Tax Credits	(0.00%)
Stock-based compensation		(2.00%)
Valuation allowance		1.00%
Other permanent items	(2.00%)	(1.00%)
Effective income tax rate	52.00%	37.00%	38.00%
Adjustments related to Income Tax Benefit from Share-Based Payment		$ 21
Union Electric Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate:	35.00%	35.00%	35.00%
Depreciation differences	1.00%	1.00%	0.00%
Amortization of deferred investment tax credit	(1.00%)	(1.00%)	(1.00%)
State tax	4.00%	3.00%	3.00%
TCJA	6.00%
Tax Credits	(1.00%)
Stock-based compensation		0.00%
Valuation allowance		0.00%
Other permanent items	0.00%	0.00%	0.00%
Effective income tax rate	44.00%	38.00%	37.00%
Ameren Illinois Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate:	35.00%	35.00%	35.00%
Depreciation differences	(1.00%)		(2.00%)
Amortization of deferred investment tax credit	(0.00%)	(0.00%)	(0.00%)
State tax	6.00%	5.00%	5.00%
TCJA	(1.00%)
Tax Credits	(0.00%)
Stock-based compensation		0.00%
Valuation allowance		0.00%
Other permanent items	(1.00%)	(2.00%)	(1.00%)
Effective income tax rate	38.00%	38.00%	37.00%

Income Taxes (Schedule Of Compo

Income Taxes (Schedule Of Components Of Income Tax Expense (Benefit)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Current Federal taxes	$ 5	$ (1)	$ (2)
Current State taxes	32	(3)	(4)
Deferred Federal Income Tax Expense (Benefit)	511	299	299
Deferred State taxes	34	92	76
Deferred investment tax credits, amortization	(6)	(5)	(6)
Total income tax expense	576	382	363
Union Electric Company
Income Taxes [Line Items]
Current Federal taxes	149	31	110
Current State taxes	23	6	17
Deferred Federal Income Tax Expense (Benefit)	76	161	71
Deferred State taxes	11	23	16
Deferred investment tax credits, amortization	(5)	(5)	(5)
Total income tax expense	254	216	209
Ameren Illinois Company
Income Taxes [Line Items]
Current Federal taxes	(34)	(8)	(83)
Current State taxes	29	12	(11)
Deferred Federal Income Tax Expense (Benefit)	185	117	193
Deferred State taxes	(13)	37	29
Deferred investment tax credits, amortization	(1)	0	(1)
Total income tax expense	166	158	127
Other
Income Taxes [Line Items]
Current Federal taxes	(110)	(24)	(29)
Current State taxes	(20)	(21)	(10)
Deferred Federal Income Tax Expense (Benefit)	250	21	35
Deferred State taxes	36	32	31
Deferred investment tax credits, amortization	0	0	0
Total income tax expense	$ 156	$ 8	$ 27

Income Taxes (Schedule Of Defer

Income Taxes (Schedule Of Deferred Tax Assets And Liabilities Resulting From Temporary Differences) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Plant related	$ 3,474	$ 5,019
Regulatory assets, net		74
Regulatory assets, net	(547)
Deferred benefit costs	(131)	(211)
Revenue Requirement Reconciliation Adjustment	20	34
Tax carryforwards	(361)	(676)
Other	51	24
Total net accumulated deferred income tax liabilities	2,506	4,264
Union Electric Company
Income Taxes [Line Items]
Plant related	2,064	3,103
Regulatory assets, net		75
Regulatory assets, net	(317)
Deferred benefit costs	(53)	(76)
Revenue Requirement Reconciliation Adjustment	0	0
Tax carryforwards	(31)	(66)
Other	(13)	(23)
Total net accumulated deferred income tax liabilities	1,650	3,013
Ameren Illinois Company
Income Taxes [Line Items]
Plant related	1,264	1,769
Regulatory assets, net	(206)	(1)
Deferred benefit costs	(17)	(38)
Revenue Requirement Reconciliation Adjustment	20	34
Tax carryforwards	(43)	(138)
Other	3	5
Total net accumulated deferred income tax liabilities	1,021	1,631
Other
Income Taxes [Line Items]
Plant related	146	147
Regulatory assets, net		0
Regulatory assets, net	(24)
Deferred benefit costs	(61)	(97)
Revenue Requirement Reconciliation Adjustment	0	0
Tax carryforwards	(287)	(472)
Other	61	42
Total net accumulated deferred income tax liabilities	$ (165)	$ (380)

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, 2017	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 235	$ 539
Tax credit carryforwards	120	130
Deferred Tax Assets, Charitable Contribution Carryforwards	11	18
Deferred Tax Assets, Valuation Allowance, Noncurrent	(5)	(11)
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	6	7
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	203	494
Tax credit carryforwards	113	109
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	32	45
Tax credit carryforwards	7	21
Union Electric Company
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	0	37
Tax credit carryforwards	31	29
Deferred Tax Assets, Charitable Contribution Carryforwards	0	0
Deferred Tax Assets, Valuation Allowance, Noncurrent	0	0
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	0	0
Union Electric Company | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	0	33
Tax credit carryforwards	31	29
Union Electric Company | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	0	4
Tax credit carryforwards	0	0
Ameren Illinois Company
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	41	137
Tax credit carryforwards	2	1
Deferred Tax Assets, Charitable Contribution Carryforwards	0	0
Deferred Tax Assets, Valuation Allowance, Noncurrent	0	0
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	0	0
Ameren Illinois Company | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	41	137
Tax credit carryforwards	2	1
Ameren Illinois Company | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	0	0
Tax credit carryforwards	0	0
Other
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	194	365
Tax credit carryforwards	87	100
Deferred Tax Assets, Charitable Contribution Carryforwards	11	18
Deferred Tax Assets, Valuation Allowance, Noncurrent	(5)	(11)
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	6	7
Other | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	162	324
Tax credit carryforwards	80	79
Other | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	32	41
Tax credit carryforwards	$ 7	$ 21
Minimum | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward, expiration period start	Jan. 1, 2033
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, 2033
Tax credit carryforward, expiration period start	Jan. 1, 2019
Minimum | Federal Contribution
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward, expiration period start	Jan. 1, 2018
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, 2037
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, 2022
Maximum | Federal Contribution
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward, expiration period start	Jan. 1, 2021

Related Party Transactions (Det

Related Party Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Operating Revenues	$ 0	$ 0	$ 0
Union Electric Company | Ameren Missouri Power Supply Agreements with Ameren Illinois
Related Party Transaction [Line Items]
Operating Revenues	23	28	15
Union Electric Company | Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	26	25	25
Union Electric Company | Ameren Illinois Transmission Services Agreement with Marketing Company [Member]
Related Party Transaction [Line Items]
Operating Revenues	1
Union Electric Company | Ameren Missouri and Ameren Illinois miscellaneous support services
Related Party Transaction [Line Items]
Operating Revenues		1	2
Union Electric Company | Total Related Party Operating Revenues [Member]
Related Party Transaction [Line Items]
Operating Revenues	49	54	42
Union Electric Company | Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	149	129	131
Union Electric Company | Money Pool
Related Party Transaction [Line Items]
Interest (Charges) Income	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	4	5	4
Ameren Illinois Company | Ameren Missouri and Ameren Illinois miscellaneous support services
Related Party Transaction [Line Items]
Operating Revenues	1	1	1
Ameren Illinois Company | Total Related Party Operating Revenues [Member]
Related Party Transaction [Line Items]
Operating Revenues	5	5	4
Ameren Illinois Company | Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Operating Expenses	23	28	15
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	25	30	17
Ameren Illinois Company | Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	139	123	119
Ameren Illinois Company | Money Pool
Related Party Transaction [Line Items]
Interest (Charges) Income	$ 1	$ 1	$ 1

Related Party Transactions (Nar

Related Party Transactions (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)MWh$ / MWh	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Union Electric Company
Related Party Transaction [Line Items]
Accounts payable - affiliates	$ 60	$ 68
Capital contribution from parent	30	44	$ 224
Noncash Or Part Noncash Capital Contribution From Parent	0	0	38
Ameren Illinois Company
Related Party Transaction [Line Items]
Accounts payable - affiliates	70	63
Capital contribution from parent	8	0	$ 25
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
Income taxes payable to parent | Union Electric Company
Related Party Transaction [Line Items]
Accounts payable - affiliates	11	16
Income taxes payable to parent | Ameren Illinois Company
Related Party Transaction [Line Items]
Accounts payable - affiliates	$ 17	$ 3
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
April 2017 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	85,600,000,000
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate | $ / MWh	34,000,000

Commitments And Contingencie107

Commitments And Contingencies (Schedule Of Lease Obligations) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments and Contingencies [Line Items]
Capital lease payments, due in one year	$ 32
Capital lease payments, due in two years	32
Capital lease payments, due in three years	32
Capital lease payments, due in four years	33
Capital lease payments, due in five years	32
Capital lease payments, After 5 Years	264
Capital lease payments, Total	425
Less Amount representing interest, due in one year	26
Less Amount representing interest, due in two years	25
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	24
Less Amount representing interest, After 5 Years	24
Less Amount representing interest, Total	149
Present value of minimum capital lease payments, due in one year	6
Present value of minimum capital lease payments, due in two years	7
Present value of minimum capital lease payments, due in three years	7
Present value of minimum capital lease payments, due in four years	8
Present value of minimum capital lease payments, due in five years	8
Present value of minimum capital lease payments, After 5 Years	240
Present value of minimum capital lease payments, Total	276
Operating leases, due in one year	10
Operating leases, due in two years	9
Operating leases, due in three years	8
Operating leases, due in four years	6
Operating leases, due in five years	6
Operating leases, After 5 Years	14
Operating leases, Total	53
Total lease obligations, due in one year	16
Total lease obligations, due in two years	16
Total lease obligations, due in three years	15
Total lease obligations, due in four years	14
Total lease obligations, due in five years	14
Total lease obligations, After 5 Years	254
Total lease obligations, Total	329
Total rental expense	11	$ 38	$ 36
Union Electric Company
Commitments and Contingencies [Line Items]
Capital lease payments, due in one year	32
Capital lease payments, due in two years	32
Capital lease payments, due in three years	32
Capital lease payments, due in four years	33
Capital lease payments, due in five years	32
Capital lease payments, After 5 Years	264
Capital lease payments, Total	425
Less Amount representing interest, due in one year	26
Less Amount representing interest, due in two years	25
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	24
Less Amount representing interest, After 5 Years	24
Less Amount representing interest, Total	149
Present value of minimum capital lease payments, due in one year	6
Present value of minimum capital lease payments, due in two years	7
Present value of minimum capital lease payments, due in three years	7
Present value of minimum capital lease payments, due in four years	8
Present value of minimum capital lease payments, due in five years	8
Present value of minimum capital lease payments, After 5 Years	240
Present value of minimum capital lease payments, Total	276
Operating leases, due in one year	8
Operating leases, due in two years	8
Operating leases, due in three years	7
Operating leases, due in four years	6
Operating leases, due in five years	6
Operating leases, After 5 Years	14
Operating leases, Total	49
Total lease obligations, due in one year	14
Total lease obligations, due in two years	15
Total lease obligations, due in three years	14
Total lease obligations, due in four years	14
Total lease obligations, due in five years	14
Total lease obligations, After 5 Years	254
Total lease obligations, Total	325
Total rental expense	10	34	34
Ameren Illinois Company
Commitments and Contingencies [Line Items]
Operating leases, due in one year	1
Operating leases, due in two years	1
Operating leases, due in three years	1
Operating leases, due in four years	1
Operating leases, due in five years	1
Operating leases, After 5 Years	1
Operating leases, Total	2
Total rental expense	$ 1	$ 30	$ 28

Commitments And Contingencie108

Commitments And Contingencies (Schedule Of Estimated Purchased Power Commitments) (Details) $ in Millions	Dec. 31, 2017USD ($)
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	$ 981
2,018	676
2,019	288
2,020	151
2,021	55
Thereafter	254
Total	2,405
Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	463
2,018	383
2,019	85
2,020	27
2,021	0
Thereafter	0
Total	958
Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	205
2,018	163
2,019	110
2,020	46
2,021	11
Thereafter	38
Total	573
Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	67
2,018	26
2,019	39
2,020	45
2,021	12
Thereafter	45
Total	234
Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	170
2,018	63
2,019	14
2,020	3
2,021	2
Thereafter	18
Total	270
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	58
Total	79
Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	73
2,018	37
2,019	36
2,020	25
2,021	25
Thereafter	95
Total	291
Union Electric Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	628
2,018	473
2,019	181
2,020	115
2,021	48
Thereafter	194
Total	1,639
Union Electric Company | Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	463
2,018	383
2,019	85
2,020	27
2,021	0
Thereafter	0
Total	958
Union Electric Company | Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	42
2,018	36
2,019	29
2,020	13
2,021	6
Thereafter	16
Total	142
Union Electric Company | Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	67
2,018	26
2,019	39
2,020	45
2,021	12
Thereafter	45
Total	234
Union Electric Company | Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	0
2,018	0
2,019	0
2,020	0
2,021	0
Thereafter	0
Total	0
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	58
Total	79
Union Electric Company | Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	53
2,018	24
2,019	24
2,020	25
2,021	25
Thereafter	75
Total	226
Ameren Illinois Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	352
2,018	203
2,019	107
2,020	36
2,021	7
Thereafter	40
Total	745
Ameren Illinois Company | Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	163
2,018	127
2,019	81
2,020	33
2,021	5
Thereafter	22
Total	431
Ameren Illinois Company | Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	170
2,018	63
2,019	14
2,020	3
2,021	2
Thereafter	18
Total	270
Ameren Illinois Company | Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
2,017	19
2,018	13
2,019	12
2,020	0
2,021	0
Thereafter	0
Total	$ 44

Commitments And Contingencie109

Commitments And Contingencies (Environmental Matters) (Details) $ in Millions	Dec. 31, 2017USD ($)site	Sep. 30, 2017scrubber	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Percentage of Rate Base Related to Carbon Dioxide Energy Centers	17.00%
Asset Retirement Obligation	$ 644		$ 650	$ 623
Union Electric Company
Percentage of Rate Base Related to Carbon Dioxide Energy Centers	33.00%
Number of Energy Center Scrubbers | scrubber		2
Asset Retirement Obligation	$ 640		644	617
Ameren Illinois Company
Asset Retirement Obligation	4		$ 6	$ 6
Manufactured Gas Plant
Accrual for environmental loss contingencies	$ 175
Manufactured Gas Plant | Ameren Illinois Company
Number of remediation sites | site	44
Accrual for environmental loss contingencies	$ 175
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	325
Minimum | Coal Combustion Residuals Estimate [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	300
Minimum | Manufactured Gas Plant | Ameren Illinois Company
Loss contingency, estimate of possible loss	175
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	425
Maximum | Coal Combustion Residuals Estimate [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	350
Maximum | Manufactured Gas Plant | Ameren Illinois Company
Loss contingency, estimate of possible loss	249
Maximum | Sauget Area 2 | Union Electric Company
Loss contingency, estimate of possible loss	2.5
New CCR Rules Estimate [Member]
Asset Retirement Obligation	150
New CCR Rules Estimate [Member] | Union Electric Company
Asset Retirement Obligation	$ 150

Segment Information (Details)

Segment Information (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017USD ($)	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Sep. 30, 2016USD ($)	Jun. 30, 2016USD ($)	Mar. 31, 2016USD ($)	Dec. 31, 2017USD ($)segment	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment									4
External revenues	$ 1,402	$ 1,723	$ 1,538	$ 1,514	$ 1,356	$ 1,859	$ 1,427	$ 1,434	$ 6,177	$ 6,076	$ 6,098
Intersegment revenues									0	0	0
Depreciation and amortization									896	845	796
Interest income									34	40	41
Interest charges									391	382	355
Income taxes (benefit)									576	382	363
Net income (loss) attributable to Ameren common shareholders from continuing operations									523	653	579
Capital expenditures									$ 2,132	2,076	1,917
Union Electric Company
Segment Reporting Information [Line Items]
Number of reportable segments | segment									1
Ameren Illinois Company
Segment Reporting Information [Line Items]
Number of reportable segments | segment									3
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	674	575	576	703	595	676	542	677	$ 2,528	2,490	2,466
Intersegment revenues									0	0	0
Depreciation and amortization									341	319	295
Interest income									7	12	12
Interest charges									144	140	131
Income taxes (benefit)									166	158	127
Net Income (Loss) Available to Common Shareholder	$ 77	$ 55	$ 57	$ 79	$ 29	$ 119	$ 45	$ 59	268	252	214
Capital expenditures									1,076	924	918
Intersegment Elimination
Segment Reporting Information [Line Items]
External revenues									0	0	0
Intersegment revenues									(98)	(105)	(87)
Depreciation and amortization									0	0	0
Interest income									(11)	(11)	(6)
Interest charges									(11)	(11)	(6)
Income taxes (benefit)									0	0	0
Net Income (Loss) Available to Common Shareholder									0	0	0
Capital expenditures									(7)	(6)	0
Intersegment Elimination | Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues									0	0	0
Intersegment revenues									(42)	(45)	(38)
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									3,490	3,469	3,566
Intersegment revenues									49	54	43
Depreciation and amortization									533	514	492
Interest income									27	28	28
Interest charges									207	211	219
Income taxes (benefit)									254	216	209
Net Income (Loss) Available to Common Shareholder									323	357	352
Capital expenditures									773	738	622
Operating Segments [Member] | Ameren Illinois Electric Distribution [Member]
Segment Reporting Information [Line Items]
External revenues									1,565	1,545	1,529
Intersegment revenues									4	4	3
Depreciation and amortization									239	226	212
Interest income									7	11	12
Interest charges									73	72	71
Income taxes (benefit)									83	78	71
Net Income (Loss) Available to Common Shareholder									131	126	123
Capital expenditures									476	470	491
Operating Segments [Member] | Ameren Illinois Gas [Member]
Segment Reporting Information [Line Items]
External revenues									742	753	782
Intersegment revenues									1	1	1
Depreciation and amortization									59	55	52
Interest income									0	0	0
Interest charges									36	34	35
Income taxes (benefit)									36	39	24
Net Income (Loss) Available to Common Shareholder									60	59	37
Capital expenditures									245	181	133
Operating Segments [Member] | Ameren Transmission [Member]
Segment Reporting Information [Line Items]
External revenues									382	309	219
Intersegment revenues									44	46	40
Depreciation and amortization									60	43	33
Interest income									0	1	0
Interest charges									67	58	35
Income taxes (benefit)									90	74	51
Net Income (Loss) Available to Common Shareholder									140	117	83
Capital expenditures									644	689	669
Operating Segments [Member] | Other Segment
Segment Reporting Information [Line Items]
External revenues									(2)	0	2
Intersegment revenues									0	0	0
Depreciation and amortization									5	7	7
Interest income									11	11	7
Interest charges									19	18	1
Income taxes (benefit)									113	(25)	8
Net Income (Loss) Available to Common Shareholder									(131)	(6)	(16)
Capital expenditures									1	4	2
Operating Segments [Member] | Ameren Illinois Company | Ameren Illinois Electric Distribution [Member]
Segment Reporting Information [Line Items]
External revenues									1,569	1,549	1,532
Intersegment revenues									0	0	0
Depreciation and amortization									239	226	212
Interest income									7	11	12
Interest charges									73	72	71
Income taxes (benefit)									83	78	71
Net Income (Loss) Available to Common Shareholder									131	126	123
Capital expenditures									476	470	491
Operating Segments [Member] | Ameren Illinois Company | Ameren Illinois Gas [Member]
Segment Reporting Information [Line Items]
External revenues									743	754	783
Intersegment revenues									0	0	0
Depreciation and amortization									59	55	52
Interest income									0	0	0
Interest charges									36	34	35
Income taxes (benefit)									36	39	24
Net Income (Loss) Available to Common Shareholder									60	59	37
Capital expenditures									245	181	133
Operating Segments [Member] | Ameren Illinois Company | Ameren Illinois Transmission [Member]
Segment Reporting Information [Line Items]
External revenues									216	187	151
Intersegment revenues									42	45	38
Depreciation and amortization									43	38	31
Interest income									0	1	0
Interest charges									35	34	25
Income taxes (benefit)									47	41	32
Net Income (Loss) Available to Common Shareholder									77	67	54
Capital expenditures									$ 355	$ 273	$ 294

Selected Quarterly Informati111

Selected Quarterly Information (Summary Of Selected Quarterly Information) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Selected Quarterly Financial Information [Line Items]
Deferred Federal Income Tax Expense (Benefit)									$ 511	$ 299	$ 299
Operating revenues	$ 1,402	$ 1,723	$ 1,538	$ 1,514	$ 1,356	$ 1,859	$ 1,427	$ 1,434	6,177	6,076	6,098
Operating Income	225	581	398	254	145	691	325	220	1,458	1,381	1,259
Net income	(59)	290	194	104	33	371	148	107	529	659	636
Net income attributable to Ameren common shareholders	$ (60)	$ 288	$ 193	$ 102	$ 32	$ 369	$ 147	$ 105	$ 523	$ 653	$ 630
Earnings per common share - basic (in dollars per share)	$ (0.24)	$ 1.19	$ 0.79	$ 0.42	$ 0.13	$ 1.52	$ 0.61	$ 0.43	$ 2.16	$ 2.69	$ 2.60
Earnings per common share - diluted (in dollars per share)	$ (0.24)	$ 1.18	$ 0.79	$ 0.42	$ 0.13	$ 1.52	$ 0.61	$ 0.43	$ 2.14	$ 2.68	$ 2.59
Union Electric Company
Selected Quarterly Financial Information [Line Items]
Deferred Federal Income Tax Expense (Benefit)									$ 76	$ 161	$ 71
Operating revenues	$ 699	$ 1,115	$ 935	$ 790	$ 750	$ 1,165	$ 867	$ 741	3,539	3,523	3,609
Operating Income	40	417	237	53	54	431	197	63	747	745	742
Net income	(36)	235	121	6	10	242	93	15	326	360	355
Net Income Available to Common Shareholder	(36)	234	120	5	10	241	92	14	323	357	352
Ameren Illinois Company
Selected Quarterly Financial Information [Line Items]
Deferred Federal Income Tax Expense (Benefit)									185	117	193
Operating revenues	674	575	576	703	595	676	542	677	2,528	2,490	2,466
Operating Income	150	128	130	172	74	230	107	133	580	544	466
Net income	78	55	58	80	30	119	46	60	271	255	217
Net Income Available to Common Shareholder	$ 77	$ 55	$ 57	$ 79	$ 29	$ 119	$ 45	$ 59	268	$ 252	$ 214
Tax Cuts and Jobs Act [Member]
Selected Quarterly Financial Information [Line Items]
Deferred Federal Income Tax Expense (Benefit)									154
Tax Cuts and Jobs Act [Member] | Union Electric Company
Selected Quarterly Financial Information [Line Items]
Deferred Federal Income Tax Expense (Benefit)									32
Tax Cuts and Jobs Act [Member] | Ameren Illinois Company
Selected Quarterly Financial Information [Line Items]
Deferred Federal Income Tax Expense (Benefit)									$ (5)

Schedule I - Condensed Finan112

Schedule I - Condensed Financial Information Of Parent (Statement of Income) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Operating revenues	$ 1,402	$ 1,723	$ 1,538	$ 1,514	$ 1,356	$ 1,859	$ 1,427	$ 1,434	$ 6,177	$ 6,076	$ 6,098
Operating expenses									4,719	4,695	4,839
Operating loss	225	581	398	254	145	691	325	220	1,458	1,381	1,259
Interest income from affiliates									8	13	14
Other Nonoperating Income (Expense)									38	42	44
Interest charges									391	382	355
Income taxes (benefit)									576	382	363
Net income attributable to Ameren common shareholders - continuing operations									523	653	579
Net Income (Loss) Attributable to Ameren common shareholders- Discontinued Operations									0	0	51
Net income attributable to Ameren common shareholders	$ (60)	$ 288	$ 193	$ 102	$ 32	$ 369	$ 147	$ 105	523	653	630
Comprehensive Income from Continuing Operations
Pension and other postretirement activity, net of income taxes (benefit)									5	(20)	6
Comprehensive Income from Continuing Operations Attributable to Ameren Common Shareholders									528	633	585
Comprehensive Income from Discontinued Operations Attributable to Ameren Common shareholders									0	0	51
Comprehensive Income (Loss) Attributable to Ameren Common Shareholders									528	633	636
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement activity, tax (benefit)									3	(7)	3
Parent Company
Condensed Financial Statements, Captions [Line Items]
Operating revenues									0	0	0
Operating expenses									13	14	14
Operating loss									(13)	(14)	(14)
Equity in earnings of subsidiaries									659	663	600
Interest income from affiliates									9	10	6
Other Nonoperating Income (Expense)									0	(5)	(5)
Interest charges									31	28	3
Income taxes (benefit)									101	(27)	5
Net income attributable to Ameren common shareholders - continuing operations									523	653	579
Net Income (Loss) Attributable to Ameren common shareholders- Discontinued Operations									0	0	51
Net income attributable to Ameren common shareholders									523	653	630
Comprehensive Income from Continuing Operations
Pension and other postretirement activity, net of income taxes (benefit)									5	(20)	6
Comprehensive Income from Continuing Operations Attributable to Ameren Common Shareholders									528	633	585
Comprehensive Income from Discontinued Operations Attributable to Ameren Common shareholders									0	0	51
Comprehensive Income (Loss) Attributable to Ameren Common Shareholders									528	633	636
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement activity, tax (benefit)									$ 3	$ (7)	$ 3

Schedule I - Condensed Finan113

Schedule I - Condensed Financial Information Of Parent (Balance Sheet) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
ASSETS
Cash and cash equivalents	$ 10	$ 9	$ 292	$ 5
Miscellaneous accounts and notes receivable	70	63
Other current assets	98	113
Total current assets	1,612	1,593
Accumulated deferred income taxes, net	6	7
Other assets	522	538
TOTAL ASSETS	25,945	24,699
LIABILITIES AND EQUITY
Short-term debt	484	558
Other current liabilities	326	274
Total current liabilities	2,940	2,674
Long-term Debt, Net	7,094	6,595
Pension and other postretirement benefits	545	769
Other deferred credits and liabilities	460	477
Commitments and Contingencies
Retained earnings	1,660	1,568
Accumulated other comprehensive loss	(18)	(23)
Total equity	7,326	7,245	7,088
TOTAL LIABILITIES AND EQUITY	25,945	24,699
Parent Company
ASSETS
Cash and cash equivalents	0	1	$ 0	$ 1
Advances to money pool	13	27
Accounts receivable - affiliates	46	31
Miscellaneous accounts and notes receivable	0	26
Other current assets	8	8
Total current assets	67	93
Investments in subsidiaries	7,944	7,498
Note receivable - affiliates	75	350
Accumulated deferred income taxes, net	222	419
Other assets	140	135
TOTAL ASSETS	8,448	8,495
LIABILITIES AND EQUITY
Short-term debt	383	507
Borrowings from money pool	28	33
Accounts payable – affiliates	6	13
Other current liabilities	27	17
Total current liabilities	444	570
Long-term Debt, Net	696	694
Pension and other postretirement benefits	37	45
Other deferred credits and liabilities	87	83
Total liabilities	1,264	1,392
Common stock, $.01 par value, 400.0 shares authorized – 242.6 shares outstanding	2	2
Other paid-in capital, principally premium on common stock	5,540	5,556
Retained earnings	1,660	1,568
Accumulated other comprehensive loss	(18)	(23)
Total equity	7,184	7,103
TOTAL LIABILITIES AND EQUITY	$ 8,448	$ 8,495

Schedule I - Condensed Finan114

Schedule I - Condensed Financial Information Of Parent (Statement of Cash Flows) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	$ 2,104	$ 2,123	$ 2,031
Cash Flows From Investing Activities:
Other	7	5	32
Net Cash Provided by (Used in) Investing Activities	(2,205)	(2,141)	(1,976)
Cash flows from financing activities:
Dividends on common stock	(431)	(416)	(402)
Short-term debt, net	(74)	257	(413)
Issuances of Long-term debt	1,345	389	1,197
Capital issuance costs	(11)	(9)	(12)
Share-based payments	(39)	(83)	(12)
Net cash provided by (used in) financing activities	102	(265)	232
Net change in cash and cash equivalents	1	(283)	287
Cash and cash equivalents at beginning of year	9	292	5
Cash and cash equivalents at end of year	10	9	292
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	454	483	551
Cash Flows From Investing Activities:
Money pool advances, net	14	(27)	55
Intercompany notes receivable, net	275	(60)	(96)
Investments in subsidiaries	(151)	(123)	(509)
Other	6	2	(12)
Net Cash Provided by (Used in) Investing Activities	144	(208)	(562)
Cash flows from financing activities:
Dividends on common stock	(431)	(416)	(402)
Short-term debt, net	(124)	206	(284)
Money pool borrowings, net	(5)	19	14
Issuances of Long-term debt	0	0	700
Capital issuance costs	0	0	(6)
Share-based payments	(39)	(83)	(12)
Net cash provided by (used in) financing activities	(599)	(274)	10
Net change in cash and cash equivalents	(1)	1	(1)
Cash and cash equivalents at beginning of year	1	0	1
Cash and cash equivalents at end of year	0	1	0
Cash dividends received from consolidated subsidiaries	362	465	575
Noncash Investing activity- investment in subsidiaries	$ 0	$ 0	$ (38)

Schedule I - Condensed Finan115

Schedule I - Condensed Financial Information Of Parent Outstanding gurarantee (Details) $ in Millions	Dec. 31, 2017USD ($)
Parent Company
Other Commitments [Line Items]
Guarantees Outstanding	$ 46

Schedule II - Valuation And 116

Schedule II - Valuation And Qualifying Accounts (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Allowance For Doubtful Accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		$ 19		$ 19	$ 21
Charged to Costs and Expenses		26		32	33
Charged to Other Accounts	[1]	7		3	5
Deductions	[2]	33		35	40
Balance at End of Period		19		19	19
Valuation Allowance of Deferred Tax Assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		11		6	10
Charged to Costs and Expenses		(6)	[3]	7	4
Charged to Other Accounts	[1]	0		(2)	(8)
Deductions		0		0	0
Balance at End of Period		5		11	6
Parent Company | Valuation Allowance of Deferred Tax Assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Charged to Costs and Expenses		3
Union Electric Company | Allowance For Doubtful Accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		7		7	8
Charged to Costs and Expenses		9		10	13
Charged to Other Accounts	[1]	0		0	0
Deductions	[2]	9		10	14
Balance at End of Period		7		7	7
Union Electric Company | Valuation Allowance of Deferred Tax Assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		0		0	1
Charged to Costs and Expenses		0		0	0
Charged to Other Accounts	[1]	0		0	(1)
Deductions		0		0	0
Balance at End of Period		0		0	0
Ameren Illinois Company | Allowance For Doubtful Accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		12		12	13
Charged to Costs and Expenses		17		22	20
Charged to Other Accounts	[1]	7		3	5
Deductions	[2]	24		25	26
Balance at End of Period		12		12	12
Ameren Illinois Company | Valuation Allowance of Deferred Tax Assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		0		0	1
Charged to Costs and Expenses		0		0	0
Charged to Other Accounts	[1]	0		0	(1)
Deductions		0		0	0
Balance at End of Period		$ 0		$ 0	$ 0
[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.
[3]	Includes an adjustment of $3 million to Ameren (parent)’s valuation allowance for certain deferred tax assets existing at December 31, 2017, for the reduction in the income tax rate.