Ameren (AEE) 10-K 26 Feb 19 2018 FY Annual report Financial data

Document And Entity Information

Document And Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Jan. 31, 2019	Jun. 30, 2018
Entity Registrant Name	AMEREN CORP
Entity Central Index Key	1,002,910
Current Fiscal Year End Date	--12-31
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Amendment Flag	false
Trading Symbol	AEE
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false
Entity Public Float			$ 14,783,320,074
Entity Common Stock, Shares Outstanding		244,638,879
Union Electric Company
Entity Registrant Name	UNION ELECTRIC CO
Entity Central Index Key	100,826
Current Fiscal Year End Date	--12-31
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Amendment Flag	false
Trading Symbol	AEE
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		102,123,834
Ameren Illinois Company
Entity Registrant Name	Ameren Illinois Co
Entity Central Index Key	18,654
Current Fiscal Year End Date	--12-31
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Amendment Flag	false
Trading Symbol	AEE
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Non-accelerated Filer
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		25,452,373

Consolidated Statement Of Incom

Consolidated Statement Of Income (Loss) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 6,291	$ 6,174	$ 6,076
Operating Expenses:
Fuel	769	737	745
Purchased Power	581	638	623
Natural gas purchased for resale	374	311	341
Other operations and maintenance	1,772	1,705	1,733
Depreciation and amortization	955	896	845
Taxes other than income taxes	483	477	467
Total operating expenses	4,934	4,764	4,754
Operating Income	1,357	1,410	1,322
Other Nonoperating Income (Expense)	102	86	101
Interest charges	401	391	382
Income Before Income Taxes	1,058	1,105	1,041
Income taxes	237	576	382
Net Income	821	529	659
Net Income (Loss) Attributable to Noncontrolling Interest	6	6	6
Net Income (Loss) Available to Common Stockholders, Basic	815	523	653
Pension and other postretirement activity, net of income taxes (benefit)	(4)	5	(20)
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	817	534	639
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	6	6	6
Comprehensive Income	$ 811	$ 528	$ 633
Earnings per Common Share – Basic:
Earnings per Common Share – Basic (in dollars per share)	$ 3.34	$ 2.16	$ 2.69
Earnings per Common Share – Diluted:
Earnings per Common Share – Diluted (in dollars per share)	$ 3.32	$ 2.14	$ 2.68
Average Common Shares Outstanding - Basic (in shares)	243.8	242.6	242.6
Average Common Shares Outstanding - Diluted (in shares)	245.8	244.2	243.4
Union Electric Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 3,589	$ 3,537	$ 3,524
Operating Expenses:
Fuel	769	737	745
Purchased Power	164	245	254
Natural gas purchased for resale	56	47	49
Other operations and maintenance	972	925	912
Depreciation and amortization	550	533	514
Taxes other than income taxes	329	328	325
Total operating expenses	2,840	2,815	2,799
Operating Income	749	722	725
Other Nonoperating Income (Expense)	56	65	62
Interest charges	200	207	211
Income Before Income Taxes	605	580	576
Income taxes	124	254	216
Net Income	481	326	360
Preferred Stock Dividends	3	3	3
Earnings per Common Share – Diluted:
Net Income (Loss) Attributable to Parent	478	323	357
Ameren Illinois Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	2,576	2,527	2,489
Operating Expenses:
Purchased Power	429	417	399
Natural gas purchased for resale	318	264	292
Other operations and maintenance	799	799	828
Depreciation and amortization	374	341	319
Taxes other than income taxes	144	137	132
Total operating expenses	2,064	1,958	1,970
Operating Income	512	569	519
Other Nonoperating Income (Expense)	42	12	34
Interest charges	149	144	140
Income Before Income Taxes	405	437	413
Income taxes	98	166	158
Net Income	307	271	255
Pension and other postretirement activity, net of income taxes (benefit)	0	0	(5)
Comprehensive Income	307	271	250
Preferred Stock Dividends	3	3	3
Earnings per Common Share – Diluted:
Net Income (Loss) Attributable to Parent	304	268	252
Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	5,339	5,307	5,196
Electricity | Union Electric Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	3,451	3,411	3,396
Electricity | Ameren Illinois Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	1,761	1,784	1,735
Natural gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	952	867	880
Natural gas | Union Electric Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	138	126	128
Natural gas | Ameren Illinois Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 815	$ 743	$ 754

Consolidated Statement Of Inc_2

Consolidated Statement Of Income (Loss) (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension and other postretirement activity, tax (benefit)	$ (1)	$ 3	$ (7)
Ameren Illinois Company
Pension and other postretirement activity, tax (benefit)	$ 0	$ 0	$ (1)

Consolidated Balance Sheet

Consolidated Balance Sheet - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Current Assets:
Cash and Cash Equivalents, at Carrying Value	$ 16	$ 10
Accounts receivable - trade (less allowance for doubtful accounts)	463	445
Unbilled revenue	295	323
Miscellaneous accounts and notes receivable	79	70
Inventories	483	522
Current regulatory assets	134	144
Other current assets	63	98
Total current assets	1,533	1,612
Property, Plant and Equipment, Net	22,810	21,466
Investments and Other Assets:
Nuclear decommissioning trust fund	684	704
Goodwill	411	411
Regulatory Assets, Noncurrent	1,127	1,230
Other assets	650	522
Total investments and other assets	2,872	2,867
TOTAL ASSETS	27,215	25,945
Current Liabilities:
Current maturities of long-term debt	580	841
Short-term debt	597	484
Accounts and wages payable	817	902
Taxes accrued	53	52
Interest accrued	93	99
Customer deposits	116	108
Current regulatory liabilities	149	128
Other current liabilities	282	326
Total current liabilities	2,687	2,940
Long-term Debt, Net	7,859	7,094
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	2,623	2,506
Accumulated deferred investment tax credits	43	49
Regulatory Liability, Noncurrent	4,637	4,387
Asset retirement obligations	627	638
Pension and other postretirement benefits	558	545
Other deferred credits and liabilities	408	460
Total deferred credits and other liabilities	8,896	8,585
Commitments and Contingencies (Notes 2, 9, and 14)
Stockholders' Equity:
Common stock	2	2
Other paid-in capital, principally premium on common stock	5,627	5,540
Retained earnings	2,024	1,660
Accumulated other comprehensive loss	(22)	(18)
Total stockholders' equity	7,631	7,184
Stockholders' Equity Attributable to Noncontrolling Interest	142	142
Total equity	7,773	7,326
TOTAL LIABILITIES AND EQUITY	27,215	25,945
Union Electric Company
Current Assets:
Cash and Cash Equivalents, at Carrying Value	0	0
Accounts receivable - trade (less allowance for doubtful accounts)	223	200
Accounts Receivable, Related Parties, Current	14	11
Unbilled revenue	155	165
Miscellaneous accounts and notes receivable	42	35
Inventories	358	388
Current regulatory assets	14	56
Other current assets	26	50
Total current assets	832	905
Property, Plant and Equipment, Net	12,103	11,751
Investments and Other Assets:
Nuclear decommissioning trust fund	684	704
Regulatory Assets, Noncurrent	366	395
Other assets	306	288
Total investments and other assets	1,356	1,387
TOTAL ASSETS	14,291	14,043
Current Liabilities:
Current maturities of long-term debt	580	384
Short-term debt	55	39
Accounts and wages payable	428	475
Accounts Payable, Related Parties, Current	69	60
Taxes accrued	27	30
Interest accrued	52	54
Current regulatory liabilities	68	19
Other current liabilities	123	103
Total current liabilities	1,402	1,164
Long-term Debt, Net	3,418	3,577
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,534	1,650
Accumulated deferred investment tax credits	42	48
Regulatory Liability, Noncurrent	2,799	2,664
Asset retirement obligations	623	634
Pension and other postretirement benefits	228	213
Other deferred credits and liabilities	16	12
Total deferred credits and other liabilities	5,242	5,221
Commitments and Contingencies (Notes 2, 9, and 14)
Stockholders' Equity:
Common stock	511	511
Other paid-in capital, principally premium on common stock	1,903	1,858
Preferred stock not subject to mandatory redemption (in shares)	80	80
Retained earnings	1,735	1,632
Total stockholders' equity	4,229	4,081
TOTAL LIABILITIES AND EQUITY	14,291	14,043
Ameren Illinois Company
Current Assets:
Cash and Cash Equivalents, at Carrying Value	0	0
Accounts receivable - trade (less allowance for doubtful accounts)	224	234
Accounts Receivable, Related Parties, Current	21	9
Unbilled revenue	140	158
Miscellaneous accounts and notes receivable	40	35
Inventories	125	134
Current regulatory assets	110	87
Other current assets	16	15
Total current assets	676	672
Property, Plant and Equipment, Net	9,198	8,293
Investments and Other Assets:
Goodwill	411	411
Regulatory Assets, Noncurrent	759	822
Other assets	275	147
Total investments and other assets	1,445	1,380
TOTAL ASSETS	11,319	10,345
Current Liabilities:
Current maturities of long-term debt	0	457
Short-term debt	72	62
Accounts and wages payable	302	337
Accounts Payable, Related Parties, Current	58	70
Taxes accrued	23	19
Interest accrued	31	33
Customer deposits	76	69
Environmental remediation	42	42
Current regulatory liabilities	62	92
Other current liabilities	130	177
Total current liabilities	796	1,358
Long-term Debt, Net	3,296	2,373
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,119	1,021
Regulatory Liability, Noncurrent	1,741	1,629
Pension and other postretirement benefits	280	285
Accrued Environmental Loss Contingencies, Noncurrent	109	134
Other deferred credits and liabilities	204	235
Total deferred credits and other liabilities	3,453	3,304
Commitments and Contingencies (Notes 2, 9, and 14)
Stockholders' Equity:
Common stock	0	0
Other paid-in capital, principally premium on common stock	2,173	2,013
Preferred stock not subject to mandatory redemption (in shares)	62	62
Retained earnings	1,539	1,235
Total stockholders' equity	3,774	3,310
TOTAL LIABILITIES AND EQUITY	$ 11,319	$ 10,345

Consolidated Balance Sheet (Par

Consolidated Balance Sheet (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Accounts receivable - trade, allowance for doubtful accounts	$ 18	$ 19
Common Stock, Par or Stated Value Per Share (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	400,000,000	400,000,000
Common Stock, Shares, Outstanding	244,500,000	242,600,000
Union Electric Company
Accounts receivable - trade, allowance for doubtful accounts	$ 7	$ 7
Common Stock, Par or Stated Value Per Share (in dollars per share)	$ 5	$ 5
Common stock, shares authorized	150,000,000	150,000,000
Common Stock, Shares, Outstanding	102,100,000	102,100,000
Ameren Illinois Company
Accounts receivable - trade, allowance for doubtful accounts	$ 11	$ 12
Common Stock, No Par Value (in dollars per share)	$ 0	$ 0
Common stock, shares authorized	45,000,000	45,000,000
Common Stock, Shares, Outstanding	25,500,000	25,500,000

Consolidated Statement Of Cash

Consolidated Statement Of Cash Flows $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Cash Flows From Operating Activities:
Net income	$ 821	$ 529	$ 659
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	938	876	835
Amortization of nuclear fuel	95	76	88
Amortization of debt issuance costs and premium/discounts	20	22	22
Deferred income taxes and investment tax credits, net	224	539	386
Allowance for equity funds used during construction	(36)	(24)	(27)
Share-based Compensation	20	17	17
Other	44	(10)	4
Changes in assets and liabilities:
Receivables	(24)	(53)	(71)
Inventories	39	17	11
Accounts and wages payable	(22)	32	19
Taxes accrued	(10)	55	13
Regulatory assets and liabilities	201	36	215
Assets, other	2	34	(21)
Liabilities, other	(117)	(7)	(17)
Pension and other postretirement benefits	(25)	(21)	(16)
Net cash provided by operating activities	2,170	2,118	2,117
Cash Flows From Investing Activities:
Capital expenditures	(2,286)	(2,132)	(2,076)
Nuclear fuel expenditures	(52)	(63)	(55)
Purchases of securities - nuclear decommissioning trust fund	(315)	(321)	(322)
Sales and maturities of securities - nuclear decommissioning trust fund	299	305	304
Other	18	7	(9)
Net cash used in investing activities	(2,336)	(2,204)	(2,158)
Cash Flows From Financing Activities:
Dividends on common stock	(451)	(431)	(416)
Dividends paid to noncontrolling interest holders	(6)	(6)	(6)
Short-term debt, net	112	(74)	257
Maturities, redemptions, and repurchases of long-term debt	(841)	(681)	(395)
Issuances of Long-term debt	1,352	1,345	396
Proceeds from Issuance of Common Stock	74	0	0
Capital issuance costs	(14)	(11)	(9)
Payments for Repurchase of Common Stock	0	(24)	(51)
Share-based payments	(19)	(15)	(32)
Other	(2)	(1)	(2)
Net cash provided by (used in) financing activities	205	102	(258)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect	39	16	(299)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	107	68	52
Stock Issued	35
Cash Paid (Refunded) During the Year:
Interest net of capitalized	387	370	358
Income taxes, net	21	(19)	(12)
Union Electric Company
Cash Flows From Operating Activities:
Net income	481	326	360
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	533	514	506
Amortization of nuclear fuel	95	76	88
Amortization of debt issuance costs and premium/discounts	6	6	6
Deferred income taxes and investment tax credits, net	(9)	82	179
Allowance for equity funds used during construction	(27)	(21)	(23)
Other	17	4	5
Changes in assets and liabilities:
Receivables	(32)	(46)	5
Inventories	30	18	(4)
Accounts and wages payable	(21)	27	(18)
Taxes accrued	(1)	(1)	11
Regulatory assets and liabilities	201	26	84
Assets, other	2	31	(25)
Liabilities, other	(13)	(23)	(1)
Pension and other postretirement benefits	(2)	(2)	(4)
Net cash provided by operating activities	1,260	1,017	1,169
Cash Flows From Investing Activities:
Capital expenditures	(914)	(773)	(738)
Nuclear fuel expenditures	(52)	(63)	(55)
Purchases of securities - nuclear decommissioning trust fund	(315)	(321)	(322)
Sales and maturities of securities - nuclear decommissioning trust fund	299	305	304
Money pool advances, net		161	(125)
Other	6	7	(1)
Net cash used in investing activities	(976)	(684)	(937)
Cash Flows From Financing Activities:
Dividends on common stock	(375)	(362)	(355)
Dividends on preferred stock	(3)	(3)	(3)
Short-term debt, net	16	39
Maturities, redemptions, and repurchases of long-term debt	(384)	(431)	(266)
Issuances of Long-term debt	423	399	149
Capital issuance costs	(5)	(3)	(3)
Proceeds from Contributions from Parent	45	30	44
Net cash provided by (used in) financing activities	(283)	(331)	(434)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect	1	2	(202)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	8	7	5
Cash Paid (Refunded) During the Year:
Interest net of capitalized	196	202	209
Income taxes, net	128	178	27
Ameren Illinois Company
Cash Flows From Operating Activities:
Net income	307	271	255
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	375	341	318
Amortization of debt issuance costs and premium/discounts	13	13	14
Deferred income taxes and investment tax credits, net	88	171	154
Allowance for equity funds used during construction	(9)	(3)	(4)
Other	11	0	(1)
Changes in assets and liabilities:
Receivables	0	(7)	(72)
Inventories	8	(1)	15
Accounts and wages payable	(13)	19	12
Taxes accrued	(13)	18	1
Regulatory assets and liabilities	1	16	120
Assets, other	(1)	(2)	(3)
Liabilities, other	(92)	3	(9)
Pension and other postretirement benefits	(25)	(14)	(8)
Net cash provided by operating activities	659	828	796
Cash Flows From Investing Activities:
Capital expenditures	(1,258)	(1,076)	(924)
Other	10	6	6
Net cash used in investing activities	(1,248)	(1,070)	(918)
Cash Flows From Financing Activities:
Dividends on common stock	0	0	(110)
Dividends on preferred stock	(3)	(3)	(3)
Short-term debt, net	10	11	51
Maturities, redemptions, and repurchases of long-term debt	(457)	(250)	(129)
Issuances of Long-term debt	929	496	247
Capital issuance costs	(9)	(6)	(4)
Proceeds from Contributions from Parent	160	8	0
Other	(2)	(1)	(1)
Net cash provided by (used in) financing activities	628	255	51
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect	39	13	(71)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	80	41	28
Cash Paid (Refunded) During the Year:
Interest net of capitalized	144	139	127
Income taxes, net	$ 28	$ (22)	$ 8

Consolidated Statement Of Cas_2

Consolidated Statement Of Cash Flows (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Interest Paid, Capitalized, Investing Activities	$ 21	$ 14	$ 15
Union Electric Company
Interest Paid, Capitalized, Investing Activities	14	10	12
Ameren Illinois Company
Interest Paid, Capitalized, Investing Activities	$ 7	$ 4	$ 3

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, 2015			$ 5,616	$ 1,331	$ (3)		$ 142				$ 1,822		$ 1,669			$ 2,005		$ 825	$ 5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Value, New Issues			0
Stock-based compensation activity			(60)
Proceeds from Contributions from Parent									$ 44		6			$ 0		0
Net income	$ 659								360				360	255				255
Net Income (Loss) Available to Common Stockholders, Basic	653			653
Common stock dividends				(416)									(355)					(110)
Preferred stock dividends													(3)					(3)
Change in deferred retirement benefit costs	20				(20)									5						$ (5)
Net income attributable to noncontrolling interest holder	$ 6						6
Dividends paid to noncontrolling interest holders							(6)
Beginning of year (shares) at Dec. 31, 2015	242.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Shares, New Issues	0
Stock Issued During Period, Shares, Other	0
End of year (shares) at Dec. 31, 2016	242.6
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Dividends, Per Share, Cash Paid	$ 1.715
Stockholders' equity, end of year at Dec. 31, 2016								$ 7,103	4,090					3,034
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Value, New Issues			0
Stock-based compensation activity			(16)
Proceeds from Contributions from Parent									30		30			8		8
Net income	$ 529								$ 326				326	271				271
Net Income (Loss) Available to Common Stockholders, Basic	523			523
Common stock dividends				(431)									(362)					0
Preferred stock dividends													(3)					(3)
Change in deferred retirement benefit costs	(5)				5									$ 0						0
Net income attributable to noncontrolling interest holder	$ 6						6
Dividends paid to noncontrolling interest holders							(6)
Stock Issued During Period, Shares, New Issues	0
Stock Issued During Period, Shares, Other	0
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Dividends, Per Share, Cash Paid	$ 1.7775
Stockholders' equity, end of year at Dec. 31, 2017	$ 7,184							7,184	$ 4,081					$ 3,310
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Value, New Issues			74
Stock-based compensation activity			13
Proceeds from Contributions from Parent									45		45			160		160
Net income	821								$ 481				481	307				307
Net Income (Loss) Available to Common Stockholders, Basic	815			815
Common stock dividends				(451)									(375)					0
Preferred stock dividends													(3)					(3)
Change in deferred retirement benefit costs	4				(4)									$ 0						0
Net income attributable to noncontrolling interest holder	$ 6						6
Dividends paid to noncontrolling interest holders							(6)
Stock Issued During Period, Shares, New Issues	1.2
Stock Issued During Period, Shares, Other	0.7
End of year (shares) at Dec. 31, 2018	244.5								102.1					25.5
End of year at Dec. 31, 2018	$ 7,773	$ 2	$ 5,627	$ 2,024	$ (22)	$ (22)	$ 142			$ 511	$ 1,903	$ 80	$ 1,735		$ 0	$ 2,173	$ 62	$ 1,539	$ 0	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common Stock, Dividends, Per Share, Cash Paid	$ 1.8475
Stockholders' equity, end of year at Dec. 31, 2018	$ 7,631							$ 7,631	$ 4,229					$ 3,774

Summary Of Significant Accounti

Summary Of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
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. Ameren also has other subsidiaries that conduct other activities, such as providing shared services. • Union Electric Company, doing business as Ameren Missouri, operates a rate-regulated electric generation, transmission, and distribution business and a rate-regulated natural gas distribution business in Missouri. Ameren Missouri was incorporated in Missouri in 1922 and is successor to a number of companies, the oldest of which was organized in 1881. It is the largest electric utility in the state of Missouri. It supplies electric and natural gas service to a 24,000 -square-mile area in central and eastern Missouri, 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. • Ameren Transmission Company of Illinois, doing business as ATXI, operates a FERC rate-regulated electric transmission business. ATXI was incorporated in Illinois in 2006. ATXI is constructing MISO-approved electric transmission projects, including the Illinois Rivers and Mark Twain projects, and operates the Spoon River project, which was placed in service in February 2018. Ameren also evaluates competitive electric transmission investment opportunities as they arise. Ameren’s financial statements are prepared on a consolidated basis and therefore include the accounts of its majority-owned subsidiaries. All intercompany transactions have been eliminated. Ameren Missouri and Ameren Illinois have no subsidiaries. All tabular dollar amounts are in millions, unless otherwise indicated. 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 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 position tracker, a tracker on certain excess deferred taxes, a renewable energy standards cost tracker, a solar rebate program tracker, PISA, and a RESRAM. Ameren Illinois’ and ATXI’s electric transmission rates are determined pursuant to formula ratemaking. Ameren Illinois participates in performance-based formula ratemaking for its electric distribution business and its electric energy-efficiency investments. Ameren Illinois also has environmental cost riders, an asbestos-related litigation rider, a natural gas energy-efficiency rider, a QIP rider, a VBA rider, a bad debt rider, and cost recovery mechanisms for renewable energy credits and zero emission credits. See Note 2 – Rate and Regulatory Matters for additional information on the regulatory assets and liabilities recorded at December 31, 2018 and 2017 . Ameren, Ameren Missouri, and Ameren Illinois continually assess the recoverability of their regulatory assets. Regulatory assets are charged to earnings when it is no longer probable that such amounts will be recovered through future revenues. To the extent that reductions in customers rates or refunds to customers related to regulatory liabilities are no longer probable, the amounts are credited to earnings. Cash, Cash Equivalents, and Restricted Cash Cash and cash equivalents include short-term, highly liquid investments purchased with an original maturity of three months or less. Cash and cash equivalents subject to legal or contractual restrictions and not readily available for use for general corporate purposes are classified as restricted cash. In November 2016, the FASB issued authoritative guidance that requires, including on a retrospective basis, restricted cash 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. Our adoption of this guidance, effective January 2018, did not result in material changes to previously reported cash flows from operating, investing, or financing activities. The changes in our restricted cash balances during the year ended December 31, 2018 , were primarily reflected as increases in cash provided by operating activities as a result of our adoption of this guidance. The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets as of December 31, 2018 and 2017 : December 31, 2018 December 31, 2017 Ameren Ameren Ameren Ameren Ameren Ameren Cash and cash equivalents $ 16 $ — $ — $ 10 $ — $ — Restricted cash included in “Other current assets” 13 4 6 21 5 6 Restricted cash included in “Other assets” 74 — 74 35 — 35 Restricted cash included in “Nuclear decommissioning trust fund” 4 4 — 2 2 — Total cash, cash equivalents, and restricted cash $ 107 $ 8 $ 80 $ 68 $ 7 $ 41 Restricted cash included in Ameren’s other current assets primarily represents participant funds from Ameren (parent)’s DRPlus and funds held by an irrevocable Voluntary Employee Beneficiary Association (VEBA) trust, which provides health care benefits for active employees. Restricted cash included in Ameren Missouri’s and Ameren Illinois’ other current assets primarily represents funds held by the VEBA trust. Restricted cash included in Ameren’s and Ameren Illinois’ other assets primarily represents amounts in a trust fund restricted for the use of funding certain asbestos-related claims and amounts collected under a cost recovery rider that are restricted for use in the procurement of renewable energy credits. Accounts Receivable “Accounts receivable – trade” on Ameren’s and Ameren Illinois’ balance sheets include certain receivables purchased at a discount from alternative retail electric suppliers that elect to participate in the utility consolidated billing program. At December 31, 2018 and 2017 , “Other current liabilities” on Ameren’s and Ameren Illinois’ balance sheets included payables for purchased receivables of $33 million and $31 million , respectively. For the years ended December 31, 2018 , 2017 , and 2016 the Ameren Companies recorded immaterial bad debt expense. 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, 2018 and 2017 : Ameren Missouri Ameren Illinois Ameren 2018 Fuel (a) $ 123 $ — $ 123 Natural gas stored underground 7 64 71 Materials, supplies, and other 228 61 289 Total inventories $ 358 $ 125 $ 483 2017 Fuel (a) $ 154 $ — $ 154 Natural gas stored underground 8 74 82 Materials, supplies, and other 226 60 286 Total inventories $ 388 $ 134 $ 522 (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. Ameren Missouri’s cost of nuclear fuel is capitalized as a part of “Property, Plant, and Equipment, Net” on the balance sheet 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. 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 2018 , 2017 , and 2016 ranged from 3% to 4% of the average depreciable cost. See Note 3 – Property, Plant, and Equipment, Net for additional information on estimated depreciable lives. 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 and GAAP. 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 average allowance for funds used during construction debt and equity blended rates that were applied to construction projects in 2018 , 2017 , and 2016 : 2018 2017 2016 Ameren Missouri 7 % 7 % 7 % Ameren Illinois 5 % 4 % 5 % 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, 2018 and 2017 . 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, 2018 and 2017 . The Ameren Transmission reporting unit had the same $93 million of goodwill as the Ameren Illinois Transmission reporting unit at December 31, 2018 and 2017 . 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. Ameren and Ameren Illinois elected to perform a qualitative assessment for their annual goodwill impairment test conducted as of October 31, 2018. 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, 2018, 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, 2018: • 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 the 30-year United States Treasury bonds; • changes 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 that 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 2018 and 2017 . Variable Interest Entities As of December 31, 2018, Ameren and Ameren Missouri had interests in unconsolidated variable interest entities that were established to construct wind generation facilities and, ultimately, sell those constructed facilities to Ameren Missouri. Neither Ameren nor Ameren Missouri are the primary beneficiary of these variable interest entities because neither has the power to direct matters that most significantly affect the entities' activities, which include designing, financing, and constructing the wind generation facilities. As a result, these variable interest entities are not required to be consolidated. As of December 31, 2018, the maximum exposure to loss related to these variable interest entities was approximately $16 million , which represents the portion of interconnection study costs that may be incurred by Ameren and Ameren Missouri. The risk of a loss was assessed to be remote and, accordingly, Ameren and Ameren Missouri have not recognized a liability associated with any portion of the maximum exposure to loss. See Note 2 – Rate and Regulatory Matters for additional information on the agreements to acquire these wind generation facilities. As of December 31, 2018 and 2017 , Ameren also had investments in unconsolidated variable interest entities totaling $22 million and $17 million , respectively, included in “Other assets” on Ameren’s consolidated balance sheet. These investments are accounted for as equity method investments. Ameren is not the primary beneficiary of these variable interest entities because it does not have the power to direct matters that most significantly affect the entities' activities. As a result, these variable interest entities are not required to be consolidated. As of December 31, 2018 , the maximum exposure to loss related to these variable interest entities is limited to the investment in these partnerships of $22 million plus associated outstanding funding commitments of $16 million . 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 $14 million , $26 million , and $31 million for the years ended December 31, 2018 , 2017 , and 2016 , respectively, which was deferred as a reduction to the net regulatory liability. The net regulatory liability also reflects deferrals of net realized and unrealized gains and losses within the nuclear decommissioning trust fund for the Callaway energy center. The depreciation deferred to the regulatory asset at Ameren Illinois was immaterial in each respective period. Uncertainties as to the probability, timing, or amount of cash expenditures associated with AROs affect our estimates of fair value. Ameren and Ameren Missouri have recorded AROs for retirement costs associated with Ameren Missouri’s Callaway energy center decommissioning, CCR facilities, and river structures. Also, Ameren, Ameren Missouri, and Ameren Illinois have recorded AROs for retirement costs associated with asbestos removal and the disposal of certain transformers. Asset removal costs that do not constitute legal obligations are classified as regulatory liabilities. See Note 2 – Rate and Regulatory Matters. The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the years ended December 31, 2018 and 2017 : Ameren Missouri Ameren Illinois Ameren Balance at December 31, 2016 $ 644 $ 6 $ 650 Liabilities settled (12 ) (1 ) (13 ) Accretion (a) 26 — 26 Change in estimates (b) (18 ) (1 ) (19 ) Balance at December 31, 2017 $ 640 (c) $ 4 (d) $ 644 (c) Liabilities settled (7 ) — (7 ) Accretion (a) 27 — 27 Change in estimates (e) (14 ) — (14 ) Balance at December 31, 2018 $ 646 (c) $ 4 (d) $ 650 (c) (a) Ameren Missouri’s accretion expense was deferred as a decrease to regulatory liabilities. (b) 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. (c) Balance included $23 million and $6 million in “Other current liabilities” on the balance sheet as of December 31, 2018 and 2017 , respectively. (d) Included in “Other deferred credits and liabilities” on the balance sheet. (e) Ameren Missouri changed its fair value estimate primarily due to a reduction in the cost estimate for closure of certain CCR storage facilities. Company-owned Life Insurance Ameren and Ameren Illinois have company-owned life insurance, which is recorded at the net cash surrender value. The net cash surrender value is the amount that can be realized under the insurance policies at the balance sheet date. As of December 31, 2018 , the cash surrender value of company-owned life insurance at Ameren and Ameren Illinois was $244 million ( December 31, 2017 – $265 million ) and $122 million ( December 31, 2017 – $129 million ), respectively, while total borrowings against the policies were $113 million ( December 31, 2017 – $120 million ) at both Ameren and Ameren Illinois. Ameren and Ameren Illinois have the right to offset the borrowings against the cash surrender value of the policies and, consequently, present the net asset in “Other assets” on their respective balance sheets. Noncontrolling Interests As of December 31, 2018 and 2017 , Ameren’s noncontrolling interests included the preferred stock of Ameren Missouri and Ameren Illinois. Operating Revenues In the first quarter of 2018, we adopted authoritative accounting guidance related to revenue from contracts with customers using the full retrospective method, with no material changes to the amount or timing of revenue recognition. We record revenues from contracts with customers for various electric and natural gas services, which primarily consist of retail distribution, electric transmission, and off-system arrangements. When more than one performance obligation exists in a contract, the consideration under the contract is allocated to the performance obligations based on the relative standalone selling price. Electric and natural gas retail distribution revenues are earned when the commodity is delivered to our customers. We accrue an estimate of electric and natural gas retail distribution revenues for service provided but unbilled at the end of each accounting period. Electric transmission revenues are earned as electric transmission services are provided. Off-system revenues are primarily comprised of MISO revenues and wholesale bilateral revenues. MISO revenues include the sale of electricity, capacity, and ancillary services. Wholesale bilateral revenues include the sale of electricity and capacity. MISO-related electricity and wholesale bilateral electricity revenues are earned as electricity is delivered. MISO-related capacity and ancillary service revenues and wholesale bilateral capacity revenues are earned as services are provided. Retail distribution, electric transmission, and off-system revenues, including the underlying components described above, represent a series of goods or services that are substantially the same and have the same pattern of transfer over time to our customers. Revenues from contracts with customers are equal to the amounts billed and our estimate of electric and natural gas retail distribution services provided but unbilled at the end of each accounting period. Revenues are billed at least monthly, and payments are due less than one month after goods and/or services are provided. See Note 15 – Segment Information for disaggregated revenue information. For certain regulatory recovery mechanisms that are alternative revenue programs rather than revenues from contracts with customers, we recognize revenues that have been authorized for rate recovery, are objectively determinable and probable of recovery, and are expected to be collected from customers within two years from the end of the year. Our alternative revenue programs include revenue requirement reconciliations, MEEIA, and VBA. These revenues are subsequently recognized as revenues from contracts with customers when billed, with an offset to alternative revenue program revenues. The Ameren Companies elected not to disclose the aggregate amount of the transaction price allocated to the performance obligations that are unsatisfied as of the end of the reporting period for contracts with an initial expected term of one year or less. As of December 31, 2018 and 2017 , our remaining performance obligations were immaterial. 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 2018 , 2017 , or 2016 . 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. Deferred Compensation As of December 31, 2018 , and 2017 , “Other deferred credits and liabilities” on Ameren’s balance sheet included deferred compensation obligations of $80 million and $86 million , respectively, recorded at the present value of future benefits to be paid. Excise Taxes Ameren Missouri and Ameren Illinois collect from their customers excise taxes, including municipal and state excise taxes and gross receipts taxes, that are levied on the sale or distribution of natural gas and electricity. The following table presents the excise taxes recorded on a gross basis in “Operating Revenues – Electric,” “Operating Revenues – Natural gas” and “Operating Expenses – Taxes other than income taxes” on the statements of income for the years ended December 31, 2018 , 2017 , and 2016 : 2018 2017 2016 Ameren Missouri $ 164 $ 153 $ 151 Ameren Illinois 118 112 (a) 108 (a) Ameren $ 282 $ 265 $ 259 (a) Amounts have been adjusted from those previously reported to reflect additional excise taxes for the years ended December 31, 2017 and 2016. 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 the revaluation of deferred taxes related to the TCJA and Missouri and Illinois state corporate income tax rate changes. 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 Earnings per basic and diluted share are computed by dividing “Net Income Attributable to Ameren Common Shareholders” by the weighted-average number of basic and diluted common shares outstanding, respectively, during the period. Earnings per diluted share reflects the potential dilution that would occur if certain stock-based performance share units and restricted stock units were s

Rate And Regulatory Matters

Rate And Regulatory Matters	12 Months Ended
	Dec. 31, 2018
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 Missouri Senate Bill 564 On June 1, 2018, Missouri Senate Bill 564 was enacted. The provision of the law applicable to the TCJA was effective immediately; the remaining provisions, including the ability to elect PISA, became effective August 28, 2018. The law required the MoPSC to authorize a reduction in Ameren Missouri’s rates to pass through the effect of the TCJA within 90 days of the law’s effective date. In July 2018, the MoPSC authorized Ameren Missouri to reduce its annual revenue requirement by $167 million and reflect that reduction in rates beginning August 1, 2018. The reduction included $74 million for the amortization of excess accumulated deferred income taxes. In addition, Ameren Missouri recorded a reduction to revenue and a corresponding regulatory liability of $60 million for the excess amounts collected in rates related to the TCJA from January 1, 2018, through July 31, 2018. The regulatory liability will be reflected in customer rates over a period of time to be determined by the MoPSC in the next regulatory rate review. Pursuant to its PISA election, Ameren Missouri is permitted to defer and recover 85% of the depreciation expense and a weighted-average cost of capital return on rate base on certain property, plant, and equipment placed in service after September 1, 2018, and not included in base rates . The rate base on which the return is calculated incorporates qualifying capital expenditures since the PISA election date, and changes in total accumulated depreciation excluding retirements and plant-related deferred income taxes. The debt return on rate base is recognized in earnings as a reduction of “Interest Charges” until PISA deferrals are reflected in customer rates, while the equity return is recognized in earnings as “Operating Revenues – Electric” when billed to customers. Accumulated PISA deferrals earn carrying costs at the weighted-average cost of capital, and all approved PISA deferrals will be added to rate base prospectively and recovered over a period of 20 years following a regulatory rate review. PISA mitigates the impacts of regulatory lag between regulatory rate reviews. The remaining 15% of certain property, plant, and equipment placed in service and not eligible for recovery under PISA, unless eligible for recovery under the RESRAM, remain subject to regulatory lag. See below for discussion of the RESRAM. Amounts deferred under PISA were immaterial as of December 31, 2018 . As a result of Ameren Missouri’s PISA election, additional provisions of Missouri law apply, including provisions limiting customer rate increases to a 2.85% compound annual growth rate in the average overall customer rate per kilowatthour, based on the electric rates that became effective in April 2017, less half of the annual savings from the TCJA that was passed on to customers as approved in the July 2018 MoPSC order. Additionally, Ameren Missouri’s electric base rates, as determined in the July 2018 MoPSC order, are frozen until April 1, 2020. Customer rates under the MEEIA, the FAC, and the RESRAM riders have not been frozen. If rate changes from the FAC or the RESRAM riders would cause rates to temporarily exceed the 2.85% rate cap, the overage would be deferred for future recovery in the next regulatory rate review; however, rates established in such regulatory rate review will be subject to the rate cap. Any deferred overages approved for recovery will be recovered in a manner consistent with costs recovered under PISA. Excluding customer rates under the MEEIA rider, which are not subject to the rate cap, Ameren Missouri would incur a penalty equal to the amount of deferred overage that would cause customer rates to exceed the 2.85% rate cap. Both the rate cap and PISA election are effective through December 2023, unless Ameren Missouri requests and receives MoPSC approval of an extension through December 2028. Wind Generation Facilities and RESRAM In the second quarter of 2018, Ameren Missouri entered into a build-transfer agreement with a subsidiary of Terra-Gen, LLC to acquire, after construction, a 400 -megawatt wind generation facility, which is expected to be located in northeastern Missouri. In October 2018, the MoPSC issued an order approving a unanimous stipulation and agreement regarding a requested certificate of convenience and necessity for the facility. In December 2018, Ameren Missouri received FERC approval to acquire the facility after construction. A transmission interconnection agreement with the MISO for this facility is expected in the fall of 2019. Also, in October 2018, Ameren Missouri entered into a build-transfer agreement with a subsidiary of EDF Renewables, Inc. to acquire, after construction, a wind generation facility of up to 157 megawatts. In February 2019, Ameren Missouri filed with the MoPSC a nonunanimous stipulation and agreement regarding a requested certificate of convenience and necessity for the facility. The up to 157 -megawatt facility is expected to be located in northwestern Missouri. A transmission interconnection agreement with the MISO for this facility is expected in early 2020. Both facilities are expected to be completed by the end of 2020 and would help Ameren Missouri comply with the Missouri renewable energy standard. Each acquisition is subject to certain conditions, including entering into a MISO transmission interconnection agreement at an acceptable cost for each facility and obtaining FERC approval and the issuance of a certificate of convenience and necessity by the MoPSC for the up to 157-megawatt facility, as well as other customary contract terms and conditions. These agreements collectively represent approximately $1 billion in capital expenditures expected in 2020, which is included in Ameren Missouri’s Smart Energy Plan. As a part of its May 2018 filing, Ameren Missouri requested the MoPSC to authorize a proposed RESRAM that would allow Ameren Missouri to adjust customer rates on an annual basis without a traditional regulatory rate review. In October and December 2018, the MoPSC issued orders approving the RESRAM. In January 2019, the MoOPC filed an appeal with the Missouri Court of Appeals, Western District, challenging the MoPSC’s December 2018 order allowing Ameren Missouri to recover, through the RESRAM, the 15% of capital investment not recovered under PISA. Ameren Missouri expects a decision by the end of 2019. The RESRAM is designed to mitigate the impacts of regulatory lag for the cost of compliance with renewable energy standards, including recovery of investments in wind and other renewable generation, by providing more timely recovery of costs and a return on investments not already provided for in customer rates or recovered under PISA. RESRAM regulatory assets earn carrying costs at short-term interest rates. Renewable Choice Program In June 2018, the MoPSC approved Ameren Missouri’s Renewable Choice Program, which allows large commercial and industrial customers and municipalities to elect to receive up to 100% of their energy from renewable resources. The tariff-based program is designed to recover the costs of the election, net of changes in the market price of such energy. Based on customer contracts, the program enables Ameren Missouri to supply up to 400 megawatts of renewable wind energy generation, up to 200 megawatts of which it could own. As applicable, the addition of generation by Ameren Missouri would be subject to the issuance of a certificate of convenience and necessity by the MoPSC, obtaining transmission interconnection agreements with MISO or other RTOs, and FERC approval. Ameren Missouri anticipates finalizing customer interest and pursuing renewable energy projects to fulfill requirements in 2019. Without extension, the option to elect into the program will terminate in the third quarter of 2023. MEEIA In July 2018, the Missouri Supreme Court overturned a 2016 decision by the Missouri Court of Appeals, Western District, which had upheld a 2015 MoPSC order regarding the determination of a certain input used to calculate the MEEIA 2013 performance incentive, and remanded the matter to the MoPSC. In January 2019, the MoPSC issued an order approving an additional $9 million MEEIA 2013 performance incentive. Accordingly, Ameren Missouri recognized the additional performance incentive in the first quarter of 2019. 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 collected $28 million of the performance incentive over a two-year period that began in February 2017. The MEEIA 2016 plan provides 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 are achieved during the three-year period beginning March 2016, with the potential to earn more if Ameren Missouri’s energy savings exceed 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 plan. In October 2018, Ameren Missouri received an order from the MoPSC approving Ameren Missouri’s energy savings results for the second year of the MEEIA 2016 plan. As a result of these orders and in accordance with revenue recognition guidance, Ameren Missouri recognized $5 million of revenues in the first quarter of 2018, $6 million of additional revenues in the fourth quarter of 2018, and $11 million of additional revenues in the first quarter of 2019 relating to the MEEIA 2016 performance incentive. In December 2018, the MoPSC issued an order approving Ameren Missouri’s MEEIA 2019 plan. The plan includes a portfolio of customer energy-efficiency programs through December 2021 and low-income customer energy-efficiency programs through December 2024, along with a regulatory recovery mechanism. Ameren Missouri intends to invest $226 million over the life of the plan, including $65 million per year through 2021. The plan includes the continued use of the MEEIA rider, which allows Ameren Missouri to collect from, or refund to, customers any difference in actual MEEIA program costs and related lost electric margins and the amounts collected from customers. In addition, the plan includes a performance incentive that provides Ameren Missouri an opportunity to earn additional revenues by achieving certain customer energy-efficiency goals, including $30 million if 100% of the goals are achieved during the period ended December 2021. Additional revenues may be earned if Ameren Missouri exceeds 100% of its energy savings goals. 2018 Natural Gas Delivery Service Regulatory Rate Review In December 2018, Ameren Missouri filed a request with the MoPSC to increase its annual revenues for natural gas delivery service by approximately $4 million . The natural gas delivery service rate increase request was based on a 10.30% return on equity, a capital structure composed of 51.84% common equity, a rate base of $259 million , and a test year ended June 30, 2018, with certain pro-forma adjustments through the anticipated true-up date of May 31, 2019. In December 2018, the MoPSC issued an order approving a stipulation and agreement for an interim rate reduction of $2 million to reflect cost of service updates including the reduction in the federal corporate income tax rate and the amortization of excess deferred taxes as a result of the TCJA. The interim rate reduction became effective January 2, 2019, and will continue until new rates are approved by the MoPSC in this regulatory rate review. Mark Twain Project Return on Equity Incentive Adder In November 2018, the FERC issued an order approving a 50 basis point return on equity incentive adder for the Mark Twain project, effective as of February 14, 2018, based on the unique nature of risks involved in the project. This incentive adder is in addition to the current 50 basis point incentive adder for participation in an RTO and the total return on equity, inclusive of all incentive adders, is subject to the top of the zone of reasonableness. The impact to Ameren’s 2018 earnings was immaterial. 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, 2018 , Ameren Illinois had recorded regulatory assets of $16 million and $54 million , including interest, to reflect its expected 2018 and its approved 2017 revenue requirement reconciliation adjustments, respectively. As of December 31, 2017 , Ameren Illinois had recorded a $24 million regulatory asset to reflect its approved 2016 revenue requirement reconciliation adjustment, which was collected, with interest, from customers during 2018. In November 2018, the ICC issued an order in Ameren Illinois’ annual update filing that approved a $72 million increase in Ameren Illinois’ electric distribution service rates beginning in January 2019. This order reflected an increase to the annual formula rate based on 2017 actual costs and expected net plant additions for 2018, and an increase to include the 2017 revenue requirement reconciliation adjustment. It also included a decrease for the conclusion of the 2016 revenue requirement reconciliation adjustment, which was fully collected from customers in 2018, consistent with the ICC’s December 2017 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. 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 its weighted-average cost of capital, with the equity return based on the annual average of the monthly yields 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. In 2018, there were no performance-related basis point adjustments. In September 2017, the ICC issued an order approving Ameren Illinois’ implementation of the FEJA electric energy-efficiency savings targets and investments for 2018 through 2021. Ameren Illinois plans to invest approximately $100 million per year in electric energy-efficiency programs through 2023, consistent with targets established by the FEJA. 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 are collected from customers through a rider and are not included in the electric distribution formula ratemaking framework. In June 2018, Ameren Illinois filed its annual electric customer energy-efficiency formula rate update to establish the revenue requirement to be used for 2019 rates with the ICC. In November 2018, the ICC issued an order that approved 2019 rates of $35 million for electric customer energy-efficiency investments, which represents an increase of $20 million from 2018 rates. 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 rates for natural gas delivery service. In November 2018, the ICC issued an order approving a stipulation and agreement that resulted in an annual natural gas rate increase of $32 million, based on a 9.87% return on common equity, a capital structure composed of 50% common equity, and a rate base of $1.6 billion. This increase reflects the reduction in the federal statutory corporate income tax rate enacted under the TCJA, as well as the increase in the Illinois corporate income tax rate that became effective in July 2017, which collectively decreased annual rates by approximately $17 million. The new customer rates were effective in November 2018. As a result of this order, the rate base under the QIP rider was reset to zero. Ameren Illinois used a 2019 future test year in this proceeding. Income Tax Regulatory Mechanisms In February 2018, the ICC granted Ameren Illinois’ request, filed in January 2018, to establish a rider to reduce Ameren Illinois’ electric distribution customer rates for the effect of 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 customer rates were reduced as a result of the rider beginning in the first quarter of 2018. The estimated reduction of approximately $50 million per year will continue through 2019, as base rates will be adjusted to reflect the current income tax rates starting in 2020. In April 2018, the ICC approved a rider for the difference between revenues billed under natural gas rates established 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 discussed above been in effect. The rider required Ameren Illinois to record this difference as a regulatory liability beginning January 25, 2018. Ameren Illinois’ natural gas customer rates were reduced as a result of the rider beginning in May 2018. As base rates were updated with the November 2018 natural gas rate order discussed above, a reduction of approximately $15 million will be reflected in customer rates substantially over a one-year period. 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. 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. Upon appeal, in October 2018, the Illinois Supreme Court reversed the Illinois Circuit Court for Edgar County’s decision and remanded the case for further proceedings. In December 2018, the Illinois Supreme Court issued an order to stay its October 2018 ruling. In February 2019, the landowners filed an appeal with the United States Supreme Court. The October 2018 ruling is further stayed pending resolution of the appeal. Construction of the Illinois Rivers project is substantially complete. Delays associated with the condemnation proceedings or a rehearing arising from the Illinois Supreme Court’s ruling will delay the expected completion date to 2020, which is not expected to materially affect 2019 earnings. The estimated line segment capital expenditure investment is approximately $81 million , of which $38 million was invested as of December 31, 2018 . 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 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, effective since September 2016. In 2017, Ameren and Ameren Illinois refunded $21 million and $17 million , respectively related to the November 2013 complaint case. 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. 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 an unrelated case in which the FERC established the allowed base return on common equity methodology subsequently used in the two MISO complaint cases described above. In October 2018, the FERC issued an order addressing the remanded issues in an unrelated case. That order proposed a new methodology for determining the base return on equity and required further briefs from the participants. In November 2018, the FERC issued an order related to the February 2015 complaint case and the September 2016 final order, which required briefs from the participants to be filed in February 2019 regarding a new methodology for determining the base return on common equity and whether and how to apply the new methodology to the two MISO complaint cases. Ameren is unable to predict the ultimate impact of the proposed methodology on these complaint cases at this time. As the FERC is under no deadline to issue a final order, the timing of the issuance of the final order in the February 2015 complaint case, or any potential impact to the amounts refunded as a result of the September 2016 final order, is uncertain. 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 now superseded 12.38% allowed base return on common equity 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 in an unrelated case, as discussed above. The FERC is under no deadline to issue an order on this motion. As of December 31, 2018 , Ameren and Ameren Illinois had recorded current regulatory liabilities of $44 million and $26 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. FERC Federal Income Tax Proceeding and Formula Rate Change In March 2018, the FERC granted a request filed in February 2018 by MISO transmission owners with forward-looking rate formulas, including Ameren Illinois and ATXI, to allow revisions to their 2018 electric transmission rates to reflect the effect of the reduction in federal income taxes enacted under the TCJA. Ameren Illinois and ATXI’s 2018 electric transmission rates were reduced by $27 million and $23 million , respectively. In May 2018, the FERC accepted Ameren Illinois, and ATXI’s tariff filings to change the formula rate calculation. The change allows for the recovery or refund of both excess deferred taxes resulting from tax law or rate changes and the effect of permanent income tax differences and were reflected in Ameren Illinois’ and ATXI’s electric transmission rates starting in January 2019. Regulatory Assets and Liabilities The following table presents our regulatory assets and regulatory liabilities at December 31, 2018 and 2017 : 2018 2017 Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Regulatory assets: Under-recovered FAC (a) $ 3 $ — $ 3 $ 47 $ — $ 47 Under-recovered PGA (b) — 7 7 1 13 14 MTM derivative losses (c) 19 197 216 12 217 229 IEIMA revenue requirement reconciliation adjustment (d)(e) — 70 70 — 78 78 FERC revenue requirement reconciliation adjustment (f) — 16 30 — 25 37 Under-recovered VBA rider (g) — — — — 15 15 Pension and postretirement benefit costs (h) 103 149 252 84 215 299 Income taxes (i) 119 68 185 139 56 197 Callaway costs (e)(j) 22 — 22 25 — 25 Unamortized loss on reacquired debt (k) 58 40 98 61 49 110 Environmental cost riders (l) — 148 148 — 173 173 Storm costs (e)(m) — 13 13 — 10 10 Demand-side costs before the MEEIA implementation (e)(n) 5 — 5 11 — 11 Workers’ compensation claims (o) 4 7 11 5 7 12 Construction accounting for pollution control equipment (e)(p) 16 — 16 18 — 18 Solar rebate program (e)(q) 14 — 14 31 — 31 FEJA energy-efficiency riders (e)(r) — 136 136 — 41 41 Other 17 18 35 17 10 27 Total regulatory assets $ 380 $ 869 $ 1,261 $ 451 $ 909 $ 1,374 Less: current regulatory assets (14 ) (110 ) (134 ) (56 ) (87 ) (144 ) Noncurrent regulatory assets $ 366 $ 759 $ 1,127 $ 395 $ 822 $ 1,230 Regulatory liabilities: Over-recovered FAC (a) $ 34 $ — $ 34 $ 4 $ — $ 4 Over-recovered Illinois electric power costs (b) — 12 12 — 16 16 Over-recovered PGA (b) 7 3 10 — 1 1 Over-recovered VBA rider (g) — 8 8 — — — MTM derivative gains (c) 5 3 8 16 — 16 FERC revenue requirement reconciliation adjustment (f) — 17 19 — — — Energy-efficiency riders (s) 19 3 22 2 40 42 Estimated refund for FERC complaint case (t) — 26 44 — 25 42 Income taxes (i) 1,484 843 2,413 1,392 842 2,323 Asset removal costs (u) 1,027 774 1,811 995 725 1,725 AROs (v) 175 — 175 223 — 223 Pension and postretirement benefit costs tracker (w) 43 — 43 35 — 35 Renewable energy credits and zero emission credits (x) — 102 102 — 58 58 Excess income taxes collected in 2018 (y) 60 — 60 — — — Other 13 12 25 16 14 30 Total regulatory liabilities $ 2,867 $ 1,803 $ 4,786 $ 2,683 $ 1,721 $ 4,515 Less: current regulatory liabilities (68 ) (62 ) (149 ) (19 ) (92 ) $ (128 ) Noncurrent regulatory liabilities $ 2,799 $ 1,741 $ 4,637 $ 2,664 $ 1,629 $ 4,387 (a) 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. (b) Under-recovered or over-recovered costs from utility customers. Amounts will be recovered from, or refunded to, customers within one year of the deferral. (c) Deferral of commodity-related derivative MTM losses or gains. See Note 7 – Derivative Financial Instruments for additional information. (d) 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. (e) These assets earn a return. (f) 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. (g) Under-recovered or over-recovered natural gas revenue caused by sales volume deviations from weather normalized sales approved by the ICC in rate regulatory reviews. Each year’s amount will be recovered from, or refunded to, customers from April through December of the following year. (h) 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. (i) 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, and the unamortized portion of investment tax credits will be amortized over the expected life of the related assets. The amortization periods for depreciation differences are determined in rate orders by the applicable regulators and range from 7 to 60 years. See Note 12 – Income Taxes for amounts related to the revaluation of deferred income taxes under the TCJA. (j) 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 original remaining life of the energy center. (k) 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. (l) 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. (m) Storm costs from 2015, 2016, and 2018 deferred in accordance with the IEIMA. These costs are being amortized over five-year periods beginning in the year the storm occurred. (n) 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

Property And Plant, Net

Property And Plant, Net	12 Months Ended
	Dec. 31, 2018
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, 2018 and 2017 : Ameren Missouri (a) Ameren Illinois Other Ameren (a) 2018 Property, plant, and equipment at original cost: (b) Electric generation $ 11,432 $ — $ — $ 11,432 Electric distribution 5,989 5,970 — 11,959 Electric transmission 1,277 2,647 1,385 5,309 Natural gas 500 2,701 — 3,201 Other (c) 1,008 863 230 2,101 20,206 12,181 1,615 34,002 Less: Accumulated depreciation and amortization 8,726 3,294 253 12,273 11,480 8,887 1,362 21,729 Construction work in progress: Nuclear fuel in process 217 — — 217 Other 406 311 147 864 Property, plant, and equipment, net $ 12,103 $ 9,198 $ 1,509 $ 22,810 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 (a) Amounts in Ameren and Ameren Missouri include two CTs under separate agreements. The gross cumulative asset value of those agreements was $235 million and $233 million at December 31, 2018 and 2017 , respectively. The total accumulated depreciation associated with the two CTs was $89 million and $83 million at December 31, 2018 and 2017 , respectively. See Note 5 – Long-term Debt and Equity Financings for additional information on these 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 amortization expense of capitalized software, the gross carrying value of capitalized software, and the related accumulated amortization by year: Amortization Expense (a) Gross Carrying Value Accumulated Amortization 2018 2017 2016 2018 2017 2018 2017 Ameren $ 71 $ 58 $ 52 $ 734 $ 655 $ (514 ) $ (466 ) Ameren Missouri 24 20 17 223 191 (125 ) (107 ) Ameren Illinois 44 36 33 297 241 (183 ) (146 ) (a) As of December 31, 2018 , 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, 2018 , 2017 , and 2016 , which represent noncash investing activity excluded from the accompanying statements of cash flows: Ameren Ameren Missouri Ameren Illinois Accrued capital expenditures: 2018 $ 272 $ 121 $ 138 2017 361 159 175 2016 251 116 87 Accrued nuclear fuel expenditures: 2018 $ 20 $ 20 $ — 2017 10 10 — 2016 20 20 —

Short-Term Debt And Liquidity

Short-Term Debt And Liquidity	12 Months Ended
	Dec. 31, 2018
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 In December 2018, the Credit Agreements, which were scheduled to mature in December 2021, were extended and now mature in December 2022. The Credit Agreements provide $2.1 billion of credit cumulatively through maturity. The maturity date may be extended for an additional one-year period 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, 2018 , based on commercial paper outstanding and letters of credit issued under the Credit Agreements, along with cash and cash equivalents, the net liquidity available to Ameren (parent), Ameren Missouri, and Ameren Illinois, collectively, was $1.5 billion . Ameren, Ameren Missouri, and Ameren Illinois did not borrow under the Credit Agreements for the years ended December 31, 2018 and 2017 . 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, 2018 and 2017 : Ameren (parent) Ameren Missouri Ameren Illinois Ameren Consolidated 2018 Average daily commercial paper outstanding $ 410 $ 61 $ 108 $ 579 Outstanding borrowings at period-end 470 55 72 597 Weighted-average interest rate 2.31 % 1.94 % 2.26 % 2.26 % Peak outstanding commercial paper during period (a) $ 543 $ 481 $ 442 $ 1,295 Peak interest rate 3.10 % 2.80 % 2.85 % 3.10 % 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 % (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, 2018 , the ratios of consolidated indebtedness to total consolidated capitalization, calculated in accordance with the provisions of the Credit Agreements, were 53% , 47% , and 48% , 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 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 the Credit Agreements at December 31, 2018 . 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 utility money pool agreement must repay the principal amount of such loan, together with accrued interest. The rate of interest depends on the composition of internal and external funds in the utility money pool. The average interest rate for borrowing under the utility money pool for the year ended December 31, 2018 , was 2.10% ( 2017 – 1.19% ). See Note 13 – Related-party Transactions for the amount of interest income and expense from the utility money pool agreement recorded by the Ameren Companies for the years ended December 31, 2018 , 2017 , and 2016 .

Long-Term Debt And Equity Finan

Long-Term Debt And Equity Financings	12 Months Ended
	Dec. 31, 2018
Long-Term Debt And Equity Financings [Abstract]
LONG-TERM DEBT AND EQUITY FINANCINGS	2.0 5.5 $ 4,688 > 2.5 140.8 $ 3,153 Ameren Illinois > 2.0 6.9 4,234 (d) > 1.5 3.2 203 (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 $2,006 million and $985 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 and ATXI may not pay any dividend on their respective stock unless, among other things, their respective earnings and earned surplus are sufficient to declare and pay a dividend after provisions are made for reasonable and proper reserves, or unless Ameren Illinois or ATXI 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, 2018 , 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. At December 31, 2018 , 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, 2018 , 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, variable interest entities, letters of credit, and Ameren (parent) guarantee arrangements on behalf of its subsidiaries. See Note 1 – Summary of Significant Accounting Policies for further detail concerning variable interest entities." 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, 2018 and 2017 : 2018 2017 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 (3 ) (4 ) Long-term debt, net $ 697 $ 696 Ameren Missouri: Bonds and notes: 6.00% Senior secured notes due 2018 (a) — 179 5.10% Senior secured notes due 2018 (a) — 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) 400 400 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 4.00% First mortgage bonds due 2048 (f) 425 — Finance obligations: City of Bowling Green agreement (Peno Creek CT) due 2022 (g) 30 36 Audrain County agreement (Audrain County CT) due 2023 (g) 240 240 Total long-term debt, gross 4,029 3,988 Less: Unamortized discount and premium (9 ) (7 ) Less: Unamortized debt issuance costs (22 ) (20 ) Less: Maturities due within one year (580 ) (384 ) Long-term debt, net $ 3,418 $ 3,577 2018 2017 Ameren Illinois: Bonds and notes: 6.25% Senior secured notes due 2018 (h) — 144 9.75% Senior secured notes due 2018 (h) — 313 2.70% Senior secured notes due 2022 (h)(i) 400 400 5.90% First mortgage bonds due 2023 (j) (j) (j) 5.70% First mortgage bonds due 2024 (k) (k) (k) 3.25% Senior secured notes due 2025 (h) 300 300 6.125% Senior secured notes due 2028 (h) 60 60 1993 Series B-1 Senior unsecured notes due 2028 (d) 17 17 3.80% First mortgage bonds due 2028 (l) 430 — 6.70% Senior secured notes due 2036 (h) 61 61 6.70% Senior secured notes due 2036 (m) 42 42 4.80% Senior secured notes due 2043 (h) 280 280 4.30% Senior secured notes due 2044 (h) 250 250 4.15% Senior secured notes due 2046 (h) 490 490 3.70% First mortgage bonds due 2047 (l) 500 500 4.50% First mortgage bonds due 2049 (l) 500 — Total long-term debt, gross 3,330 2,857 Less: Unamortized discount and premium (3 ) (3 ) Less: Unamortized debt issuance costs (31 ) (24 ) Less: Maturities due within one year — (457 ) Long-term debt, net $ 3,296 $ 2,373 ATXI: 3.43% Senior notes due 2050 (n) $ 450 $ 450 Total long-term debt, gross 450 450 Less: Unamortized debt issuance costs (2 ) (2 ) Long-term debt, net $ 448 $ 448 Ameren consolidated long-term debt, net $ 7,859 $ 7,094 (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 2048 maturity of the 4.00% first mortgage bonds and the restrictions preventing a release date to occur that are attached to certain senior secured notes described in footnote (b) below, Ameren Missouri does not expect the first mortgage lien protection associated with these notes to fall away. (b) Ameren Missouri has agreed that so long as any of the 3.90% senior secured notes due 2042 are outstanding, Ameren Missouri will not permit a release date to occur, and so long as any of the 6.70% senior secured notes due 2019 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 average interest rates for 2018 and 2017 were as follows: 2018 2017 Ameren Missouri 1992 Series due 2022 2.37% 1.43% Ameren Missouri 1998 Series A due 2033 2.76% 1.77% Ameren Missouri 1998 Series B due 2033 2.79% 1.75% Ameren Missouri 1998 Series C due 2033 2.83% 1.73% Ameren Illinois 1993 Series B-1 due 2028 1.58% 1.08% (e) These bonds are first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage bond indenture. They are secured by substantially all Ameren Missouri property and franchises. Less than $1 million principal amount of the bonds remain outstanding. (f) These bonds are first mortgage bonds issued by Ameren Missouri under the Ameren Missouri bond indenture. They are secured by substantially all Ameren Missouri property and franchises. (g) Payments due related to these financing 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 city/county and held by Ameren Missouri. The timing and amounts of payments due from Ameren Missouri under the 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 financing obligations and the related investments in debt securities, recorded in "Other Assets," was $270 million and $276 million , respectively, as of December 31, 2018 and 2017. (h) 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 2049 maturity date of the 4.50% first mortgage bonds, Ameren Illinois does not expect the first mortgage lien protection associated with these notes to fall away. (i) 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. (j) 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. (k) 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 also backed by an insurance guarantee policy. Less than $1 million principal amount of the bonds remains outstanding. (l) 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. (m) 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). (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, 2018 : Ameren (parent) (a) Ameren Missouri (a) Ameren Illinois (a) ATXI (a) Ameren Consolidated 2019 $ — $ 580 $ — $ — $ 580 2020 350 92 — — 442 2021 — 8 — — 8 2022 — 55 400 50 505 2023 — 240 — — 240 Thereafter 350 3,054 2,930 400 6,734 Total $ 700 $ 4,029 $ 3,330 $ 450 $ 8,509 (a) Excludes unamortized discount, unamortized premium, and debt issuance costs of $3 million , $31 million , $34 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, 2018 and 2017 : Redemption Price (per share) 2018 2017 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 2018, Ameren issued a total of 1.2 million shares of common stock under its DRPlus and 401(k) plan, and received proceeds of $74 million . In addition, in 2018, Ameren issued 0.7 million shares of common stock valued at $35 million upon the vesting of stock-based compensation. Ameren did not issue any common stock in 2017 or 2016. In October 2018, 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 issuable 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. 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 the DRPlus, which expires in 2020. Shares of common stock sold under the 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, 2018 and 2017, the DRPlus participant funds of $1 million and $8 million , respectively, were reflected on Ameren’s consolidated balance sheets in “Other current assets.” Ameren Missouri In April 2018, Ameren Missouri issued $425 million of 4.00% first mortgage bonds due April 2048, with interest payable semiannually on April 1 and October 1 of each year, beginning October 1, 2018. Ameren Missouri received proceeds of $419 million , which were used to repay outstanding short-term debt, including short-term debt that Ameren Missouri incurred in connection with the repayment of $179 million of its 6.00% senior secured notes that matured April 1, 2018. In August 2018, $199 million principal amount of Ameren Missouri’s 5.10% senior secured notes matured and were repaid with cash on hand. 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. For information on Ameren Missouri’s capital contributions, refer to Capital Contributions in Note 13 – Related-party Transactions. Ameren Illinois In May 2018, Ameren Illinois issued $430 million of 3.80% first mortgage bonds due May 2028, with interest payable semiannually on May 15 and November 15 of each year, beginning November 15, 2018. Ameren Illinois received proceeds of $427 million , which were used to repay outstanding short-term debt, including short-term debt that Ameren Illinois incurred in connection with the repayment of $144 million of its 6.25% senior secured notes that matured April 1, 2018. In November 2018, Ameren Illinois issued $500 million of 4.50% first mortgage bonds due March 2049, with interest payable semiannually on March 15 and September 15 of each year, beginning March 15, 2019. Ameren Illinois received proceeds of $495 million , which were used to repay outstanding short-term debt, including short-term debt that Ameren Illinois incurred in connection with the repayment of $313 million of its 9.75% senior secured notes that matured November 15, 2018. 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. 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. 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, 2018 , 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 5.5 $ 4,688 > 2.5 140.8 $ 3,153 Ameren Illinois > 2.0 6.9 4,234 (d) > 1.5 3.2 203 (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 $2,006 million and $985 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 and ATXI may not pay any dividend on their respective stock unless, among other things, their respective earnings and earned surplus are sufficient to declare and pay a dividend after provisions are made for reasonable and proper reserves, or unless Ameren Illinois or ATXI 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, 2018 , 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. At December 31, 2018 , 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, 2018 , 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, variable interest entities, letters of credit, and Ameren (parent) guarantee arrangements on behalf of its subsidiaries. See Note 1 – Summary of Significant Accounting Policies for further detail concerning variable interest entities.

Other Income, Net

Other Income, Net	12 Months Ended
	Dec. 31, 2018
Other Nonoperating Income (Expense) [Abstract]
OTHER INCOME AND EXPENSES	OTHER INCOME, NET The following table presents the components of “Other Income, Net” in the Ameren Companies’ statements of income for the years ended December 31, 2018 , 2017 , and 2016 : 2018 2017 2016 Ameren: Other Income, Net Allowance for equity funds used during construction $ 36 $ 24 $ 27 Interest income on industrial development revenue bonds 26 26 27 Other interest income 7 8 13 Non-service cost components of net periodic benefit income 70 (a) 44 56 Other income 8 5 10 Donations (33 ) (8 ) (16 ) Other expense (12 ) (13 ) (16 ) Total Other Income, Net $ 102 $ 86 $ 101 Ameren Missouri: Other Income, Net Allowance for equity funds used during construction $ 27 $ 21 $ 23 Interest income on industrial development revenue bonds 26 26 27 Other interest income 2 1 1 Non-service cost components of net periodic benefit income 17 (a) 22 18 Other income 4 3 3 Donations (14 ) (2 ) (4 ) Other expense (6 ) (6 ) (6 ) Total Other Income, Net $ 56 $ 65 $ 62 Ameren Illinois: Other Income, Net Allowance for equity funds used during construction $ 9 $ 3 $ 4 Interest income 6 7 12 Non-service cost components of net periodic benefit income 34 10 24 Other income 3 2 6 Donations (6 ) (5 ) (6 ) Other expense (4 ) (5 ) (6 ) Total Other Income, Net $ 42 $ 12 $ 34 . (a) For the year ended December 31, 2018 , the non-service cost components of net periodic benefit income were partially offset by a $17 million deferral due to a regulatory tracking mechanism for the difference between the level of such costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates.

Derivative Financial Instrument

Derivative Financial Instruments	12 Months Ended
	Dec. 31, 2018
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 and power, 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 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, 2018 and 2017 . As of December 31, 2018 , these contracts extended through October 2021, March 2023, and May 2032 for fuel oils, natural gas, and power, respectively. Quantity (in millions) 2018 2017 Commodity Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils (in gallons) (a) 66 — 66 28 — 28 Natural gas (in mmbtu) 19 154 173 24 139 163 Power (in megawatthours) 1 8 9 3 9 12 (a) Consists of ultra-low-sulfur diesel products. 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, 2018 and 2017 , 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, 2018 and 2017 : 2018 2017 Commodity Balance Sheet Location Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils Other current assets $ 3 $ — $ 3 $ 5 $ — $ 5 Other assets 5 — 5 2 — 2 Natural gas Other current assets — 1 1 — — — Other assets — 2 2 1 — 1 Power Other current assets 4 — 4 9 — 9 Total assets $ 12 $ 3 $ 15 $ 17 $ — $ 17 Fuel oils Other current liabilities $ 4 $ — $ 4 $ — $ — $ — Other deferred credits and liabilities 9 — 9 — — — Natural gas Other current liabilities 4 8 12 5 12 17 Other deferred credits and liabilities 1 6 7 3 10 13 Power Other current liabilities 4 14 18 1 13 14 Other deferred credits and liabilities — 169 169 — 182 182 Total liabilities $ 22 $ 197 $ 219 $ 9 $ 217 $ 226 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 at the gross amounts 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, 2018 and 2017 . 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, 2018 , 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 below investment grade, 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, 2018 , 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, 2018 , 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) Ameren Missouri $ 76 $ 4 $ 64 Ameren Illinois 37 — 30 Ameren $ 113 $ 4 $ 94 (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, 2018
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 (quoted prices in active markets for identical assets or liabilities): 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. 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 (significant other observable inputs) : 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 United States Treasury and agency securities, corporate bonds and other fixed-income 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. 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 derivative 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 (significant other unobservable inputs): 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. 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 material gains or losses related to valuation adjustments for counterparty default risk were recorded at Ameren, Ameren Missouri, or Ameren Illinois in 2018, 2017, or 2016. At December 31, 2018 and 2017 , 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, 2018 and 2017 : December 31, 2018 December 31, 2017 Level 1 Level 2 Level 3 Total Level 1 Level 2 Level 3 Total Assets: Ameren Derivative assets – commodity contracts (a) : Fuel oils $ 1 $ — $ 7 $ 8 $ 4 $ — $ 3 $ 7 Natural gas — 2 1 3 — — 1 1 Power — 1 3 4 — 1 8 9 Total derivative assets – commodity contracts $ 1 $ 3 $ 11 $ 15 $ 4 $ 1 $ 12 $ 17 Nuclear decommissioning trust fund: Equity securities: U.S. large capitalization $ 427 $ — $ — $ 427 $ 468 $ — $ — $ 468 Debt securities: U.S. Treasury and agency securities — 148 — 148 — 125 — 125 Corporate bonds — 72 — 72 — 82 — 82 Other — 32 — 32 — 25 — 25 Total nuclear decommissioning trust fund $ 427 $ 252 $ — $ 679 (b) $ 468 $ 232 $ — $ 700 (b) Total Ameren $ 428 $ 255 $ 11 $ 694 $ 472 $ 233 $ 12 $ 717 Ameren Missouri Derivative assets – commodity contracts (a) : Fuel oils $ 1 $ — $ 7 $ 8 $ 4 $ — $ 3 $ 7 Natural gas — — — — — — 1 1 Power — 1 3 4 — 1 8 9 Total derivative assets – commodity contracts $ 1 $ 1 $ 10 $ 12 $ 4 $ 1 $ 12 $ 17 Nuclear decommissioning trust fund: Equity securities: U.S. large capitalization $ 427 $ — $ — $ 427 $ 468 $ — $ — $ 468 Debt securities: U.S. Treasury and agency securities — 148 — 148 — 125 — 125 Corporate bonds — 72 — 72 — 82 — 82 Other — 32 — 32 — 25 — 25 Total nuclear decommissioning trust fund $ 427 $ 252 $ — $ 679 (b) $ 468 $ 232 $ — $ 700 (b) Total Ameren Missouri $ 428 $ 253 $ 10 $ 691 $ 472 $ 233 $ 12 $ 717 Ameren Illinois Derivative assets – commodity contracts (a) : Natural gas $ — $ 2 $ 1 $ 3 $ — $ — $ — $ — Liabilities: Ameren Derivative liabilities – commodity contracts (a): Fuel oils $ 2 $ — $ 11 $ 13 $ — $ — $ — $ — Natural gas — 15 4 19 1 25 4 30 Power — 1 186 187 — — 196 196 Total Ameren $ 2 $ 16 $ 201 $ 219 $ 1 $ 25 $ 200 $ 226 Ameren Missouri Derivative liabilities – commodity contracts (a) : Fuel oils $ 2 $ — $ 11 $ 13 $ — $ — $ — $ — Natural gas — 5 — 5 — 7 1 8 Power — 1 3 4 — — 1 1 Total Ameren Missouri $ 2 $ 6 $ 14 $ 22 $ — $ 7 $ 2 $ 9 Ameren Illinois Derivative liabilities – commodity contracts (a) : Natural gas $ — $ 10 $ 4 $ 14 $ 1 $ 18 $ 3 $ 22 Power — — 183 183 — — 195 195 Total Ameren Illinois $ — $ 10 $ 187 $ 197 $ 1 $ 18 $ 198 $ 217 (a) The derivative asset and liability balances are presented net of registrant and counterparty credit considerations. (b) Balance excludes $5 million and $4 million of cash and cash equivalents, receivables, payables, and accrued income, net for December 31, 2018 and 2017 , respectively. See Note 10 – Retirement Benefits for tables that set forth, by level within the fair value hierarchy, Ameren’s pension and postretirement plan assets as of December 31, 2018 and 2017 . Level 3 fuel oils and natural gas derivative contract assets and liabilities measured at fair value on a recurring basis were immaterial for all periods presented. The following table presents the fair value reconciliation of Level 3 power derivative contract assets and liabilities measured at fair value on a recurring basis for the years ended December 31, 2018 and 2017 : 2018 2017 Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Beginning balance at January 1 $ 7 $ (195 ) $ (188 ) $ 7 $ (185 ) $ (178 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (6 ) — (6 ) (4 ) (21 ) (25 ) Purchases 5 — 5 14 — 14 Sales — — — 1 — 1 Settlements (5 ) 12 7 (11 ) 11 — Transfers out of Level 3 (1 ) — (1 ) — — — Ending balance at December 31 $ — $ (183 ) $ (183 ) $ 7 $ (195 ) $ (188 ) Change in unrealized gains (losses) related to assets/liabilities held at December 31 $ (1 ) $ (2 ) $ (3 ) $ — $ (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, 2018 and 2017 , 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. All gains or losses related to our Level 3 derivative commodity contracts are expected to be recovered or returned through customer rates; therefore, there is no impact to net income resulting from changes in the fair value of these instruments. The following table describes the valuation techniques and significant unobservable inputs utilized for the fair value of our Level 3 power derivative contract assets and liabilities as of December 31, 2018 and 2017 : Fair Value (a) Weighted Commodity Assets Liabilities Valuation Technique(s) Unobservable Input Range Average 2018 Power (b) $ 3 $ (186 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps($/MWh) (c) 23 – 39 28 Nodal basis($/MWh) (c) (9) – 0 (2) Fundamental energy production model Estimated future natural gas prices($/mmbtu) (c) 3 – 4 3 Escalation rate(%) (c)(d) 4 – 5 4 2017 Power (b) $ 8 $ (196 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps($/MWh) (c) 24 – 46 28 Nodal basis($/MWh) (c) (10) – 0 (2) Fundamental energy production model Estimated future natural gas prices($/mmbtu) (c) 3 – 4 3 Escalation rate(%) (c)(d) 5 5 (a) The derivative asset and liability balances are presented net of registrant and counterparty credit considerations. (b) Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2022 and 2021 for December 31, 2018 and 2017, respectively. Valuations beyond 2022 and 2021 for December 31, 2018 and 2017, respectively, use fundamentally modeled pricing by month for peak and off-peak demand. (c) Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement. (d) Escalation rate applies to power prices in 2031 and beyond. The following table sets forth, by level within the fair value hierarchy, the carrying amount and fair value of financial assets and liabilities disclosed, but not carried, at fair value as of December 31, 2018 and 2017 : December 31, 2018 Carrying Amount Fair Value Level 1 Level 2 Level 3 Total Ameren: Cash, cash equivalents, and restricted cash $ 107 $ 107 $ — $ — $ 107 Investments in held-to-maturity debt securities (a) 270 — 270 — 270 Short-term debt 597 — 597 — 597 Long-term debt (including current portion) (a) 8,439 (b) — 8,240 429 (c) 8,669 Ameren Missouri: Cash, cash equivalents, and restricted cash $ 8 $ 8 $ — $ — $ 8 Investments in held-to-maturity debt securities (a) 270 — 270 — 270 Short-term debt 55 — 55 — 55 Long-term debt (including current portion) (a) 3,998 (b) — 4,156 — 4,156 Ameren Illinois: Cash, cash equivalents, and restricted cash $ 80 $ 80 $ — $ — $ 80 Short-term debt 72 — 72 — 72 Long-term debt (including current portion) 3,296 (b) — 3,391 — 3,391 December 31, 2017 Ameren: Cash, cash equivalents, and restricted cash $ 68 $ 68 $ — $ — $ 68 Investments in held-to-maturity debt securities (a) 276 — 276 — 276 Short-term debt 484 — 484 — 484 Long-term debt (including current portion) (a) 7,935 (b) — 8,531 — 8,531 Ameren Missouri: Cash, cash equivalents, and restricted cash $ 7 $ 7 $ — $ — $ 7 Investments in held-to-maturity debt securities (a) 276 — 276 — 276 Short-term debt 39 — 39 — 39 Long-term debt (including current portion) (a) 3,961 (b) — 4,348 — 4,348 Ameren Illinois: Cash, cash equivalents, and restricted cash $ 41 $ 41 $ — $ — $ 41 Short-term debt 62 — 62 — 62 Long-term debt (including current portion) 2,830 (b) — 3,028 — 3,028 (a) Ameren and Ameren Missouri have investments in industrial development revenue bonds, classified as held-to-maturity and recorded in “Other Assets,” that are equal to the finance obligations for the Peno Creek and Audrain CT energy centers. As of December 31, 2018 and 2017 , the carrying amount of both the investments in industrial development revenue bonds and the finance obligations approximated fair value. (b) Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $58 million , $22 million , and $31 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2018 . Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $50 million , $20 million , and $24 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2017 . (c) The Level 3 fair value amount consists of ATXI’s senior unsecured notes. In the first quarter of 2018, the amount was transferred to Level 3 because inputs to the valuation model became unobservable during the period.

Callaway Energy Center

Callaway Energy Center	12 Months Ended
	Dec. 31, 2018
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. As required by the NWPA, Ameren Missouri and other utilities have entered into standard contracts with the DOE, which stated that the DOE would begin to dispose of spent nuclear fuel by 1998. However, the DOE has failed to fulfill its disposal obligations, and Ameren Missouri and other nuclear energy center owners sued the DOE to recover costs incurred for ongoing storage of their spent fuel. Ameren Missouri’s lawsuit against the DOE 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 $11 million , $3 million , and $24 million in 2018, 2017, and 2016, respectively. Ameren Missouri will continue to apply for reimbursement from the DOE for allowable costs associated with the ongoing storage of spent fuel. 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. 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 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 consistent with 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, 2018 and 2017 . 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, 2018 , 2017 , and 2016 : 2018 2017 2016 Proceeds from sales and maturities $ 299 $ 305 $ 304 Gross realized gains 18 13 7 Gross realized losses 5 5 4 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, 2018 and 2017 : Security Type Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value 2018 Debt securities $ 253 $ 3 $ 4 $ 252 Equity securities 162 277 12 427 Cash and cash equivalents 3 — — 3 Other (a) 2 — — 2 Total $ 420 $ 280 $ 16 $ 684 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 (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, 2018 : Cost Fair Value Less than 5 years $ 140 $ 140 5 years to 10 years 48 47 Due after 10 years 65 65 Total $ 253 $ 252 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, 2018 . The property coverage and the nuclear liability coverage renewal dates are April 1 and January 1, respectively, of each year. Both coverages were renewed in 2018. Type and Source of Coverage Maximum Coverages Maximum Assessments for Single Incidents Public liability and nuclear worker liability: American Nuclear Insurers $ 450 $ — Pool participation 13,623 (a) 138 (b) $ 14,073 (c) $ 138 Property damage: NEIL and EMANI $ 3,200 (d) $ 27 (e) Replacement power: NEIL $ 490 (f) $ 7 (e) (a) Provided through mandatory participation in an industrywide retrospective premium assessment program. The maximum coverage available is dependent on the number of United States commercial reactors participating in the 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 $21 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 November 2018. 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 insured by NEIL are subject to industrywide aggregates, such that terrorist acts against one or more commercial nuclear power plants within a stated time period would be treated as a single event, and the owners of the nuclear power plants would share the 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 are not subject to industrywide aggregates in the event of terrorist attacks on nuclear facilities. If losses from a nuclear incident at the Callaway energy center exceed the limits of, or are not covered by insurance, or if coverage is unavailable, Ameren Missouri is at risk for any uninsured losses. If a serious nuclear incident were to occur, it could have a material adverse effect on Ameren’s and Ameren Missouri’s results of operations, financial position, or liquidity.

Retirement Benefits

Retirement Benefits	12 Months Ended
	Dec. 31, 2018
Defined Benefit Plan [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. Only Ameren subsidiaries participate in the plans listed above. Ameren’s unfunded obligation under its pension and other postretirement benefit plans was $481 million and $551 million as of December 31, 2018 and 2017 , respectively. These net liabilities are recorded in “Other current liabilities,” “Pension and other postretirement benefits,” and “Other assets” on Ameren’s consolidated balance sheet. The decrease in the unfunded obligation during 2018 was the result of a 75 basis point increase in the pension and other postretirement benefit plan discount rates used to determine the present value of the obligation offset by a decrease in the return on plan assets of the pension and postretirement trusts. 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, 2018 and 2017 : 2018 2017 Ameren (a) $ 481 $ 551 Ameren Missouri 229 215 Ameren Illinois (a) 120 213 (a) Assets associated with other postretirement benefits are 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, 2018 and 2017 . It also provides the amounts included in regulatory assets and accumulated OCI at December 31, 2018 and 2017 , that have not been recognized in net periodic benefit costs. 2018 2017 Pension Benefits Postretirement Benefits Pension Benefits Postretirement Benefits Accumulated benefit obligation at end of year $ 4,258 $ (a) $ 4,577 $ (a) Change in benefit obligation: Net benefit obligation at beginning of year $ 4,827 $ 1,240 $ 4,518 $ 1,170 Service cost 100 21 93 21 Interest cost 169 40 179 47 Plan amendments — (49 ) — — Participant contributions — 9 — 8 Actuarial (gain) loss (401 ) (163 ) 255 53 Benefits paid (236 ) (64 ) (218 ) (59 ) Net benefit obligation at end of year 4,459 1,034 4,827 1,240 Change in plan assets: Fair value of plan assets at beginning of year 4,293 1,223 3,813 1,101 Actual return on plan assets (218 ) (57 ) 634 171 Employer contributions 60 2 64 2 Participant contributions — 9 — 8 Benefits paid (236 ) (64 ) (218 ) (59 ) Fair value of plan assets at end of year 3,899 1,113 4,293 1,223 Funded status – deficiency (surplus) 560 (79 ) 534 17 Accrued benefit cost (asset) at December 31 $ 560 $ (79 ) $ 534 $ 17 Amounts recognized in the balance sheet consist of: Noncurrent asset (b) $ — $ (79 ) $ — $ — Current liability (c) 2 — 3 3 Noncurrent liability 558 — 531 14 Net liability (asset) recognized $ 560 $ (79 ) $ 534 $ 17 Amounts recognized in regulatory assets consist of: Net actuarial (gain) loss $ 393 $ (91 ) $ 374 $ (69 ) Prior service credit (2 ) (48 ) (3 ) (3 ) Amounts (pretax) recognized in accumulated OCI consist of: Net actuarial loss 35 3 30 2 Total $ 426 $ (136 ) $ 401 $ (70 ) (a) Not applicable. (b) Included in “Other assets” on Ameren’s consolidated balance sheet. (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, 2018 and 2017 : Pension Benefits Postretirement Benefits 2018 2017 2018 2017 Discount rate at measurement date 4.25 % 3.50 % 4.25 % 3.50 % 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 nearly 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 2018, Ameren adopted the Society of Actuaries 2018 Mortality Improvement Scale. The updated scale assumes a lower rate of mortality improvement as compared to the 2017 Mortality Improvement Scale that Ameren used in 2017, 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. Based on its assumptions at December 31, 2018 , its investment performance in 2018, and its pension funding policy, Ameren expects to make annual contributions of approximately $20 million to $70 million in each of the next five years, with aggregate estimated contributions of $200 million . Ameren Missouri and Ameren Illinois estimate that their portion of the future funding requirements will be 30% and 60% , respectively. These estimates 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 2018 , 2017 , and 2016 : Pension Benefits Postretirement Benefits 2018 2017 2016 2018 2017 2016 Ameren Missouri $ 18 $ 19 $ 21 $ 1 $ 1 $ 1 Ameren Illinois 35 37 30 1 1 1 Other 7 8 6 — — — Ameren $ 60 $ 64 $ 57 $ 2 $ 2 $ 2 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 2019 . No plan assets are expected to be returned to Ameren during 2019 . 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 2019 and our pension and postretirement plans’ asset categories as of December 31, 2018 and 2017 : Asset Category Target Allocation 2019 Percentage of Plan Assets at December 31, 2018 2017 Pension Plan: Cash and cash equivalents 0% – 5% 1 % 1 % Equity securities: U.S. large-capitalization 21% – 31% 24 % 34 % U.S. small- and mid-capitalization 3% – 13% 7 % 9 % International 9% – 19% 13 % 14 % Global 3% – 13% 8 % — % Total equity 51% – 61% 52 % 57 % Debt securities 35% – 45% 42 % 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 % 2 % Equity securities: U.S. large-capitalization 34% – 44% 40 % 41 % U.S. small- and mid-capitalization 2% – 12% 7 % 8 % International 9% – 19% 13 % 14 % Total equity 55% – 65% 60 % 63 % Debt securities 33% – 43% 38 % 35 % 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, global, 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 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, 2018 . 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, 2018 : 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 $ — $ — $ — $ 41 $ 41 Equity securities: U.S. large-capitalization — — — 955 955 U.S. small- and mid-capitalization 272 — — — 272 International 224 — — 298 522 Global — — — 321 321 Debt securities: Corporate bonds — 701 — 19 720 Municipal bonds — 87 — — 87 U.S. Treasury and agency securities — 891 — — 891 Other 1 11 — — 12 Real estate — — — 202 202 Private equity — — — 3 3 Total $ 497 $ 1,690 $ — $ 1,839 $ 4,026 Less: Medical benefit assets at December 31 (a) (144 ) Plus: Net receivables at December 31 (b) 17 Fair value of pension plans’ assets at December 31 $ 3,899 (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, 2017 : 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 $ — $ — $ — $ 25 $ 25 Equity securities: U.S. large-capitalization — — — 1,523 1,523 U.S. small- and mid-capitalization 379 — — — 379 International 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 postretirement benefit plans’ assets measured at fair value as of December 31, 2018 : 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 $ 32 $ — $ — $ — $ 32 Equity securities: U.S. large-capitalization 297 — — 89 386 U.S. small- and mid-capitalization 63 — — — 63 International 45 — — 84 129 Other — 12 — — 12 Debt securities: Corporate bonds — 144 — — 144 Municipal bonds — 107 — — 107 U.S. Treasury and agency securities — 62 — — 62 Other — 7 — 34 41 Total $ 437 $ 332 $ — $ 207 $ 976 Plus: Medical benefit assets at December 31 (a) 144 Less: Net payables at December 31 (b) (7 ) Fair value of postretirement benefit plans’ assets at December 31 $ 1,113 (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, 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 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. Net Periodic 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 periodic benefit cost separately from the service cost component and outside of operating income. The Ameren Companies adopted this guidance, effective January 1, 2018, and as a result, $44 million , $22 million , and $10 million of net benefit income has been retrospectively reclassified from “Operating Expenses – Other operations and maintenance” to “Other Income, Net” on Ameren's, Ameren Missouri’s, and Ameren Illinois’ respective statements of income for the year ended December 31, 2017. Net benefit income of $56 million , $18 million , and $24 million has been similarly retrospectively reclassified on Ameren’s, Ameren Missouri’s, and Ameren Illinois’ respective statements of income for the year ended December 31, 2016. The guidance also requires 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. The following table presents the components of the net periodic benefit cost of Ameren’s pension and postretirement benefit plans during 2018 , 2017 , and 2016 : Pension Benefits Postretirement Benefits 2018 Service cost (a) $ 100 $ 21 Non-service cost components: Interest cost 169 40 Expected return on plan assets (276 ) (77 ) Amortization of: Prior service credit (1 ) (4 ) Actuarial (gain) loss 68 (6 ) Total non-service cost components (b) $ (40 ) $ (47 ) Net periodic benefit cost (income) $ 60 $ (26 ) 2017 Service cost (a) $ 93 $ 21 Non-service cost components: Interest cost 179 47 Expected return on plan assets (262 ) (75 ) Amortization of: Prior service credit (1 ) (5 ) Actuarial (gain) loss 55 (6 ) Total non-service cost components (b) $ (29 ) $ (39 ) Net periodic benefit cost (income) $ 64 $ (18 ) 2016 Service cost (a) $ 81 $ 19 Non-service cost components: Interest cost 185 50 Expected return on plan assets (253 ) (72 ) Amortization of: Prior service credit (1 ) (5 ) Actuarial (gain) loss 32 (11 ) Total non-service cost components (b) $ (37 ) $ (38 ) Net periodic benefit cost (income) $ 44 $ (19 ) (a) Service cost, net of capitalization, is reflected in “Operating Expenses - Other operations and maintenance” on Ameren’s statement of income. (b) 2018 amounts and the non-capitalized portion of 2017 and 2016’s non-service cost components, as discussed above, are reflected in “Other Income, Net” on Ameren’s statement of income. See Note 5 - Other Income, Net for additional information. The estimated amounts that will be amortized from regulatory assets and accumulated OCI into Ameren’s net periodic benefit cost in 2019 are as follows: Pension Benefits Postretirement Benefits Regulatory assets: Prior service credit $ (1 ) $ (5 ) Net actuarial (gain) loss 24 (15 ) Accumulated OCI: Net actuarial loss 2 — Total $ 25 $ (20 ) 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, 2018 , 2017 , and 2016 : Pension Costs Postretirement Costs 2018 2017 2016 2018 2017 2016 Ameren Missouri (a) $ 22 $ 24 $ 26 $ (1 ) $ (4 ) $ (5 ) Ameren Illinois 39 41 22 (25 ) (14 ) (13 ) Other (1 ) (1 ) (4 ) — — (1 ) Ameren 60 64 44 (26 ) (18 ) (19 ) (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, 2018 , 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 2019 $ 267 $ 3 $ 57 $ 2 2020 272 3 59 2 2021 282 3 61 2 2022 285 3 62 2 2023 286 3 64 2 2024 – 2028 1,439 12 315 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, 2018 , 2017 , and 2016 : Pension Benefits Postretirement Benefits 2018 2017 2016 2018 2017 2016 Discount rate at measurement date 3.50 % 4.00 % 4.50 % 3.50 % 4.00 % 4.50 % Expected return on plan assets 7.00 7.00 7.00 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 $ (2 ) $ 135 $ — $ 33 0.25% increase in salary scale 2 12 — — 1.00% increase in annual medical trend — — 4 58 1.00% decrease in annual medical trend — — (4 ) (58 ) Other Ameren sponsors a 401(k) plan for eligible employees. The Ameren 401(k) plan covered all eligible Ameren employees at December 31, 2018 . 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 each of the Ameren Companies for the years ended December 31, 2018 , 2017 , and 2016 : 2018 2017 2016 Ameren Missouri $ 17 $ 16 $ 16 Ameren Illinois 15 13 12 Other 1 1 1 Ameren $ 33 $ 30 $ 29

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2018
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-based 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, 2018 , there were 3.8 million common shares remaining for grant under the 2014 Incentive Plan. The 2014 Incentive Plan awards may be stock options, stock appreciation rights, restricted stock, restricted stock units, performance shares, performance share units, cash-based awards, and other stock-based awards. Ameren used newly issued shares to fulfill its stock-based compensation obligations for 2018 and intends to use newly issued shares to fulfill its stock-based compensation obligations for 2019. The following table summarizes Ameren’s nonvested performance share unit and restricted stock unit activity for the year ended December 31, 2018 : Performance Share Units Restricted Stock Units Share Units Weighted-average Fair Value per Share Unit Stock Units Weighted-average Fair Value per Stock Unit Nonvested at January 1, 2018 (a) 895,489 $ 52.28 — $ — Granted 316,875 62.88 187,273 57.66 Forfeitures (65,106 ) 51.11 (5,463 ) 58.99 Vested and undistributed (b) (288,404 ) 53.63 (26,557 ) 59.02 Vested and distributed (176,043 ) 52.88 — — Nonvested at December 31, 2018 (c) 682,811 $ 56.58 155,253 $ 57.38 (a) Does not include 712,572 undistributed vested performance share units. (b) Vested and undistributed units are awards that vested due to attainment of retirement eligibility by certain employees, but have not yet been distributed. For vested and undistributed performance share units, the number of shares issued for retirement-eligible employees will vary depending on actual performance over the three-year performance period. (c) Does not include 619,783 of vested and undistributed performance share units and 26,557 of vested and undistributed restricted stock units. Performance Share Units A performance 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 market conditions have been met and if the individual remains employed by Ameren through the required vesting period. The vesting period for share units awarded extends beyond the three -year performance period to the payout date, which is approximately 38 months after the grant date. In the event of a participant’s death or retirement at age 55 or older with five or more years of service, awards vest on a pro rata basis over the three -year performance 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 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 three -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, 2018 , 2017 , and 2016 : 2018 2017 2016 Fair value of share units awarded $62.88 $59.16 $44.13 Ameren’s closing common share price at December 31 of the prior year $61.69 $52.46 $43.23 Three-year risk-free rate 1.98% 1.47% 1.31% Volatility range for the peer group (a) 15% – 23% 15% – 21% 15% – 20% (a) Based on a historical period that is equal to the remaining term of the performance period as of the grant date. Restricted Stock Units Restricted stock units vest and entitle an employee to receive shares of Ameren common stock (plus accumulated dividends) if the individual remains employed with Ameren through the payment date of the awards. Generally, in the event of a participant’s death or retirement at age 55 or older with five or more years of service, awards vest on a pro rata basis. The payout date of the awards is approximately 38 months after the grant date. The fair value of each restricted stock unit is determined by Ameren’s closing common share price on the grant date. Stock-Based Compensation Expense The following table presents the stock-based compensation expense for the years ended December 31, 2018, 2017, and 2016: 2018 2017 2016 Ameren Missouri $ 4 $ 4 $ 4 Ameren Illinois 3 2 2 Other (a) 13 12 11 Ameren 20 18 17 Less income tax benefit 6 7 6 Stock-based compensation expense, net $ 14 $ 11 $ 11 (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 and restricted stock units of $54 million , $39 million , and $83 million for the years ended December 31, 2018 , 2017 , and 2016 . There were no significant stock-based compensation costs capitalized during the years ended December 31, 2018 , 2017 , and 2016 . As of December 31, 2018 , 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 . Ameren realized income tax benefits for settled performance share units of $13 million , $15 million , and $31 million for the years ended December 31, 2018 , 2017 , and 2016 .

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
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) is subject to provisions of the TCJA that limit the deductibility of interest expense, but such limitation did not affect Ameren in 2018. 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 of December 31, 2017, the Ameren Companies made reasonable estimates for the measurement and accounting of certain effects of the TCJA, which have been reflected in their financial statements. We 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. The TCJA had the following provisional effects on the Ameren Companies 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) 32 (5 ) 127 154 Noncurrent regulatory assets (89 ) (24 ) (1 ) (114 ) Noncurrent regulatory liabilities 1,362 842 89 2,293 During the year ended December 31, 2018, Ameren, Ameren Missouri, and Ameren Illinois updated their respective provisional estimates in accordance with SEC staff guidance and recorded $13 million , $4 million , and $4 million , respectively, of income tax expense, primarily due to the application of proposed IRS regulations on depreciation transition rules. The offsetting impact to Ameren’s, Ameren Missouri’s, and Ameren Illinois’ regulatory asset and regulatory liability balances was immaterial. As of December 31, 2018, Ameren, Ameren Missouri, and Ameren Illinois have completed their accounting for certain effects of the TCJA. 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 30 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. Missouri Income Tax Rate In 2018, legislation modifying Missouri tax law was enacted to decrease the state's corporate income tax rate from 6.25% to 4% , effective January 1, 2020. As a result, in 2018, Ameren’s and Ameren Missouri’s accumulated deferred tax balances were revalued, resulting in a net decrease of $122 million to their accumulated deferred tax liability, which was offset by a regulatory liability. Additionally, Ameren recorded an immaterial amount to income tax expense. As a result of its PISA election under Missouri Senate Bill 564, which prohibits a change in electric base rates prior to April 2020, Ameren Missouri anticipates that the effect of this tax decrease will be reflected in customer rates upon completion of its next regulatory rate review. Ameren (parent) and nonregistrant subsidiaries do not expect this income tax decrease to have a material impact on net income. 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 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 and Ameren Illinois do not expect this tax increase to have a material impact on their net income prospectively. The tax increase is not expected to materially affect 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 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, 2018 , 2017 , and 2016 : Ameren Missouri Ameren Illinois Ameren 2018 Federal statutory corporate income tax rate: 21 % 21 % 21 % Increases (decreases) from: Amortization of excess deferred taxes (4 ) (4 ) (4 ) Other depreciation differences — (1 ) — Amortization of deferred investment tax credit (1 ) — (1 ) State tax 4 7 6 TCJA 1 1 1 Tax credits (1 ) — — Other permanent items — — (1 ) Effective income tax rate 20 % 24 % 22 % 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 % (a) Reflects the adoption of authoritative accounting guidance related to stock-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, 2018 , 2017 , and 2016 : Ameren Missouri Ameren Illinois Other Ameren 2018 Current taxes: Federal $ 104 $ 4 $ (118 ) $ (10 ) State 29 6 (12 ) 23 Deferred taxes: Federal 22 75 123 220 State (2 ) 28 23 49 Amortization of excess deferred taxes (24 ) (15 ) (1 ) (40 ) Amortization of deferred investment tax credits (5 ) — — (5 ) Total income tax expense $ 124 $ 98 $ 15 $ 237 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 The following table presents the accumulated deferred income tax assets and liabilities recorded as a result of temporary differences at December 31, 2018 and 2017 : Ameren Missouri Ameren Illinois Other Ameren 2018 Accumulated deferred income taxes, net liability (asset): Plant-related $ 2,010 $ 1,345 $ 179 $ 3,534 Regulatory assets and liabilities, net (343 ) (221 ) (25 ) (589 ) Deferred employee benefit costs (58 ) (4 ) (64 ) (126 ) Tax carryforwards (35 ) (26 ) (166 ) (227 ) Other (40 ) 25 46 31 Total net accumulated deferred income tax liabilities (assets) $ 1,534 $ 1,119 $ (30 ) $ 2,623 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 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, 2018 and 2017 : Ameren Missouri Ameren Illinois Other Ameren 2018 Net operating loss carryforwards: Federal (a) $ — $ 23 $ 55 $ 78 State (a) — — 13 13 Total net operating loss carryforwards $ — $ 23 $ 68 $ 91 Tax credit carryforwards: Federal (b) $ 35 $ 3 $ 79 $ 117 State (c) — — 10 10 Total tax credit carryforwards $ 35 $ 3 $ 89 $ 127 Charitable contribution carryforwards (d) $ — $ — $ 14 $ 14 Valuation allowance (e) — — (5 ) (5 ) Total charitable contribution carryforwards $ — $ — $ 9 $ 9 2017 Net operating loss carryforwards: Federal $ — $ 41 $ 162 $ 203 State — — 32 32 Total net operating loss carryforwards $ — $ 41 $ 194 $ 235 Tax credit carryforwards: Federal $ 31 $ 2 $ 80 $ 113 State — — 7 7 Total tax credit carryforwards $ 31 $ 2 $ 87 $ 120 Charitable contribution carryforwards $ — $ — $ 11 $ 11 Valuation allowance — — (5 ) (5 ) Total charitable contribution carryforwards $ — $ — $ 6 $ 6 (a) Will expire between 2034 and 2037 . 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 2019 and 2023 . (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, 2018 and 2017 , the Ameren Companies did not record any uncertain tax positions. The Internal Revenue Service is currently examining Ameren’s 2018 and 2017 income tax returns. 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 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 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, 2018
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 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 specified performance period, price, and amount of megawatthours included in the agreements: 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 April 2018 June 2019 – September 2020 110,000 32 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, 2018 and 2017 , 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 with access to their facilities for administrative purposes and with use of other assets. The costs of the rent and facility services and other assets 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. Electric Transmission Maintenance and Construction Agreements ATXI entered into separate agreements with Ameren Missouri and Ameren Illinois in which Ameren Missouri or Ameren Illinois, as applicable, may perform certain maintenance and construction services related to ATXI’s electric transmission assets. The Ameren Missouri and Ameren Illinois agreements are effective from August 2017 through June 2019 and from August 2018 through July 2019, respectively. 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. The following table presents the affiliate balances related to income taxes for Ameren Missouri and Ameren Illinois as of December 31, 2018 and 2017 : 2018 2017 Ameren Missouri Ameren Illinois Ameren Missouri Ameren Illinois Income taxes payable to parent (a) $ 16 $ 7 $ 11 $ 17 Income taxes receivable from parent (b) — 6 — — (a) Included in “Accounts payable – affiliates” on the balance sheet. (b) Included in “Accounts receivable – affiliates” on the balance sheet. Capital Contributions The following table presents cash capital contributions received from Ameren (parent) by Ameren Missouri and Ameren Illinois for the years ended December 31, 2018 , 2017 , and 2016 : 2018 2017 2016 Ameren Missouri (a) $ 45 $ 30 $ 44 (b) Ameren Illinois 160 8 — (a) As a result of the tax allocation agreement. (b) Included a $38 million accrued capital contribution from 2015. The following table presents the impact on Ameren Missouri and Ameren Illinois of related-party transactions for the years ended December 31, 2018 , 2017 , and 2016 . 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 2018 $ 11 $ (a) with Ameren Illinois 2017 23 (a) 2016 28 (a) Ameren Missouri and Ameren Illinois Operating Revenues 2018 22 3 rent and facility services 2017 26 4 2016 25 5 Ameren Missouri and Ameren Illinois miscellaneous Operating Revenues 2018 1 1 support services and services provided to ATXI 2017 (b) 1 2016 1 (b) Total Operating Revenues 2018 $ 34 $ 4 2017 49 5 2016 54 5 Ameren Illinois power supply Purchased Power 2018 $ (a) $ 11 agreements with Ameren Missouri 2017 (a) 23 2016 (a) 28 Ameren Illinois transmission Purchased Power 2018 (a) 1 services from ATXI 2017 (a) 2 2016 (a) 2 Total Purchased Power 2018 $ (a) $ 12 2017 (a) 25 2016 (a) 30 Ameren Missouri and Ameren Illinois Other Operations and 2018 $ 3 $ 6 rent and facility services Maintenance 2017 (b) (b) 2016 (b) (b) Ameren Services support services Other Operations and 2018 136 126 agreement Maintenance 2017 149 139 2016 129 123 Total Other Operations and 2018 $ 139 $ 132 Maintenance Expenses 2017 149 139 2016 129 123 Money pool borrowings (advances) (Interest Charges) 2018 $ 1 $ (b) Other Income, Net 2017 1 (b) 2016 (b) (b) (a) Not applicable. (b) Amount less than $1 million.

Commitments And Contingencies

Commitments And Contingencies	12 Months Ended
	Dec. 31, 2018
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, 2018 : 2019 2020 2021 2022 2023 After 5 Years Total Ameren: Minimum capital lease payments (a)(b) $ 32 $ 32 $ 33 $ 32 $ 264 $ — $ 393 Less amount representing interest 25 25 25 24 24 — 123 Present value of minimum capital lease payments $ 7 $ 7 $ 8 $ 8 $ 240 $ — $ 270 Operating leases 10 8 7 6 5 9 45 Total lease obligations $ 17 $ 15 $ 15 $ 14 $ 245 $ 9 $ 315 Ameren Missouri: Minimum capital lease payments (b)(c) $ 32 $ 32 $ 33 $ 32 $ 264 $ — $ 393 Less amount representing interest 25 25 25 24 24 — 123 Present value of minimum capital lease payments $ 7 $ 7 $ 8 $ 8 $ 240 $ — $ 270 Operating leases 8 7 6 5 5 9 40 Total lease obligations $ 15 $ 14 $ 14 $ 13 $ 245 $ 9 $ 310 Ameren Illinois: Operating leases $ 1 $ — $ — $ — $ — $ — $ 1 (a) See Note 3 – Property, Plant, and Equipment, Net for additional information. (b) See Note 5 – Long-term Debt and Equity Financings for additional information on Ameren’s and Ameren Missouri’s capital lease agreements. The following table presents total operating lease expenses included in “Operating Expenses” in the statement of income for the years ended December 31, 2018 , 2017 , and 2016 : 2018 2017 2016 Ameren $ 9 $ 11 $ 38 Ameren Missouri 8 10 34 Ameren Illinois 1 1 30 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, 2018 . 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, 2018 . Coal Natural Gas (a) Nuclear Fuel Purchased Power (b)(c) Methane Gas Other Total Ameren: 2019 $ 349 $ 197 $ 25 $ 157 $ 4 $ 67 $ 799 2020 160 143 43 54 4 41 445 2021 121 77 59 10 4 30 301 2022 72 27 14 — 3 26 142 2023 — 7 42 — 3 27 79 Thereafter — 34 31 — 29 72 166 Total $ 702 $ 485 $ 214 $ 221 $ 47 $ 263 $ 1,932 Ameren Missouri: 2019 $ 349 $ 40 $ 25 $ — $ 4 $ 49 $ 467 2020 160 31 43 — 4 26 264 2021 121 15 59 — 4 26 225 2022 72 5 14 — 3 26 120 2023 — 3 42 — 3 27 75 Thereafter — 14 31 — 29 56 130 Total $ 702 $ 108 $ 214 $ — $ 47 $ 210 $ 1,281 Ameren Illinois: 2019 $ — $ 157 $ — $ 157 $ — $ 8 $ 322 2020 — 112 — 54 — 5 171 2021 — 62 — 10 — — 72 2022 — 22 — — — — 22 2023 — 4 — — — — 4 Thereafter — 20 — — — — 20 Total $ — $ 377 $ — $ 221 $ — $ 13 $ 611 (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 2034 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. In January 2018, as required by the FEJA, Ameren Illinois entered into 10-year agreements to acquire zero emission credits. Annual zero emission credit commitment amounts will be published by the IPA each May prior to the start of the subsequent planning year. The amounts above reflect Ameren Illinois’ commitment to acquire approximately $26 million of zero emission credits through May 2019. Environmental Matters We are subject to various environmental laws, including statutes 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. These laws address emissions, discharges to water, water intake, 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, 2018, Ameren Missouri’s fossil fuel-fired energy centers represented 16% and 32% 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 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 and Ameren Missouri estimate that they will need to make capital expenditures of $300 million to $400 million from 2019 through 2023 in order to comply with existing environmental regulations. Additional environmental controls beyond 2023 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. 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. 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 proposed amendments to regulations and guidelines, including to 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, including CSAPR, regulate emissions of SO 2 and NO x through emission source reductions and 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 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 August 2018, the EPA proposed to repeal and replace the Clean Power Plan with a proposed new rule known as the Affordable Clean Energy Rule, which establishes emission guidelines for states to follow in developing plans to limit CO 2 emissions from power plants. The EPA proposes to use certain efficiency measures as the best system of emission reduction for coal-fired power plants. The EPA is expected to finalize the Affordable Clean Energy rule in the first half of 2019. We cannot predict the outcome of EPA’s rulemaking or the outcome of legal challenges related to such rulemaking. 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 the first phase, in January 2017, the district court issued a liability ruling that the projects violated provisions of the Clean Air Act and Missouri law. In the second phase, the district court will determine the actions required to remedy the violations found in the liability phase. The EPA previously withdrew all claims for penalties and fines. Hearings on remedy-related issues are scheduled for April 2019. 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 July 2018, the United States Court of Appeals for the Second Circuit upheld the EPA’s Section 316(b) 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 a power plant’s cooling water 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 by Ameren Missouri during the permit renewal process of each energy center’s water discharge permit, which is expected to be completed by 2023. 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. Ameren Missouri is in the process of constructing wastewater treatment facilities that meet the limitations in these guidelines at three of its energy centers. CCR Management In 2015, the EPA issued the CCR rule, which established 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 July 2018, the EPA issued revisions to the CCR rule that extended certain compliance deadlines and indicated that additional revisions to the CCR rule are likely. Ameren and Ameren Missouri have AROs of $135 million recorded on their respective balance sheets as of December 31, 2018 , associated with CCR storage facilities that reflect the regulations issued in 2015. Ameren plans to close these CCR storage facilities between 2019 and 2023. The recent EPA revisions do not affect Ameren Missouri’s closure schedule. Ameren Missouri estimates it will need to make capital expenditures of $150 million to $200 million from 2019 through 2023 to implement its CCR management compliance plan, which includes installation of dry ash handling systems, waste water treatment facilities, and groundwater monitoring equipment. Remediation The Ameren Companies are involved in a number of remediation actions to clean up sites impacted 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, 2018 , Ameren Illinois had investigated and remediated the majority of the 44 former MGP sites in Illinois it owned or for which it was otherwise responsible. Ameren Illinois estimates it could substantially conclude remediation efforts at its remaining sites 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, 2018 , Ameren Illinois estimated the obligation related to these former MGP sites at $150 million to $212 million . Ameren and Ameren Illinois recorded a liability of $150 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 December 2018, Ameren Missouri signed a consent decree with the EPA to implement certain remedial measures at one of the disposal sites and reached an agreement with Solutia, Inc., the primary potentially responsible party for the Sauget Area 2, limiting Ameren Missouri’s cleanup obligations with respect to the other disposal sites. Remediation efforts at the site are expected to occur in 2020. As of December 31, 2018 , Ameren Missouri recorded a liability of $1 million to represent its estimated minimum obligation for this site. Our operations or those of our predecessor companies involve the use of, disposal of, and in appropriate circumstances, the cleanup of substances regulated under environmental laws. We are unable to determine whether such practices will result in future environmental commitments or will affect our results of operations, financial position, or liquidity.

Segment Information

Segment Information	12 Months Ended
	Dec. 31, 2018
Segment Reporting [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 primarily consists of the aggregated electric transmission businesses of Ameren Illinois and ATXI . The category called Other primarily includes Ameren (parent) 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, other retail electric suppliers, 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 information about the reported revenue and specified items reflected in net income attributable to common shareholders and capital expenditures by segment at Ameren and Ameren Illinois for the years ended December 31, 2018 , 2017 , and 2016 . 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 Ameren 2018 External revenues $ 3,555 $ 1,544 $ 814 $ 378 $ — $ — $ 6,291 Intersegment revenues 34 3 1 55 (a) — (93 ) — Depreciation and amortization 550 259 65 77 4 — 955 Interest income 28 6 — — 4 (5 ) 33 Interest charges 200 73 38 75 (b) 19 (4 ) 401 Income taxes 124 41 25 56 (9 ) — 237 Net income (loss) attributable to Ameren common shareholders 478 136 70 164 (33 ) — 815 Capital expenditures 914 503 311 562 5 (9 ) 2,286 2017 External revenues $ 3,488 $ 1,564 $ 742 $ 382 $ (2 ) $ — $ 6,174 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 323 131 60 140 (131 ) — 523 Capital expenditures 773 476 245 644 1 (7 ) 2,132 2016 External revenues $ 3,470 $ 1,544 $ 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 357 126 59 117 (6 ) — 653 Capital expenditures 738 470 181 689 4 (6 ) 2,076 (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 Ameren Illinois 2018 External revenues $ 1,547 $ 815 $ 214 $ — $ 2,576 Intersegment revenues — — 53 (a) (53 ) — Depreciation and amortization 259 65 50 — 374 Interest income 6 — — — 6 Interest charges 73 38 38 — 149 Income taxes 41 25 32 — 98 Net income available to common shareholder 136 70 98 — 304 Capital expenditures 503 311 444 — 1,258 2017 External revenues $ 1,568 $ 743 $ 216 $ — $ 2,527 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 $ 1,548 $ 754 $ 187 $ — $ 2,489 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 (a) Ameren Illinois Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above. The following tables present disaggregated revenues by segment at Ameren and Ameren Illinois for the years ended December 31, 2018 , 2017 , and 2016 . Economic factors affect the nature, timing, amount, and uncertainty of revenues and cash flows in a similar manner across customer classes. Revenues from alternative revenue programs have a similar distribution among customer classes as revenues from contracts with customers. Other revenues not associated with contracts with customers are presented in the Other customer classification, along with electric transmission and off-system revenues. Ameren Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Other Intersegment Eliminations Ameren 2018 Residential $ 1,560 $ 867 $ — $ — $ — $ — $ 2,427 Commercial 1,271 511 — — — — 1,782 Industrial 312 130 — — — — 442 Other 308 (a) 39 — 433 — (92 ) 688 (a) Total electric revenues $ 3,451 $ 1,547 $ — $ 433 $ — $ (92 ) $ 5,339 Residential $ 90 $ — $ 581 $ — $ — $ — $ 671 Commercial 37 — 159 — — — 196 Industrial 4 — 17 — — — 21 Other 7 — 58 — — (1 ) 64 Total gas revenues $ 138 $ — $ 815 $ — $ — $ (1 ) $ 952 Total revenues (b) $ 3,589 $ 1,547 $ 815 $ 433 $ — $ (93 ) $ 6,291 2017 Residential $ 1,417 $ 870 $ — $ — $ — $ — $ 2,287 Commercial 1,208 527 — — — — 1,735 Industrial 305 113 — — — — 418 Other 481 58 — 426 (2 ) (96 ) 867 Total electric revenues $ 3,411 $ 1,568 $ — $ 426 $ (2 ) $ (96 ) $ 5,307 Residential $ 77 $ — $ 531 $ — $ — $ — $ 608 Commercial 31 — 146 — — — 177 Industrial 4 — 12 — — — 16 Other 14 — 54 — — (2 ) 66 Total gas revenues $ 126 $ — $ 743 $ — $ — $ (2 ) $ 867 Total revenues (b) $ 3,537 $ 1,568 $ 743 $ 426 $ (2 ) $ (98 ) $ 6,174 2016 Residential $ 1,422 $ 895 $ — $ — $ — $ — $ 2,317 Commercial 1,224 517 — — — — 1,741 Industrial 315 96 — — — — 411 Other 435 40 — 355 1 (104 ) 727 Total electric revenues $ 3,396 $ 1,548 $ — $ 355 $ 1 $ (104 ) $ 5,196 Residential $ 77 $ — $ 530 $ — $ — $ — $ 607 Commercial 30 — 153 — — — 183 Industrial 4 — 10 — — — 14 Other 17 — 61 — — (2 ) 76 Total gas revenues $ 128 $ — $ 754 $ — $ — $ (2 ) $ 880 Total revenues (b) $ 3,524 $ 1,548 $ 754 $ 355 $ 1 $ (106 ) $ 6,076 (a) Includes $60 million for the year ended December 31, 2018 , for the reduction to revenue for the excess amounts collected in rates related to the TCJA from January 1, 2018, through July 31, 2018. See Note 2 – Rate and Regulatory Matters for additional information. (b) The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the years ended December 31, 2018 , 2017 , and 2016 : Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Ameren 2018 Revenues from alternative revenue programs $ (8 ) $ (3 ) $ (23 ) $ (25 ) $ (59 ) Other revenues not from contracts with customers 24 16 2 — 42 2017 Revenues from alternative revenue programs $ (28 ) $ (5 ) $ 5 $ 13 $ (15 ) Other revenues not from contracts with customers 15 6 2 — 23 2016 Revenues from alternative revenue programs $ 8 $ (70 ) $ 11 $ (1 ) $ (52 ) Other revenues not from contracts with customers 16 6 2 — 24 Ameren Illinois Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Ameren Illinois 2018 Residential $ 867 $ 581 $ — $ — $ 1,448 Commercial 511 159 — — 670 Industrial 130 17 — — 147 Other 39 58 267 (53 ) 311 Total revenues (a) $ 1,547 $ 815 $ 267 $ (53 ) $ 2,576 2017 Residential $ 870 $ 531 $ — $ — $ 1,401 Commercial 527 146 — — 673 Industrial 113 12 — — 125 Other 58 54 258 (42 ) 328 Total revenues (a) $ 1,568 $ 743 $ 258 $ (42 ) $ 2,527 2016 Residential $ 895 $ 530 $ — $ — $ 1,425 Commercial 517 153 — — 670 Industrial 96 10 — — 106 Other 40 61 232 (45 ) 288 Total revenues (a) $ 1,548 $ 754 $ 232 $ (45 ) $ 2,489 (a) The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the Ameren Illinois segments for the years ended December 31, 2018 , 2017 , and 2016 : Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Ameren Illinois 2018 Revenues from alternative revenue programs $ (3 ) $ (23 ) $ (25 ) $ (51 ) Other revenues not from contracts with customers 16 2 — 18 2017 Revenues from alternative revenue programs $ (5 ) $ 5 $ 9 $ 9 Other revenues not from contracts with customers 6 2 — 8 2016 Revenues from alternative revenue programs $ (70 ) $ 11 $ 2 $ (57 ) Other revenues not from contracts with customers 6 2 — 8

Selected Quarterly Information

Selected Quarterly Information	12 Months Ended
	Dec. 31, 2018
Selected Quarterly Financial Information [Abstract]
SELECTED QUARTERLY INFORMATION	Ameren 2018 2017 Quarter ended March 31 June 30 September 30 December 31 March 31 June 30 September 30 December 31 Operating revenues (a) $ 1,585 $ 1,563 $ 1,724 $ 1,419 $ 1,515 $ 1,537 $ 1,723 $ 1,399 Operating income (a) 273 385 533 166 242 387 569 212 Net income (loss) 153 240 359 69 104 194 290 (59 ) (b) Net income (loss) attributable to Ameren common shareholders $ 151 $ 239 $ 357 $ 68 $ 102 $ 193 $ 288 $ (60 ) Earnings (loss) per common share – basic $ 0.62 $ 0.98 $ 1.46 $ 0.28 $ 0.42 $ 0.79 $ 1.19 $ (0.24 ) Earnings (loss) per common share – diluted (c) $ 0.62 $ 0.97 $ 1.45 $ 0.28 $ 0.42 $ 0.79 $ 1.18 $ (0.24 ) (a) 2017 amounts have been revised to reflect the adoption of accounting guidance on revenue from contracts with customers and the presentation of net periodic pension and postretirement benefit cost, effective for the Ameren Companies as of January 1, 2018. See Note 1 – Summary of Significant Accounting Policies and Note 10 – Retirement Benefits under Part II, Item 8, of this report for additional information. (b) Includes an increase to income tax expense of $154 million recorded in 2017 as a result of the TCJA. (c) 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 (a) Operating Income (a) Net Income (Loss) Net Income (Loss) Available to Common Shareholder March 31, 2018 $ 792 $ 90 $ 39 $ 38 March 31, 2017 791 47 6 5 June 30, 2018 955 258 169 168 June 30, 2017 934 230 121 120 September 30, 2018 1,129 394 295 294 September 30, 2017 1,116 412 235 234 December 31, 2018 713 7 (22 ) (22 ) December 31, 2017 696 33 (36 ) (b) (36 ) (a) 2017 amounts have been revised to reflect the adoption of accounting guidance on revenue from contracts with customers and the presentation of net periodic pension and postretirement benefit cost, effective for the Ameren Companies as of January 1, 2018. See Note 1 – Summary of Significant Accounting Policies and Note 10 – Retirement Benefits under Part II, Item 8, of this report for additional information. (b) Includes an increase to income tax expense of $32 million recorded in 2017 as a result of the TCJA. Ameren Illinois Quarter ended Operating Revenues (a) Operating Income (a) Net Income Net Income Available to Common Shareholder March 31, 2018 $ 760 $ 159 $ 96 $ 95 March 31, 2017 703 169 80 79 June 30, 2018 578 105 63 62 June 30, 2017 576 128 58 57 September 30, 2018 564 113 63 63 September 30, 2017 574 124 55 55 December 31, 2018 674 135 85 84 December 31, 2017 674 148 78 77 (a) 2017 amounts have been revised to reflect the adoption of accounting guidance on revenue from contracts with customers and the presentation of net periodic pension and postretirement benefit cost, effective for the Ameren Companies as of January 1, 2018. See Note 1 – Summary of Significant Accounting Policies and Note 10 – Retirement Benefits under Part II, Item 8, of this report for additional information.

Schedule I - Condensed Financia

Schedule I - Condensed Financial Information Of Parent	12 Months Ended
	Dec. 31, 2018
Condensed Financial Information Disclosure [Abstract]
Condensed Financial Information Of Parent	SCHEDULE I – CONDENSED FINANCIAL INFORMATION OF PARENT (In millions) 2018 2017 2016 Operating revenues $ — $ — $ — Operating expenses 11 15 19 Operating loss (11 ) (15 ) (19 ) Equity in earnings of subsidiaries 857 659 663 Interest income from affiliates 3 9 10 Total other income (expense), net (12 ) 2 — Interest charges 34 31 28 Income tax (benefit) (12 ) 101 (27 ) Net Income Attributable to Ameren Common Shareholders $ 815 $ 523 $ 653 Net Income Attributable to Ameren Common Shareholders $ 815 $ 523 $ 653 Other Comprehensive Income (Loss), Net of Taxes: Pension and other postretirement benefit plan activity, net of income taxes (benefit) of $(1), $3, and $(7), respectively (4 ) 5 (20 ) Comprehensive Income Attributable to Ameren Common Shareholders $ 811 $ 528 $ 633 SCHEDULE I – CONDENSED FINANCIAL INFORMATION OF PARENT (In millions) December 31, 2018 December 31, 2017 Assets: Cash and cash equivalents $ — $ — Advances to money pool 76 13 Accounts receivable – affiliates 43 46 Other current assets 4 8 Total current assets 123 67 Investments in subsidiaries 8,559 7,944 Note receivable – ATXI 75 75 Accumulated deferred income taxes, net 108 222 Other assets 126 140 Total assets $ 8,991 $ 8,448 Liabilities and Shareholders’ Equity: Short-term debt $ 470 $ 383 Borrowings from money pool 46 28 Accounts payable – affiliates 10 6 Other current liabilities 12 27 Total current liabilities 538 444 Long-term debt 697 696 Pension and other postretirement benefits 43 37 Other deferred credits and liabilities 82 87 Total liabilities 1,360 1,264 Commitments and Contingencies (Note 5) Shareholders’ Equity: Common stock, $.01 par value, 400.0 shares authorized – shares outstanding of 244.5 and 242.6, respectively 2 2 Other paid-in capital, principally premium on common stock 5,627 5,540 Retained earnings 2,024 1,660 Accumulated other comprehensive loss (22 ) (18 ) Total shareholders’ equity 7,631 7,184 Total liabilities and shareholders’ equity $ 8,991 $ 8,448 SCHEDULE I – CONDENSED FINANCIAL INFORMATION OF PARENT (In millions) 2018 2017 2016 Net cash flows provided by operating activities $ 550 $ 454 $ 483 Cash flows from investing activities: Money pool advances, net (63 ) 14 (27 ) Notes receivable – ATXI, net — 275 (60 ) Investments in subsidiaries (208 ) (151 ) (123 ) Other 5 6 2 Net cash flows provided by (used in) investing activities (266 ) 144 (208 ) Cash flows from financing activities: Dividends on common stock (451 ) (431 ) (416 ) Short-term debt, net 87 (124 ) 206 Money pool borrowings, net 18 (5 ) 19 Issuances of common stock 74 — — Repurchases of common stock for stock-based compensation — (24 ) (51 ) Employee payroll taxes related to stock-based compensation (19 ) (15 ) (32 ) Net cash flows used in financing activities (291 ) (599 ) (274 ) Net change in cash, cash equivalents, and restricted cash $ (7 ) $ (1 ) $ 1 Cash, cash equivalents, and restricted cash at beginning of year 8 9 8 Cash, cash equivalents, and restricted cash at end of year $ 1 $ 8 $ 9 Cash dividends received from consolidated subsidiaries $ 450 $ 362 $ 465 Noncash financing activity – Issuance of common stock for stock-based compensation $ 35 $ — $ — AMEREN CORPORATION (parent company only) NOTES TO CONDENSED FINANCIAL STATEMENTS December 31, 2018 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 – CASH AND CASH EQUIVALENTS The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheet as of December 31, 2018 and 2017 : 2018 2017 Cash and cash equivalents $ — $ — Restricted cash included in “Other current assets” 1 8 Total cash, cash equivalents, and restricted cash $ 1 $ 8 See Note 1 – Summary of Significant Accounting Policies under Part II, Item 8, of this report for additional information. NOTE 3 – 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 2016 , 2017 , and 2018 . Ameren Corporation (parent company only) had a total of $11 million in guarantees outstanding, primarily for ATXI, that were not recorded on its December 31, 2018 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 4 – 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 5 – 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 6 – OTHER INCOME (EXPENSE), NET The following table presents the components of “Other Income (Expense), Net” in the Condensed Statement of Income and Comprehensive Income for the years ended December 31, 2018 , 2017 , and 2016 : 2018 2017 2016 Other Income (Expense), Net Non-service cost components of net periodic benefit income $ 2 $ 2 $ 5 Donations (13 ) — (5 ) Other expense, net (1 ) — — Total Other Income (Expense), Net $ (12 ) $ 2 $ — Based on authoritative accounting guidance described in Note 10 - Retirement Benefits under Part II, Item 8, of this report, Ameren Corporation (parent company only) has retrospectively reclassified $2 million and $5 million of net benefit income from “Operating Expenses” to “Other Income (Expense), Net” in the Condensed Statement of Income and Comprehensive Income for the years ended December 31, 2017 , and December 31, 2016 , respectively. NOTE 7 – INCOME TAXES During the year ended December 31, 2017, Ameren (parent) recorded $110 million in income tax expense and reduction in accumulated deferred income taxes as a result of the TCJA. During the year ended December 31, 2018, Ameren (parent) updated its provisional estimate and recorded $5 million of income tax expense and reduction in accumulated deferred income taxes, primarily due to the application of proposed IRS regulations on depreciation transition rules.
Schedule of Cash and Cash Equivalents Including Restricted Cash [Table Text Block]	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets as of December 31, 2018 and 2017 : December 31, 2018 December 31, 2017 Ameren Ameren Ameren Ameren Ameren Ameren Cash and cash equivalents $ 16 $ — $ — $ 10 $ — $ — Restricted cash included in “Other current assets” 13 4 6 21 5 6 Restricted cash included in “Other assets” 74 — 74 35 — 35 Restricted cash included in “Nuclear decommissioning trust fund” 4 4 — 2 2 — Total cash, cash equivalents, and restricted cash $ 107 $ 8 $ 80 $ 68 $ 7 $ 41 The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheet as of December 31, 2018 and 2017 : 2018 2017 Cash and cash equivalents $ — $ — Restricted cash included in “Other current assets” 1 8 Total cash, cash equivalents, and restricted cash $ 1 $ 8
Other Income And Expenses	The following table presents the components of “Other Income, Net” in the Ameren Companies’ statements of income for the years ended December 31, 2018 , 2017 , and 2016 : 2018 2017 2016 Ameren: Other Income, Net Allowance for equity funds used during construction $ 36 $ 24 $ 27 Interest income on industrial development revenue bonds 26 26 27 Other interest income 7 8 13 Non-service cost components of net periodic benefit income 70 (a) 44 56 Other income 8 5 10 Donations (33 ) (8 ) (16 ) Other expense (12 ) (13 ) (16 ) Total Other Income, Net $ 102 $ 86 $ 101 Ameren Missouri: Other Income, Net Allowance for equity funds used during construction $ 27 $ 21 $ 23 Interest income on industrial development revenue bonds 26 26 27 Other interest income 2 1 1 Non-service cost components of net periodic benefit income 17 (a) 22 18 Other income 4 3 3 Donations (14 ) (2 ) (4 ) Other expense (6 ) (6 ) (6 ) Total Other Income, Net $ 56 $ 65 $ 62 Ameren Illinois: Other Income, Net Allowance for equity funds used during construction $ 9 $ 3 $ 4 Interest income 6 7 12 Non-service cost components of net periodic benefit income 34 10 24 Other income 3 2 6 Donations (6 ) (5 ) (6 ) Other expense (4 ) (5 ) (6 ) Total Other Income, Net $ 42 $ 12 $ 34 . (a) For the year ended December 31, 2018 , the non-service cost components of net periodic benefit income were partially offset by a $17 million deferral due to a regulatory tracking mechanism for the difference between the level of such costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. The following table presents the components of “Other Income (Expense), Net” in the Condensed Statement of Income and Comprehensive Income for the years ended December 31, 2018 , 2017 , and 2016 : 2018 2017 2016 Other Income (Expense), Net Non-service cost components of net periodic benefit income $ 2 $ 2 $ 5 Donations (13 ) — (5 ) Other expense, net (1 ) — — Total Other Income (Expense), Net $ (12 ) $ 2 $ —

Schedule II - Valuation And Qua

Schedule II - Valuation And Qualifying Accounts	12 Months Ended
	Dec. 31, 2018
SEC Schedule, 12-09, 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: 2018 $ 19 $ 27 $ 4 $ 32 $ 18 2017 19 26 7 33 19 2016 19 32 3 35 19 Deferred tax valuation allowance: 2018 $ 5 $ — $ — $ — $ 5 2017 11 (6 ) (c) — — 5 2016 6 7 (2 ) — 11 Ameren Missouri: Deducted from assets – allowance for doubtful accounts: 2018 $ 7 $ 9 $ — $ 9 $ 7 2017 7 9 — 9 7 2016 7 10 — 10 7 Ameren Illinois: Deducted from assets – allowance for doubtful accounts: 2018 $ 12 $ 18 $ 4 $ 23 $ 11 2017 12 17 7 24 12 2016 12 22 3 25 12 (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 Accoun_2

Summary Of Significant Accounting Policies (Policy)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Life Insurance, Corporate or Bank Owned [Text Block]	Company-owned Life Insurance Ameren and Ameren Illinois have company-owned life insurance, which is recorded at the net cash surrender value. The net cash surrender value is the amount that can be realized under the insurance policies at the balance sheet date. As of December 31, 2018 , the cash surrender value of company-owned life insurance at Ameren and Ameren Illinois was $244 million ( December 31, 2017 – $265 million ) and $122 million ( December 31, 2017 – $129 million ), respectively, while total borrowings against the policies were $113 million ( December 31, 2017 – $120 million ) at both Ameren and Ameren Illinois. Ameren and Ameren Illinois have the right to offset the borrowings against the cash surrender value of the policies and, consequently, present the net asset in “Other assets” on their respective balance sheets.
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. Ameren also has other subsidiaries that conduct other activities, such as providing shared services. • Union Electric Company, doing business as Ameren Missouri, operates a rate-regulated electric generation, transmission, and distribution business and a rate-regulated natural gas distribution business in Missouri. Ameren Missouri was incorporated in Missouri in 1922 and is successor to a number of companies, the oldest of which was organized in 1881. It is the largest electric utility in the state of Missouri. It supplies electric and natural gas service to a 24,000 -square-mile area in central and eastern Missouri, 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. • Ameren Transmission Company of Illinois, doing business as ATXI, operates a FERC rate-regulated electric transmission business. ATXI was incorporated in Illinois in 2006. ATXI is constructing MISO-approved electric transmission projects, including the Illinois Rivers and Mark Twain projects, and operates the Spoon River project, which was placed in service in February 2018. Ameren also evaluates competitive electric transmission investment opportunities as they arise.
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. 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	Environmental Costs Liabilities for environmental costs are recorded on an undiscounted basis when it is probable that a liability has been incurred and the amount of the liability can be reasonably estimated. Costs are expensed or deferred as a regulatory asset when it is expected that the costs will be recovered from customers in future rates. Regulation We are regulated by the MoPSC, the ICC, and the FERC. We defer certain costs as assets pursuant to actions of rate regulators or because of expectations that we will be able to recover such costs in future rates charged to customers. We also defer certain amounts as liabilities pursuant to actions of rate regulators or based on the expectation that such amounts will be returned to customers in future rates. Regulatory assets and liabilities are amortized consistent with the period of expected regulatory treatment. 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 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 position tracker, a tracker on certain excess deferred taxes, a renewable energy standards cost tracker, a solar rebate program tracker, PISA, and a RESRAM. Ameren Illinois’ and ATXI’s electric transmission rates are determined pursuant to formula ratemaking. Ameren Illinois participates in performance-based formula ratemaking for its electric distribution business and its electric energy-efficiency investments. Ameren Illinois also has environmental cost riders, an asbestos-related litigation rider, a natural gas energy-efficiency rider, a QIP rider, a VBA rider, a bad debt rider, and cost recovery mechanisms for renewable energy credits and zero emission credits. See Note 2 – Rate and Regulatory Matters for additional information on the regulatory assets and liabilities recorded at December 31, 2018 and 2017 . Ameren, Ameren Missouri, and Ameren Illinois continually assess the recoverability of their regulatory assets. Regulatory assets are charged to earnings when it is no longer probable that such amounts will be recovered through future revenues. To the extent that reductions in customers rates or refunds to customers related to regulatory liabilities are no longer probable, the amounts are credited to earnings.
Cash and Cash Equivalents	Cash, Cash Equivalents, and Restricted Cash Cash and cash equivalents include short-term, highly liquid investments purchased with an original maturity of three months or less. Cash and cash equivalents subject to legal or contractual restrictions and not readily available for use for general corporate purposes are classified as restricted cash. In November 2016, the FASB issued authoritative guidance that requires, including on a retrospective basis, restricted cash 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. Our adoption of this guidance, effective January 2018, did not result in material changes to previously reported cash flows from operating, investing, or financing activities. The changes in our restricted cash balances during the year ended December 31, 2018 , were primarily reflected as increases in cash provided by operating activities as a result of our adoption of this guidance. The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets as of December 31, 2018 and 2017 : December 31, 2018 December 31, 2017 Ameren Ameren Ameren Ameren Ameren Ameren Cash and cash equivalents $ 16 $ — $ — $ 10 $ — $ — Restricted cash included in “Other current assets” 13 4 6 21 5 6 Restricted cash included in “Other assets” 74 — 74 35 — 35 Restricted cash included in “Nuclear decommissioning trust fund” 4 4 — 2 2 — Total cash, cash equivalents, and restricted cash $ 107 $ 8 $ 80 $ 68 $ 7 $ 41 Restricted cash included in Ameren’s other current assets primarily represents participant funds from Ameren (parent)’s DRPlus and funds held by an irrevocable Voluntary Employee Beneficiary Association (VEBA) trust, which provides health care benefits for active employees. Restricted cash included in Ameren Missouri’s and Ameren Illinois’ other current assets primarily represents funds held by the VEBA trust. Restricted cash included in Ameren’s and Ameren Illinois’ other assets primarily represents amounts in a trust fund restricted for the use of funding certain asbestos-related claims and amounts collected under a cost recovery rider that are restricted for use in the procurement of renewable energy credits.
Allowance for Doubtful Accounts Receivable	Accounts Receivable “Accounts receivable – trade” on Ameren’s and Ameren Illinois’ balance sheets include certain receivables purchased at a discount from alternative retail electric suppliers that elect to participate in the utility consolidated billing program. At December 31, 2018 and 2017 , “Other current liabilities” on Ameren’s and Ameren Illinois’ balance sheets included payables for purchased receivables of $33 million and $31 million , respectively. For the years ended December 31, 2018 , 2017 , and 2016 the Ameren Companies recorded immaterial bad debt expense. 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. Ameren Missouri’s cost of nuclear fuel is capitalized as a part of “Property, Plant, and Equipment, Net” on the balance sheet 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. 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 2018 , 2017 , and 2016 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 and GAAP. 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, 2018 and 2017 . 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, 2018 and 2017 . The Ameren Transmission reporting unit had the same $93 million of goodwill as the Ameren Illinois Transmission reporting unit at December 31, 2018 and 2017 . 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. Ameren and Ameren Illinois elected to perform a qualitative assessment for their annual goodwill impairment test conducted as of October 31, 2018. 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, 2018, 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, 2018: • 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 the 30-year United States Treasury bonds; • changes 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 that 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 2018 and 2017 .
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 $14 million , $26 million , and $31 million for the years ended December 31, 2018 , 2017 , and 2016 , respectively, which was deferred as a reduction to the net regulatory liability. The net regulatory liability also reflects deferrals of net realized and unrealized gains and losses within the nuclear decommissioning trust fund for the Callaway energy center. The depreciation deferred to the regulatory asset at Ameren Illinois was immaterial in each respective period. Uncertainties as to the probability, timing, or amount of cash expenditures associated with AROs affect our estimates of fair value. Ameren and Ameren Missouri have recorded AROs for retirement costs associated with Ameren Missouri’s Callaway energy center decommissioning, CCR facilities, and river structures. Also, Ameren, Ameren Missouri, and Ameren Illinois have recorded AROs for retirement costs associated with asbestos removal and the disposal of certain transformers. Asset removal costs that do not constitute legal obligations are classified as regulatory liabilities. See Note 2 – Rate and Regulatory Matters.
Noncontrolling Interest	Noncontrolling Interests As of December 31, 2018 and 2017 , Ameren’s noncontrolling interests included the preferred stock of Ameren Missouri and Ameren Illinois.
Revenue	Operating Revenues In the first quarter of 2018, we adopted authoritative accounting guidance related to revenue from contracts with customers using the full retrospective method, with no material changes to the amount or timing of revenue recognition. We record revenues from contracts with customers for various electric and natural gas services, which primarily consist of retail distribution, electric transmission, and off-system arrangements. When more than one performance obligation exists in a contract, the consideration under the contract is allocated to the performance obligations based on the relative standalone selling price. Electric and natural gas retail distribution revenues are earned when the commodity is delivered to our customers. We accrue an estimate of electric and natural gas retail distribution revenues for service provided but unbilled at the end of each accounting period. Electric transmission revenues are earned as electric transmission services are provided. Off-system revenues are primarily comprised of MISO revenues and wholesale bilateral revenues. MISO revenues include the sale of electricity, capacity, and ancillary services. Wholesale bilateral revenues include the sale of electricity and capacity. MISO-related electricity and wholesale bilateral electricity revenues are earned as electricity is delivered. MISO-related capacity and ancillary service revenues and wholesale bilateral capacity revenues are earned as services are provided. Retail distribution, electric transmission, and off-system revenues, including the underlying components described above, represent a series of goods or services that are substantially the same and have the same pattern of transfer over time to our customers. Revenues from contracts with customers are equal to the amounts billed and our estimate of electric and natural gas retail distribution services provided but unbilled at the end of each accounting period. Revenues are billed at least monthly, and payments are due less than one month after goods and/or services are provided. See Note 15 – Segment Information for disaggregated revenue information. For certain regulatory recovery mechanisms that are alternative revenue programs rather than revenues from contracts with customers, we recognize revenues that have been authorized for rate recovery, are objectively determinable and probable of recovery, and are expected to be collected from customers within two years from the end of the year. Our alternative revenue programs include revenue requirement reconciliations, MEEIA, and VBA. These revenues are subsequently recognized as revenues from contracts with customers when billed, with an offset to alternative revenue program revenues. The Ameren Companies elected not to disclose the aggregate amount of the transaction price allocated to the performance obligations that are unsatisfied as of the end of the reporting period for contracts with an initial expected term of one year or less. As of December 31, 2018 and 2017 , our remaining performance obligations were immaterial.
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. There were no material MISO resettlements in 2018 , 2017 , or 2016 .
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. See Note 11 – Stock-based Compensation for additional information. Deferred Compensation As of December 31, 2018 , and 2017 , “Other deferred credits and liabilities” on Ameren’s balance sheet included deferred compensation obligations of $80 million and $86 million , respectively, recorded at the present value of future benefits to be paid.
Excise Taxes	Excise Taxes Ameren Missouri and Ameren Illinois collect from their customers excise taxes, including municipal and state excise taxes and gross receipts taxes, that are levied on the sale or distribution of natural gas and electricity.
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 the revaluation of deferred taxes related to the TCJA and Missouri and Illinois state corporate income tax rate changes. 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 Earnings per basic and diluted share are computed by dividing “Net Income Attributable to Ameren Common Shareholders” by the weighted-average number of basic and diluted common shares outstanding, respectively, during the period. Earnings per diluted share reflects the potential dilution that would occur if certain stock-based performance share units and restricted stock units were settled. The number of shares from performance share units assumed to be settled was 2.0 million , 1.6 million , and 0.8 million for the years ended December 31, 2018 , 2017 , and 2016 , respectively. The number of shares from restricted stock units assumed to be settled was immaterial for the year ended December 31, 2018, and not applicable for the years ended December 31, 2017 and 2016. There were no potentially dilutive securities excluded from the diluted earnings per share calculations for the years ended December 31, 2018 , 2017 , and 2016 .
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. In the first quarter of 2018, the Ameren Companies adopted authoritative accounting guidance on various topics. See the Operating Revenues section above for more information on our adoption of the guidance on revenue from contracts with customers. See Note 10 – Retirement Benefits for more information on our adoption of the guidance on the presentation of net periodic pension and postretirement benefit cost. See the Cash, Cash Equivalents, and Restricted Cash section above for more information on our adoption of the guidance on restricted cash. Our adoption of the guidance on the recognition and measurement of financial assets and financial liabilities did not have a material impact on our results of operations or financial position. 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. This guidance will affect the Ameren Companies’ financial position by increasing the assets and liabilities recorded relating to operating leases. Ameren expects both its assets and liabilities to increase by approximately $40 million , largely due to an increase at Ameren Missouri. We do not expect the impacts of this guidance to be material to our results of operations or cash flows. 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. We will adopt this guidance using the January 1, 2019 effective date as the date of our application of the standard. No adjustment to comparative periods will be made. 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. Measurement of Credit Losses on Financial Instruments 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. This guidance will be effective for the Ameren Companies in the first quarter of 2020, and will require changes to be applied retrospectively with a cumulative effect adjustment to retained earnings as of the adoption date. Fair Value Measurement Disclosures In August 2018, the FASB issued authoritative guidance that affects disclosure requirements for fair value measurements. This guidance will be effective for the Ameren Companies in the first quarter of 2020, with early adoption permitted. We are currently assessing the impacts of this guidance on our disclosures. Defined Benefit Plan Disclosures In August 2018, the FASB issued authoritative guidance that affects disclosure requirements for defined benefit plans. This guidance will be effective for the Ameren Companies in the fourth quarter of 2020, and will require changes to be applied retrospectively to each period presented. Early adoption is permitted. We are currently assessing the impacts of this guidance on our disclosures. Implementation Costs Incurred in Certain Cloud Computing Arrangements In August 2018, the FASB issued authoritative guidance that aligns the requirements for capitalizing implementation costs incurred in certain hosting arrangements with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software. This guidance requires capitalized implementation costs to be expensed over the term of the hosting arrangement and presented in the same line item in the statement of income as the fees of the associated hosting arrangement. Capitalized implementation costs must be presented in the balance sheet in the same line item that a prepayment for the fees of the associated hosting arrangement would be presented, and payments for capitalized implementation costs must be classified in the statement of cash flows in the same manner as payments for hosting arrangement fees. The Ameren Companies early adopted this guidance in the third quarter of 2018 and applied the guidance prospectively to all implementation costs incurred after the date of adoption. Implementation costs capitalized in 2018 were immaterial. 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 was effective for the Ameren Companies in the first quarter of 2018 and was applied retrospectively. Our adoption of this guidance did not result in material changes to previously reported cash flows from operating, investing, or financing activities.

Derivative Financial Instrume_2

Derivative Financial Instruments (Policies)	12 Months Ended
	Dec. 31, 2018
Derivative Instrument Detail [Abstract]
Derivatives, Policy [Policy Text Block]	The Ameren Companies elect to present the fair value amounts of derivative assets and derivative liabilities subject to an enforceable master netting arrangement or similar agreement at the gross amounts on the balance sheet. • an unrealized appreciation or depreciation of our contracted commitments to purchase or sell when purchase or sale prices under the commitments are compared with current commodity prices; • market values of natural gas inventories that differ from the cost of those commodities in inventory; and • actual cash outlays for the purchase of these commodities that differ from anticipated cash outlays. The derivatives that we use to hedge these risks are governed by our risk management policies for forward contracts, futures, options, and swaps. Our net positions are continually assessed within our structured hedging programs to determine whether new or offsetting transactions are required. The goal of the hedging program is generally to mitigate financial risks while ensuring that sufficient volumes are available to meet our requirements. Contracts we enter into as part of our risk management program may be settled financially, settled by physical delivery, or net settled with the counterparty.

Summary Of Significant Accoun_3

Summary Of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Schedule of Cash and Cash Equivalents Including Restricted Cash [Table Text Block]	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets as of December 31, 2018 and 2017 : December 31, 2018 December 31, 2017 Ameren Ameren Ameren Ameren Ameren Ameren Cash and cash equivalents $ 16 $ — $ — $ 10 $ — $ — Restricted cash included in “Other current assets” 13 4 6 21 5 6 Restricted cash included in “Other assets” 74 — 74 35 — 35 Restricted cash included in “Nuclear decommissioning trust fund” 4 4 — 2 2 — Total cash, cash equivalents, and restricted cash $ 107 $ 8 $ 80 $ 68 $ 7 $ 41 The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheet as of December 31, 2018 and 2017 : 2018 2017 Cash and cash equivalents $ — $ — Restricted cash included in “Other current assets” 1 8 Total cash, cash equivalents, and restricted cash $ 1 $ 8
Schedule Of Inventories	The following table presents a breakdown of inventories for each of the Ameren Companies at December 31, 2018 and 2017 : Ameren Missouri Ameren Illinois Ameren 2018 Fuel (a) $ 123 $ — $ 123 Natural gas stored underground 7 64 71 Materials, supplies, and other 228 61 289 Total inventories $ 358 $ 125 $ 483 2017 Fuel (a) $ 154 $ — $ 154 Natural gas stored underground 8 74 82 Materials, supplies, and other 226 60 286 Total inventories $ 388 $ 134 $ 522 (a) Consists of coal, oil, and propane.
Schedule Of Rates Used For Allowance For Funds Used During Construction	The following table presents the average allowance for funds used during construction debt and equity blended rates that were applied to construction projects in 2018 , 2017 , and 2016 : 2018 2017 2016 Ameren Missouri 7 % 7 % 7 % Ameren Illinois 5 % 4 % 5 %
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, 2018 and 2017 : Ameren Missouri Ameren Illinois Ameren Balance at December 31, 2016 $ 644 $ 6 $ 650 Liabilities settled (12 ) (1 ) (13 ) Accretion (a) 26 — 26 Change in estimates (b) (18 ) (1 ) (19 ) Balance at December 31, 2017 $ 640 (c) $ 4 (d) $ 644 (c) Liabilities settled (7 ) — (7 ) Accretion (a) 27 — 27 Change in estimates (e) (14 ) — (14 ) Balance at December 31, 2018 $ 646 (c) $ 4 (d) $ 650 (c) (a) Ameren Missouri’s accretion expense was deferred as a decrease to regulatory liabilities. (b) 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. (c) Balance included $23 million and $6 million in “Other current liabilities” on the balance sheet as of December 31, 2018 and 2017 , respectively. (d) Included in “Other deferred credits and liabilities” on the balance sheet. (e) Ameren Missouri changed its fair value estimate primarily due to a reduction in the cost estimate for closure of certain CCR storage facilities.
Schedule Of Excise Taxes	The following table presents the excise taxes recorded on a gross basis in “Operating Revenues – Electric,” “Operating Revenues – Natural gas” and “Operating Expenses – Taxes other than income taxes” on the statements of income for the years ended December 31, 2018 , 2017 , and 2016 : 2018 2017 2016 Ameren Missouri $ 164 $ 153 $ 151 Ameren Illinois 118 112 (a) 108 (a) Ameren $ 282 $ 265 $ 259 (a) Amounts have been adjusted from those previously reported to reflect additional excise taxes for the years ended December 31, 2017 and 2016.

Rate And Regulatory Matters (Ta

Rate And Regulatory Matters (Tables)	12 Months Ended
	Dec. 31, 2018
Public Utilities, General Disclosures [Abstract]
Schedule Of Regulatory Assets And Liabilities	The following table presents our regulatory assets and regulatory liabilities at December 31, 2018 and 2017 : 2018 2017 Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Regulatory assets: Under-recovered FAC (a) $ 3 $ — $ 3 $ 47 $ — $ 47 Under-recovered PGA (b) — 7 7 1 13 14 MTM derivative losses (c) 19 197 216 12 217 229 IEIMA revenue requirement reconciliation adjustment (d)(e) — 70 70 — 78 78 FERC revenue requirement reconciliation adjustment (f) — 16 30 — 25 37 Under-recovered VBA rider (g) — — — — 15 15 Pension and postretirement benefit costs (h) 103 149 252 84 215 299 Income taxes (i) 119 68 185 139 56 197 Callaway costs (e)(j) 22 — 22 25 — 25 Unamortized loss on reacquired debt (k) 58 40 98 61 49 110 Environmental cost riders (l) — 148 148 — 173 173 Storm costs (e)(m) — 13 13 — 10 10 Demand-side costs before the MEEIA implementation (e)(n) 5 — 5 11 — 11 Workers’ compensation claims (o) 4 7 11 5 7 12 Construction accounting for pollution control equipment (e)(p) 16 — 16 18 — 18 Solar rebate program (e)(q) 14 — 14 31 — 31 FEJA energy-efficiency riders (e)(r) — 136 136 — 41 41 Other 17 18 35 17 10 27 Total regulatory assets $ 380 $ 869 $ 1,261 $ 451 $ 909 $ 1,374 Less: current regulatory assets (14 ) (110 ) (134 ) (56 ) (87 ) (144 ) Noncurrent regulatory assets $ 366 $ 759 $ 1,127 $ 395 $ 822 $ 1,230 Regulatory liabilities: Over-recovered FAC (a) $ 34 $ — $ 34 $ 4 $ — $ 4 Over-recovered Illinois electric power costs (b) — 12 12 — 16 16 Over-recovered PGA (b) 7 3 10 — 1 1 Over-recovered VBA rider (g) — 8 8 — — — MTM derivative gains (c) 5 3 8 16 — 16 FERC revenue requirement reconciliation adjustment (f) — 17 19 — — — Energy-efficiency riders (s) 19 3 22 2 40 42 Estimated refund for FERC complaint case (t) — 26 44 — 25 42 Income taxes (i) 1,484 843 2,413 1,392 842 2,323 Asset removal costs (u) 1,027 774 1,811 995 725 1,725 AROs (v) 175 — 175 223 — 223 Pension and postretirement benefit costs tracker (w) 43 — 43 35 — 35 Renewable energy credits and zero emission credits (x) — 102 102 — 58 58 Excess income taxes collected in 2018 (y) 60 — 60 — — — Other 13 12 25 16 14 30 Total regulatory liabilities $ 2,867 $ 1,803 $ 4,786 $ 2,683 $ 1,721 $ 4,515 Less: current regulatory liabilities (68 ) (62 ) (149 ) (19 ) (92 ) $ (128 ) Noncurrent regulatory liabilities $ 2,799 $ 1,741 $ 4,637 $ 2,664 $ 1,629 $ 4,387 (a) 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. (b) Under-recovered or over-recovered costs from utility customers. Amounts will be recovered from, or refunded to, customers within one year of the deferral. (c) Deferral of commodity-related derivative MTM losses or gains. See Note 7 – Derivative Financial Instruments for additional information. (d) 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. (e) These assets earn a return. (f) 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. (g) Under-recovered or over-recovered natural gas revenue caused by sales volume deviations from weather normalized sales approved by the ICC in rate regulatory reviews. Each year’s amount will be recovered from, or refunded to, customers from April through December of the following year. (h) 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. (i) 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, and the unamortized portion of investment tax credits will be amortized over the expected life of the related assets. The amortization periods for depreciation differences are determined in rate orders by the applicable regulators and range from 7 to 60 years. See Note 12 – Income Taxes for amounts related to the revaluation of deferred income taxes under the TCJA. (j) 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 original remaining life of the energy center. (k) 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. (l) 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. (m) Storm costs from 2015, 2016, and 2018 deferred in accordance with the IEIMA. These costs are being amortized over five-year periods beginning in the year the storm occurred. (n) 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’s 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. (o) The period of recovery will depend on the timing of actual expenditures. (p) 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. (q) Costs associated with Ameren Missouri’s solar rebate program to fulfill its renewable energy requirements. Costs incurred from 2010 to 2014 are being amortized over a two-year period that began in April 2017 as modified per the MoPSC’s March 2017 electric rate order. Costs incurred from 2015 to 2016 are being amortized over a three-year period that began in April 2017. (r) Electric energy-efficiency program investment deferrals which earn a return at Ameren Illinois’ weighted-average cost of capital with the equity return based on the annual average of the monthly yields 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, with current remaining amortization periods ranging from 8 to 12 years. (s) 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, lost electric margins, and the performance incentive. Under the MEEIA rider, collections from or refunds to customers occur one year after the program costs, and lost electric margins are incurred or any performance incentive are earned. The Ameren Illinois balance relates to a regulatory tracking mechanism to recover its electric pre-FEJA costs 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. (t) Estimated refunds to transmission customers related to the February 2015 FERC complaint case discussed above. (u) Estimated funds collected for the eventual dismantling and removal of plant retired from service, net of salvage value. (v) 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. (w) 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 2015 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. (x) Funds collected for the purchase of renewable energy credits and zero emission credits through IPA procurements. The balance will be amortized as the credits are purchased. (y) The excess amount collected in rates related to the TCJA from January 1, 2018, through July 31, 2018. The regulatory liability will be reflected in customer rates over a period of time to be determined by the MoPSC in the next regulatory rate review.

Property And Plant, Net (Tables

Property And Plant, Net (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 and 2017 : Ameren Missouri (a) Ameren Illinois Other Ameren (a) 2018 Property, plant, and equipment at original cost: (b) Electric generation $ 11,432 $ — $ — $ 11,432 Electric distribution 5,989 5,970 — 11,959 Electric transmission 1,277 2,647 1,385 5,309 Natural gas 500 2,701 — 3,201 Other (c) 1,008 863 230 2,101 20,206 12,181 1,615 34,002 Less: Accumulated depreciation and amortization 8,726 3,294 253 12,273 11,480 8,887 1,362 21,729 Construction work in progress: Nuclear fuel in process 217 — — 217 Other 406 311 147 864 Property, plant, and equipment, net $ 12,103 $ 9,198 $ 1,509 $ 22,810 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 (a) Amounts in Ameren and Ameren Missouri include two CTs under separate agreements. The gross cumulative asset value of those agreements was $235 million and $233 million at December 31, 2018 and 2017 , respectively. The total accumulated depreciation associated with the two CTs was $89 million and $83 million at December 31, 2018 and 2017 , respectively. See Note 5 – Long-term Debt and Equity Financings for additional information on these 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, 2018 , 2017 , and 2016 , which represent noncash investing activity excluded from the accompanying statements of cash flows: Ameren Ameren Missouri Ameren Illinois Accrued capital expenditures: 2018 $ 272 $ 121 $ 138 2017 361 159 175 2016 251 116 87 Accrued nuclear fuel expenditures: 2018 $ 20 $ 20 $ — 2017 10 10 — 2016 20 20 —
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 amortization expense of capitalized software, the gross carrying value of capitalized software, and the related accumulated amortization by year: Amortization Expense (a) Gross Carrying Value Accumulated Amortization 2018 2017 2016 2018 2017 2018 2017 Ameren $ 71 $ 58 $ 52 $ 734 $ 655 $ (514 ) $ (466 ) Ameren Missouri 24 20 17 223 191 (125 ) (107 ) Ameren Illinois 44 36 33 297 241 (183 ) (146 ) (a) As of December 31, 2018 , 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, 2018
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, 2018 and 2017 : Ameren (parent) Ameren Missouri Ameren Illinois Ameren Consolidated 2018 Average daily commercial paper outstanding $ 410 $ 61 $ 108 $ 579 Outstanding borrowings at period-end 470 55 72 597 Weighted-average interest rate 2.31 % 1.94 % 2.26 % 2.26 % Peak outstanding commercial paper during period (a) $ 543 $ 481 $ 442 $ 1,295 Peak interest rate 3.10 % 2.80 % 2.85 % 3.10 % 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 % (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 Fin_2

Long-Term Debt And Equity Financings (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 and 2017 : 2018 2017 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 (3 ) (4 ) Long-term debt, net $ 697 $ 696 Ameren Missouri: Bonds and notes: 6.00% Senior secured notes due 2018 (a) — 179 5.10% Senior secured notes due 2018 (a) — 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) 400 400 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 4.00% First mortgage bonds due 2048 (f) 425 — Finance obligations: City of Bowling Green agreement (Peno Creek CT) due 2022 (g) 30 36 Audrain County agreement (Audrain County CT) due 2023 (g) 240 240 Total long-term debt, gross 4,029 3,988 Less: Unamortized discount and premium (9 ) (7 ) Less: Unamortized debt issuance costs (22 ) (20 ) Less: Maturities due within one year (580 ) (384 ) Long-term debt, net $ 3,418 $ 3,577 2018 2017 Ameren Illinois: Bonds and notes: 6.25% Senior secured notes due 2018 (h) — 144 9.75% Senior secured notes due 2018 (h) — 313 2.70% Senior secured notes due 2022 (h)(i) 400 400 5.90% First mortgage bonds due 2023 (j) (j) (j) 5.70% First mortgage bonds due 2024 (k) (k) (k) 3.25% Senior secured notes due 2025 (h) 300 300 6.125% Senior secured notes due 2028 (h) 60 60 1993 Series B-1 Senior unsecured notes due 2028 (d) 17 17 3.80% First mortgage bonds due 2028 (l) 430 — 6.70% Senior secured notes due 2036 (h) 61 61 6.70% Senior secured notes due 2036 (m) 42 42 4.80% Senior secured notes due 2043 (h) 280 280 4.30% Senior secured notes due 2044 (h) 250 250 4.15% Senior secured notes due 2046 (h) 490 490 3.70% First mortgage bonds due 2047 (l) 500 500 4.50% First mortgage bonds due 2049 (l) 500 — Total long-term debt, gross 3,330 2,857 Less: Unamortized discount and premium (3 ) (3 ) Less: Unamortized debt issuance costs (31 ) (24 ) Less: Maturities due within one year — (457 ) Long-term debt, net $ 3,296 $ 2,373 ATXI: 3.43% Senior notes due 2050 (n) $ 450 $ 450 Total long-term debt, gross 450 450 Less: Unamortized debt issuance costs (2 ) (2 ) Long-term debt, net $ 448 $ 448 Ameren consolidated long-term debt, net $ 7,859 $ 7,094 (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 2048 maturity of the 4.00% first mortgage bonds and the restrictions preventing a release date to occur that are attached to certain senior secured notes described in footnote (b) below, Ameren Missouri does not expect the first mortgage lien protection associated with these notes to fall away. (b) Ameren Missouri has agreed that so long as any of the 3.90% senior secured notes due 2042 are outstanding, Ameren Missouri will not permit a release date to occur, and so long as any of the 6.70% senior secured notes due 2019 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 average interest rates for 2018 and 2017 were as follows: 2018 2017 Ameren Missouri 1992 Series due 2022 2.37% 1.43% Ameren Missouri 1998 Series A due 2033 2.76% 1.77% Ameren Missouri 1998 Series B due 2033 2.79% 1.75% Ameren Missouri 1998 Series C due 2033 2.83% 1.73% Ameren Illinois 1993 Series B-1 due 2028 1.58% 1.08% (e) These bonds are first mortgage bonds issued by Ameren Missouri under the Ameren Missouri mortgage bond indenture. They are secured by substantially all Ameren Missouri property and franchises. Less than $1 million principal amount of the bonds remain outstanding. (f) These bonds are first mortgage bonds issued by Ameren Missouri under the Ameren Missouri bond indenture. They are secured by substantially all Ameren Missouri property and franchises. (g) Payments due related to these financing 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 city/county and held by Ameren Missouri. The timing and amounts of payments due from Ameren Missouri under the 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 financing obligations and the related investments in debt securities, recorded in "Other Assets," was $270 million and $276 million , respectively, as of December 31, 2018 and 2017. (h) 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 2049 maturity date of the 4.50% first mortgage bonds, Ameren Illinois does not expect the first mortgage lien protection associated with these notes to fall away. (i) 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. (j) 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. (k) 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 also backed by an insurance guarantee policy. Less than $1 million principal amount of the bonds remains outstanding. (l) 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. (m) 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).
Schedule Of Maturities Of Long-Term Debt	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, 2018 : Ameren (parent) (a) Ameren Missouri (a) Ameren Illinois (a) ATXI (a) Ameren Consolidated 2019 $ — $ 580 $ — $ — $ 580 2020 350 92 — — 442 2021 — 8 — — 8 2022 — 55 400 50 505 2023 — 240 — — 240 Thereafter 350 3,054 2,930 400 6,734 Total $ 700 $ 4,029 $ 3,330 $ 450 $ 8,509 (a) Excludes unamortized discount, unamortized premium, and debt issuance costs of $3 million , $31 million , $34 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, 2018 and 2017 : Redemption Price (per share) 2018 2017 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, 2018 , 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 5.5 $ 4,688 > 2.5 140.8 $ 3,153 Ameren Illinois > 2.0 6.9 4,234 (d) > 1.5 3.2 203 (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 $2,006 million and $985 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, Net (Tables)

Other Income, Net (Tables)	12 Months Ended
	Dec. 31, 2018
Other Nonoperating Income (Expense) [Abstract]
Other Income And Expenses	The following table presents the components of “Other Income, Net” in the Ameren Companies’ statements of income for the years ended December 31, 2018 , 2017 , and 2016 : 2018 2017 2016 Ameren: Other Income, Net Allowance for equity funds used during construction $ 36 $ 24 $ 27 Interest income on industrial development revenue bonds 26 26 27 Other interest income 7 8 13 Non-service cost components of net periodic benefit income 70 (a) 44 56 Other income 8 5 10 Donations (33 ) (8 ) (16 ) Other expense (12 ) (13 ) (16 ) Total Other Income, Net $ 102 $ 86 $ 101 Ameren Missouri: Other Income, Net Allowance for equity funds used during construction $ 27 $ 21 $ 23 Interest income on industrial development revenue bonds 26 26 27 Other interest income 2 1 1 Non-service cost components of net periodic benefit income 17 (a) 22 18 Other income 4 3 3 Donations (14 ) (2 ) (4 ) Other expense (6 ) (6 ) (6 ) Total Other Income, Net $ 56 $ 65 $ 62 Ameren Illinois: Other Income, Net Allowance for equity funds used during construction $ 9 $ 3 $ 4 Interest income 6 7 12 Non-service cost components of net periodic benefit income 34 10 24 Other income 3 2 6 Donations (6 ) (5 ) (6 ) Other expense (4 ) (5 ) (6 ) Total Other Income, Net $ 42 $ 12 $ 34 . (a) For the year ended December 31, 2018 , the non-service cost components of net periodic benefit income were partially offset by a $17 million deferral due to a regulatory tracking mechanism for the difference between the level of such costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. The following table presents the components of “Other Income (Expense), Net” in the Condensed Statement of Income and Comprehensive Income for the years ended December 31, 2018 , 2017 , and 2016 : 2018 2017 2016 Other Income (Expense), Net Non-service cost components of net periodic benefit income $ 2 $ 2 $ 5 Donations (13 ) — (5 ) Other expense, net (1 ) — — Total Other Income (Expense), Net $ (12 ) $ 2 $ —

Derivative Financial Instrume_3

Derivative Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 and 2017 . As of December 31, 2018 , these contracts extended through October 2021, March 2023, and May 2032 for fuel oils, natural gas, and power, respectively. Quantity (in millions) 2018 2017 Commodity Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils (in gallons) (a) 66 — 66 28 — 28 Natural gas (in mmbtu) 19 154 173 24 139 163 Power (in megawatthours) 1 8 9 3 9 12 (a) Consists of ultra-low-sulfur diesel products.
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, 2018 and 2017 : 2018 2017 Commodity Balance Sheet Location Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils Other current assets $ 3 $ — $ 3 $ 5 $ — $ 5 Other assets 5 — 5 2 — 2 Natural gas Other current assets — 1 1 — — — Other assets — 2 2 1 — 1 Power Other current assets 4 — 4 9 — 9 Total assets $ 12 $ 3 $ 15 $ 17 $ — $ 17 Fuel oils Other current liabilities $ 4 $ — $ 4 $ — $ — $ — Other deferred credits and liabilities 9 — 9 — — — Natural gas Other current liabilities 4 8 12 5 12 17 Other deferred credits and liabilities 1 6 7 3 10 13 Power Other current liabilities 4 14 18 1 13 14 Other deferred credits and liabilities — 169 169 — 182 182 Total liabilities $ 22 $ 197 $ 219 $ 9 $ 217 $ 226
Derivative Instruments With Credit Risk-Related Contingent Features	The following table presents, as of December 31, 2018 , 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, 2018 , 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) Ameren Missouri $ 76 $ 4 $ 64 Ameren Illinois 37 — 30 Ameren $ 113 $ 4 $ 94 (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, 2018
Fair Value Disclosures [Abstract]
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, 2018 and 2017 : December 31, 2018 December 31, 2017 Level 1 Level 2 Level 3 Total Level 1 Level 2 Level 3 Total Assets: Ameren Derivative assets – commodity contracts (a) : Fuel oils $ 1 $ — $ 7 $ 8 $ 4 $ — $ 3 $ 7 Natural gas — 2 1 3 — — 1 1 Power — 1 3 4 — 1 8 9 Total derivative assets – commodity contracts $ 1 $ 3 $ 11 $ 15 $ 4 $ 1 $ 12 $ 17 Nuclear decommissioning trust fund: Equity securities: U.S. large capitalization $ 427 $ — $ — $ 427 $ 468 $ — $ — $ 468 Debt securities: U.S. Treasury and agency securities — 148 — 148 — 125 — 125 Corporate bonds — 72 — 72 — 82 — 82 Other — 32 — 32 — 25 — 25 Total nuclear decommissioning trust fund $ 427 $ 252 $ — $ 679 (b) $ 468 $ 232 $ — $ 700 (b) Total Ameren $ 428 $ 255 $ 11 $ 694 $ 472 $ 233 $ 12 $ 717 Ameren Missouri Derivative assets – commodity contracts (a) : Fuel oils $ 1 $ — $ 7 $ 8 $ 4 $ — $ 3 $ 7 Natural gas — — — — — — 1 1 Power — 1 3 4 — 1 8 9 Total derivative assets – commodity contracts $ 1 $ 1 $ 10 $ 12 $ 4 $ 1 $ 12 $ 17 Nuclear decommissioning trust fund: Equity securities: U.S. large capitalization $ 427 $ — $ — $ 427 $ 468 $ — $ — $ 468 Debt securities: U.S. Treasury and agency securities — 148 — 148 — 125 — 125 Corporate bonds — 72 — 72 — 82 — 82 Other — 32 — 32 — 25 — 25 Total nuclear decommissioning trust fund $ 427 $ 252 $ — $ 679 (b) $ 468 $ 232 $ — $ 700 (b) Total Ameren Missouri $ 428 $ 253 $ 10 $ 691 $ 472 $ 233 $ 12 $ 717 Ameren Illinois Derivative assets – commodity contracts (a) : Natural gas $ — $ 2 $ 1 $ 3 $ — $ — $ — $ — Liabilities: Ameren Derivative liabilities – commodity contracts (a): Fuel oils $ 2 $ — $ 11 $ 13 $ — $ — $ — $ — Natural gas — 15 4 19 1 25 4 30 Power — 1 186 187 — — 196 196 Total Ameren $ 2 $ 16 $ 201 $ 219 $ 1 $ 25 $ 200 $ 226 Ameren Missouri Derivative liabilities – commodity contracts (a) : Fuel oils $ 2 $ — $ 11 $ 13 $ — $ — $ — $ — Natural gas — 5 — 5 — 7 1 8 Power — 1 3 4 — — 1 1 Total Ameren Missouri $ 2 $ 6 $ 14 $ 22 $ — $ 7 $ 2 $ 9 Ameren Illinois Derivative liabilities – commodity contracts (a) : Natural gas $ — $ 10 $ 4 $ 14 $ 1 $ 18 $ 3 $ 22 Power — — 183 183 — — 195 195 Total Ameren Illinois $ — $ 10 $ 187 $ 197 $ 1 $ 18 $ 198 $ 217 (a) The derivative asset and liability balances are presented net of registrant and counterparty credit considerations. (b) Balance excludes $5 million and $4 million of cash and cash equivalents, receivables, payables, and accrued income, net for December 31, 2018 and 2017 , respectively.
Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level 3 In The Fair Value Hierarchy	The following table presents the fair value reconciliation of Level 3 power derivative contract assets and liabilities measured at fair value on a recurring basis for the years ended December 31, 2018 and 2017 : 2018 2017 Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Beginning balance at January 1 $ 7 $ (195 ) $ (188 ) $ 7 $ (185 ) $ (178 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (6 ) — (6 ) (4 ) (21 ) (25 ) Purchases 5 — 5 14 — 14 Sales — — — 1 — 1 Settlements (5 ) 12 7 (11 ) 11 — Transfers out of Level 3 (1 ) — (1 ) — — — Ending balance at December 31 $ — $ (183 ) $ (183 ) $ 7 $ (195 ) $ (188 ) Change in unrealized gains (losses) related to assets/liabilities held at December 31 $ (1 ) $ (2 ) $ (3 ) $ — $ (22 ) $ (22 )
Schedule of Carrying Values and Estimated Fair Values of Debt Instruments [Table Text Block]	The following table sets forth, by level within the fair value hierarchy, the carrying amount and fair value of financial assets and liabilities disclosed, but not carried, at fair value as of December 31, 2018 and 2017 : December 31, 2018 Carrying Amount Fair Value Level 1 Level 2 Level 3 Total Ameren: Cash, cash equivalents, and restricted cash $ 107 $ 107 $ — $ — $ 107 Investments in held-to-maturity debt securities (a) 270 — 270 — 270 Short-term debt 597 — 597 — 597 Long-term debt (including current portion) (a) 8,439 (b) — 8,240 429 (c) 8,669 Ameren Missouri: Cash, cash equivalents, and restricted cash $ 8 $ 8 $ — $ — $ 8 Investments in held-to-maturity debt securities (a) 270 — 270 — 270 Short-term debt 55 — 55 — 55 Long-term debt (including current portion) (a) 3,998 (b) — 4,156 — 4,156 Ameren Illinois: Cash, cash equivalents, and restricted cash $ 80 $ 80 $ — $ — $ 80 Short-term debt 72 — 72 — 72 Long-term debt (including current portion) 3,296 (b) — 3,391 — 3,391 December 31, 2017 Ameren: Cash, cash equivalents, and restricted cash $ 68 $ 68 $ — $ — $ 68 Investments in held-to-maturity debt securities (a) 276 — 276 — 276 Short-term debt 484 — 484 — 484 Long-term debt (including current portion) (a) 7,935 (b) — 8,531 — 8,531 Ameren Missouri: Cash, cash equivalents, and restricted cash $ 7 $ 7 $ — $ — $ 7 Investments in held-to-maturity debt securities (a) 276 — 276 — 276 Short-term debt 39 — 39 — 39 Long-term debt (including current portion) (a) 3,961 (b) — 4,348 — 4,348 Ameren Illinois: Cash, cash equivalents, and restricted cash $ 41 $ 41 $ — $ — $ 41 Short-term debt 62 — 62 — 62 Long-term debt (including current portion) 2,830 (b) — 3,028 — 3,028 (a) Ameren and Ameren Missouri have investments in industrial development revenue bonds, classified as held-to-maturity and recorded in “Other Assets,” that are equal to the finance obligations for the Peno Creek and Audrain CT energy centers. As of December 31, 2018 and 2017 , the carrying amount of both the investments in industrial development revenue bonds and the finance obligations approximated fair value. (b) Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $58 million , $22 million , and $31 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2018 . Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $50 million , $20 million , and $24 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2017 . (c) The Level 3 fair value amount consists of ATXI’s senior unsecured notes. In the first quarter of 2018, the amount was transferred to Level 3 because inputs to the valuation model became unobservable during the period.
Fair Value Inputs, Assets and Liabilities, Quantitative Information	The following table describes the valuation techniques and significant unobservable inputs utilized for the fair value of our Level 3 power derivative contract assets and liabilities as of December 31, 2018 and 2017 : Fair Value (a) Weighted Commodity Assets Liabilities Valuation Technique(s) Unobservable Input Range Average 2018 Power (b) $ 3 $ (186 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps($/MWh) (c) 23 – 39 28 Nodal basis($/MWh) (c) (9) – 0 (2) Fundamental energy production model Estimated future natural gas prices($/mmbtu) (c) 3 – 4 3 Escalation rate(%) (c)(d) 4 – 5 4 2017 Power (b) $ 8 $ (196 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps($/MWh) (c) 24 – 46 28 Nodal basis($/MWh) (c) (10) – 0 (2) Fundamental energy production model Estimated future natural gas prices($/mmbtu) (c) 3 – 4 3 Escalation rate(%) (c)(d) 5 5 (a) The derivative asset and liability balances are presented net of registrant and counterparty credit considerations. (b) Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2022 and 2021 for December 31, 2018 and 2017, respectively. Valuations beyond 2022 and 2021 for December 31, 2018 and 2017, respectively, use fundamentally modeled pricing by month for peak and off-peak demand. (c) Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement. (d) Escalation rate applies to power prices in 2031 and beyond.

Callaway Energy Center (Tables)

Callaway Energy Center (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 , 2017 , and 2016 : 2018 2017 2016 Proceeds from sales and maturities $ 299 $ 305 $ 304 Gross realized gains 18 13 7 Gross realized losses 5 5 4
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, 2018 and 2017 : Security Type Cost Gross Unrealized Gain Gross Unrealized Loss Fair Value 2018 Debt securities $ 253 $ 3 $ 4 $ 252 Equity securities 162 277 12 427 Cash and cash equivalents 3 — — 3 Other (a) 2 — — 2 Total $ 420 $ 280 $ 16 $ 684 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 (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, 2018 : Cost Fair Value Less than 5 years $ 140 $ 140 5 years to 10 years 48 47 Due after 10 years 65 65 Total $ 253 $ 252
Schedule Of Insurance Coverage At Callaway Energy Center	The following table presents insurance coverage at Ameren Missouri’s Callaway energy center at December 31, 2018 . The property coverage and the nuclear liability coverage renewal dates are April 1 and January 1, respectively, of each year. Both coverages were renewed in 2018. Type and Source of Coverage Maximum Coverages Maximum Assessments for Single Incidents Public liability and nuclear worker liability: American Nuclear Insurers $ 450 $ — Pool participation 13,623 (a) 138 (b) $ 14,073 (c) $ 138 Property damage: NEIL and EMANI $ 3,200 (d) $ 27 (e) Replacement power: NEIL $ 490 (f) $ 7 (e) (a) Provided through mandatory participation in an industrywide retrospective premium assessment program. The maximum coverage available is dependent on the number of United States commercial reactors participating in the 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 $21 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, 2018
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, 2018 and 2017 : 2018 2017 Ameren (a) $ 481 $ 551 Ameren Missouri 229 215 Ameren Illinois (a) 120 213 (a) Assets associated with other postretirement benefits are 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, 2018 and 2017 . It also provides the amounts included in regulatory assets and accumulated OCI at December 31, 2018 and 2017 , that have not been recognized in net periodic benefit costs. 2018 2017 Pension Benefits Postretirement Benefits Pension Benefits Postretirement Benefits Accumulated benefit obligation at end of year $ 4,258 $ (a) $ 4,577 $ (a) Change in benefit obligation: Net benefit obligation at beginning of year $ 4,827 $ 1,240 $ 4,518 $ 1,170 Service cost 100 21 93 21 Interest cost 169 40 179 47 Plan amendments — (49 ) — — Participant contributions — 9 — 8 Actuarial (gain) loss (401 ) (163 ) 255 53 Benefits paid (236 ) (64 ) (218 ) (59 ) Net benefit obligation at end of year 4,459 1,034 4,827 1,240 Change in plan assets: Fair value of plan assets at beginning of year 4,293 1,223 3,813 1,101 Actual return on plan assets (218 ) (57 ) 634 171 Employer contributions 60 2 64 2 Participant contributions — 9 — 8 Benefits paid (236 ) (64 ) (218 ) (59 ) Fair value of plan assets at end of year 3,899 1,113 4,293 1,223 Funded status – deficiency (surplus) 560 (79 ) 534 17 Accrued benefit cost (asset) at December 31 $ 560 $ (79 ) $ 534 $ 17 Amounts recognized in the balance sheet consist of: Noncurrent asset (b) $ — $ (79 ) $ — $ — Current liability (c) 2 — 3 3 Noncurrent liability 558 — 531 14 Net liability (asset) recognized $ 560 $ (79 ) $ 534 $ 17 Amounts recognized in regulatory assets consist of: Net actuarial (gain) loss $ 393 $ (91 ) $ 374 $ (69 ) Prior service credit (2 ) (48 ) (3 ) (3 ) Amounts (pretax) recognized in accumulated OCI consist of: Net actuarial loss 35 3 30 2 Total $ 426 $ (136 ) $ 401 $ (70 ) (a) Not applicable. (b) Included in “Other assets” on Ameren’s consolidated balance sheet. (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, 2018 and 2017 : Pension Benefits Postretirement Benefits 2018 2017 2018 2017 Discount rate at measurement date 4.25 % 3.50 % 4.25 % 3.50 % 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 2018 , 2017 , and 2016 : Pension Benefits Postretirement Benefits 2018 2017 2016 2018 2017 2016 Ameren Missouri $ 18 $ 19 $ 21 $ 1 $ 1 $ 1 Ameren Illinois 35 37 30 1 1 1 Other 7 8 6 — — — Ameren $ 60 $ 64 $ 57 $ 2 $ 2 $ 2
Target Allocation Of The Plans' Asset Categories	The following table presents our target allocations for 2019 and our pension and postretirement plans’ asset categories as of December 31, 2018 and 2017 : Asset Category Target Allocation 2019 Percentage of Plan Assets at December 31, 2018 2017 Pension Plan: Cash and cash equivalents 0% – 5% 1 % 1 % Equity securities: U.S. large-capitalization 21% – 31% 24 % 34 % U.S. small- and mid-capitalization 3% – 13% 7 % 9 % International 9% – 19% 13 % 14 % Global 3% – 13% 8 % — % Total equity 51% – 61% 52 % 57 % Debt securities 35% – 45% 42 % 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 % 2 % Equity securities: U.S. large-capitalization 34% – 44% 40 % 41 % U.S. small- and mid-capitalization 2% – 12% 7 % 8 % International 9% – 19% 13 % 14 % Total equity 55% – 65% 60 % 63 % Debt securities 33% – 43% 38 % 35 % 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 2018 , 2017 , and 2016 : Pension Benefits Postretirement Benefits 2018 Service cost (a) $ 100 $ 21 Non-service cost components: Interest cost 169 40 Expected return on plan assets (276 ) (77 ) Amortization of: Prior service credit (1 ) (4 ) Actuarial (gain) loss 68 (6 ) Total non-service cost components (b) $ (40 ) $ (47 ) Net periodic benefit cost (income) $ 60 $ (26 ) 2017 Service cost (a) $ 93 $ 21 Non-service cost components: Interest cost 179 47 Expected return on plan assets (262 ) (75 ) Amortization of: Prior service credit (1 ) (5 ) Actuarial (gain) loss 55 (6 ) Total non-service cost components (b) $ (29 ) $ (39 ) Net periodic benefit cost (income) $ 64 $ (18 ) 2016 Service cost (a) $ 81 $ 19 Non-service cost components: Interest cost 185 50 Expected return on plan assets (253 ) (72 ) Amortization of: Prior service credit (1 ) (5 ) Actuarial (gain) loss 32 (11 ) Total non-service cost components (b) $ (37 ) $ (38 ) Net periodic benefit cost (income) $ 44 $ (19 ) (a) Service cost, net of capitalization, is reflected in “Operating Expenses - Other operations and maintenance” on Ameren’s statement of income. (b) 2018 amounts and the non-capitalized portion of 2017 and 2016’s non-service cost components, as discussed above, are reflected in “Other Income, Net” on Ameren’s statement of income. See Note 5 - Other Income, Net for additional information.
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 2019 are as follows: Pension Benefits Postretirement Benefits Regulatory assets: Prior service credit $ (1 ) $ (5 ) Net actuarial (gain) loss 24 (15 ) Accumulated OCI: Net actuarial loss 2 — Total $ 25 $ (20 )
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, 2018 , 2017 , and 2016 : Pension Costs Postretirement Costs 2018 2017 2016 2018 2017 2016 Ameren Missouri (a) $ 22 $ 24 $ 26 $ (1 ) $ (4 ) $ (5 ) Ameren Illinois 39 41 22 (25 ) (14 ) (13 ) Other (1 ) (1 ) (4 ) — — (1 ) Ameren 60 64 44 (26 ) (18 ) (19 ) (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, 2018 , 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 2019 $ 267 $ 3 $ 57 $ 2 2020 272 3 59 2 2021 282 3 61 2 2022 285 3 62 2 2023 286 3 64 2 2024 – 2028 1,439 12 315 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, 2018 , 2017 , and 2016 : Pension Benefits Postretirement Benefits 2018 2017 2016 2018 2017 2016 Discount rate at measurement date 3.50 % 4.00 % 4.50 % 3.50 % 4.00 % 4.50 % Expected return on plan assets 7.00 7.00 7.00 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 $ (2 ) $ 135 $ — $ 33 0.25% increase in salary scale 2 12 — — 1.00% increase in annual medical trend — — 4 58 1.00% decrease in annual medical trend — — (4 ) (58 )
Schedule Of Matching Contributions	The following table presents the portion of the matching contribution to the Ameren 401(k) plan attributable to each of the Ameren Companies for the years ended December 31, 2018 , 2017 , and 2016 : 2018 2017 2016 Ameren Missouri $ 17 $ 16 $ 16 Ameren Illinois 15 13 12 Other 1 1 1 Ameren $ 33 $ 30 $ 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, 2018 : 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 $ — $ — $ — $ 41 $ 41 Equity securities: U.S. large-capitalization — — — 955 955 U.S. small- and mid-capitalization 272 — — — 272 International 224 — — 298 522 Global — — — 321 321 Debt securities: Corporate bonds — 701 — 19 720 Municipal bonds — 87 — — 87 U.S. Treasury and agency securities — 891 — — 891 Other 1 11 — — 12 Real estate — — — 202 202 Private equity — — — 3 3 Total $ 497 $ 1,690 $ — $ 1,839 $ 4,026 Less: Medical benefit assets at December 31 (a) (144 ) Plus: Net receivables at December 31 (b) 17 Fair value of pension plans’ assets at December 31 $ 3,899 (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, 2017 : 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 $ — $ — $ — $ 25 $ 25 Equity securities: U.S. large-capitalization — — — 1,523 1,523 U.S. small- and mid-capitalization 379 — — — 379 International 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.
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, 2018 : 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 $ 32 $ — $ — $ — $ 32 Equity securities: U.S. large-capitalization 297 — — 89 386 U.S. small- and mid-capitalization 63 — — — 63 International 45 — — 84 129 Other — 12 — — 12 Debt securities: Corporate bonds — 144 — — 144 Municipal bonds — 107 — — 107 U.S. Treasury and agency securities — 62 — — 62 Other — 7 — 34 41 Total $ 437 $ 332 $ — $ 207 $ 976 Plus: Medical benefit assets at December 31 (a) 144 Less: Net payables at December 31 (b) (7 ) Fair value of postretirement benefit plans’ assets at December 31 $ 1,113 (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, 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 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.

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Summary Of Nonvested Shares Related To Long-Term Incentive Plan	nonvested performance share unit and restricted stock unit activity for the year ended December 31, 2018 : Performance Share Units Restricted Stock Units Share Units Weighted-average Fair Value per Share Unit Stock Units Weighted-average Fair Value per Stock Unit Nonvested at January 1, 2018 (a) 895,489 $ 52.28 — $ — Granted 316,875 62.88 187,273 57.66 Forfeitures (65,106 ) 51.11 (5,463 ) 58.99 Vested and undistributed (b) (288,404 ) 53.63 (26,557 ) 59.02 Vested and distributed (176,043 ) 52.88 — — Nonvested at December 31, 2018 (c) 682,811 $ 56.58 155,253 $ 57.38 (a) Does not include 712,572 undistributed vested performance share units. (b) Vested and undistributed units are awards that vested due to attainment of retirement eligibility by certain employees, but have not yet been distributed. For vested and undistributed performance share units, the number of shares issued for retirement-eligible employees will vary depending on actual performance over the three-year performance period. (c)
Disclosure of Share-based Compensation Arrangements by Share-based Payment Award	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, 2018 , 2017 , and 2016 : 2018 2017 2016 Fair value of share units awarded $62.88 $59.16 $44.13 Ameren’s closing common share price at December 31 of the prior year $61.69 $52.46 $43.23 Three-year risk-free rate 1.98% 1.47% 1.31% Volatility range for the peer group (a) 15% – 23% 15% – 21% 15% – 20% (a) Based on a historical period that is equal to the remaining term of the performance period as of the grant date. The following table presents the stock-based compensation expense for the years ended December 31, 2018, 2017, and 2016: 2018 2017 2016 Ameren Missouri $ 4 $ 4 $ 4 Ameren Illinois 3 2 2 Other (a) 13 12 11 Ameren 20 18 17 Less income tax benefit 6 7 6 Stock-based compensation expense, net $ 14 $ 11 $ 11 (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, 2018
Income Tax Disclosure [Abstract]
Schedule of Remeasurement of Deferred Incomes Taxes due to the Tax Cuts and Jobs Act [Table Text Block]	Ameren Missouri Ameren Illinois Other Ameren Increase (Decrease) Accumulated deferred income taxes, net $ (1,419 ) $ (871 ) $ 37 $ (2,253 ) Income tax expense (benefit) 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, 2018 , 2017 , and 2016 : Ameren Missouri Ameren Illinois Ameren 2018 Federal statutory corporate income tax rate: 21 % 21 % 21 % Increases (decreases) from: Amortization of excess deferred taxes (4 ) (4 ) (4 ) Other depreciation differences — (1 ) — Amortization of deferred investment tax credit (1 ) — (1 ) State tax 4 7 6 TCJA 1 1 1 Tax credits (1 ) — — Other permanent items — — (1 ) Effective income tax rate 20 % 24 % 22 % 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 % (a) Reflects the adoption of authoritative accounting guidance related to stock-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, 2018 , 2017 , and 2016 : Ameren Missouri Ameren Illinois Other Ameren 2018 Current taxes: Federal $ 104 $ 4 $ (118 ) $ (10 ) State 29 6 (12 ) 23 Deferred taxes: Federal 22 75 123 220 State (2 ) 28 23 49 Amortization of excess deferred taxes (24 ) (15 ) (1 ) (40 ) Amortization of deferred investment tax credits (5 ) — — (5 ) Total income tax expense $ 124 $ 98 $ 15 $ 237 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
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, 2018 and 2017 : Ameren Missouri Ameren Illinois Other Ameren 2018 Accumulated deferred income taxes, net liability (asset): Plant-related $ 2,010 $ 1,345 $ 179 $ 3,534 Regulatory assets and liabilities, net (343 ) (221 ) (25 ) (589 ) Deferred employee benefit costs (58 ) (4 ) (64 ) (126 ) Tax carryforwards (35 ) (26 ) (166 ) (227 ) Other (40 ) 25 46 31 Total net accumulated deferred income tax liabilities (assets) $ 1,534 $ 1,119 $ (30 ) $ 2,623 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
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, 2018 and 2017 : Ameren Missouri Ameren Illinois Other Ameren 2018 Net operating loss carryforwards: Federal (a) $ — $ 23 $ 55 $ 78 State (a) — — 13 13 Total net operating loss carryforwards $ — $ 23 $ 68 $ 91 Tax credit carryforwards: Federal (b) $ 35 $ 3 $ 79 $ 117 State (c) — — 10 10 Total tax credit carryforwards $ 35 $ 3 $ 89 $ 127 Charitable contribution carryforwards (d) $ — $ — $ 14 $ 14 Valuation allowance (e) — — (5 ) (5 ) Total charitable contribution carryforwards $ — $ — $ 9 $ 9 2017 Net operating loss carryforwards: Federal $ — $ 41 $ 162 $ 203 State — — 32 32 Total net operating loss carryforwards $ — $ 41 $ 194 $ 235 Tax credit carryforwards: Federal $ 31 $ 2 $ 80 $ 113 State — — 7 7 Total tax credit carryforwards $ 31 $ 2 $ 87 $ 120 Charitable contribution carryforwards $ — $ — $ 11 $ 11 Valuation allowance — — (5 ) (5 ) Total charitable contribution carryforwards $ — $ — $ 6 $ 6 (a) Will expire between 2034 and 2037 . 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 2019 and 2023 . (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, 2018
Related Party Transactions [Abstract]
Schedule of Related Party Transactions	The following table presents the specified performance period, price, and amount of megawatthours included in the agreements: 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 April 2018 June 2019 – September 2020 110,000 32 The following table presents the impact on Ameren Missouri and Ameren Illinois of related-party transactions for the years ended December 31, 2018 , 2017 , and 2016 . 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 2018 $ 11 $ (a) with Ameren Illinois 2017 23 (a) 2016 28 (a) Ameren Missouri and Ameren Illinois Operating Revenues 2018 22 3 rent and facility services 2017 26 4 2016 25 5 Ameren Missouri and Ameren Illinois miscellaneous Operating Revenues 2018 1 1 support services and services provided to ATXI 2017 (b) 1 2016 1 (b) Total Operating Revenues 2018 $ 34 $ 4 2017 49 5 2016 54 5 Ameren Illinois power supply Purchased Power 2018 $ (a) $ 11 agreements with Ameren Missouri 2017 (a) 23 2016 (a) 28 Ameren Illinois transmission Purchased Power 2018 (a) 1 services from ATXI 2017 (a) 2 2016 (a) 2 Total Purchased Power 2018 $ (a) $ 12 2017 (a) 25 2016 (a) 30 Ameren Missouri and Ameren Illinois Other Operations and 2018 $ 3 $ 6 rent and facility services Maintenance 2017 (b) (b) 2016 (b) (b) Ameren Services support services Other Operations and 2018 136 126 agreement Maintenance 2017 149 139 2016 129 123 Total Other Operations and 2018 $ 139 $ 132 Maintenance Expenses 2017 149 139 2016 129 123 Money pool borrowings (advances) (Interest Charges) 2018 $ 1 $ (b) Other Income, Net 2017 1 (b) 2016 (b) (b) (a) Not applicable. (b) Amount less than $1 million.
Schedule of affiliate receivables and payables	The following table presents the affiliate balances related to income taxes for Ameren Missouri and Ameren Illinois as of December 31, 2018 and 2017 : 2018 2017 Ameren Missouri Ameren Illinois Ameren Missouri Ameren Illinois Income taxes payable to parent (a) $ 16 $ 7 $ 11 $ 17 Income taxes receivable from parent (b) — 6 — — (a) Included in “Accounts payable – affiliates” on the balance sheet. (b) Included in “Accounts receivable – affiliates” on the balance sheet
Schedule of Capital Contributions	The following table presents cash capital contributions received from Ameren (parent) by Ameren Missouri and Ameren Illinois for the years ended December 31, 2018 , 2017 , and 2016 : 2018 2017 2016 Ameren Missouri (a) $ 45 $ 30 $ 44 (b) Ameren Illinois 160 8 — (a) As a result of the tax allocation agreement. (b) Included a $38 million accrued capital contribution from 2015.

Commitments And Contingencies (

Commitments And Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
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, 2018 : 2019 2020 2021 2022 2023 After 5 Years Total Ameren: Minimum capital lease payments (a)(b) $ 32 $ 32 $ 33 $ 32 $ 264 $ — $ 393 Less amount representing interest 25 25 25 24 24 — 123 Present value of minimum capital lease payments $ 7 $ 7 $ 8 $ 8 $ 240 $ — $ 270 Operating leases 10 8 7 6 5 9 45 Total lease obligations $ 17 $ 15 $ 15 $ 14 $ 245 $ 9 $ 315 Ameren Missouri: Minimum capital lease payments (b)(c) $ 32 $ 32 $ 33 $ 32 $ 264 $ — $ 393 Less amount representing interest 25 25 25 24 24 — 123 Present value of minimum capital lease payments $ 7 $ 7 $ 8 $ 8 $ 240 $ — $ 270 Operating leases 8 7 6 5 5 9 40 Total lease obligations $ 15 $ 14 $ 14 $ 13 $ 245 $ 9 $ 310 Ameren Illinois: Operating leases $ 1 $ — $ — $ — $ — $ — $ 1 (a) See Note 3 – Property, Plant, and Equipment, Net for additional information. (b) See Note 5 – Long-term Debt and Equity Financings for additional information on Ameren’s and Ameren Missouri’s capital lease agreements.
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, 2018 , 2017 , and 2016 : 2018 2017 2016 Ameren $ 9 $ 11 $ 38 Ameren Missouri 8 10 34 Ameren Illinois 1 1 30
Schedule Of Estimated Purchased Commitments	The table below presents our estimated minimum fuel, purchased power, and other commitments at December 31, 2018 . 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, 2018 . Coal Natural Gas (a) Nuclear Fuel Purchased Power (b)(c) Methane Gas Other Total Ameren: 2019 $ 349 $ 197 $ 25 $ 157 $ 4 $ 67 $ 799 2020 160 143 43 54 4 41 445 2021 121 77 59 10 4 30 301 2022 72 27 14 — 3 26 142 2023 — 7 42 — 3 27 79 Thereafter — 34 31 — 29 72 166 Total $ 702 $ 485 $ 214 $ 221 $ 47 $ 263 $ 1,932 Ameren Missouri: 2019 $ 349 $ 40 $ 25 $ — $ 4 $ 49 $ 467 2020 160 31 43 — 4 26 264 2021 121 15 59 — 4 26 225 2022 72 5 14 — 3 26 120 2023 — 3 42 — 3 27 75 Thereafter — 14 31 — 29 56 130 Total $ 702 $ 108 $ 214 $ — $ 47 $ 210 $ 1,281 Ameren Illinois: 2019 $ — $ 157 $ — $ 157 $ — $ 8 $ 322 2020 — 112 — 54 — 5 171 2021 — 62 — 10 — — 72 2022 — 22 — — — — 22 2023 — 4 — — — — 4 Thereafter — 20 — — — — 20 Total $ — $ 377 $ — $ 221 $ — $ 13 $ 611 (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 2034 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.

Segment Information (Tables)

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2018
Segment Reporting [Abstract]
Schedule Of Segment Reporting Information, By Segment	The following tables present information about the reported revenue and specified items reflected in net income attributable to common shareholders and capital expenditures by segment at Ameren and Ameren Illinois for the years ended December 31, 2018 , 2017 , and 2016 . 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 Ameren 2018 External revenues $ 3,555 $ 1,544 $ 814 $ 378 $ — $ — $ 6,291 Intersegment revenues 34 3 1 55 (a) — (93 ) — Depreciation and amortization 550 259 65 77 4 — 955 Interest income 28 6 — — 4 (5 ) 33 Interest charges 200 73 38 75 (b) 19 (4 ) 401 Income taxes 124 41 25 56 (9 ) — 237 Net income (loss) attributable to Ameren common shareholders 478 136 70 164 (33 ) — 815 Capital expenditures 914 503 311 562 5 (9 ) 2,286 2017 External revenues $ 3,488 $ 1,564 $ 742 $ 382 $ (2 ) $ — $ 6,174 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 323 131 60 140 (131 ) — 523 Capital expenditures 773 476 245 644 1 (7 ) 2,132 2016 External revenues $ 3,470 $ 1,544 $ 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 357 126 59 117 (6 ) — 653 Capital expenditures 738 470 181 689 4 (6 ) 2,076 (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 Ameren Illinois 2018 External revenues $ 1,547 $ 815 $ 214 $ — $ 2,576 Intersegment revenues — — 53 (a) (53 ) — Depreciation and amortization 259 65 50 — 374 Interest income 6 — — — 6 Interest charges 73 38 38 — 149 Income taxes 41 25 32 — 98 Net income available to common shareholder 136 70 98 — 304 Capital expenditures 503 311 444 — 1,258 2017 External revenues $ 1,568 $ 743 $ 216 $ — $ 2,527 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 $ 1,548 $ 754 $ 187 $ — $ 2,489 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 (a) Ameren Illinois Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above.
Disaggregation of Revenue	The following tables present disaggregated revenues by segment at Ameren and Ameren Illinois for the years ended December 31, 2018 , 2017 , and 2016 . Economic factors affect the nature, timing, amount, and uncertainty of revenues and cash flows in a similar manner across customer classes. Revenues from alternative revenue programs have a similar distribution among customer classes as revenues from contracts with customers. Other revenues not associated with contracts with customers are presented in the Other customer classification, along with electric transmission and off-system revenues. Ameren Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Other Intersegment Eliminations Ameren 2018 Residential $ 1,560 $ 867 $ — $ — $ — $ — $ 2,427 Commercial 1,271 511 — — — — 1,782 Industrial 312 130 — — — — 442 Other 308 (a) 39 — 433 — (92 ) 688 (a) Total electric revenues $ 3,451 $ 1,547 $ — $ 433 $ — $ (92 ) $ 5,339 Residential $ 90 $ — $ 581 $ — $ — $ — $ 671 Commercial 37 — 159 — — — 196 Industrial 4 — 17 — — — 21 Other 7 — 58 — — (1 ) 64 Total gas revenues $ 138 $ — $ 815 $ — $ — $ (1 ) $ 952 Total revenues (b) $ 3,589 $ 1,547 $ 815 $ 433 $ — $ (93 ) $ 6,291 2017 Residential $ 1,417 $ 870 $ — $ — $ — $ — $ 2,287 Commercial 1,208 527 — — — — 1,735 Industrial 305 113 — — — — 418 Other 481 58 — 426 (2 ) (96 ) 867 Total electric revenues $ 3,411 $ 1,568 $ — $ 426 $ (2 ) $ (96 ) $ 5,307 Residential $ 77 $ — $ 531 $ — $ — $ — $ 608 Commercial 31 — 146 — — — 177 Industrial 4 — 12 — — — 16 Other 14 — 54 — — (2 ) 66 Total gas revenues $ 126 $ — $ 743 $ — $ — $ (2 ) $ 867 Total revenues (b) $ 3,537 $ 1,568 $ 743 $ 426 $ (2 ) $ (98 ) $ 6,174 2016 Residential $ 1,422 $ 895 $ — $ — $ — $ — $ 2,317 Commercial 1,224 517 — — — — 1,741 Industrial 315 96 — — — — 411 Other 435 40 — 355 1 (104 ) 727 Total electric revenues $ 3,396 $ 1,548 $ — $ 355 $ 1 $ (104 ) $ 5,196 Residential $ 77 $ — $ 530 $ — $ — $ — $ 607 Commercial 30 — 153 — — — 183 Industrial 4 — 10 — — — 14 Other 17 — 61 — — (2 ) 76 Total gas revenues $ 128 $ — $ 754 $ — $ — $ (2 ) $ 880 Total revenues (b) $ 3,524 $ 1,548 $ 754 $ 355 $ 1 $ (106 ) $ 6,076 (a) Includes $60 million for the year ended December 31, 2018 , for the reduction to revenue for the excess amounts collected in rates related to the TCJA from January 1, 2018, through July 31, 2018. See Note 2 – Rate and Regulatory Matters for additional information. (b) The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the years ended December 31, 2018 , 2017 , and 2016 : Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Ameren 2018 Revenues from alternative revenue programs $ (8 ) $ (3 ) $ (23 ) $ (25 ) $ (59 ) Other revenues not from contracts with customers 24 16 2 — 42 2017 Revenues from alternative revenue programs $ (28 ) $ (5 ) $ 5 $ 13 $ (15 ) Other revenues not from contracts with customers 15 6 2 — 23 2016 Revenues from alternative revenue programs $ 8 $ (70 ) $ 11 $ (1 ) $ (52 ) Other revenues not from contracts with customers 16 6 2 — 24 Ameren Illinois Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Ameren Illinois 2018 Residential $ 867 $ 581 $ — $ — $ 1,448 Commercial 511 159 — — 670 Industrial 130 17 — — 147 Other 39 58 267 (53 ) 311 Total revenues (a) $ 1,547 $ 815 $ 267 $ (53 ) $ 2,576 2017 Residential $ 870 $ 531 $ — $ — $ 1,401 Commercial 527 146 — — 673 Industrial 113 12 — — 125 Other 58 54 258 (42 ) 328 Total revenues (a) $ 1,568 $ 743 $ 258 $ (42 ) $ 2,527 2016 Residential $ 895 $ 530 $ — $ — $ 1,425 Commercial 517 153 — — 670 Industrial 96 10 — — 106 Other 40 61 232 (45 ) 288 Total revenues (a) $ 1,548 $ 754 $ 232 $ (45 ) $ 2,489 (a) The following table presents increases/(decreases) in revenues from alternative revenue programs and other revenues not from contracts with customers for the Ameren Illinois segments for the years ended December 31, 2018 , 2017 , and 2016 : Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Ameren Illinois 2018 Revenues from alternative revenue programs $ (3 ) $ (23 ) $ (25 ) $ (51 ) Other revenues not from contracts with customers 16 2 — 18 2017 Revenues from alternative revenue programs $ (5 ) $ 5 $ 9 $ 9 Other revenues not from contracts with customers 6 2 — 8 2016 Revenues from alternative revenue programs $ (70 ) $ 11 $ 2 $ (57 ) Other revenues not from contracts with customers 6 2 — 8

Selected Quarterly Information

Selected Quarterly Information (Tables)	12 Months Ended
	Dec. 31, 2018
Selected Quarterly Financial Information [Abstract]
Summary Of Selected Quarterly Information	SELECTED QUARTERLY INFORMATION (Unaudited) (In millions, except per share amounts) Ameren 2018 2017 Quarter ended March 31 June 30 September 30 December 31 March 31 June 30 September 30 December 31 Operating revenues (a) $ 1,585 $ 1,563 $ 1,724 $ 1,419 $ 1,515 $ 1,537 $ 1,723 $ 1,399 Operating income (a) 273 385 533 166 242 387 569 212 Net income (loss) 153 240 359 69 104 194 290 (59 ) (b) Net income (loss) attributable to Ameren common shareholders $ 151 $ 239 $ 357 $ 68 $ 102 $ 193 $ 288 $ (60 ) Earnings (loss) per common share – basic $ 0.62 $ 0.98 $ 1.46 $ 0.28 $ 0.42 $ 0.79 $ 1.19 $ (0.24 ) Earnings (loss) per common share – diluted (c) $ 0.62 $ 0.97 $ 1.45 $ 0.28 $ 0.42 $ 0.79 $ 1.18 $ (0.24 ) (a) 2017 amounts have been revised to reflect the adoption of accounting guidance on revenue from contracts with customers and the presentation of net periodic pension and postretirement benefit cost, effective for the Ameren Companies as of January 1, 2018. See Note 1 – Summary of Significant Accounting Policies and Note 10 – Retirement Benefits under Part II, Item 8, of this report for additional information. (b) Includes an increase to income tax expense of $154 million recorded in 2017 as a result of the TCJA. (c) 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 (a) Operating Income (a) Net Income (Loss) Net Income (Loss) Available to Common Shareholder March 31, 2018 $ 792 $ 90 $ 39 $ 38 March 31, 2017 791 47 6 5 June 30, 2018 955 258 169 168 June 30, 2017 934 230 121 120 September 30, 2018 1,129 394 295 294 September 30, 2017 1,116 412 235 234 December 31, 2018 713 7 (22 ) (22 ) December 31, 2017 696 33 (36 ) (b) (36 ) (a) 2017 amounts have been revised to reflect the adoption of accounting guidance on revenue from contracts with customers and the presentation of net periodic pension and postretirement benefit cost, effective for the Ameren Companies as of January 1, 2018. See Note 1 – Summary of Significant Accounting Policies and Note 10 – Retirement Benefits under Part II, Item 8, of this report for additional information. (b) Includes an increase to income tax expense of $32 million recorded in 2017 as a result of the TCJA. Ameren Illinois Quarter ended Operating Revenues (a) Operating Income (a) Net Income Net Income Available to Common Shareholder March 31, 2018 $ 760 $ 159 $ 96 $ 95 March 31, 2017 703 169 80 79 June 30, 2018 578 105 63 62 June 30, 2017 576 128 58 57 September 30, 2018 564 113 63 63 September 30, 2017 574 124 55 55 December 31, 2018 674 135 85 84 December 31, 2017 674 148 78 77 (a) 2017 amounts have been revised to reflect the adoption of accounting guidance on revenue from contracts with customers and the presentation of net periodic pension and postretirement benefit cost, effective for the Ameren Companies as of January 1, 2018. See Note 1 – Summary of Significant Accounting Policies and Note 10 – Retirement Benefits under Part II, Item 8, of this report for additional information.

Summary Of Significant Accoun_4

Summary Of Significant Accounting Policies (Narrative) (Details) customer in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)mi²customer	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Accounting Policies [Line Items]
Balance Sheet impact of ASC 842	$ 40
Payables for purchased receivables	33	$ 31
Deferred Compensation Liability, Classified, Noncurrent	80	86
Cash Surrender Value of Life Insurance	244	265
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets and Liabilities, Net	22	17
Goodwill	411	411
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	16
Defined Benefit Plan, Non-service Cost or Income Components	70	44	$ 56
Corporate owned life insurance, borrowings	113	120
Ameren Illinois Company
Accounting Policies [Line Items]
Cash Surrender Value of Life Insurance	$ 122	129
Public Utilities, Area Serviced | mi²	40,000
Goodwill	$ 411	411
Defined Benefit Plan, Non-service Cost or Income Components	$ 34	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	$ 17	$ 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%
Ameren Illinois Electric Distribution [Member]
Accounting Policies [Line Items]
Goodwill	$ 238
Ameren Illinois Gas [Member]
Accounting Policies [Line Items]
Goodwill	80
Ameren Illinois Transmission [Member]
Accounting Policies [Line Items]
Goodwill	93
Ameren Transmission [Member]
Accounting Policies [Line Items]
Goodwill	$ 93

Summary Of Significant Accoun_5

Summary Of Significant Accounting Policies (Schedule Of Inventories) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Accounting Policies [Line Items]
Fuel	$ 123	$ 154
Gas stored underground	71	82
Other materials and supplies	289	286
Total Inventories	483	522
Union Electric Company
Accounting Policies [Line Items]
Fuel	123	154
Gas stored underground	7	8
Other materials and supplies	228	226
Total Inventories	358	388
Ameren Illinois Company
Accounting Policies [Line Items]
Fuel	0	0
Gas stored underground	64	74
Other materials and supplies	61	60
Total Inventories	$ 125	$ 134

Summary Of Significant Accoun_6

Summary Of Significant Accounting Policies (Schedule Of Rates Used For Allowance For Funds Used During Construction) (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Union Electric Company
Accounting Policies [Line Items]
Allowance for funds used during construction, rate	7.00%	7.00%	7.00%
Ameren Illinois Company
Accounting Policies [Line Items]
Allowance for funds used during construction, rate	5.00%	4.00%	5.00%

Summary Of Significant Accoun_7

Summary Of Significant Accounting Policies (Schedule Of Asset Retirement Obligations) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance	$ 644	$ 650
Liabilities settled	(7)	(13)
Accretion in period	27	26
Change in estimates	(14)	(19)
Balance	650	644	$ 650
Other current liabilities	282	326
Asset Retirement Obligation Balance [Member]
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Other current liabilities	23	6
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance	640	644
Liabilities settled	(7)	(12)
Accretion in period	27	26
Change in estimates	(14)	(18)
Balance	646	640	644
Other current liabilities	123	103
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance	4	6
Liabilities settled	0	(1)
Accretion in period	0	0
Change in estimates	0	(1)
Balance	4	4	6
Other current liabilities	130	177
Asset Retirement Obligation | Union Electric Company
Asset Retirement Obligation [Line Items]
Noncash Depreciation related to ARO	$ 14	$ 26	$ 31

Summary Of Significant Accoun_8

Summary Of Significant Accounting Policies (Schedule Of Excise Taxes) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Line Items]
Excise tax expense	$ 282	$ 265	$ 259
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	164	153	151
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$ 118	$ 112	$ 108

Summary Of Significant Accoun_9

Summary Of Significant Accounting Policies (Basic and Diluted Earnings Per Share Calculations) (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Assumed Settlement of Performance Share Units	2,000,000	1,600,000	800,000
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	0	0	0

Summary Of Significant Accou_10

Summary Of Significant Accounting Policies (Cash and Cash Equivalents) (Details) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 16	$ 10
Restricted Cash and Cash Equivalents, Current	13	21
Restricted Cash and Cash Equivalents, Noncurrent	74	35
Restricted Cash and Cash Equivalents, Nuclear Decommissioning Trust Fund	4	2
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	107	68	$ 52	$ 351
Union Electric Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and Cash Equivalents, at Carrying Value	0	0
Restricted Cash and Cash Equivalents, Current	4	5
Restricted Cash and Cash Equivalents, Noncurrent	0	0
Restricted Cash and Cash Equivalents, Nuclear Decommissioning Trust Fund	4	2
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	8	7	5	207
Ameren Illinois Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and Cash Equivalents, at Carrying Value	0	0
Restricted Cash and Cash Equivalents, Current	6	6
Restricted Cash and Cash Equivalents, Noncurrent	74	35
Restricted Cash and Cash Equivalents, Nuclear Decommissioning Trust Fund	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 80	$ 41	$ 28	$ 99

Rate and Regulatory Matters (Na

Rate and Regulatory Matters (Narrative-Missouri) (Details) $ in Millions	3 Months Ended									12 Months Ended
	Mar. 31, 2019USD ($)	Dec. 31, 2018USD ($)MWh	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)MWh	Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2018USD ($)MWh	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Public Utilities, General Disclosures [Line Items]
Regulatory Liability, Noncurrent		$ 4,637				$ 4,387				$ 4,637	$ 4,387
Revenues		1,419	$ 1,724	$ 1,563	$ 1,585	1,399	$ 1,723	$ 1,537	$ 1,515	6,291	6,174	$ 6,076
Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liability, Noncurrent		2,799				2,664				$ 2,799	$ 2,664
PISA Deferral Percentage										85.00%
Recovery period for plant in service accounting deferrals										20 years
Depreciation Percentage Not Included in PISA Deferral										15.00%
Revenues		713	$ 1,129	$ 955	792	$ 696	$ 1,116	$ 934	$ 791
Union Electric Company | Electricity
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount										$ 167
Ameren Transmission Company of Illinois
Public Utilities, General Disclosures [Line Items]
Estimated Capital Project Costs										$ 81
Ameren Transmission Company of Illinois | Mark Twain Project
Public Utilities, General Disclosures [Line Items]
Incentive adder to FERC allowed base return on common equity										0.50%
TCJA - Excess Accumulated Deferred Income Tax Amortization | Union Electric Company | Electricity
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount										$ 74
TCJA - Excess Amounts Collected in Rates
Public Utilities, General Disclosures [Line Items]
Regulatory Liability, Noncurrent		60								60
Final Rate Order | Union Electric Company | Natural gas
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		2
Final Rate Order | Union Electric Company | MEEIA 2013 | Electricity
Public Utilities, General Disclosures [Line Items]
Incentive Award if Energy Efficiency Goals Are Achieved										28
Final Rate Order | Union Electric Company | MEEIA 2016 | Electricity
Public Utilities, General Disclosures [Line Items]
Incentive Award if Energy Efficiency Goals Are Achieved										$ 27
Revenues		$ 6			$ 5
Achieved Percentage of Energy Efficiency Earnings For Incentive Award		100.00%								100.00%
Final Rate Order | Union Electric Company | MEEIA 2019 | Electricity
Public Utilities, General Disclosures [Line Items]
Incentive Award if Energy Efficiency Goals Are Achieved		$ 30
MEEIAEnergyEfficiencyInvestments		226
AnnualMEEIAEnergyEfficiencyInvestments		$ 65
Achieved Percentage of Energy Efficiency Earnings For Incentive Award		100.00%								100.00%
Final Rate Order | Ameren Transmission Company of Illinois | Mark Twain Project
Public Utilities, General Disclosures [Line Items]
Incentive adder to FERC allowed base return on common equity										0.50%
Pending Rate Case | Union Electric Company | Natural gas
Public Utilities, General Disclosures [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 4
Public Utilities, Requested Return on Equity, Percentage		10.30%
Public Utilities, Requested Equity Capital Structure, Percentage		51.84%
Rate Base		$ 259
Wind Generation Facility | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Amount of Megawatts | MWh		157		400
Estimated Capital Project Costs										$ 1,000
Renewable Choice Program | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Amount of Megawatts | MWh										400
Renewable Choice Program | Union Electric Company | Electricity
Public Utilities, General Disclosures [Line Items]
Percentage of Energy Received From Renewable Resources										100.00%
Renewable Choice Program - Megawatts Owned | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Amount of Megawatts | MWh										200
Maximum
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage										2.85%
Subsequent Event [Member] | Union Electric Company | MEEIA 2013 | Electricity
Public Utilities, General Disclosures [Line Items]
Revenues	$ 9
Subsequent Event [Member] | Final Rate Order | Union Electric Company | MEEIA 2016 | Electricity
Public Utilities, General Disclosures [Line Items]
Revenues	$ 11

Rate And Regulatory Matters (_2

Rate And Regulatory Matters (Narrative-Illinois) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Public Utilities, General Disclosures [Line Items]
Regulatory Assets, Noncurrent	$ 1,127	$ 1,127	$ 1,230
Current regulatory assets	134	134	144
Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets, Noncurrent	759	759	822
Current regulatory assets	110	110	87
Ameren Transmission Company of Illinois
Public Utilities, General Disclosures [Line Items]
Estimated Capital Project Costs		81
Property, Plant and Equipment, Additions		38
Electricity | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		50
Natural gas | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 15
Final Rate Order | Electricity | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Electric Energy-Efficiency Revenue Requirement	35
Final Rate Order | Natural gas | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 32
Public Utilities, Approved Return on Equity, Percentage	9.87%
Public Utilities, Approved Equity Capital Structure, Percentage	50.00%
Rate Base	$ 1,600
TCJA - Reduction in Federal Statutory Income Tax Rate | Natural gas | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	17
FEJA | Electricity | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Return on Equity, Percentage		5.80%
Public Utilities, Approved Equity Capital Structure, Percentage		50.00%
Increase Decrease in Return on Equity for Energy Savings Goals		2.00%
FEJA 2019 through 2023 | Electricity | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
FEJA Energy Efficiency Investments		$ 100
IEIMA | 2018 IEIMA Revenue Requirement Reconciliation | Electricity | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets, Noncurrent	16	16
IEIMA | 2017 IEIMA Revenue Requirement Reconciliation | Electricity | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets	54	$ 54
IEIMA | 2016 IEIMA Revenue Requirement Reconciliation | Electricity | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory assets			$ 24
IEIMA | Final Rate Order | Electricity | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	72
FEJA energy-efficiency rider [Domain] | Final Rate Order | Electricity | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 20

Rate and Regulatory Matters (_3

Rate and Regulatory Matters (Narrative-Federal) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities		$ 149	$ 128
Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities		$ 62	92
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
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 27
Midwest Independent Transmission System Operator, Inc [Member] | Ameren Transmission Company of Illinois
Public Utilities, General Disclosures [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 23
Administrative Law Judge | 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 | 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		$ 44
Pending Ferc Case | Midwest Independent Transmission System Operator, Inc [Member] | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Current regulatory liabilities		$ 26
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%

Rate And Regulatory Matters (Sc

Rate And Regulatory Matters (Schedule Of Regulatory Assets And Liabilities) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	$ 1,261	$ 1,374
Current regulatory assets	(134)	(144)
Regulatory Assets, Noncurrent	1,127	1,230
Regulatory Liabilities	4,786	4,515
Current regulatory liabilities	(149)	(128)
Regulatory Liability, Noncurrent	4,637	4,387
Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	380	451
Current regulatory assets	(14)	(56)
Regulatory Assets, Noncurrent	366	395
Regulatory Liabilities	2,867	2,683
Current regulatory liabilities	(68)	(19)
Regulatory Liability, Noncurrent	2,799	2,664
Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	869	909
Current regulatory assets	(110)	(87)
Regulatory Assets, Noncurrent	759	822
Regulatory Liabilities	1,803	1,721
Current regulatory liabilities	(62)	(92)
Regulatory Liability, Noncurrent	1,741	1,629
Under-Recovered FAC
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	3	47
Under-Recovered FAC | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	3	47
Under-Recovered PGA
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	7	14
Under-Recovered PGA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	1
Under-Recovered PGA | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	7	13
MTM Derivative Losses
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	216	229
MTM Derivative Losses | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	19	12
MTM Derivative Losses | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	197	217
Energy Efficiency Rider
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	22	42
Energy Efficiency Rider | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	19	2
Energy Efficiency Rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	3	40
IEIMA
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	70	78
IEIMA | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	70	78
FERC Revenue Requirement Reconciliation
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	30	37
Regulatory Liabilities	19	0
FERC Revenue Requirement Reconciliation | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	16	25
Regulatory Liabilities	17	0
VBA Rider
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	15
Regulatory Liabilities	8	0
VBA Rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	0	15
Regulatory Liabilities	8	0
Pension And Postretirement Benefit Costs
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	252	299
Pension And Postretirement Benefit Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	103	84
Pension And Postretirement Benefit Costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	149	215
Income Taxes
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	185	197
Regulatory Liabilities	2,413	2,323
Income Taxes | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	119	139
Regulatory Liabilities	1,484	1,392
Income Taxes | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	68	56
Regulatory Liabilities	843	842
Asset Retirement Obligation
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	175	223
Asset Retirement Obligation | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	175	223
Callaway Costs
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	22	25
Callaway Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	22	25
Unamortized Loss On Reacquired Debt
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	98	110
Unamortized Loss On Reacquired Debt | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	58	61
Unamortized Loss On Reacquired Debt | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	40	49
Environmental cost riders
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	148	173
Environmental cost riders | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	148	173
Storm Costs
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	13	10
Storm Costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	13	10
Demand-Side Costs
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	5	11
Demand-Side Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	5	11
Reserve For Workers' Compensation Liabilities
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	11	12
Reserve For Workers' Compensation Liabilities | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	4	5
Reserve For Workers' Compensation Liabilities | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	7	7
Construction Accounting For Pollution Control Equipment
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	16	18
Construction Accounting For Pollution Control Equipment | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	16	18
Solar Rebates
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	14	31
Solar Rebates | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	14	31
FEJA energy-efficiency rider
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	136	41
FEJA energy-efficiency rider | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	136	41
Other Regulatory Assets
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	35	27
Other Regulatory Assets | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	17	17
Other Regulatory Assets | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Assets	18	10
Over-Recovered FAC
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	34	4
Over-Recovered FAC | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	34	4
Over-Recovered Illinois Electric Power Costs
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	12	16
Over-Recovered Illinois Electric Power Costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	12	16
Over-Recovered PGA
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	10	1
Over-Recovered PGA | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	7	0
Over-Recovered PGA | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	3	1
MTM Derivative Gains
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	8	16
MTM Derivative Gains | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	5	16
MTM Derivative Gains | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	3	0
Refund Reserves for FERC Orders and Audit Findings
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	44	42
Refund Reserves for FERC Orders and Audit Findings | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	26	25
Removal Costs
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	1,811	1,725
Removal Costs | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	1,027	995
Removal Costs | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	774	725
Pension And Postretirement Benefit Costs Tracker
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	43	35
Pension And Postretirement Benefit Costs Tracker | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	43	35
Renewable Energy Credits
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	102	58
Renewable Energy Credits | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	102	58
Excess Income Taxes Collected - TCJA [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	60	0
Excess Income Taxes Collected - TCJA [Member] | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	60	0
Other Regulatory Liabilities
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	25	30
Other Regulatory Liabilities | Union Electric Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	13	16
Other Regulatory Liabilities | Ameren Illinois Company
Public Utilities, General Disclosures [Line Items]
Regulatory Liabilities	$ 12	$ 14
Minimum
Public Utilities, General Disclosures [Line Items]
Regulatory Liability, Amortization Period	30 years
Minimum | Income Taxes
Public Utilities, General Disclosures [Line Items]
Regulatory Liability, Amortization Period	7 years
Minimum | FEJA energy-efficiency rider
Public Utilities, General Disclosures [Line Items]
Regulatory Asset, Amortization Period	8 years
Maximum
Public Utilities, General Disclosures [Line Items]
Regulatory Liability, Amortization Period	60 years
Maximum | Income Taxes
Public Utilities, General Disclosures [Line Items]
Regulatory Liability, Amortization Period	60 years
Maximum | FEJA energy-efficiency rider
Public Utilities, General Disclosures [Line Items]
Regulatory Asset, Amortization Period	12 years

Property And Plant, Net (Schedu

Property And Plant, Net (Schedule Of Property And Plant, Net) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)equipment	Dec. 31, 2017USD ($)
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	$ 34,002	$ 32,027
Accumulated depreciation and amortization	12,273	11,633
Property and plant, before construction work in progress	21,729	20,394
Property, Plant and Equipment, Net	$ 22,810	21,466
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	$ 235	233
Total accumulated depreciation, capital lease agreements	89	83
Electric Generation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	11,432	11,132
Electric distribution equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	11,959	11,415
Electric transmission equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	5,309	4,666
Gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	3,201	2,893
Property, Plant and Equipment, Other Types [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	2,101	1,921
Construction work in progress	864	924
Nuclear Fuel
Property, Plant and Equipment [Line Items]
Construction work in progress	217	148
Union Electric Company
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	20,206	19,495
Accumulated depreciation and amortization	8,726	8,305
Property and plant, before construction work in progress	11,480	11,190
Property, Plant and Equipment, Net	12,103	11,751
Union Electric Company | Electric Generation Equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	11,432	11,132
Union Electric Company | Electric distribution equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	5,989	5,766
Union Electric Company | Electric transmission equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	1,277	1,201
Union Electric Company | Gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	500	474
Union Electric Company | Property, Plant and Equipment, Other Types [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	1,008	922
Construction work in progress	406	413
Union Electric Company | Nuclear Fuel
Property, Plant and Equipment [Line Items]
Construction work in progress	217	148
Ameren Illinois Company
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	12,181	11,123
Accumulated depreciation and amortization	3,294	3,082
Property and plant, before construction work in progress	8,887	8,041
Property, Plant and Equipment, Net	9,198	8,293
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	5,970	5,649
Ameren Illinois Company | Electric transmission equipment [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	2,647	2,298
Ameren Illinois Company | Gas
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	2,701	2,419
Ameren Illinois Company | Property, Plant and Equipment, Other Types [Member]
Property, Plant and Equipment [Line Items]
Property and plant, at original cost	863	757
Construction work in progress	311	252
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	1,615	1,409
Accumulated depreciation and amortization	253	246
Property and plant, before construction work in progress	1,362	1,163
Property, Plant and Equipment, Net	1,509	1,422
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	1,385	1,167
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	230	242
Construction work in progress	147	259
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, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	$ 272	$ 361	$ 251
Union Electric Company
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	121	159	116
Ameren Illinois Company
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	138	175	87
Nuclear Fuel
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	20	10	20
Nuclear Fuel | Union Electric Company
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	20	10	20
Nuclear Fuel | Ameren Illinois Company
Property, Plant and Equipment [Line Items]
Accrued capital expenditures	$ 0	$ 0	$ 0

Property and Plant, Net (Sche_2

Property and Plant, Net (Schedule of Capitalized Software) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Capitalized Computer Software, Amortization	$ 71	$ 58	$ 52
Capitalized Computer Software, Gross	734	655
Capitalized Computer Software, Accumulated Amortization	$ (514)	(466)
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	$ 24	20	17
Capitalized Computer Software, Gross	223	191
Capitalized Computer Software, Accumulated Amortization	(125)	(107)
Ameren Illinois Company
Property, Plant and Equipment [Line Items]
Capitalized Computer Software, Amortization	44	36	$ 33
Capitalized Computer Software, Gross	297	241
Capitalized Computer Software, Accumulated Amortization	$ (183)	$ (146)

Short-Term Debt And Liquidity_2

Short-Term Debt And Liquidity (Narrative) (Details)	12 Months Ended
	Dec. 31, 2018USD ($)lender	Dec. 31, 2017
Credit Agreements
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,500,000,000
Covenant terms, default provisions, maximum indebtedness	100,000,000
Multiyear Credit Facility
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
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,300,000,000
Missouri Credit Agreement 2012
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	0.47
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	0.48
Utilities [Member]
Short-term Debt [Line Items]
Short Term Debt, Weighted Average Interest Rate During Period	2.10%	1.19%

Short-Term Debt And Liquidity_3

Short-Term Debt And Liquidity (Schedule Of Maximum Aggregate Amount Available On Credit Agreements) (Details)	Dec. 31, 2018USD ($)
Illinois Credit Agreement 2012
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,300,000,000
Illinois Credit Agreement 2012 | Parent Company
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	500,000,000
Illinois Credit Agreement 2012 | Ameren Illinois Company
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	800,000,000
Missouri Credit Agreement 2012
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	1,200,000,000
Missouri Credit Agreement 2012 | Parent Company
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	700,000,000
Missouri Credit Agreement 2012 | Union Electric Company
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 800,000,000

Short-Term Debt And Liquidity_4

Short-Term Debt And Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 579	$ 668
Short-term debt	$ 597	$ 484
Short-term Debt, Weighted Average Interest Rate, at Point in Time	2.26%	1.31%
Peak short-term borrowings	$ 1,295	$ 948
Peak short-term borrowings interest rate	3.10%	2.00%
Parent Company
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 410	$ 573
Short-term debt	$ 470	$ 383
Short-term Debt, Weighted Average Interest Rate, at Point in Time	2.31%	1.30%
Peak short-term borrowings	$ 543	$ 841
Peak short-term borrowings interest rate	3.10%	1.90%
Union Electric Company
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 61	$ 5
Short-term debt	$ 55	$ 39
Short-term Debt, Weighted Average Interest Rate, at Point in Time	1.94%	1.24%
Peak short-term borrowings	$ 481	$ 64
Peak short-term borrowings interest rate	2.80%	1.78%
Ameren Illinois Company
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 108	$ 90
Short-term debt	$ 72	$ 62
Short-term Debt, Weighted Average Interest Rate, at Point in Time	2.26%	1.35%
Peak short-term borrowings	$ 442	$ 469
Peak short-term borrowings interest rate	2.85%	2.00%

Long-Term Debt And Equity Fin_3

Long-Term Debt And Equity Financings (Narrative) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized (in shares)		100,000,000
Stock Issued During Period, Shares, New Issues		1,200,000	0	0
Stock Issued During Period, Shares, Other		700,000	0	0
Stock Issued		$ 35,000,000
Common Stock, Shares Authorized Under 401(k) Plan		4,000,000
Proceeds from Issuance of Common Stock		$ 74,000,000	$ 0	$ 0
Preferred stock, par value (in dollars per share)		$ 0.01
Preferred stock, shares outstanding (in shares)		0
Common stock, shares authorized		6,000,000
Restricted Cash and Cash Equivalents		$ 1,000,000	8,000,000
Repayments of Other Long-term Debt		841,000,000	681,000,000	395,000,000
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
Preferred stock, par value (in dollars per share)		$ 1
Repayments of Other Long-term Debt		$ 384,000,000	431,000,000	266,000,000
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized (in shares)		2,600,000
Repayments of Other Long-term Debt		$ 457,000,000	250,000,000	$ 129,000,000
Common stock equity to capitalization ratio		51.00%
Ameren Missouri and Ameren Illinois
Long-Term Debt And Equity Financings [Line Items]
Bonds interest rate assumption		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%
Proceeds from issuance of secured debt		$ 396,000,000
6.40% Senior secured notes due 2017 | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate		6.40%
Repayments of Other Long-term Debt		$ 425,000,000
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
Long-term debt interest rate		3.65%
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	500,000,000
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 | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate	6.125%
Repayments of Other Long-term Debt	$ 250,000,000
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
Long-term debt interest rate		4.15%
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%
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	450,000,000
Long-term debt interest rate		3.43%
Proceeds from Issuance of Unsecured Debt		$ 449,000,000
Ratio of Indebtedness to Net Capital		0.70
Ratio of Indebtedness to Total Assets		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
Long-term debt interest rate		5.90%
First Mortgage Bonds, 4.00%, Due 2048 - $425 Issuance [Member] | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 425,000,000	0
Long-term debt interest rate		4.00%
Proceeds from issuance of secured debt		$ 419,000,000
6.00% Senior secured notes due 2018 | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 0	179,000,000
Long-term debt interest rate		6.00%
Repayments of Other Long-term Debt		$ 179,000,000
Senior Secured Notes 5.10% Due 2018 [Member] | Union Electric Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Long-term debt interest rate		5.10%
Repayments of Other Long-term Debt		$ 199,000,000
First Mortgage Bonds, 3.80%, Due 2028 [Member] | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 430,000,000	0
Long-term debt interest rate		3.80%
Proceeds from issuance of secured debt		$ 427,000,000
Senior Secured Notes 6.25% Due 2018 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 0	144,000,000
Long-term debt interest rate		6.25%
Repayments of Other Long-term Debt		$ 144,000,000
First Mortgage Bonds, 4.50%, Due 2049 [Member] | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 500,000,000	0
Long-term debt interest rate		4.50%
Proceeds from issuance of secured debt		$ 495,000,000
Senior Secured Notes 9.75% Due 2018 | Ameren Illinois Company | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Debt instrument face amount		$ 0	$ 313,000,000
Long-term debt interest rate		9.75%
Repayments of Other Long-term Debt		$ 313,000,000
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
Minimum | Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Common stock equity to capitalization ratio		30.00%
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

Long-Term Debt And Equity Fin_4

Long-Term Debt And Equity Financings (Schedule Of Long-Term Debt Outstanding) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Less: Maturities due within one year	$ (580,000,000)		$ (841,000,000)
Long-term Debt, Net	7,859,000,000		7,094,000,000
2,022	505,000,000
Thereafter	6,734,000,000
Ameren (parent)
Debt Instrument [Line Items]
Long-term debt, gross	700,000,000		700,000,000
Debt Issuance Costs, Net	(3,000,000)		(4,000,000)
Long-term Debt, Net	697,000,000		696,000,000
Thereafter	350,000,000
Union Electric Company
Debt Instrument [Line Items]
Long-term debt, gross	4,029,000,000		3,988,000,000
Less: Unamortized discount and premium	(9,000,000)		(7,000,000)
Debt Issuance Costs, Net	(22,000,000)		(20,000,000)
Less: Maturities due within one year	(580,000,000)		(384,000,000)
Long-term Debt, Net	3,418,000,000		3,577,000,000
2,022	55,000,000
Thereafter	3,054,000,000
Union Electric Company | City Of Bowling Green Capital Lease Peno Creek Ct
Debt Instrument [Line Items]
Capital Lease Obligations	30,000,000		36,000,000
Union Electric Company | Audrain County Capital Lease Audrain County Ct
Debt Instrument [Line Items]
Capital Lease Obligations	240,000,000		240,000,000
Ameren Illinois Company
Debt Instrument [Line Items]
Long-term debt, gross	3,330,000,000		2,857,000,000
Less: Unamortized discount and premium	(3,000,000)		(3,000,000)
Debt Issuance Costs, Net	(31,000,000)		(24,000,000)
Less: Maturities due within one year	0		(457,000,000)
Long-term Debt, Net	3,296,000,000		2,373,000,000
2,022	400,000,000
Thereafter	2,930,000,000
Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Long-term debt, gross	450,000,000		450,000,000
Debt Issuance Costs, Net	(2,000,000)		(2,000,000)
Long-term Debt, Net	448,000,000		448,000,000
2,022	50,000,000
Thereafter	400,000,000
Unsecured Debt | Ameren (parent) | 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%
Unsecured Debt | Ameren (parent) | 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%
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		450,000,000
Long-term debt interest rate	3.43%
Secured Debt | Union Electric Company | 6.40% Senior secured notes due 2017
Debt Instrument [Line Items]
Long-term debt interest rate	6.40%
Secured Debt | Union Electric Company | 6.00% Senior secured notes due 2018
Debt Instrument [Line Items]
Debt instrument face amount	$ 0		179,000,000
Long-term debt interest rate	6.00%
Secured Debt | Union Electric Company | 5.10% Senior secured notes due 2018
Debt Instrument [Line Items]
Debt instrument face amount	$ 0		199,000,000
Long-term debt interest rate	5.10%
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%
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%
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%
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%
Secured Debt | Union Electric Company | Senior Secured Notes, 2.95%, Due 2027
Debt Instrument [Line Items]
Debt instrument face amount	$ 400,000,000		400,000,000
Long-term debt interest rate	2.95%
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%
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%
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%
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%
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%
Secured Debt | Union Electric Company | First Mortgage Bonds, 4.00%, Due 2048 - $425 Issuance [Member]
Debt Instrument [Line Items]
Debt instrument face amount	$ 425,000,000		0
Long-term debt interest rate	4.00%
Secured Debt | Ameren Illinois Company | Senior Secured Notes6125 Due2017
Debt Instrument [Line Items]
Long-term debt interest rate		6.125%
Secured Debt | Ameren Illinois Company | Senior Secured Notes 6.25% Due 2018
Debt Instrument [Line Items]
Debt instrument face amount	$ 0		144,000,000
Long-term debt interest rate	6.25%
Secured Debt | Ameren Illinois Company | Senior Secured Notes 9.75% Due 2018
Debt Instrument [Line Items]
Debt instrument face amount	$ 0		313,000,000
Long-term debt interest rate	9.75%
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%
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%
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%
Secured Debt | Ameren Illinois Company | First Mortgage Bonds, 3.80%, Due 2028 [Member]
Debt Instrument [Line Items]
Debt instrument face amount	$ 430,000,000		0
Long-term debt interest rate	3.80%
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%
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%
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%
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%
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%
Secured Debt | Ameren Illinois Company | Senior Secured Notes, 3.70%, Due 2047
Debt Instrument [Line Items]
Debt instrument face amount	$ 500,000,000	$ 500,000,000	500,000,000
Long-term debt interest rate	3.70%	3.70%
Secured Debt | Ameren Illinois Company | First Mortgage Bonds, 4.50%, Due 2049 [Member]
Debt Instrument [Line Items]
Debt instrument face amount	$ 500,000,000		0
Long-term debt interest rate	4.50%
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%
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
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%
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
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
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
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%
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%
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
Redemption price, percentage	100.00%
Capital Lease Obligations [Member] | Union Electric Company
Debt Instrument [Line Items]
Debt Securities, Held-to-maturity	$ 270,000,000		$ 276,000,000
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]
2,022	$ 49,500,000
Debt Instrument, Redemption, Period Two | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Thereafter	49,500,000
Debt Instrument, Redemption, Period Three | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Thereafter	49,500,000
Debt Instrument, Redemption, Period Four | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Thereafter	49,500,000
Debt Instrument, Redemption, Period Five | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Thereafter	49,500,000
Debt Instrument, Redemption, Period Six | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Thereafter	49,500,000
Debt Instrument, Redemption, Period Seven | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Thereafter	76,500,000
Debt Instrument, Redemption, Period Eight | Unsecured Debt | Ameren Transmission Company of Illinois
Debt Instrument [Line Items]
Thereafter	$ 76,500,000

Long-Term Debt And Equity Fin_5

Long-Term Debt And Equity Financings (Schedule Of Average Interest Rates) (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
1998 Series A due 2033 | Union Electric Company
Debt Instrument [Line Items]
Debt instrument, interest rate during period	2.76%	1.77%
1998 Series B due 2033 | Union Electric Company
Debt Instrument [Line Items]
Debt instrument, interest rate during period	2.79%	1.75%
1998 Series C due 2033 | Union Electric Company
Debt Instrument [Line Items]
Debt instrument, interest rate during period	2.83%	1.73%
Series B-1 1993 Due 2028 | Ameren Illinois Company
Debt Instrument [Line Items]
Debt instrument, interest rate during period	1.58%	1.08%
1992 Series due 2022 | Union Electric Company
Debt Instrument [Line Items]
Debt instrument, interest rate during period	2.37%	1.43%

Long-Term Debt And Equity Fin_6

Long-Term Debt And Equity Financings (Schedule Of Maturities Of Long-Term Debt) (Details) $ in Millions	Dec. 31, 2018USD ($)
Debt Instrument [Line Items]
2,019	$ 580
2,020	442
2,021	8
2,022	505
2,023	240
Thereafter	6,734
Total	8,509
Ameren (parent)
Debt Instrument [Line Items]
2,020	350
Thereafter	350
Total	700
Debt Instrument, Unamortized Discount or Premium and Debt Issuance Costs	3
Union Electric Company
Debt Instrument [Line Items]
2,019	580
2,020	92
2,021	8
2,022	55
2,023	240
Thereafter	3,054
Total	4,029
Debt Instrument, Unamortized Discount or Premium and Debt Issuance Costs	31
Ameren Illinois Company
Debt Instrument [Line Items]
2,022	400
Thereafter	2,930
Total	3,330
Debt Instrument, Unamortized Discount or Premium and Debt Issuance Costs	34
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 Fin_7

Long-Term Debt And Equity Financings (Schedule Of Outstanding Preferred Stock) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
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 (in shares)	0
Preferred stock, voluntary liquidation (in dollars per share)	$ 105.50
Union Electric Company and Ameren Illinois [Member]
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, issued (in shares)	$ 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 (in shares)	$ 80	80
Union Electric Company | Par Value $100
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 (in dollars per share)	$ 3.50
Preferred stock, shares outstanding (in shares)	130,000
Preferred stock, redemption price per share (in dollars per share)	$ 110
Preferred stock, issued (in shares)	$ 13	13
Union Electric Company | $3.70 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount (in dollars per share)	$ 3.70
Preferred stock, shares outstanding (in shares)	40,000
Preferred stock, redemption price per share (in dollars per share)	$ 104.75
Preferred stock, issued (in shares)	$ 4	4
Union Electric Company | $4.00 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount (in dollars per share)	$ 4
Preferred stock, shares outstanding (in shares)	150,000
Preferred stock, redemption price per share (in dollars per share)	$ 105.625
Preferred stock, issued (in shares)	$ 15	15
Union Electric Company | $4.30 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount (in dollars per share)	$ 4.30
Preferred stock, shares outstanding (in shares)	40,000
Preferred stock, redemption price per share (in dollars per share)	$ 105
Preferred stock, issued (in shares)	$ 4	4
Union Electric Company | $4.50 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount (in dollars per share)	$ 4.50
Preferred stock, shares outstanding (in shares)	213,595
Preferred stock, redemption price per share (in dollars per share)	$ 110
Preferred stock, issued (in shares)	$ 21	21
Union Electric Company | $4.56 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount (in dollars per share)	$ 4.56
Preferred stock, shares outstanding (in shares)	200,000
Preferred stock, redemption price per share (in dollars per share)	$ 102.47
Preferred stock, issued (in shares)	$ 20	20
Union Electric Company | $4.75 Series
Long-Term Debt And Equity Financings [Line Items]
Dividend rate on preferred shares, per-dollar amount (in dollars per share)	$ 4.75
Preferred stock, shares outstanding (in shares)	20,000
Preferred stock, redemption price per share (in dollars per share)	$ 102.176
Preferred stock, issued (in shares)	$ 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 (in dollars per share)	$ 5.50
Preferred stock, shares outstanding (in shares)	14,000
Preferred stock, redemption price per share (in dollars per share)	$ 110
Preferred stock, issued (in shares)	$ 1	1
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Preferred stock, authorized (in shares)	2,600,000
Preferred stock, issued (in shares)	$ 62	62
Ameren Illinois Company | Par Value $100
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 (in shares)	144,275
Preferred stock, redemption price per share (in dollars per share)	$ 101
Preferred stock, issued (in shares)	$ 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 (in shares)	45,224
Preferred stock, redemption price per share (in dollars per share)	$ 103
Preferred stock, issued (in shares)	$ 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 (in shares)	23,655
Preferred stock, redemption price per share (in dollars per share)	$ 104
Preferred stock, issued (in shares)	$ 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 (in shares)	50,000
Preferred stock, redemption price per share (in dollars per share)	$ 102
Preferred stock, issued (in shares)	$ 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 (in shares)	16,621
Preferred stock, redemption price per share (in dollars per share)	$ 103
Preferred stock, issued (in shares)	$ 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 (in shares)	16,190
Preferred stock, redemption price per share (in dollars per share)	$ 103
Preferred stock, issued (in shares)	$ 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 (in shares)	18,429
Preferred stock, redemption price per share (in dollars per share)	$ 103
Preferred stock, issued (in shares)	$ 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 (in shares)	73,825
Preferred stock, redemption price per share (in dollars per share)	$ 102
Preferred stock, issued (in shares)	$ 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 (in shares)	49,289
Preferred stock, redemption price per share (in dollars per share)	$ 103.50
Preferred stock, issued (in shares)	$ 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 (in shares)	50,000
Preferred stock, redemption price per share (in dollars per share)	$ 102
Preferred stock, issued (in shares)	$ 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 (in shares)	124,274
Preferred stock, redemption price per share (in dollars per share)	$ 100
Preferred stock, issued (in shares)	$ 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 (in shares)	4,542
Preferred stock, redemption price per share (in dollars per share)	$ 100
Preferred stock, issued (in shares)	$ 1	$ 1

Long-Term Debt and Equity Fin_8

Long-Term Debt and Equity Financings (Schedule of Required and Actual Debt Ratios) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)
Union Electric Company
Debt Instrument [Line Items]
Bonds Issuable Based On Coverage Ratio	$ 4,688
Preferred Stock Issuable Based On Coverage Ratio	3,153
Retired Bond Capacity	$ 2,006
Union Electric Company | Actual Interest Coverage Ratio
Debt Instrument [Line Items]
Interest Coverage Ratio	5.5
Dividend Coverage Ratio	140.8
Ameren Illinois Company
Debt Instrument [Line Items]
Bonds Issuable Based On Coverage Ratio	$ 4,234
Preferred Stock Issuable Based On Coverage Ratio	203
Retired Bond Capacity	$ 985
Ameren Illinois Company | Actual Interest Coverage Ratio
Debt Instrument [Line Items]
Interest Coverage Ratio	6.9
Dividend Coverage Ratio	3.2
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, Net (Other Income

Other Income, Net (Other Income And Expenses) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		$ 36	$ 24	$ 27
Interest income on industrial development revenue bonds		26	26	27
Interest and dividend income		7	8	13
Defined Benefit Plan, Non-service Cost or Income Components		70	44	56
Other		8	5	10
Donations		(33)	(8)	(16)
Other		(12)	(13)	(16)
Other Nonoperating Income (Expense)		102	86	101
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	$ 17
Allowance for equity funds used during construction		27	21	23
Interest income on industrial development revenue bonds		26	26	27
Interest and dividend income		2	1	1
Defined Benefit Plan, Non-service Cost or Income Components		17	22	18
Other		4	3	3
Donations		(14)	(2)	(4)
Other		(6)	(6)	(6)
Other Nonoperating Income (Expense)		56	65	62
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		9	3	4
Interest and dividend income		6	7	12
Defined Benefit Plan, Non-service Cost or Income Components		34	10	24
Other		3	2	6
Donations		(6)	(5)	(6)
Other		(4)	(5)	(6)
Other Nonoperating Income (Expense)		$ 42	$ 12	$ 34

Derivative Financial Instrume_4

Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Details) gal in Millions, MWh in Millions, MMBTU in Millions	Dec. 31, 2018MWhgalMMBTU	Dec. 31, 2017MWhgalMMBTU
Fuel Oils
Derivative [Line Items]
Quantity | gal	66	28
Natural gas
Derivative [Line Items]
Quantity | MMBTU	173	163
Power
Derivative [Line Items]
Quantity | MWh	9	12
Union Electric Company | Fuel Oils
Derivative [Line Items]
Quantity | gal	66	28
Union Electric Company | Natural gas
Derivative [Line Items]
Quantity | MMBTU	19	24
Union Electric Company | Power
Derivative [Line Items]
Quantity | MWh	1	3
Ameren Illinois Company | Natural gas
Derivative [Line Items]
Quantity | MMBTU	154	139
Ameren Illinois Company | Power
Derivative [Line Items]
Quantity | MWh	8	9

Derivative Financial Instrume_5

Derivative Financial Instruments (Derivative Instruments Carrying Value) (Details) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative assets	$ 15	$ 17
Derivative liabilities	219	226
Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets	3	5
Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets	5	2
Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities	4	0
Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	9	0
Natural gas | Other current assets
Derivative [Line Items]
Derivative assets	1	0
Natural gas | Other assets
Derivative [Line Items]
Derivative assets	2	1
Natural gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	12	17
Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	7	13
Power | Other current assets
Derivative [Line Items]
Derivative assets	4	9
Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	18	14
Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	169	182
Union Electric Company
Derivative [Line Items]
Derivative assets	12	17
Derivative liabilities	22	9
Union Electric Company | Fuel Oils | Other current assets
Derivative [Line Items]
Derivative assets	3	5
Union Electric Company | Fuel Oils | Other assets
Derivative [Line Items]
Derivative assets	5	2
Union Electric Company | Fuel Oils | Other current liabilities
Derivative [Line Items]
Derivative liabilities	4	0
Union Electric Company | Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	9	0
Union Electric Company | Natural gas | Other current assets
Derivative [Line Items]
Derivative assets	0	0
Union Electric Company | Natural gas | Other assets
Derivative [Line Items]
Derivative assets	0	1
Union Electric Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	4	5
Union Electric Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	1	3
Union Electric Company | Power | Other current assets
Derivative [Line Items]
Derivative assets	4	9
Union Electric Company | Power | Other current liabilities
Derivative [Line Items]
Derivative liabilities	4	1
Union Electric Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	3	0
Derivative liabilities	197	217
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	0
Ameren Illinois Company | Fuel Oils | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Natural gas | Other current assets
Derivative [Line Items]
Derivative assets	1	0
Ameren Illinois Company | Natural gas | Other assets
Derivative [Line Items]
Derivative assets	2	0
Ameren Illinois Company | Natural gas | Other current liabilities
Derivative [Line Items]
Derivative liabilities	8	12
Ameren Illinois Company | Natural gas | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	6	10
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	14	13
Ameren Illinois Company | Power | Other deferred credits and liabilities
Derivative [Line Items]
Derivative liabilities	$ 169	$ 182

Derivative Financial Instrume_6

Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Details) $ in Millions	Dec. 31, 2018USD ($)
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	$ 113
Cash Collateral Posted	4
Potential Aggregate Amount of Additional Collateral Required	94
Union Electric Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	76
Cash Collateral Posted	4
Potential Aggregate Amount of Additional Collateral Required	64
Ameren Illinois Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	37
Cash Collateral Posted	0
Potential Aggregate Amount of Additional Collateral Required	$ 30

Fair Value Measurements (Schedu

Fair Value Measurements (Schedule of Valuation Process and Unobservable Inputs) (Details) - Power $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)$ / MMBTU$ / MWh	Dec. 31, 2017USD ($)$ / MMBTU$ / MWh
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative assets | $	$ 4	$ 9
Minimum | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Average bid/ask consensus peak and off-peak pricing	23	24
Nodal basis	(9)	(10)
Minimum | Fundamental Energy Production Model
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Escalation rate	4.00%	5.00%
Estimated future gas prices | $ / MMBTU	3	3
Maximum | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Average bid/ask consensus peak and off-peak pricing	39	46
Nodal basis	0	0
Maximum | Fundamental Energy Production Model
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Escalation rate	5.00%	5.00%
Estimated future gas prices | $ / MMBTU	4	4
Weighted Average | Discounted Cash Flow
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Average bid/ask consensus peak and off-peak pricing	28	28
Nodal basis	(2)	(2)
Weighted Average | Fundamental Energy Production Model
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Escalation rate	4.00%	5.00%
Estimated future gas prices | $ / MMBTU	3	3
Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative assets | $	$ 3	$ 8
Derivative Liability, Fair Value, Gross Asset | $	$ (186)	$ (196)

Fair Value Measurements (Sche_2

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, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	$ 679	$ 700
Assets	694	717
Excluded receivables, payables, and accrued income, net	5	4
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	427	468
Assets	428	472
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	252	232
Assets	255	233
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Assets	11	12
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	15	17
Derivative liabilities	219	226
Commodity Contract | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	4
Derivative liabilities	2	1
Commodity Contract | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	1
Derivative liabilities	16	25
Commodity Contract | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	11	12
Derivative liabilities	201	200
Fuel Oils
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	8	7
Derivative liabilities	13	0
Fuel Oils | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	4
Derivative liabilities	2	0
Fuel Oils | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Fuel Oils | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	7	3
Derivative liabilities	11	0
Natural gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	1
Derivative liabilities	19	30
Natural gas | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	1
Natural gas | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	0
Derivative liabilities	15	25
Natural gas | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	1
Derivative liabilities	4	4
Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	4	9
Derivative liabilities	187	196
Power | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Power | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	1
Derivative liabilities	1	0
Power | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	8
Derivative liabilities	186	196
Equity Securities | U.S. Large Capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	427	468
Equity Securities | U.S. Large Capitalization | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	427	468
Equity Securities | U.S. Large Capitalization | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Equity Securities | U.S. Large Capitalization | 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	72	82
Debt Securities | Corporate Bonds | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Debt Securities | Corporate Bonds | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	72	82
Debt Securities | Corporate Bonds | 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	148	125
Debt Securities | U.S. treasury and agency securities | 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 | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	148	125
Debt Securities | U.S. treasury and agency securities | 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	32	25
Debt Securities | Other debt securities | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Debt Securities | Other debt securities | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	32	25
Debt Securities | Other debt securities | 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	679	700
Assets	691	717
Union Electric Company | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	427	468
Assets	428	472
Union Electric Company | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	252	232
Assets	253	233
Union Electric Company | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Assets	10	12
Union Electric Company | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	12	17
Derivative liabilities	22	9
Union Electric Company | Commodity Contract | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	4
Derivative liabilities	2	0
Union Electric Company | Commodity Contract | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	1
Derivative liabilities	6	7
Union Electric Company | Commodity Contract | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	10	12
Derivative liabilities	14	2
Union Electric Company | Fuel Oils
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	8	7
Derivative liabilities	13	0
Union Electric Company | Fuel Oils | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	4
Derivative liabilities	2	0
Union Electric Company | Fuel Oils | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Union Electric Company | Fuel Oils | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	7	3
Derivative liabilities	11	0
Union Electric Company | Natural gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	1
Derivative liabilities	5	8
Union Electric Company | Natural gas | 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 | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	5	7
Union Electric Company | Natural gas | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	1
Derivative liabilities	0	1
Union Electric Company | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	4	9
Derivative liabilities	4	1
Union Electric Company | Power | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Union Electric Company | Power | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	1
Derivative liabilities	1	0
Union Electric Company | Power | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	8
Derivative liabilities	3	1
Union Electric Company | Equity Securities | U.S. Large Capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	427	468
Union Electric Company | Equity Securities | U.S. Large Capitalization | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	427	468
Union Electric Company | Equity Securities | U.S. Large Capitalization | 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 | 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	72	82
Union Electric Company | Debt Securities | Corporate Bonds | 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 | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	72	82
Union Electric Company | Debt Securities | Corporate Bonds | 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	148	125
Union Electric Company | Debt Securities | U.S. treasury and agency securities | 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 | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	148	125
Union Electric Company | Debt Securities | U.S. treasury and agency securities | 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	32	25
Union Electric Company | Debt Securities | Other debt securities | 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 | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	32	25
Union Electric Company | Debt Securities | Other debt securities | 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	197	217
Ameren Illinois Company | Commodity Contract | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	1
Ameren Illinois Company | Commodity Contract | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	10	18
Ameren Illinois Company | Commodity Contract | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	187	198
Ameren Illinois Company | Natural gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	0
Derivative liabilities	14	22
Ameren Illinois Company | Natural gas | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	1
Ameren Illinois Company | Natural gas | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	0
Derivative liabilities	10	18
Ameren Illinois Company | Natural gas | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	0
Derivative liabilities	4	3
Ameren Illinois Company | Power
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	183	195
Ameren Illinois Company | Power | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Power | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Power | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	$ 183	$ 195

Fair Value Measurements (Sche_3

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	$ (1)	$ 0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	(183)	(188)	$ (178)
Included in regulatory assets/liabilities	(6)	(25)
Purchases	5	14
Sales	0	1
Settlements	7	0
Change in unrealized gains (losses) related to assets/liabilities still held	(3)	(22)
Union Electric Company
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(1)	0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	0	7	7
Included in regulatory assets/liabilities	(6)	(4)
Purchases	5	14
Sales	0	1
Settlements	(5)	(11)
Change in unrealized gains (losses) related to assets/liabilities still held	(1)	0
Ameren Illinois Company
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	(183)	(195)	$ (185)
Included in regulatory assets/liabilities	0	(21)
Purchases	0	0
Sales	0	0
Settlements	12	11
Change in unrealized gains (losses) related to assets/liabilities still held	$ (2)	$ (22)

Fair Value Measurements (Sche_4

Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Financial Assets and Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term debt	$ 597	$ 484
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	107	68	$ 52	$ 351
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term debt	55	39
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	8	7	5	207
Debt Issuance Costs, Net	22	20
Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term debt	72	62
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	80	41	$ 28	$ 99
Debt Issuance Costs, Net	31	24
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities and Held-to-maturity Securities	270	276
Short-term debt	597	484
Long-term debt (including current portion)	8,439	7,935
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	107	68
Debt Issuance Costs, Net	58	50
Carrying Amount | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term debt		39
Long-term debt (including current portion)	3,998	3,961
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	8	7
Debt Issuance Costs, Net	22	20
Carrying Amount | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term debt	72	62
Long-term debt (including current portion)	3,296	2,830
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	80	41
Debt Issuance Costs, Net	31	24
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	597	484
Investments, Fair Value Disclosure	270	276
Long-term Debt, Fair Value	8,669	8,531
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	107	68
Fair Value | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	107	68
Fair Value | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	597	484
Investments, Fair Value Disclosure	270	276
Long-term Debt, Fair Value	8,240	8,531
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Fair Value | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	429	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Fair Value | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	55	39
Investments, Fair Value Disclosure	270	276
Long-term Debt, Fair Value	4,156	4,348
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	8	7
Fair Value | Union Electric Company | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	8	7
Fair Value | Union Electric Company | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	55	39
Investments, Fair Value Disclosure	270	276
Long-term Debt, Fair Value	4,156	4,348
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Fair Value | Union Electric Company | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Fair Value | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	72	62
Long-term Debt, Fair Value	3,391	3,028
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	80	41
Fair Value | Ameren Illinois Company | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	80	41
Fair Value | Ameren Illinois Company | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	72	62
Long-term Debt, Fair Value	3,391	3,028
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Fair Value | Ameren Illinois Company | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 0	$ 0

Callaway Energy Center (Details

Callaway Energy Center (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Nuclear Waste Matters [Line Items]
Litigation Settlement, Amount (Deprecated 2017-01-31)	$ 11	$ 3	$ 24
Nuclear Plant
Nuclear Waste Matters [Line Items]
Annual decommissioning costs included in costs of service	$ 7

Callaway Energy Center (Proceed

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Nuclear Waste Matters [Line Items]
Sales and maturities of securities - nuclear decommissioning trust fund	$ 299	$ 305	$ 304
Gross realized gains	18	13	7
Gross realized losses	5	5	4
Union Electric Company
Nuclear Waste Matters [Line Items]
Sales and maturities of securities - nuclear decommissioning trust fund	$ 299	$ 305	$ 304
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 (Fair Va

Callaway Energy Center (Fair Values Of Investments In Debt And Equity Securities In Nuclear Decommissioning Trust Fund) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Nuclear Waste Matters [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	$ 253
Cost	420	$ 387
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	107	68	$ 52	$ 351
Cash and Cash Equivalents, at Carrying Value	16	10
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax		323
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	16	6
Fair Value	684	704
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	280
Debt Securities
Nuclear Waste Matters [Line Items]
Debt Securities, Available-for-sale, Amortized Cost	253
Cost		228
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	3	5
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	4	1
Fair Value	252	232
Equity Securities
Nuclear Waste Matters [Line Items]
Cost		155
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax	277	318
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax	12	5
Fair Value	427	468
Available-for-sale Equity Securities, Amortized Cost Basis	162
Cash
Nuclear Waste Matters [Line Items]
Cost	3	2
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents		0
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	0	0
Cash and Cash Equivalents, at Carrying Value	3	2
Other Debt And Equity Securities
Nuclear Waste Matters [Line Items]
Cost	2
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax		0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax		0
Fair Value	2	$ 2
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	$ 0

Callaway Energy Center (Cost an

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, 2018USD ($)
Nuclear Waste Matters [Abstract]
Cost, Less than 5 years	$ 140
Cost, 5 years to 10 years	48
Cost, Due after 10 years	65
Cost, Total	253
Fair Value, Less than 5 years	140
Fair Value, 5 years to 10 years	47
Fair Value, Due after 10 years	65
Fair Value, Total	$ 252

Callaway Energy Center (Insuran

Callaway Energy Center (Insurance Disclosure) (Details)	12 Months Ended
	Dec. 31, 2018USD ($)
Nuclear Waste Matters [Line Items]
Number of weeks of coverage after the first eight weeks of an outage	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	21,000,000
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
Aggregate nuclear power industry insurance policy limit for losses from terrorist attacks within twelve month period	$ 3,200,000,000
Number Of Additional Weeks After Initial Indemnity Coverage For Power Outage	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	13,623,000,000
Maximum assessments for single incidents	138,000,000
Public Liability
Nuclear Waste Matters [Line Items]
Maximum coverages	14,073,000,000
Maximum assessments for single incidents	138,000,000
Property Damage - Nuclear Electric Insurance Ltd
Nuclear Waste Matters [Line Items]
Maximum coverages	3,200,000,000
Maximum assessments for single incidents	27,000,000
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, 2018USD ($)bond	Dec. 31, 2017USD ($)	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	$ 481	$ 551
Defined Benefit Plan Assumptions Used Calculating Benefit Obligation, Change in Discount Rate	0.75%
Number of high-quality corporate bonds | bond	600
Defined benefit plan, estimated future employer contributions during the next five years	$ 200
Amortization basis, straight line, in years	10 years
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	$ (560)	$ (534)
Expected return on plan assets	7.00%	7.00%	7.00%
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	$ 79	$ (17)
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	$ 20
Maximum
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, estimated future employer contributions during the next five years	$ 70
Union Electric Company | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Future funding requirement, percentage	30.00%
Ameren Illinois Company | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Future funding requirement, percentage	60.00%

Retirement Benefits (Summary Of

Retirement Benefits (Summary Of Benefit Liability Recorded) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	$ 481	$ 551
Union Electric Company
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	229	215
Ameren Illinois Company
Defined Benefit Plan Disclosure [Line Items]
Benefit liability recorded on the balance sheet	$ 120	$ 213

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Change in plan assets:
Funded status – deficiency (surplus)	$ (481)	$ (551)
Amounts recognized in the balance sheet consist of:
Noncurrent liability	558	545
Net liability (asset) recognized	481	551
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation at end of year	4,258	4,577
Change in benefit obligation:
Net benefit obligation at beginning of year	4,827	4,518
Service cost	100	93	$ 81
Interest cost	169	179	185
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment	0	0
Participant contributions	0	0
Actuarial (gain) loss	(401)	255
Benefits paid	(236)	(218)
Net benefit obligation at end of year	4,459	4,827	4,518
Change in plan assets:
Fair value of plan assets at beginning of year	4,293	3,813
Actual return on plan assets	(218)	634
Employer contributions	60	64	57
Participant contributions	0	0
Benefits paid	(236)	(218)
Fair value of plan assets at end of year	3,899	4,293	3,813
Funded status – deficiency (surplus)	560	534
Accrued benefit cost (asset) at December 31	560	534
Amounts recognized in the balance sheet consist of:
Noncurrent asset	0	0
Current liability(c)	2	3
Noncurrent liability	558	531
Net liability (asset) recognized	560	534
Amounts recognized in regulatory assets consist of:
Net actuarial (gain) loss	393	374
Prior service credit	(2)	(3)
Amounts (pretax) recognized in accumulated OCI consist of:
Net actuarial loss	35	30
Total	426	401
Postretirement Benefits
Change in benefit obligation:
Net benefit obligation at beginning of year	1,240	1,170
Service cost	21	21	19
Interest cost	40	47	50
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment	(49)	0
Participant contributions	9	8
Actuarial (gain) loss	(163)	53
Benefits paid	(64)	(59)
Net benefit obligation at end of year	1,034	1,240	1,170
Change in plan assets:
Fair value of plan assets at beginning of year	1,223	1,101
Actual return on plan assets	(57)	171
Employer contributions	2	2	2
Participant contributions	9	8
Benefits paid	(64)	(59)
Fair value of plan assets at end of year	1,113	1,223	$ 1,101
Funded status – deficiency (surplus)	(79)	17
Accrued benefit cost (asset) at December 31	(79)	17
Amounts recognized in the balance sheet consist of:
Noncurrent asset	(79)	0
Current liability(c)	0	3
Noncurrent liability	0	14
Net liability (asset) recognized		17
Amounts recognized in regulatory assets consist of:
Net actuarial (gain) loss	(91)	(69)
Prior service credit	(48)	(3)
Amounts (pretax) recognized in accumulated OCI consist of:
Net actuarial loss	3	2
Total	$ (136)	$ (70)

Retirement Benefits (Assumption

Retirement Benefits (Assumptions Used To Determine Benefit Obligations) (Details)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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	4.25%	3.50%
Increase in future compensation	3.50%	3.50%
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate at measurement date	4.25%	3.50%
Increase in future compensation	3.50%	3.50%
Medical cost trend rate (initial)	5.00%	5.00%	5.00%
Medical cost trend rate (ultimate)	5.00%	5.00%	5.00%

Retirement Benefits (Cash Contr

Retirement Benefits (Cash Contributions Made To Benefit Plans) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	$ 60	$ 64	$ 57
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	2	2	2
Union Electric Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	18	19	21
Union Electric Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	1	1	1
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	35	37	30
Ameren Illinois Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	1	1	1
Other | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	7	8	6
Other | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Cash contributions to benefit plans	$ 0	$ 0	$ 0

Retirement Benefits (Target All

Retirement Benefits (Target Allocation Of The Plans' Asset Categories) (Details)	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	100.00%	100.00%
Pension Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	1.00%	1.00%
Pension Benefits | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	52.00%	57.00%
Pension Benefits | U.S. large capitalization
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	24.00%	34.00%
Pension Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	7.00%	9.00%
Pension Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	13.00%	14.00%
Pension Benefits | Global
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	8.00%	0.00%
Pension Benefits | Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	42.00%	37.00%
Pension Benefits | Real estate
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	5.00%	5.00%
Pension Benefits | Private equity
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	1.00%	1.00%
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	100.00%	100.00%
Postretirement Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	2.00%	2.00%
Postretirement Benefits | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	60.00%	63.00%
Postretirement Benefits | U.S. large capitalization
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	40.00%	41.00%
Postretirement Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	7.00%	8.00%
Postretirement Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	13.00%	14.00%
Postretirement Benefits | Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Percentage of Plan Assets at December 31,	38.00%	35.00%
Minimum | Pension Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%
Minimum | Pension Benefits | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	51.00%
Minimum | Pension Benefits | U.S. large capitalization
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	21.00%
Minimum | Pension Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	3.00%
Minimum | Pension Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	9.00%
Minimum | Pension Benefits | Global
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	3.00%
Minimum | Pension Benefits | Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	35.00%
Minimum | Pension Benefits | Real estate
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%
Minimum | Pension Benefits | Private equity
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%
Minimum | Postretirement Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	0.00%
Minimum | Postretirement Benefits | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	55.00%
Minimum | Postretirement Benefits | U.S. large capitalization
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	34.00%
Minimum | Postretirement Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	2.00%
Minimum | Postretirement Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	9.00%
Minimum | Postretirement Benefits | Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	33.00%
Maximum | Pension Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	5.00%
Maximum | Pension Benefits | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	61.00%
Maximum | Pension Benefits | U.S. large capitalization
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	31.00%
Maximum | Pension Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	13.00%
Maximum | Pension Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	19.00%
Maximum | Pension Benefits | Global
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	13.00%
Maximum | Pension Benefits | Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	45.00%
Maximum | Pension Benefits | Real estate
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	9.00%
Maximum | Pension Benefits | Private equity
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	5.00%
Maximum | Postretirement Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	7.00%
Maximum | Postretirement Benefits | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	65.00%
Maximum | Postretirement Benefits | U.S. large capitalization
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	44.00%
Maximum | Postretirement Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	12.00%
Maximum | Postretirement Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	19.00%
Maximum | Postretirement Benefits | Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Target Allocation, Percentage	43.00%

Retirement Benefits (Fair Value

Retirement Benefits (Fair Value Of Plan Assets Utilizing Fair Value Hierarchy) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 3,899	$ 4,293	$ 3,813
Pension Benefits | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	497	566
Pension Benefits | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,690	1,640
Pension Benefits | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,839	2,213
Pension Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	41	25
Pension Benefits | Cash And Cash Equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Cash And Cash Equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Cash And Cash Equivalents | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Cash And Cash Equivalents | Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	41	25
Pension Benefits | U.S. large capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	955	1,523
Pension Benefits | U.S. large capitalization | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | U.S. large capitalization | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | U.S. large capitalization | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | U.S. large capitalization | Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	955	1,523
Pension Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	272	379
Pension Benefits | U.S. small and mid-capitalization | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	272	379
Pension Benefits | U.S. small and mid-capitalization | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | U.S. small and mid-capitalization | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | U.S. small and mid-capitalization | Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	522	629
Pension Benefits | International | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	224	179
Pension Benefits | International | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | International | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | International | Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	298	450
Pension Benefits | Global
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	321
Pension Benefits | Global | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0
Pension Benefits | Global | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0
Pension Benefits | Global | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0
Pension Benefits | Global | Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	321
Pension Benefits | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	720	741
Pension Benefits | Corporate Bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Corporate Bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	701	726
Pension Benefits | Corporate Bonds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Corporate Bonds | Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	19	15
Pension Benefits | Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	87	91
Pension Benefits | Municipal Bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Municipal Bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	87	91
Pension Benefits | Municipal Bonds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Municipal Bonds | Measured at NAV
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	891	824
Pension Benefits | U.S. treasury and agency securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	8
Pension Benefits | U.S. treasury and agency securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	891	816
Pension Benefits | U.S. treasury and agency securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | U.S. treasury and agency securities | Measured at NAV
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	12	7
Pension Benefits | Other | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	0
Pension Benefits | Other | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11	7
Pension Benefits | Other | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Other | Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Real estate
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	202	196
Pension Benefits | Real estate | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Real estate | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Real estate | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Real estate | Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	202	196
Pension Benefits | Private equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	4
Pension Benefits | Private equity | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Private equity | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Private equity | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Pension Benefits | Private equity | Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	4
Pension Benefits | Medical benefit assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(144)	(153)
Pension Benefits | Net receivables
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	17	27
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	4,026	4,419
Pension Benefits | Excludes Medical Benefit Component Under Section401 H And Includes Receivables Related To Pending Security Sales [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,899	4,293
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,113	1,223	$ 1,101
Postretirement Benefits | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	437	509
Postretirement Benefits | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	332	342
Postretirement Benefits | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	207	245
Postretirement Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	32	44
Postretirement Benefits | Cash And Cash Equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	32	44
Postretirement Benefits | Cash And Cash Equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | Cash And Cash Equivalents | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | Cash And Cash Equivalents | Measured at NAV
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	386	442
Postretirement Benefits | U.S. large capitalization | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	297	332
Postretirement Benefits | U.S. large capitalization | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | U.S. large capitalization | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | U.S. large capitalization | Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	89	110
Postretirement Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	63	80
Postretirement Benefits | U.S. small and mid-capitalization | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	63	80
Postretirement Benefits | U.S. small and mid-capitalization | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | U.S. small and mid-capitalization | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | U.S. small and mid-capitalization | Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	129	154
Postretirement Benefits | International | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	45	53
Postretirement Benefits | International | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | International | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | International | Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	84	101
Postretirement Benefits | Other Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12	8
Postretirement Benefits | Other Equity | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | Other Equity | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12	8
Postretirement Benefits | Other Equity | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | Other Equity | Measured at NAV
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	144
Postretirement Benefits | Corporate Bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | Corporate Bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	144	144
Postretirement Benefits | Corporate Bonds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | Corporate Bonds | Measured at NAV
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	107	110
Postretirement Benefits | Municipal Bonds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | Municipal Bonds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	107	110
Postretirement Benefits | Municipal Bonds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | Municipal Bonds | Measured at NAV
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	62	76
Postretirement Benefits | U.S. treasury and agency securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | U.S. treasury and agency securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	62	76
Postretirement Benefits | U.S. treasury and agency securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | U.S. treasury and agency securities | Measured at NAV
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	41	38
Postretirement Benefits | Other | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | Other | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7	4
Postretirement Benefits | Other | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Postretirement Benefits | Other | Measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	34	34
Postretirement Benefits | Medical benefit assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	144	153
Postretirement Benefits | Net payables
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	(7)	(26)
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	976	1,096
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	$ 1,113	$ 1,223

Retirement Benefits (Components

Retirement Benefits (Components Of Net Periodic Benefit Cost) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Non-service Cost or Income Components	$ 70	$ 44	$ 56
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Non-service Cost or Income Components	(40)	(29)	(37)
Service cost	100	93	81
Interest cost	169	179	185
Expected return on plan assets	(276)	(262)	(253)
Prior service credit	(1)	(1)	(1)
Actuarial (gain) loss	68	55	32
Net periodic benefit cost (income)	60	64	44
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Non-service Cost or Income Components	(47)	(39)	(38)
Service cost	21	21	19
Interest cost	40	47	50
Expected return on plan assets	(77)	(75)	(72)
Prior service credit	(4)	(5)	(5)
Actuarial (gain) loss	(6)	(6)	(11)
Net periodic benefit cost (income)	(26)	(18)	(19)
Ameren Illinois Company
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Non-service Cost or Income Components	34	10	24
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (income)	39	41	22
Ameren Illinois Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (income)	(25)	(14)	(13)
Union Electric Company
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Non-service Cost or Income Components	17	22	18
Union Electric Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (income)	22	24	26
Union Electric Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost (income)	$ (1)	$ (4)	$ (5)

Retirement Benefits (Summary _2

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, 2018USD ($)
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	$ (1)
Net actuarial (gain) loss	24
Net actuarial loss	2
Net periodic benefit cost	25
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Prior service credit	(5)
Net actuarial (gain) loss	(15)
Net actuarial loss	0
Net periodic benefit cost	$ (20)

Retirement Benefits (Summary _3

Retirement Benefits (Summary Of Benefit Plan Costs Incurred) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 60	$ 64	$ 44
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	(26)	(18)	(19)
Union Electric Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	22	24	26
Union Electric Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	(1)	(4)	(5)
Ameren Illinois Company | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	39	41	22
Ameren Illinois Company | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	(25)	(14)	(13)
Other | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	(1)	(1)	(4)
Other | Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 0	$ 0	$ (1)

Retirement Benefits (Schedule O

Retirement Benefits (Schedule Of Expected Payments From Qualified Trust And Company Funds) (Details) $ in Millions	Dec. 31, 2018USD ($)
Pension Benefits | Paid From Qualified Trust
Defined Benefit Plan Disclosure [Line Items]
2,019	$ 267
2,020	272
2,021	282
2,022	285
2,023	286
2024 - 2028	1,439
Pension Benefits | Paid From Company Funds
Defined Benefit Plan Disclosure [Line Items]
2,019	3
2,020	3
2,021	3
2,022	3
2,023	3
2024 - 2028	12
Postretirement Benefits | Paid From Qualified Trust
Defined Benefit Plan Disclosure [Line Items]
2,019	57
2,020	59
2,021	61
2,022	62
2,023	64
2024 - 2028	315
Postretirement Benefits | Paid From Company Funds
Defined Benefit Plan Disclosure [Line Items]
2,019	2
2,020	2
2,021	2
2,022	2
2,023	2
2024 - 2028	$ 12

Retirement Benefits (Assumpti_2

Retirement Benefits (Assumptions Used To Determine Net Periodic Benefit Cost) (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate at measurement date	3.50%	4.00%	4.50%
Expected return on plan assets	7.00%	7.00%	7.00%
Increase in future compensation	3.50%	3.50%	3.50%
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate at measurement date	3.50%	4.00%	4.50%
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 (Schedule_2

Retirement Benefits (Schedule Of Potential Changes In Key Assumptions) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service Cost and Interest Cost, .25% decrease in discount rate	$ (2)
Benefit Obligation, .25% decrease in discount rate	135
Service Cost and Interest Cost, .25% increase in salary rate	2
Benefit Obligation, .25% increase in salary rate	12
Service Cost and Interest Cost, 1.00% increase in annual medical trend	0
Benefit Obligation, 1.00% increase in annual medical trend	0
Service Cost and Interest Cost, 1.00% decrease in annual medical trend	0
Benefit Obligation, 1.00% decrease in annual medical trend	0
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	33
Service Cost and Interest Cost, .25% increase in salary rate	0
Benefit Obligation, .25% increase in salary rate	0
Service Cost and Interest Cost, 1.00% increase in annual medical trend	4
Benefit Obligation, 1.00% increase in annual medical trend	58
Service Cost and Interest Cost, 1.00% decrease in annual medical trend	(4)
Benefit Obligation, 1.00% decrease in annual medical trend	$ (58)

Retirement Benefits (Schedule_3

Retirement Benefits (Schedule Of Matching Contributions) (Details) - United States Postretirement Benefit Plan of US Entity - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 33	$ 30	$ 29
Union Electric Company
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	17	16	16
Ameren Illinois Company
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	15	13	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, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized (in shares)	8,000,000
Maximum shares available for grants (in shares)	3,800,000
Performance Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award Requisite Service Period	5 years
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	$ 54	$ 39	$ 83
Compensation cost not yet recognized	$ 29
Expected weighted average recognition period for share-based compensation expense, in months	22 months
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	38 months
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award Requisite Service Period	5 years
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period	38 months
Income Taxes
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Employee Service Share-based Compensation, Tax Benefit from Exercise of Stock Options	$ 13	$ 15	$ 31

Stock-Based Compensation (Summa

Stock-Based Compensation (Summary Of Nonvested Shares) (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Performance Share Units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award Requisite Service Period	5 years
Fully Vested Undistributed Retirement-eligible units (in shares)	619,783	712,572
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 (in shares)	895,489
Share Units, Granted (in shares)	316,875
Share Units, Unearned or forfeited (in shares)	(65,106)
Undistributed Vested Units (in shares)	(288,404)
Share Units, Vested and distributed (in shares)	(176,043)
Share Units, Nonvested at end of year (in shares)	682,811	895,489
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 (in dollars per share)	$ 52.28
Weighted-average Fair Value per Unit, Granted (in dollars per share)	62.88	$ 59.16	$ 44.13
Weighted-average Fair Value per Unit, Unearned or forfeited (in dollars per share)	51.11
Weighted-average Fair Value per Unit, Vested and undistributed (in dollars per share)	53.63
Weighted-average Fair Value per Unit, Vested and distributed (in dollars per share0	52.88
Weighted-average Fair Value per Unit, Nonvested at end of year (in dollars per share)	$ 56.58	$ 52.28
Performance period	3 years
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Fully Vested Undistributed Retirement-eligible units (in shares)	26,557
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 (in shares)	0
Share Units, Granted (in shares)	187,273
Share Units, Unearned or forfeited (in shares)	(5,463)
Undistributed Vested Units (in shares)	(26,557)
Share Units, Vested and distributed (in shares)	0
Share Units, Nonvested at end of year (in shares)	155,253	0
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 (in dollars per share)	$ 0
Weighted-average Fair Value per Unit, Granted (in dollars per share)	57.66
Weighted-average Fair Value per Unit, Unearned or forfeited (in dollars per share)	58.99
Weighted-average Fair Value per Unit, Vested and undistributed (in dollars per share)	59.02
Weighted-average Fair Value per Unit, Vested and distributed (in dollars per share0	0
Weighted-average Fair Value per Unit, Nonvested at end of year (in dollars per share)	$ 57.38	$ 0

Stock-Based Compensation (Sum_2

Stock-Based Compensation (Summary of Expense) (Details) - Performance Share Units - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 20	$ 18	$ 17
Employee service share-based compensation, tax benefit from compensation expense	6	7	6
Share-based Compensation Expense, Net of Tax	$ 14	$ 11	$ 11
Performance period	3 years
Weighted-average Fair Value per Unit, Granted (in dollars per share)	$ 62.88	$ 59.16	$ 44.13
Closing common share price (in dollars per share)		$ 61.69	$ 52.46	$ 43.23
Three-year risk-free rate	1.98%	1.47%	1.31%
Volatility rate, minimum	15.00%	15.00%	15.00%
Volatility rate, maximum	23.00%	21.00%	20.00%
Ameren Missouri [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 4	$ 4	$ 4
Ameren Illinois Company
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	3	2	2
Other
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation expense	$ 13	$ 12	$ 11

Income Taxes (Narrative) (Detai

Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Federal statutory corporate income tax rate:	21.00%	35.00%	35.00%
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	$ 13
Deferred Tax Liabilities, Net	$ 2,623	$ 2,506
Minimum
Income Taxes [Line Items]
Regulatory Liability, Amortization Period	30 years
Maximum
Income Taxes [Line Items]
Regulatory Liability, Amortization Period	60 years
Parent Company
Income Taxes [Line Items]
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	$ 5
Other
Income Taxes [Line Items]
Deferred Tax Liabilities, Net	$ (30)	$ (165)
Union Electric Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate:	21.00%	35.00%	35.00%
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	$ 4
Deferred Tax Liabilities, Net	$ 1,534	$ 1,650
Ameren Illinois Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate:	21.00%	35.00%	35.00%
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	$ 4
Deferred Tax Liabilities, Net	$ 1,119	$ 1,021
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 | Union Electric Company
Income Taxes [Line Items]
State Income Tax Statutory Rate	4.00%
Deferred Tax Liabilities, Net	$ 122
State | Union Electric Company | Maximum
Income Taxes [Line Items]
State Income Tax Statutory Rate	6.25%
State | Ameren Illinois Company
Income Taxes [Line Items]
Deferred Tax Liabilities, Net		79
Increase (Decrease) in Income Taxes		$ 1

Income Taxes (Schedule of Remea

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, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Deferred Federal Income Tax Expense (Benefit)	$ 220	$ 511	$ 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
Parent Company | Tax Cuts and Jobs Act [Member]
Income Taxes [Line Items]
Deferred Federal Income Tax Expense (Benefit)		110
Union Electric Company
Income Taxes [Line Items]
Deferred Federal Income Tax Expense (Benefit)	22	76	161
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)	75	185	117
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)	$ 123	250	$ 21
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, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Federal statutory corporate income tax rate:	21.00%	35.00%	35.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent	(4.00%)
Other depreciation differences	0.00%
Amortization of deferred investment tax credit	(1.00%)	(1.00%)	(0.00%)
State tax	6.00%	6.00%	4.00%
TCJA	1.00%	14.00%
Tax Credits	(0.00%)	(0.00%)
Stock-based compensation			(2.00%)
Other permanent items	(1.00%)	(2.00%)	(1.00%)
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent			1.00%
Effective income tax rate	22.00%	52.00%	37.00%
Adjustments related to Income Tax Benefit from Share-Based Payment			$ 21
Union Electric Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate:	21.00%	35.00%	35.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent	(4.00%)
Other depreciation differences	0.00%	1.00%	1.00%
Amortization of deferred investment tax credit	(1.00%)	(1.00%)	(1.00%)
State tax	4.00%	4.00%	3.00%
TCJA	1.00%	6.00%
Tax Credits	(1.00%)	(1.00%)
Stock-based compensation			0.00%
Other permanent items	0.00%	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent			0.00%
Effective income tax rate	20.00%	44.00%	38.00%
Ameren Illinois Company
Income Taxes [Line Items]
Federal statutory corporate income tax rate:	21.00%	35.00%	35.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent	(4.00%)
Other depreciation differences	(1.00%)	(1.00%)
Amortization of deferred investment tax credit	(0.00%)	(0.00%)	(0.00%)
State tax	7.00%	6.00%	5.00%
TCJA	1.00%	(1.00%)
Tax Credits	(0.00%)	(0.00%)
Stock-based compensation			0.00%
Other permanent items		(1.00%)	(2.00%)
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent			0.00%
Effective income tax rate	24.00%	38.00%	38.00%

Income Taxes (Schedule Of Compo

Income Taxes (Schedule Of Components Of Income Tax Expense (Benefit)) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Current Federal taxes	$ (10)	$ 5	$ (1)
Current State taxes	23	32	(3)
Deferred Federal Income Tax Expense (Benefit)	220	511	299
Deferred State taxes	49	34	92
Amortization of Excess Deferred Taxes	(40)
Deferred investment tax credits, amortization	(5)	(6)	(5)
Total income tax expense	237	576	382
Union Electric Company
Income Taxes [Line Items]
Current Federal taxes	104	149	31
Current State taxes	29	23	6
Deferred Federal Income Tax Expense (Benefit)	22	76	161
Deferred State taxes	(2)	11	23
Amortization of Excess Deferred Taxes	(24)
Deferred investment tax credits, amortization	(5)	(5)	(5)
Total income tax expense	124	254	216
Ameren Illinois Company
Income Taxes [Line Items]
Current Federal taxes	4	(34)	(8)
Current State taxes	6	29	12
Deferred Federal Income Tax Expense (Benefit)	75	185	117
Deferred State taxes	28	(13)	37
Amortization of Excess Deferred Taxes	(15)
Deferred investment tax credits, amortization	0	(1)	0
Total income tax expense	98	166	158
Other
Income Taxes [Line Items]
Current Federal taxes	(118)	(110)	(24)
Current State taxes	(12)	(20)	(21)
Deferred Federal Income Tax Expense (Benefit)	123	250	21
Deferred State taxes	23	36	32
Amortization of Excess Deferred Taxes	(1)
Deferred investment tax credits, amortization	0	0	0
Total income tax expense	$ 15	$ 156	$ 8

Income Taxes (Schedule Of Defer

Income Taxes (Schedule Of Deferred Tax Assets And Liabilities Resulting From Temporary Differences) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Income Taxes [Line Items]
Plant related	$ 3,534	$ 3,474
Regulatory assets, net	(589)	(547)
Deferred benefit costs	(126)	(131)
Revenue Requirement Reconciliation Adjustment		20
Tax carryforwards	(227)	(361)
Other	31	51
Total net accumulated deferred income tax liabilities	2,623	2,506
Union Electric Company
Income Taxes [Line Items]
Plant related	2,010	2,064
Regulatory assets, net	(343)	(317)
Deferred benefit costs	(58)	(53)
Revenue Requirement Reconciliation Adjustment		0
Tax carryforwards	(35)	(31)
Other	(40)	(13)
Total net accumulated deferred income tax liabilities	1,534	1,650
Ameren Illinois Company
Income Taxes [Line Items]
Plant related	1,345	1,264
Regulatory assets, net	(221)	(206)
Deferred benefit costs	(4)	(17)
Revenue Requirement Reconciliation Adjustment		20
Tax carryforwards	(26)	(43)
Other	25	3
Total net accumulated deferred income tax liabilities	1,119	1,021
Other
Income Taxes [Line Items]
Plant related	179	146
Regulatory assets, net	(25)	(24)
Deferred benefit costs	(64)	(61)
Revenue Requirement Reconciliation Adjustment		0
Tax carryforwards	(166)	(287)
Other	46	61
Total net accumulated deferred income tax liabilities	$ (30)	$ (165)

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, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	$ 91	$ 235
Tax credit carryforwards	127	120
Deferred Tax Assets, Charitable Contribution Carryforwards	14	11
Deferred Tax Assets, Valuation Allowance, Noncurrent	(5)	(5)
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	9	6
Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	78	203
Tax credit carryforwards	117	113
State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	13	32
Tax credit carryforwards	10	7
Union Electric Company
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	0	0
Tax credit carryforwards	35	31
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	0
Tax credit carryforwards	35	31
Union Electric Company | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	0	0
Tax credit carryforwards	0	0
Ameren Illinois Company
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	23	41
Tax credit carryforwards	3	2
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	23	41
Tax credit carryforwards	3	2
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	68	194
Tax credit carryforwards	89	87
Deferred Tax Assets, Charitable Contribution Carryforwards	14	11
Deferred Tax Assets, Valuation Allowance, Noncurrent	(5)	(5)
Deferred Tax Assets, Net of Valuation Allowance, Noncurrent	9	6
Other | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	55	162
Tax credit carryforwards	79	80
Other | State
Operating Loss Carryforwards [Line Items]
Net operating loss carryforwards	13	32
Tax credit carryforwards	$ 10	$ 7
Minimum | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward, expiration period start	Jan. 1, 2034
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, 2034
Tax credit carryforward, expiration period start	Jan. 1, 2019
Minimum | Federal Contribution
Operating Loss Carryforwards [Line Items]
Tax credit carryforward, expiration period start	Jan. 1, 2019
Maximum | Federal
Operating Loss Carryforwards [Line Items]
Net operating loss carryforward, expiration period start	Jan. 1, 2037
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, 2037
Tax credit carryforward, expiration period start	Jan. 1, 2022
Maximum | Federal Contribution
Operating Loss Carryforwards [Line Items]
Tax credit carryforward, expiration period start	Jan. 1, 2023

Related Party Transactions (Det

Related Party Transactions (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Related Party Transaction [Line Items]
Operating Revenues	$ 0	$ 0	$ 0
Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	139	149	129
Union Electric Company | Ameren Missouri Power Supply Agreements with Ameren Illinois
Related Party Transaction [Line Items]
Operating Revenues	11	23	28
Union Electric Company | Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	22	26	25
Operating Expenses	3	1	1
Union Electric Company | Ameren Missouri and Ameren Illinois miscellaneous support services and services provided to ATXI
Related Party Transaction [Line Items]
Operating Revenues	1	1	1
Union Electric Company | Total Operating Revenues
Related Party Transaction [Line Items]
Operating Revenues	34	49	54
Union Electric Company | Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	136	149	129
Union Electric Company | Money Pool borrowings (advances)
Related Party Transaction [Line Items]
Interest (Charges) Income	1	1	1
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	0	0	0
Operating Expenses	132	139	123
Ameren Illinois Company | Ameren Missouri and Ameren Illinois Rent and Facility Services
Related Party Transaction [Line Items]
Operating Revenues	3	4	5
Operating Expenses	6	1	1
Ameren Illinois Company | Ameren Missouri and Ameren Illinois miscellaneous support services and services provided to ATXI
Related Party Transaction [Line Items]
Operating Revenues	1	1	1
Ameren Illinois Company | Total Operating Revenues
Related Party Transaction [Line Items]
Operating Revenues	4	5	5
Ameren Illinois Company | Ameren Illinois Power Supply Agreements with Ameren Missouri
Related Party Transaction [Line Items]
Operating Expenses	11	23	28
Ameren Illinois Company | Ameren Illinois transmission services from ATXI
Related Party Transaction [Line Items]
Operating Expenses	1	2	2
Ameren Illinois Company | Purchased Power
Related Party Transaction [Line Items]
Operating Expenses	12	25	30
Ameren Illinois Company | Ameren Services Support Services Agreement
Related Party Transaction [Line Items]
Operating Expenses	126	139	123
Ameren Illinois Company | Money Pool borrowings (advances)
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, 2018USD ($)MWh$ / MWh
May 31, 2017 | Union Electric Company
Related Party Transaction [Line Items]
Energy Supply Agreements Amount | $	$ 15
May 2014 Procurement | 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 | 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 | 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 | 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 | 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 | 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
April 2018 Procurement | 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	110,000,000,000
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate | $ / MWh	32,000,000

Related Party Transactions (Sch

Related Party Transactions (Schedule of affiliate receivables and payables) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Union Electric Company
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	$ 69	$ 60
Accounts Receivable, Related Parties, Current	14	11
Union Electric Company | Income taxes payable to parent
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	16	11
Union Electric Company | Income taxes receivable from parent
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	0	0
Ameren Illinois Company
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	58	70
Accounts Receivable, Related Parties, Current	21	9
Ameren Illinois Company | Income taxes payable to parent
Related Party Transaction [Line Items]
Accounts Payable, Related Parties, Current	7	17
Ameren Illinois Company | Income taxes receivable from parent
Related Party Transaction [Line Items]
Accounts Receivable, Related Parties, Current	$ 6	$ 0

Related Party Transactions (S_2

Related Party Transactions (Schedule of Capital Contributions) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Union Electric Company
Related Party Transaction [Line Items]
Proceeds from Contributions from Parent	$ 45	$ 30	$ 44
Noncash Or Part Noncash Capital Contribution From Parent			38
Ameren Illinois Company
Related Party Transaction [Line Items]
Proceeds from Contributions from Parent	$ 160	$ 8	$ 0

Commitments And Contingencies_2

Commitments And Contingencies (Schedule Of Lease Obligations) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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	33
Capital lease payments, due in four years	32
Capital lease payments, due in five years	264
Capital lease payments, After 5 Years	0
Capital lease payments, Total	393
Less Amount representing interest, due in one year	25
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	24
Less Amount representing interest, due in 5 years	24
Less Amount representing interest, After 5 Years	0
Less Amount representing interest, Total	123
Present value of minimum capital lease payments, due in one year	7
Present value of minimum capital lease payments, due in two years	7
Present value of minimum capital lease payments, due in three years	8
Present value of minimum capital lease payments, due in four years	8
Present value of minimum capital lease payments, due in five years	240
Present value of minimum capital lease payments, After 5 Years	0
Present value of minimum capital lease payments, Total	270
Operating leases, due in one year	10
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	5
Operating leases, After 5 Years	9
Operating leases, Total	45
Total lease obligations, due in one year	17
Total lease obligations, due in two years	15
Total lease obligations, due in three years	15
Total lease obligations, due in four years	14
Total lease obligations, due in five years	245
Total lease obligations, After 5 Years	9
Total lease obligations, Total	315
Total rental expense	9	$ 11	$ 38
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	33
Capital lease payments, due in four years	32
Capital lease payments, due in five years	264
Capital lease payments, After 5 Years	0
Capital lease payments, Total	393
Less Amount representing interest, due in one year	25
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	24
Less Amount representing interest, due in 5 years	24
Less Amount representing interest, After 5 Years	0
Less Amount representing interest, Total	123
Present value of minimum capital lease payments, due in one year	7
Present value of minimum capital lease payments, due in two years	7
Present value of minimum capital lease payments, due in three years	8
Present value of minimum capital lease payments, due in four years	8
Present value of minimum capital lease payments, due in five years	240
Present value of minimum capital lease payments, After 5 Years	0
Present value of minimum capital lease payments, Total	270
Operating leases, due in one year	8
Operating leases, due in two years	7
Operating leases, due in three years	6
Operating leases, due in four years	5
Operating leases, due in five years	5
Operating leases, After 5 Years	9
Operating leases, Total	40
Total lease obligations, due in one year	15
Total lease obligations, due in two years	14
Total lease obligations, due in three years	14
Total lease obligations, due in four years	13
Total lease obligations, due in five years	245
Total lease obligations, After 5 Years	9
Total lease obligations, Total	310
Total rental expense	8	10	34
Ameren Illinois Company
Commitments and Contingencies [Line Items]
Operating leases, due in one year	1
Operating leases, due in two years	0
Operating leases, due in three years	0
Operating leases, due in four years	0
Operating leases, due in five years	0
Operating leases, After 5 Years	0
Operating leases, Total	1
Total rental expense	$ 1	$ 1	$ 30

Commitments And Contingencies_3

Commitments And Contingencies (Schedule Of Estimated Purchased Power Commitments) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018USD ($)MWh
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	$ 799
Unrecorded Unconditional Purchase Obligation, Due within Two Years	445
Unrecorded Unconditional Purchase Obligation, Due within Three Years	301
Unrecorded Unconditional Purchase Obligation, Due within Four Years	142
Unrecorded Unconditional Purchase Obligation, Due within Five Years	79
Unrecorded Unconditional Purchase Obligation, Due after Five Years	166
Unrecorded Unconditional Purchase Obligation	1,932
Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	349
Unrecorded Unconditional Purchase Obligation, Due within Two Years	160
Unrecorded Unconditional Purchase Obligation, Due within Three Years	121
Unrecorded Unconditional Purchase Obligation, Due within Four Years	72
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	702
Natural gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	197
Unrecorded Unconditional Purchase Obligation, Due within Two Years	143
Unrecorded Unconditional Purchase Obligation, Due within Three Years	77
Unrecorded Unconditional Purchase Obligation, Due within Four Years	27
Unrecorded Unconditional Purchase Obligation, Due within Five Years	7
Unrecorded Unconditional Purchase Obligation, Due after Five Years	34
Unrecorded Unconditional Purchase Obligation	485
Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	25
Unrecorded Unconditional Purchase Obligation, Due within Two Years	43
Unrecorded Unconditional Purchase Obligation, Due within Three Years	59
Unrecorded Unconditional Purchase Obligation, Due within Four Years	14
Unrecorded Unconditional Purchase Obligation, Due within Five Years	42
Unrecorded Unconditional Purchase Obligation, Due after Five Years	31
Unrecorded Unconditional Purchase Obligation	214
Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	157
Unrecorded Unconditional Purchase Obligation, Due within Two Years	54
Unrecorded Unconditional Purchase Obligation, Due within Three Years	10
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	221
Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	4
Unrecorded Unconditional Purchase Obligation, Due within Two Years	4
Unrecorded Unconditional Purchase Obligation, Due within Three Years	4
Unrecorded Unconditional Purchase Obligation, Due within Four Years	3
Unrecorded Unconditional Purchase Obligation, Due within Five Years	3
Unrecorded Unconditional Purchase Obligation, Due after Five Years	29
Unrecorded Unconditional Purchase Obligation	47
Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	67
Unrecorded Unconditional Purchase Obligation, Due within Two Years	41
Unrecorded Unconditional Purchase Obligation, Due within Three Years	30
Unrecorded Unconditional Purchase Obligation, Due within Four Years	26
Unrecorded Unconditional Purchase Obligation, Due within Five Years	27
Unrecorded Unconditional Purchase Obligation, Due after Five Years	72
Unrecorded Unconditional Purchase Obligation	263
Union Electric Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	467
Unrecorded Unconditional Purchase Obligation, Due within Two Years	264
Unrecorded Unconditional Purchase Obligation, Due within Three Years	225
Unrecorded Unconditional Purchase Obligation, Due within Four Years	120
Unrecorded Unconditional Purchase Obligation, Due within Five Years	75
Unrecorded Unconditional Purchase Obligation, Due after Five Years	130
Unrecorded Unconditional Purchase Obligation	1,281
Union Electric Company | Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	349
Unrecorded Unconditional Purchase Obligation, Due within Two Years	160
Unrecorded Unconditional Purchase Obligation, Due within Three Years	121
Unrecorded Unconditional Purchase Obligation, Due within Four Years	72
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	702
Union Electric Company | Natural gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	40
Unrecorded Unconditional Purchase Obligation, Due within Two Years	31
Unrecorded Unconditional Purchase Obligation, Due within Three Years	15
Unrecorded Unconditional Purchase Obligation, Due within Four Years	5
Unrecorded Unconditional Purchase Obligation, Due within Five Years	3
Unrecorded Unconditional Purchase Obligation, Due after Five Years	14
Unrecorded Unconditional Purchase Obligation	108
Union Electric Company | Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	25
Unrecorded Unconditional Purchase Obligation, Due within Two Years	43
Unrecorded Unconditional Purchase Obligation, Due within Three Years	59
Unrecorded Unconditional Purchase Obligation, Due within Four Years	14
Unrecorded Unconditional Purchase Obligation, Due within Five Years	42
Unrecorded Unconditional Purchase Obligation, Due after Five Years	31
Unrecorded Unconditional Purchase Obligation	214
Union Electric Company | Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	0
Unrecorded Unconditional Purchase Obligation, Due within Two Years	0
Unrecorded Unconditional Purchase Obligation, Due within Three Years	0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	$ 0
Amount of Megawatts | MWh	102
Union Electric Company | Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	$ 4
Unrecorded Unconditional Purchase Obligation, Due within Two Years	4
Unrecorded Unconditional Purchase Obligation, Due within Three Years	4
Unrecorded Unconditional Purchase Obligation, Due within Four Years	3
Unrecorded Unconditional Purchase Obligation, Due within Five Years	3
Unrecorded Unconditional Purchase Obligation, Due after Five Years	29
Unrecorded Unconditional Purchase Obligation	47
Union Electric Company | Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	49
Unrecorded Unconditional Purchase Obligation, Due within Two Years	26
Unrecorded Unconditional Purchase Obligation, Due within Three Years	26
Unrecorded Unconditional Purchase Obligation, Due within Four Years	26
Unrecorded Unconditional Purchase Obligation, Due within Five Years	27
Unrecorded Unconditional Purchase Obligation, Due after Five Years	56
Unrecorded Unconditional Purchase Obligation	210
Ameren Illinois Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	322
Unrecorded Unconditional Purchase Obligation, Due within Two Years	171
Unrecorded Unconditional Purchase Obligation, Due within Three Years	72
Unrecorded Unconditional Purchase Obligation, Due within Four Years	22
Unrecorded Unconditional Purchase Obligation, Due within Five Years	4
Unrecorded Unconditional Purchase Obligation, Due after Five Years	20
Unrecorded Unconditional Purchase Obligation	611
Ameren Illinois Company | Natural gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	157
Unrecorded Unconditional Purchase Obligation, Due within Two Years	112
Unrecorded Unconditional Purchase Obligation, Due within Three Years	62
Unrecorded Unconditional Purchase Obligation, Due within Four Years	22
Unrecorded Unconditional Purchase Obligation, Due within Five Years	4
Unrecorded Unconditional Purchase Obligation, Due after Five Years	20
Unrecorded Unconditional Purchase Obligation	377
Ameren Illinois Company | Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	157
Unrecorded Unconditional Purchase Obligation, Due within Two Years	54
Unrecorded Unconditional Purchase Obligation, Due within Three Years	10
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	221
Ameren Illinois Company | Other
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	8
Unrecorded Unconditional Purchase Obligation, Due within Two Years	5
Unrecorded Unconditional Purchase Obligation, Due within Three Years	0
Unrecorded Unconditional Purchase Obligation, Due within Four Years	0
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	13
Ameren Illinois Company | Zero Emission Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation	$ 26

Commitments And Contingencies_4

Commitments And Contingencies (Environmental Matters) (Details) $ in Millions	Dec. 31, 2018USD ($)site	Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Sep. 30, 2017scrubber	Dec. 31, 2016USD ($)
Percentage of Rate Base Related to Carbon Dioxide Energy Centers	17.00%
Asset Retirement Obligation	$ 650		$ 644		$ 650
Union Electric Company
Percentage of Rate Base Related to Carbon Dioxide Energy Centers	32.00%
Number of Energy Center Scrubbers | scrubber				2
Asset Retirement Obligation	$ 646		640		644
Ameren Illinois Company
Asset Retirement Obligation	4		$ 4		$ 6
Manufactured Gas Plant
Accrual for environmental loss contingencies	$ 150
Manufactured Gas Plant | Ameren Illinois Company
Number of remediation sites | site	44
Accrual for environmental loss contingencies	$ 150
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	300
Minimum | Coal Combustion Residuals Estimate [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	150
Minimum | Manufactured Gas Plant | Ameren Illinois Company
Loss contingency, estimate of possible loss	150
Maximum
Estimated capital costs to comply with existing and known federal and state air emissions regulations	400
Maximum | Coal Combustion Residuals Estimate [Member]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	200
Maximum | Manufactured Gas Plant | Ameren Illinois Company
Loss contingency, estimate of possible loss	212
New CCR Rules Estimate [Member]
Asset Retirement Obligation		$ 135
New CCR Rules Estimate [Member] | Union Electric Company
Asset Retirement Obligation	$ 150

Segment Information (Schedule O

Segment Information (Schedule Of Segment Reporting Information By Segment) (Details) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018USD ($)	Sep. 30, 2018USD ($)	Jun. 30, 2018USD ($)	Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2018USD ($)segment	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | segment									4
Revenues	$ 1,419	$ 1,724	$ 1,563	$ 1,585	$ 1,399	$ 1,723	$ 1,537	$ 1,515	$ 6,291	$ 6,174	$ 6,076
Intersegment revenues									0	0	0
Depreciation and amortization									955	896	845
Interest income									33	34	40
Interest charges									401	391	382
Income taxes (benefit)									237	576	382
Net Income (Loss) Available to Common Stockholders, Basic	68	357	239	151	(60)	288	193	102	815	523	653
Capital expenditures									$ 2,286	2,132	2,076
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]
Revenues	674	564	578	760	674	574	576	703	$ 2,576	2,527	2,489
Intersegment revenues									0	0	0
Depreciation and amortization									374	341	319
Interest income									6	7	12
Interest charges									149	144	140
Income taxes (benefit)									98	166	158
Net Income (Loss) Attributable to Parent									304	268	252
Net Income (Loss) Available to Common Stockholders, Basic	$ 84	$ 63	$ 62	$ 95	$ 77	$ 55	$ 57	$ 79
Capital expenditures									1,258	1,076	924
Operating Segments | Union Electric Company
Segment Reporting Information [Line Items]
Revenues									3,555	3,488	3,470
Intersegment revenues									34	49	54
Depreciation and amortization									550	533	514
Interest income									28	27	28
Interest charges									200	207	211
Income taxes (benefit)									124	254	216
Net Income (Loss) Attributable to Parent									478	323	357
Capital expenditures									914	773	738
Operating Segments | Ameren Illinois Electric Distribution [Member]
Segment Reporting Information [Line Items]
Revenues									1,544	1,564	1,544
Intersegment revenues									3	4	4
Depreciation and amortization									259	239	226
Interest income									6	7	11
Interest charges									73	73	72
Income taxes (benefit)									41	83	78
Net Income (Loss) Attributable to Parent									136	131	126
Capital expenditures									503	476	470
Operating Segments | Ameren Illinois Gas [Member]
Segment Reporting Information [Line Items]
Revenues									814	742	753
Intersegment revenues									1	1	1
Depreciation and amortization									65	59	55
Interest income									0	0	0
Interest charges									38	36	34
Income taxes (benefit)									25	36	39
Net Income (Loss) Attributable to Parent									70	60	59
Capital expenditures									311	245	181
Operating Segments | Ameren Transmission [Member]
Segment Reporting Information [Line Items]
Revenues									378	382	309
Intersegment revenues									55	44	46
Depreciation and amortization									77	60	43
Interest income									0	0	1
Interest charges									75	67	58
Income taxes (benefit)									56	90	74
Net Income (Loss) Attributable to Parent									164	140	117
Capital expenditures									562	644	689
Operating Segments | Other Segment
Segment Reporting Information [Line Items]
Revenues									0	(2)	0
Intersegment revenues									0	0	0
Depreciation and amortization									4	5	7
Interest income									4	11	11
Interest charges									19	19	18
Income taxes (benefit)									(9)	113	(25)
Net Income (Loss) Attributable to Parent									(33)	(131)	(6)
Capital expenditures									5	1	4
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution [Member]
Segment Reporting Information [Line Items]
Revenues									1,547	1,568	1,548
Intersegment revenues									0	0	0
Depreciation and amortization									259	239	226
Interest income									6	7	11
Interest charges									73	73	72
Income taxes (benefit)									41	83	78
Net Income (Loss) Attributable to Parent									136	131	126
Capital expenditures									503	476	470
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas [Member]
Segment Reporting Information [Line Items]
Revenues									815	743	754
Intersegment revenues									0	0	0
Depreciation and amortization									65	59	55
Interest income									0	0	0
Interest charges									38	36	34
Income taxes (benefit)									25	36	39
Net Income (Loss) Attributable to Parent									70	60	59
Capital expenditures									311	245	181
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission [Member]
Segment Reporting Information [Line Items]
Revenues									214	216	187
Intersegment revenues									53	42	45
Depreciation and amortization									50	43	38
Interest income									0	0	1
Interest charges									38	35	34
Income taxes (benefit)									32	47	41
Net Income (Loss) Attributable to Parent									98	77	67
Capital expenditures									444	355	273
Intersegment Elimination
Segment Reporting Information [Line Items]
Revenues									0	0	0
Intersegment revenues									(93)	(98)	(105)
Depreciation and amortization									0	0	0
Interest income									(5)	(11)	(11)
Interest charges									(4)	(11)	(11)
Income taxes (benefit)									0	0	0
Net Income (Loss) Attributable to Parent									0	0	0
Capital expenditures									(9)	(7)	(6)
Intersegment Elimination | Ameren Illinois Company
Segment Reporting Information [Line Items]
Revenues									0	0	0
Intersegment revenues									(53)	(42)	(45)
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) Attributable to Parent									0	0	0
Capital expenditures									$ 0	$ 0	$ 0

Segment Information (Disaggrega

Segment Information (Disaggregation of Revenues) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ 6,291	$ 6,174	$ 6,076
Revenues from alternative revenue programs	(59)	(15)	(52)
Other revenues not from contracts with customers	42	23	24
TCJA - Excess Amounts Collected in Rates
Disaggregation of Revenue [Line Items]
TCJA Revenue Reduction	60
Intersegment Elimination
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(93)	(98)	(106)
Electricity
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	5,339	5,307	5,196
Electricity | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	2,427	2,287	2,317
Electricity | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,782	1,735	1,741
Electricity | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	442	418	411
Electricity | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	688	867	727
Electricity | Intersegment Elimination
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(92)	(96)	(104)
Electricity | Intersegment Elimination | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Electricity | Intersegment Elimination | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Electricity | Intersegment Elimination | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Electricity | Intersegment Elimination | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(92)	(96)	(104)
Natural gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	952	867	880
Natural gas | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	671	608	607
Natural gas | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	196	177	183
Natural gas | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	21	16	14
Natural gas | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	64	66	76
Natural gas | Intersegment Elimination
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(1)	(2)	(2)
Natural gas | Intersegment Elimination | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Natural gas | Intersegment Elimination | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Natural gas | Intersegment Elimination | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Natural gas | Intersegment Elimination | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(1)	(2)	(2)
Ameren Illinois Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	2,576	2,527	2,489
Ameren Illinois Company | Intersegment Elimination
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(53)	(42)	(45)
Ameren Illinois Company | Intersegment Elimination | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Ameren Illinois Company | Intersegment Elimination | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Ameren Illinois Company | Intersegment Elimination | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Ameren Illinois Company | Intersegment Elimination | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(53)	(42)	(45)
Ameren Illinois Company | Ameren Illinois Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	2,576	2,527	2,489
Revenues from alternative revenue programs	(51)	9	(57)
Other revenues not from contracts with customers	18	8	8
Ameren Illinois Company | Ameren Illinois Company | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,448	1,401	1,425
Ameren Illinois Company | Ameren Illinois Company | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	670	673	670
Ameren Illinois Company | Ameren Illinois Company | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	147	125	106
Ameren Illinois Company | Ameren Illinois Company | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	311	328	288
Ameren Illinois Company | Electricity
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,761	1,784	1,735
Ameren Illinois Company | Natural gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	815	743	754
Operating Segments | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	3,589	3,537	3,524
Revenues from alternative revenue programs	(8)	(28)	8
Other revenues not from contracts with customers	24	15	16
Operating Segments | Ameren Illinois Electric Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,547	1,568	1,548
Revenues from alternative revenue programs	(3)	(5)	(70)
Other revenues not from contracts with customers	16	6	6
Operating Segments | Ameren Illinois Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	815	743	754
Revenues from alternative revenue programs	(23)	5	11
Other revenues not from contracts with customers	2	2	2
Operating Segments | Ameren Transmission [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	433	426	355
Revenues from alternative revenue programs	(25)	13	(1)
Other revenues not from contracts with customers	0	0	0
Operating Segments | Other Segment
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	(2)	1
Operating Segments | Electricity | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	3,451	3,411	3,396
Operating Segments | Electricity | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,560	1,417	1,422
Operating Segments | Electricity | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,271	1,208	1,224
Operating Segments | Electricity | Union Electric Company | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	312	305	315
Operating Segments | Electricity | Union Electric Company | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	308	481	435
Operating Segments | Electricity | Ameren Illinois Electric Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,547	1,568	1,548
Operating Segments | Electricity | Ameren Illinois Electric Distribution [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	867	870	895
Operating Segments | Electricity | Ameren Illinois Electric Distribution [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	511	527	517
Operating Segments | Electricity | Ameren Illinois Electric Distribution [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	130	113	96
Operating Segments | Electricity | Ameren Illinois Electric Distribution [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	39	58	40
Operating Segments | Electricity | Ameren Illinois Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Electricity | Ameren Illinois Gas [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Electricity | Ameren Illinois Gas [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Electricity | Ameren Illinois Gas [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Electricity | Ameren Illinois Gas [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Electricity | Ameren Transmission [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	433	426	355
Operating Segments | Electricity | Ameren Transmission [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Electricity | Ameren Transmission [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Electricity | Ameren Transmission [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Electricity | Ameren Transmission [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	433	426	355
Operating Segments | Electricity | Other Segment
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	(2)	1
Operating Segments | Electricity | Other Segment | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Electricity | Other Segment | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Electricity | Other Segment | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Electricity | Other Segment | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	(2)	1
Operating Segments | Natural gas | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	138	126	128
Operating Segments | Natural gas | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	90	77	77
Operating Segments | Natural gas | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	37	31	30
Operating Segments | Natural gas | Union Electric Company | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	4	4	4
Operating Segments | Natural gas | Union Electric Company | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	7	14	17
Operating Segments | Natural gas | Ameren Illinois Electric Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Ameren Illinois Electric Distribution [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Ameren Illinois Electric Distribution [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Ameren Illinois Electric Distribution [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Ameren Illinois Electric Distribution [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Ameren Illinois Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	815	743	754
Operating Segments | Natural gas | Ameren Illinois Gas [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	581	531	530
Operating Segments | Natural gas | Ameren Illinois Gas [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	159	146	153
Operating Segments | Natural gas | Ameren Illinois Gas [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	17	12	10
Operating Segments | Natural gas | Ameren Illinois Gas [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	58	54	61
Operating Segments | Natural gas | Ameren Transmission [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Ameren Transmission [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Ameren Transmission [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Ameren Transmission [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Ameren Transmission [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Other Segment
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Other Segment | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Other Segment | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Other Segment | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Natural gas | Other Segment | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(3)	(5)	(70)
Other revenues not from contracts with customers	16	6	6
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas [Member]
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(23)	5	11
Other revenues not from contracts with customers	2	2	2
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	267	258	232
Revenues from alternative revenue programs	(25)	9	2
Other revenues not from contracts with customers	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	267	258	232
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,547	1,568	1,548
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	867	870	895
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	511	527	517
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	130	113	96
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	39	58	40
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	815	743	754
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas [Member] | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	581	531	530
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas [Member] | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	159	146	153
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas [Member] | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	17	12	10
Operating Segments | Ameren Illinois Company | Natural gas | Ameren Illinois Gas [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ 58	$ 54	$ 61

Selected Quarterly Informatio_2

Selected Quarterly Information (Summary Of Selected Quarterly Information) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Selected Quarterly Financial Information [Line Items]
Revenues	$ 1,419	$ 1,724	$ 1,563	$ 1,585	$ 1,399	$ 1,723	$ 1,537	$ 1,515	$ 6,291	$ 6,174	$ 6,076
Deferred Federal Income Tax Expense (Benefit)									220	511	299
Operating Income	166	533	385	273	212	569	387	242	1,357	1,410	1,322
Net income	69	359	240	153	(59)	290	194	104	821	529	659
Net Income Available to Common Shareholder	$ 68	$ 357	$ 239	$ 151	$ (60)	$ 288	$ 193	$ 102	$ 815	$ 523	$ 653
Earnings per Common Share – Basic (in dollars per share)	$ 0.28	$ 1.46	$ 0.98	$ 0.62	$ (0.24)	$ 1.19	$ 0.79	$ 0.42	$ 3.34	$ 2.16	$ 2.69
Earnings per Common Share – Diluted (in dollars per share)	$ 0.28	$ 1.45	$ 0.97	$ 0.62	$ (0.24)	$ 1.18	$ 0.79	$ 0.42	$ 3.32	$ 2.14	$ 2.68
Union Electric Company
Selected Quarterly Financial Information [Line Items]
Revenues	$ 713	$ 1,129	$ 955	$ 792	$ 696	$ 1,116	$ 934	$ 791
Deferred Federal Income Tax Expense (Benefit)									$ 22	$ 76	$ 161
Operating Income	7	394	258	90	33	412	230	47	749	722	725
Net income	(22)	295	169	39	(36)	235	121	6	481	326	360
Net Income Available to Common Shareholder	(22)	294	168	38	(36)	234	120	5
Ameren Illinois Company
Selected Quarterly Financial Information [Line Items]
Revenues	674	564	578	760	674	574	576	703	2,576	2,527	2,489
Deferred Federal Income Tax Expense (Benefit)									75	185	117
Operating Income	135	113	105	159	148	124	128	169	512	569	519
Net income	85	63	63	96	78	55	58	80	$ 307	271	$ 255
Net Income Available to Common Shareholder	$ 84	$ 63	$ 62	$ 95	$ 77	$ 55	$ 57	$ 79
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 Financ_2

Schedule I - Condensed Financial Information Of Parent (Statement of Income) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Operating revenues	$ 1,419	$ 1,724	$ 1,563	$ 1,585	$ 1,399	$ 1,723	$ 1,537	$ 1,515	$ 6,291	$ 6,174	$ 6,076
Operating expenses									4,934	4,764	4,754
Operating loss	166	533	385	273	212	569	387	242	1,357	1,410	1,322
Interest income from affiliates									7	8	13
Other Nonoperating Income (Expense)									102	86	101
Interest charges									401	391	382
Income taxes (benefit)									237	576	382
Net Income (Loss) Available to Common Stockholders, Basic	$ 68	$ 357	$ 239	$ 151	$ (60)	$ 288	$ 193	$ 102	815	523	653
Comprehensive Income from Continuing Operations
Pension and other postretirement activity, net of income taxes (benefit)									(4)	5	(20)
Comprehensive Income (Loss) Attributable to Ameren Common Shareholders									811	528	633
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement activity, tax (benefit)									(1)	3	(7)
Parent Company
Condensed Financial Statements, Captions [Line Items]
Operating revenues									0	0	0
Operating expenses									11	15	19
Operating loss									(11)	(15)	(19)
Equity in earnings of subsidiaries									857	659	663
Interest income from affiliates									3	9	10
Other Nonoperating Income (Expense)									(12)	2	0
Interest charges									34	31	28
Income taxes (benefit)									(12)	101	(27)
Net income attributable to Ameren common shareholders - continuing operations									815	523	653
Net Income (Loss) Available to Common Stockholders, Basic									815	523	653
Comprehensive Income from Continuing Operations
Comprehensive Income (Loss) Attributable to Ameren Common Shareholders									811	528	633
Other Comprehensive Income (Loss), Taxes:
Pension and other postretirement activity, tax (benefit)									$ (1)	$ 3	$ (7)

Schedule I - Condensed Financ_3

Schedule I - Condensed Financial Information Of Parent (Balance Sheet) (Details) - USD ($) $ / shares in Units, $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
ASSETS
Cash and Cash Equivalents, at Carrying Value	$ 16	$ 10
Miscellaneous accounts and notes receivable	79	70
Other current assets	63	98
Total current assets	1,533	1,612
Accumulated deferred income taxes, net	9	6
Other assets	650	522
TOTAL ASSETS	27,215	25,945
LIABILITIES AND EQUITY
Short-term debt	597	484
Other current liabilities	282	326
Total current liabilities	2,687	2,940
Long-term Debt, Net	7,859	7,094
Pension and other postretirement benefits	558	545
Other deferred credits and liabilities	408	460
Commitments and Contingencies
Retained earnings	2,024	1,660
Accumulated other comprehensive loss	(22)	(18)
Total equity	7,773	7,326	$ 7,245
TOTAL LIABILITIES AND EQUITY	$ 27,215	$ 25,945
Common Stock, Par or Stated Value Per Share (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	400,000,000	400,000,000
Common Stock, Shares, Outstanding	244,500,000	242,600,000	242,600,000	242,600,000
Parent Company
ASSETS
Cash and Cash Equivalents, at Carrying Value	$ 0	$ 0
Advances to money pool	76	13
Accounts Receivable, Related Parties, Current	43	46
Other current assets	4	8
Total current assets	123	67
Investments in subsidiaries	8,559	7,944
Note receivable - affiliates	75	75
Accumulated deferred income taxes, net	108	222
Other assets	126	140
TOTAL ASSETS	8,991	8,448
LIABILITIES AND EQUITY
Short-term debt	470	383
Borrowings from money pool	46	28
Accounts payable – affiliates	10	6
Other current liabilities	12	27
Total current liabilities	538	444
Long-term Debt, Net	697	696
Pension and other postretirement benefits	43	37
Other deferred credits and liabilities	82	87
Total liabilities	1,360	1,264
Common stock, $.01 par value, 400.0 shares authorized – shares outstanding of 244.5 and 242.6, respectively	2	2
Other paid-in capital, principally premium on common stock	5,627	5,540
Retained earnings	2,024	1,660
Accumulated other comprehensive loss	(22)	(18)
Total equity	7,631	7,184
TOTAL LIABILITIES AND EQUITY	$ 8,991	$ 8,448
Common Stock, Par or Stated Value Per Share (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	400,000,000	400,000,000
Common Stock, Shares, Outstanding	244,500,000	242,600,000

Schedule I - Condensed Financ_4

Schedule I - Condensed Financial Information Of Parent (Statement of Cash Flows) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	$ 2,170	$ 2,118	$ 2,117
Cash Flows From Investing Activities:
Other	18	7	(9)
Net Cash Provided by (Used in) Investing Activities	(2,336)	(2,204)	(2,158)
Cash flows from financing activities:
Dividends on common stock	(451)	(431)	(416)
Short-term debt, net	112	(74)	257
Proceeds from Issuance of Common Stock	74	0	0
Issuances of Long-term debt	1,352	1,345	396
Capital issuance costs	(14)	(11)	(9)
Payments for Repurchase of Common Stock	0	(24)	(51)
Share-based payments	(19)	(15)	(32)
Net cash provided by (used in) financing activities	205	102	(258)
Net change in cash and cash equivalents	39	16	(299)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	107	68	52	$ 351
Stock Issued	35
Parent Company
Condensed Financial Statements, Captions [Line Items]
Net cash provided by operating activities	550	454	483
Cash Flows From Investing Activities:
Money pool advances, net	(63)	14	(27)
Intercompany notes receivable, net		275	(60)
Investments in subsidiaries	(208)	(151)	(123)
Other	5	6	2
Net Cash Provided by (Used in) Investing Activities	(266)	144	(208)
Cash flows from financing activities:
Dividends on common stock	(451)	(431)	(416)
Short-term debt, net	87	(124)	206
Money pool borrowings, net	18	(5)	19
Proceeds from Issuance of Common Stock	74		0
Payments for Repurchase of Common Stock	0	(24)	(51)
Share-based payments	(19)	(15)	(32)
Net cash provided by (used in) financing activities	(291)	(599)	(274)
Net change in cash and cash equivalents	(7)	(1)	1
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	1	8	9	$ 8
Cash dividends received from consolidated subsidiaries	450	$ 362	$ 465
Stock Issued	$ 35

Schedule I - Condensed Financ_5

Schedule I - Condensed Financial Information Of Parent Cash and Cash Equivalents (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 16	$ 10
Restricted Cash and Cash Equivalents, Noncurrent	74	35
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	107	68	$ 52	$ 351
Parent Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and Cash Equivalents, at Carrying Value	0	0
Restricted Cash and Cash Equivalents, Noncurrent	1	8
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 1	$ 8	$ 9	$ 8

Schedule I - Condensed Financ_6

Schedule I - Condensed Financial Information Of Parent Guarantees (Details) $ in Millions	Dec. 31, 2018USD ($)
Parent Company
Other Commitments [Line Items]
Guarantees Outstanding	$ 11

Schedule I - Condensed Financ_7

Schedule I - Condensed Financial Information Of Parent Other Income (Expense), Net (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Nonoperating Income (Expense) [Line Items]
Defined Benefit Plan, Non-service Cost or Income Components	$ 70	$ 44	$ 56
Donations	33	8	16
Other Nonoperating Income (Expense)	102	86	101
Parent Company
Other Nonoperating Income (Expense) [Line Items]
Defined Benefit Plan, Non-service Cost or Income Components	2	2	5
Donations	(13)	0	(5)
Other Expenses	(1)	0	0
Other Nonoperating Income (Expense)	$ (12)	$ 2	$ 0

Schedule I - Condensed Financ_8

Schedule I - Condensed Financial Information Of Parent Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes [Line Items]
Deferred Federal Income Tax Expense (Benefit)	$ 220	$ 511	$ 299
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	13
Tax Cuts and Jobs Act [Member]
Income Taxes [Line Items]
Deferred Federal Income Tax Expense (Benefit)		154
Parent Company
Income Taxes [Line Items]
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability	$ 5
Parent Company | Tax Cuts and Jobs Act [Member]
Income Taxes [Line Items]
Deferred Federal Income Tax Expense (Benefit)		$ 110

Schedule II - Valuation And Q_2

Schedule II - Valuation And Qualifying Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Allowance For Doubtful Accounts
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	$ 19	$ 19	$ 19
Charged to Costs and Expenses	27	26	32
Charged to Other Accounts	4	7	3
Deductions	32	33	35
Balance at End of Period	18	19	19
Valuation Allowance of Deferred Tax Assets
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	5	11	6
Charged to Costs and Expenses		(6)	7
Charged to Other Accounts	0	0	(2)
Deductions	0	0	0
Balance at End of Period	5	5	11
Parent Company | Valuation Allowance of Deferred Tax Assets
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Charged to Costs and Expenses	3
Union Electric Company | Allowance For Doubtful Accounts
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	7	7	7
Charged to Costs and Expenses	9	9	10
Charged to Other Accounts	0	0	0
Deductions	9	9	10
Balance at End of Period	7	7	7
Ameren Illinois Company | Allowance For Doubtful Accounts
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	12	12	12
Charged to Costs and Expenses	18	17	22
Charged to Other Accounts	4	7	3
Deductions	23	24	25
Balance at End of Period	$ 11	$ 12	$ 12