Ameren Illinois (AILIN) 10-Q 2 Nov 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2018	Oct. 31, 2018
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Trading Symbol	AEE
Entity Registrant Name	AMEREN CORP
Entity Central Index Key	1,002,910
Current Fiscal Year End Date	--12-31
Entity Emerging Growth Company	0
Entity Small Business	0
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		244,295,792
Union Electric Company
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Entity Registrant Name	UNION ELECTRIC CO
Entity Central Index Key	100,826
Current Fiscal Year End Date	--12-31
Entity Emerging Growth Company	0
Entity Small Business	0
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		102,123,834
Ameren Illinois Company
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Entity Registrant Name	AMEREN ILLINOIS CO
Entity Central Index Key	18,654
Current Fiscal Year End Date	--12-31
Entity Emerging Growth Company	0
Entity Small Business	0
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		25,452,373

Consolidated Statement of Incom

Consolidated Statement of Income (Loss) and Comprehensive Income (Loss) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,724	$ 1,723	$ 4,872	$ 4,775
Operating Expenses:
Fuel	216	199	590	594
Utilities Operating Expense, Purchased Power	148	163	453	493
Utilities Operating Expense, Gas and Petroleum Purchased	30	25	252	196
Other operations and maintenance	429	413	1,299	1,262
Depreciation and amortization	241	225	713	668
Taxes other than income taxes	127	129	374	364
Total operating expenses	1,191	1,154	3,681	3,577
Operating Income	533	569	1,191	1,198
Other Income (Expense), Net	32	23	84	61
Interest Charges	101	97	302	295
Income Before Income Taxes	464	495	973	964
Income Taxes	105	205	221	376
Net income	359	290	752	588
Less: Net Income Attributable to Noncontrolling Interests	2	2	5	5
Net Income (Loss) Available to Common Stockholders, Basic	357	288	747	583
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	2	0	1	2
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	361	290	753	590
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	2	2	5	5
Comprehensive Income	$ 359	$ 288	$ 748	$ 585
Earnings Per Share, Basic and Diluted [Abstract]
Earnings Per Share, Basic	$ 1.46	$ 1.19	$ 3.06	$ 2.40
Earnings Per Share, Diluted	1.45	1.18	3.04	2.39
Dividends per Common Share (in dollars per share)	$ 0.4575	$ 0.44	$ 1.3725	$ 1.320
Average Common Shares Outstanding - Basic (in shares)	244.1	242.6	243.6	242.6
Weighted Average Number of Shares Outstanding, Diluted	246.3	244.7	245.5	244
Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 1,590	$ 1,594	$ 4,209	$ 4,183
Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	134	129	663	592
Union Electric Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	1,129	1,116	2,876	2,841
Operating Expenses:
Fuel	216	199	590	594
Utilities Operating Expense, Purchased Power	49	43	131	203
Utilities Operating Expense, Gas and Petroleum Purchased	5	4	37	29
Other operations and maintenance	234	229	707	672
Depreciation and amortization	137	134	411	399
Taxes other than income taxes	94	95	258	255
Total operating expenses	735	704	2,134	2,152
Operating Income	394	412	742	689
Other Income (Expense), Net	16	16	45	48
Interest Charges	50	50	152	157
Income Before Income Taxes	360	378	635	580
Income Taxes	65	143	132	218
Net income	295	235	503	362
Preferred Stock Dividends	1	1	3	3
Net Income (Loss) Attributable to Parent	294	234	500	359
Union Electric Company | Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	1,111	1,099	2,782	2,758
Union Electric Company | Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	18	17	94	83
Ameren Illinois Company
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	564	574	1,902	1,853
Operating Expenses:
Utilities Operating Expense, Purchased Power	105	124	334	312
Utilities Operating Expense, Gas and Petroleum Purchased	25	21	215	167
Other operations and maintenance	195	186	590	598
Depreciation and amortization	94	86	278	254
Taxes other than income taxes	32	33	108	101
Total operating expenses	451	450	1,525	1,432
Operating Income	113	124	377	421
Other Income (Expense), Net	11	5	30	8
Interest Charges	38	36	112	109
Income Before Income Taxes	86	93	295	320
Income Taxes	23	38	73	127
Net income	63	55	222	193
Preferred Stock Dividends	0	0	2	2
Net Income (Loss) Attributable to Parent	63	55	220	191
Ameren Illinois Company | Electricity
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	448	462	1,333	1,343
Ameren Illinois Company | Natural Gas
Operating Revenues:
Revenue from Contract with Customer, Including Assessed Tax	$ 116	$ 112	$ 569	$ 510

Consolidated Statement of Inc_2

Consolidated Statement of Income (Loss) and Comprehensive Income (Loss) (Parenthetical) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Statement [Abstract]
Pension and other postretirement benefit plan activity, tax expense (benefit)	$ 0	$ 0	$ 0	$ 1

Consolidated Balance Sheet

Consolidated Balance Sheet - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Current Assets:
Cash and cash equivalents	$ 11	$ 10
Accounts receivable - trade (less allowance for doubtful accounts)	605	445
Unbilled revenue	260	323
Miscellaneous accounts and notes receivable	84	70
Inventories	525	522
Current regulatory assets	72	144
Other current assets	83	98
Total current assets	1,640	1,612
Property, Plant, and Equipment, Net	22,379	21,466
Investments and Other Assets:
Nuclear decommissioning trust fund	752	704
Goodwill	411	411
Regulatory assets	1,130	1,230
Other assets	647	522
Total investments and other assets	2,940	2,867
TOTAL ASSETS	26,959	25,945
Current Liabilities:
Current maturities of long-term debt	649	841
Short-term Debt	521	484
Accounts and wages payable	591	902
Taxes accrued	154	52
Interest accrued	108	99
Contract with Customer, Liability, Current	126	108
Current regulatory liabilities	114	128
Other current liabilities	317	326
Total current liabilities	2,580	2,940
Long-term Debt, Net	7,614	7,094
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	2,692	2,506
Accumulated Deferred Investment Tax Credit	45	49
Regulatory Liability, Noncurrent	4,652	4,387
Asset retirement obligations	640	638
Pension and other postretirement benefits	529	545
Other deferred credits and liabilities	409	460
Total deferred credits and other liabilities	8,967	8,585
Commitments and Contingencies
Stockholders' Equity:
Common Stock	2	2
Other paid-in capital	5,598	5,540
Retained earnings	2,073	1,660
Accumulated other comprehensive loss	(17)	(18)
Stockholder's equity	7,656	7,184
Noncontrolling Interest	142	142
Total equity	7,798	7,326
TOTAL LIABILITIES AND EQUITY	26,959	25,945
Union Electric Company
Current Assets:
Cash and cash equivalents	0	0
Due from Related Parties, Current	28	0
Accounts receivable - trade (less allowance for doubtful accounts)	331	200
Accounts receivable - affiliates	16	11
Unbilled revenue	153	165
Miscellaneous accounts and notes receivable	61	35
Inventories	385	388
Current regulatory assets	28	56
Other current assets	40	50
Total current assets	1,042	905
Property, Plant, and Equipment, Net	11,933	11,751
Investments and Other Assets:
Nuclear decommissioning trust fund	752	704
Regulatory assets	351	395
Other assets	305	288
Total investments and other assets	1,408	1,387
TOTAL ASSETS	14,383	14,043
Current Liabilities:
Current maturities of long-term debt	336	384
Short-term Debt	0	39
Accounts and wages payable	246	475
Accounts payable - affiliates	90	60
Taxes accrued	138	30
Interest accrued	62	54
Current regulatory liabilities	48	19
Other current liabilities	115	103
Total current liabilities	1,035	1,164
Long-term Debt, Net	3,668	3,577
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,636	1,650
Accumulated Deferred Investment Tax Credit	44	48
Regulatory Liability, Noncurrent	2,799	2,664
Asset retirement obligations	636	634
Pension and other postretirement benefits	204	213
Other deferred credits and liabilities	5	12
Total deferred credits and other liabilities	5,324	5,221
Commitments and Contingencies
Stockholders' Equity:
Common Stock	511	511
Other paid-in capital	1,858	1,858
Preferred stock	80	80
Retained earnings	1,907	1,632
Stockholder's equity	4,356	4,081
TOTAL LIABILITIES AND EQUITY	14,383	14,043
Ameren Illinois Company
Current Assets:
Cash and cash equivalents	0	0
Accounts receivable - trade (less allowance for doubtful accounts)	251	234
Accounts receivable - affiliates	36	9
Unbilled revenue	107	158
Miscellaneous accounts and notes receivable	33	35
Inventories	140	134
Current regulatory assets	44	87
Other current assets	18	15
Total current assets	629	672
Property, Plant, and Equipment, Net	8,969	8,293
Investments and Other Assets:
Goodwill	411	411
Regulatory assets	743	822
Other assets	258	147
Total investments and other assets	1,412	1,380
TOTAL ASSETS	11,010	10,345
Current Liabilities:
Current maturities of long-term debt	313	457
Short-term Debt	108	62
Due to Related Parties, Current	45	0
Accounts and wages payable	275	337
Accounts payable - affiliates	44	70
Taxes accrued	15	19
Interest accrued	39	33
Contract with Customer, Liability, Current	84	69
Current environmental remediation	50	42
Current regulatory liabilities	48	92
Other current liabilities	161	177
Total current liabilities	1,182	1,358
Long-term Debt, Net	2,801	2,373
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,081	1,021
Regulatory Liability, Noncurrent	1,732	1,629
Pension and other postretirement benefits	284	285
Environmental remediation	113	134
Other deferred credits and liabilities	207	235
Total deferred credits and other liabilities	3,417	3,304
Commitments and Contingencies
Stockholders' Equity:
Common Stock	0	0
Other paid-in capital	2,093	2,013
Preferred stock	62	62
Retained earnings	1,455	1,235
Stockholder's equity	3,610	3,310
TOTAL LIABILITIES AND EQUITY	$ 11,010	$ 10,345

Consolidated Balance Sheet (Par

Consolidated Balance Sheet (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Accounts receivable - trade allowance for doubtful accounts	$ 22	$ 19
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	400,000,000	400,000,000
Common stock, shares outstanding (in shares)	244,200,000	242,600,000
Union Electric Company
Accounts receivable - trade allowance for doubtful accounts	$ 8	$ 7
Common stock, par value (in dollars per share)	$ 5	$ 5
Common stock, shares authorized (in shares)	150,000,000	150,000,000
Common stock, shares outstanding (in shares)	102,100,000	102,100,000
Ameren Illinois Company
Accounts receivable - trade allowance for doubtful accounts	$ 14	$ 12
Common stock, no par value (in dollars per share)	$ 0	$ 0
Common stock, shares authorized (in shares)	45,000,000	45,000,000
Common stock, shares outstanding (in shares)	25,500,000	25,500,000

Consolidated Statement of Cash

Consolidated Statement of Cash Flows - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash Flows From Operating Activities:
Net income	$ 752	$ 588
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	699	653
Amortization of nuclear fuel	71	71
Amortization of debt issuance costs and premium/discounts	16	16
Deferred income taxes and investment tax credits, net	212	366
Allowance for equity funds used during construction	(25)	(16)
Stock-based compensation costs	15	12
Other	21	(7)
Changes in assets and liabilities:
Receivables	(129)	(59)
Inventories	(4)	(20)
Accounts and wages payable	(198)	(183)
Taxes accrued	92	138
Regulatory assets and liabilities	213	89
Assets, other	(2)	18
Liabilities, other	(45)	12
Pension and other postretirement benefits	(2)	(31)
Net cash provided by operating activities	1,686	1,647
Cash Flows From Investing Activities:
Capital expenditures	(1,689)	(1,523)
Nuclear fuel expenditures	(30)	(52)
Payments to Acquire Investments to be Held in Decommissioning Trust Fund	(172)	(187)
Proceeds from Decommissioning Trust Fund Assets	159	175
Other	13	3
Net cash used in investing activities	(1,719)	(1,584)
Cash Flows From Financing Activities:
Dividends on common stock	(334)	(320)
Dividends paid to noncontrolling interest holders	(5)	(5)
Short-term debt, net	36	(112)
Repayments of Long-term Debt	(522)	(425)
Proceeds from Issuance of Long-term Debt	853	849
Issuance of common stock	56	0
Repurchases of common stock for stock-based compensation	0	(24)
Employee payroll taxes related to stock-based compensation	(19)	(15)
Payment of Financing and Stock Issuance Costs	(9)	(5)
Other	1	(1)
Net cash provided by (used in) financing activities	57	(58)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect	24	5
Noncash financing activity - Issuance of common stock for stock-based compensation	35	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	92	57
Union Electric Company
Cash Flows From Operating Activities:
Net income	503	362
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	398	384
Amortization of nuclear fuel	71	71
Amortization of debt issuance costs and premium/discounts	4	5
Deferred income taxes and investment tax credits, net	4	55
Allowance for equity funds used during construction	(19)	(15)
Other	14	4
Changes in assets and liabilities:
Receivables	(156)	(117)
Inventories	3	(3)
Accounts and wages payable	(168)	(151)
Taxes accrued	148	160
Regulatory assets and liabilities	149	48
Assets, other	0	19
Liabilities, other	7	4
Pension and other postretirement benefits	3	(7)
Net cash provided by operating activities	961	819
Cash Flows From Investing Activities:
Capital expenditures	(664)	(533)
Nuclear fuel expenditures	(30)	(52)
Payments to Acquire Investments to be Held in Decommissioning Trust Fund	(172)	(187)
Proceeds from Decommissioning Trust Fund Assets	159	175
Money pool advances, net	(28)	143
Net cash used in investing activities	(735)	(454)
Cash Flows From Financing Activities:
Dividends on common stock	(225)	(332)
Dividends on preferred stock	(3)	(3)
Short-term debt, net	(39)
Repayments of Long-term Debt	(378)	(425)
Proceeds from Issuance of Long-term Debt	423	399
Payment of Financing and Stock Issuance Costs	(4)	(3)
Net cash provided by (used in) financing activities	(226)	(364)
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect	0	1
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	7	6
Ameren Illinois Company
Cash Flows From Operating Activities:
Net income	222	193
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	278	254
Amortization of debt issuance costs and premium/discounts	10	10
Deferred income taxes and investment tax credits, net	56	161
Allowance for equity funds used during construction	(6)	(1)
Other	5	(1)
Changes in assets and liabilities:
Receivables	21	59
Inventories	(7)	(17)
Accounts and wages payable	(44)	(24)
Taxes accrued	(40)	(22)
Regulatory assets and liabilities	63	45
Assets, other	0	(5)
Liabilities, other	(40)	(2)
Pension and other postretirement benefits	(8)	(19)
Net cash provided by operating activities	516	632
Cash Flows From Investing Activities:
Capital expenditures	(947)	(760)
Other	10	6
Net cash used in investing activities	(937)	(754)
Cash Flows From Financing Activities:
Dividends on preferred stock	(2)	(2)
Short-term debt, net	46	118
Proceeds from (Repayments of) Short-term Debt, Maturing in More than Three Months	45	11
Repayments of Long-term Debt	(144)	0
Proceeds from Issuance of Long-term Debt	430
Payment of Financing and Stock Issuance Costs	(5)
Capital contribution from parent	80
Other	1	(1)
Net cash provided by (used in) financing activities	451	126
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect	30	4
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 71	$ 32

Summary Of Significant Accounti

Summary Of Significant Accounting Policies	9 Months Ended
	Sep. 30, 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 has other subsidiaries that conduct other activities, such as providing shared services. Ameren evaluates competitive electric transmission investment opportunities as they arise. • Union Electric Company, doing business as Ameren Missouri, operates a rate-regulated electric generation, transmission, and distribution business and a rate-regulated natural gas distribution business in Missouri. • Ameren Illinois Company, doing business as Ameren Illinois, operates rate-regulated electric transmission, electric distribution, and natural gas distribution businesses in Illinois. • ATXI operates a FERC rate-regulated electric transmission business. ATXI is developing MISO-approved electric transmission projects, including the Illinois Rivers and Mark Twain projects, and placed the Spoon River project in service in February 2018. Ameren’s financial statements are prepared on a consolidated basis and therefore include the accounts of its majority-owned subsidiaries. All intercompany transactions have been eliminated. Ameren Missouri and Ameren Illinois have no subsidiaries. All tabular dollar amounts are in millions, unless otherwise indicated. As of September 30, 2018 , and December 31, 2017 , Ameren had unconsolidated variable interests as a limited partner in various equity method investments, totaling $20 million and $17 million , respectively, included in “Other assets” on Ameren’s consolidated balance sheet. Ameren is not the primary beneficiary of these investments because it does not have the power to direct matters that most significantly affect the activities of these variable interest entities. As of September 30, 2018 , the maximum exposure to loss related to these variable interests is limited to the investment in these partnerships of $20 million plus associated outstanding funding commitments of $17 million . Our accounting policies conform to GAAP. Our financial statements reflect all adjustments (which include normal, recurring adjustments) that are necessary, in our opinion, for a fair presentation of our results. The preparation of financial statements in conformity with GAAP requires management to make certain estimates and assumptions. Such estimates and assumptions affect reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the dates of financial statements, and the reported amounts of revenues and expenses during the reported periods. Actual results could differ from those estimates. The results of operations of an interim period may not give a true indication of results that may be expected for a full year. These financial statements should be read in conjunction with the financial statements and accompanying notes included in the Form 10-K. 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 following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets and the statements of cash flows as of September 30, 2018 and 2017 , and December 31, 2017 and 2016 : September 30, 2018 December 31, 2017 September 30, 2017 December 31, 2016 Ameren Ameren Ameren Ameren Ameren Ameren Ameren Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Cash and cash equivalents (a) $ 11 $ — $ — $ 10 $ — $ — $ 9 $ — $ — $ 9 $ — $ — Restricted cash included in “Other current assets” 15 4 8 21 5 6 19 4 5 20 4 6 Restricted cash included in “Other assets” 63 — 63 35 — 35 27 — 27 22 — 22 Restricted cash included in “Nuclear decommissioning trust fund” 3 3 (b) 2 2 (b) 2 2 (b) 1 1 (b) Total cash, cash equivalents, and restricted cash (c) $ 92 $ 7 $ 71 $ 68 $ 7 $ 41 $ 57 $ 6 $ 32 $ 52 $ 5 $ 28 (a) As presented on the balance sheets. (b) Not applicable. (c) As presented on the statements of cash flows. 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. Supplemental Cash Flow Information The following table provides noncash investing activity excluded from the statements of cash flows for the nine months ended September 30, 2018 and 2017 : September 30, 2018 September 30, 2017 Ameren (a) Ameren Missouri Ameren Illinois Ameren (a) Ameren Missouri Ameren Illinois Accrued capital expenditures $ 240 $ 94 $ 133 $ 202 $ 70 $ 100 Net realized and unrealized gain – nuclear decommissioning trust fund 33 33 (b) 53 53 (b) (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries. (b) Not applicable. 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 September 30, 2018 , and December 31, 2017 , “Other current liabilities” on Ameren’s and Ameren Illinois’ balance sheets included payables for purchased receivables of $40 million and $31 million , respectively. For the three and nine months ended September 30, 2018 and 2017 , the Ameren Companies recorded immaterial bad debt expense. Asset Retirement Obligations The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the nine months ended September 30, 2018 : Ameren Missouri Ameren Illinois (a) Ameren Balance at December 31, 2017 $ 640 (b) $ 4 $ 644 (b) Liabilities settled (4 ) (c) (4 ) Accretion (d) 20 (c) 20 Change in estimates (e) (14 ) — (14 ) Balance at September 30, 2018 $ 642 (b) $ 4 $ 646 (b) (a) Included in “Other deferred credits and liabilities” on the balance sheet. (b) Balance included $6 million in “Other current liabilities” on the balance sheet as of both December 31, 2017 , and September 30, 2018 . (c) Less than $1 million. (d) Accretion expense attributable to Ameren Missouri was recorded as a decrease to regulatory liabilities. (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 September 30, 2018 , the cash surrender value of company-owned life insurance at Ameren and Ameren Illinois was $ 256 million (December 31, 2017 – $265 million ) and $ 120 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. Stock-based Compensation The following table summarizes Ameren's nonvested performance share unit and restricted stock unit activity for the nine months ended September 30, 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 313,984 62.88 186,728 57.65 Forfeitures (62,865 ) 50.78 (4,964 ) 58.99 Undistributed vested units (b) (217,350 ) 53.57 (19,742 ) 59.01 Vested and distributed (176,043 ) 52.88 — — Nonvested at September 30, 2018 (c) 753,215 $ 56.31 162,022 $ 57.44 (a) Does not include 712,572 undistributed vested performance share units. (b) Undistributed vested units are awards that vested due to attainment of retirement eligibility by certain employees, but have not yet been distributed. For undistributed vested 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 548,542 undistributed vested performance share units and 19,742 undistributed vested 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 performance share unit granted in 2018 was determined to be $62.88 , which was based on Ameren’s closing common share price of $58.99 at December 31, 2017 , 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 beginning January 1, 2018, relative to the designated peer group. The significant assumptions used to calculate fair value included a three -year risk-free rate of 1.98% and volatility of 15% to 23% for the peer group. 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. Deferred Compensation As of both September 30, 2018 , and December 31, 2017 , “Other deferred credits and liabilities” on Ameren’s balance sheet included deferred compensation obligations of $86 million recorded at the present value of future benefits to be paid. 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 is 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 13 – 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 include disclosure related to 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 September 30, 2018 and 2017 , our remaining performance obligations were immaterial. 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 three and nine months ended September 30, 2018 and 2017 : Three Months Nine Months 2018 2017 2018 2017 Ameren Missouri $ 52 $ 51 $ 133 $ 122 Ameren Illinois 26 26 (a) 89 82 (a) Ameren $ 78 $ 77 (a) $ 222 $ 204 (a) (a) Amounts have been adjusted from those previously reported to reflect additional excise taxes for the three and nine months ended September 30, 2017. 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 is computed by dividing “Net Income Attributable to Ameren Common Shareholders” by the weighted-average number of diluted common shares outstanding during the period. Earnings per diluted share reflects the dilution that would occur if certain stock-based performance share units and restricted stock units were assumed to be settled. The number of potential common shares assumed to have been issued was 2.2 million and 1.9 million in the three and nine months ended September 30, 2018 , respectively, and 2.1 million and 1.4 million , respectively, in the year-ago periods. There were no potentially dilutive securities excluded from the earnings per diluted share calculations for the three and nine months ended September 30, 2018 and 2017 . Accounting and Reporting Developments 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 11 – 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. See Note 1 – Summary of Significant Accounting Policies under Part II, Item 8, of the Form 10-K for additional information about recently issued authoritative accounting standards relating to the measurement of credit losses on financial instruments and the reclassification of certain tax effects from accumulated OCI. Leases In February 2016, the FASB issued authoritative guidance that requires an entity to recognize assets and liabilities arising from all leases with a term greater than one year. Consistent with current GAAP, the recognition, measurement, and presentation of expenses and cash flows arising from a lease will depend on its classification as a finance lease or operating lease. The guidance also requires additional disclosures to enable users of financial statements to understand the amount, timing, and uncertainty of cash flows arising from leases. This guidance will affect the Ameren Companies’ financial position by increasing the assets and liabilities recorded relating to their operating leases. We are also assessing the impacts of this guidance on our results of operations, cash flows, and disclosures. We have selected a software vendor and are in the process of implementing system changes required for the implementation of this guidance. We are currently assessing our agreements to determine those that are within the scope of this guidance. This guidance will be effective for the Ameren Companies in the first quarter of 2019. In July 2018, the FASB issued authoritative guidance that provides entities with an optional transition method for adopting the new leases standard. Under this optional transition method, the Ameren Companies may adopt the new leases standard by recognizing a cumulative-effect adjustment to our January 1, 2019, retained earnings balances. Periods prior to 2019 would continue to be presented and disclosed in the financial statements in accordance with current GAAP. We are currently assessing whether we will elect this optional transition method. Fair Value Measurement Disclosures In August 2018, the FASB issued authoritative guidance that affects disclosure requirements for fair value measurements. The 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. The 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. The 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. The amount of implementation costs that were capitalized in the third quarter of 2018 was immaterial. SEC Disclosure Update and Simplification In August 2018, the SEC adopted a final rule that requires, among other things, inclusion of a statement of changes in shareholders’ equity, or disclosure of such changes, and disclosure of the amount of dividends per share for each class of shares with respect to interim periods. The guidance will be effective for the Ameren Companies in the fourth quarter of 2018. We are currently assessing the impact of this guidance on our disclosures.

Rate And Regulatory Matters

Rate And Regulatory Matters	9 Months Ended
	Sep. 30, 2018
Public Utilities, General Disclosures [Abstract]
RATE AND REGULATORY MATTERS	RATE AND REGULATORY MATTERS Below is a summary of updates to significant regulatory proceedings and related lawsuits. See also Note 2 – Rate and Regulatory Matters under Part II, Item 8, of the Form 10-K. We are unable to predict the ultimate outcome of these matters, the timing of the final decisions of the various agencies and courts, or the impact on our results of operations, financial position, or liquidity. Missouri Missouri Senate Bill 564 On June 1, 2018, Missouri Senate Bill 564 was enacted. The section of the law applicable to the TCJA was effective immediately; the remaining sections, including the ability to elect PISA, became effective August 28, 2018. The law resulted in certain changes to the regulation of Ameren Missouri’s electric service business. These changes include the reduction of customer rates to pass through the effect of the reduction in the federal statutory corporate income tax rate enacted under the TCJA and, at each electric utility’s election, the use of PISA. 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. Ameren Missouri filed a notification with the MoPSC on September 1, 2018, to elect PISA. Under PISA, 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 as well as 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. Costs not included in the PISA deferral, including the remaining 15% of the depreciation expense and return on rate base, remain subject to regulatory lag. Qualifying PISA capital expenditures exclude amounts related to new coal-fired, nuclear, and natural gas generating units and service to new customer premises. Amounts deferred under PISA were immaterial as of September 30, 2018 . As a result of Ameren Missouri’s PISA election, additional provisions of Missouri Senate Bill 564 apply, including 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 2018 savings from the TCJA that was passed on to customers. Additionally, Ameren Missouri’s electric base rates, as determined in the July 2018 MoPSC rate order, are frozen until April 1, 2020. Recoveries under the MEEIA, the FAC, and the RESRAM riders have not been frozen; however, except for costs recoverable under the MEEIA rider, Ameren Missouri will be unable to recover any amounts above the 2.85% cap from customers. If rate changes from the FAC or the RESRAM riders would cause rates to temporarily exceed the 2.85% cap, the overage will 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. Both the rate cap and PISA election will be effective through December 2023, unless Ameren Missouri requests and receives MoPSC approval of an extension through December 2028. Ameren Missouri’s PISA election supports Ameren Missouri’s ability to invest approximately $1 billion of incremental capital over the 2019 to 2023 period to strengthen and modernize Missouri’s electric grid. Wind Generation Facilities and RESRAM In the second quarter of 2018, Ameren Missouri entered into an 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 May 2018, Ameren Missouri filed for a certificate of convenience and necessity with the MoPSC for the 400-megawatt facility. The MoPSC issued an order approving a unanimous stipulation and agreement regarding that requested certificate in October 2018. Also in October 2018, Ameren Missouri entered into an agreement with a subsidiary of EDF Renewables, Inc. to acquire, after construction, a wind generation facility of up to 157 megawatts, and filed for a certificate of convenience and necessity with the MoPSC. The MoPSC is expected to issue an order regarding that certificate by May 2019. The up to 157-megawatt facility is expected to be located in northwestern Missouri. Both facilities are expected to be completed in 2020 and would help Ameren Missouri comply with the state renewable energy standard. Each acquisition is subject to certain conditions, including the issuance of a certificate of convenience and necessity by the MoPSC, obtaining a MISO transmission interconnection agreement, approval by the FERC, and other customary contract terms and conditions. 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. The October 2018 MoPSC order included approval of the RESRAM, without addressing recovery through the RESRAM of the 15% of capital investment not recovered under PISA, which was an objection raised by the MoOPC. Ameren Missouri anticipates a MoPSC decision resolving this remaining issue and approving the RESRAM tariff by December 2018. The RESRAM is designed to mitigate the impacts of regulatory lag for the cost of compliance with renewable energy requirements, including recovery of investments in wind generation and other renewables, by providing more timely recovery of costs and a return on investments not already provided for in customer rates or any other recovery mechanism. RESRAM regulatory assets will 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 percent 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 approval by the FERC. This generation would be incremental to the expected renewable generation included in the 2017 IRP. Without extension, the option to elect into the program will terminate in the third quarter of 2023. MEEIA In June 2018, Ameren Missouri filed a proposed customer energy-efficiency plan with the MoPSC under the MEEIA. In October 2018, Ameren Missouri, the MoPSC staff, the MoOPC, and certain other intervenors filed a stipulation and agreement with the MoPSC with respect to that proposed plan. The proposed plan includes a three-year plan for a portfolio of customer energy-efficiency programs and a six-year plan for low-income energy-efficiency programs, along with a cost recovery mechanism. If the proposal is approved, Ameren Missouri intends to invest $226 million over the life of the plan, including $65 million per program year for the three-year period beginning March 2019. The proposed 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 revenues and the amounts collected from customers. In addition, similar to the MEEIA 2016 plan ending in February 2019, the proposed plan includes a performance incentive that would provide 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 three-year period beginning March 2019. Additional revenues may be earned if Ameren Missouri exceeds 100% of its energy savings goals. A decision by the MoPSC in this proceeding is anticipated in November 2018. The MEEIA 2016 program provided Ameren Missouri with a performance incentive to earn additional revenues by achieving certain customer energy-efficiency goals, including $27 million if 100% of the goals were achieved during the three-year period beginning March 2016, with the potential to earn more if Ameren Missouri’s energy savings exceeded those goals. In September 2017, Ameren Missouri received an order from the MoPSC approving Ameren Missouri’s energy savings results for the first year of the MEEIA 2016 programs. As a result of this order and in accordance with revenue recognition guidance, Ameren Missouri recognized $5 million of revenues in the first quarter of 2018 relating to the MEEIA 2016 performance incentive. 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 programs. As a result of this order, Ameren Missouri will recognize $6 million of additional revenues in the fourth quarter of 2018 relating to the MEEIA 2016 performance incentive. 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. Upon issuance of a MoPSC order, Ameren Missouri expects to recognize an additional $9 million MEEIA 2013 performance incentive. Illinois Electric Distribution Service Rates In April 2018, Ameren Illinois filed its annual electric distribution service 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 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 will be fully collected from customers in 2018 , consistent with the ICC’s December 2017 annual update filing order. As of September 30, 2018 , Ameren Illinois had recorded a regulatory liability of $25 million to reflect the difference between Ameren Illinois’ estimate of its 2018 revenue requirement and the revenue requirement reflected in customer rates, including interest. Electric Customer Energy-Efficiency Investments 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. 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 $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, with an estimated reduction of up to $17 million to be reflected substantially over a one-year period. 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 will result 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. The new rates will be effective starting in November 2018. This increase reflects the reduction in the federal corporate income tax rate as a result of the TCJA, as well as the increase in the Illinois corporate income tax rate that became effective in July 2017, which collectively decreased annual rates by approximately $17 million. As a result of this order, rate base under the QIP rider has been reset to zero. Ameren Illinois used a 2019 future test year in this proceeding. 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. In November 2017, ATXI appealed this decision to the Illinois Supreme Court. In October 2018, the Illinois Supreme Court reversed the Illinois Circuit Court for Edgar County’s decision and remanded the case for further proceedings. Absent the landowners pursuing rehearing, or a voluntary settlement, ATXI intends to file a motion to reinstate the condemnation cases in the Illinois Circuit Court for Edgar County in the fourth quarter of 2018. ATXI plans to complete the project by the end of 2019; however, delays associated with the condemnation proceedings or a rehearing arising from the Illinois Supreme Court’s ruling could delay the completion date. The estimated line segment capital expenditure investment is approximately $81 million , of which $38 million was invested as of September 30, 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 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. 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 used in the two MISO complaint cases described above. In October 2018, the FERC issued an order addressing the remanded issues, which proposed a new methodology for determining the base return on equity and required further briefs from the participants. A final order is not expected until 2019. While this order provides insight on how the FERC may determine the return on equity in the MISO complaint cases, Ameren is unable to predict the impact of the outcome on the MISO FERC 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 November 2013 complaint case, 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, as discussed above. The FERC is under no deadline to issue an order on this motion. As of September 30, 2018 , Ameren and Ameren Illinois had recorded current regulatory liabilities of $43 million and $25 million , respectively, to reflect the expected refunds, including interest, associated with the reduced allowed returns on common equity in the initial decision in the February 2015 complaint case. Ameren Missouri does not expect that a reduction in the FERC-allowed base return on common equity would be material to its results of operations, financial position, or liquidity. 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 have been reduced by $27 million and $23 million , respectively. In May 2018, the FERC accepted Ameren Illinois and ATXI tariff filings to change the formula rate calculation. The change allows for the recovery or refund of both excess deferred income taxes resulting from tax law or rate changes and effect of permanent income tax differences and will be reflected in Ameren Illinois and ATXI’s electric transmission rates starting in January 2019.

Short-Term Debt And Liquidity

Short-Term Debt And Liquidity	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [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. See Note 4 – Short-term Debt and Liquidity under Part II, Item 8, in the Form 10-K for a description of our indebtedness provisions and other covenants as well as a description of money pool arrangements. The Missouri Credit Agreement and the Illinois Credit Agreement were not utilized for direct borrowings during the nine months ended September 30, 2018 , but were used to support commercial paper issuances and to issue letters of credit. Based on commercial paper outstanding and letters of credit issued under the Credit Agreements, the aggregate credit capacity available under the Credit Agreements to Ameren (parent), Ameren Missouri, and Ameren Illinois, collectively, at September 30, 2018 , was $1.6 billion . The Ameren Companies were in compliance with the covenants in their Credit Agreements as of September 30, 2018 . As of September 30, 2018 , the ratios of consolidated indebtedness to consolidated total capitalization, calculated in accordance with the provisions of the Credit Agreements, were 52% , 46% , and 48% for Ameren, Ameren Missouri, and Ameren Illinois, respectively. Commercial Paper The following table presents commercial paper outstanding, net of issuance discounts, as of September 30, 2018 , and December 31, 2017 : September 30, 2018 December 31, 2017 Ameren (parent) $ 413 $ 383 Ameren Missouri — 39 Ameren Illinois 108 62 Ameren Consolidated $ 521 $ 484 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 nine months ended September 30, 2018 and 2017: Ameren (parent) Ameren Missouri Ameren Illinois Ameren Consolidated 2018 Average daily commercial paper outstanding at par value $ 431 $ 81 $ 117 $ 629 Weighted-average interest rate 2.23 % 1.94 % 2.21 % 2.18 % Peak commercial paper during period at par value (a) $ 543 $ 481 $ 442 $ 1,295 Peak interest rate 2.45 % 2.42 % 2.55 % 2.55 % 2017 Average daily commercial paper outstanding at par value $ 669 $ 7 $ 78 $ 754 Weighted-average interest rate 1.27 % 1.20 % 1.28 % 1.27 % Peak commercial paper during period at par value (a) $ 841 $ 64 $ 193 $ 948 Peak interest rate 1.50 % 1.41 % 1.50 % 1.50 % (a) The timing of peak outstanding commercial paper issuances varies by company. Therefore, the sum of individual company peak amounts may not equal the Ameren Consolidated peak commercial paper issuances for the period. 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. The average interest rate for borrowings under the money pool for the three and nine months ended September 30, 2018 , was 2.00% and 2.02% , respectively (2017 – 1.24% and 1.18% , respectively). See Note 8 – Related-party Transactions for the amount of interest income and expense from the money pool arrangements recorded by the Ameren Companies for the three and nine months ended September 30, 2018 and 2017 .

Long-Term Debt And Equity Finan

Long-Term Debt And Equity Financings	9 Months Ended
	Sep. 30, 2018
Long-Term Debt And Equity Financings [Abstract]
LONG-TERM DEBT AND EQUITY FINANCINGS	LONG-TERM DEBT AND EQUITY FINANCINGS Ameren For the three and nine months ended September 30, 2018 , Ameren issued a total of 0.2 million and 0.9 million shares, respectively, of common stock under its DRPlus and 401(k) plan, and received proceeds of $16 million and $56 million , respectively. In addition, in the first quarter of 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 during the first nine months of 2017. In October 2018, Ameren filed a Form S-8 registration statement with the SEC, authorizing the offering of four 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. 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. 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. Indenture Provisions and Other Covenants See Note 5 – Long-Term Debt and Equity Financings under Part II, Item 8, in the Form 10-K for a description of our indenture provisions and other covenants, as well as restrictions on the payment of dividends. At September 30, 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. Off-balance-sheet Arrangements At September 30, 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, letters of credit, and Ameren (parent) guarantee arrangements on behalf of its subsidiaries.

Other Income and Expenses

Other Income and Expenses	9 Months Ended
	Sep. 30, 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 three and nine months ended September 30, 2018 and 2017 : Three Months Nine Months 2018 2017 2018 2017 Ameren: (a) Other Income, Net Allowance for equity funds used during construction $ 11 $ 6 $ 25 $ 16 Interest income on industrial development revenue bonds 6 7 19 20 Other interest income 2 — 6 5 Non-service cost components of net periodic benefit income 17 (b) 11 52 (b) 33 Other income 2 1 5 3 Donations (4 ) — (15 ) (7 ) Other expense (2 ) (2 ) (8 ) (9 ) Total Other Income, Net $ 32 $ 23 $ 84 $ 61 Ameren Missouri: Other Income, Net Allowance for equity funds used during construction $ 8 $ 6 $ 19 $ 15 Interest income on industrial development revenue bonds 6 7 19 20 Other interest income 1 — 2 1 Non-service cost components of net periodic benefit income 4 (b) 5 13 (b) 17 Other income 2 — 3 1 Donations (3 ) — (6 ) (2 ) Other expense (2 ) (2 ) (5 ) (4 ) Total Other Income, Net $ 16 $ 16 $ 45 $ 48 Ameren Illinois: Other Income, Net Allowance for equity funds used during construction $ 3 $ — $ 6 $ 1 Interest income 1 1 4 5 Non-service cost components of net periodic benefit income 8 4 25 8 Other income 1 — 3 2 Donations — — (5 ) (5 ) Other expense (2 ) — (3 ) (3 ) Total Other Income, Net $ 11 $ 5 $ 30 $ 8 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations. (b) For the three and nine months ended September 30, 2018 , the non-service cost components of net periodic benefit income were partially offset by a $5 million and $13 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	9 Months Ended
	Sep. 30, 2018
Derivative Instruments and Hedging Activities Disclosure [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 September 30, 2018 , and December 31, 2017 . As of September 30, 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) 42 (b) 42 28 (b) 28 Natural gas (in mmbtu) 20 160 180 24 139 163 Power (in megawatthours) 2 8 10 3 9 12 (a) Consists of ultra-low-sulfur diesel products. (b) Not applicable. All contracts considered to be derivative instruments are recorded on the balance sheet at their fair values, unless the NPNS exception applies. See Note 7 – Fair Value Measurements for a discussion of our methods of assessing the fair value of derivative instruments. Many of our physical contracts, such as our purchased power contracts, qualify for the NPNS exception to derivative accounting rules. The revenue or expense on NPNS contracts is recognized at the contract price upon physical delivery. If we determine that a contract meets the definition of a derivative and is not eligible for the NPNS exception, we review the contract to determine whether the resulting gains or losses qualify for regulatory deferral. Derivative contracts that qualify for regulatory deferral are recorded at fair value, with changes in fair value recorded as regulatory assets or 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 September 30, 2018 , and December 31, 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 September 30, 2018 , and December 31, 2017 : Balance Sheet Location Ameren Missouri Ameren Illinois Ameren 2018 Fuel oils Other current assets $ 10 $ — $ 10 Other assets 6 — 6 Natural gas Other current assets — 1 1 Other assets — 1 1 Power Other current assets 3 — 3 Total assets (a) $ 19 $ 2 $ 21 Fuel oils Other deferred credits and liabilities $ 1 $ — $ 1 Natural gas Other current liabilities 4 12 16 Other deferred credits and liabilities 2 9 11 Power Other current liabilities 3 13 16 Other deferred credits and liabilities — 174 174 Total liabilities (b) $ 10 $ 208 $ 218 2017 Fuel oils Other current assets $ 5 $ — $ 5 Other assets 2 — 2 Natural gas Other assets 1 — 1 Power Other current assets 9 — 9 Total assets (a) $ 17 $ — $ 17 Natural gas Other current liabilities $ 5 $ 12 $ 17 Other deferred credits and liabilities 3 10 13 Power Other current liabilities 1 13 14 Other deferred credits and liabilities — 182 182 Total liabilities (b) $ 9 $ 217 $ 226 (a) The cumulative amount of pretax net gains on all derivative instruments is deferred as a regulatory liability. (b) The cumulative amount of pretax net losses on all derivative instruments is deferred as a regulatory asset. Derivative instruments are subject to various credit-related losses in the event of nonperformance by counterparties to the transaction. Exchange-traded contracts are supported by the financial and credit quality of the clearing members of the respective exchanges; these contracts have nominal credit risk. In all other transactions, we are exposed to credit risk. Our credit risk management program involves establishing credit limits and collateral requirements for counterparties, using master netting arrangements or similar agreements, and reporting daily exposure to senior management. We believe that entering into master netting arrangements or similar agreements mitigates the level of financial loss that could result from default by allowing net settlement of derivative assets and liabilities. These master netting arrangements allow the counterparties to net settle sale and purchase transactions. Further, collateral requirements are calculated at the master netting arrangement or similar agreement level by counterparty. The Ameren Companies elect to present the fair value amounts of derivative assets and derivative liabilities subject to an enforceable master netting arrangement or similar agreement gross on the balance sheet. However, if the gross amounts recognized on the balance sheet were netted with derivative instruments and cash collateral received or posted, the net amounts would not be materially different from the gross amounts at September 30, 2018 , and December 31, 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 September 30, 2018 , if counterparty groups were to fail completely to perform on contracts, Ameren, Ameren Missouri, and Ameren Illinois’ maximum exposures were $37 million , $34 million and $3 million , respectively. The potential loss on counterparty exposures may be reduced or eliminated by the application of master netting arrangements or similar agreements and collateral held. As of September 30, 2018 , the potential loss after consideration of the application of master netting arrangements or similar agreements and collateral held for Ameren, Ameren Missouri, and Ameren Illinois was $32 million , $31 million , and $1 million , respectively. Derivative Instruments with Credit Risk-related Contingent Features Our commodity contracts contain collateral provisions tied to the Ameren Companies’ credit ratings. If our credit ratings were downgraded, or if a counterparty with reasonable grounds for uncertainty regarding our ability to satisfy an obligation requested adequate assurance of performance, additional collateral postings might be required. The following table presents, as of September 30, 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 September 30, 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 $ 71 $ 4 $ 51 Ameren Illinois 49 — 43 Ameren $ 120 $ 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.

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2018
Related Party Transactions [Abstract]
RELATED PARTY TRANSACTIONS	RELATED-PARTY TRANSACTIONS In the normal course of business, the Ameren Companies 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. For a discussion of our material related-party agreements and money pool arrangements, see Note 13 – Related-party Transactions and Note 4 – Short-term Debt and Liquidity under Part II, Item 8, of the Form 10-K. Electric Power Supply Agreement In April 2018, Ameren Illinois conducted a procurement event, administered by the IPA, to purchase energy products. Ameren Missouri was among the winning suppliers in this event. As a result, in April 2018, Ameren Missouri and Ameren Illinois entered into an energy product agreement by which Ameren Missouri agreed to sell, and Ameren Illinois agreed to purchase, 110,000 megawatthours at an average price of $32 per megawatthour during the period of June 2019 through September 2020. The following table presents the impact on Ameren Missouri and Ameren Illinois of related-party transactions for the three and nine months ended September 30, 2018 and 2017 : Three Months Nine Months Agreement Income Statement Line Item Ameren Missouri Ameren Illinois Ameren Missouri Ameren Illinois Ameren Missouri power supply Operating Revenues 2018 $ 5 $ (a) $ 11 $ (a) agreements with Ameren Illinois 2017 4 (a) 21 (a) Ameren Missouri and Ameren Illinois Operating Revenues 2018 6 (b) 17 2 rent and facility services 2017 7 1 20 3 Ameren Missouri and Ameren Illinois Operating Revenues 2018 (b) (b) (b) (b) miscellaneous support services 2017 (b) (b) (b) 1 Total Operating Revenues 2018 $ 11 $ (b) $ 28 $ 2 2017 11 1 41 4 Ameren Illinois power supply Purchased Power 2018 $ (a) $ 5 $ (a) $ 11 agreements with Ameren Missouri 2017 (a) 4 (a) 21 Ameren Illinois transmission Purchased Power 2018 (a) (b) (a) 1 services with ATXI 2017 (a) (b) (a) 1 Total Purchased Power 2018 $ (a) $ 5 $ (a) $ 12 2017 (a) 4 (a) 22 Ameren Services support services Other Operations and Maintenance 2018 $ 36 $ 33 $ 101 $ 93 agreement 2017 34 33 103 99 Money pool borrowings (advances) Interest Charges/ Other Income, Net 2018 $ (b) $ (b) $ (b) $ (b) 2017 (b) (b) (b) (b) (a) Not applicable. (b) Amount less than $1 million.

Commitments And Contingencies

Commitments And Contingencies	9 Months Ended
	Sep. 30, 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 the notes to our financial statements in this report and in the Form 10-K, will not have a material adverse effect on our results of operations, financial position, or liquidity. Reference is made to Note 1 – Summary of Significant Accounting Policies, Note 2 – Rate and Regulatory Matters, Note 13 – Related-party Transactions, and Note 14 – Commitments and Contingencies under Part II, Item 8, of the Form 10-K. See also Note 1 – Summary of Significant Accounting Policies, Note 2 – Rate and Regulatory Matters, Note 8 – Related-party Transactions, and Note 10 – Callaway Energy Center of this report. 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 for fuel at September 30, 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 September 30, 2018 . Coal Natural Gas (a) Nuclear Fuel Purchased Power (b)(c) Methane Gas Other Total Ameren: (d) 2018 $ 86 $ 55 $ 36 $ 60 $ 1 $ 38 $ 276 2019 319 189 26 157 4 51 746 2020 159 131 38 53 4 41 426 2021 121 70 56 10 5 30 292 2022 73 19 13 — 5 25 135 Thereafter — 39 72 — 58 92 261 Total $ 758 $ 503 $ 241 $ 280 $ 77 $ 277 $ 2,136 Ameren Missouri: 2018 $ 86 $ 11 $ 36 $ — $ 1 $ 31 $ 165 2019 319 39 26 — 4 35 423 2020 159 30 38 — 4 25 256 2021 121 14 56 — 5 25 221 2022 73 5 13 — 5 25 121 Thereafter — 17 72 — 58 75 222 Total $ 758 $ 116 $ 241 $ — $ 77 $ 216 $ 1,408 Ameren Illinois: 2018 $ — $ 44 $ — $ 60 $ — $ 4 $ 108 2019 — 150 — 157 — 7 314 2020 — 100 — 53 — 7 160 2021 — 56 — 10 — — 66 2022 — 14 — — — — 14 Thereafter — 22 — — — — 22 Total $ — $ 386 $ — $ 280 $ — $ 18 $ 684 (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. (d) Includes amounts for Ameren registrant and nonregistrant subsidiaries. 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 $42 million of zero emission credits through May 2019. In April and September 2018, Ameren Illinois conducted procurement events, administered by the IPA, to purchase energy products and capacity through May 2021. In the April 2018 procurement event, Ameren Illinois contracted to purchase 3,956,200 megawatthours of energy products for $112 million from June 2018 through May 2021. In the September 2018 procurement event, Ameren Illinois contracted to purchase approximately 2,221,400 megawatthours of energy products for $63 million from October 2018 through May 2021. In addition, in the September procurement event, Ameren Illinois contracted to purchase 653 megawatts of capacity for $7 million from June 1, 2019, through May 31, 2020. The results of both procurement events are reflected in the amounts above. See Note 8 – Related-party Transactions for additional information regarding energy product agreements between Ameren Missouri and Ameren Illinois as a result of the April procurement event. Environmental Matters We are subject to various environmental laws and regulations enforced by federal, state, and local authorities. The development and operation of electric generation, transmission, and distribution facilities and natural gas storage, transmission, and distribution facilities can trigger compliance obligations with respect to environmental laws and regulations. These laws and regulations address emissions, discharges to water, water usage, impacts to air, land, and water, and chemical and waste handling. Complex and lengthy processes are required to obtain and renew approvals, permits, and licenses for new, existing or modified facilities. Additionally, the use and handling of various chemicals or hazardous materials require release prevention plans and emergency response procedures. The EPA has promulgated environmental regulations that have a significant impact on the electric utility industry. Over time, compliance with these regulations could be costly for Ameren Missouri, which operates coal-fired power plants. As of December 31, 2017, Ameren Missouri’s fossil fuel-fired energy centers represented 17% and 33% of Ameren’s and Ameren Missouri’s rate base, respectively. Regulations that apply to air emissions from the electric utility industry include the NSPS, the CSAPR, the MATS, and the National Ambient Air Quality Standards, which are subject to periodic review for certain pollutants. Collectively, these regulations cover a variety of pollutants, such as SO 2 , particulate matter, NO x, mercury, toxic metals, and acid gases, and CO 2 emissions from new power plants. Water intake and discharges from power plants are regulated under the Clean Water Act. Such regulation could require modifications to water intake structures or more stringent limitations on wastewater discharges at Ameren Missouri’s energy centers, either of which could result in significant capital expenditures. The management and disposal of coal ash is regulated under the CCR Rule, which will require the closure of surface impoundments and the installations of dry ash handling systems at several of Ameren Missouri’s energy centers. The individual or combined effects of existing environmental regulations could result in significant capital expenditures, increased operating costs, or the closure or alteration of operations at some of Ameren Missouri’s energy centers. Ameren and Ameren Missouri expect that such compliance costs would be recoverable through rates, subject to MoPSC prudence review, but the timing of costs and their recovery could be subject to regulatory lag. Ameren Missouri’s current plan for compliance with existing air emission regulations includes burning ultra-low-sulfur coal and installing new or optimizing existing pollution control equipment. Ameren and Ameren Missouri estimate that they will need to make capital expenditures of $325 million to $425 million from 2018 through 2022 in order to comply with existing environmental regulations. Additional environmental controls beyond 2022 could be required. This estimate of capital expenditures includes expenditures required by CCR regulations, by the Clean Water Act rule applicable to cooling water intake structures at existing power plants, and by effluent limitation guidelines applicable to steam electric generating units, all of which are discussed below. The actual amount of capital expenditures required to comply with existing environmental regulations may vary substantially from the above estimate because of uncertainty as to whether the EPA will substantially revise regulatory obligations, exactly which compliance strategies will be used and their ultimate cost, among other things. The following sections describe the more significant environmental laws and rules and environmental enforcement and remediation matters that affect or could affect our operations. The EPA has initiated an administrative review of several regulations and 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. The EPA has proposed to repeal and replace the Clean Power Plan and is currently taking public comments on a 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 quarter 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 January 2017, the district court issued a liability ruling that the projects violated provisions of the Clean Air Act and Missouri law. The case then proceeded to the second phase to determine the actions required to remedy the violations found in the liability phase. The EPA previously withdrew all claims for penalties and fines. No date has been set by the district court for a trial on the remedy phase of the litigation. At the conclusion of both phases of the litigation, Ameren Missouri intends to appeal the liability ruling to the United States Court of Appeals for the Eighth Circuit. The ultimate resolution of this matter could have a material adverse effect on the results of operations, financial position, and liquidity of Ameren and Ameren Missouri. Among other things and subject to economic and regulatory considerations, resolution of this matter could result in increased capital expenditures for the installation of pollution control equipment, as well as increased operations and maintenance expenses. We are unable to predict the ultimate resolution of this matter or the costs that might be incurred. Clean Water Act In 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 the facility’s intake screens or entrained through the plant’s cooling water system. All of Ameren Missouri’s coal-fired and nuclear energy centers are subject to the cooling water intake structures rule. The rule will be implemented between 2018 and 2023, during the permit renewal process of each energy center’s water discharge permit. Additionally, in 2015, the EPA issued a rule to revise the effluent limitation guidelines applicable to steam electric generating units. These guidelines established national standards for water discharges that are based on the effectiveness of available control technology. The EPA’s 2015 rule prohibits effluent discharges of certain waste streams and imposes more stringent limitations on certain water discharges from power plants. In September 2017, the EPA published a rule that postponed the compliance dates by two years for the limitations applicable to two specific waste streams so that it could potentially revise those standards. Ameren Missouri is in the process of constructing wastewater treatment facilities 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 September 30, 2018 , associated with CCR storage facilities that reflect the regulations issued in 2015. Ameren plans to close these CCR storage facilities between 2018 and 2023. The recent EPA revisions do not affect Ameren Missouri’s plan. Ameren Missouri estimates it will need to make capital expenditures of $300 million to $350 million from 2018 through 2022 to implement its CCR management compliance plan, 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 September 30, 2018 , Ameren Illinois owned or was otherwise responsible for 44 former MGP sites in Illinois, the majority of which have been investigated, remediated, and closed. Ameren Illinois estimates it could substantially conclude remediation efforts by 2023. The ICC allows Ameren Illinois to recover such remediation and related litigation costs from its electric and natural gas utility customers through environmental cost riders. Costs are subject to annual prudence review by the ICC. As of September 30, 2018 , Ameren Illinois estimated the obligation related to these former MGP sites at $162 million to $226 million . Ameren and Ameren Illinois recorded a liability of $162 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, technical feasibility of certain remediation measures, regulatory changes, disposal costs, and site accessibility. The actual costs and timing of completion may vary substantially from these estimates. Ameren Missouri participated in the investigation of various sites known as Sauget Area 2, located in Sauget, Illinois. In 2000, the EPA notified Ameren Missouri and numerous other companies that former landfills and lagoons at those sites may contain soil and groundwater contamination. In 2013, the EPA issued its record of decision for Sauget Area 2 approving the investigation and the remediation actions recommended by the potentially responsible parties. Ameren Missouri is the owner of one of the sites and in July 2018 reached an agreement with the EPA and Solutia, Inc., the primary potentially responsible party for Sauget Area 2, which limits Ameren Missouri’s cleanup obligation to the site it owns. Remediation efforts at the site are expected to occur in 2019. As of September 30, 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.

Callaway Energy Center

Callaway Energy Center	9 Months Ended
	Sep. 30, 2018
Nuclear Waste Matters [Abstract]
CALLAWAY ENERGY CENTER	CALLAWAY ENERGY CENTER Spent Nuclear Fuel Under the NWPA, the DOE is responsible for disposing of spent nuclear fuel from the Callaway energy center and other commercial nuclear energy centers. The NWPA established the fee paid by Ameren Missouri and other utilities that own and operate those energy centers to the federal government for disposing of the spent nuclear fuel at one mill, or one-tenth of one cent, for each kilowatthour generated and sold by those plants. The NWPA also requires the DOE to review the nuclear waste fee annually against the cost of the nuclear waste disposal program and to propose to the United States Congress any fee adjustment necessary to offset the costs of the program. As required by the NWPA, Ameren Missouri and other utilities have entered into standard contracts with the DOE. Consistent with the NWPA and its standard contract, which stated that the DOE would begin to dispose of spent nuclear fuel by 1998, Ameren Missouri had historically collected one mill from its electric customers for each kilowatthour of electricity that it generated and sold from its Callaway energy center. Because the federal government is not meeting its disposal obligation, the collection of this fee was suspended in 2014. As a result of the DOE’s failure to fulfill its contractual obligations, Ameren Missouri and other nuclear energy center owners sued the DOE to recover costs incurred for ongoing storage of their spent fuel. 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 and $3 million in September 2018 and October 2017, 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. Supplier of Fuel Assemblies The Callaway energy center uses nuclear fuel assemblies fabricated by Westinghouse, which is the only NRC-licensed supplier authorized to provide fuel assemblies to the Callaway energy center. During the first quarter of 2017, Westinghouse filed voluntary petitions for a court-supervised restructuring process under Chapter 11 of the United States Bankruptcy Code. As part of its bankruptcy plan, Westinghouse filed a schedule of assumed contracts, which includes all current contracts between Westinghouse and Ameren Missouri, including the contract for fabrication of fuel assemblies for the Callaway energy center. In April 2018, the bankruptcy court approved Westinghouse’s bankruptcy plan, which included the assumption of its contracts with Ameren Missouri. In August 2018, the plan became effective and Westinghouse emerged from bankruptcy. This restructuring did not affect Westinghouse’s performance under the terms of its existing contracts with Ameren Missouri. 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 based on Ameren Missouri’s updated cost study and funding analysis. The fair value of the trust fund for Ameren Missouri’s Callaway energy center is reported as “Nuclear decommissioning trust fund” in Ameren’s and Ameren Missouri’s balance sheets. This amount is legally restricted and may be used only to fund the costs of nuclear decommissioning. Changes in the fair value of the trust fund are recorded as an increase or decrease to the nuclear decommissioning trust fund, with an offsetting adjustment to the related regulatory liability. If the assumed return on trust assets is not earned, Ameren Missouri believes that it is probable that any such earnings deficiency will be recovered in rates. Insurance The following table presents insurance coverage at Ameren Missouri’s Callaway energy center as of November 1, 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 twelve weeks of an outage, plus up to $3.6 million per week for a minimum of 71 weeks thereafter for a total not exceeding the policy limit of $490 million . Nonradiation events are 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	9 Months Ended
	Sep. 30, 2018
Defined Benefit Plan [Abstract]
RETIREMENT BENEFITS	RETIREMENT BENEFITS 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, $33 million , $17 million , and $8 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 nine months ended September 30, 2017 . Net benefit income of $11 million , $5 million , and $4 million has been similarly retrospectively reclassified on Ameren’s, Ameren Missouri’s, and Ameren Illinois’ respective statements of income for the three months ended September 30, 2017 . 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 (income), prior to capitalization, incurred for Ameren’s pension and postretirement benefit plans for the three and nine months ended September 30, 2018 and 2017 : Pension Benefits Postretirement Benefits Three Months Nine Months Three Months Nine Months 2018 2017 2018 2017 2018 2017 2018 2017 Service cost (a) $ 25 $ 24 $ 75 $ 70 $ 6 $ 6 $ 16 $ 16 Non-service cost components: Interest cost 42 44 126 134 10 12 30 35 Expected return on plan assets (68 ) (65 ) (206 ) (196 ) (20 ) (19 ) (58 ) (56 ) Amortization of: Prior service benefit — (1 ) — (1 ) (1 ) (2 ) (3 ) (4 ) Actuarial loss (gain) 17 14 51 41 (2 ) (2 ) (5 ) (5 ) Total non-service cost components (b) (9 ) (8 ) (29 ) (22 ) (13 ) (11 ) (36 ) (30 ) Net periodic benefit cost (income) $ 16 $ 16 $ 46 $ 48 $ (7 ) $ (5 ) $ (20 ) $ (14 ) (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’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. Ameren Missouri and Ameren Illinois are responsible for their respective shares of Ameren’s pension and postretirement costs. The following table presents the respective share of net periodic pension and other postretirement benefit costs (income) incurred for the three and nine months ended September 30, 2018 and 2017 : Pension Benefits Postretirement Benefits Three Months Nine Months Three Months Nine Months 2018 2017 2018 2017 2018 2017 2018 2017 Ameren Missouri (a) $ 6 $ 6 $ 17 $ 18 $ (1 ) $ (1 ) $ (1 ) $ (3 ) Ameren Illinois 11 10 30 30 (6 ) (3 ) (19 ) (10 ) Other (1 ) — (1 ) — — (1 ) — (1 ) Ameren (a) $ 16 $ 16 $ 46 $ 48 $ (7 ) $ (5 ) $ (20 ) $ (14 ) (a) Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	INCOME TAXES The following table presents a reconciliation of the federal statutory corporate income tax rate to the effective income tax rate for the three and nine months ended September 30, 2018 and 2017 : Ameren Ameren Missouri Ameren Illinois Three Months 2018 2017 2018 2017 2018 2017 Federal statutory corporate income tax rate: 21% 35% 21% 35% 21% 35% Increases (decreases) from: Amortization of excess deferred taxes (6) — (7) (a) — (3) — Other depreciation differences — 1 — 1 (1) (2) Amortization of deferred investment tax credit (1) (1) — (1) — — State tax 6 7 4 3 5 7 TCJA 3 (b) — — — 4 (b) — Other permanent items — (1) — — — — Effective income tax rate 23% 41% 18% 38% 26% 40% Nine Months Federal statutory corporate income tax rate: 21% 35% 21% 35% 21% 35% Increases (decreases) from: Amortization of excess deferred taxes (3) — (4) (a) — (4) — Other depreciation differences — — — 1 — — Amortization of deferred investment tax credit (1) (1) — (1) — — State tax 6 6 4 3 7 5 TCJA 1 (b) — — — 1 (b) — Other permanent items (1) (1) — — — — Effective income tax rate 23% 39% 21% 38% 25% 40% (a) Based on an order issued by the MoPSC in July 2018, Ameren Missouri began amortizing excess deferred taxes in August 2018. See Note 2 – Rate and Regulatory Matters for additional information. (b) The Ameren Companies updated their respective provisional estimates recorded related to TCJA, as discussed below. Federal Tax Reform As of December 31, 2017, the Ameren Companies made provisional estimates for the measurement and accounting of certain effects of the TCJA in accordance with SEC guidance, which provides for a one-year period in which to complete the required analysis and update provisional estimates. During the three and nine months ended September 30, 2018 , Ameren, Ameren Missouri, and Ameren Illinois updated their respective provisional estimates 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. As of September 30, 2018, our provisional estimates also include amounts related to compensation-related deductions, which remain subject to adjustment based on any additional guidance that may be issued. Missouri Income Tax Rate In June 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 the second quarter of 2018, Ameren’s and Ameren Missouri’s accumulated deferred tax balances were revalued, resulting in a net decrease of $33 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.

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 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 comprises 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 at 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 revenues, net income attributable to common shareholders, and capital expenditures by segment at Ameren and Ameren Illinois for the three and nine months ended September 30, 2018 and 2017 . Ameren, Ameren Missouri, and Ameren Illinois management review segment capital expenditure information rather than any individual or total asset amount. Ameren Three Months Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Other Intersegment Eliminations Consolidated 2018 External revenues $ 1,118 $ 392 $ 116 $ 98 $ — $ — $ 1,724 Intersegment revenues 11 — — 15 — (26 ) — Net income attributable to Ameren common shareholders 294 35 — 48 (a) (20 ) — 357 Capital expenditures 210 135 111 124 1 (4 ) 577 2017 External revenues $ 1,105 $ 404 $ 111 $ 105 $ (2 ) $ — $ 1,723 Intersegment revenues 11 — 1 14 — (26 ) — Net income attributable to Ameren common shareholders 234 31 2 38 (a) (17 ) — 288 Capital expenditures 178 112 71 173 (2 ) (7 ) 525 Nine Months 2018 External revenues $ 2,848 $ 1,177 $ 569 $ 278 $ — $ — $ 4,872 Intersegment revenues 28 2 — 42 — (72 ) — Net income attributable to Ameren common shareholders 500 101 49 121 (a) (24 ) — 747 Capital expenditures 664 389 237 399 6 (6 ) 1,689 2017 External revenues $ 2,800 $ 1,175 $ 509 $ 293 $ (2 ) $ — $ 4,775 Intersegment revenues 41 3 1 33 — (78 ) — Net income attributable to Ameren common shareholders 359 94 40 106 (a) (16 ) — 583 Capital expenditures 533 354 180 463 3 (10 ) 1,523 (a) Ameren Transmission earnings include an allocation of financing costs from Ameren (parent). Ameren Illinois Three Months Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Total 2018 External revenues $ 392 $ 116 $ 56 $ — $ 564 Intersegment revenues — — 15 (15 ) — Net income available to common shareholder 35 — 28 — 63 Capital expenditures 135 111 99 — 345 2017 External revenues $ 404 $ 112 $ 58 $ — $ 574 Intersegment revenues — — 14 (14 ) — Net income available to common shareholder 31 2 22 — 55 Capital expenditures 112 71 93 — 276 Nine Months 2018 External revenues $ 1,179 $ 569 $ 154 $ — $ 1,902 Intersegment revenues — — 41 (41 ) — Net income available to common shareholder 101 49 70 — 220 Capital expenditures 389 237 321 — 947 2017 External revenues $ 1,178 $ 510 $ 165 $ — $ 1,853 Intersegment revenues — — 32 (32 ) — Net income available to common shareholder 94 40 57 — 191 Capital expenditures 354 180 226 — 760 The following tables present disaggregated revenues by segment at Ameren and Ameren Illinois for the three and nine months ended September 30, 2018 and 2017 . 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 Three Months Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Other Intersegment Eliminations Consolidated 2018 Residential $ 508 $ 223 $ — $ — $ — $ — $ 731 Commercial 417 131 — — — — 548 Industrial 101 28 — — — — 129 Other 85 (a) 10 — 113 — (26 ) 182 (a) Total electric revenues $ 1,111 $ 392 $ — $ 113 $ — $ (26 ) $ 1,590 Residential $ 8 $ — $ 68 $ — $ — $ — $ 76 Commercial 3 — 20 — — — 23 Industrial 1 — 1 — — — 2 Other 6 — 27 — — — 33 Total gas revenues $ 18 $ — $ 116 $ — $ — $ — $ 134 Total revenues (b) $ 1,129 $ 392 $ 116 $ 113 $ — $ (26 ) $ 1,724 2017 Residential $ 491 $ 224 $ — $ — $ — $ — $ 715 Commercial 409 133 — — — — 542 Industrial 100 27 — — — — 127 Other 99 20 — 119 (2 ) (26 ) 210 Total electric revenues $ 1,099 $ 404 $ — $ 119 $ (2 ) $ (26 ) $ 1,594 Residential $ 9 $ — $ 72 $ — $ — $ — $ 81 Commercial 4 — 21 — — — 25 Industrial 1 — 2 — — — 3 Other 3 — 17 — — — 20 Total gas revenues $ 17 $ — $ 112 $ — $ — $ — $ 129 Total revenues (b) $ 1,116 $ 404 $ 112 $ 119 $ (2 ) $ (26 ) $ 1,723 Nine Months 2018 Residential $ 1,272 $ 663 $ — $ — $ — $ — $ 1,935 Commercial 1,033 381 — — — — 1,414 Industrial 249 96 — — — — 345 Other 228 (a) 39 — 320 — (72 ) 515 (a) Total electric revenues $ 2,782 $ 1,179 $ — $ 320 $ — $ (72 ) $ 4,209 Residential $ 62 $ — $ 408 $ — $ — $ — $ 470 Commercial 25 — 113 — — — 138 Industrial 3 — 12 — — — 15 Other 4 — 36 — — — 40 Total gas revenues $ 94 $ — $ 569 $ — $ — $ — $ 663 Total revenues (b) $ 2,876 $ 1,179 $ 569 $ 320 $ — $ (72 ) $ 4,872 2017 Residential $ 1,135 $ 651 $ — $ — $ — $ — $ 1,786 Commercial 971 395 — — — — 1,366 Industrial 242 83 — — — — 325 Other 410 49 — 326 (2 ) (77 ) 706 Total electric revenues $ 2,758 $ 1,178 $ — $ 326 $ (2 ) $ (77 ) $ 4,183 Residential $ 49 $ — $ 359 $ — $ — $ — $ 408 Commercial 20 — 100 — — — 120 Industrial 3 — 7 — — — 10 Other 11 — 44 — — (1 ) 54 Total gas revenues $ 83 $ — $ 510 $ — $ — $ (1 ) $ 592 Total revenues (b) $ 2,841 $ 1,178 $ 510 $ 326 $ (2 ) $ (78 ) $ 4,775 (a) Includes $13 million and $60 million for the three and nine months ended September 30, 2018 , respectively, for the reduction to revenue for the excess amounts collected in rates related to the TCJA from January 1, 2018, through September 30, 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 three and nine months ended September 30, 2018 and 2017 : Three Months Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Consolidated 2018 Revenues from alternative revenue programs $ 1 $ (98 ) $ (2 ) $ (12 ) $ (111 ) Other revenues not from contracts with customers 3 1 1 — 5 2017 Revenues from alternative revenue programs $ (6 ) $ (96 ) $ (1 ) $ (2 ) $ (105 ) Other revenues not from contracts with customers 4 2 — — 6 Nine Months 2018 Revenues from alternative revenue programs $ (8 ) $ (52 ) $ (10 ) $ (21 ) $ (91 ) Other revenues not from contracts with customers 22 14 2 — 38 2017 Revenues from alternative revenue programs $ (20 ) $ (47 ) $ 11 $ 5 $ (51 ) Other revenues not from contracts with customers 11 5 2 — 18 Ameren Illinois Three Months Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Total Ameren Illinois 2018 Residential $ 223 $ 68 $ — $ — $ 291 Commercial 131 20 — — 151 Industrial 28 1 — — 29 Other 10 27 71 (15 ) 93 Total revenues (a) $ 392 $ 116 $ 71 $ (15 ) $ 564 2017 Residential $ 224 $ 72 $ — $ — $ 296 Commercial 133 21 — — 154 Industrial 27 2 — — 29 Other 20 17 72 (14 ) 95 Total revenues (a) $ 404 $ 112 $ 72 $ (14 ) $ 574 Nine Months 2018 Residential $ 663 $ 408 $ — $ — $ 1,071 Commercial 381 113 — — 494 Industrial 96 12 — — 108 Other 39 36 195 (41 ) 229 Total revenues (a) $ 1,179 $ 569 $ 195 $ (41 ) $ 1,902 2017 Residential $ 651 $ 359 $ — $ — $ 1,010 Commercial 395 100 — — 495 Industrial 83 7 — — 90 Other 49 44 197 (32 ) 258 Total revenues (a) $ 1,178 $ 510 $ 197 $ (32 ) $ 1,853 (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 three and nine months ended September 30, 2018 and 2017 : Three Months Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Consolidated 2018 Revenues from alternative revenue programs $ (98 ) $ (2 ) $ (10 ) $ (110 ) Other revenues not from contracts with customers 1 1 — 2 2017 Revenues from alternative revenue programs $ (96 ) $ (1 ) $ (2 ) $ (99 ) Other revenues not from contracts with customers 2 — — 2 Nine Months 2018 Revenues from alternative revenue programs $ (52 ) $ (10 ) $ (19 ) $ (81 ) Other revenues not from contracts with customers 14 2 — 16 2017 Revenues from alternative revenue programs $ (47 ) $ 11 $ 3 $ (33 ) Other revenues not from contracts with customers 5 2 — 7

Summary Of Significant Accoun_2

Summary Of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Nature of Operations	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 has other subsidiaries that conduct other activities, such as providing shared services. Ameren evaluates competitive electric transmission investment opportunities as they arise. • Union Electric Company, doing business as Ameren Missouri, operates a rate-regulated electric generation, transmission, and distribution business and a rate-regulated natural gas distribution business in Missouri. • Ameren Illinois Company, doing business as Ameren Illinois, operates rate-regulated electric transmission, electric distribution, and natural gas distribution businesses in Illinois. • ATXI operates a FERC rate-regulated electric transmission business. ATXI is developing MISO-approved electric transmission projects, including the Illinois Rivers and Mark Twain projects, and placed the Spoon River project in service in February 2018.
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. As of September 30, 2018 , and December 31, 2017 , Ameren had unconsolidated variable interests as a limited partner in various equity method investments, totaling $20 million and $17 million , respectively, included in “Other assets” on Ameren’s consolidated balance sheet. Ameren is not the primary beneficiary of these investments because it does not have the power to direct matters that most significantly affect the activities of these variable interest entities. As of September 30, 2018 , the maximum exposure to loss related to these variable interests is limited to the investment in these partnerships of $20 million plus associated outstanding funding commitments of $17 million . Our accounting policies conform to GAAP. Our financial statements reflect all adjustments (which include normal, recurring adjustments) that are necessary, in our opinion, for a fair presentation of our results. The preparation of financial statements in conformity with GAAP requires management to make certain estimates and assumptions. Such estimates and assumptions affect reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the dates of financial statements, and the reported amounts of revenues and expenses during the reported periods. Actual results could differ from those estimates. The results of operations of an interim period may not give a true indication of results that may be expected for a full year. These financial statements should be read in conjunction with the financial statements and accompanying notes included in the Form 10-K.
Cash and Cash Equivalents and Restricted Cash	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.
Company-owned Life Insurance	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 September 30, 2018 , the cash surrender value of company-owned life insurance at Ameren and Ameren Illinois was $ 256 million (December 31, 2017 – $265 million ) and $ 120 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.
Performance Shares and Restricted Stock Units Policy	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 performance share unit granted in 2018 was determined to be $62.88 , which was based on Ameren’s closing common share price of $58.99 at December 31, 2017 , 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 beginning January 1, 2018, relative to the designated peer group. The significant assumptions used to calculate fair value included a three -year risk-free rate of 1.98% and volatility of 15% to 23% for the peer group. 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.
Operating Revenues	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 is 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 13 – 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 include disclosure related to 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 September 30, 2018 and 2017 , our remaining performance obligations were immaterial.
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.
Accounting and Reporting Developments	Accounting and Reporting Developments 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 11 – 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. See Note 1 – Summary of Significant Accounting Policies under Part II, Item 8, of the Form 10-K for additional information about recently issued authoritative accounting standards relating to the measurement of credit losses on financial instruments and the reclassification of certain tax effects from accumulated OCI. Leases In February 2016, the FASB issued authoritative guidance that requires an entity to recognize assets and liabilities arising from all leases with a term greater than one year. Consistent with current GAAP, the recognition, measurement, and presentation of expenses and cash flows arising from a lease will depend on its classification as a finance lease or operating lease. The guidance also requires additional disclosures to enable users of financial statements to understand the amount, timing, and uncertainty of cash flows arising from leases. This guidance will affect the Ameren Companies’ financial position by increasing the assets and liabilities recorded relating to their operating leases. We are also assessing the impacts of this guidance on our results of operations, cash flows, and disclosures. We have selected a software vendor and are in the process of implementing system changes required for the implementation of this guidance. We are currently assessing our agreements to determine those that are within the scope of this guidance. This guidance will be effective for the Ameren Companies in the first quarter of 2019. In July 2018, the FASB issued authoritative guidance that provides entities with an optional transition method for adopting the new leases standard. Under this optional transition method, the Ameren Companies may adopt the new leases standard by recognizing a cumulative-effect adjustment to our January 1, 2019, retained earnings balances. Periods prior to 2019 would continue to be presented and disclosed in the financial statements in accordance with current GAAP. We are currently assessing whether we will elect this optional transition method. Fair Value Measurement Disclosures In August 2018, the FASB issued authoritative guidance that affects disclosure requirements for fair value measurements. The 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. The 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. The 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. The amount of implementation costs that were capitalized in the third quarter of 2018 was immaterial. SEC Disclosure Update and Simplification In August 2018, the SEC adopted a final rule that requires, among other things, inclusion of a statement of changes in shareholders’ equity, or disclosure of such changes, and disclosure of the amount of dividends per share for each class of shares with respect to interim periods. The guidance will be effective for the Ameren Companies in the fourth quarter of 2018. We are currently assessing the impact of this guidance on our disclosures.
Derivatives	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.

Summary Of Significant Accoun_3

Summary Of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Schedule of Cash and Cash Equivalents Including Restricted Cash	The following table provides a reconciliation of cash, cash equivalents, and restricted cash reported within the balance sheets and the statements of cash flows as of September 30, 2018 and 2017 , and December 31, 2017 and 2016 : September 30, 2018 December 31, 2017 September 30, 2017 December 31, 2016 Ameren Ameren Ameren Ameren Ameren Ameren Ameren Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Cash and cash equivalents (a) $ 11 $ — $ — $ 10 $ — $ — $ 9 $ — $ — $ 9 $ — $ — Restricted cash included in “Other current assets” 15 4 8 21 5 6 19 4 5 20 4 6 Restricted cash included in “Other assets” 63 — 63 35 — 35 27 — 27 22 — 22 Restricted cash included in “Nuclear decommissioning trust fund” 3 3 (b) 2 2 (b) 2 2 (b) 1 1 (b) Total cash, cash equivalents, and restricted cash (c) $ 92 $ 7 $ 71 $ 68 $ 7 $ 41 $ 57 $ 6 $ 32 $ 52 $ 5 $ 28 (a) As presented on the balance sheets. (b) Not applicable. (c) As presented on the statements of cash flows.
Schedule of Cash Flow, Supplemental Disclosures	The following table provides noncash investing activity excluded from the statements of cash flows for the nine months ended September 30, 2018 and 2017 : September 30, 2018 September 30, 2017 Ameren (a) Ameren Missouri Ameren Illinois Ameren (a) Ameren Missouri Ameren Illinois Accrued capital expenditures $ 240 $ 94 $ 133 $ 202 $ 70 $ 100 Net realized and unrealized gain – nuclear decommissioning trust fund 33 33 (b) 53 53 (b) (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries. (b) Not applicable.
Asset Retirement Obligation Disclosure	The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the nine months ended September 30, 2018 : Ameren Missouri Ameren Illinois (a) Ameren Balance at December 31, 2017 $ 640 (b) $ 4 $ 644 (b) Liabilities settled (4 ) (c) (4 ) Accretion (d) 20 (c) 20 Change in estimates (e) (14 ) — (14 ) Balance at September 30, 2018 $ 642 (b) $ 4 $ 646 (b) (a) Included in “Other deferred credits and liabilities” on the balance sheet. (b) Balance included $6 million in “Other current liabilities” on the balance sheet as of both December 31, 2017 , and September 30, 2018 . (c) Less than $1 million. (d) Accretion expense attributable to Ameren Missouri was recorded as a decrease to regulatory liabilities. (e) Ameren Missouri changed its fair value estimate primarily due to a reduction in the cost estimate for closure of certain CCR storage facilities
Summary Of Nonvested Shares Related To Long-Term Incentive Plan	The following table summarizes Ameren's nonvested performance share unit and restricted stock unit activity for the nine months ended September 30, 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 313,984 62.88 186,728 57.65 Forfeitures (62,865 ) 50.78 (4,964 ) 58.99 Undistributed vested units (b) (217,350 ) 53.57 (19,742 ) 59.01 Vested and distributed (176,043 ) 52.88 — — Nonvested at September 30, 2018 (c) 753,215 $ 56.31 162,022 $ 57.44 (a) Does not include 712,572 undistributed vested performance share units. (b) Undistributed vested units are awards that vested due to attainment of retirement eligibility by certain employees, but have not yet been distributed. For undistributed vested 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 548,542 undistributed vested performance share units and 19,742 undistributed vested restricted stock units.
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 three and nine months ended September 30, 2018 and 2017 : Three Months Nine Months 2018 2017 2018 2017 Ameren Missouri $ 52 $ 51 $ 133 $ 122 Ameren Illinois 26 26 (a) 89 82 (a) Ameren $ 78 $ 77 (a) $ 222 $ 204 (a) (a) Amounts have been adjusted from those previously reported to reflect additional excise taxes for the three and nine months ended September 30, 2017.

Short-Term Debt And Liquidity S

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Tables)	9 Months Ended
	Sep. 30, 2018
Debt Disclosure [Abstract]
Schedule of Short-term Debt	The following table presents commercial paper outstanding, net of issuance discounts, as of September 30, 2018 , and December 31, 2017 : September 30, 2018 December 31, 2017 Ameren (parent) $ 413 $ 383 Ameren Missouri — 39 Ameren Illinois 108 62 Ameren Consolidated $ 521 $ 484 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 nine months ended September 30, 2018 and 2017: Ameren (parent) Ameren Missouri Ameren Illinois Ameren Consolidated 2018 Average daily commercial paper outstanding at par value $ 431 $ 81 $ 117 $ 629 Weighted-average interest rate 2.23 % 1.94 % 2.21 % 2.18 % Peak commercial paper during period at par value (a) $ 543 $ 481 $ 442 $ 1,295 Peak interest rate 2.45 % 2.42 % 2.55 % 2.55 % 2017 Average daily commercial paper outstanding at par value $ 669 $ 7 $ 78 $ 754 Weighted-average interest rate 1.27 % 1.20 % 1.28 % 1.27 % Peak commercial paper during period at par value (a) $ 841 $ 64 $ 193 $ 948 Peak interest rate 1.50 % 1.41 % 1.50 % 1.50 % (a) The timing of peak outstanding commercial paper issuances varies by company. Therefore, the sum of individual company peak amounts may not equal the Ameren Consolidated peak commercial paper issuances for the period.

Other Income and Expenses (Tabl

Other Income and Expenses (Tables)	9 Months Ended
	Sep. 30, 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 three and nine months ended September 30, 2018 and 2017 : Three Months Nine Months 2018 2017 2018 2017 Ameren: (a) Other Income, Net Allowance for equity funds used during construction $ 11 $ 6 $ 25 $ 16 Interest income on industrial development revenue bonds 6 7 19 20 Other interest income 2 — 6 5 Non-service cost components of net periodic benefit income 17 (b) 11 52 (b) 33 Other income 2 1 5 3 Donations (4 ) — (15 ) (7 ) Other expense (2 ) (2 ) (8 ) (9 ) Total Other Income, Net $ 32 $ 23 $ 84 $ 61 Ameren Missouri: Other Income, Net Allowance for equity funds used during construction $ 8 $ 6 $ 19 $ 15 Interest income on industrial development revenue bonds 6 7 19 20 Other interest income 1 — 2 1 Non-service cost components of net periodic benefit income 4 (b) 5 13 (b) 17 Other income 2 — 3 1 Donations (3 ) — (6 ) (2 ) Other expense (2 ) (2 ) (5 ) (4 ) Total Other Income, Net $ 16 $ 16 $ 45 $ 48 Ameren Illinois: Other Income, Net Allowance for equity funds used during construction $ 3 $ — $ 6 $ 1 Interest income 1 1 4 5 Non-service cost components of net periodic benefit income 8 4 25 8 Other income 1 — 3 2 Donations — — (5 ) (5 ) Other expense (2 ) — (3 ) (3 ) Total Other Income, Net $ 11 $ 5 $ 30 $ 8 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations. (b) For the three and nine months ended September 30, 2018 , the non-service cost components of net periodic benefit income were partially offset by a $5 million and $13 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 Instrume_2

Derivative Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Open Gross Derivative Volumes By Commodity Type	The following table presents open gross commodity contract volumes by commodity type for derivative assets and liabilities as of September 30, 2018 , and December 31, 2017 . As of September 30, 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) 42 (b) 42 28 (b) 28 Natural gas (in mmbtu) 20 160 180 24 139 163 Power (in megawatthours) 2 8 10 3 9 12 (a) Consists of ultra-low-sulfur diesel products. (b) Not applicable.
Derivative Instruments Carrying Value	The following table presents the carrying value and balance sheet location of all derivative commodity contracts, none of which were designated as hedging instruments, as of September 30, 2018 , and December 31, 2017 : Balance Sheet Location Ameren Missouri Ameren Illinois Ameren 2018 Fuel oils Other current assets $ 10 $ — $ 10 Other assets 6 — 6 Natural gas Other current assets — 1 1 Other assets — 1 1 Power Other current assets 3 — 3 Total assets (a) $ 19 $ 2 $ 21 Fuel oils Other deferred credits and liabilities $ 1 $ — $ 1 Natural gas Other current liabilities 4 12 16 Other deferred credits and liabilities 2 9 11 Power Other current liabilities 3 13 16 Other deferred credits and liabilities — 174 174 Total liabilities (b) $ 10 $ 208 $ 218 2017 Fuel oils Other current assets $ 5 $ — $ 5 Other assets 2 — 2 Natural gas Other assets 1 — 1 Power Other current assets 9 — 9 Total assets (a) $ 17 $ — $ 17 Natural gas Other current liabilities $ 5 $ 12 $ 17 Other deferred credits and liabilities 3 10 13 Power Other current liabilities 1 13 14 Other deferred credits and liabilities — 182 182 Total liabilities (b) $ 9 $ 217 $ 226 (a) The cumulative amount of pretax net gains on all derivative instruments is deferred as a regulatory liability. (b) The cumulative amount of pretax net losses on all derivative instruments is deferred as a regulatory asset.
Derivative Instruments With Credit Risk-Related Contingent Features	The following table presents, as of September 30, 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 September 30, 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 $ 71 $ 4 $ 51 Ameren Illinois 49 — 43 Ameren $ 120 $ 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)	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value Disclosures	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 valuations 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 provides a fair value hierarchy that prioritizes the inputs used to measure fair value. On a quarterly basis, all financial assets and liabilities carried at fair value are classified and disclosed in one of three hierarchy levels. Financial assets and liabilities are classified in their entirety according to the lowest level of input that is significant to the fair value measurement. See Note 8 – Fair Value Measurements under Part II, Item 8, of the Form 10-K for information related to hierarchy levels. 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). We have also factored the impact of our credit standing, as well as any potential credit enhancements, into the fair value measurement of both derivative assets and derivative liabilities. Included in our valuation, and based on current market conditions, is a valuation adjustment for counterparty default derived from market data such as the price of credit default swaps, bond yields, and credit ratings. No gains or losses related to valuation adjustments for counterparty default risk were recorded at Ameren, Ameren Missouri, or Ameren Illinois in the three and nine months ended September 30, 2018 or 2017 . At September 30, 2018 , and December 31, 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 September 30, 2018 : Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Total Assets: Ameren Derivative assets – commodity contracts (a) : Fuel oils $ 8 $ — $ 8 $ 16 Natural gas — 1 1 2 Power — — 3 3 Total derivative assets – commodity contracts $ 8 $ 1 $ 12 $ 21 Nuclear decommissioning trust fund: Equity securities: U.S. large capitalization $ 497 $ — $ — $ 497 Debt securities: U.S. Treasury and agency securities — 139 — 139 Corporate bonds — 77 — 77 Other — 33 — 33 Total nuclear decommissioning trust fund $ 497 $ 249 $ — $ 746 (b) Total Ameren $ 505 $ 250 $ 12 $ 767 Ameren Missouri Derivative assets – commodity contracts (a) : Fuel oils $ 8 $ — $ 8 $ 16 Power — — 3 3 Total derivative assets – commodity contracts $ 8 $ — $ 11 $ 19 Nuclear decommissioning trust fund: Equity securities: U.S. large capitalization $ 497 $ — $ — $ 497 Debt securities: U.S. Treasury and agency securities — 139 — 139 Corporate bonds — 77 — 77 Other — 33 — 33 Total nuclear decommissioning trust fund $ 497 $ 249 $ — $ 746 (b) Total Ameren Missouri $ 505 $ 249 $ 11 $ 765 Ameren Illinois Derivative assets – commodity contracts (a) : Natural gas $ — $ 1 $ 1 $ 2 Liabilities: Ameren Derivative liabilities – commodity contracts (a) : Fuel oils $ — $ — $ 1 $ 1 Natural gas 1 21 5 27 Power — — 190 190 Total Ameren $ 1 $ 21 $ 196 $ 218 Ameren Missouri Derivative liabilities – commodity contracts (a) : Fuel oils $ — $ — $ 1 $ 1 Natural gas — 6 — 6 Power — — 3 3 Total Ameren Missouri $ — $ 6 $ 4 $ 10 Ameren Illinois Derivative liabilities – commodity contracts (a) : Natural gas $ 1 $ 15 $ 5 $ 21 Power — — 187 187 Total Ameren Illinois $ 1 $ 15 $ 192 $ 208 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Balance excludes $6 million of cash and cash equivalents, receivables, payables, and accrued income, net. The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of December 31, 2017 : Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Total Assets: Ameren Derivative assets – commodity contracts (a) : Fuel oils $ 4 $ — $ 3 $ 7 Natural gas — — 1 1 Power — 1 8 9 Total derivative assets – commodity contracts $ 4 $ 1 $ 12 $ 17 Nuclear decommissioning trust fund: Equity securities: U.S. large capitalization $ 468 $ — $ — $ 468 Debt securities: U.S. Treasury and agency securities — 125 — 125 Corporate bonds — 82 — 82 Other — 25 — 25 Total nuclear decommissioning trust fund $ 468 $ 232 $ — $ 700 (b) Total Ameren $ 472 $ 233 $ 12 $ 717 Ameren Missouri Derivative assets – commodity contracts (a) : Fuel oils $ 4 $ — $ 3 $ 7 Natural gas — — 1 1 Power — 1 8 9 Total derivative assets – commodity contracts $ 4 $ 1 $ 12 $ 17 Nuclear decommissioning trust fund: Equity securities: U.S. large capitalization $ 468 $ — $ — $ 468 Debt securities: U.S. Treasury and agency securities — 125 — 125 Corporate bonds — 82 — 82 Other — 25 — 25 Total nuclear decommissioning trust fund $ 468 $ 232 $ — $ 700 (b) Total Ameren Missouri $ 472 $ 233 $ 12 $ 717 Liabilities: Ameren Derivative liabilities – commodity contracts (a) : Natural gas $ 1 $ 25 $ 4 $ 30 Power — — 196 196 Total Ameren $ 1 $ 25 $ 200 $ 226 Ameren Missouri Derivative liabilities – commodity contracts (a) : Natural gas $ — $ 7 $ 1 $ 8 Power — — 1 1 Total Ameren Missouri $ — $ 7 $ 2 $ 9 Ameren Illinois Derivative liabilities – commodity contracts (a) : Natural gas $ 1 $ 18 $ 3 $ 22 Power — — 195 195 Total Ameren Illinois $ 1 $ 18 $ 198 $ 217 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Balance excludes $4 million of cash and cash equivalents, receivables, payables, and accrued income, net. All costs related to financial assets and liabilities classified as Level 3 in the fair value hierarchy are expected to be recoverable through customer rates; therefore, there is no impact to net income resulting from changes in the fair value of these instruments. For the three and nine months ended September 30, 2018 and 2017 , the balances and changes in the fair value of Level 3 financial assets and liabilities associated with fuel oils and natural gas were immaterial. The following table summarizes the changes in the fair value of power financial assets and liabilities classified as Level 3 in the fair value hierarchy: Net derivative commodity contracts Ameren Missouri Ameren Illinois Ameren For the three months ended September 30, 2018 Beginning balance at July 1, 2018 $ 5 $ (190 ) $ (185 ) Realized and unrealized losses included in regulatory assets/liabilities (4 ) — (4 ) Purchases 1 — 1 Settlements (1 ) 3 2 Transfers out of Level 3 (1 ) — (1 ) Ending balance at September 30, 2018 $ — $ (187 ) $ (187 ) Change in unrealized losses related to assets/liabilities held at September 30, 2018 $ — $ — $ — For the three months ended September 30, 2017 Beginning balance at July 1, 2017 $ 14 $ (192 ) $ (178 ) Realized and unrealized losses included in regulatory assets/liabilities (2 ) (3 ) (5 ) Sales 1 — 1 Settlements (3 ) 3 — Ending balance at September 30, 2017 $ 10 $ (192 ) $ (182 ) Change in unrealized losses related to assets/liabilities held at September 30, 2017 $ — $ (2 ) $ (2 ) For the nine months ended September 30, 2018 Beginning balance at January 1, 2018 $ 7 $ (195 ) $ (188 ) Realized and unrealized losses included in regulatory assets/liabilities (7 ) (1 ) (8 ) Purchases 5 — 5 Settlements (4 ) 9 5 Transfers out of Level 3 (1 ) — (1 ) Ending balance at September 30, 2018 $ — $ (187 ) $ (187 ) Change in unrealized losses related to assets/liabilities held at September 30, 2018 $ (1 ) $ (2 ) $ (3 ) For the nine months ended September 30, 2017 Beginning balance at January 1, 2017 $ 7 $ (185 ) $ (178 ) Realized and unrealized losses included in regulatory assets/liabilities (3 ) (14 ) (17 ) Purchases 15 — 15 Sales 1 — 1 Settlements (10 ) 7 (3 ) Ending balance at September 30, 2017 $ 10 $ (192 ) $ (182 ) Change in unrealized losses related to assets/liabilities held at September 30, 2017 $ — $ (15 ) $ (15 ) 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 three and nine months ended September 30, 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. The following table describes the valuation techniques and unobservable inputs utilized by the Ameren Companies for the fair value of financial assets and liabilities measured at fair value on a recurring basis and classified as Level 3 in the fair value hierarchy as of September 30, 2018 , and December 31, 2017 : Fair Value Weighted Average Assets Liabilities Valuation Technique(s) Unobservable Input Range Level 3 Derivative asset and liability – commodity contracts (a) : 2018 Fuel oils $ 8 $ (1 ) Option model Volatilities(%) (b) 20 – 34 23 Discounted cash flow Counterparty credit risk(%) (c)(d) 0.21 – 0.79 0.55 Ameren Missouri credit risk(%) (c)(d) 0.35 0.35 Natural gas 1 (5 ) Discounted cash flow Nodal basis ($/mmbtu) (b) (1.30) – 0.70 (0.80) Counterparty credit risk (%) (c)(d) 0.32 – 0.95 0.77 Ameren Illinois credit risk (%) (c)(d) 0.35 0.35 Power (e) 3 (190 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps ($/MWh) (f) 23 – 40 28 Estimated auction price for FTRs ($/MW) (b) (911) – 1,504 19 Nodal basis ($/MWh) (f) (10) – 0 (2) Counterparty credit risk (%) (c)(d) 0.95 0.95 Ameren Illinois credit risk (%) (c)(d) 0.35 0.35 Fundamental energy production model Estimated future natural gas prices ($/mmbtu) (b) 2 - 3 3 Escalation rate (%) (b)(g) 4 4 Contract price allocation Estimated renewable energy credit costs ($/credit) (b) 5 – 7 6 2017 Fuel oils $ 3 $ — Option model Volatilities (%) (b) 20 – 26 22 Discounted cash flow Counterparty credit risk (%) (c)(d) 0.12 – 0.72 0.41 Ameren Missouri credit risk (%) (c)(d) 0.37 0.37 Natural gas 1 (4 ) Option model Volatilities (%) (b) 26 – 46 37 Nodal basis ($/mmbtu) (c) (0.50) – (0.30) (0.40) Discounted cash flow Nodal basis ($/mmbtu) (b) (1.20) – 0.10 (1) Counterparty credit risk (%) (c)(d) 0.37 – 0.92 0.53 Ameren credit risk (%) (c)(d) 0.37 0.37 Power (e) 8 (196 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps ($/MWh) (f) 24 – 46 28 Estimated auction price for FTRs ($/MW) (b) (65) – 1,823 251 Nodal basis ($/MWh) (f) (10) – 0 (2) Counterparty credit risk (%) (c)(d) 0.28 0.28 Ameren Illinois credit risk (%) (c)(d) 0.37 0.37 Fundamental energy production model Estimated future natural gas prices ($/mmbtu) (b) 3 – 4 3 Escalation rate (%) (b)(g) 5 5 Contract price allocation Estimated renewable energy credit costs ($/credit) (b) 5 – 7 6 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement. (c) Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement. (d) Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances. (e) Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2022 for September 30, 2018, and through 2021 for December 31, 2017. Valuations beyond 2022 for September 30, 2018, and 2021 for December 31, 2017, use fundamentally modeled pricing by month for peak and off-peak demand. (f) The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes due to their opposing positions. (g) 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 September 30, 2018 , and December 31, 2017 : September 30, 2018 Carrying Amount Fair Value Level 1 Level 2 Level 3 Total Ameren: Cash, cash equivalents, and restricted cash $ 92 $ 92 $ — $ — $ 92 Investments in held-to-maturity debt securities (a) 276 — 276 — 276 Short-term debt 521 — 521 — 521 Long-term debt (including current portion) (a) 8,263 (b) — 7,981 412 (c) 8,393 Preferred stock (d) 142 — 139 — 139 Ameren Missouri: Cash, cash equivalents, and restricted cash $ 7 $ 7 $ — $ — $ 7 Advances to money pool 28 — 28 — 28 Investments in held-to-maturity debt securities (a) 276 — 276 — 276 Long-term debt (including current portion) (a) 4,004 (b) — 4,173 — 4,173 Preferred stock 80 — 78 — 78 Ameren Illinois: Cash, cash equivalents, and restricted cash $ 71 $ 71 $ — $ — $ 71 Short-term debt 108 — 108 — 108 Borrowings from money pool 45 — 45 — 45 Long-term debt (including current portion) 3,114 (b) — 3,134 — 3,134 Preferred stock 62 — 61 — 61 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 Preferred stock (d) 142 — 131 — 131 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 Preferred stock 80 — 80 — 80 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 Preferred stock 62 — 51 — 51 (a) Ameren and Ameren Missouri have investments in industrial revenue bonds, classified as held-to-maturity and recorded in “Other Assets,” that are equal to the debt obligation for CTs leased from the city of Bowling Green and Audrain County. As of September 30, 2018 , and December 31, 2017 , the carrying amount of both the investments in industrial revenue bonds and the debt obligations approximated fair value. (b) Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $56 million , $23 million , and $27 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of September 30, 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. (d) Preferred stock is recorded in “Noncontrolling Interests” on the consolidated balance sheet.
Schedule of Financial Assets and Liabilities	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 September 30, 2018 , and December 31, 2017 : September 30, 2018 Carrying Amount Fair Value Level 1 Level 2 Level 3 Total Ameren: Cash, cash equivalents, and restricted cash $ 92 $ 92 $ — $ — $ 92 Investments in held-to-maturity debt securities (a) 276 — 276 — 276 Short-term debt 521 — 521 — 521 Long-term debt (including current portion) (a) 8,263 (b) — 7,981 412 (c) 8,393 Preferred stock (d) 142 — 139 — 139 Ameren Missouri: Cash, cash equivalents, and restricted cash $ 7 $ 7 $ — $ — $ 7 Advances to money pool 28 — 28 — 28 Investments in held-to-maturity debt securities (a) 276 — 276 — 276 Long-term debt (including current portion) (a) 4,004 (b) — 4,173 — 4,173 Preferred stock 80 — 78 — 78 Ameren Illinois: Cash, cash equivalents, and restricted cash $ 71 $ 71 $ — $ — $ 71 Short-term debt 108 — 108 — 108 Borrowings from money pool 45 — 45 — 45 Long-term debt (including current portion) 3,114 (b) — 3,134 — 3,134 Preferred stock 62 — 61 — 61 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 Preferred stock (d) 142 — 131 — 131 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 Preferred stock 80 — 80 — 80 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 Preferred stock 62 — 51 — 51 (a) Ameren and Ameren Missouri have investments in industrial revenue bonds, classified as held-to-maturity and recorded in “Other Assets,” that are equal to the debt obligation for CTs leased from the city of Bowling Green and Audrain County. As of September 30, 2018 , and December 31, 2017 , the carrying amount of both the investments in industrial revenue bonds and the debt obligations approximated fair value. (b) Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $56 million , $23 million , and $27 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of September 30, 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. (d) Preferred stock is recorded in “Noncontrolling Interests” on the consolidated balance sheet.
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis, Valuation Techniques	The following table describes the valuation techniques and unobservable inputs utilized by the Ameren Companies for the fair value of financial assets and liabilities measured at fair value on a recurring basis and classified as Level 3 in the fair value hierarchy as of September 30, 2018 , and December 31, 2017 : Fair Value Weighted Average Assets Liabilities Valuation Technique(s) Unobservable Input Range Level 3 Derivative asset and liability – commodity contracts (a) : 2018 Fuel oils $ 8 $ (1 ) Option model Volatilities(%) (b) 20 – 34 23 Discounted cash flow Counterparty credit risk(%) (c)(d) 0.21 – 0.79 0.55 Ameren Missouri credit risk(%) (c)(d) 0.35 0.35 Natural gas 1 (5 ) Discounted cash flow Nodal basis ($/mmbtu) (b) (1.30) – 0.70 (0.80) Counterparty credit risk (%) (c)(d) 0.32 – 0.95 0.77 Ameren Illinois credit risk (%) (c)(d) 0.35 0.35 Power (e) 3 (190 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps ($/MWh) (f) 23 – 40 28 Estimated auction price for FTRs ($/MW) (b) (911) – 1,504 19 Nodal basis ($/MWh) (f) (10) – 0 (2) Counterparty credit risk (%) (c)(d) 0.95 0.95 Ameren Illinois credit risk (%) (c)(d) 0.35 0.35 Fundamental energy production model Estimated future natural gas prices ($/mmbtu) (b) 2 - 3 3 Escalation rate (%) (b)(g) 4 4 Contract price allocation Estimated renewable energy credit costs ($/credit) (b) 5 – 7 6 2017 Fuel oils $ 3 $ — Option model Volatilities (%) (b) 20 – 26 22 Discounted cash flow Counterparty credit risk (%) (c)(d) 0.12 – 0.72 0.41 Ameren Missouri credit risk (%) (c)(d) 0.37 0.37 Natural gas 1 (4 ) Option model Volatilities (%) (b) 26 – 46 37 Nodal basis ($/mmbtu) (c) (0.50) – (0.30) (0.40) Discounted cash flow Nodal basis ($/mmbtu) (b) (1.20) – 0.10 (1) Counterparty credit risk (%) (c)(d) 0.37 – 0.92 0.53 Ameren credit risk (%) (c)(d) 0.37 0.37 Power (e) 8 (196 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps ($/MWh) (f) 24 – 46 28 Estimated auction price for FTRs ($/MW) (b) (65) – 1,823 251 Nodal basis ($/MWh) (f) (10) – 0 (2) Counterparty credit risk (%) (c)(d) 0.28 0.28 Ameren Illinois credit risk (%) (c)(d) 0.37 0.37 Fundamental energy production model Estimated future natural gas prices ($/mmbtu) (b) 3 – 4 3 Escalation rate (%) (b)(g) 5 5 Contract price allocation Estimated renewable energy credit costs ($/credit) (b) 5 – 7 6 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement. (c) Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement. (d) Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances. (e) Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2022 for September 30, 2018, and through 2021 for December 31, 2017. Valuations beyond 2022 for September 30, 2018, and 2021 for December 31, 2017, use fundamentally modeled pricing by month for peak and off-peak demand. (f) The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes due to their opposing positions. (g) Escalation rate applies to power prices in 2031 and beyond.
Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis	The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of September 30, 2018 : Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Total Assets: Ameren Derivative assets – commodity contracts (a) : Fuel oils $ 8 $ — $ 8 $ 16 Natural gas — 1 1 2 Power — — 3 3 Total derivative assets – commodity contracts $ 8 $ 1 $ 12 $ 21 Nuclear decommissioning trust fund: Equity securities: U.S. large capitalization $ 497 $ — $ — $ 497 Debt securities: U.S. Treasury and agency securities — 139 — 139 Corporate bonds — 77 — 77 Other — 33 — 33 Total nuclear decommissioning trust fund $ 497 $ 249 $ — $ 746 (b) Total Ameren $ 505 $ 250 $ 12 $ 767 Ameren Missouri Derivative assets – commodity contracts (a) : Fuel oils $ 8 $ — $ 8 $ 16 Power — — 3 3 Total derivative assets – commodity contracts $ 8 $ — $ 11 $ 19 Nuclear decommissioning trust fund: Equity securities: U.S. large capitalization $ 497 $ — $ — $ 497 Debt securities: U.S. Treasury and agency securities — 139 — 139 Corporate bonds — 77 — 77 Other — 33 — 33 Total nuclear decommissioning trust fund $ 497 $ 249 $ — $ 746 (b) Total Ameren Missouri $ 505 $ 249 $ 11 $ 765 Ameren Illinois Derivative assets – commodity contracts (a) : Natural gas $ — $ 1 $ 1 $ 2 Liabilities: Ameren Derivative liabilities – commodity contracts (a) : Fuel oils $ — $ — $ 1 $ 1 Natural gas 1 21 5 27 Power — — 190 190 Total Ameren $ 1 $ 21 $ 196 $ 218 Ameren Missouri Derivative liabilities – commodity contracts (a) : Fuel oils $ — $ — $ 1 $ 1 Natural gas — 6 — 6 Power — — 3 3 Total Ameren Missouri $ — $ 6 $ 4 $ 10 Ameren Illinois Derivative liabilities – commodity contracts (a) : Natural gas $ 1 $ 15 $ 5 $ 21 Power — — 187 187 Total Ameren Illinois $ 1 $ 15 $ 192 $ 208 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Balance excludes $6 million of cash and cash equivalents, receivables, payables, and accrued income, net. The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of December 31, 2017 : Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Total Assets: Ameren Derivative assets – commodity contracts (a) : Fuel oils $ 4 $ — $ 3 $ 7 Natural gas — — 1 1 Power — 1 8 9 Total derivative assets – commodity contracts $ 4 $ 1 $ 12 $ 17 Nuclear decommissioning trust fund: Equity securities: U.S. large capitalization $ 468 $ — $ — $ 468 Debt securities: U.S. Treasury and agency securities — 125 — 125 Corporate bonds — 82 — 82 Other — 25 — 25 Total nuclear decommissioning trust fund $ 468 $ 232 $ — $ 700 (b) Total Ameren $ 472 $ 233 $ 12 $ 717 Ameren Missouri Derivative assets – commodity contracts (a) : Fuel oils $ 4 $ — $ 3 $ 7 Natural gas — — 1 1 Power — 1 8 9 Total derivative assets – commodity contracts $ 4 $ 1 $ 12 $ 17 Nuclear decommissioning trust fund: Equity securities: U.S. large capitalization $ 468 $ — $ — $ 468 Debt securities: U.S. Treasury and agency securities — 125 — 125 Corporate bonds — 82 — 82 Other — 25 — 25 Total nuclear decommissioning trust fund $ 468 $ 232 $ — $ 700 (b) Total Ameren Missouri $ 472 $ 233 $ 12 $ 717 Liabilities: Ameren Derivative liabilities – commodity contracts (a) : Natural gas $ 1 $ 25 $ 4 $ 30 Power — — 196 196 Total Ameren $ 1 $ 25 $ 200 $ 226 Ameren Missouri Derivative liabilities – commodity contracts (a) : Natural gas $ — $ 7 $ 1 $ 8 Power — — 1 1 Total Ameren Missouri $ — $ 7 $ 2 $ 9 Ameren Illinois Derivative liabilities – commodity contracts (a) : Natural gas $ 1 $ 18 $ 3 $ 22 Power — — 195 195 Total Ameren Illinois $ 1 $ 18 $ 198 $ 217 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Balance excludes $4 million of cash and cash equivalents, receivables, payables, and accrued income, net.
Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level Three In The Fair Value Hierarchy	The following table summarizes the changes in the fair value of power financial assets and liabilities classified as Level 3 in the fair value hierarchy: Net derivative commodity contracts Ameren Missouri Ameren Illinois Ameren For the three months ended September 30, 2018 Beginning balance at July 1, 2018 $ 5 $ (190 ) $ (185 ) Realized and unrealized losses included in regulatory assets/liabilities (4 ) — (4 ) Purchases 1 — 1 Settlements (1 ) 3 2 Transfers out of Level 3 (1 ) — (1 ) Ending balance at September 30, 2018 $ — $ (187 ) $ (187 ) Change in unrealized losses related to assets/liabilities held at September 30, 2018 $ — $ — $ — For the three months ended September 30, 2017 Beginning balance at July 1, 2017 $ 14 $ (192 ) $ (178 ) Realized and unrealized losses included in regulatory assets/liabilities (2 ) (3 ) (5 ) Sales 1 — 1 Settlements (3 ) 3 — Ending balance at September 30, 2017 $ 10 $ (192 ) $ (182 ) Change in unrealized losses related to assets/liabilities held at September 30, 2017 $ — $ (2 ) $ (2 ) For the nine months ended September 30, 2018 Beginning balance at January 1, 2018 $ 7 $ (195 ) $ (188 ) Realized and unrealized losses included in regulatory assets/liabilities (7 ) (1 ) (8 ) Purchases 5 — 5 Settlements (4 ) 9 5 Transfers out of Level 3 (1 ) — (1 ) Ending balance at September 30, 2018 $ — $ (187 ) $ (187 ) Change in unrealized losses related to assets/liabilities held at September 30, 2018 $ (1 ) $ (2 ) $ (3 ) For the nine months ended September 30, 2017 Beginning balance at January 1, 2017 $ 7 $ (185 ) $ (178 ) Realized and unrealized losses included in regulatory assets/liabilities (3 ) (14 ) (17 ) Purchases 15 — 15 Sales 1 — 1 Settlements (10 ) 7 (3 ) Ending balance at September 30, 2017 $ 10 $ (192 ) $ (182 ) Change in unrealized losses related to assets/liabilities held at September 30, 2017 $ — $ (15 ) $ (15 )

Related Party Transactions (Tab

Related Party Transactions (Tables)	9 Months Ended
	Sep. 30, 2018
Related Party Transactions [Abstract]
Schedule of Related Party Transactions	The following table presents the impact on Ameren Missouri and Ameren Illinois of related-party transactions for the three and nine months ended September 30, 2018 and 2017 : Three Months Nine Months Agreement Income Statement Line Item Ameren Missouri Ameren Illinois Ameren Missouri Ameren Illinois Ameren Missouri power supply Operating Revenues 2018 $ 5 $ (a) $ 11 $ (a) agreements with Ameren Illinois 2017 4 (a) 21 (a) Ameren Missouri and Ameren Illinois Operating Revenues 2018 6 (b) 17 2 rent and facility services 2017 7 1 20 3 Ameren Missouri and Ameren Illinois Operating Revenues 2018 (b) (b) (b) (b) miscellaneous support services 2017 (b) (b) (b) 1 Total Operating Revenues 2018 $ 11 $ (b) $ 28 $ 2 2017 11 1 41 4 Ameren Illinois power supply Purchased Power 2018 $ (a) $ 5 $ (a) $ 11 agreements with Ameren Missouri 2017 (a) 4 (a) 21 Ameren Illinois transmission Purchased Power 2018 (a) (b) (a) 1 services with ATXI 2017 (a) (b) (a) 1 Total Purchased Power 2018 $ (a) $ 5 $ (a) $ 12 2017 (a) 4 (a) 22 Ameren Services support services Other Operations and Maintenance 2018 $ 36 $ 33 $ 101 $ 93 agreement 2017 34 33 103 99 Money pool borrowings (advances) Interest Charges/ Other Income, Net 2018 $ (b) $ (b) $ (b) $ (b) 2017 (b) (b) (b) (b) (a) Not applicable. (b) Amount less than $1 million.

Commitments and Contingencies (

Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Long-term Purchase Commitment	The table below presents our estimated minimum fuel, purchased power, and other commitments for fuel at September 30, 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 September 30, 2018 . Coal Natural Gas (a) Nuclear Fuel Purchased Power (b)(c) Methane Gas Other Total Ameren: (d) 2018 $ 86 $ 55 $ 36 $ 60 $ 1 $ 38 $ 276 2019 319 189 26 157 4 51 746 2020 159 131 38 53 4 41 426 2021 121 70 56 10 5 30 292 2022 73 19 13 — 5 25 135 Thereafter — 39 72 — 58 92 261 Total $ 758 $ 503 $ 241 $ 280 $ 77 $ 277 $ 2,136 Ameren Missouri: 2018 $ 86 $ 11 $ 36 $ — $ 1 $ 31 $ 165 2019 319 39 26 — 4 35 423 2020 159 30 38 — 4 25 256 2021 121 14 56 — 5 25 221 2022 73 5 13 — 5 25 121 Thereafter — 17 72 — 58 75 222 Total $ 758 $ 116 $ 241 $ — $ 77 $ 216 $ 1,408 Ameren Illinois: 2018 $ — $ 44 $ — $ 60 $ — $ 4 $ 108 2019 — 150 — 157 — 7 314 2020 — 100 — 53 — 7 160 2021 — 56 — 10 — — 66 2022 — 14 — — — — 14 Thereafter — 22 — — — — 22 Total $ — $ 386 $ — $ 280 $ — $ 18 $ 684 (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. (d) Includes amounts for Ameren registrant and nonregistrant subsidiaries.

Callaway Energy Center (Tables)

Callaway Energy Center (Tables)	9 Months Ended
	Sep. 30, 2018
Nuclear Waste Matters [Abstract]
Schedule of Insurance Coverage at Callaway Energy Center	The following table presents insurance coverage at Ameren Missouri’s Callaway energy center as of November 1, 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 twelve weeks of an outage, plus up to $3.6 million per week for a minimum of 71 weeks thereafter for a total not exceeding the policy limit of $490 million . Nonradiation events are limited to $328 million .

Retirement Benefits (Tables)

Retirement Benefits (Tables)	9 Months Ended
	Sep. 30, 2018
Defined Benefit Plan [Abstract]
Components Of Net Periodic Benefit Cost	The following table presents the components of the net periodic benefit cost (income), prior to capitalization, incurred for Ameren’s pension and postretirement benefit plans for the three and nine months ended September 30, 2018 and 2017 : Pension Benefits Postretirement Benefits Three Months Nine Months Three Months Nine Months 2018 2017 2018 2017 2018 2017 2018 2017 Service cost (a) $ 25 $ 24 $ 75 $ 70 $ 6 $ 6 $ 16 $ 16 Non-service cost components: Interest cost 42 44 126 134 10 12 30 35 Expected return on plan assets (68 ) (65 ) (206 ) (196 ) (20 ) (19 ) (58 ) (56 ) Amortization of: Prior service benefit — (1 ) — (1 ) (1 ) (2 ) (3 ) (4 ) Actuarial loss (gain) 17 14 51 41 (2 ) (2 ) (5 ) (5 ) Total non-service cost components (b) (9 ) (8 ) (29 ) (22 ) (13 ) (11 ) (36 ) (30 ) Net periodic benefit cost (income) $ 16 $ 16 $ 46 $ 48 $ (7 ) $ (5 ) $ (20 ) $ (14 ) (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’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 Benefit Plan Costs Incurred	Ameren Missouri and Ameren Illinois are responsible for their respective shares of Ameren’s pension and postretirement costs. The following table presents the respective share of net periodic pension and other postretirement benefit costs (income) incurred for the three and nine months ended September 30, 2018 and 2017 : Pension Benefits Postretirement Benefits Three Months Nine Months Three Months Nine Months 2018 2017 2018 2017 2018 2017 2018 2017 Ameren Missouri (a) $ 6 $ 6 $ 17 $ 18 $ (1 ) $ (1 ) $ (1 ) $ (3 ) Ameren Illinois 11 10 30 30 (6 ) (3 ) (19 ) (10 ) Other (1 ) — (1 ) — — (1 ) — (1 ) Ameren (a) $ 16 $ 16 $ 46 $ 48 $ (7 ) $ (5 ) $ (20 ) $ (14 ) (a) Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates.

Income Taxes Income Taxes (Tabl

Income Taxes Income Taxes (Tables)	9 Months Ended
	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Schedule of Effective Income Tax Rate Reconciliation	The following table presents a reconciliation of the federal statutory corporate income tax rate to the effective income tax rate for the three and nine months ended September 30, 2018 and 2017 : Ameren Ameren Missouri Ameren Illinois Three Months 2018 2017 2018 2017 2018 2017 Federal statutory corporate income tax rate: 21% 35% 21% 35% 21% 35% Increases (decreases) from: Amortization of excess deferred taxes (6) — (7) (a) — (3) — Other depreciation differences — 1 — 1 (1) (2) Amortization of deferred investment tax credit (1) (1) — (1) — — State tax 6 7 4 3 5 7 TCJA 3 (b) — — — 4 (b) — Other permanent items — (1) — — — — Effective income tax rate 23% 41% 18% 38% 26% 40% Nine Months Federal statutory corporate income tax rate: 21% 35% 21% 35% 21% 35% Increases (decreases) from: Amortization of excess deferred taxes (3) — (4) (a) — (4) — Other depreciation differences — — — 1 — — Amortization of deferred investment tax credit (1) (1) — (1) — — State tax 6 6 4 3 7 5 TCJA 1 (b) — — — 1 (b) — Other permanent items (1) (1) — — — — Effective income tax rate 23% 39% 21% 38% 25% 40% (a) Based on an order issued by the MoPSC in July 2018, Ameren Missouri began amortizing excess deferred taxes in August 2018. See Note 2 – Rate and Regulatory Matters for additional information. (b) The Ameren Companies updated their respective provisional estimates recorded related to TCJA, as discussed below.

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2018
Segment Reporting [Abstract]
Schedule Of Segment Reporting Information By Segment	The following tables present revenues, net income attributable to common shareholders, and capital expenditures by segment at Ameren and Ameren Illinois for the three and nine months ended September 30, 2018 and 2017 . Ameren, Ameren Missouri, and Ameren Illinois management review segment capital expenditure information rather than any individual or total asset amount. Ameren Three Months Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Other Intersegment Eliminations Consolidated 2018 External revenues $ 1,118 $ 392 $ 116 $ 98 $ — $ — $ 1,724 Intersegment revenues 11 — — 15 — (26 ) — Net income attributable to Ameren common shareholders 294 35 — 48 (a) (20 ) — 357 Capital expenditures 210 135 111 124 1 (4 ) 577 2017 External revenues $ 1,105 $ 404 $ 111 $ 105 $ (2 ) $ — $ 1,723 Intersegment revenues 11 — 1 14 — (26 ) — Net income attributable to Ameren common shareholders 234 31 2 38 (a) (17 ) — 288 Capital expenditures 178 112 71 173 (2 ) (7 ) 525 Nine Months 2018 External revenues $ 2,848 $ 1,177 $ 569 $ 278 $ — $ — $ 4,872 Intersegment revenues 28 2 — 42 — (72 ) — Net income attributable to Ameren common shareholders 500 101 49 121 (a) (24 ) — 747 Capital expenditures 664 389 237 399 6 (6 ) 1,689 2017 External revenues $ 2,800 $ 1,175 $ 509 $ 293 $ (2 ) $ — $ 4,775 Intersegment revenues 41 3 1 33 — (78 ) — Net income attributable to Ameren common shareholders 359 94 40 106 (a) (16 ) — 583 Capital expenditures 533 354 180 463 3 (10 ) 1,523 (a) Ameren Transmission earnings include an allocation of financing costs from Ameren (parent). Ameren Illinois Three Months Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Total 2018 External revenues $ 392 $ 116 $ 56 $ — $ 564 Intersegment revenues — — 15 (15 ) — Net income available to common shareholder 35 — 28 — 63 Capital expenditures 135 111 99 — 345 2017 External revenues $ 404 $ 112 $ 58 $ — $ 574 Intersegment revenues — — 14 (14 ) — Net income available to common shareholder 31 2 22 — 55 Capital expenditures 112 71 93 — 276 Nine Months 2018 External revenues $ 1,179 $ 569 $ 154 $ — $ 1,902 Intersegment revenues — — 41 (41 ) — Net income available to common shareholder 101 49 70 — 220 Capital expenditures 389 237 321 — 947 2017 External revenues $ 1,178 $ 510 $ 165 $ — $ 1,853 Intersegment revenues — — 32 (32 ) — Net income available to common shareholder 94 40 57 — 191 Capital expenditures 354 180 226 — 760
Disaggregation of Revenue	The following tables present disaggregated revenues by segment at Ameren and Ameren Illinois for the three and nine months ended September 30, 2018 and 2017 . 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 Three Months Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Other Intersegment Eliminations Consolidated 2018 Residential $ 508 $ 223 $ — $ — $ — $ — $ 731 Commercial 417 131 — — — — 548 Industrial 101 28 — — — — 129 Other 85 (a) 10 — 113 — (26 ) 182 (a) Total electric revenues $ 1,111 $ 392 $ — $ 113 $ — $ (26 ) $ 1,590 Residential $ 8 $ — $ 68 $ — $ — $ — $ 76 Commercial 3 — 20 — — — 23 Industrial 1 — 1 — — — 2 Other 6 — 27 — — — 33 Total gas revenues $ 18 $ — $ 116 $ — $ — $ — $ 134 Total revenues (b) $ 1,129 $ 392 $ 116 $ 113 $ — $ (26 ) $ 1,724 2017 Residential $ 491 $ 224 $ — $ — $ — $ — $ 715 Commercial 409 133 — — — — 542 Industrial 100 27 — — — — 127 Other 99 20 — 119 (2 ) (26 ) 210 Total electric revenues $ 1,099 $ 404 $ — $ 119 $ (2 ) $ (26 ) $ 1,594 Residential $ 9 $ — $ 72 $ — $ — $ — $ 81 Commercial 4 — 21 — — — 25 Industrial 1 — 2 — — — 3 Other 3 — 17 — — — 20 Total gas revenues $ 17 $ — $ 112 $ — $ — $ — $ 129 Total revenues (b) $ 1,116 $ 404 $ 112 $ 119 $ (2 ) $ (26 ) $ 1,723 Nine Months 2018 Residential $ 1,272 $ 663 $ — $ — $ — $ — $ 1,935 Commercial 1,033 381 — — — — 1,414 Industrial 249 96 — — — — 345 Other 228 (a) 39 — 320 — (72 ) 515 (a) Total electric revenues $ 2,782 $ 1,179 $ — $ 320 $ — $ (72 ) $ 4,209 Residential $ 62 $ — $ 408 $ — $ — $ — $ 470 Commercial 25 — 113 — — — 138 Industrial 3 — 12 — — — 15 Other 4 — 36 — — — 40 Total gas revenues $ 94 $ — $ 569 $ — $ — $ — $ 663 Total revenues (b) $ 2,876 $ 1,179 $ 569 $ 320 $ — $ (72 ) $ 4,872 2017 Residential $ 1,135 $ 651 $ — $ — $ — $ — $ 1,786 Commercial 971 395 — — — — 1,366 Industrial 242 83 — — — — 325 Other 410 49 — 326 (2 ) (77 ) 706 Total electric revenues $ 2,758 $ 1,178 $ — $ 326 $ (2 ) $ (77 ) $ 4,183 Residential $ 49 $ — $ 359 $ — $ — $ — $ 408 Commercial 20 — 100 — — — 120 Industrial 3 — 7 — — — 10 Other 11 — 44 — — (1 ) 54 Total gas revenues $ 83 $ — $ 510 $ — $ — $ (1 ) $ 592 Total revenues (b) $ 2,841 $ 1,178 $ 510 $ 326 $ (2 ) $ (78 ) $ 4,775 (a) Includes $13 million and $60 million for the three and nine months ended September 30, 2018 , respectively, for the reduction to revenue for the excess amounts collected in rates related to the TCJA from January 1, 2018, through September 30, 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 three and nine months ended September 30, 2018 and 2017 : Three Months Ameren Missouri Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Transmission Consolidated 2018 Revenues from alternative revenue programs $ 1 $ (98 ) $ (2 ) $ (12 ) $ (111 ) Other revenues not from contracts with customers 3 1 1 — 5 2017 Revenues from alternative revenue programs $ (6 ) $ (96 ) $ (1 ) $ (2 ) $ (105 ) Other revenues not from contracts with customers 4 2 — — 6 Nine Months 2018 Revenues from alternative revenue programs $ (8 ) $ (52 ) $ (10 ) $ (21 ) $ (91 ) Other revenues not from contracts with customers 22 14 2 — 38 2017 Revenues from alternative revenue programs $ (20 ) $ (47 ) $ 11 $ 5 $ (51 ) Other revenues not from contracts with customers 11 5 2 — 18 Ameren Illinois Three Months Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Total Ameren Illinois 2018 Residential $ 223 $ 68 $ — $ — $ 291 Commercial 131 20 — — 151 Industrial 28 1 — — 29 Other 10 27 71 (15 ) 93 Total revenues (a) $ 392 $ 116 $ 71 $ (15 ) $ 564 2017 Residential $ 224 $ 72 $ — $ — $ 296 Commercial 133 21 — — 154 Industrial 27 2 — — 29 Other 20 17 72 (14 ) 95 Total revenues (a) $ 404 $ 112 $ 72 $ (14 ) $ 574 Nine Months 2018 Residential $ 663 $ 408 $ — $ — $ 1,071 Commercial 381 113 — — 494 Industrial 96 12 — — 108 Other 39 36 195 (41 ) 229 Total revenues (a) $ 1,179 $ 569 $ 195 $ (41 ) $ 1,902 2017 Residential $ 651 $ 359 $ — $ — $ 1,010 Commercial 395 100 — — 495 Industrial 83 7 — — 90 Other 49 44 197 (32 ) 258 Total revenues (a) $ 1,178 $ 510 $ 197 $ (32 ) $ 1,853 (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 three and nine months ended September 30, 2018 and 2017 : Three Months Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Consolidated 2018 Revenues from alternative revenue programs $ (98 ) $ (2 ) $ (10 ) $ (110 ) Other revenues not from contracts with customers 1 1 — 2 2017 Revenues from alternative revenue programs $ (96 ) $ (1 ) $ (2 ) $ (99 ) Other revenues not from contracts with customers 2 — — 2 Nine Months 2018 Revenues from alternative revenue programs $ (52 ) $ (10 ) $ (19 ) $ (81 ) Other revenues not from contracts with customers 14 2 — 16 2017 Revenues from alternative revenue programs $ (47 ) $ 11 $ 3 $ (33 ) Other revenues not from contracts with customers 5 2 — 7

Summary of Significant Accoun_4

Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Basis Of Presentation And Significant Accounting Policies [Line Items]
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets and Liabilities, Net	$ 20		$ 20		$ 17
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	17		17
Payables for purchased receivables	40		40		31
Cash Surrender Value of Life Insurance	256		256		265
Corporate owned life insurance, borrowings	113		113		120
Deferred Compensation Liability, Classified, Noncurrent	$ 86		$ 86		86
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements (in shares)	2.2	2.1	1.9	1.4
Ameren Illinois Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Payables for purchased receivables	$ 40		$ 40		31
Cash Surrender Value of Life Insurance	120		120		129
Corporate owned life insurance, borrowings	$ 113		$ 113		$ 120

Summary of Significant Accoun_5

Summary of Significant Accounting Policies (Cash and Cash Equivalents) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	$ 11	$ 10	$ 9	$ 9
Restricted Cash and Cash Equivalents	15	21	19	20
Restricted Cash and Cash Equivalents, Noncurrent	63	35	27	22
Restricted Cash and Cash Equivalents, Nuclear Decommissioning Trust Fund	3	2	2	1
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	92	68	57	52
Union Electric Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	0	0	0	0
Restricted Cash and Cash Equivalents	4	5	4	4
Restricted Cash and Cash Equivalents, Noncurrent	0	0	0	0
Restricted Cash and Cash Equivalents, Nuclear Decommissioning Trust Fund	3	2	2	1
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	7	7	6	5
Ameren Illinois Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	0	0	0	0
Restricted Cash and Cash Equivalents	8	6	5	6
Restricted Cash and Cash Equivalents, Noncurrent	63	35	27	22
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 71	$ 41	$ 32	$ 28

Summary of Significant Account

Summary of Significant Account Policies (Supplemental Cash Flow Information) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	$ 240	$ 202
Net Realized and Unrealized Gain (Loss) on Trading Securities	33	53
Union Electric Company
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	94	70
Net Realized and Unrealized Gain (Loss) on Trading Securities	33	53
Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	$ 133	$ 100

Summary of Significant Accoun_6

Summary of Significant Accounting Policies (Schedule of Asset Retirement Obligations) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	$ 646	$ 644
Liabilities settled	(4)
Accretion	20
Change of estimates	(14)
Other current liabilities	317	326
Asset Retirement Obligation Balance
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Other current liabilities	6	6
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	642	640
Liabilities settled	(4)
Accretion	20
Change of estimates	(14)
Other current liabilities	115	103
Union Electric Company | Asset Retirement Obligation Balance
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Other current liabilities	6	6
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset Retirement Obligation	4	4
Liabilities settled	(1)
Accretion	1
Change of estimates	0
Other current liabilities	$ 161	$ 177

Summary of Significant Accoun_7

Summary of Significant Accounting Policies (Summary of Nonvested Shares Related To Long-Term Incentive Plan) (Details) - $ / shares	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Weighted-average Fair Value per Unit at Grant Date
Closing common share price (in dollars per share)			$ 58.99
Performance Shares
Share Units
Nonvested at the beginning of the period (in shares)		895,489
Granted (in shares)		313,984
Forfeitures (in shares)		(62,865)
Undistributed vested units (in shares)		(217,350)
Vested and distributed (in shares)		(176,043)
Nonvested at the end of the period (in shares)	753,215	753,215
Fully Vested Undistributed to Retirement-eligible employees (in shares)	548,542	548,542	712,572
Weighted-average Fair Value per Unit at Grant Date
Weighted-average Fair Value per Unit, Nonvested as of Beginning of year (in dollars per share)		$ 52.28
Weighted-average Fair Value per Unit, Granted (in dollars per share)		62.88
Weighted-average Fair Value per Unit, Forfeitures (in dollars per share)		50.78
Weighted-average Fair Value per Unit, Undistributed Vested Units (in dollars per share)		53.57
Weighted-average Fair Value per Unit, Vested (in dollars per share)		52.88
Weighted-average Fair Value per Unit, Nonvested as of Current period end (in dollars per share)	$ 56.31	$ 56.31
Performance period		3 years
Award Vesting Period		38 months
Award Requisite Service Period		5 years
Stock Issued During Period Percentage Conversion Of Units, Low End	0.00%
Stock Issued During Period Percentage Conversion Of Units, High End	200.00%
Risk free interest rate period		3 years
Three-year risk-free rate		1.98%
Volatility rate, minimum		15.00%
Volatility rate, maximum		23.00%
Restricted Stock Units (RSUs) [Member]
Share Units
Nonvested at the beginning of the period (in shares)		0
Granted (in shares)		186,728
Forfeitures (in shares)		(4,964)
Undistributed vested units (in shares)		(19,742)
Vested and distributed (in shares)		0
Nonvested at the end of the period (in shares)	162,022	162,022
Fully Vested Undistributed to Retirement-eligible employees (in shares)	19,742	19,742
Weighted-average Fair Value per Unit at Grant Date
Weighted-average Fair Value per Unit, Nonvested as of Beginning of year (in dollars per share)		$ 0
Weighted-average Fair Value per Unit, Granted (in dollars per share)		57.65
Weighted-average Fair Value per Unit, Forfeitures (in dollars per share)		58.99
Weighted-average Fair Value per Unit, Undistributed Vested Units (in dollars per share)		59.01
Weighted-average Fair Value per Unit, Vested (in dollars per share)		0
Weighted-average Fair Value per Unit, Nonvested as of Current period end (in dollars per share)	$ 57.44	$ 57.44
Award Vesting Period		38 months
Award Requisite Service Period		5 years

Summary Of Significant Accoun_8

Summary Of Significant Accounting Policies (Schedule Of Excise Taxes) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Accounting Policies [Line Items]
Excise tax expense	$ 78	$ 77	$ 222	$ 204
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	52	51	133	122
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$ 26	$ 26	$ 89	$ 82

Rate And Regulatory Matters (Na

Rate And Regulatory Matters (Narrative-Missouri) (Detail) $ in Millions	3 Months Ended			9 Months Ended
	Dec. 31, 2018USD ($)MWh	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)MWh	Sep. 30, 2017USD ($)	Dec. 31, 2017USD ($)
Rate And Regulatory Matters [Line Items]
Regulatory Liability, Noncurrent		$ 4,652		$ 4,652		$ 4,387
Total operating revenues		1,724	$ 1,723	4,872	$ 4,775
Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory Liability, Noncurrent		$ 2,799		$ 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%
MEEIAEnergyEfficiencyInvestments				$ 226
AnnualMEEIAEnergyEfficiencyInvestments				65
Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount				167
Estimated incremental capital Investment				1,000
Incentive Award if Energy Efficiency Goals Are Achieved				$ 30
Achieved Percentage of Energy Efficiency Earnings For Incentive Award		100.00%		100.00%
TCJA - Excess Amounts Collected in Rates
Rate And Regulatory Matters [Line Items]
Regulatory Liability, Noncurrent		$ 60		$ 60
TCJA - Excess Accumulated Deferred Income Tax Amortization | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount				$ 74
Wind Generation Facility | Union Electric Company
Rate And Regulatory Matters [Line Items]
Amount of Megawatts | MWh				400
Renewable Choice Program | Union Electric Company
Rate And Regulatory Matters [Line Items]
Amount of Megawatts | MWh				400
Renewable Choice Program | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Percentage of Energy Received From Renewable Resources				100.00%
Renewable Choice Program - Megawatts Owned | Union Electric Company
Rate And Regulatory Matters [Line Items]
Amount of Megawatts | MWh				200
MEEIA 2016 | Final Rate Order | Union Electric Company
Rate And Regulatory Matters [Line Items]
Total operating revenues				$ 5
MEEIA 2016 | Final Rate Order | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Incentive Award if Energy Efficiency Goals Are Achieved				$ 27
Achieved Percentage of Energy Efficiency Earnings For Incentive Award		100.00%		100.00%
MEEIA 2013 | Electricity | Union Electric Company
Rate And Regulatory Matters [Line Items]
Total operating revenues				$ 9
Maximum
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Percentage				2.85%
Subsequent Event | Wind Generation Facility | Union Electric Company
Rate And Regulatory Matters [Line Items]
Amount of Megawatts | MWh	157
Subsequent Event | MEEIA 2016 | Final Rate Order | Union Electric Company
Rate And Regulatory Matters [Line Items]
Total operating revenues	$ 6

Rate And Regulatory Matters (_2

Rate And Regulatory Matters (Narrative-Illinois) (Detail) - USD ($) $ in Millions	Nov. 01, 2018	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Rate And Regulatory Matters [Line Items]
Regulatory Liability, Noncurrent		$ 4,652	$ 4,652	$ 4,387
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory Liability, Noncurrent		$ 1,732	1,732	$ 1,629
Ameren Illinois Company | Natural Gas
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount			17
Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount			50
Ameren Transmission Company of Illinois
Rate And Regulatory Matters [Line Items]
Estimated Capital Project Costs			81
Property, Plant and Equipment, Additions			$ 38
Final Rate Order | Ameren Illinois Company | Natural Gas
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage		9.87%
Public Utilities, Requested Equity Capital Structure, Percentage			50.00%
Rate Base			$ 1,600
TCJA - Reduction in Federal Statutory Income Tax Rate | Ameren Illinois Company | Natural Gas
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount			17
Subsequent Event | Final Rate Order | Ameren Illinois Company | Natural Gas
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 32
Subsequent Event | Final Rate Order | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Electric Energy-Efficiency Revenue Requirement	35
Subsequent Event | Final Rate Order | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	72
Subsequent Event | Final Rate Order | FEJA energy-efficiency rider [Domain] | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 20
IEIMA | 2018 IEIMA Revenue Requirement Reconciliation | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Regulatory Liability, Noncurrent		$ 25	$ 25

Rate And Regulatory Matters (_3

Rate And Regulatory Matters (Narrative-Federal) (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	$ 114	$ 128
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	$ 48	$ 92
Midwest Independent Transmission System Operator, Inc
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage	12.38%
Midwest Independent Transmission System Operator, Inc | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 27
Midwest Independent Transmission System Operator, Inc | Ameren Transmission Company of Illinois
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 23
Midwest Independent Transmission System Operator, Inc | Final Rate Order
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested 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%
Midwest Independent Transmission System Operator, Inc | Administrative Law Judge
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage	9.70%
Midwest Independent Transmission System Operator, Inc | Pending Ferc Case
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	$ 43
Midwest Independent Transmission System Operator, Inc | Pending Ferc Case | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	$ 25
Maximum | Midwest Independent Transmission System Operator, Inc | Final Rate Order
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage	10.82%
Maximum | Midwest Independent Transmission System Operator, Inc | Administrative Law Judge
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Return on Equity, Percentage	10.20%

Short-Term Debt And Liquidity (

Short-Term Debt And Liquidity (Narrative) (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2018USD ($)	Sep. 30, 2017	Sep. 30, 2018USD ($)	Sep. 30, 2017
Credit Agreements
Short-term Debt [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,578,000,000		$ 1,578,000,000
Actual debt-to-capital ratio	0.52		0.52
Utilities
Short-term Debt [Line Items]
Short-term Debt, Weighted Average Interest Rate, over Time	2.00%	1.24%	2.02%	1.18%
Union Electric Company | Missouri Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.46		0.46
Ameren Illinois Company | Illinois Credit Agreement
Short-term Debt [Line Items]
Actual debt-to-capital ratio	0.48		0.48

Short-Term Debt and Liquidity_2

Short-Term Debt and Liquidity (Commercial Paper outstanding) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Commercial paper outstanding	$ 521	$ 484
Ameren (parent)
Short-term Debt [Line Items]
Commercial paper outstanding	413	383
Union Electric Company
Short-term Debt [Line Items]
Commercial paper outstanding	0	39
Ameren Illinois Company
Short-term Debt [Line Items]
Commercial paper outstanding	$ 108	$ 62

Short-Term Debt and Liquidity_3

Short-Term Debt and Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 629	$ 754
Weighted-average interest rate	2.18%	1.27%
Short-term Debt, Maximum Amount Outstanding During Period	$ 1,295	$ 948
Peak interest rate	2.55%	1.50%
Ameren (parent)
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 431	$ 669
Weighted-average interest rate	2.23%	1.27%
Short-term Debt, Maximum Amount Outstanding During Period	$ 543	$ 841
Peak interest rate	2.45%	1.50%
Union Electric Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 81	$ 7
Weighted-average interest rate	1.94%	1.20%
Short-term Debt, Maximum Amount Outstanding During Period	$ 481	$ 64
Peak interest rate	2.42%	1.41%
Ameren Illinois Company
Short-term Debt [Line Items]
Short-term Debt, Average Outstanding Amount	$ 117	$ 78
Weighted-average interest rate	2.21%	1.28%
Short-term Debt, Maximum Amount Outstanding During Period	$ 442	$ 193
Peak interest rate	2.55%	1.50%

Long-Term debt and Equity Fin_2

Long-Term debt and Equity Financings (Narrative) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Oct. 31, 2018	Dec. 31, 2017
Long-Term Debt And Equity Financings [Line Items]
Stock Issued During Period, Shares, New Issues	200,000	700,000	900,000
Issuance of common stock	$ 16,000,000		$ 56,000,000	$ 0
Noncash financing activity - Issuance of common stock for stock-based compensation		$ 35,000,000	$ 35,000,000	$ 0
Common stock, shares authorized (in shares)	400,000,000		400,000,000			400,000,000
Ameren (parent) | Subsequent Event
Long-Term Debt And Equity Financings [Line Items]
Common stock, shares authorized (in shares)					4,000,000
Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Common stock, shares authorized (in shares)	150,000,000		150,000,000			150,000,000
Union Electric Company | Secured Debt | First Mortgage Bonds, 4.00%, Due 2048 - $425 Issuance
Long-Term Debt And Equity Financings [Line Items]
Total principal amount of notes	$ 425,000,000		$ 425,000,000
State interest rate	4.00%		4.00%
Proceeds from Issuance of Secured Debt			$ 419,000,000
Union Electric Company | Secured Debt | Senior Secured Notes 6 Point 00 Due 2018
Long-Term Debt And Equity Financings [Line Items]
State interest rate	6.00%		6.00%
Repayments of other long-term debt			$ 179,000,000
Union Electric Company | Secured Debt | Senior Secured Notes 5.10% Due 2018
Long-Term Debt And Equity Financings [Line Items]
State interest rate	5.10%		5.10%
Repayments of other long-term debt			$ 199,000,000
Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Common stock, shares authorized (in shares)	45,000,000		45,000,000			45,000,000
Ameren Illinois Company | Secured Debt | First Mortgage Bonds, 3.80%, Due 2028
Long-Term Debt And Equity Financings [Line Items]
Total principal amount of notes	$ 430,000,000		$ 430,000,000
State interest rate	3.80%		3.80%
Proceeds from Issuance of Secured Debt			$ 427,000,000
Ameren Illinois Company | Secured Debt | Senior Secured Notes6250 Due2018
Long-Term Debt And Equity Financings [Line Items]
State interest rate	6.25%		6.25%
Repayments of other long-term debt			$ 144,000,000

Other Income, Net (Detail)

Other Income, Net (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 11	$ 6	$ 25	$ 16
Interest income on industrial development revenue bonds	6	7	19	20
Interest income	2	0	6	5
Non-service cost components of net periodic benefit income	17	11	52	33
Other income	2	1	5	3
Donations	(4)	0	(15)	(7)
Other expense	(2)	(2)	(8)	(9)
Other Income (Expense), Net	32	23	84	61
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	8	6	19	15
Interest income on industrial development revenue bonds	6	7	19	20
Interest income	1	0	2	1
Non-service cost components of net periodic benefit income	4	5	13	17
Other income	2	0	3	1
Donations	(3)	0	(6)	2
Other expense	(2)	(2)	(5)	(4)
Other Income (Expense), Net	16	16	45	48
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	5		13
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	3	0	6	1
Interest income	1	1	4	5
Non-service cost components of net periodic benefit income	8	4	25	8
Other income	1	0	3	2
Donations	0	0	(5)	(5)
Other expense	(2)	0	(3)	(3)
Other Income (Expense), Net	$ 11	$ 5	$ 30	$ 8

Derivative Financial Instrume_3

Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) gal in Millions, MWh in Millions, MMBTU in Millions	Sep. 30, 2018MWhgalMMBTU	Dec. 31, 2017MWhgalMMBTU
Fuel Oils
Derivative [Line Items]
Quantity | gal	42	28
Natural Gas
Derivative [Line Items]
Quantity | MMBTU	180	163
Power
Derivative [Line Items]
Quantity | MWh	10	12
Union Electric Company | Fuel Oils
Derivative [Line Items]
Quantity | gal	42	28
Union Electric Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU	20	24
Union Electric Company | Power
Derivative [Line Items]
Quantity | MWh	2	3
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU	160	139
Ameren Illinois Company | Power
Derivative [Line Items]
Quantity | MWh	8	9

Derivative Financial Instrume_4

Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative assets	$ 1
Derivative liabilities	218
Not Designated As Hedging Instrument
Derivative [Line Items]
Derivative assets	21	$ 17
Derivative liabilities		226
Not Designated As Hedging Instrument | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	10	5
Not Designated As Hedging Instrument | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	6	2
Not Designated As Hedging Instrument | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	1
Not Designated As Hedging Instrument | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	1
Not Designated As Hedging Instrument | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	1	1
Not Designated As Hedging Instrument | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	16	17
Not Designated As Hedging Instrument | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	11	13
Not Designated As Hedging Instrument | Power | Other Current Assets
Derivative [Line Items]
Derivative assets	3	9
Not Designated As Hedging Instrument | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	16	14
Not Designated As Hedging Instrument | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	174	182
Union Electric Company | Not Designated As Hedging Instrument
Derivative [Line Items]
Derivative assets	19	17
Derivative liabilities	10	9
Union Electric Company | Not Designated As Hedging Instrument | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	10	5
Union Electric Company | Not Designated As Hedging Instrument | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	6	2
Union Electric Company | Not Designated As Hedging Instrument | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	1
Union Electric Company | Not Designated As Hedging Instrument | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	0
Union Electric Company | Not Designated As Hedging Instrument | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	0	1
Union Electric Company | Not Designated As Hedging Instrument | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	4	5
Union Electric Company | Not Designated As Hedging Instrument | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	2	3
Union Electric Company | Not Designated As Hedging Instrument | Power | Other Current Assets
Derivative [Line Items]
Derivative assets	3	9
Union Electric Company | Not Designated As Hedging Instrument | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	3	1
Union Electric Company | Not Designated As Hedging Instrument | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Not Designated As Hedging Instrument
Derivative [Line Items]
Derivative assets	2	0
Derivative liabilities	208	217
Ameren Illinois Company | Not Designated As Hedging Instrument | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Not Designated As Hedging Instrument | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Not Designated As Hedging Instrument | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0
Ameren Illinois Company | Not Designated As Hedging Instrument | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	1
Ameren Illinois Company | Not Designated As Hedging Instrument | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	1	0
Ameren Illinois Company | Not Designated As Hedging Instrument | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	12	12
Ameren Illinois Company | Not Designated As Hedging Instrument | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	9	10
Ameren Illinois Company | Not Designated As Hedging Instrument | Power | Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Not Designated As Hedging Instrument | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	13	13
Ameren Illinois Company | Not Designated As Hedging Instrument | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	$ 174	$ 182

Derivative Financial Instrume_5

Derivative Financial Instruments (Maximum Exposure if Counterparties Fail to Perform on Contracts (Details) $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)
Derivative [Line Items]
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure	$ 37
Union Electric Company
Derivative [Line Items]
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure	34
Ameren Illinois Company
Derivative [Line Items]
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure	$ 3

Derivative Financial Instrume_6

Derivative Financial Instruments (Potential Loss on Counterparty Exposures) (Details) $ in Millions	Sep. 30, 2018USD ($)
Derivative [Line Items]
Derivative Potential Loss	$ 32
Union Electric Company
Derivative [Line Items]
Derivative Potential Loss	31
Ameren Illinois Company
Derivative [Line Items]
Derivative Potential Loss	$ 1

Derivative Financial Instrume_7

Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)
Derivative [Line Items]
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure	$ 37
Aggregate Fair Value of Derivative Liabilities	120
Cash Collateral Posted	4
Potential Aggregate Amount of Additional Collateral Required	94
Union Electric Company
Derivative [Line Items]
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure	34
Aggregate Fair Value of Derivative Liabilities	71
Cash Collateral Posted	4
Potential Aggregate Amount of Additional Collateral Required	51
Ameren Illinois Company
Derivative [Line Items]
Concentration Risk, Credit Risk, Financial Instrument, Maximum Exposure	3
Aggregate Fair Value of Derivative Liabilities	49
Cash Collateral Posted	0
Potential Aggregate Amount of Additional Collateral Required	$ 43

Fair Value Measurements (Schedu

Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	$ 1
Nuclear Decommissioning Trust Fund	746	$ 700
Assets fair value	767	717
Derivative liabilities	218
Excluded receivables, payables, and accrued income, net	6	4
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	746	700
Assets fair value	765	717
Excluded receivables, payables, and accrued income, net	6	4
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	21	17
Derivative liabilities	218	226
Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	19	17
Derivative liabilities	10	9
Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	208	217
Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	497	468
Equity Securities | U.S. large capitalization | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	497	468
Debt Securities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	77	82
Debt Securities | Corporate bonds | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	77	82
Debt Securities | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	139	125
Debt Securities | US treasury and government securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	139	125
Debt Securities | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	33	25
Debt Securities | Other Debt Securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	33	25
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	497	468
Assets fair value	505	472
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	497	468
Assets fair value	505	472
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	8	4
Derivative liabilities	1	1
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	8	4
Derivative liabilities	0	0
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	1	1
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	497	468
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Equity Securities | U.S. large capitalization | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	497	468
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Debt Securities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Debt Securities | Corporate bonds | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Debt Securities | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Debt Securities | US treasury and government securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Debt Securities | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Debt Securities | Other Debt Securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	249	232
Assets fair value	250	233
Significant Other Observable Inputs (Level 2) | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	249	232
Assets fair value	249	233
Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	1
Derivative liabilities	21	25
Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	1
Derivative liabilities	6	7
Significant Other Observable Inputs (Level 2) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	15	18
Significant Other Observable Inputs (Level 2) | Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Significant Other Observable Inputs (Level 2) | Equity Securities | U.S. large capitalization | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Significant Other Observable Inputs (Level 2) | Debt Securities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	77	82
Significant Other Observable Inputs (Level 2) | Debt Securities | Corporate bonds | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	77	82
Significant Other Observable Inputs (Level 2) | Debt Securities | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	139	125
Significant Other Observable Inputs (Level 2) | Debt Securities | US treasury and government securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	139	125
Significant Other Observable Inputs (Level 2) | Debt Securities | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	33	25
Significant Other Observable Inputs (Level 2) | Debt Securities | Other Debt Securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	33	25
Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Assets fair value	12	12
Significant Other Unobservable Inputs (Level 3) | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Assets fair value	11	12
Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	12	12
Derivative liabilities	196	200
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	11	12
Derivative liabilities	4	2
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	192	198
Significant Other Unobservable Inputs (Level 3) | Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Significant Other Unobservable Inputs (Level 3) | Equity Securities | U.S. large capitalization | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Significant Other Unobservable Inputs (Level 3) | Debt Securities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Significant Other Unobservable Inputs (Level 3) | Debt Securities | Corporate bonds | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Significant Other Unobservable Inputs (Level 3) | Debt Securities | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Significant Other Unobservable Inputs (Level 3) | Debt Securities | US treasury and government securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Significant Other Unobservable Inputs (Level 3) | Debt Securities | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Significant Other Unobservable Inputs (Level 3) | Debt Securities | Other Debt Securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund	0	0
Fuel Oils | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	16	7
Derivative liabilities	1
Fuel Oils | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	16	7
Derivative liabilities	1
Fuel Oils | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	8	4
Derivative liabilities	0
Fuel Oils | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	8	4
Derivative liabilities	0
Fuel Oils | Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0
Fuel Oils | Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0
Fuel Oils | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	8	3
Derivative liabilities	1
Fuel Oils | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	8	3
Derivative liabilities	1
Power | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	9
Derivative liabilities	190	196
Power | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	9
Derivative liabilities	3	1
Power | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	187	195
Power | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Power | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	0	0
Power | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Power | Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	1
Derivative liabilities	0	0
Power | Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	1
Derivative liabilities	0	0
Power | Significant Other Observable Inputs (Level 2) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	0	0
Power | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	8
Derivative liabilities	190	196
Power | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	3	8
Derivative liabilities	3	1
Power | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	187	195
Natural Gas | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2	1
Derivative liabilities	27	30
Natural Gas | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1
Derivative liabilities	6	8
Natural Gas | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	2
Derivative liabilities	21	22
Natural Gas | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0	0
Derivative liabilities	1	1
Natural Gas | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0
Derivative liabilities	0	0
Natural Gas | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	0
Derivative liabilities	1	1
Natural Gas | Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1	0
Derivative liabilities	21	25
Natural Gas | Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0
Derivative liabilities	6	7
Natural Gas | Significant Other Observable Inputs (Level 2) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1
Derivative liabilities	15	18
Natural Gas | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1
Derivative liabilities	5	4
Natural Gas | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1
Derivative liabilities	0	1
Natural Gas | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	1
Derivative liabilities	$ 5	$ 3

Fair Value Measurements (Sche_2

Fair Value Measurements (Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level Three In The Fair Value Hierarchy) (Detail) - Power - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	$ (187)	$ (182)	$ (187)	$ (182)	$ (185)	$ (188)	$ (178)	$ (178)
Included in regulatory assets/liabilities	(4)	(5)	(8)	(17)
Purchases	1		5	15
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		1		1
Settlements	2	0	5	(3)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(1)		(1)
Change in unrealized gains (losses) related to assets/liabilities held at period end	0	(2)	(3)	(15)
Union Electric Company
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	0	10	0	10	5	7	14	7
Included in regulatory assets/liabilities	(4)	(2)	(7)	(3)
Purchases	1		5	15
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		1		1
Settlements	(1)	(3)	(4)	(10)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	(1)		(1)
Change in unrealized gains (losses) related to assets/liabilities held at period end	0	0	(1)	0
Ameren Illinois Company
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	(187)	(192)	(187)	(192)	$ (190)	$ (195)	$ (192)	$ (185)
Included in regulatory assets/liabilities	0	(3)	(1)	(14)
Purchases	0		0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		0		0
Settlements	3	3	9	7
Change in unrealized gains (losses) related to assets/liabilities held at period end	$ 0	$ (2)	$ (2)	$ (15)

Fair Value Measurements (Sche_3

Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Detail) $ in Millions	Sep. 30, 2018USD ($)$ / credit$ / MMBTU$ / MWh$ / MW	Dec. 31, 2017USD ($)	Sep. 30, 2017$ / credit$ / MMBTU$ / MWh$ / MW
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	$ 1
Derivative liabilities | $	(218)
Natural Gas | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	1	$ 1
Natural Gas | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	5	(4)
Fuel Oils | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	8	3
Fuel Oils | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	(1)	0
Power | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	3	8
Power | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	$ 190	$ (196)
Counterparty Credit Risk | Natural Gas | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.0032		0.0037
Counterparty Credit Risk | Natural Gas | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.0095		0.0092
Counterparty Credit Risk | Natural Gas | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.0077		0.0053
Counterparty Credit Risk | Fuel Oils | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.0021		0.0012
Counterparty Credit Risk | Fuel Oils | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.0079		0.0072
Counterparty Credit Risk | Fuel Oils | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.0055		0.0041
Counterparty Credit Risk | Power | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.0095		0.0028
Counterparty Credit Risk | Power | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.0095		0.0028
Counterparty Credit Risk | Power | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.0095		0.0028
Entity Credit Risk | Natural Gas | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.35		0.0037
Entity Credit Risk | Natural Gas | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.35		0.0037
Entity Credit Risk | Natural Gas | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.35		0.0037
Entity Credit Risk | Fuel Oils | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.35		0.0037
Entity Credit Risk | Fuel Oils | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.35		0.0037
Entity Credit Risk | Fuel Oils | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.35		0.0037
Entity Credit Risk | Power | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.0035		0.0037
Entity Credit Risk | Power | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.0035		0.0037
Entity Credit Risk | Power | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.0035		0.0037
Commodity Forward Price | Power | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MWh			24
Commodity Forward Price | Power | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MWh			46
Commodity Forward Price | Power | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MWh			28
Price Volatility | Natural Gas | Option Model | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input			0.26
Price Volatility | Natural Gas | Option Model | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input			0.46
Price Volatility | Natural Gas | Option Model | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input			0.37
Price Volatility | Fuel Oils | Option Model | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.20		0.20
Price Volatility | Fuel Oils | Option Model | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.34		0.26
Price Volatility | Fuel Oils | Option Model | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.23		0.22
Nodal Basis | Natural Gas | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU	(1.3)		(1.20)
Nodal Basis | Natural Gas | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU	0.7		0.10
Nodal Basis | Natural Gas | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU	(0.8)		(1)
Nodal Basis | Natural Gas | Option Model | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU			(0.50)
Nodal Basis | Natural Gas | Option Model | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU			(0.30)
Nodal Basis | Natural Gas | Option Model | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU			(0.40)
Nodal Basis | Power | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MWh	(10)		(10)
Nodal Basis | Power | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MWh	0		0
Nodal Basis | Power | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MWh	(2)		(2)
Quoted Price | Power | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MWh	23
Quoted Price | Power | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MWh	40
Quoted Price | Power | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MWh	28
Auction Pricing | Power | Discounted Cash Flow | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MW	(911)		(65)
Auction Pricing | Power | Discounted Cash Flow | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MW	1,504		1,823
Auction Pricing | Power | Discounted Cash Flow | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MW	19		251
Commodity Future Price | Power | Fundamental Energy Production Model | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU	2		3
Commodity Future Price | Power | Fundamental Energy Production Model | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU	3		4
Commodity Future Price | Power | Fundamental Energy Production Model | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / MMBTU	3		3
Escalation Rate | Power | Fundamental Energy Production Model | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.04		0.05
Escalation Rate | Power | Fundamental Energy Production Model | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.04		0.05
Escalation Rate | Power | Fundamental Energy Production Model | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input	0.04		0.05
Estimated Renewable Energy Credit Costs | Power | Contract Price Allocation | Minimum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / credit	5		5
Estimated Renewable Energy Credit Costs | Power | Contract Price Allocation | Maximum
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / credit	7		7
Estimated Renewable Energy Credit Costs | Power | Contract Price Allocation | Weighted Average
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Measurement input | $ / credit	6		6

Fair Value Measurements (Sche_4

Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Financial Assets and Liabilities) (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt	$ 521	$ 484
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	92	68	$ 57	$ 52
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt	0	39
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	7	7	6	5
Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt	108	62
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	71	41	$ 32	$ 28
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available-for-sale Securities and Held-to-maturity Securities	276	276
Short-term Debt	521	484
Long-term debt (including current portion)	8,263	7,935
Preferred stock	142	142
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	92	68
Debt Issuance Costs, Net	56	50
Carrying Amount | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Advances to Money Pool	28
Short-term Debt		39
Long-term debt (including current portion)	4,004	3,961
Preferred stock	80	80
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	7	7
Debt Issuance Costs, Net	23	20
Carrying Amount | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Borrowings from Money Pool	45
Short-term Debt	108	62
Long-term debt (including current portion)	3,114	2,830
Preferred stock	62	62
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	71	41
Debt Issuance Costs, Net	27	24
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	521	484
Investments, Fair Value Disclosure	276	276
Long-term Debt, Fair Value	8,393	8,531
Preferred stock	139	131
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	92	68
Fair Value | Quoted Prices In Active Markets For Identical Assets or Liabilities (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
Preferred stock	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	92	68
Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	521	484
Investments, Fair Value Disclosure	276	276
Long-term Debt, Fair Value	7,981	8,531
Preferred stock	139	131
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Fair Value | Significant Other Unobservable Inputs (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	412	0
Preferred stock	0	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]
Advances to Money Pool	28
Short-term Debt, Fair Value		39
Investments, Fair Value Disclosure	276	276
Long-term Debt, Fair Value	4,173	4,348
Preferred stock	78	80
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	7	7
Fair Value | Union Electric Company | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Advances to Money Pool	0
Short-term Debt, Fair Value		0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	0	0
Preferred stock	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	7	7
Fair Value | Union Electric Company | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Advances to Money Pool	28
Short-term Debt, Fair Value		39
Investments, Fair Value Disclosure	276	276
Long-term Debt, Fair Value	4,173	4,348
Preferred stock	78	80
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Fair Value | Union Electric Company | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Advances to Money Pool	0
Short-term Debt, Fair Value		0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	0	0
Preferred stock	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	108	62
Borrowings from Money Pool	45
Long-term Debt, Fair Value	3,134	3,028
Preferred stock	61	51
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	71	41
Fair Value | Ameren Illinois Company | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	0	0
Borrowings from Money Pool	0
Long-term Debt, Fair Value	0	0
Preferred stock	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	71	41
Fair Value | Ameren Illinois Company | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	108	62
Borrowings from Money Pool	45
Long-term Debt, Fair Value	3,134	3,028
Preferred stock	61	51
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0
Fair Value | Ameren Illinois Company | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term Debt, Fair Value	0	0
Borrowings from Money Pool	0
Long-term Debt, Fair Value	0	0
Preferred stock	0	0
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	$ 0	$ 0

Related Party Transactions (Sch

Related Party Transactions (Schedule of Related Party Transactions) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Related Party Transaction [Line Items]
Operating Revenues	$ 0	$ 0	$ 0	$ 0
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	0	0	0	0
Ameren Missouri Power Supply Agreements with Ameren Illinois | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	5	4	11	21
Ameren Missouri and Ameren Illinois Rent and Facility Services | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	6	7	17	20
Ameren Missouri and Ameren Illinois Rent and Facility Services | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	1	2	3
Ameren Missouri and Ameren Illinois miscellaneous support services | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	1	1	1	1
Ameren Missouri and Ameren Illinois miscellaneous support services | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	1	1	1
Total Related Party Operating Revenues | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	11	11	28	41
Total Related Party Operating Revenues | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	1	1	2	4
Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	5	4	11	21
Ameren Illinois Transmission Agreements with ATXI | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	1	1	1	1
Purchased Power | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	5	4	12	22
Ameren Services Support Services Agreement | Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses	36	34	101	103
Ameren Services Support Services Agreement | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	33	33	93	99
Related Party Money Pool Interest | Union Electric Company
Related Party Transaction [Line Items]
Interest Charges	1	1	1	1
Related Party Money Pool Interest | Ameren Illinois Company
Related Party Transaction [Line Items]
Interest Charges	$ 1	$ 1	$ 1	$ 1

Related Party Transactions Narr

Related Party Transactions Narrative (Details) - April 2018 Procurement - Ameren Illinois Company - Ameren Illinois Power Supply Agreements with Ameren Missouri	3 Months Ended	9 Months Ended
	Sep. 30, 2018$ / MWh	Sep. 30, 2018MWh
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power | MWh		110,000
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate | $ / MWh	32

Commitments And Contingencies_2

Commitments And Contingencies (Other Obligations) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)MWh
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	$ 276
Unrecorded Unconditional Purchase Obligation, Due within Two Years	746
Unrecorded Unconditional Purchase Obligation, Due within Three Years	426
Unrecorded Unconditional Purchase Obligation, Due within Four Years	292
Unrecorded Unconditional Purchase Obligation, Due within Five Years	135
Unrecorded Unconditional Purchase Obligation, Due after Five Years	261
Unrecorded Unconditional Purchase Obligation	2,136
Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	86
Unrecorded Unconditional Purchase Obligation, Due within Two Years	319
Unrecorded Unconditional Purchase Obligation, Due within Three Years	159
Unrecorded Unconditional Purchase Obligation, Due within Four Years	121
Unrecorded Unconditional Purchase Obligation, Due within Five Years	73
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	758
Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	55
Unrecorded Unconditional Purchase Obligation, Due within Two Years	189
Unrecorded Unconditional Purchase Obligation, Due within Three Years	131
Unrecorded Unconditional Purchase Obligation, Due within Four Years	70
Unrecorded Unconditional Purchase Obligation, Due within Five Years	19
Unrecorded Unconditional Purchase Obligation, Due after Five Years	39
Unrecorded Unconditional Purchase Obligation	503
Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	36
Unrecorded Unconditional Purchase Obligation, Due within Two Years	26
Unrecorded Unconditional Purchase Obligation, Due within Three Years	38
Unrecorded Unconditional Purchase Obligation, Due within Four Years	56
Unrecorded Unconditional Purchase Obligation, Due within Five Years	13
Unrecorded Unconditional Purchase Obligation, Due after Five Years	72
Unrecorded Unconditional Purchase Obligation	241
Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	60
Unrecorded Unconditional Purchase Obligation, Due within Two Years	157
Unrecorded Unconditional Purchase Obligation, Due within Three Years	53
Unrecorded Unconditional Purchase Obligation, Due within Four Years	10
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	$ 280
Amount of Megawatts | MWh	102
Methane Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	$ 1
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	5
Unrecorded Unconditional Purchase Obligation, Due within Five Years	5
Unrecorded Unconditional Purchase Obligation, Due after Five Years	58
Unrecorded Unconditional Purchase Obligation	77
Other Purchased Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	38
Unrecorded Unconditional Purchase Obligation, Due within Two Years	51
Unrecorded Unconditional Purchase Obligation, Due within Three Years	41
Unrecorded Unconditional Purchase Obligation, Due within Four Years	30
Unrecorded Unconditional Purchase Obligation, Due within Five Years	25
Unrecorded Unconditional Purchase Obligation, Due after Five Years	92
Unrecorded Unconditional Purchase Obligation	277
Union Electric Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	165
Unrecorded Unconditional Purchase Obligation, Due within Two Years	423
Unrecorded Unconditional Purchase Obligation, Due within Three Years	256
Unrecorded Unconditional Purchase Obligation, Due within Four Years	221
Unrecorded Unconditional Purchase Obligation, Due within Five Years	121
Unrecorded Unconditional Purchase Obligation, Due after Five Years	222
Unrecorded Unconditional Purchase Obligation	1,408
Union Electric Company | Coal
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	86
Unrecorded Unconditional Purchase Obligation, Due within Two Years	319
Unrecorded Unconditional Purchase Obligation, Due within Three Years	159
Unrecorded Unconditional Purchase Obligation, Due within Four Years	121
Unrecorded Unconditional Purchase Obligation, Due within Five Years	73
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	758
Union Electric Company | Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	11
Unrecorded Unconditional Purchase Obligation, Due within Two Years	39
Unrecorded Unconditional Purchase Obligation, Due within Three Years	30
Unrecorded Unconditional Purchase Obligation, Due within Four Years	14
Unrecorded Unconditional Purchase Obligation, Due within Five Years	5
Unrecorded Unconditional Purchase Obligation, Due after Five Years	17
Unrecorded Unconditional Purchase Obligation	116
Union Electric Company | Nuclear Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	36
Unrecorded Unconditional Purchase Obligation, Due within Two Years	26
Unrecorded Unconditional Purchase Obligation, Due within Three Years	38
Unrecorded Unconditional Purchase Obligation, Due within Four Years	56
Unrecorded Unconditional Purchase Obligation, Due within Five Years	13
Unrecorded Unconditional Purchase Obligation, Due after Five Years	72
Unrecorded Unconditional Purchase Obligation	241
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	$ 1
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	5
Unrecorded Unconditional Purchase Obligation, Due within Five Years	5
Unrecorded Unconditional Purchase Obligation, Due after Five Years	58
Unrecorded Unconditional Purchase Obligation	77
Union Electric Company | Other Purchased Fuel
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	31
Unrecorded Unconditional Purchase Obligation, Due within Two Years	35
Unrecorded Unconditional Purchase Obligation, Due within Three Years	25
Unrecorded Unconditional Purchase Obligation, Due within Four Years	25
Unrecorded Unconditional Purchase Obligation, Due within Five Years	25
Unrecorded Unconditional Purchase Obligation, Due after Five Years	75
Unrecorded Unconditional Purchase Obligation	216
Ameren Illinois Company
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	108
Unrecorded Unconditional Purchase Obligation, Due within Two Years	314
Unrecorded Unconditional Purchase Obligation, Due within Three Years	160
Unrecorded Unconditional Purchase Obligation, Due within Four Years	66
Unrecorded Unconditional Purchase Obligation, Due within Five Years	14
Unrecorded Unconditional Purchase Obligation, Due after Five Years	22
Unrecorded Unconditional Purchase Obligation	684
Ameren Illinois Company | Coal
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
Ameren Illinois Company | Natural Gas
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	44
Unrecorded Unconditional Purchase Obligation, Due within Two Years	150
Unrecorded Unconditional Purchase Obligation, Due within Three Years	100
Unrecorded Unconditional Purchase Obligation, Due within Four Years	56
Unrecorded Unconditional Purchase Obligation, Due within Five Years	14
Unrecorded Unconditional Purchase Obligation, Due after Five Years	22
Unrecorded Unconditional Purchase Obligation	386
Ameren Illinois Company | Nuclear Fuel
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
Ameren Illinois Company | Purchased Power
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation, Due in Next Twelve Months	60
Unrecorded Unconditional Purchase Obligation, Due within Two Years	157
Unrecorded Unconditional Purchase Obligation, Due within Three Years	53
Unrecorded Unconditional Purchase Obligation, Due within Four Years	10
Unrecorded Unconditional Purchase Obligation, Due within Five Years	0
Unrecorded Unconditional Purchase Obligation, Due after Five Years	0
Unrecorded Unconditional Purchase Obligation	280
Ameren Illinois Company | Methane Gas
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
Ameren Illinois Company | Other Purchased Fuel
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	7
Unrecorded Unconditional Purchase Obligation, Due within Three Years	7
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	18
Ameren Illinois Company | Zero Emission Credits
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Unrecorded Unconditional Purchase Obligation	$ 42
Ameren Illinois Company | April 2018 Procurement
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Amount of Megawatthours | MWh	3,956,200
Long-term Purchase Commitment, Amount	$ 112
Ameren Illinois Company | September 2018 Procurement
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Amount of Megawatthours | MWh	2,221,400
Long-term Purchase Commitment, Amount	$ 63
Ameren Illinois Company | September 2018 Procurement - Capacity
Unrecorded Unconditional Purchase Obligation, Fiscal Year Maturity [Abstract]
Amount of Megawatts | MWh	653
Long-term Purchase Commitment, Amount	$ 7

Commitments And Contingencies_3

Commitments And Contingencies (Environmental Matters) (Detail) $ in Millions	Sep. 30, 2018USD ($)scrubbersite	Dec. 31, 2017USD ($)
Loss Contingencies [Line Items]
Percentage of Rate Base Related to Carbon Dioxide Energy Centers		17.00%
Asset Retirement Obligation	$ 646	$ 644
Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	325
Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	425
Union Electric Company
Loss Contingencies [Line Items]
Percentage of Rate Base Related to Carbon Dioxide Energy Centers		33.00%
Asset Retirement Obligation	$ 642	$ 640
Number of Energy Center Scrubbers | scrubber	2
Union Electric Company | Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 325
Union Electric Company | Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	425
Ameren Illinois Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	4	$ 4
Coal Combustion Residuals Estimate | Union Electric Company | Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	300
Coal Combustion Residuals Estimate | Union Electric Company | Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	350
Manufactured Gas Plant
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 162
Manufactured Gas Plant | Ameren Illinois Company
Loss Contingencies [Line Items]
Number of remediation sites | site	44
Accrual for environmental loss contingencies	$ 162
Manufactured Gas Plant | Ameren Illinois Company | Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	162
Manufactured Gas Plant | Ameren Illinois Company | Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	226
Sauget Area Two | Union Electric Company
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	1
New CCR Rules Estimate
Loss Contingencies [Line Items]
Asset Retirement Obligation	135
New CCR Rules Estimate | Union Electric Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	$ 135

Callaway Energy Center (Narrati

Callaway Energy Center (Narrative) (Detail) - Nuclear Plant $ in Millions	9 Months Ended
	Sep. 30, 2018USD ($)mill
Nuclear Waste Matters [Line Items]
Number of mills charged for NWF fee | mill	1
Annual decommissioning costs included in costs of service | $	$ 7

Callaway Energy Center (Insuran

Callaway Energy Center (Insurance Disclosure) (Detail)	9 Months Ended
	Sep. 30, 2018USD ($)
Nuclear Waste Matters [Line Items]
Number of weeks of coverage after the first eight weeks of an outage	1 year
Threshold 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
Inflationary adjustment prescribed by most recent Price-Anderson Act renewal, in years	5 years
Aggregate nuclear power industry insurance policy limit for losses from terrorist attacks within twelve month period	$ 3,200,000,000
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) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	$ 17	$ 11	$ 52	$ 33
Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	4	5	13	17
Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	$ 8	$ 4	$ 25	$ 8

Retirement Benefits (Components

Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	$ (17)	$ (11)	$ (52)	$ (33)
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	25	24	75	70
Interest cost	42	44	126	134
Expected return on plan assets	(68)	(65)	(206)	(196)
Prior service cost (benefit)	0	(1)	0	(1)
Actuarial loss	17	14	51	41
Total non-service cost components	(9)	(8)	(29)	(22)
Net periodic benefit cost	16	16	46	48
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	6	6	16	16
Interest cost	10	12	30	35
Expected return on plan assets	(20)	(19)	(58)	(56)
Prior service cost (benefit)	(1)	(2)	(3)	(4)
Actuarial loss	(2)	(2)	(5)	(5)
Total non-service cost components	(13)	(11)	(36)	(30)
Net periodic benefit cost	(7)	(5)	(20)	(14)
Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(4)	(5)	(13)	(17)
Union Electric Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	6	6	17	18
Union Electric Company | Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(1)	(1)	(1)	(3)
Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(8)	(4)	(25)	(8)
Ameren Illinois Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	11	10	30	30
Ameren Illinois Company | Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ (6)	$ (3)	$ (19)	$ (10)

Retirement Benefits (Summary Of

Retirement Benefits (Summary Of Benefit Plan Costs Incurred) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 16	$ 16	$ 46	$ 48
Pension Plan | Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	6	6	17	18
Pension Plan | Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	11	10	30	30
Pension Plan | Other Affiliated Entities and Intercompany Eliminations
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(1)	0	(1)	0
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(7)	(5)	(20)	(14)
Other Postretirement Benefit Plan, Defined Benefit | Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(1)	(1)	(1)	(3)
Other Postretirement Benefit Plan, Defined Benefit | Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	(6)	(3)	(19)	(10)
Other Postretirement Benefit Plan, Defined Benefit | Other Affiliated Entities and Intercompany Eliminations
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	$ 0	$ (1)	$ 0	$ (1)

Income Taxes Income Taxes (Sche

Income Taxes Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018Rate	Sep. 30, 2017Rate	Sep. 30, 2018Rate	Sep. 30, 2017Rate
Income Taxes [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	35.00%	21.00%	35.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent	(6.00%)	0.00%	(3.00%)	0.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent	0.00%	1.00%	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Percent	(1.00%)	(1.00%)	(1.00%)	(1.00%)
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent	6.00%	7.00%	6.00%	6.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent	3.00%	0.00%	1.00%	0.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent	0.00%	(1.00%)	(1.00%)	(1.00%)
Effective Income Tax Rate Reconciliation, Percent	23.00%	41.00%	23.00%	39.00%
Union Electric Company
Income Taxes [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	35.00%	21.00%	35.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent	(7.00%)	0.00%	(4.00%)	0.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent	0.00%	1.00%	0.00%	1.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Percent	(0.00%)	(1.00%)	(0.00%)	(1.00%)
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent	4.00%	3.00%	4.00%	3.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent	0.00%	0.00%	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent	0.00%	0.00%	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Percent	18.00%	38.00%	21.00%	38.00%
Ameren Illinois Company
Income Taxes [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	35.00%	21.00%	35.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amortization of Excess Deferred Taxes, Percent	(3.00%)	0.00%	(4.00%)	0.00%
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Depreciation, Percent	(1.00%)	(2.00%)	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Investment, Percent	(0.00%)	(0.00%)	(0.00%)	(0.00%)
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Percent	5.00%	7.00%	7.00%	5.00%
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Percent	4.00%	0.00%	1.00%	0.00%
Effective Income Tax Rate Reconciliation, Other Adjustments, Percent	0.00%	0.00%	0.00%	0.00%
Effective Income Tax Rate Reconciliation, Percent	26.00%	40.00%	25.00%	40.00%

Income Taxes Income Taxes (Narr

Income Taxes Income Taxes (Narrative) (Details) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018USD ($)	Sep. 30, 2018USD ($)
Income Taxes [Line Items]
Adjustment of deferred tax (asset) liability	$ 13	$ 13
Union Electric Company
Income Taxes [Line Items]
Adjustment of deferred tax (asset) liability	4	4
Ameren Illinois Company
Income Taxes [Line Items]
Adjustment of deferred tax (asset) liability	4	$ 4
State and Local Jurisdiction | Union Electric Company
Income Taxes [Line Items]
State Income Tax Statutory Rate		4.00%
Deferred tax liabilities, net	$ 33	$ 33
State and Local Jurisdiction | Maximum | Union Electric Company
Income Taxes [Line Items]
State Income Tax Statutory Rate		6.25%

Segment Information (Schedule O

Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)segment	Sep. 30, 2017USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment			4
External revenues	$ 1,724	$ 1,723	$ 4,872	$ 4,775
Intersegment revenues	0	0	0	0
Net Income (Loss) Available to Common Stockholders, Basic	357	288	747	583
Capital expenditures	577	525	$ 1,689	1,523
Union Electric Company
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment			1
Ameren Illinois Company
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment			3
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	564	574	$ 1,902	1,853
Intersegment revenues	0	0	0	0
Net Income Available to Common Shareholder	63	55	220	191
Capital expenditures	345	276	947	760
Operating Segments | Union Electric Company
Segment Reporting Information [Line Items]
External revenues	1,118	1,105	2,848	2,800
Intersegment revenues	11	11	28	41
Net Income Available to Common Shareholder	294	234	500	359
Capital expenditures	210	178	664	533
Operating Segments | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	392	404	1,177	1,175
Intersegment revenues	0	0	2	3
Net Income Available to Common Shareholder	35	31	101	94
Capital expenditures	135	112	389	354
Operating Segments | Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues	116	111	569	509
Intersegment revenues	0	1	0	1
Net Income Available to Common Shareholder	0	2	49	40
Capital expenditures	111	71	237	180
Operating Segments | Ameren Transmission
Segment Reporting Information [Line Items]
External revenues	98	105	278	293
Intersegment revenues	15	14	42	33
Net Income Available to Common Shareholder	48	38	121	106
Capital expenditures	124	173	399	463
Operating Segments | Other
Segment Reporting Information [Line Items]
External revenues	0	(2)	0	(2)
Intersegment revenues	0	0	0	0
Net Income Available to Common Shareholder	(20)	(17)	(24)	(16)
Capital expenditures	1	(2)	6	3
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	392	404	1,179	1,178
Intersegment revenues	0	0	0	0
Net Income Available to Common Shareholder	35	31	101	94
Capital expenditures	135	112	389	354
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues	116	112	569	510
Intersegment revenues	0	0	0	0
Net Income Available to Common Shareholder	0	2	49	40
Capital expenditures	111	71	237	180
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Segment Reporting Information [Line Items]
External revenues	56	58	154	165
Intersegment revenues	15	14	41	32
Net Income Available to Common Shareholder	28	22	70	57
Capital expenditures	99	93	321	226
Intersegment Eliminations
Segment Reporting Information [Line Items]
External revenues	0	0	0	0
Intersegment revenues	(26)	(26)	(72)	(78)
Net Income Available to Common Shareholder	0	0	0	0
Capital expenditures	(4)	(7)	(6)	(10)
Intersegment Eliminations | Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	0	0	0	0
Intersegment revenues	(15)	(14)	(41)	(32)
Net Income Available to Common Shareholder	0	0	0	0
Capital expenditures	$ 0	$ 0	$ 0	$ 0

Segment Information (Disaggrega

Segment Information (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ 1,724	$ 1,723	$ 4,872	$ 4,775
Revenues from alternative revenue programs	(111)	(105)	(91)	(51)
Other revenues not from contracts with customers	5	6	38	18
TCJA - Excess Amounts Collected in Rates
Disaggregation of Revenue [Line Items]
TCJA Revenue Reduction	13		60
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(26)	(26)	(72)	(78)
Electricity
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,590	1,594	4,209	4,183
Electricity | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	731	715	1,935	1,786
Electricity | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	548	542	1,414	1,366
Electricity | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	129	127	345	325
Electricity | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	182	210	515	706
Electricity | Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(26)	(26)	(72)	(77)
Electricity | Intersegment Eliminations | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Electricity | Intersegment Eliminations | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Electricity | Intersegment Eliminations | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Electricity | Intersegment Eliminations | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(26)	(26)	(72)	(77)
Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	134	129	663	592
Natural Gas | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	76	81	470	408
Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	23	25	138	120
Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	2	3	15	10
Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	33	20	40	54
Natural Gas | Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	(1)
Natural Gas | Intersegment Eliminations | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Natural Gas | Intersegment Eliminations | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Natural Gas | Intersegment Eliminations | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Natural Gas | Intersegment Eliminations | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	(1)
Ameren Illinois Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	564	574	1,902	1,853
Ameren Illinois Company | Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(15)	(14)	(41)	(32)
Ameren Illinois Company | Intersegment Eliminations | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Ameren Illinois Company | Intersegment Eliminations | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Ameren Illinois Company | Intersegment Eliminations | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Ameren Illinois Company | Intersegment Eliminations | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(15)	(14)	(41)	(32)
Ameren Illinois Company | Ameren Illinois Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	564	574	1,902	1,853
Revenues from alternative revenue programs	(110)	(99)	(81)	(33)
Other revenues not from contracts with customers	2	2	16	7
Ameren Illinois Company | Ameren Illinois Company | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	291	296	1,071	1,010
Ameren Illinois Company | Ameren Illinois Company | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	151	154	494	495
Ameren Illinois Company | Ameren Illinois Company | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	29	29	108	90
Ameren Illinois Company | Ameren Illinois Company | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	93	95	229	258
Ameren Illinois Company | Electricity
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	448	462	1,333	1,343
Ameren Illinois Company | Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	116	112	569	510
Operating Segments | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,129	1,116	2,876	2,841
Revenues from alternative revenue programs	1	(6)	(8)	(20)
Other revenues not from contracts with customers	3	4	22	11
Operating Segments | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	392	404	1,179	1,178
Revenues from alternative revenue programs	(98)	(96)	(52)	(47)
Other revenues not from contracts with customers	1	2	14	5
Operating Segments | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	116	112	569	510
Revenues from alternative revenue programs	(2)	(1)	(10)	11
Other revenues not from contracts with customers	1	0	2	2
Operating Segments | Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	113	119	320	326
Revenues from alternative revenue programs	(12)	(2)	(21)	5
Other revenues not from contracts with customers	0	0	0	0
Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	(2)	0	(2)
Operating Segments | Electricity | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,111	1,099	2,782	2,758
Operating Segments | Electricity | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	508	491	1,272	1,135
Operating Segments | Electricity | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	417	409	1,033	971
Operating Segments | Electricity | Union Electric Company | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	101	100	249	242
Operating Segments | Electricity | Union Electric Company | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	85	99	228	410
Operating Segments | Electricity | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	392	404	1,179	1,178
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	223	224	663	651
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	131	133	381	395
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	28	27	96	83
Operating Segments | Electricity | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	10	20	39	49
Operating Segments | Electricity | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Electricity | Ameren Illinois Gas | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Electricity | Ameren Illinois Gas | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Electricity | Ameren Illinois Gas | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Electricity | Ameren Illinois Gas | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Electricity | Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	113	119	320	326
Operating Segments | Electricity | Ameren Transmission | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Electricity | Ameren Transmission | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Electricity | Ameren Transmission | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Electricity | Ameren Transmission | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	113	119	320	326
Operating Segments | Electricity | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	(2)	0	(2)
Operating Segments | Electricity | Other | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Electricity | Other | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Electricity | Other | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Electricity | Other | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	(2)	0	(2)
Operating Segments | Natural Gas | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	18	17	94	83
Operating Segments | Natural Gas | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	8	9	62	49
Operating Segments | Natural Gas | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	3	4	25	20
Operating Segments | Natural Gas | Union Electric Company | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1	1	3	3
Operating Segments | Natural Gas | Union Electric Company | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	6	3	4	11
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Natural Gas | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	116	112	569	510
Operating Segments | Natural Gas | Ameren Illinois Gas | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	68	72	408	359
Operating Segments | Natural Gas | Ameren Illinois Gas | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	20	21	113	100
Operating Segments | Natural Gas | Ameren Illinois Gas | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1	2	12	7
Operating Segments | Natural Gas | Ameren Illinois Gas | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	27	17	36	44
Operating Segments | Natural Gas | Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Natural Gas | Ameren Transmission | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Natural Gas | Ameren Transmission | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Natural Gas | Ameren Transmission | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Natural Gas | Ameren Transmission | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Natural Gas | Other | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Natural Gas | Other | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Natural Gas | Other | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Natural Gas | Other | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(98)	(96)	(52)	(47)
Other revenues not from contracts with customers	1	2	14	5
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenues from alternative revenue programs	(2)	(1)	(10)	11
Other revenues not from contracts with customers	1	0	2	2
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	71	72	195	197
Revenues from alternative revenue programs	(10)	(2)	(19)	3
Other revenues not from contracts with customers	0	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	71	72	195	197
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	392	404	1,179	1,178
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	223	224	663	651
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	131	133	381	395
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	28	27	96	83
Operating Segments | Ameren Illinois Company | Electricity | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	10	20	39	49
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	116	112	569	510
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Gas | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	68	72	408	359
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Gas | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	20	21	113	100
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Gas | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1	2	12	7
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Gas | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ 27	$ 17	$ 36	$ 44