Ameren (AEE) 10-Q 9 May 18 2018 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2018	Apr. 30, 2018
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q1
Trading Symbol	AEE
Entity Registrant Name	AMEREN CORP
Entity Central Index Key	1,002,910
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		243,653,807
Union Electric Company
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q1
Entity Registrant Name	UNION ELECTRIC CO
Entity Central Index Key	100,826
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		102,123,834
Ameren Illinois Company
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q1
Entity Registrant Name	AMEREN ILLINOIS CO
Entity Central Index Key	18,654
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		25,452,373

Consolidated Statement of Incom

Consolidated Statement of Income (Loss) and Comprehensive Income (Loss) - USD ($) shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Operating Revenues:
Electric	$ 1,223	$ 1,207
Natural gas	362	308
Total operating revenues	1,585	1,515
Operating Expenses:
Fuel	188	206
Purchased power	163	180
Natural gas purchased for resale	171	130
Other operations and maintenance	431	418
Depreciation and amortization	234	221
Taxes other than income taxes	125	118
Total operating expenses	1,312	1,273
Operating Income	273	242
Other Income (Expense), Net	23	18
Interest Charges	101	99
Income Before Income Taxes	195	161
Income Taxes	42	57
Net income	153	104
Less: Net Income Attributable to Noncontrolling Interests	2	2
Net Income Attributable to Ameren Common Shareholders	151	102
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	1	0
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	154	104
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	2	2
Comprehensive Income	$ 152	$ 102
Earnings Per Share, Basic and Diluted [Abstract]
Earnings Per Share, Basic and Diluted (In dollars per share)	$ 0.62	$ 0.42
Dividends per Common Share (in dollars per share)	$ 0.4575	$ 0.44
Average Common Shares Outstanding - Basic (in shares)	242.9	242.6
Union Electric Company
Operating Revenues:
Electric	$ 741	$ 747
Natural gas	51	44
Total operating revenues	792	791
Operating Expenses:
Fuel	188	206
Purchased power	42	91
Natural gas purchased for resale	24	20
Other operations and maintenance	232	219
Depreciation and amortization	136	133
Taxes other than income taxes	80	75
Total operating expenses	702	744
Operating Income	90	47
Other Income (Expense), Net	13	16
Interest Charges	51	54
Income Before Income Taxes	52	9
Income Taxes	13	3
Net income	39	6
Preferred Stock Dividends	1	1
Net Income Available to Common Shareholder	38	5
Ameren Illinois Company
Operating Revenues:
Electric	449	439
Natural gas	311	264
Total operating revenues	760	703
Operating Expenses:
Purchased power	124	101
Natural gas purchased for resale	147	110
Other operations and maintenance	199	200
Depreciation and amortization	90	83
Taxes other than income taxes	41	40
Total operating expenses	601	534
Operating Income	159	169
Other Income (Expense), Net	6	0
Interest Charges	37	37
Income Before Income Taxes	128	132
Income Taxes	32	52
Net income	96	80
Preferred Stock Dividends	1	1
Net Income Available to Common Shareholder	$ 95	$ 79

Consolidated Statement of Inco3

Consolidated Statement of Income (Loss) and Comprehensive Income (Loss) (Parenthetical) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Income Statement [Abstract]
Pension and other postretirement benefit plan activity, tax expense (benefit)	$ 0	$ 0

Consolidated Balance Sheet

Consolidated Balance Sheet - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Current Assets:
Cash and cash equivalents	$ 30	$ 10
Accounts receivable - trade (less allowance for doubtful accounts)	514	445
Unbilled revenue	258	323
Miscellaneous accounts and notes receivable	98	70
Inventories	453	522
Current regulatory assets	130	144
Other current assets	84	98
Total current assets	1,567	1,612
Property, Plant, and Equipment, Net	21,666	21,466
Investments and Other Assets:
Nuclear decommissioning trust fund	698	704
Goodwill	411	411
Regulatory assets	1,205	1,230
Other assets	532	522
Total investments and other assets	2,846	2,867
TOTAL ASSETS	26,079	25,945
Current Liabilities:
Current maturities of long-term debt	1,170	841
Short-term Debt	960	484
Accounts and wages payable	497	902
Taxes accrued	91	52
Interest accrued	97	99
Customer deposits	115	108
Current regulatory liabilities	130	128
Other current liabilities	285	326
Total current liabilities	3,345	2,940
Long-term Debt, Net	6,766	7,094
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	2,564	2,506
Accumulated deferred investment tax credits	47	49
Regulatory liabilities	4,363	4,387
Asset retirement obligations	636	638
Pension and other postretirement benefits	541	545
Other deferred credits and liabilities	445	460
Total deferred credits and other liabilities	8,596	8,585
Commitments and Contingencies
Stockholders' Equity:
Common Stock	2	2
Other paid-in capital	5,546	5,540
Retained earnings	1,699	1,660
Accumulated other comprehensive loss	(17)	(18)
Stockholder's equity	7,230	7,184
Noncontrolling Interest	142	142
Total equity	7,372	7,326
TOTAL LIABILITIES AND EQUITY	26,079	25,945
Union Electric Company
Current Assets:
Cash and cash equivalents	3	0
Accounts receivable - trade (less allowance for doubtful accounts)	205	200
Accounts receivable - affiliates	14	11
Unbilled revenue	134	165
Miscellaneous accounts and notes receivable	54	35
Inventories	383	388
Current regulatory assets	60	56
Other current assets	48	50
Total current assets	901	905
Property, Plant, and Equipment, Net	11,768	11,751
Investments and Other Assets:
Nuclear decommissioning trust fund	698	704
Regulatory assets	380	395
Other assets	293	288
Total investments and other assets	1,371	1,387
TOTAL ASSETS	14,040	14,043
Current Liabilities:
Current maturities of long-term debt	713	384
Short-term Debt	282	39
Accounts and wages payable	203	475
Accounts payable - affiliates	40	60
Taxes accrued	64	30
Interest accrued	48	54
Current regulatory liabilities	46	19
Other current liabilities	99	103
Total current liabilities	1,495	1,164
Long-term Debt, Net	3,249	3,577
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,680	1,650
Accumulated deferred investment tax credits	46	48
Regulatory liabilities	2,644	2,664
Asset retirement obligations	632	634
Pension and other postretirement benefits	212	213
Other deferred credits and liabilities	13	12
Total deferred credits and other liabilities	5,227	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	1,620	1,632
Stockholder's equity	4,069	4,081
TOTAL LIABILITIES AND EQUITY	14,040	14,043
Ameren Illinois Company
Current Assets:
Cash and cash equivalents	3	0
Accounts receivable - trade (less allowance for doubtful accounts)	298	234
Accounts receivable - affiliates	11	9
Unbilled revenue	124	158
Miscellaneous accounts and notes receivable	39	35
Inventories	70	134
Current regulatory assets	69	87
Other current assets	12	15
Total current assets	626	672
Property, Plant, and Equipment, Net	8,463	8,293
Investments and Other Assets:
Goodwill	411	411
Regulatory assets	812	822
Other assets	159	147
Total investments and other assets	1,382	1,380
TOTAL ASSETS	10,471	10,345
Current Liabilities:
Current maturities of long-term debt	457	457
Short-term Debt	224	62
Accounts and wages payable	243	337
Accounts payable - affiliates	59	70
Taxes accrued	18	19
Interest accrued	42	33
Customer deposits	77	69
Current environmental remediation	47	42
Current regulatory liabilities	67	92
Other current liabilities	141	177
Total current liabilities	1,375	1,358
Long-term Debt, Net	2,373	2,373
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,035	1,021
Accumulated deferred investment tax credits	1	1
Regulatory liabilities	1,625	1,629
Pension and other postretirement benefits	285	285
Environmental remediation	122	134
Other deferred credits and liabilities	230	234
Total deferred credits and other liabilities	3,298	3,304
Commitments and Contingencies
Stockholders' Equity:
Common Stock	0	0
Other paid-in capital	2,033	2,013
Preferred stock	62	62
Retained earnings	1,330	1,235
Stockholder's equity	3,425	3,310
TOTAL LIABILITIES AND EQUITY	$ 10,471	$ 10,345

Consolidated Balance Sheet (Par

Consolidated Balance Sheet (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Accounts receivable - trade allowance for doubtful accounts	$ 20	$ 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)	243,600,000	242,600,000
Union Electric Company
Accounts receivable - trade allowance for doubtful accounts	$ 7	$ 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	$ 13	$ 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	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Cash Flows From Operating Activities:
Net income	$ 153	$ 104
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	230	217
Amortization of nuclear fuel	24	24
Amortization of debt issuance costs and premium/discounts	5	6
Deferred income taxes and investment tax credits, net	26	51
Allowance for equity funds used during construction	(5)	(6)
Stock-based compensation costs	6	4
Other	2	(4)
Changes in assets and liabilities:
Receivables	(26)	44
Inventories	68	60
Accounts and wages payable	(249)	(231)
Taxes accrued	49	36
Regulatory assets and liabilities	20	7
Assets, other	1	7
Liabilities, other	(57)	3
Pension and other postretirement benefits	11	9
Net cash provided by operating activities	258	331
Cash Flows From Investing Activities:
Capital expenditures	(579)	(504)
Nuclear fuel expenditures	(12)	(27)
Purchases of securities - nuclear decommissioning trust fund	(38)	(40)
Sales and maturities of securities - nuclear decommissioning trust fund	34	34
Other	(2)	(2)
Net cash used in investing activities	(597)	(539)
Cash Flows From Financing Activities:
Dividends on common stock	(111)	(107)
Dividends paid to noncontrolling interest holders	(2)	(2)
Short-term debt, net	475	356
Issuance of common stock	17	0
Repurchases of common stock for stock-based compensation	0	(24)
Employee payroll taxes related to stock-based compensation	(19)	(15)
Other	0	(1)
Net cash provided by financing activities	360	207
Net change in cash, cash equivalents, and restricted cash	21	(1)
Cash, cash equivalents, and restricted cash at beginning of year	68	52
Cash, cash equivalents, and restricted cash at end of period	89	51
Noncash financing activity - Issuance of common stock for stock-based compensation	35	0
Union Electric Company
Cash Flows From Operating Activities:
Net income	39	6
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	132	127
Amortization of nuclear fuel	24	24
Amortization of debt issuance costs and premium/discounts	2	2
Deferred income taxes and investment tax credits, net	(1)	2
Allowance for equity funds used during construction	(4)	(5)
Other	4	1
Changes in assets and liabilities:
Receivables	4	28
Inventories	5	11
Accounts and wages payable	(192)	(186)
Taxes accrued	36	35
Regulatory assets and liabilities	38	29
Assets, other	(5)	11
Liabilities, other	(10)	4
Pension and other postretirement benefits	4	4
Net cash provided by operating activities	76	93
Cash Flows From Investing Activities:
Capital expenditures	(249)	(196)
Nuclear fuel expenditures	(12)	(27)
Purchases of securities - nuclear decommissioning trust fund	(38)	(40)
Sales and maturities of securities - nuclear decommissioning trust fund	34	34
Money pool advances, net	0	161
Other	0	0
Net cash used in investing activities	(265)	(68)
Cash Flows From Financing Activities:
Dividends on common stock	(50)	(60)
Dividends on preferred stock	(1)	(1)
Short-term debt, net	243	36
Net cash provided by financing activities	192	(25)
Net change in cash, cash equivalents, and restricted cash	3	0
Cash, cash equivalents, and restricted cash at beginning of year	7	5
Cash, cash equivalents, and restricted cash at end of period	10	5
Ameren Illinois Company
Cash Flows From Operating Activities:
Net income	96	80
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	90	83
Amortization of debt issuance costs and premium/discounts	3	3
Deferred income taxes and investment tax credits, net	9	51
Allowance for equity funds used during construction	(1)	(1)
Other	(2)	0
Changes in assets and liabilities:
Receivables	(34)	16
Inventories	63	49
Accounts and wages payable	(52)	(51)
Taxes accrued	1	(2)
Regulatory assets and liabilities	(16)	(19)
Assets, other	2	2
Liabilities, other	(36)	(5)
Pension and other postretirement benefits	6	5
Net cash provided by operating activities	130	212
Cash Flows From Investing Activities:
Capital expenditures	(300)	(227)
Net cash used in investing activities	(300)	(227)
Cash Flows From Financing Activities:
Dividends on preferred stock	(1)	(1)
Short-term debt, net	162	17
Capital contribution from parent	20
Other	0	(1)
Net cash provided by financing activities	181	15
Net change in cash, cash equivalents, and restricted cash	11	0
Cash, cash equivalents, and restricted cash at beginning of year	41	28
Cash, cash equivalents, and restricted cash at end of period	$ 52	$ 28

Summary Of Significant Accounti

Summary Of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES General Ameren, headquartered in St. Louis, Missouri, is a public utility holding company whose primary assets are its equity interests in its subsidiaries. Ameren’s subsidiaries are separate, independent legal entities with separate businesses, assets, and liabilities. Dividends on Ameren’s common stock and the payment of expenses by Ameren depend on distributions made to it by its subsidiaries. Ameren’s principal subsidiaries are listed below. Ameren also has other subsidiaries that conduct other activities, such as providing shared services. 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 March 31, 2018 , and December 31, 2017 , Ameren had unconsolidated variable interests as a limited partner in various equity method investments, totaling $19 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 March 31, 2018 , the maximum exposure to loss related to these variable interests is limited to the investment in these partnerships of $19 million plus associated outstanding funding commitments of $18 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 in the first quarter of 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 March 31, 2018 and 2017 , and December 31, 2017 and 2016 : March 31, 2018 December 31, 2017 March 31, 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) $ 30 $ 3 $ 3 $ 10 $ — $ — $ 8 $ — $ — $ 9 $ — $ — Restricted cash included in “Other current assets” 12 4 5 21 5 6 19 4 5 20 4 6 Restricted cash included in “Other assets” 44 — 44 35 — 35 23 — 23 22 — 22 Restricted cash included in “Nuclear decommissioning trust fund” 3 3 (b) 2 2 (b) 1 1 (b) 1 1 (b) Total cash, cash equivalents, and restricted cash (c) $ 89 $ 10 $ 52 $ 68 $ 7 $ 41 $ 51 $ 5 $ 28 $ 52 $ 5 $ 28 (a) As presented on the balance sheet. (b) Not applicable. (c) As presented on the statement 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 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 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 which 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 three months ended March 31, 2018 and 2017 : March 31, 2018 March 31, 2017 Ameren (a) Ameren Missouri Ameren Illinois Ameren (a) Ameren Missouri Ameren Illinois Accrued capital expenditures $ 202 $ 73 $ 114 $ 164 $ 50 $ 74 Accrued nuclear fuel expenditures (b) (b) (c) 13 13 (c) Net realized gain – nuclear decommissioning trust fund (b) (b) (c) 4 4 (c) Net unrealized gain (loss) – nuclear decommissioning trust fund (11 ) (11 ) (c) 19 19 (c) (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries. (b) Less than $1 million. (c) Not applicable. Accounts Receivable "Accounts receivable – trade" on Ameren's and Ameren Illinois' balance sheet includes certain receivables purchased at a discount from alternative retail electric suppliers that elect to participate in the utility consolidated billing program. At March 31, 2018 , and December 31, 2017 , "Other current liabilities" on Ameren's and Ameren Illinois' balance sheet included payables for purchased receivables of $33 million and $31 million , respectively. For the three months ended March 31, 2018 and 2017 , the Ameren Companies recorded immaterial expense related to doubtful accounts. Asset Retirement Obligations The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the three months ended March 31, 2018 : Ameren Missouri Ameren Illinois (a) Ameren Balance at December 31, 2017 $ 640 (b) $ 4 $ 644 (b) Liabilities settled (c) (c) (c) Accretion (d) 7 (c) 7 Change in estimates (e) (9 ) — (9 ) Balance at March 31, 2018 $ 638 (b) $ 4 $ 642 (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 March 31, 2018 , respectively. (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 Company-owned life insurance is recorded at the net cash surrender value, which is the amount that can be realized under the insurance policies at the balance sheet date. As of March 31, 2018 , the cash surrender value of company-owned life insurance at Ameren and Ameren Illinois was $ 251 million and $ 120 million , respectively, while total borrowings against the policies were $ 111 million at both Ameren and Ameren Illinois. As of December 31, 2017 , the cash surrender value of company-owned life insurance at Ameren and Ameren Illinois was $ 265 million and $ 129 million , respectively, while total borrowings against the policies were $ 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 A summary of nonvested performance share units and restricted stock units at March 31, 2018 , and changes during the three months ended March 31, 2018 , under the 2014 Incentive Plan are presented below: 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 298,774 62.88 181,145 57.62 Forfeitures (37,337 ) 46.08 (643 ) 58.99 Undistributed vested units (b) (72,392 ) 53.50 (6,458 ) 58.99 Earned and vested (176,043 ) 52.88 — — Nonvested at March 31, 2018 (c) 908,491 $ 55.81 174,044 $ 57.56 (a) Excludes 712,572 of 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 measurement period. (c) Excludes 403,584 undistributed vested performance share units and 6,458 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. In the event of a participant’s death or retirement, awards vest on a pro rata basis. 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 payout date of the awards is approximately 38 months after the grant date. 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 simulations can produce a greater fair value for the performance share unit than the December 31 applicable closing common share price because they include the weighted payout scenarios in which an increase in the share price has occurred. The significant assumptions used to calculate fair value also included a three -year risk-free rate of 1.98% 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, 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 date the restricted stock unit is granted. Deferred Compensation As of March 31, 2018 , and December 31, 2017 , “Other deferred credits and liabilities” on Ameren’s balance sheet included deferred compensation obligations of $84 million and $86 million , respectively, 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. MISO-related electricity revenues are earned as electricity is delivered. MISO-related capacity and ancillary service revenues are earned as services are provided. Wholesale bilateral revenues include the sale of electricity and capacity. Wholesale bilateral electricity revenues are earned as electricity is delivered. Wholesale bilateral capacity revenues are earned as services are provided. Retail distribution, electric transmission, and off-system revenues, including each respective underlying component 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 services are provided. The Ameren Companies have elected the optional exemption to exclude 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 March 31, 2018 and 2017, our remaining performance obligations were immaterial. See Note 12 – 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. Excise Taxes Ameren Missouri and Ameren Illinois collect from their customers certain excise taxes that are levied on the sale or distribution of natural gas and electricity. Excise taxes are levied on Ameren Missouri’s electric and natural gas businesses and on Ameren Illinois’ natural gas business. They are recorded gross in “Operating Revenues – Electric,” “Operating Revenues – Natural gas” and “Operating Expenses – Taxes other than income taxes” on the statements of income. Excise taxes for electric service in Illinois are levied on customers and are therefore not included in Ameren Illinois’ revenues and expenses. The following table presents the excise taxes recorded in “Operating Revenues – Electric,” “Operating Revenues – Natural gas” and “Operating Expenses – Taxes other than income taxes” for the three months ended March 31, 2018 and 2017 : Three Months 2018 2017 Ameren Missouri $ 34 $ 31 Ameren Illinois 22 19 Ameren $ 56 $ 50 Earnings Per Share There were no material differences between Ameren’s basic and diluted earnings per share amounts for the three months ended March 31, 2018 and 2017 . The assumed settlement of dilutive performance share units had an immaterial impact on earnings per share. There were no potentially dilutive securities excluded from the earnings per diluted share calculations for the three months ended March 31, 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 leases, the measurement of credit losses on financial instruments, and the reclassification of certain tax effects from accumulated OCI.

Rate And Regulatory Matters

Rate And Regulatory Matters	3 Months Ended
	Mar. 31, 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 MoPSC Federal Income Tax Proceedings In February 2018, the MoPSC initiated proceedings to investigate how the effect of the reduction in the federal statutory corporate income tax rate enacted under the TCJA should be reflected in rates paid by customers of Missouri’s regulated utilities, including rates paid by electric and natural gas customers of Ameren Missouri. As of March 31, 2018, Ameren Missouri recorded an immaterial decrease in revenues as a regulatory liability for a potential reduction in customer rates. The MoPSC is under no deadline to issue an order in these proceedings. MEEIA The MEEIA 2016 program provided Ameren Missouri with a performance incentive to earn additional revenues by achieving certain customer energy-efficiency goals, including $27 million if 100% of the goals were achieved during the three-year period, with the potential to earn more if Ameren Missouri’s energy savings exceeded those goals. In September 2017, Ameren Missouri received an order from the MoPSC approving Ameren Missouri’s energy savings results for the first year of the MEEIA 2016 programs. As a result of this order and in accordance with revenue recognition guidance, Ameren Missouri recognized $5 million of revenues in the first quarter of 2018 relating to the MEEIA 2016 performance incentive. Illinois Electric Distribution Service Rates Under a formula ratemaking framework effective through 2022, Ameren Illinois’ electric distribution service rates are subject to an annual revenue requirement reconciliation to its actual recoverable costs and allowed return on equity. The formula ratemaking framework qualifies as an alternative revenue program under GAAP. Each year, Ameren Illinois records a regulatory asset or a regulatory liability and a corresponding increase or decrease to operating revenues for any differences between the revenue requirement reflected in customer rates for that year and its estimate of the probable increase or decrease in the revenue requirement expected to ultimately be approved by the ICC. As of March 31, 2018 , Ameren Illinois had recorded regulatory assets of $52 million to reflect its 2017 revenue requirement reconciliation adjustment, which was included in the April 2018 formula rate update discussed below, and $19 million for the approved 2016 revenue requirement reconciliation adjustment, each with interest. As of March 31, 2018 , Ameren Illinois had recorded a regulatory asset of $42 million to reflect the difference between Ameren Illinois’ estimate of its 2018 revenue requirement and the revenue requirement reflected in customer rates, including interest. In April 2018, Ameren Illinois filed with the ICC its annual electric distribution service formula rate update to establish the revenue requirement to be used for 2019 rates. Pending ICC approval, this update filing will result in a $72 million increase in Ameren Illinois’ electric distribution service rates beginning in January 2019. This update reflects an increase to the annual formula rate based on 2017 actual costs and expected net plant additions for 2018, an increase to include the 2017 revenue requirement reconciliation adjustment, and 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. An ICC decision regarding the revenue requirement to be used for customer rates in 2019 is expected by December 2018. 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 through 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 reflect the current income tax rates starting in 2020. In April 2018, the ICC approved a rider for the net amount of the difference between revenues billed under natural gas rates in effect, pursuant to Ameren Illinois’ most recent natural gas rate order, and the revenues that would have been billed had the state and federal tax rate changes discussed above been in effect. The rider required Ameren Illinois to record this regulatory liability beginning January 25, 2018. The rider calculates such differences by evaluating the return of excess deferred taxes and income taxes included in the revenue requirement prior to the reduction in the federal statutory corporate income tax rate enacted under the TCJA and the increase in state income taxes enacted in July 2017. Ameren Illinois’ natural gas customer rates were reduced through the rider beginning in May 2018, with an estimated reduction of up to $16 million reflected over a one-year period. As of March 31, 2018, Ameren Illinois recorded an immaterial reduction in revenues as a regulatory liability for an anticipated reduction in customer rates. 2018 Natural Gas Delivery Service Regulatory Rate Review In January 2018, Ameren Illinois filed a request with the ICC seeking approval to increase its annual rates for natural gas delivery service by $49 million. In the second quarter of 2018, Ameren Illinois and the ICC staff entered into agreements to use a 9.87% return on common equity and a capital structure composed of up to and including 50% common equity in this regulatory rate review. The return on common equity and the common equity ratio are subject to ICC approval. The impact of a 9.87% return on common equity would lower the requested annual natural gas rate increase to an estimated $44 million, which includes an estimated $42 million of annual rates that would otherwise be recovered under the QIP rider. This estimated increase in annual rates includes a capital structure composed of 50% common equity and a rate base of $1.6 billion. It also reflects the reduction in the federal corporate income tax rate as a result of the TCJA, as well as the increase in the Illinois corporate income tax rate that became effective in July 2017. In an attempt to reduce regulatory lag, Ameren Illinois used a 2019 future test year in this proceeding. A decision by the ICC in this proceeding is required by December 2018, with new rates expected to be effective in January 2019. Ameren Illinois cannot predict the level of any delivery service rate changes the ICC may approve, nor whether any rate changes that may eventually be approved will be sufficient to enable Ameren Illinois to recover its costs and to earn a reasonable return on investments when the rate changes go into effect. Federal FERC Complaint Cases In November 2013, a customer group filed a complaint case with the FERC seeking a reduction in the allowed base return on common equity for FERC-regulated transmission rate base under the MISO tariff from 12.38% to 9.15% . In September 2016, the FERC issued a final order in the November 2013 complaint case, which lowered the allowed base return on common equity to 10.32%, or a 10.82% total allowed return on common equity with the inclusion of a 50 basis point incentive adder for participation in an RTO, effective since September 2016. 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 a separate case in which the FERC established the allowed base return on common equity methodology used in the two MISO complaint cases described above. Ameren is unable to predict the impact of the outcome of the United States Court of Appeals for the District of Columbia Circuit’s remand on the MISO FERC complaint cases at this time. 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 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 March 31, 2018 , Ameren and Ameren Illinois had recorded current regulatory liabilities of $42 million and $25 million , respectively, to reflect the expected refunds, including interest, associated with the reduced allowed returns on common equity in the initial decision in the February 2015 complaint case. Ameren Missouri does not expect that a reduction in the FERC-allowed base return on common equity would be material to its results of operations, financial position, or liquidity. FERC Federal Income Tax Proceeding In March 2018, the FERC granted a request filed in February 2018 by MISO transmission owners with forward-looking rate formulas, which included 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.

Short-Term Debt And Liquidity

Short-Term Debt And Liquidity	3 Months Ended
	Mar. 31, 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, both of which expire in December 2021, were not utilized for direct borrowings during the three months ended March 31, 2018 , but were used to support commercial paper issuances and to issue letters of credit. Based on commercial paper outstanding, letters of credit issued under the Credit Agreements, and cash on hand, the aggregate amount of credit capacity available under the Credit Agreements to Ameren (parent), Ameren Missouri, and Ameren Illinois, collectively, at March 31, 2018 , was $1.2 billion . The Ameren Companies were in compliance with the covenants in their Credit Agreements as of March 31, 2018 . As of March 31, 2018 , the ratios of consolidated indebtedness to consolidated total capitalization, calculated in accordance with the provisions of the Credit Agreements, were 54% , 50% , 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 March 31, 2018 , and December 31, 2017 : 2018 2017 Ameren (parent) $ 454 $ 383 Ameren Missouri 282 39 Ameren Illinois 224 62 Ameren Consolidated $ 960 $ 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 three months ended March 31, 2018 and 2017: Ameren (parent) Ameren Missouri Ameren Illinois Ameren Consolidated 2018 Average daily commercial paper outstanding at par value $ 378 $ 213 $ 137 $ 728 Weighted-average interest rate 1.90 % 1.88 % 1.96 % 1.90 % Peak commercial paper during period at par value (a) $ 454 $ 282 $ 238 $ 960 Peak interest rate 2.35 % 2.40 % 2.55 % 2.55 % 2017 Average daily commercial paper outstanding at par value $ 682 $ 4 $ 50 $ 736 Weighted-average interest rate 1.07 % 0.93 % 0.95 % 1.06 % Peak commercial paper during period at par value (a) $ 810 $ 45 $ 74 $ 914 Peak interest rate 1.30 % 1.15 % 1.15 % 1.30 % (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 months ended March 31, 2018 and 2017 , was 1.90% and 1.01% , 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 months ended March 31, 2018 and 2017 .

Long-Term Debt And Equity Finan

Long-Term Debt And Equity Financings	3 Months Ended
	Mar. 31, 2018
Long-Term Debt And Equity Financings [Abstract]
LONG-TERM DEBT AND EQUITY FINANCINGS	LONG-TERM DEBT AND EQUITY FINANCINGS Ameren Under DRPlus and our 401(k) plan, Ameren issued a total of 0.3 million shares of common stock and received $17 million for the three months ended March 31, 2018 . In addition, 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 three months of 2017. Ameren Missouri and Ameren Illinois did not issue any common stock during the first three months of 2018 or 2017. 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. Ameren Illinois In April 2018, Ameren Illinois repaid $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 March 31, 2018 , the Ameren Companies were in compliance with the provisions and covenants contained in their indentures and articles of incorporation, as applicable, and ATXI was in compliance with the provisions and covenants contained in its note purchase agreement. Off-balance-sheet Arrangements At March 31, 2018 , none of the Ameren Companies had any significant off-balance-sheet financing arrangements, other than operating leases entered into in the ordinary course of business, letters of credit, and Ameren (parent) guarantee arrangements on behalf of its subsidiaries.

Other Income and Expenses

Other Income and Expenses	3 Months Ended
	Mar. 31, 2018
Other Nonoperating Income (Expense) [Abstract]
OTHER INCOME AND EXPENSES	OTHER INCOME, NET The following table presents the components of “Other Income, Net” in the Ameren Companies’ statements of income for the three months ended March 31, 2018 and 2017 : Three Months 2018 2017 Ameren: (a) Other Income, Net Allowance for equity funds used during construction $ 5 $ 6 Interest income on industrial development revenue bonds 6 7 Interest income 2 2 Non-service cost components of net periodic benefit income 16 (b) 12 Other income 1 — Donations (5 ) (5 ) Other expense (2 ) (4 ) Total Other Income, Net $ 23 $ 18 Ameren Missouri: Other Income, Net Allowance for equity funds used during construction $ 4 $ 5 Interest income on industrial development revenue bonds 6 7 Non-service cost components of net periodic benefit income 5 (b) 6 Other income 1 — Donations (1 ) — Other expense (2 ) (2 ) Total Other Income, Net $ 13 $ 16 Ameren Illinois: Other Income, Net Allowance for equity funds used during construction $ 1 $ 1 Interest income 2 2 Non-service cost components of net periodic benefit income 7 3 Donations (4 ) (4 ) Other expense — (2 ) Total Other Income, Net $ 6 $ — (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations. (b) For the three months ended March 31, 2018, the non-service cost components of net periodic benefit income were partially offset by a $4 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.

Financial Instruments

Financial Instruments	3 Months Ended
	Mar. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
DERIVATIVE FINANCIAL INSTRUMENTS	NOTE 6 – DERIVATIVE FINANCIAL INSTRUMENTS We use derivatives to manage the risk of changes in market prices for natural gas 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 March 31, 2018 , and December 31, 2017 . As of March 31, 2018 , these contracts extended through October 2019, March 2023, and May 2032 for fuel oils, natural gas, and power, respectively. Quantity (in millions, except as indicated) 2018 2017 Commodity Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils (in gallons) (a) 27 (b) 27 28 (b) 28 Natural gas (in mmbtu) 25 144 169 24 139 163 Power (in megawatthours) 2 9 11 3 9 12 (a) Consists of ultra-low-sulfur diesel products. (b) Not applicable. All contracts considered to be derivative instruments are required to be recorded on the balance sheet at their fair values, unless the NPNS exception applies. See Note 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 March 31, 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 March 31, 2018 , and December 31, 2017 : Balance Sheet Location Ameren Missouri Ameren Illinois Ameren 2018 Fuel oils Other current assets $ 6 $ — $ 6 Other assets 2 — 2 Natural gas Other assets — 1 1 Power Other current assets 4 — 4 Other assets 1 — 1 Total assets (a) $ 13 $ 1 $ 14 Natural gas Other current liabilities $ 5 $ 13 $ 18 Other deferred credits and liabilities 4 16 20 Power Other current liabilities 1 14 15 Other deferred credits and liabilities — 177 177 Total liabilities (b) $ 10 $ 220 $ 230 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 March 31, 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 March 31, 2018 , if counterparty groups were to fail completely to perform on contracts, the Ameren Companies’ maximum exposure would have been immaterial with or without consideration of the application of master netting arrangements or similar agreements and collateral held. Derivative Instruments with Credit Risk-Related Contingent Features Our commodity contracts contain collateral provisions tied to the Ameren Companies’ credit ratings. If our credit ratings were downgraded, 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 March 31, 2018 , the aggregate fair value of all derivative instruments with credit risk-related contingent features in a gross liability position, the cash collateral posted, and the aggregate amount of additional collateral that counterparties could require. The additional collateral required is the net liability position allowed under the master netting arrangements or similar agreements, assuming (1) the credit risk-related contingent features underlying these arrangements were triggered on March 31, 2018 , and (2) those counterparties with rights to do so requested collateral. Aggregate Fair Value of Derivative Liabilities (a) Cash Collateral Posted Potential Aggregate Amount of Additional Collateral Required (b) Ameren Missouri $ 55 $ 5 $ 44 Ameren Illinois 50 — 44 Ameren $ 105 $ 5 $ 88 (a) Before consideration of master netting arrangements or similar agreements and including NPNS and other accrual contract exposures. (b) As collateral requirements with certain counterparties are based on master netting arrangements or similar agreements, the aggregate amount of additional collateral required to be posted is determined after consideration of the effects of such arrangements.

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2018
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	FAIR VALUE MEASUREMENTS Fair value is defined as the price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. We use various methods to determine fair value, including market, income, and cost approaches. With these approaches, we adopt certain assumptions that market participants would use in pricing the asset or liability, including assumptions about market risk or the risks inherent in the inputs to the valuation. Inputs to valuation can be readily observable, market-corroborated, or unobservable. We use valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs. Authoritative accounting guidance established a fair value hierarchy that prioritizes the inputs used to measure fair value. All financial assets and liabilities carried at fair value are classified and disclosed in one of three hierarchy levels. See Note 8 – Fair Value Measurements under Part II, Item 8, of the Form 10-K for information related to hierarchy levels. We perform an analysis each quarter to determine the appropriate hierarchy level of the assets and liabilities subject to fair value measurements. Financial assets and liabilities are classified in their entirety according to the lowest level of input that is significant to the fair value measurement. All assets and liabilities whose fair value measurement is based on significant unobservable inputs are classified as Level 3. We consider nonperformance risk in our valuation of derivative instruments by analyzing the credit standing of our counterparties and considering any counterparty credit enhancements (e.g., collateral). The guidance also requires that the fair value measurement of liabilities reflect the nonperformance risk of the reporting entity, as applicable. Therefore, we have factored the impact of our credit standing, as well as any potential credit enhancements, into the fair value measurement of both derivative assets and derivative liabilities. Included in our valuation, and based on current market conditions, is a valuation adjustment for counterparty default derived from market data such as the price of credit default swaps, bond yields, and credit ratings. No gains or losses related to valuation adjustments for counterparty default risk were recorded at Ameren, Ameren Missouri, or Ameren Illinois in the three months ended March 31, 2018 or 2017 . At March 31, 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 March 31, 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 $ 6 $ — $ 2 $ 8 Natural gas — 1 — 1 Power — — 5 5 Total derivative assets – commodity contracts $ 6 $ 1 $ 7 $ 14 Nuclear decommissioning trust fund: Equity securities: U.S. large capitalization $ 464 $ — $ — $ 464 Debt securities: U.S. treasury and agency securities — 118 — 118 Corporate bonds — 80 — 80 Other — 32 — 32 Total nuclear decommissioning trust fund $ 464 $ 230 $ — $ 694 (b) Total Ameren $ 470 $ 231 $ 7 $ 708 Ameren Missouri Derivative assets – commodity contracts (a) : Fuel oils $ 6 $ — $ 2 $ 8 Power — — 5 5 Total derivative assets – commodity contracts $ 6 $ — $ 7 $ 13 Nuclear decommissioning trust fund: Equity securities: U.S. large capitalization $ 464 $ — $ — $ 464 Debt securities: U.S. treasury and agency securities — 118 — 118 Corporate bonds — 80 — 80 Other — 32 — 32 Total nuclear decommissioning trust fund $ 464 $ 230 $ — $ 694 (b) Total Ameren Missouri $ 470 $ 230 $ 7 $ 707 Ameren Illinois Derivative assets – commodity contracts (a) : Natural gas $ — $ 1 $ — $ 1 Liabilities: Ameren Derivative liabilities – commodity contracts (a) : Natural gas $ — $ 32 $ 6 $ 38 Power — — 192 192 Total Ameren $ — $ 32 $ 198 $ 230 Ameren Missouri Derivative liabilities – commodity contracts (a) : Natural gas $ — $ 9 $ — $ 9 Power — — 1 1 Total Ameren Missouri $ — $ 9 $ 1 $ 10 Ameren Illinois Derivative liabilities – commodity contracts (a) : Natural gas $ — $ 23 $ 6 $ 29 Power — — 191 191 Total Ameren Illinois $ — $ 23 $ 197 $ 220 (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. 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 months ended March 31, 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 March 31, 2018 Beginning balance at January 1, 2018 $ 7 $ (195 ) $ (188 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (2 ) 1 (1 ) Settlements (1 ) 3 2 Ending balance at March 31, 2018 $ 4 $ (191 ) $ (187 ) Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2018 $ (1 ) $ 1 $ — For the three months ended March 31, 2017 Beginning balance at January 1, 2017 $ 7 $ (185 ) $ (178 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities — (10 ) (10 ) Settlements (3 ) 1 (2 ) Ending balance at March 31, 2017 $ 4 $ (194 ) $ (190 ) Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2017 $ — $ (11 ) $ (11 ) 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 months ended March 31, 2018 and 2017 , there were no material transfers between Level 1 and Level 2, Level 1 and Level 3, or Level 2 and Level 3 related to derivative commodity contracts. 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 for the periods ended March 31, 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 $ 2 $ — Option model Volatilities(%) (b) 21 – 33 24 Discounted cash flow Counterparty credit risk(%) (c)(d) 0.12 – 0.91 0.49 Ameren Missouri credit risk(%) (c)(d) 0.35 (e) Natural gas — (6 ) Discounted cash flow Nodal basis ($/mmbtu) (b) (1.40) – (0.10) (1) Ameren Illinois credit risk (%) (c)(d) 0.35 (e) Power (f) 5 (192 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps ($/MWh) (g) 22 – 37 31 Estimated auction price for FTRs ($/MW) (b) (479) – 1,608 58 Nodal basis ($/MWh) (g) (10) – 0 (2) Counterparty credit risk (%) (c)(d) 0.92 (e) Ameren Illinois credit risk (%) (c)(d) 0.35 (e) Fundamental energy production model Estimated future natural gas prices ($/mmbtu) (b) 3 – 4 3 Escalation rate (%) (b)(h) 4 (e) Contract price allocation Estimated renewable energy credit costs ($/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 (e) Natural gas 1 (4 ) Option model Volatilities (%) (b) 26 – 46 37 Nodal basis ($/mmbtu) (b) (0.50) – (0.30) (0.40) Discounted cash flow Nodal basis ($/mmbtu) (b) (1.20) – 0.10 (1) Counterparty credit risk (%) (c)(d) 0.37 – 0.92 0.53 Ameren credit risk (%) (c)(d) 0.37 (e) Power (f) 8 (196 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps ($/MWh) (g) 24 – 46 28 Estimated auction price for FTRs ($/MW) (b) (65) – 1,823 251 Nodal basis ($/MWh) (g) (10) – 0 (2) Counterparty credit risk (%) (c)(d) 0.28 (e) Ameren Illinois credit risk (%) (c)(d) 0.37 (e) Fundamental energy production model Estimated future natural gas prices ($/mmbtu) (b) 3 – 4 3 Escalation rate (%) (b)(h) 5 (e) Contract price allocation Estimated renewable energy credit costs ($/credit) (b) 5 – 7 6 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement. (c) Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement. (d) Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances. (e) Not applicable. (f) Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2021. Valuations beyond 2021 use fundamentally modeled pricing by month for peak and off-peak demand. (g) 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. (h) 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 March 31, 2018 , and December 31, 2017 : March 31, 2018 Carrying Amount Fair Value Level 1 Level 2 Level 3 Total Ameren: Cash, cash equivalents, and restricted cash $ 89 $ 89 $ — $ — $ 89 Investments in held-to-maturity debt securities (a) 276 — 276 — 276 Short-term debt 960 — 960 — 960 Long-term debt (including current portion) (a) 7,936 (b) — 7,820 439 (c) 8,259 Preferred stock (d) 142 — 130 — 130 Ameren Missouri: Cash, cash equivalents, and restricted cash $ 10 $ 10 $ — $ — $ 10 Investments in held-to-maturity debt securities (a) 276 — 276 — 276 Short-term debt 282 — 282 — 282 Long-term debt (including current portion) (a) 3,962 (b) — 4,201 — 4,201 Preferred stock 80 — 78 — 78 Ameren Illinois: Cash, cash equivalents, and restricted cash $ 52 $ 52 $ — $ — $ 52 Short-term debt 224 — 224 — 224 Long-term debt (including current portion) 2,830 (b) — 2,928 — 2,928 Preferred stock 62 — 52 — 52 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 (c) 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 capital lease obligation for CTs leased from the city of Bowling Green and Audrain County. As of March 31, 2018 and December 31, 2017, the carrying amount of both the investments in industrial revenue bonds and the capital lease obligations approximated fair value. (b) Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $49 million , $20 million , and $24 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of March 31, 2018 . Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $50 million , $20 million , and $24 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2017 . (c) The Level 3 fair value amount consists of ATXI’s senior unsecured notes. For the three months ended March 31, 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.

Related Party Transactions

Related Party Transactions	3 Months Ended
	Mar. 31, 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 months ended March 31, 2018 and 2017 : Three Months Agreement Income Statement Line Item Ameren Missouri Ameren Illinois Ameren Missouri power supply Operating Revenues 2018 $ 3 $ (a) agreements with Ameren Illinois 2017 11 (a) Ameren Missouri and Ameren Illinois Operating Revenues 2018 5 1 rent and facility services 2017 7 1 Ameren Missouri and Ameren Illinois Operating Revenues 2018 (b) (b) miscellaneous support services 2017 (b) (b) Total Operating Revenues 2018 $ 8 $ 1 2017 18 1 Ameren Illinois power supply Purchased Power 2018 $ (a) $ 3 agreements with Ameren Missouri 2017 (a) 11 Ameren Illinois transmission Purchased Power 2018 (a) (b) services with ATXI 2017 (a) (b) Total Purchased Power 2018 $ (a) $ 3 2017 (a) 11 Ameren Services support services Other Operations and Maintenance 2018 $ 33 $ 30 agreement 2017 35 32 Money pool borrowings (advances) Interest Charges/ Other Income, Net 2018 $ (b) $ (b) 2017 (b) (b) (a) Not applicable. (b) Amount less than $1 million.

Commitments And Contingencies

Commitments And Contingencies	3 Months Ended
	Mar. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	COMMITMENTS AND CONTINGENCIES We are involved in legal, tax, and regulatory proceedings before various courts, regulatory commissions, authorities, and governmental agencies with respect to matters that arise in the ordinary course of business, some of which involve substantial amounts of money. We believe that the final disposition of these proceedings, except as otherwise disclosed in 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 In April 2018, Ameren Illinois conducted procurement events, administered by the IPA, to purchase energy products 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. 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. 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. At March 31, 2018 , total obligations related to commitments for coal, natural gas, nuclear fuel, purchased power, methane gas, equipment, and meter reading services, among other agreements, at Ameren, Ameren Missouri, and Ameren Illinois were $2,154 million , $1,509 million , and $627 million , respectively. 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 above amounts reflect Ameren Illinois’ commitment of approximately $10 million through May 2018. 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 the CCR regulations, by the Clean Water Act rule applicable to cooling water intake structures at existing power plants, and by effluent limitation guidelines applicable to steam electric generating units, all of which are discussed below. The actual amount of capital expenditures required to comply with existing environmental regulations may vary substantially from the above estimate because of uncertainty as to whether the EPA will substantially revise regulatory obligations, exactly which compliance strategies will be used and their ultimate cost, among other things. The following sections describe the more significant environmental laws and rules and environmental enforcement and remediation matters that affect or could affect our operations. The EPA has initiated an administrative review of several regulations and proposed regulation amendments, 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 require significant reductions in SO 2 and NO x through either emission source reductions or the use and retirement of emission allowances. The first phase of the CSAPR emission reduction requirements became effective in 2015. The second phase of emission reduction requirements, which were revised by the EPA in 2016, became effective in 2017; additional emission reduction requirements may apply in subsequent years. To achieve compliance with the CSAPR, Ameren Missouri burns ultra-low-sulfur coal, operates two scrubbers at its Sioux energy center, and optimizes other existing pollution control equipment. Ameren Missouri did not make additional capital investments to comply with the 2017 CSAPR requirements. However, Ameren Missouri expects to incur additional costs to lower its emissions at one or more of its energy centers to comply with the CSAPR in future years. These higher costs are expected to be recovered from customers through the FAC or higher base rates. CO 2 Emissions Standards In 2015, the EPA issued the Clean Power Plan, which would have established CO 2 emissions standards applicable to existing power plants. The United States Supreme Court stayed the rule in February 2016, pending various legal challenges. In October 2017, the EPA announced a proposal to repeal the Clean Power Plan. In December 2017, the EPA issued an advanced notice of proposed rulemaking to solicit input from stakeholders as to how the EPA should regulate CO 2 emissions from existing power plants under the Clean Air Act. Accordingly, we no longer expect the Clean Power Plan to take effect. However, the EPA may issue new requirements that would regulate CO 2 emissions from existing power plants. We cannot predict the outcome of the EPA’s future rulemaking or the outcome of any legal challenges relating to such future rulemakings, any of which could have an adverse effect on our results of operations, financial position, and liquidity. NSR and Clean Air Litigation In January 2011, the Department of Justice, on behalf of the EPA, filed a complaint against Ameren Missouri in the United States District Court for the Eastern District of Missouri. The complaint, as amended in October 2013, alleged that in performing projects at its Rush Island coal-fired energy center in 2007 and 2010, Ameren Missouri violated provisions of the Clean Air Act and Missouri law. The litigation has been divided into two phases: liability and remedy. In January 2017, the district court issued a liability ruling that the projects violated provisions of the Clean Air Act and Missouri law. The case then proceeded to the second phase to determine the actions required to remedy the violations found in the liability phase. The EPA previously withdrew all claims for penalties and fines. No date has been set by the district court for a trial on the remedy phase of the litigation. At the conclusion of both phases of the litigation, Ameren Missouri intends to appeal the liability ruling to the United States Court of Appeals for the Eighth Circuit. The ultimate resolution of this matter could have a material adverse effect on the results of operations, financial position, and liquidity of Ameren and Ameren Missouri. Among other things and subject to economic and regulatory considerations, resolution of this matter could result in increased capital expenditures for the installation of pollution control equipment, as well as increased operations and maintenance expenses. We are unable to predict the ultimate resolution of this matter or the costs that might be incurred. Clean Water Act In 2014, the EPA issued its final rule applicable to cooling water intake structures at existing power plants. The rule requires a case-by-case evaluation and plan for reducing aquatic organisms impinged on the facility’s intake screens or entrained through the plant’s cooling water system. All of Ameren Missouri’s coal-fired and nuclear energy centers are subject to the cooling water intake structures rule. The rule will be implemented 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. Both the intake and effluent rules, if implemented as enacted, could have an adverse effect on Ameren’s and Ameren Missouri’s results of operations, financial position, and liquidity should such implementation require extensive modifications to the cooling water systems and water discharge systems at Ameren Missouri’s energy centers, and if such investments are not recovered on a timely basis in electric rates charged to Ameren Missouri’s customers. CCR Management In 2015, the EPA issued regulations regarding the management and disposal of CCR from coal-fired energy centers. These regulations affect CCR disposal and handling costs at Ameren Missouri’s energy centers. They require closure of impoundments if performance criteria relating to groundwater impacts and location restrictions are not achieved. In September 2017, the EPA granted petitions filed on behalf of coal-fired electricity generators in which the EPA agreed to reconsider certain provisions of the CCR rules. In March 2018, the EPA issued proposed revisions to the CCR Rule that, if finalized, would allow state agencies greater flexibility in establishing state CCR programs. We cannot predict the outcome of the EPA’s proposed revisions or the requirements of state programs if the proposed revisions are adopted. Ameren and Ameren Missouri have AROs of $141 million recorded on their respective balance sheets as of March 31, 2018 , associated with CCR storage facilities that reflect the regulations issued in 2015. Ameren plans to close these CCR storage facilities between 2018 and 2024. Ameren Missouri 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 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 March 31, 2018 , Ameren Illinois owned or was otherwise responsible for 44 former MGP sites in Illinois, which are in various stages of investigation, remediation, and closure. Ameren Illinois estimates it could substantially conclude remediation efforts by 2023. The ICC allows Ameren Illinois to recover such remediation and related litigation costs from its electric and natural gas utility customers through environmental cost riders. Costs are subject to annual prudence review by the ICC. As of March 31, 2018 , Ameren Illinois estimated the obligation related to these former MGP sites at $168 million to $240 million . Ameren and Ameren Illinois recorded a liability of $168 million to represent the estimated minimum obligation for these sites, as no other amount within the range was a better estimate. The scope of the remediation activities at these former MGP sites may increase as remediation efforts continue. Considerable uncertainty remains in these estimates because many site-specific factors can influence the ultimate actual costs, including unanticipated underground structures, the degree to which groundwater is encountered, regulatory changes, local ordinances, and site accessibility. The actual costs and timing of completion may vary substantially from these estimates. Ameren Missouri participated in the investigation of various sites known as Sauget Area 2, located in Sauget, Illinois. In 2000, the EPA notified Ameren Missouri and numerous other companies that former landfills and lagoons at those sites may contain soil and groundwater contamination. In 2013, the EPA issued its record of decision for Sauget Area 2 approving the investigation and the remediation actions recommended by the potentially responsible parties. As of March 31, 2018 , Ameren Missouri estimated its obligation related to Sauget Area 2 at $1 million to $2.5 million . Ameren Missouri recorded a liability of $1 million to represent its estimated minimum obligation for this site, as no other amount within the range was a better estimate. Our operations or those of our predecessor companies involve the use of, disposal of, and, in appropriate circumstances, the cleanup of substances regulated under environmental laws. We are unable to determine whether such practices will result in future environmental commitments or will affect our results of operations, financial position, or liquidity.

Callaway Energy Center

Callaway Energy Center	3 Months Ended
	Mar. 31, 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 May 2014. The DOE’s delay in carrying out its obligation to dispose of spent nuclear fuel from the Callaway energy center is not expected to adversely affect the continued operations of the energy center. As a result of the DOE's failure to fulfill its contractual obligations, Ameren Missouri and other nuclear energy center owners sued the DOE to recover costs incurred for ongoing storage of their spent fuel. 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. For the three months ended March 31, 2018, Ameren Missouri did not receive any such reimbursements. Ameren Missouri will continue to apply for reimbursement from the DOE for allowable costs associated with the ongoing storage of spent fuel. Supplier of Fuel Assemblies The Callaway energy center uses nuclear fuel assemblies fabricated by Westinghouse, which is the only NRC-licensed supplier authorized to provide fuel assemblies to the Callaway energy center. During the first quarter of 2017, Westinghouse filed voluntary petitions for a court-supervised restructuring process under Chapter 11 of the United States Bankruptcy Code. 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 Ameren Missouri’s contracts. At this time, Ameren and Ameren Missouri believe the remainder of the restructuring proceeding will not affect Westinghouse’s performance under the terms of its existing contracts with Ameren Missouri, and therefore do not expect any material impact to Ameren Missouri’s operations. A change of fuel suppliers or a change in the type of fuel assembly design that is currently licensed for use at the Callaway energy center could take an estimated three years of analysis and NRC licensing efforts to implement. Decommissioning Electric rates charged to customers provide for the recovery of the Callaway energy center’s decommissioning costs, which include decontamination, dismantling, and site restoration costs, over the expected life of the nuclear energy center. Amounts collected from customers are deposited into the external nuclear decommissioning trust fund to provide for the Callaway energy center’s decommissioning. It is assumed that the Callaway energy center site will be 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 March 31, 2018 . The property coverage and the nuclear liability coverage renewal dates are April 1 and January 1, respectively, of each year. Both coverages were renewed in 2018. Type and Source of Coverage Maximum Coverages Maximum Assessments for Single Incidents Public liability and nuclear worker liability: American Nuclear Insurers $ 450 $ — Pool participation 12,604 (a) 127 (b) $ 13,054 (c) $ 127 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 $19 million per year. (c) Limit of liability for each incident under the Price-Anderson liability provisions of the Atomic Energy Act of 1954, as amended. This limit is subject to change to account for the effects of inflation and changes in the number of licensed reactors. (d) NEIL provides $2.7 billion in property damage, stabilization, decontamination, and premature decommissioning insurance for radiation events and $2.3 billion in property damage insurance for nonradiation events. EMANI provides $490 million in property damage insurance for both radiation and nonradiation events. (e) All NEIL insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL. (f) Provides replacement power cost insurance in the event of a prolonged accidental outage. Weekly indemnity up to $4.5 million for 52 weeks, which commences after the first 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 September 2013. Owners of a nuclear reactor cover this exposure through a combination of private insurance and mandatory participation in a financial protection pool, as established by the Price-Anderson Act. Losses resulting from terrorist attacks on nuclear facilities are subject to industrywide aggregates, such that terrorist acts against one or more commercial nuclear power plants insured by NEIL or EMANI within a stated time period would be treated as a single event, and the owners of the nuclear power plants would share one full limit of liability. NEIL policies have an aggregate limit of $3.2 billion within a 12-month period for radiation events, or $1.8 billion for events not involving radiation contamination. The EMANI policies have an aggregate limit of €600 million for radiation and nonradiation events within a period of 72 hours. If losses from a nuclear incident at the Callaway energy center exceed the 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	3 Months Ended
	Mar. 31, 2018
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [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 benefit cost separately from the service cost component and outside of operating income. The Ameren Companies’ adoption of this guidance in the first quarter of 2018 resulted in the reclassification from "Operating Expenses – Other operations and maintenance" to “Other Income, Net” of first quarter 2017 net benefit income of $12 million , $6 million , and $3 million on Ameren's, Ameren Missouri's, and Ameren Illinois' respective statements of income. 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 months ended March 31, 2018 and 2017 : Pension Benefits Postretirement Benefits Three Months Three Months 2018 2017 2018 2017 Service cost (a) $ 25 $ 23 $ 5 $ 5 Non-service cost components: Interest cost 42 45 11 12 Expected return on plan assets (69 ) (66 ) (19 ) (19 ) Amortization of: Prior service benefit — — (1 ) (1 ) Actuarial loss (gain) 16 14 — (2 ) Total non-service cost components (b) (11 ) (7 ) (9 ) (10 ) Net periodic benefit cost (income) $ 14 $ 16 $ (4 ) $ (5 ) (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. 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 pension costs and the postretirement benefit costs (income) incurred for the three months ended March 31, 2018 and 2017 : Pension Benefits Postretirement Benefits Three Months Three Months 2018 2017 2018 2017 Ameren Missouri (a) $ 5 $ 6 $ — $ (1 ) Ameren Illinois 9 10 (4 ) (4 ) Ameren (a) $ 14 $ 16 $ (4 ) $ (5 ) (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.

Segment Information

Segment Information	3 Months Ended
	Mar. 31, 2018
Segment Reporting [Abstract]
SEGMENT INFORMATION	SEGMENT INFORMATION Ameren has four segments: Ameren Missouri, Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Transmission. The Ameren Missouri segment includes all of the operations of Ameren Missouri. Ameren Illinois Electric Distribution consists of the electric distribution business of Ameren Illinois. Ameren Illinois Natural Gas consists of the natural gas business of Ameren Illinois. Ameren Transmission is primarily composed of the aggregated electric transmission businesses of Ameren Illinois and ATXI. The category called Other primarily includes Ameren (parent) activities and Ameren Services. Ameren Missouri has one segment. Ameren Illinois has three segments: Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Illinois Transmission. See Note 1 – Summary of Significant Accounting Policies for additional information regarding the operations of Ameren Missouri, Ameren Illinois, and ATXI. Segment operating revenues and a majority of operating expenses are directly recognized and incurred by Ameren Illinois 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 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 months ended March 31, 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 $ 784 $ 399 $ 311 $ 91 $ — $ — $ 1,585 Intersegment revenues 8 1 — 13 — (22 ) — Net income attributable to Ameren common shareholders 38 33 42 37 (a) 1 — 151 Capital expenditures 249 122 60 145 7 (4 ) 579 2017 External revenues $ 773 $ 384 $ 264 $ 96 $ (2 ) $ — $ 1,515 Intersegment revenues 18 1 — 6 — (25 ) — Net income attributable to Ameren common shareholders 5 30 33 34 (a) — — 102 Capital expenditures 196 120 51 134 4 (1 ) 504 (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 $ 400 $ 311 $ 49 $ — $ 760 Intersegment revenues — — 13 (13 ) — Net income available to common shareholder 33 42 20 — 95 Capital expenditures 122 60 118 — 300 2017 External revenues $ 385 $ 264 $ 54 $ — $ 703 Intersegment revenues — — 6 (6 ) — Net income available to common shareholder 30 33 16 — 79 Capital expenditures 120 51 56 — 227 The following tables present disaggregated revenues by segment at Ameren and Ameren Illinois for the three months ended March 31, 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 $ 332 $ 219 $ — $ — $ — $ — $ 551 Commercial 252 124 — — — — 376 Industrial 61 35 — — — — 96 Other 96 22 — 104 — (22 ) 200 Total electric revenues $ 741 $ 400 $ — $ 104 $ — $ (22 ) $ 1,223 Residential $ 41 $ — $ 243 $ — $ — $ — $ 284 Commercial 16 — 67 — — — 83 Industrial 2 — 6 — — — 8 Other (8 ) — (5 ) — — — (13 ) Total gas revenues $ 51 $ — $ 311 $ — $ — $ — $ 362 Total revenues (a) $ 792 $ 400 $ 311 $ 104 $ — $ (22 ) $ 1,585 2017 Residential $ 290 $ 219 $ — $ — $ — $ — $ 509 Commercial 232 133 — — — — 365 Industrial 58 28 — — — — 86 Other 167 5 — 102 (2 ) (25 ) 247 Total electric revenues $ 747 $ 385 $ — $ 102 $ (2 ) $ (25 ) $ 1,207 Residential $ 30 $ — $ 203 $ — $ — $ — $ 233 Commercial 12 — 55 — — — 67 Industrial 1 — 3 — — — 4 Other 1 — 3 — — — 4 Total gas revenues $ 44 $ — $ 264 $ — $ — $ — $ 308 Total revenues (b) $ 791 $ 385 $ 264 $ 102 $ (2 ) $ (25 ) $ 1,515 (a) Includes revenues from alternative revenue programs of $(4) million , $31 million , $(3) million , $(4) million , and $20 million at Ameren Missouri, Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, Ameren Transmission, and Ameren, respectively. Also includes other revenues not from contracts with customers of $14 million , $10 million , $1 million , $- million, and $25 million at Ameren Missouri, Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, Ameren Transmission, and Ameren, respectively. (b) Includes revenues from alternative revenue programs of $(7) million , $33 million , $11 million , $5 million , and $42 million at Ameren Missouri, Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, Ameren Transmission, and Ameren, respectively. Also includes other revenues not from contracts with customers of $4 million , $2 million , $1 million , $- million, and $7 million at Ameren Missouri, Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, Ameren Transmission, and Ameren, respectively. Ameren Illinois Three Months Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Total Ameren Illinois 2018 Residential $ 219 $ 243 $ — $ — $ 462 Commercial 124 67 — — 191 Industrial 35 6 — — 41 Other 22 (5 ) 62 (13 ) 66 Total revenues (a) $ 400 $ 311 $ 62 $ (13 ) $ 760 2017 Residential $ 219 $ 203 $ — $ — $ 422 Commercial 133 55 — — 188 Industrial 28 3 — — 31 Other 5 3 60 (6 ) 62 Total revenues (b) $ 385 $ 264 $ 60 $ (6 ) $ 703 (a) Includes revenues from alternative revenue programs of $31 million , $(3) million , $(4) million , and $24 million at Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Illinois Transmission, and Ameren Illinois, respectively. Also includes other revenues not from contracts with customers of $10 million , $1 million , $- million, and $11 million at Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, Ameren Illinois Transmission, and Ameren Illinois, respectively. (b) Includes revenues from alternative revenue programs of $33 million , $11 million , $3 million , and $47 million at Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Illinois Transmission, and Ameren Illinois, respectively. Also includes other revenues not from contracts with customers of $2 million , $1 million , $- million, and $3 million at Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, Ameren Illinois Transmission, and Ameren Illinois, respectively.

Summary Of Significant Accoun19

Summary Of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 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 also 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 March 31, 2018 , and December 31, 2017 , Ameren had unconsolidated variable interests as a limited partner in various equity method investments, totaling $19 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 March 31, 2018 , the maximum exposure to loss related to these variable interests is limited to the investment in these partnerships of $19 million plus associated outstanding funding commitments of $18 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, Policy	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 in the first quarter of 2018 did not result in material changes to previously reported cash flows from operating, investing, or financing activities.
Life Insurance, Corporate or Bank Owned	Company-owned Life Insurance Company-owned life insurance is recorded at the net cash surrender value, which is the amount that can be realized under the insurance policies at the balance sheet date. As of March 31, 2018 , the cash surrender value of company-owned life insurance at Ameren and Ameren Illinois was $ 251 million and $ 120 million , respectively, while total borrowings against the policies were $ 111 million at both Ameren and Ameren Illinois. As of December 31, 2017 , the cash surrender value of company-owned life insurance at Ameren and Ameren Illinois was $ 265 million and $ 129 million , respectively, while total borrowings against the policies were $ 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.
Disclosure of Compensation Related Costs, Share-based Payments	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. In the event of a participant’s death or retirement, awards vest on a pro rata basis. 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 payout date of the awards is approximately 38 months after the grant date. 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 simulations can produce a greater fair value for the performance share unit than the December 31 applicable closing common share price because they include the weighted payout scenarios in which an increase in the share price has occurred. The significant assumptions used to calculate fair value also included a three -year risk-free rate of 1.98% 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, 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 date the restricted stock unit is granted.
Revenue Recognition	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. MISO-related electricity revenues are earned as electricity is delivered. MISO-related capacity and ancillary service revenues are earned as services are provided. Wholesale bilateral revenues include the sale of electricity and capacity. Wholesale bilateral electricity revenues are earned as electricity is delivered. Wholesale bilateral capacity revenues are earned as services are provided. Retail distribution, electric transmission, and off-system revenues, including each respective underlying component 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 services are provided. The Ameren Companies have elected the optional exemption to exclude 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 March 31, 2018 and 2017, our remaining performance obligations were immaterial. See Note 12 – 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.
Excise Taxes	Ameren Missouri and Ameren Illinois collect from their customers certain excise taxes that are levied on the sale or distribution of natural gas and electricity. Excise taxes are levied on Ameren Missouri’s electric and natural gas businesses and on Ameren Illinois’ natural gas business. They are recorded gross in “Operating Revenues – Electric,” “Operating Revenues – Natural gas” and “Operating Expenses – Taxes other than income taxes” on the statements of income. Excise taxes for electric service in Illinois are levied on customers and are therefore not included in Ameren Illinois’ revenues and expenses.
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 leases, the measurement of credit losses on financial instruments, and the reclassification of certain tax effects from accumulated OCI.

Financial Instruments Financial

Financial Instruments Financial Instruments (Policies)	3 Months Ended
	Mar. 31, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivatives	• an unrealized appreciation or depreciation of our contracted commitments to purchase or sell when purchase or sale prices under the commitments are compared with current commodity prices; • market values of natural gas 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 Accoun21

Summary Of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 2018 and 2017 , and December 31, 2017 and 2016 : March 31, 2018 December 31, 2017 March 31, 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) $ 30 $ 3 $ 3 $ 10 $ — $ — $ 8 $ — $ — $ 9 $ — $ — Restricted cash included in “Other current assets” 12 4 5 21 5 6 19 4 5 20 4 6 Restricted cash included in “Other assets” 44 — 44 35 — 35 23 — 23 22 — 22 Restricted cash included in “Nuclear decommissioning trust fund” 3 3 (b) 2 2 (b) 1 1 (b) 1 1 (b) Total cash, cash equivalents, and restricted cash (c) $ 89 $ 10 $ 52 $ 68 $ 7 $ 41 $ 51 $ 5 $ 28 $ 52 $ 5 $ 28 (a) As presented on the balance sheet. (b) Not applicable. (c) As presented on the statement 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 three months ended March 31, 2018 and 2017 : March 31, 2018 March 31, 2017 Ameren (a) Ameren Missouri Ameren Illinois Ameren (a) Ameren Missouri Ameren Illinois Accrued capital expenditures $ 202 $ 73 $ 114 $ 164 $ 50 $ 74 Accrued nuclear fuel expenditures (b) (b) (c) 13 13 (c) Net realized gain – nuclear decommissioning trust fund (b) (b) (c) 4 4 (c) Net unrealized gain (loss) – nuclear decommissioning trust fund (11 ) (11 ) (c) 19 19 (c) (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries. (b) Less than $1 million. (c) Not applicable.
Asset Retirement Obligation Disclosure	The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the three months ended March 31, 2018 : Ameren Missouri Ameren Illinois (a) Ameren Balance at December 31, 2017 $ 640 (b) $ 4 $ 644 (b) Liabilities settled (c) (c) (c) Accretion (d) 7 (c) 7 Change in estimates (e) (9 ) — (9 ) Balance at March 31, 2018 $ 638 (b) $ 4 $ 642 (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 March 31, 2018 , respectively. (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	A summary of nonvested performance share units and restricted stock units at March 31, 2018 , and changes during the three months ended March 31, 2018 , under the 2014 Incentive Plan are presented below: 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 298,774 62.88 181,145 57.62 Forfeitures (37,337 ) 46.08 (643 ) 58.99 Undistributed vested units (b) (72,392 ) 53.50 (6,458 ) 58.99 Earned and vested (176,043 ) 52.88 — — Nonvested at March 31, 2018 (c) 908,491 $ 55.81 174,044 $ 57.56 (a) Excludes 712,572 of 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 measurement period. (c) Excludes 403,584 undistributed vested performance share units and 6,458 undistributed vested restricted stock units.
Schedule of Excise Taxes	The following table presents the excise taxes recorded in “Operating Revenues – Electric,” “Operating Revenues – Natural gas” and “Operating Expenses – Taxes other than income taxes” for the three months ended March 31, 2018 and 2017 : Three Months 2018 2017 Ameren Missouri $ 34 $ 31 Ameren Illinois 22 19 Ameren $ 56 $ 50

Short-Term Debt And Liquidity S

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Tables)	3 Months Ended
	Mar. 31, 2018
Debt Disclosure [Abstract]
Schedule of Short-term Debt	The following table presents commercial paper outstanding, net of issuance discounts, as of March 31, 2018 , and December 31, 2017 : 2018 2017 Ameren (parent) $ 454 $ 383 Ameren Missouri 282 39 Ameren Illinois 224 62 Ameren Consolidated $ 960 $ 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 three months ended March 31, 2018 and 2017: Ameren (parent) Ameren Missouri Ameren Illinois Ameren Consolidated 2018 Average daily commercial paper outstanding at par value $ 378 $ 213 $ 137 $ 728 Weighted-average interest rate 1.90 % 1.88 % 1.96 % 1.90 % Peak commercial paper during period at par value (a) $ 454 $ 282 $ 238 $ 960 Peak interest rate 2.35 % 2.40 % 2.55 % 2.55 % 2017 Average daily commercial paper outstanding at par value $ 682 $ 4 $ 50 $ 736 Weighted-average interest rate 1.07 % 0.93 % 0.95 % 1.06 % Peak commercial paper during period at par value (a) $ 810 $ 45 $ 74 $ 914 Peak interest rate 1.30 % 1.15 % 1.15 % 1.30 % (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)	3 Months Ended
	Mar. 31, 2018
Other Nonoperating Income (Expense) [Abstract]
Other Income And Expenses	The following table presents the components of “Other Income, Net” in the Ameren Companies’ statements of income for the three months ended March 31, 2018 and 2017 : Three Months 2018 2017 Ameren: (a) Other Income, Net Allowance for equity funds used during construction $ 5 $ 6 Interest income on industrial development revenue bonds 6 7 Interest income 2 2 Non-service cost components of net periodic benefit income 16 (b) 12 Other income 1 — Donations (5 ) (5 ) Other expense (2 ) (4 ) Total Other Income, Net $ 23 $ 18 Ameren Missouri: Other Income, Net Allowance for equity funds used during construction $ 4 $ 5 Interest income on industrial development revenue bonds 6 7 Non-service cost components of net periodic benefit income 5 (b) 6 Other income 1 — Donations (1 ) — Other expense (2 ) (2 ) Total Other Income, Net $ 13 $ 16 Ameren Illinois: Other Income, Net Allowance for equity funds used during construction $ 1 $ 1 Interest income 2 2 Non-service cost components of net periodic benefit income 7 3 Donations (4 ) (4 ) Other expense — (2 ) Total Other Income, Net $ 6 $ — (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations. (b) For the three months ended March 31, 2018, the non-service cost components of net periodic benefit income were partially offset by a $4 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.

Financial Instruments (Tables)

Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 2018 , and December 31, 2017 . As of March 31, 2018 , these contracts extended through October 2019, March 2023, and May 2032 for fuel oils, natural gas, and power, respectively. Quantity (in millions, except as indicated) 2018 2017 Commodity Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils (in gallons) (a) 27 (b) 27 28 (b) 28 Natural gas (in mmbtu) 25 144 169 24 139 163 Power (in megawatthours) 2 9 11 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 March 31, 2018 , and December 31, 2017 : Balance Sheet Location Ameren Missouri Ameren Illinois Ameren 2018 Fuel oils Other current assets $ 6 $ — $ 6 Other assets 2 — 2 Natural gas Other assets — 1 1 Power Other current assets 4 — 4 Other assets 1 — 1 Total assets (a) $ 13 $ 1 $ 14 Natural gas Other current liabilities $ 5 $ 13 $ 18 Other deferred credits and liabilities 4 16 20 Power Other current liabilities 1 14 15 Other deferred credits and liabilities — 177 177 Total liabilities (b) $ 10 $ 220 $ 230 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 March 31, 2018 , the aggregate fair value of all derivative instruments with credit risk-related contingent features in a gross liability position, the cash collateral posted, and the aggregate amount of additional collateral that counterparties could require. The additional collateral required is the net liability position allowed under the master netting arrangements or similar agreements, assuming (1) the credit risk-related contingent features underlying these arrangements were triggered on March 31, 2018 , and (2) those counterparties with rights to do so requested collateral. Aggregate Fair Value of Derivative Liabilities (a) Cash Collateral Posted Potential Aggregate Amount of Additional Collateral Required (b) Ameren Missouri $ 55 $ 5 $ 44 Ameren Illinois 50 — 44 Ameren $ 105 $ 5 $ 88 (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)	3 Months Ended
	Mar. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value, Assets and Liabilities Measured on Nonrecurring Basis, Valuation Techniques	The following table describes the valuation techniques and unobservable inputs 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 for the periods ended March 31, 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 $ 2 $ — Option model Volatilities(%) (b) 21 – 33 24 Discounted cash flow Counterparty credit risk(%) (c)(d) 0.12 – 0.91 0.49 Ameren Missouri credit risk(%) (c)(d) 0.35 (e) Natural gas — (6 ) Discounted cash flow Nodal basis ($/mmbtu) (b) (1.40) – (0.10) (1) Ameren Illinois credit risk (%) (c)(d) 0.35 (e) Power (f) 5 (192 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps ($/MWh) (g) 22 – 37 31 Estimated auction price for FTRs ($/MW) (b) (479) – 1,608 58 Nodal basis ($/MWh) (g) (10) – 0 (2) Counterparty credit risk (%) (c)(d) 0.92 (e) Ameren Illinois credit risk (%) (c)(d) 0.35 (e) Fundamental energy production model Estimated future natural gas prices ($/mmbtu) (b) 3 – 4 3 Escalation rate (%) (b)(h) 4 (e) Contract price allocation Estimated renewable energy credit costs ($/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 (e) Natural gas 1 (4 ) Option model Volatilities (%) (b) 26 – 46 37 Nodal basis ($/mmbtu) (b) (0.50) – (0.30) (0.40) Discounted cash flow Nodal basis ($/mmbtu) (b) (1.20) – 0.10 (1) Counterparty credit risk (%) (c)(d) 0.37 – 0.92 0.53 Ameren credit risk (%) (c)(d) 0.37 (e) Power (f) 8 (196 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps ($/MWh) (g) 24 – 46 28 Estimated auction price for FTRs ($/MW) (b) (65) – 1,823 251 Nodal basis ($/MWh) (g) (10) – 0 (2) Counterparty credit risk (%) (c)(d) 0.28 (e) Ameren Illinois credit risk (%) (c)(d) 0.37 (e) Fundamental energy production model Estimated future natural gas prices ($/mmbtu) (b) 3 – 4 3 Escalation rate (%) (b)(h) 5 (e) Contract price allocation Estimated renewable energy credit costs ($/credit) (b) 5 – 7 6 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement. (c) Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement. (d) Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances. (e) Not applicable. (f) Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2021. Valuations beyond 2021 use fundamentally modeled pricing by month for peak and off-peak demand. (g) 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. (h) 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 March 31, 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 $ 6 $ — $ 2 $ 8 Natural gas — 1 — 1 Power — — 5 5 Total derivative assets – commodity contracts $ 6 $ 1 $ 7 $ 14 Nuclear decommissioning trust fund: Equity securities: U.S. large capitalization $ 464 $ — $ — $ 464 Debt securities: U.S. treasury and agency securities — 118 — 118 Corporate bonds — 80 — 80 Other — 32 — 32 Total nuclear decommissioning trust fund $ 464 $ 230 $ — $ 694 (b) Total Ameren $ 470 $ 231 $ 7 $ 708 Ameren Missouri Derivative assets – commodity contracts (a) : Fuel oils $ 6 $ — $ 2 $ 8 Power — — 5 5 Total derivative assets – commodity contracts $ 6 $ — $ 7 $ 13 Nuclear decommissioning trust fund: Equity securities: U.S. large capitalization $ 464 $ — $ — $ 464 Debt securities: U.S. treasury and agency securities — 118 — 118 Corporate bonds — 80 — 80 Other — 32 — 32 Total nuclear decommissioning trust fund $ 464 $ 230 $ — $ 694 (b) Total Ameren Missouri $ 470 $ 230 $ 7 $ 707 Ameren Illinois Derivative assets – commodity contracts (a) : Natural gas $ — $ 1 $ — $ 1 Liabilities: Ameren Derivative liabilities – commodity contracts (a) : Natural gas $ — $ 32 $ 6 $ 38 Power — — 192 192 Total Ameren $ — $ 32 $ 198 $ 230 Ameren Missouri Derivative liabilities – commodity contracts (a) : Natural gas $ — $ 9 $ — $ 9 Power — — 1 1 Total Ameren Missouri $ — $ 9 $ 1 $ 10 Ameren Illinois Derivative liabilities – commodity contracts (a) : Natural gas $ — $ 23 $ 6 $ 29 Power — — 191 191 Total Ameren Illinois $ — $ 23 $ 197 $ 220 (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. 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 March 31, 2018 Beginning balance at January 1, 2018 $ 7 $ (195 ) $ (188 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (2 ) 1 (1 ) Settlements (1 ) 3 2 Ending balance at March 31, 2018 $ 4 $ (191 ) $ (187 ) Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2018 $ (1 ) $ 1 $ — For the three months ended March 31, 2017 Beginning balance at January 1, 2017 $ 7 $ (185 ) $ (178 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities — (10 ) (10 ) Settlements (3 ) 1 (2 ) Ending balance at March 31, 2017 $ 4 $ (194 ) $ (190 ) Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2017 $ — $ (11 ) $ (11 )
Schedule Of Carrying Amounts And Estimated Fair Values Of Long-Term Debt, Capital Lease Obligations And Preferred Stock	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 March 31, 2018 , and December 31, 2017 : March 31, 2018 Carrying Amount Fair Value Level 1 Level 2 Level 3 Total Ameren: Cash, cash equivalents, and restricted cash $ 89 $ 89 $ — $ — $ 89 Investments in held-to-maturity debt securities (a) 276 — 276 — 276 Short-term debt 960 — 960 — 960 Long-term debt (including current portion) (a) 7,936 (b) — 7,820 439 (c) 8,259 Preferred stock (d) 142 — 130 — 130 Ameren Missouri: Cash, cash equivalents, and restricted cash $ 10 $ 10 $ — $ — $ 10 Investments in held-to-maturity debt securities (a) 276 — 276 — 276 Short-term debt 282 — 282 — 282 Long-term debt (including current portion) (a) 3,962 (b) — 4,201 — 4,201 Preferred stock 80 — 78 — 78 Ameren Illinois: Cash, cash equivalents, and restricted cash $ 52 $ 52 $ — $ — $ 52 Short-term debt 224 — 224 — 224 Long-term debt (including current portion) 2,830 (b) — 2,928 — 2,928 Preferred stock 62 — 52 — 52 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 (c) 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 capital lease obligation for CTs leased from the city of Bowling Green and Audrain County. As of March 31, 2018 and December 31, 2017, the carrying amount of both the investments in industrial revenue bonds and the capital lease obligations approximated fair value. (b) Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $49 million , $20 million , and $24 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of March 31, 2018 . Included unamortized debt issuance costs, which were excluded from the fair value measurement, of $50 million , $20 million , and $24 million for Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of December 31, 2017 . (c) The Level 3 fair value amount consists of ATXI’s senior unsecured notes. For the three months ended March 31, 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.

Related Party Transactions (Tab

Related Party Transactions (Tables)	3 Months Ended
	Mar. 31, 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 months ended March 31, 2018 and 2017 : Three Months Agreement Income Statement Line Item Ameren Missouri Ameren Illinois Ameren Missouri power supply Operating Revenues 2018 $ 3 $ (a) agreements with Ameren Illinois 2017 11 (a) Ameren Missouri and Ameren Illinois Operating Revenues 2018 5 1 rent and facility services 2017 7 1 Ameren Missouri and Ameren Illinois Operating Revenues 2018 (b) (b) miscellaneous support services 2017 (b) (b) Total Operating Revenues 2018 $ 8 $ 1 2017 18 1 Ameren Illinois power supply Purchased Power 2018 $ (a) $ 3 agreements with Ameren Missouri 2017 (a) 11 Ameren Illinois transmission Purchased Power 2018 (a) (b) services with ATXI 2017 (a) (b) Total Purchased Power 2018 $ (a) $ 3 2017 (a) 11 Ameren Services support services Other Operations and Maintenance 2018 $ 33 $ 30 agreement 2017 35 32 Money pool borrowings (advances) Interest Charges/ Other Income, Net 2018 $ (b) $ (b) 2017 (b) (b) (a) Not applicable. (b) Amount less than $1 million.

Callaway Energy Center (Tables)

Callaway Energy Center (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 2018 . The property coverage and the nuclear liability coverage renewal dates are April 1 and January 1, respectively, of each year. Both coverages were renewed in 2018. Type and Source of Coverage Maximum Coverages Maximum Assessments for Single Incidents Public liability and nuclear worker liability: American Nuclear Insurers $ 450 $ — Pool participation 12,604 (a) 127 (b) $ 13,054 (c) $ 127 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 $19 million per year. (c) Limit of liability for each incident under the Price-Anderson liability provisions of the Atomic Energy Act of 1954, as amended. This limit is subject to change to account for the effects of inflation and changes in the number of licensed reactors. (d) NEIL provides $2.7 billion in property damage, stabilization, decontamination, and premature decommissioning insurance for radiation events and $2.3 billion in property damage insurance for nonradiation events. EMANI provides $490 million in property damage insurance for both radiation and nonradiation events. (e) All NEIL insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL. (f) Provides replacement power cost insurance in the event of a prolonged accidental outage. Weekly indemnity up to $4.5 million for 52 weeks, which commences after the first 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)	3 Months Ended
	Mar. 31, 2018
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]
Components Of Net Periodic Benefit Cost	The following table presents the components of the net periodic benefit cost (income), prior to capitalization, incurred for Ameren’s pension and postretirement benefit plans for the three months ended March 31, 2018 and 2017 : Pension Benefits Postretirement Benefits Three Months Three Months 2018 2017 2018 2017 Service cost (a) $ 25 $ 23 $ 5 $ 5 Non-service cost components: Interest cost 42 45 11 12 Expected return on plan assets (69 ) (66 ) (19 ) (19 ) Amortization of: Prior service benefit — — (1 ) (1 ) Actuarial loss (gain) 16 14 — (2 ) Total non-service cost components (b) (11 ) (7 ) (9 ) (10 ) Net periodic benefit cost (income) $ 14 $ 16 $ (4 ) $ (5 ) (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.
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 pension costs and the postretirement benefit costs (income) incurred for the three months ended March 31, 2018 and 2017 : Pension Benefits Postretirement Benefits Three Months Three Months 2018 2017 2018 2017 Ameren Missouri (a) $ 5 $ 6 $ — $ (1 ) Ameren Illinois 9 10 (4 ) (4 ) Ameren (a) $ 14 $ 16 $ (4 ) $ (5 ) (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.

Segment Information (Tables)

Segment Information (Tables)	3 Months Ended
	Mar. 31, 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 months ended March 31, 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 $ 784 $ 399 $ 311 $ 91 $ — $ — $ 1,585 Intersegment revenues 8 1 — 13 — (22 ) — Net income attributable to Ameren common shareholders 38 33 42 37 (a) 1 — 151 Capital expenditures 249 122 60 145 7 (4 ) 579 2017 External revenues $ 773 $ 384 $ 264 $ 96 $ (2 ) $ — $ 1,515 Intersegment revenues 18 1 — 6 — (25 ) — Net income attributable to Ameren common shareholders 5 30 33 34 (a) — — 102 Capital expenditures 196 120 51 134 4 (1 ) 504 (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 $ 400 $ 311 $ 49 $ — $ 760 Intersegment revenues — — 13 (13 ) — Net income available to common shareholder 33 42 20 — 95 Capital expenditures 122 60 118 — 300 2017 External revenues $ 385 $ 264 $ 54 $ — $ 703 Intersegment revenues — — 6 (6 ) — Net income available to common shareholder 30 33 16 — 79 Capital expenditures 120 51 56 — 227
Disaggregation of Revenue	The following tables present disaggregated revenues by segment at Ameren and Ameren Illinois for the three months ended March 31, 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 $ 332 $ 219 $ — $ — $ — $ — $ 551 Commercial 252 124 — — — — 376 Industrial 61 35 — — — — 96 Other 96 22 — 104 — (22 ) 200 Total electric revenues $ 741 $ 400 $ — $ 104 $ — $ (22 ) $ 1,223 Residential $ 41 $ — $ 243 $ — $ — $ — $ 284 Commercial 16 — 67 — — — 83 Industrial 2 — 6 — — — 8 Other (8 ) — (5 ) — — — (13 ) Total gas revenues $ 51 $ — $ 311 $ — $ — $ — $ 362 Total revenues (a) $ 792 $ 400 $ 311 $ 104 $ — $ (22 ) $ 1,585 2017 Residential $ 290 $ 219 $ — $ — $ — $ — $ 509 Commercial 232 133 — — — — 365 Industrial 58 28 — — — — 86 Other 167 5 — 102 (2 ) (25 ) 247 Total electric revenues $ 747 $ 385 $ — $ 102 $ (2 ) $ (25 ) $ 1,207 Residential $ 30 $ — $ 203 $ — $ — $ — $ 233 Commercial 12 — 55 — — — 67 Industrial 1 — 3 — — — 4 Other 1 — 3 — — — 4 Total gas revenues $ 44 $ — $ 264 $ — $ — $ — $ 308 Total revenues (b) $ 791 $ 385 $ 264 $ 102 $ (2 ) $ (25 ) $ 1,515 (a) Includes revenues from alternative revenue programs of $(4) million , $31 million , $(3) million , $(4) million , and $20 million at Ameren Missouri, Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, Ameren Transmission, and Ameren, respectively. Also includes other revenues not from contracts with customers of $14 million , $10 million , $1 million , $- million, and $25 million at Ameren Missouri, Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, Ameren Transmission, and Ameren, respectively. (b) Includes revenues from alternative revenue programs of $(7) million , $33 million , $11 million , $5 million , and $42 million at Ameren Missouri, Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, Ameren Transmission, and Ameren, respectively. Also includes other revenues not from contracts with customers of $4 million , $2 million , $1 million , $- million, and $7 million at Ameren Missouri, Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, Ameren Transmission, and Ameren, respectively. Ameren Illinois Three Months Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Total Ameren Illinois 2018 Residential $ 219 $ 243 $ — $ — $ 462 Commercial 124 67 — — 191 Industrial 35 6 — — 41 Other 22 (5 ) 62 (13 ) 66 Total revenues (a) $ 400 $ 311 $ 62 $ (13 ) $ 760 2017 Residential $ 219 $ 203 $ — $ — $ 422 Commercial 133 55 — — 188 Industrial 28 3 — — 31 Other 5 3 60 (6 ) 62 Total revenues (b) $ 385 $ 264 $ 60 $ (6 ) $ 703 (a) Includes revenues from alternative revenue programs of $31 million , $(3) million , $(4) million , and $24 million at Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Illinois Transmission, and Ameren Illinois, respectively. Also includes other revenues not from contracts with customers of $10 million , $1 million , $- million, and $11 million at Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, Ameren Illinois Transmission, and Ameren Illinois, respectively. (b) Includes revenues from alternative revenue programs of $33 million , $11 million , $3 million , and $47 million at Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Illinois Transmission, and Ameren Illinois, respectively. Also includes other revenues not from contracts with customers of $2 million , $1 million , $- million, and $3 million at Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, Ameren Illinois Transmission, and Ameren Illinois, respectively.

Summary Of Significant Accoun30

Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Basis Of Presentation And Significant Accounting Policies [Line Items]
Payables for purchased receivables	$ 33	$ 31
Deferred Compensation Liability, Classified, Noncurrent	84	86
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets and Liabilities, Net	19	17
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	18
Cash Surrender Value of Life Insurance	251	265
Corporate owned life insurance, borrowings	111	120
Ameren Illinois Company
Basis Of Presentation And Significant Accounting Policies [Line Items]
Payables for purchased receivables	33	31
Cash Surrender Value of Life Insurance	120	129
Corporate owned life insurance, borrowings	$ 111	$ 120

Summary Of Significant Accoun31

Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Cash and Cash Equivalents) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	$ 30	$ 10	$ 8	$ 9
Restricted Cash and Cash Equivalents	12	21	19	20
Restricted Cash and Cash Equivalents, Noncurrent	44	35	23	22
Restricted Cash and Cash Equivalents, Nuclear Decommissioning Trust Fund	3	2	1	1
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	89	68	51	52
Union Electric Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	3	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	1	1
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	10	7	5	5
Ameren Illinois Company
Schedule of Cash and Cash Equivalents Including Restricted Cash [Line Items]
Cash and cash equivalents	3	0	0	0
Restricted Cash and Cash Equivalents	5	6	5	6
Restricted Cash and Cash Equivalents, Noncurrent	44	35	23	22
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	$ 52	$ 41	$ 28	$ 28

Summary Of Significant Accoun32

Summary Of Significant Accounting Policies Summary of Significant Account Policies (Supplemental Cash Flow Information) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018		Mar. 31, 2017
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	$ 202		$ 164
Available-for-sale Securities, Gross Realized Gains	1	[1]	4
Unrealized Gain (Loss) on Investments	(11)		19
Union Electric Company
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	73		50
Available-for-sale Securities, Gross Realized Gains	1	[1]	4
Unrealized Gain (Loss) on Investments	(11)		19
Ameren Illinois Company
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	114		74
Nuclear Fuel
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	1	[1]	13
Nuclear Fuel | Union Electric Company
Supplemental Cash Flow Information [Line Items]
Capital Expenditures Incurred but Not yet Paid	$ 1	[1]	$ 13
[1]	Less than $1 million.

Summary Of Significant Accoun33

Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Schedule of Asset Retirement Obligations) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Dec. 31, 2017
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance		$ 642	$ 644
Liabilities settled	[1]	(1)
Accretion		7
Change of estimates		(9)
Other current liabilities		285	326
Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance		638	640
Liabilities settled	[1]	(1)
Accretion		7
Change of estimates		(9)
Other current liabilities		99	103
Ameren Illinois Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Balance		4	4
Liabilities settled	[1]	(1)
Accretion	[1]	1
Change of estimates		0
Other current liabilities		141	177
Asset Retirement Obligation Balance
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Other current liabilities		6	6
Asset Retirement Obligation Balance | Union Electric Company
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Other current liabilities		$ 6	$ 6
[1]	(c)Less than $1 million.

Summary Of Significant Accoun34

Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Summary of Nonvested Shares Related To Long-Term Incentive Plan) (Details) - $ / shares	3 Months Ended
	Mar. 31, 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-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Fully Vested Undistributed to Retirement-eligible employees.	403,584	712,572
Share Units
Share Units, Nonvested as of beginning of year	895,489
Share Units, Granted	298,774
Share Units, Forfeitures	(37,337)
Share Units, Undistributed Vested	(72,392)
Share Units, Vested	(176,043)
Share Units, Nonvested as of Current period end	908,491
Weighted-average Fair Value per Unit at Grant Date
Weighted-average Fair Value per Unit, Nonvested as of Beginning of year	$ 52.28
Weighted-average Fair Value per Unit, Granted	62.88
Weighted-average Fair Value per Unit, Forfeitures	46.08
Weighted-average Fair Value per Unit, Undistributed Vested Units	53.50
Weighted-average Fair Value per Unit, Vested	52.88
Weighted-average Fair Value per Unit, Nonvested as of Current period end	$ 55.81
Performance period	3 years
Risk free interest rate period	3 years
Three-year risk-free rate	1.98%
Volatility rate, minimum	15.00%
Volatility rate, maximum	23.00%
Stock Issued During Period Percentage Conversion Of Units, Low End	0.00%
Stock Issued During Period Percentage Conversion Of Units, High End	200.00%
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Fully Vested Undistributed to Retirement-eligible employees.	6,458
Share Units
Share Units, Nonvested as of beginning of year	0
Share Units, Granted	181,145
Share Units, Forfeitures	(643)
Share Units, Undistributed Vested	(6,458)
Share Units, Vested	0
Share Units, Nonvested as of Current period end	174,044
Weighted-average Fair Value per Unit at Grant Date
Weighted-average Fair Value per Unit, Nonvested as of Beginning of year	$ 0
Weighted-average Fair Value per Unit, Granted	57.62
Weighted-average Fair Value per Unit, Forfeitures	58.99
Weighted-average Fair Value per Unit, Undistributed Vested Units	58.99
Weighted-average Fair Value per Unit, Vested	0
Weighted-average Fair Value per Unit, Nonvested as of Current period end	$ 57.56

Summary Of Significant Accoun35

Summary Of Significant Accounting Policies (Schedule Of Excise Taxes) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Accounting Policies [Line Items]
Excise tax expense	$ 56	$ 50
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	34	31
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$ 22	$ 19

Rate And Regulatory Matters (Na

Rate And Regulatory Matters (Narrative-Missouri) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Rate And Regulatory Matters [Line Items]
Electric	$ 1,223	$ 1,207
Union Electric Company
Rate And Regulatory Matters [Line Items]
Electric	741	$ 747
MEEIA 2016 [Member] | Final Rate Order | Electric Distribution | 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%
Electric	$ 5

Rate And Regulatory Matters (37

Rate And Regulatory Matters (Narrative-Illinois) (Detail) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Rate And Regulatory Matters [Line Items]
Regulatory assets		$ 1,205	$ 1,230
Current regulatory assets		130	144
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets		812	822
Current regulatory assets		69	$ 87
Ameren Illinois Company | Natural Gas
Rate And Regulatory Matters [Line Items]
Tax Reform Refunds From the Tax Cuts and Jobs Act		16
Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Tax Reform Refunds From the Tax Cuts and Jobs Act		50
2017 IEIMA Revenue Requirement Reconciliation [Member] | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Regulatory assets		52
2016 IEIMA Revenue Requirement Reconciliation | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Current regulatory assets		19
2018 IEIMA Revenue Requirement Reconciliation | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Regulatory assets		42
Pending Rate Case | Ameren Illinois Company | Natural Gas
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 49
Public Utilities, Requested Equity Capital Structure, Percentage		50.00%
Rate Base		$ 1,600
Components of Rate Increase | Ameren Illinois Company | Natural Gas
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 42
Subsequent Event | Pending Rate Case | Ameren Illinois Company | Natural Gas
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 44
Public Utilities, Requested Return on Equity, Percentage	9.87%
Subsequent Event | Pending Rate Case | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 72

Rate And Regulatory Matters (38

Rate And Regulatory Matters (Narrative-Federal) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	$ 130	$ 128
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	$ 67	$ 92
Midwest Independent Transmission System Operator, Inc
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Return on Equity, Percentage	12.38%
Midwest Independent Transmission System Operator, Inc | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Tax Reform Refunds From the Tax Cuts and Jobs Act	$ 27
Midwest Independent Transmission System Operator, Inc | Ameren Transmission Company of Illinois
Rate And Regulatory Matters [Line Items]
Tax Reform Refunds From the Tax Cuts and Jobs Act	$ 23
Midwest Independent Transmission System Operator, Inc | Final Rate Order
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Return on Equity, Percentage	10.32%
Customer Requested Rate on Equity	9.15%
Incentive adder to FERC allowed base return on common equity	0.50%
Midwest Independent Transmission System Operator, Inc | Administrative Law Judge
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Return on Equity, Percentage	9.70%
Midwest Independent Transmission System Operator, Inc | Pending Ferc Case
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	$ 42
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, Approved Return on Equity, Percentage	10.82%
Maximum | Midwest Independent Transmission System Operator, Inc | Administrative Law Judge
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Return on Equity, Percentage	10.20%

Short-Term Debt And Liquidity (

Short-Term Debt And Liquidity (Narrative) (Detail)	3 Months Ended
	Mar. 31, 2018USD ($)	Mar. 31, 2017
Utilities
Line of Credit Facility [Line Items]
Short Term Debt, Weighted Average Interest Rate During Period	1.90%	1.01%
Credit Agreements
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,200,000,000
Actual debt-to-capital ratio	0.54
Missouri Credit Agreement | Union Electric Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.50
Illinois Credit Agreement | Ameren Illinois Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.48

Short-Term Debt And Liquidity40

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper outstanding) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Commercial paper outstanding	$ 960	$ 484
Ameren (parent)
Short-term Debt [Line Items]
Commercial paper outstanding	454	383
Union Electric Company
Short-term Debt [Line Items]
Commercial paper outstanding	282	39
Ameren Illinois Company
Short-term Debt [Line Items]
Commercial paper outstanding	$ 224	$ 62

Short-Term Debt And Liquidity41

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 728	$ 736
Weighted-average interest rate	1.90%	1.06%
Peak commercial paper during period	$ 960	$ 914
Peak interest rate	2.55%	1.30%
Ameren (parent)
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 378	$ 682
Weighted-average interest rate	1.90%	1.07%
Peak commercial paper during period	$ 454	$ 810
Peak interest rate	2.35%	1.30%
Union Electric Company
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 213	$ 4
Weighted-average interest rate	1.88%	0.93%
Peak commercial paper during period	$ 282	$ 45
Peak interest rate	2.40%	1.15%
Ameren Illinois Company
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding	$ 137	$ 50
Weighted-average interest rate	1.96%	0.95%
Peak commercial paper during period	$ 238	$ 74
Peak interest rate	2.55%	1.15%

Long-Term debt and Equity Fin42

Long-Term debt and Equity Financings (Narrative) (Details) - USD ($) shares in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2018	Mar. 31, 2017	Jun. 30, 2018
Long-Term Debt And Equity Financings [Line Items]
Stock Issued During Period, Shares, New Issues	0.7	0.3
Noncash financing activity - Issuance of common stock for stock-based compensation		$ 35,000,000	$ 0
Issuance of common stock		$ 17,000,000	$ 0
Subsequent Event | Union Electric Company | First Mortgage Bonds, 4.00%, Due 2048 - $425 Issuance [Member] | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Total principal amount of notes				$ 425,000,000
State interest rate				4.00%
Proceeds from Issuance of Secured Debt				$ 419,000,000
Subsequent Event | Union Electric Company | Senior Secured Notes 6 Point 00 Due 2018 [Member] | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
State interest rate				6.00%
Repayments of other long-term debt				$ 179,000,000
Subsequent Event | Ameren Illinois Company | Senior Secured Notes6250 Due2018 [Member] | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
State interest rate				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
	Mar. 31, 2018	Mar. 31, 2017
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	$ 5	$ 6
Interest income on industrial development revenue bonds	6	7
Interest income	2	2
Non-service cost components of net periodic benefit income	16	12
Other income	1	0
Donations	(5)	(5)
Other expense	(2)	(4)
Other Income (Expense), Net	23	18
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	4	5
Interest income on industrial development revenue bonds	6	7
Non-service cost components of net periodic benefit income	5	6
Other income	1	0
Donations	(1)	0
Other expense	(2)	(2)
Other Income (Expense), Net	13	16
Defined Benefit Plan, Non-service Cost or Income Components - Tracker	4
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	1	1
Interest income	2	2
Non-service cost components of net periodic benefit income	7	3
Donations	(4)	(4)
Other expense	0	(2)
Other Income (Expense), Net	$ 6	$ 0

Financial Instruments (Open Gro

Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) gal in Millions, MWh in Millions		Mar. 31, 2018galMWh	Dec. 31, 2017galMWh
Fuel Oils
Derivative [Line Items]
Quantity | gal	[1]	27	28
Natural Gas
Derivative [Line Items]
Quantity		169	163
Power
Derivative [Line Items]
Quantity		11	12
Union Electric Company | Fuel Oils
Derivative [Line Items]
Quantity | gal	[1]	27	28
Union Electric Company | Natural Gas
Derivative [Line Items]
Quantity		25	24
Union Electric Company | Power
Derivative [Line Items]
Quantity		2	3
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Quantity		144	139
Ameren Illinois Company | Power
Derivative [Line Items]
Quantity		9	9
[1]	Consists of ultra-low-sulfur diesel products.

Financial Instruments (Derivati

Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative assets	[1]	$ 14	$ 17
Derivative liabilities	[2]	230	226
Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets		6	5
Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets		2	2
Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets		1
Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		18	17
Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		20	13
Power | Other Current Assets
Derivative [Line Items]
Derivative assets		4	9
Power | Other Assets
Derivative [Line Items]
Derivative assets		1
Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		15	14
Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		177	182
Union Electric Company
Derivative [Line Items]
Derivative assets	[1]	13	17
Derivative liabilities	[2]	10	9
Union Electric Company | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets		6	5
Union Electric Company | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets		2	2
Union Electric Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets		0	1
Union Electric Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		5	5
Union Electric Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		4	3
Union Electric Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets		4	9
Union Electric Company | Power | Other Assets
Derivative [Line Items]
Derivative assets		1
Union Electric Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		1	1
Union Electric Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		0	0
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	[1]	1	0
Derivative liabilities	[2]	220	217
Ameren Illinois Company | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets		0	0
Ameren Illinois Company | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets		0	0
Ameren Illinois Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets		1	0
Ameren Illinois Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		13	12
Ameren Illinois Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		16	10
Ameren Illinois Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets		0	0
Ameren Illinois Company | Power | Other Assets
Derivative [Line Items]
Derivative assets		0
Ameren Illinois Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		14	13
Ameren Illinois Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		$ 177	$ 182
[1]	The cumulative amount of pretax net gains on all derivative instruments is deferred as a regulatory liability.
[2]	The cumulative amount of pretax net losses on all derivative instruments is deferred as a regulatory asset.

Financial Instruments (Deriva46

Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Detail) $ in Millions	Mar. 31, 2018USD ($)
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	$ 105	[1]
Cash Collateral Posted	5
Potential Aggregate Amount of Additional Collateral Required	88	[2]
Union Electric Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	55	[1]
Cash Collateral Posted	5
Potential Aggregate Amount of Additional Collateral Required	44	[2]
Ameren Illinois Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	50	[1]
Cash Collateral Posted	0
Potential Aggregate Amount of Additional Collateral Required	$ 44	[2]
[1]	Before consideration of master netting arrangements or similar agreements and including NPNS and other accrual contract exposures.
[2]	As collateral requirements with certain counterparties are based on master netting arrangements or similar agreements, the aggregate amount of additional collateral required to be posted is determined after consideration of the effects of such arrangements

Fair Value Measurements (Schedu

Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions		Mar. 31, 2018		Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		$ 694	[1]	$ 700	[2]
Assets fair value		708	[3]	717	[4]
Excluded receivables, payables, and accrued income, net		4		4
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		694	[1]	700	[2]
Assets fair value		707	[3]	717	[4]
Excluded receivables, payables, and accrued income, net		4		4
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		14	[3]	17	[4]
Derivative liabilities		230	[3]	226	[4]
Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		13	[3]	17	[4]
Derivative liabilities		10	[3]	9	[4]
Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		220	[3]	217	[4]
Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		464		468
Equity Securities | U.S. large capitalization | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		464		468
Debt Securities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		80		82
Debt Securities | Corporate bonds | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		80		82
Debt Securities | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		118		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		118		125
Debt Securities | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		32		25
Debt Securities | Other Debt Securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		32		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		464		468
Assets fair value		470	[3]	472	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		464		468
Assets fair value		470	[3]	472	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		6	[3]	4	[4]
Derivative liabilities		0	[3]	1	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		6	[3]	4	[4]
Derivative liabilities		0	[3]	0	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0	[3]	1	[4]
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		464		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		464		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		230		232
Assets fair value		231	[3]	233	[4]
Significant Other Observable Inputs (Level 2) | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		230		232
Assets fair value		230	[3]	233	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	1	[4]
Derivative liabilities		32	[3]	25	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	1	[4]
Derivative liabilities		9	[3]	7	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		23	[3]	18	[4]
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		80		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		80		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		118		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		118		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		32		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		32		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		7	[3]	12	[4]
Significant Other Unobservable Inputs (Level 3) | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Assets fair value		7	[3]	12	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		7	[3]	12	[4]
Derivative liabilities		198	[3]	200	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		7	[3]	12	[4]
Derivative liabilities		1	[3]	2	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		197	[3]	198	[4]
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		8	[3]	7	[4]
Fuel Oils | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		8	[3]	7	[4]
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		6	[3]	4	[4]
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		6	[3]	4	[4]
Fuel Oils | Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
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	[3]	0	[4]
Fuel Oils | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		2	[3]	3	[4]
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		2	[3]	3	[4]
Power | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		5	[3]	9	[4]
Derivative liabilities		192	[3]	196	[4]
Power | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		5	[3]	9	[4]
Derivative liabilities		1	[3]	1	[4]
Power | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		191	[3]	195	[4]
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	[3]	0	[4]
Derivative liabilities		0	[3]	0	[4]
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	[3]	0	[4]
Derivative liabilities		0	[3]	0	[4]
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	[3]	0	[4]
Power | Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	1	[4]
Derivative liabilities		0	[3]	0	[4]
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	[3]	1	[4]
Derivative liabilities		0	[3]	0	[4]
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	[3]	0	[4]
Power | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		5	[3]	8	[4]
Derivative liabilities		192	[3]	196	[4]
Power | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		5	[3]	8	[4]
Derivative liabilities		1	[3]	1	[4]
Power | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		191	[3]	195	[4]
Natural Gas | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	1	[4]
Derivative liabilities		38	[3]	30	[4]
Natural Gas | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			1
Derivative liabilities		9	[3]	8	[4]
Natural Gas | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]	1
Derivative liabilities		29	[3]	22	[4]
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	[3]	0	[4]
Derivative liabilities		0	[3]	1	[4]
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	[4]			0
Derivative liabilities		0	[3]	0	[4]
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	[3]	0
Derivative liabilities		0	[3]	1	[4]
Natural Gas | Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	0	[4]
Derivative liabilities		32	[3]	25	[4]
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	[4]			0
Derivative liabilities		9	[3]	7	[4]
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	[3]	1
Derivative liabilities		23	[3]	18	[4]
Natural Gas | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	1	[4]
Derivative liabilities		6	[3]	4	[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	[4]			1
Derivative liabilities		0	[3]	1	[4]
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	[3]	0
Derivative liabilities		$ 6	[3]	$ 3	[4]
[1]	Balance excludes $4 million of cash and cash equivalents, receivables, payables, and accrued income, net.
[2]	Balance excludes $4 million of cash and cash equivalents, receivables, payables, and accrued income, net.
[3]	The derivative asset and liability balances are presented net of counterparty credit considerations.
[4]	The derivative asset and liability balances are presented net of counterparty credit considerations.

Fair Value Measurements (Sche48

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
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 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)	$ (190)	$ (188)	$ (178)
Included in regulatory assets/liabilities	(1)	(10)
Settlements	2	(2)
Change in unrealized gains (losses) related to assets/liabilities held at period end	0	(11)
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	4	4	7	7
Included in regulatory assets/liabilities	(2)	0
Settlements	(1)	(3)
Change in unrealized gains (losses) related to assets/liabilities held at period end	(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	(191)	(194)	$ (195)	$ (185)
Included in regulatory assets/liabilities	1	(10)
Settlements	3	1
Change in unrealized gains (losses) related to assets/liabilities held at period end	$ 1	$ (11)

Fair Value Measurements (Sche49

Fair Value Measurements (Schedule Of Valuation Process And Unobservable Inputs) (Detail) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018USD ($)$ / credit$ / MMBTU$ / MWh$ / MW	Dec. 31, 2017USD ($)$ / credit$ / MMBTU$ / MWh$ / MW
Natural Gas | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk		0.37%
Nodal basis	(1.4)	(1.2)
Credit risk	35.00%	0.37%
Natural Gas | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk		0.92%
Nodal basis	(0.1)	0.1
Credit risk	35.00%	0.37%
Natural Gas | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk		0.53%
Nodal basis	(1)	(1)
Credit risk	35.00%	0.37%
Natural Gas | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities		26.00%
Nodal basis		(0.5)
Natural Gas | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities		46.00%
Nodal basis		(0.3)
Natural Gas | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities		37.00%
Nodal basis		(0.4)
Natural Gas | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	$ 0	$ 1
Natural Gas | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	$ (6)	$ (4)
Fuel Oils | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.12%	0.12%
Credit risk	35.00%	0.37%
Fuel Oils | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.91%	0.72%
Credit risk	35.00%	0.37%
Fuel Oils | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	0.49%	0.41%
Credit risk	35.00%	0.37%
Fuel Oils | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities	21.00%	20.00%
Fuel Oils | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities	33.00%	26.00%
Fuel Oils | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities	24.00%	22.00%
Fuel Oils | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	$ 2	$ 3
Fuel Oils | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	$ 0	$ 0
Power | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.92%	0.28%
Nodal basis | $ / MWh	(10)	(10)
Credit risk	0.35%	0.37%
Average forward peak and off-peak pricing | $ / MWh	22	24
Estimated auction price | $ / MW	(479)	(65)
Power | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	0.92%	0.28%
Nodal basis | $ / MWh	0	0
Credit risk	0.35%	0.37%
Average forward peak and off-peak pricing | $ / MWh	37	46
Estimated auction price | $ / MW	1,608	1,823
Power | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	0.92%	0.28%
Nodal basis | $ / MWh	(2)	(2)
Credit risk	0.35%	0.37%
Average forward peak and off-peak pricing | $ / MWh	31	28
Estimated auction price | $ / MW	58	251
Power | Fundamental Energy Production Model | Minimum
Fair Value Inputs [Abstract]
Escalation rate	4.00%	5.00%
Estimated future gas prices	3	3
Power | Fundamental Energy Production Model | Maximum
Fair Value Inputs [Abstract]
Escalation rate	4.00%	5.00%
Estimated future gas prices	4	4
Power | Fundamental Energy Production Model | Weighted Average
Fair Value Inputs [Abstract]
Escalation rate	4.00%	5.00%
Estimated future gas prices	3	3
Power | Contract Price Allocation | Minimum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit	5	5
Power | Contract Price Allocation | Maximum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit	7	7
Power | Contract Price Allocation | Weighted Average
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit	6	6
Power | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	$ 5	$ 8
Power | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	$ (192)	$ (196)

Fair Value Measurements (Sche50

Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Financial Assets and Liabilities) (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	$ 89	$ 68	$ 51	$ 52
Short-term Debt	960	484
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	10	7	5	5
Short-term Debt	282	39
Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	52	41	$ 28	$ 28
Short-term Debt	224	62
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	89	68
Short-term Debt	960	484
Long-term debt (including current portion)	7,936	7,935
Preferred stock	142	142
Debt Issuance Costs, Net	49	50
Carrying Amount | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	10	7
Available-for-sale Securities and Held-to-maturity Securities	276	276
Short-term Debt	282	39
Long-term debt (including current portion)	3,962	3,961
Preferred stock	80	80
Debt Issuance Costs, Net	20	20
Carrying Amount | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents		41
Short-term Debt	224	62
Long-term debt (including current portion)	2,830	2,830
Preferred stock	62	62
Fair Value, Concentration of Risk, Cash and Cash Equivalents	52
Debt Issuance Costs, Net	24	24
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	89	68
Short-term Debt, Fair Value	960	484
Investments, Fair Value Disclosure	276	276
Long-term Debt, Fair Value	8,259	8,531
Preferred stock	130	131
Fair Value | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	89	68
Short-term Debt, Fair Value	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	0	0
Preferred stock	0	0
Fair Value | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	0	0
Short-term Debt, Fair Value	960	484
Investments, Fair Value Disclosure	276	276
Long-term Debt, Fair Value	7,820	8,531
Preferred stock	130	131
Fair Value | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	0	0
Short-term Debt, Fair Value	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	439	0
Preferred stock	0	0
Fair Value | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	10	7
Short-term Debt, Fair Value	282	39
Investments, Fair Value Disclosure	276	276
Long-term Debt, Fair Value	4,201	4,348
Preferred stock	78	80
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]
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	10	7
Short-term Debt, Fair Value	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	0	0
Preferred stock	0	0
Fair Value | Union Electric Company | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	0	0
Short-term Debt, Fair Value	282	39
Investments, Fair Value Disclosure	276	276
Long-term Debt, Fair Value	4,201	4,348
Preferred stock	78	80
Fair Value | Union Electric Company | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	0	0
Short-term Debt, Fair Value	0	0
Investments, Fair Value Disclosure	0	0
Long-term Debt, Fair Value	0	0
Preferred stock	0	0
Fair Value | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	52	41
Short-term Debt, Fair Value	224	62
Long-term Debt, Fair Value	2,928	3,028
Preferred stock	52	51
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]
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	52	41
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	0	0
Preferred stock	0	0
Fair Value | Ameren Illinois Company | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	0	0
Short-term Debt, Fair Value	224	62
Long-term Debt, Fair Value	2,928	3,028
Preferred stock	52	51
Fair Value | Ameren Illinois Company | Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents and Restricted Cash and Cash Equivalents	0	0
Short-term Debt, Fair Value	0	0
Long-term Debt, Fair Value	0	0
Preferred stock	$ 0	$ 0

Related Party Transactions (Sch

Related Party Transactions (Schedule of Related Party Transactions) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Related Party Transaction [Line Items]
Operating Revenues		$ 0	$ 0
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues		0	0
Purchased Power | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses		3	11
Ameren Missouri Power Supply Agreements with Ameren Illinois | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues		3	11
Ameren Missouri and Ameren Illinois Rent and Facility Services | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues		5	7
Ameren Missouri and Ameren Illinois Rent and Facility Services | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues		1	1
Ameren Missouri and Ameren Illinois miscellaneous support services | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues	[1]	1	1
Ameren Missouri and Ameren Illinois miscellaneous support services | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	[1]	1	1
Total Related Party Operating Revenues [Member] | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues		8	18
Total Related Party Operating Revenues [Member] | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues		1	1
Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses		3	11
Ameren Illinois Transmission Agreements with ATXI | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses	[1]	1	1
Ameren Services Support Services Agreement | Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses		33	35
Ameren Services Support Services Agreement | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses		30	32
Related Party Money Pool Interest | Union Electric Company
Related Party Transaction [Line Items]
Interest Charges	[1]	1	1
Related Party Money Pool Interest | Ameren Illinois Company
Related Party Transaction [Line Items]
Interest Charges	[1]	$ 1	$ 1
[1]	Amount less than $1 million.

Related Party Transactions Narr

Related Party Transactions Narrative (Details) - Subsequent Event - April 2018 Procurement [Member] - Ameren Illinois Company - Ameren Illinois Power Supply Agreements with Ameren Missouri	3 Months Ended
	Jun. 30, 2018MWh$ / MWh
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 (

Commitments And Contingencies (Other Obligations) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2018USD ($)MWh
Long-term Purchase Commitment [Line Items]
Total Other Obligations	$ 2,154
Union Electric Company
Long-term Purchase Commitment [Line Items]
Total Other Obligations	1,509
Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Total Other Obligations	$ 627
April 2018 Procurement [Member] | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Amount of Megawatthours | MWh	3,956,200
Long-term Purchase Commitment, Amount	$ 112
Purchased Power | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Total Other Obligations	$ 10

Commitments And Contingencies54

Commitments And Contingencies (Environmental Matters) (Detail) $ in Millions	Mar. 31, 2018USD ($)scrubbersite	Dec. 31, 2017USD ($)
Loss Contingencies [Line Items]
Percentage of Rate Base Related to Carbon Dioxide Energy Centers		17.00%
Asset Retirement Obligation	$ 642	$ 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%
Number of Energy Center Scrubbers | scrubber	2
Asset Retirement Obligation	$ 638	$ 640
Ameren Illinois Company
Loss Contingencies [Line Items]
Asset Retirement Obligation	4	$ 4
Coal Combustion Residuals Estimate [Member] | 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 [Member] | 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	$ 168
Manufactured Gas Plant | Ameren Illinois Company
Loss Contingencies [Line Items]
Number of remediation sites | site	44
Accrual for environmental loss contingencies	$ 168
Manufactured Gas Plant | Ameren Illinois Company | Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	168
Manufactured Gas Plant | Ameren Illinois Company | Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	240
Sauget Area Two | Union Electric Company
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	1
Sauget Area Two | Union Electric Company | Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	1
Sauget Area Two | Union Electric Company | Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	2.5
New CCR Rules Estimate
Loss Contingencies [Line Items]
Asset Retirement Obligation	$ 141

Callaway Energy Center (Narrati

Callaway Energy Center (Narrative) (Detail) - Nuclear Plant $ in Millions	3 Months Ended
	Mar. 31, 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) € in Millions	3 Months Ended
	Mar. 31, 2018USD ($)		Mar. 31, 2018EUR (€)
Nuclear Waste Matters [Line Items]
Number of weeks of coverage after the first eight weeks of an outage	1 year		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	19,000,000
Amount of weekly indemnity coverage commencing eight weeks after power outage	4,500,000
Amount of additional weekly indemnity coverage commencing after initial indemnity coverage	$ 3,600,000
Inflationary adjustment prescribed by most recent Price-Anderson Act renewal, in years	5 years		5 years
Aggregate nuclear power industry insurance policy limit for losses from terrorist attacks within twelve month period	$ 3,200,000,000		€ 600
Number Of Additional Weeks After Initial Indemnity Coverage For Power Outage	1 year 4 months 10 days		1 year 4 months 10 days
Public Liability And Nuclear Worker Liability - American Nuclear Insurers
Nuclear Waste Matters [Line Items]
Maximum Coverages	$ 450,000,000
Maximum Assessments for Single Incidents	0
Public Liability And Nuclear Worker Liability - Pool Participation
Nuclear Waste Matters [Line Items]
Maximum Coverages	12,604,000,000	[1]
Maximum Assessments for Single Incidents	127,000,000	[2]
Public Liability
Nuclear Waste Matters [Line Items]
Maximum Coverages	13,054,000,000	[3]
Maximum Assessments for Single Incidents	127,000,000
Property Damage - Nuclear Electric Insurance Ltd
Nuclear Waste Matters [Line Items]
Maximum Coverages	3,200,000,000	[4]
Maximum Assessments for Single Incidents	27,000,000	[5]
Replacement Power - Nuclear Electric Insurance Ltd
Nuclear Waste Matters [Line Items]
Maximum Coverages	490,000,000	[6]
Maximum Assessments for Single Incidents	7,000,000	[5]
Amount of weekly indemnity coverage thereafter not exceeding policy limit	490,000,000
Sub-limit of for non-nuclear events	328,000,000
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
[1]	Provided through mandatory participation in an industrywide retrospective premium assessment program.
[2]	Retrospective premium under the Price-Anderson Act. This is subject to retrospective assessment with respect to a covered loss in excess of $450 million in the event of an incident at any licensed United States commercial reactor, payable at $19 million per year.
[3]	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.
[4]	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.
[5]	All NEIL insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL.
[6]	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 Retirement

Retirement Benefits Retirement Benefits (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	$ 16	$ 12
Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	5	6
Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	$ 7	$ 3

Retirement Benefits (Components

Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	$ (16)	$ (12)
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	25	23
Interest cost	42	45
Expected return on plan assets	(69)	(66)
Prior service cost (benefit)	0	0
Actuarial loss	16	14
Total non-service cost components	(11)	(7)
Net periodic benefit cost	14	16
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	5	5
Interest cost	11	12
Expected return on plan assets	(19)	(19)
Prior service cost (benefit)	(1)	(1)
Actuarial loss	0	(2)
Total non-service cost components	(9)	(10)
Net periodic benefit cost	(4)	(5)
Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(5)	(6)
Union Electric Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	5	6
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	0	(1)
Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Total non-service cost components	(7)	(3)
Ameren Illinois Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	9	10
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	$ (4)	$ (4)

Retirement Benefits (Summary Of

Retirement Benefits (Summary Of Benefit Plan Costs Incurred) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost	$ 14	$ 16
Pension Plan | Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost	5	6
Pension Plan | Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost	9	10
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost	(4)	(5)
Other Postretirement Benefit Plan, Defined Benefit | Union Electric Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost	0	(1)
Other Postretirement Benefit Plan, Defined Benefit | Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost	$ (4)	$ (4)

Segment Information (Schedule O

Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2018USD ($)segment	Mar. 31, 2017USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment	4
External revenues	$ 1,585	$ 1,515
Intersegment revenues	0	0
Net Income Attributable to Ameren Common Shareholders	151	102
Capital expenditures	$ 579	504
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	$ 760	703
Intersegment revenues	0	0
Net Income Available to Common Shareholder	95	79
Capital expenditures	300	227
Intersegment Eliminations
Segment Reporting Information [Line Items]
External revenues	0	0
Intersegment revenues	(22)	(25)
Net Income Available to Common Shareholder	0	0
Capital expenditures	(4)	(1)
Intersegment Eliminations | Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues	0	0
Intersegment revenues	(13)	(6)
Net Income Available to Common Shareholder	0	0
Capital expenditures	0	0
Operating Segments | Union Electric Company
Segment Reporting Information [Line Items]
External revenues	784	773
Intersegment revenues	8	18
Net Income Available to Common Shareholder	38	5
Capital expenditures	249	196
Operating Segments | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	399	384
Intersegment revenues	1	1
Net Income Available to Common Shareholder	33	30
Capital expenditures	122	120
Operating Segments | Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues	311	264
Intersegment revenues	0	0
Net Income Available to Common Shareholder	42	33
Capital expenditures	60	51
Operating Segments | Ameren Transmission
Segment Reporting Information [Line Items]
External revenues	91	96
Intersegment revenues	13	6
Net Income Available to Common Shareholder	37	34
Capital expenditures	145	134
Operating Segments | Other
Segment Reporting Information [Line Items]
External revenues	0	(2)
Intersegment revenues	0	0
Net Income Available to Common Shareholder	1	0
Capital expenditures	7	4
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues	400	385
Intersegment revenues	0	0
Net Income Available to Common Shareholder	33	30
Capital expenditures	122	120
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues	311	264
Intersegment revenues	0	0
Net Income Available to Common Shareholder	42	33
Capital expenditures	60	51
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Segment Reporting Information [Line Items]
External revenues	49	54
Intersegment revenues	13	6
Net Income Available to Common Shareholder	20	16
Capital expenditures	$ 118	$ 56

Segment Information Segment Inf

Segment Information Segment Information (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ 1,585	$ 1,515
Regulated Operating Revenue, Other	20	42
Other Revenue, Net	25	7
Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(22)	(25)
Electric
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	1,223	1,207
Electric | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	551	509
Electric | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	376	365
Electric | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	96	86
Electric | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	200	247
Electric | Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(22)	(25)
Electric | Intersegment Eliminations | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Electric | Intersegment Eliminations | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Electric | Intersegment Eliminations | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Electric | Intersegment Eliminations | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(22)	(25)
Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	362	308
Natural Gas | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	284	233
Natural Gas | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	83	67
Natural Gas | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	8	4
Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(13)	4
Natural Gas | Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Natural Gas | Intersegment Eliminations | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Natural Gas | Intersegment Eliminations | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Natural Gas | Intersegment Eliminations | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Natural Gas | Intersegment Eliminations | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Ameren Illinois Company | Ameren Illinois Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	760	703
Regulated Operating Revenue, Other	24	47
Other Revenue, Net	11	3
Ameren Illinois Company | Ameren Illinois Company | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	462	422
Ameren Illinois Company | Ameren Illinois Company | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	191	188
Ameren Illinois Company | Ameren Illinois Company | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	41	31
Ameren Illinois Company | Ameren Illinois Company | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	66	62
Ameren Illinois Company | Intersegment Eliminations
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(13)	(6)
Ameren Illinois Company | Intersegment Eliminations | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Ameren Illinois Company | Intersegment Eliminations | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Ameren Illinois Company | Intersegment Eliminations | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Ameren Illinois Company | Intersegment Eliminations | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(13)	(6)
Operating Segments | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	792	791
Regulated Operating Revenue, Other	(4)	(7)
Other Revenue, Net	14	4
Operating Segments | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	400	385
Regulated Operating Revenue, Other	31	33
Other Revenue, Net	10	2
Operating Segments | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	311	264
Regulated Operating Revenue, Other	(3)	11
Other Revenue, Net	1	1
Operating Segments | Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	104	102
Regulated Operating Revenue, Other	(4)	5
Other Revenue, Net	0	0
Operating Segments | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	(2)
Operating Segments | Electric | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	741	747
Operating Segments | Electric | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	332	290
Operating Segments | Electric | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	252	232
Operating Segments | Electric | Union Electric Company | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	61	58
Operating Segments | Electric | Union Electric Company | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	96	167
Operating Segments | Electric | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	400	385
Operating Segments | Electric | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	219	219
Operating Segments | Electric | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	124	133
Operating Segments | Electric | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	35	28
Operating Segments | Electric | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	22	5
Operating Segments | Electric | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments | Electric | Ameren Illinois Gas | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments | Electric | Ameren Illinois Gas | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments | Electric | Ameren Illinois Gas | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments | Electric | Ameren Illinois Gas | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments | Electric | Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	104	102
Operating Segments | Electric | Ameren Transmission | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments | Electric | Ameren Transmission | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments | Electric | Ameren Transmission | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments | Electric | Ameren Transmission | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	104	102
Operating Segments | Electric | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	(2)
Operating Segments | Electric | Other | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments | Electric | Other | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments | Electric | Other | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments | Electric | Other | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	(2)
Operating Segments | Natural Gas | Union Electric Company
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	51	44
Operating Segments | Natural Gas | Union Electric Company | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	41	30
Operating Segments | Natural Gas | Union Electric Company | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	16	12
Operating Segments | Natural Gas | Union Electric Company | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	2	1
Operating Segments | Natural Gas | Union Electric Company | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(8)	1
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	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
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	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
Operating Segments | Natural Gas | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments | Natural Gas | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	311	264
Operating Segments | Natural Gas | Ameren Illinois Gas | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	243	203
Operating Segments | Natural Gas | Ameren Illinois Gas | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	67	55
Operating Segments | Natural Gas | Ameren Illinois Gas | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	6	3
Operating Segments | Natural Gas | Ameren Illinois Gas | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	(5)	3
Operating Segments | Natural Gas | Ameren Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments | Natural Gas | Ameren Transmission | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments | Natural Gas | Ameren Transmission | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments | Natural Gas | Ameren Transmission | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments | Natural Gas | Ameren Transmission | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments | Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments | Natural Gas | Other | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments | Natural Gas | Other | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments | Natural Gas | Other | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments | Natural Gas | Other | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	0	0
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Regulated Operating Revenue, Other	31	33
Other Revenue, Net	10	2
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Regulated Operating Revenue, Other	(3)	11
Other Revenue, Net	1	1
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	62	60
Regulated Operating Revenue, Other	(4)	3
Other Revenue, Net	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
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	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
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	62	60
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Electric Distribution
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	400	385
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Electric Distribution | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	219	219
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Electric Distribution | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	124	133
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Electric Distribution | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	35	28
Operating Segments | Ameren Illinois Company | Electric | Ameren Illinois Electric Distribution | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	22	5
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Gas
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	311	264
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Gas | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	243	203
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Gas | Commercial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	67	55
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Gas | Industrial
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	6	3
Operating Segments | Ameren Illinois Company | Natural Gas | Ameren Illinois Gas | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax	$ (5)	$ 3