Ameren Illinois (AILLM) 10-Q 3 Nov 17 2017 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2017	Oct. 31, 2017
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q3
Trading Symbol	AEE
Entity Registrant Name	AMEREN CORP
Entity Central Index Key	1,002,910
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		242,634,798
Union Electric Company
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q3
Entity Registrant Name	UNION ELECTRIC CO
Entity Central Index Key	100,826
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		102,123,834
Ameren Illinois Company
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q3
Entity Registrant Name	AMEREN ILLINOIS CO
Entity Central Index Key	18,654
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		25,452,373

Consolidated Statement of Incom

Consolidated Statement of Income (Loss) - USD ($) shares in Millions, $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Operating Revenues:
Electric		$ 1,594	$ 1,725	$ 4,183	$ 4,101
Gas		129	134	592	619
Total operating revenues		1,723	1,859	4,775	4,720
Operating Expenses:
Fuel		199	205	594	574
Purchased power		162	178	491	451
Gas purchased for resale		25	34	196	227
Other operations and maintenance		402	411	1,229	1,246
Depreciation and amortization		225	211	668	628
Taxes other than income taxes		129	129	364	358
Total operating expenses		1,142	1,168	3,542	3,484
Operating Income		581	691	1,233	1,236
Other Income and Expenses:
Miscellaneous income	[1]	13	18	42	54
Miscellaneous expense	[1]	2	8	16	21
Total other income (expense)		11	10	26	33
Interest Charges		97	97	295	287
Income Before Income Taxes		495	604	964	982
Income Taxes		205	233	376	356
Net income		290	371	588	626
Pension and other postretirement benefit plan activity, net of income taxes (benefit)		0	(1)	2	1
Comprehensive Income		288	368	585	622
Less: Net Income Attributable to Noncontrolling Interests		2	2	5	5
Net Income Attributable to Ameren Common Shareholders		$ 288	$ 369	$ 583	$ 621
Earnings Per Share, Basic and Diluted [Abstract]
Earnings Per Share, Basic		$ 1.19	$ 1.52	$ 2.40	$ 2.56
Earnings Per Share, Diluted		1.18	1.52	2.39	2.56
Dividends per Common Share		$ 0.44	$ 0.425	$ 1.32	$ 1.275
Average Common Shares Outstanding - Basic		242.6	242.6	242.6	242.6
Average Common Shares Outstanding - Diluted		244.7	242.9	244	243
Union Electric Company
Operating Revenues:
Electric		$ 1,098	$ 1,144	$ 2,757	$ 2,682
Gas		17	20	83	90
Other Revenue, Net		0	1	0	1
Total operating revenues		1,115	1,165	2,840	2,773
Operating Expenses:
Fuel		199	205	594	574
Purchased power		42	77	201	169
Gas purchased for resale		4	6	29	33
Other operations and maintenance		224	220	655	670
Depreciation and amortization		134	130	399	384
Taxes other than income taxes		95	96	255	252
Total operating expenses		698	734	2,133	2,082
Operating Income		417	431	707	691
Other Income and Expenses:
Miscellaneous income		13	14	36	38
Miscellaneous expense		2	2	6	6
Total other income (expense)		11	12	30	32
Interest Charges		50	53	157	158
Income Before Income Taxes		378	390	580	565
Income Taxes		143	148	218	215
Net income		235	242	362	350
Other Comprehensive Income		0	0	0	0
Comprehensive Income		235	242	362	350
Earnings Per Share, Basic and Diluted [Abstract]
Preferred Stock Dividends		1	1	3	3
Net Income Available to Common Stockholder		234	241	359	347
Ameren Illinois Company
Operating Revenues:
Electric		463	562	1,343	1,365
Gas		112	114	510	530
Other Revenue, Net		0	0	1	0
Total operating revenues		575	676	1,854	1,895
Operating Expenses:
Purchased power		124	110	312	304
Gas purchased for resale		21	28	167	194
Other operations and maintenance		183	198	590	592
Depreciation and amortization		86	80	254	237
Taxes other than income taxes		33	30	101	98
Total operating expenses		447	446	1,424	1,425
Operating Income		128	230	430	470
Other Income and Expenses:
Miscellaneous income		1	4	7	15
Miscellaneous expense		0	3	8	11
Total other income (expense)		1	1	(1)	4
Interest Charges		36	35	109	105
Income Before Income Taxes		93	196	320	369
Income Taxes		38	77	127	144
Net income		55	119	193	225
Pension and other postretirement benefit plan activity, net of income taxes (benefit)		0	(1)	0	(3)
Comprehensive Income		55	118	193	222
Earnings Per Share, Basic and Diluted [Abstract]
Preferred Stock Dividends		0	0	2	2
Net Income Available to Common Stockholder		$ 55	$ 119	$ 191	$ 223
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Consolidated Statement of Inco3

Consolidated Statement of Income (Loss) (Parenthetical) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension and other postretirement benefit plan activity, tax expense (benefit)	$ 0	$ 0	$ 1	$ 4
Ameren Illinois Company
Pension and other postretirement benefit plan activity, tax expense (benefit)	$ 0	$ (1)	$ 0	$ (2)

Consolidated Statement of Compr

Consolidated Statement of Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 290	$ 371	$ 588	$ 626
Other Comprehensive Income (Loss), Net of Taxes
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	0	(1)	2	1
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	290	370	590	627
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	2	2	5	5
Comprehensive Income Attributable to Ameren Common Shareholders	$ 288	$ 368	$ 585	$ 622

Consolidated Statement of Comp5

Consolidated Statement of Comprehensive Income (Loss) Consolidated Statement of Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Pension and other postretirement benefit plan activity, tax expense (benefit)	$ 0	$ 0	$ 1	$ 4

Consolidated Balance Sheet

Consolidated Balance Sheet - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Current Assets:
Cash and cash equivalents	$ 9	$ 9
Accounts receivable - trade (less allowance for doubtful accounts)	507	437
Unbilled revenue	262	295
Miscellaneous accounts and notes receivable	85	63
Inventories	547	527
Current regulatory assets	75	149
Other current assets	96	113
Total current assets	1,581	1,593
Property, Plant, and Equipment, Net	20,906	20,113
Investments and Other Assets:
Nuclear decommissioning trust fund	672	607
Goodwill	411	411
Regulatory assets	1,509	1,437
Other assets	538	538
Total investments and other assets	3,130	2,993
TOTAL ASSETS	25,617	24,699
Current Liabilities:
Current maturities of long-term debt	777	681
Short-term Debt	446	558
Accounts and wages payable	548	805
Taxes accrued	159	46
Interest accrued	106	93
Customer deposits	108	107
Current regulatory liabilities	119	110
Other current liabilities	318	274
Total current liabilities	2,581	2,674
Long-term Debt, Net	6,922	6,595
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	4,721	4,264
Accumulated deferred investment tax credits	50	55
Regulatory liabilities	2,045	1,985
Asset retirement obligations	631	635
Pension and other postretirement benefits	711	769
Other deferred credits and liabilities	469	477
Total deferred credits and other liabilities	8,627	8,185
Commitments and Contingencies
Stockholders' Equity:
Common Stock	2	2
Other paid-in capital	5,534	5,556
Retained earnings	1,830	1,568
Accumulated other comprehensive loss	(21)	(23)
Stockholder's equity	7,345	7,103
Noncontrolling Interest	142	142
Total equity	7,487	7,245
TOTAL LIABILITIES AND EQUITY	25,617	24,699
Union Electric Company
Current Assets:
Cash and cash equivalents	0	0
Advances to money pool	18	161
Accounts receivable - trade (less allowance for doubtful accounts)	274	187
Accounts receivable - affiliates	14	12
Unbilled revenue	151	154
Miscellaneous accounts and notes receivable	45	14
Inventories	396	392
Current regulatory assets	23	35
Other current assets	43	49
Total current assets	964	1,004
Property, Plant, and Equipment, Net	11,538	11,478
Investments and Other Assets:
Nuclear decommissioning trust fund	672	607
Regulatory assets	576	619
Other assets	318	327
Total investments and other assets	1,566	1,553
TOTAL ASSETS	14,068	14,035
Current Liabilities:
Current maturities of long-term debt	383	431
Short-term Debt	0	0
Accounts and wages payable	226	444
Accounts payable - affiliates	102	68
Taxes accrued	148	30
Interest accrued	61	54
Current regulatory liabilities	18	12
Other current liabilities	118	123
Total current liabilities	1,056	1,162
Long-term Debt, Net	3,584	3,563
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	3,073	3,013
Accumulated deferred investment tax credits	49	53
Regulatory liabilities	1,275	1,215
Asset retirement obligations	627	629
Pension and other postretirement benefits	274	291
Other deferred credits and liabilities	13	19
Total deferred credits and other liabilities	5,311	5,220
Commitments and Contingencies
Stockholders' Equity:
Common Stock	511	511
Other paid-in capital	1,828	1,828
Preferred stock	80	80
Retained earnings	1,698	1,671
Stockholder's equity	4,117	4,090
TOTAL LIABILITIES AND EQUITY	14,068	14,035
Ameren Illinois Company
Current Assets:
Cash and cash equivalents	0	0
Accounts receivable - trade (less allowance for doubtful accounts)	219	242
Accounts receivable - affiliates	21	10
Unbilled revenue	111	141
Miscellaneous accounts and notes receivable	31	22
Inventories	151	135
Current regulatory assets	51	108
Other current assets	18	25
Total current assets	602	683
Property, Plant, and Equipment, Net	7,987	7,469
Investments and Other Assets:
Goodwill	411	411
Regulatory assets	921	816
Other assets	101	95
Total investments and other assets	1,433	1,322
TOTAL ASSETS	10,022	9,474
Current Liabilities:
Current maturities of long-term debt	394	250
Short-term Debt	169	51
Borrowings from money pool	11	0
Accounts and wages payable	247	264
Accounts payable - affiliates	50	63
Taxes accrued	8	16
Interest accrued	37	33
Customer deposits	69	69
Current environmental remediation	43	38
Current regulatory liabilities	85	78
Other current liabilities	153	109
Total current liabilities	1,266	971
Long-term Debt, Net	2,196	2,338
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,874	1,631
Accumulated deferred investment tax credits	1	2
Regulatory liabilities	766	768
Pension and other postretirement benefits	322	346
Environmental remediation	143	162
Other deferred credits and liabilities	229	222
Total deferred credits and other liabilities	3,335	3,131
Commitments and Contingencies
Stockholders' Equity:
Common Stock	0	0
Other paid-in capital	2,005	2,005
Preferred stock	62	62
Retained earnings	1,158	967
Stockholder's equity	3,225	3,034
TOTAL LIABILITIES AND EQUITY	$ 10,022	$ 9,474

Consolidated Balance Sheet (Par

Consolidated Balance Sheet (Parenthetical) - USD ($) shares in Millions, $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Accounts receivable - trade allowance for doubtful accounts	$ 20	$ 19
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	400	400
Common stock, shares outstanding	242.6	242.6
Union Electric Company
Accounts receivable - trade allowance for doubtful accounts	$ 8	$ 7
Common stock, par value	$ 5	$ 5
Common stock, shares authorized	150	150
Common stock, shares outstanding	102.1	102.1
Ameren Illinois Company
Accounts receivable - trade allowance for doubtful accounts	$ 12	$ 12
Common stock, no par value	$ 0	$ 0
Common stock, shares authorized	45	45
Common stock, shares outstanding	25.5	25.5

Consolidated Statement of Cash

Consolidated Statement of Cash Flows - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016
Cash Flows From Operating Activities:
Net income		$ 588	$ 626
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization		653	625
Amortization of nuclear fuel		71	63
Amortization of debt issuance costs and premium/discounts		16	17
Deferred income taxes and investment tax credits, net		366	364
Allowance for equity funds used during construction	[1]	(16)	(20)
Share-based compensation costs		12	17
Other		(7)	(9)
Changes in assets and liabilities:
Receivables		(59)	(134)
Inventories		(20)	(13)
Accounts and wages payable		(183)	(196)
Taxes accrued		138	119
Regulatory assets and liabilities		89	146
Assets, other		14	9
Liabilities, other		12	(29)
Pension and other postretirement benefits		(31)	(26)
Net cash provided by operating activities		1,643	1,559
Cash Flows From Investing Activities:
Capital expenditures		(1,523)	(1,496)
Nuclear fuel expenditures		(52)	(41)
Purchases of securities - nuclear decommissioning trust fund		(248)	(310)
Sales and maturities of securities - nuclear decommissioning trust fund		235	297
Other		3	(1)
Net cash used in investing activities		(1,585)	(1,551)
Cash Flows From Financing Activities:
Dividends on common stock		(320)	(309)
Dividends paid to noncontrolling interest holders		(5)	(5)
Short-term debt, net		(112)	307
Maturities of Long-term Debt		(425)	(389)
Proceeds from Issuance of Long-term Debt		849	149
Share-based payments		(39)	(32)
Debt issuance costs		(5)	(1)
Other		(1)	(2)
Net cash used in financing activities		(58)	(282)
Net change in cash and cash equivalents		0	(274)
Cash and cash equivalents at beginning of period		9	292
Cash and cash equivalents at end of period		9	18
Union Electric Company
Cash Flows From Operating Activities:
Net income		362	350
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization		384	381
Amortization of nuclear fuel		71	63
Amortization of debt issuance costs and premium/discounts		5	5
Deferred income taxes and investment tax credits, net		55	159
Allowance for equity funds used during construction		(15)	(16)
Other		4	0
Changes in assets and liabilities:
Receivables		(117)	(95)
Inventories		(3)	(5)
Accounts and wages payable		(151)	(176)
Taxes accrued		160	165
Regulatory assets and liabilities		48	60
Assets, other		19	(8)
Liabilities, other		4	13
Pension and other postretirement benefits		(7)	(8)
Net cash provided by operating activities		819	888
Cash Flows From Investing Activities:
Capital expenditures		(533)	(500)
Nuclear fuel expenditures		(52)	(41)
Purchases of securities - nuclear decommissioning trust fund		(248)	(310)
Sales and maturities of securities - nuclear decommissioning trust fund		235	297
Money pool advances, net		143	(165)
Other		0	(5)
Net cash used in investing activities		(455)	(724)
Cash Flows From Financing Activities:
Dividends on common stock		(332)	(285)
Dividends on preferred stock		(3)	(3)
Maturities of Long-term Debt		(425)	(260)
Proceeds from Issuance of Long-term Debt		399	149
Capital contribution from parent		0	38
Debt issuance costs		(3)	(1)
Net cash used in financing activities		(364)	(362)
Net change in cash and cash equivalents		0	(198)
Cash and cash equivalents at beginning of period		0	199
Cash and cash equivalents at end of period		0	1
Ameren Illinois Company
Cash Flows From Operating Activities:
Net income		193	225
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization		254	236
Amortization of debt issuance costs and premium/discounts		10	11
Deferred income taxes and investment tax credits, net		161	141
Allowance for equity funds used during construction		(1)	(4)
Other		(1)	(8)
Changes in assets and liabilities:
Receivables		59	(36)
Inventories		(17)	(8)
Accounts and wages payable		(24)	(17)
Taxes accrued		(22)	5
Regulatory assets and liabilities		45	75
Assets, other		(9)	11
Liabilities, other		(2)	6
Pension and other postretirement benefits		(19)	(14)
Net cash provided by operating activities		628	627
Cash Flows From Investing Activities:
Capital expenditures		(760)	(683)
Other		6	4
Net cash used in investing activities		(754)	(679)
Cash Flows From Financing Activities:
Dividends on common stock		0	(95)
Dividends on preferred stock		(2)	(2)
Short-term debt, net		118	157
Money pool borrowings, net		11	54
Maturities of Long-term Debt		0	(129)
Other		(1)	(1)
Net cash used in financing activities		126	(16)
Net change in cash and cash equivalents		0	(68)
Cash and cash equivalents at beginning of period		0	71
Cash and cash equivalents at end of period		$ 0	$ 3
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Summary Of Significant Accounti

Summary Of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES General Ameren, headquartered in St. Louis, Missouri, is a public utility holding company whose primary assets are its equity interests in its subsidiaries. Ameren’s subsidiaries are separate, independent legal entities with separate businesses, assets, and liabilities. Dividends on Ameren’s common stock and the payment of expenses by Ameren depend on distributions made to it by its subsidiaries. Ameren’s principal subsidiaries, Ameren Missouri, Ameren Illinois, and ATXI, are described below. Ameren also has other subsidiaries that conduct other activities, such as the provision of shared services. Ameren evaluates competitive electric transmission investment opportunities outside of MISO 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, Spoon River, and Mark Twain projects. 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. Also see the Glossary of Terms and Abbreviations at the front of this report and in the Form 10-K. As of September 30, 2017 and December 31, 2016, Ameren had unconsolidated variable interests as a limited partner in various equity method investments, totaling $14 million and $9 million , respectively, included in “Other assets” on Ameren’s consolidated balance sheet. Ameren is not the primary beneficiary of these investments because it does not have the power to direct matters that most significantly impact the activities of these variable interest entities. As of September 30, 2017, the maximum exposure to loss related to these variable interests is limited to the investment in these partnerships of $14 million plus associated outstanding funding commitments of $23 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 statement 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. See Note 2 – Rate and Regulatory Matters for information regarding the 2017 change in Ameren Illinois' method used to recognize interim period revenue in connection with the revenue decoupling provisions of the FEJA. These financial statements should be read in conjunction with the financial statements and the notes thereto included in the Form 10-K. Discontinued operations were immaterial to all periods presented in Ameren’s financial statements. As such, the “Assets of discontinued operations” and “Liabilities of discontinued operations” included on the December 31, 2016 balance sheet have been reclassified in this report to “Other current assets” and “Other current liabilities,” respectively. See Note 1 – Summary of Significant Accounting Policies under Part II, Item 8, of the Form 10-K for additional information. Asset Retirement Obligations The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the nine months ended September 30, 2017 : Ameren Missouri Ameren Illinois (a) Ameren Balance at December 31, 2016 $ 644 (b) $ 6 $ 650 (b) Liabilities settled (4 ) (1 ) (5 ) Accretion (c) 20 (d) 20 Change in estimates (e) (18 ) (1 ) (19 ) Balance at September 30, 2017 $ 642 (b) $ 4 $ 646 (b) (a) Included in “Other deferred credits and liabilities” on the balance sheet. (b) Balance included $15 million in “Other current liabilities” on the balance sheet as of both December 31, 2016 and September 30, 2017, respectively. (c) Accretion expense was recorded as a decrease to regulatory liabilities. (d) Less than $1 million. (e) Ameren Missouri changed its fair value estimate primarily due to an extension of the remediation period of certain CCR storage facilities, an update to the decommissioning of the Callaway energy center to reflect the cost study and funding analysis filed with the MoPSC in 2017, and an increase in the discount rate assumption. Share-based Compensation A summary of nonvested performance share units at September 30, 2017 , and changes during the nine months ended September 30, 2017 , under the 2014 Incentive Plan are presented below: Performance Share Units Share Units Weighted-average Fair Value per Share Unit Nonvested at January 1, 2017 1,059,639 $ 48.04 Granted (a) 500,943 59.16 Forfeitures (48,661 ) 52.54 Vested (b) (27,446 ) 52.88 Nonvested at September 30, 2017 1,484,475 $ 51.55 (a) Performance share units granted to certain executive and nonexecutive officers and other eligible employees under the 2014 Incentive Plan. (b) Performance share units vested due to the attainment of retirement eligibility by certain employees. Actual shares issued for retirement-eligible employees vary depending on actual performance over the three -year measurement period. The fair value of each performance share unit awarded in 2017 under the 2014 Incentive Plan was determined to be $59.16 , which was based on Ameren’s closing common share price of $52.46 at December 31, 2016 , 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, 2017, 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 1.47% , volatility of 15% to 21% for the peer group, and Ameren’s attainment of a three-year average earnings per share threshold during the performance period. Operating Revenue The Ameren Companies record operating revenue for electric or natural gas service when it is delivered to customers. We accrue an estimate of electric and natural gas revenues for service rendered but unbilled at the end of each accounting period. For certain regulatory recovery mechanisms qualifying as alternative revenue programs, such as revenue requirement reconciliations, the Ameren Companies 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. Excise Taxes Ameren Missouri and Ameren Illinois collect certain excise taxes from customers 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 and are recorded gross in “Operating Revenues – Electric,” “Operating Revenues – Natural gas” and “Operating Expenses – Taxes other than income taxes” on the statement of income or the statement of income and comprehensive income. Excise taxes for electric service in Illinois are levied on the customer and therefore are not included in Ameren Illinois’ revenues and expenses. The following table presents excise taxes recorded in “Operating Revenues – Electric,” “Operating Revenues – Natural gas” and “Operating Expenses – Taxes other than income taxes” for the three and nine months ended September 30, 2017 and 2016 : Three Months Nine Months 2017 2016 2017 2016 Ameren Missouri $ 51 $ 52 $ 122 $ 122 Ameren Illinois 10 9 40 40 Ameren $ 61 $ 61 $ 162 $ 162 Earnings Per Share Basic earnings per share is computed by dividing “Net Income Attributable to Ameren Common Shareholders” by the weighted-average number of common shares outstanding during the period. Earnings per diluted share is computed by dividing “Net Income Attributable to Ameren Common Shareholders” by the weighted-average number of diluted common shares outstanding during the period. Earnings per diluted share reflects the dilution that would occur if certain stock-based performance share units were settled. The number of performance share units assumed to be settled was 2.1 million and 1.4 million in the three and nine months ended September 30, 2017 , respectively, and 0.3 million and 0.4 million , respectively, in the year-ago periods. There were no potentially dilutive securities excluded from the earnings per diluted share calculations for the three and nine months ended September 30, 2017 and 2016 . Income Taxes In July 2017, Illinois enacted a law that increased the state's corporate income tax rate from 7.75% to 9.5% as of July 1, 2017. The law made the increase in the state’s corporate income tax rate, which was previously scheduled to decrease to 7.3% in 2025, permanent. In July 2017, Ameren recorded an expense of $14 million at Ameren (parent) due to the revaluation of accumulated deferred taxes and the estimated state apportionment of such taxes. Beyond this expense, Ameren does not expect this tax increase to have a material impact on its consolidated net income prospectively. The tax increase is not expected to materially impact the earnings of the Ameren Illinois Electric Distribution, the Ameren Transmission, or the Ameren Illinois Transmission segments, since these businesses operate under formula ratemaking frameworks. The tax increase is expected to unfavorably affect 2017 net income of the Ameren Illinois Natural Gas segment by less than $1 million. In addition, in the third quarter of 2017, Ameren’s and Ameren Illinois’ accumulated deferred tax balances were revalued using the state’s new corporate income tax rate, which resulted in a net increase to the liability balances of $97 million and $79 million, respectively. These increased liabilities were offset by a regulatory asset, as well as income tax expense, as discussed above. Accounting and Reporting Developments Below is a summary of updates related to our adoption of recently issued authoritative accounting standards. 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, financial instruments, and restricted cash. Revenue from Contracts with Customers In May 2014, the FASB issued authoritative guidance that changes the criteria for recognizing revenue from a contract with a customer. The underlying principle of the guidance is that an entity will recognize revenue for the transfer of promised goods or services to customers at an amount that the entity expects to be entitled to in exchange for those goods or services. The guidance requires additional disclosures to enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers, as well as separate presentation of alternative revenue programs on the income statement. Entities can apply the guidance to each reporting period presented (the full retrospective method) or by recording a cumulative effect adjustment to retained earnings in the period of initial adoption (the modified retrospective method). We have substantially completed the evaluation of our contracts and do not expect material changes to the amount or timing of revenue recognition. We will finalize our contract assessments by the end of 2017. We will apply the guidance using the full retrospective method and include disaggregated revenue disclosures by segment and customer class in the combined notes to the financial statements in the first quarter of 2018. Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost In March 2017, the FASB issued authoritative guidance that requires an entity to retrospectively report the service cost component of net benefit cost in the same line item(s) as other compensation costs arising from services rendered by employees during the period and to present the other components of net benefit cost in the income statement separately from the service cost component and outside of operating income. The guidance also requires that an entity only capitalize 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 will not affect our ability to continue to obtain recovery of net benefit costs through customer rates. See Note 11 – Retirement Benefits for the components of net benefit cost. This guidance will be effective for the Ameren Companies in the first quarter of 2018. We are currently assessing the impacts of this guidance on our results of operations, financial position, and disclosures.

Rate And Regulatory Matters

Rate And Regulatory Matters	9 Months Ended
	Sep. 30, 2017
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 March 2017 Electric Rate Order In March 2017, the MoPSC issued an order approving a unanimous stipulation and agreement in Ameren Missouri’s July 2016 regulatory rate review. The order resulted in a $3.4 billion revenue requirement, which is a $92 million increase in Ameren Missouri’s annual revenue requirement for electric service, compared to its prior revenue requirement established in the MoPSC's April 2015 electric rate order. The new rates, base level of expenses, and amortizations became effective on April 1, 2017. The order authorized the continued use of the FAC and the regulatory tracking mechanisms for pension and postretirement benefits, uncertain income tax positions, and renewable energy standards that the MoPSC authorized in earlier electric rate orders. These regulatory tracking mechanisms provide for a base level of expense to be reflected in Ameren Missouri’s base electric rates with differences in the actual expenses incurred recorded as a regulatory asset or liability. Excluding cost reductions associated with reduced sales volumes, the base level of net energy costs decreased by $54 million from the base level established in the MoPSC's April 2015 electric rate order. Changes in amortizations and the base level of expenses for the other regulatory tracking mechanisms, including extending the amortization period of certain regulatory assets, reduced expenses by $26 million from the base levels established in the MoPSC's April 2015 electric rate order. ATXI’s Mark Twain Project The Mark Twain project is a MISO-approved transmission line to be located in northeast Missouri. I n the third quarter of 2017, ATXI finalized an alternative project route and reached agreements with a cooperative electric company in northeast Missouri and Ameren Missouri to locate nearly all of the Mark Twain project on existing transmission line corridors. It also received assents for road crossings from the five affected counties in northeast Missouri. ATXI had previously filed suit in the circuit courts to obtain assents for the original project route. ATXI has since withdrawn one of the lawsuits. The other lawsuits remain pending but have been stayed until the first quarter of 2018. In September 2017, ATXI filed for a certificate of convenience and necessity with the MoPSC and anticipates a decision from the MoPSC in the first half of 2018. ATXI plans to complete the project in December 2019; however, delays in obtaining approval from the MoPSC could delay completion. Illinois IEIMA & FEJA Ameren Illinois’ electric distribution service rates are subject to an annual revenue requirement reconciliation to its actual recoverable costs and allowed return on equity under a formula ratemaking process effective through 2022. This 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 based on that year's actual recoverable costs incurred and investment return. As of September 30, 2017 , Ameren Illinois had recorded regulatory assets of $24 million to reflect its 2016 revenue requirement reconciliation adjustment, which was included in the April 2017 formula rate update discussed below, and $16 million for the approved 2015 revenue requirement reconciliation adjustment, each with interest. As of September 30, 2017 , Ameren Illinois had recorded a regulatory liability of $1 million to reflect the difference between Ameren Illinois’ estimate of its 2017 revenue requirement and the revenue requirement reflected in customer rates, including interest. In April 2017, Ameren Illinois filed with the ICC its annual electric distribution service formula rate update to establish the revenue requirement used for 2018 rates. In June 2017, the ICC staff submitted its calculation of the revenue requirement, which Ameren Illinois supported in its revised July 2017 filing, and recommended a decrease to the electric distribution service revenue requirement. Pending ICC approval, this update filing will result in a $17 million decrease in Ameren Illinois’ electric distribution service revenue requirement beginning in January 2018. This update reflects an increase to the annual formula rate based on 2016 actual costs and expected net plant additions for 2017, as well as an increase to include the 2016 revenue requirement reconciliation adjustment. The increases in the update filing are more than offset by a decrease for the conclusion of the 2015 revenue requirement reconciliation adjustment, which will be fully collected from customers in 2017 , consistent with the ICC’s December 2016 annual update filing order. In November 2017, an administrative law judge issued a proposed order that was consistent with Ameren Illinois’ revised July 2017 filing. An ICC decision regarding the revenue requirement to be used for customer rates in 2018 is expected by December 2017. The FEJA revised certain portions of the IEIMA, including extending the IEIMA formula ratemaking process through 2022 and clarifying that a common equity ratio of up to, and including, 50% is prudent. Beginning in 2017, the FEJA provides that Ameren Illinois will recover, within the following two years, its electric distribution revenue requirement for a given year, independent of actual sales volumes. Prior to the FEJA, Ameren Illinois’ interim period revenue recognition was volume-based, as revenues were affected by the timing of sales volumes due to seasonal rates and changes in volumes resulting from, among other things, weather and energy efficiency. This previous revenue recognition method resulted in more revenues during the third quarter and less revenues during the other quarters of each year. Beginning in 2017, in connection with the decoupling provisions of the FEJA, Ameren Illinois changed its method used to recognize interim period revenue. Ameren Illinois now recognizes revenue consistent with the timing of actual incurred electric distribution recoverable costs and recognizes revenue associated with the expected return on its rate base ratably over the year. Ameren Illinois recognized a reduction to electric revenue to reflect the difference between the estimate of its revenue requirement and the revenue requirement reflected in customer rates of $76 million and $1 million for the three and nine months ended September 30, 2017 , respectively. Comparative electric revenues at Ameren Illinois for the three and nine months ended September 30, 2016, were increased $11 million and $24 million , respectively, for the difference between the estimate of its revenue requirement and the revenue requirement reflected in customer rates. In June 2017, pursuant to the FEJA, Ameren Illinois filed with the ICC an energy efficiency plan for 2018 through 2021. In September 2017, the ICC issued an order approving Ameren Illinois' implementation of FEJA electric energy efficiency savings targets and investments. Ameren Illinois plans to invest up to $99 million in electric energy efficiency programs per year from 2018 through 2021 that will earn a return. The electric energy efficiency program investments and the return on those investments will be collected from customers through a rider and will not be included in the IEIMA formula ratemaking process. ATXI’s Illinois Rivers Project In August 2017, the Illinois Circuit Court for Edgar County dismissed several of ATXI’s condemnation cases related to one segment in the Illinois Rivers project, which has an estimated segment cost of approximately $85 million , of which $32 million was invested as of September 30, 2017 . These cases had been filed in order to obtain necessary easements and rights of way to complete the segment. The court found that required notice was not given to the relevant landowners during the underlying ICC proceeding. ATXI intends to appeal this decision. ATXI plans to complete the project in 2019; however, delays associated with the condemnation proceedings or an appeal arising from the order dismissing the Edgar County cases could delay the completion date. The other eight segments of the Illinois Rivers project are not affected by these proceedings. Federal FERC Complaint Cases In November 2013, a customer group filed a complaint case with the FERC seeking a reduction in the allowed base return on common equity for FERC-regulated transmission rate base under the MISO tariff from 12.38% to 9.15% . In September 2016, the FERC issued a final order in the November 2013 complaint case, which lowered the allowed base return on common equity for the 15-month period of November 2013 to February 2015 to 10.32%, or a 10.82% total allowed return on common equity with the inclusion of a 50 basis point incentive adder for participation in an RTO. The order required customer refunds, with interest, to be issued for that 15-month period. In the first six months of 2017, Ameren and Ameren Illinois refunded $21 million and $17 million , respectively, related to the November 2013 complaint case. In addition, the 10.82% allowed return on common equity has been reflected in rates since September 2016. The 10.82% allowed return on common equity may be replaced prospectively after the FERC issues a final order in the February 2015 complaint case, discussed below. Since the maximum FERC-allowed refund period for the November 2013 complaint case ended in February 2015, another customer complaint case was filed in February 2015. MISO transmission owners have since filed a motion to dismiss the February 2015 complaint. See below for additional information about the motion. 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, which, if approved by the FERC, 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, and require customer refunds, with interest, for that 15-month period. The timing of the issuance of the final order in the February 2015 complaint case is uncertain for two reasons. First, while the FERC reestablished a quorum of commissioners in August 2017 after six months without a quorum, the FERC is under no deadline to issue a final order. Second, in the second quarter of 2017, the United States Court of Appeals for the District of Columbia Circuit vacated and remanded to the FERC an order in a separate case in which the FERC established the allowed base return on common equity methodology used in the two MISO complaint cases described above. Ameren is unable to predict the impact of the outcome of the United States Court of Appeals for the District of Columbia Circuit’s remand on the MISO FERC complaint cases at this time. In September 2017, MISO transmission owners, including Ameren Missouri, Ameren Illinois, and ATXI, filed a motion to dismiss the February 2015 complaint case with the FERC. The MISO transmission owners maintain that the February 2015 complaint was predicated on the now superseded 12.38% allowed base return on common equity being an unjust and unreasonable return and is not applicable given the currently effective 10.32% allowed base return on common equity. The MISO transmission owners further maintain that the currently effective 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 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 is insufficient. That same approach was rejected by the United States Court of Appeals for the District of Columbia Circuit, as discussed above. FERC is under no deadline to issue an order on this motion. As of September 30, 2017 , Ameren and Ameren Illinois had recorded current regulatory liabilities of $41 million and $24 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.

Short-Term Debt And Liquidity

Short-Term Debt And Liquidity	9 Months Ended
	Sep. 30, 2017
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 intercompany 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 nine months ended September 30, 2017 , but were used to support commercial paper issuances and to issue letters of credit. Based on commercial paper outstanding, as well as letters of credit issued under the Credit Agreements, the aggregate amount of credit capacity available under the Credit Agreements to Ameren (parent), Ameren Missouri, and Ameren Illinois, collectively, at September 30, 2017 , was $1.7 billion . The Ameren Companies were in compliance with the covenants in their Credit Agreements as of September 30, 2017. As of September 30, 2017 , the ratios of consolidated indebtedness to consolidated total capitalization, calculated in accordance with the provisions of the Credit Agreements, were 51% , 47% , and 46% for Ameren, Ameren Missouri, and Ameren Illinois, respectively. Commercial Paper The following table presents commercial paper outstanding as of September 30, 2017 , and December 31, 2016 : 2017 2016 Ameren (parent) $ 277 $ 507 Ameren Missouri — — Ameren Illinois 169 51 Ameren Consolidated $ 446 $ 558 The following table summarizes the borrowing activity and relevant interest rates under Ameren’s (parent), Ameren Missouri’s, and Ameren Illinois’ commercial paper programs for the nine months ended September 30, 2017 and 2016: Ameren (parent) Ameren Missouri Ameren Illinois Ameren Consolidated 2017 Average daily commercial paper outstanding $ 669 $ 7 $ 78 $ 754 Weighted-average interest rate 1.27 % 1.20 % 1.28 % 1.27 % Peak commercial paper during period (a) $ 841 $ 64 $ 193 $ 948 Peak interest rate 1.50 % 1.41 % 1.50 % 1.50 % 2016 Average daily commercial paper outstanding $ 435 $ 80 $ 48 $ 563 Weighted-average interest rate 0.81 % 0.74 % 0.72 % 0.79 % Peak commercial paper during period (a) $ 574 $ 208 $ 195 $ 839 Peak interest rate 0.95 % 0.85 % 0.85 % 0.95 % (a) The timing of peak commercial paper issuances varies by company. Therefore, the sum of peak commercial paper issuances presented by company does 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 short-term cash and working capital requirements. The average interest rate for borrowing under the utility money pool for the three and nine months ended September 30, 2017 , was 1.24% and 1.18% , respectively (2016 – 0.53% and 0.54% , respectively). See Note 8 – Related Party Transactions for the amount of interest income and expense from the money pool arrangements recorded by the Ameren Companies for the three and nine months ended September 30, 2017 and 2016 .

Long-Term Debt And Equity Finan

Long-Term Debt And Equity Financings	9 Months Ended
	Sep. 30, 2017
Long-Term Debt And Equity Financings [Abstract]
LONG-TERM DEBT AND EQUITY FINANCINGS	LONG-TERM DEBT AND EQUITY FINANCINGS Ameren Missouri In June 2017, Ameren Missouri issued $400 million principal amount of 2.95% senior secured notes, due June 2027, with interest payable semiannually on June 15 and December 15 of each year, beginning in December 2017. Ameren Missouri received proceeds of $396 million , which were used, in conjunction with other available funds, to repay at maturity in June 2017 $425 million principal amount of its 6.40% senior secured notes. ATXI In June 2017, pursuant to a note purchase agreement, ATXI agreed to issue $450 million principal amount of 3.43% senior unsecured notes, due 2050, through a private placement offering exempt from registration under the Securities Act of 1933, as amended. ATXI issued $150 million principal amount of the notes in June 2017 and the remaining $300 million principal amount of the notes in August 2017. ATXI received proceeds of $449 million from the notes, which were used by ATXI to repay existing short-term and long-term affiliate debt owed to Ameren (parent). ATXI may prepay at any time not less than 5% of the principal amount of notes then outstanding at 100% of the principal amount plus a make-whole premium. In the event of a change of control, as defined in the agreement, each holder of notes may require ATXI to prepay the entire unpaid principal amount of the notes held by such holder at a price equal to 100% of the principal amount of such notes together with accrued and unpaid interest thereon. The following table presents the principal maturities schedule for the notes: Payment Date Principal Payment August 2022 $ 49.5 August 2024 49.5 August 2027 49.5 August 2030 49.5 August 2032 49.5 August 2038 49.5 August 2043 76.5 August 2050 76.5 Total Principal Amount of Notes $ 450.0 The note purchase agreement includes financial covenants that require ATXI to not permit at any time: (i) debt to exceed 70% of total capitalization or (ii) secured debt to exceed 10% of total assets. The note purchase agreement also contains restrictive covenants that, among other things, restrict the ability of ATXI to: (i) enter into transactions with affiliates; (ii) consolidate, merge, transfer or lease all or substantially all of its assets; and (iii) create liens. Indenture Provisions and Other Covenants Ameren Missouri’s and Ameren Illinois’ indentures and articles of incorporation include covenants and provisions related to issuances of first mortgage bonds and preferred stock. Ameren Missouri and Ameren Illinois are required to meet certain ratios to issue additional first mortgage bonds and preferred stock. A failure to achieve these ratios would not result in a default under these covenants and provisions, but would restrict the companies’ ability to issue first mortgage bonds or preferred stock. 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. See the discussion above for covenants related to ATXI’s note purchase agreement. At September 30, 2017 , the Ameren Companies were in compliance with the provisions and covenants contained in their indentures and articles of incorporation, as applicable, and ATXI was in compliance with the provisions and covenants contained in its note purchase agreement. Off-Balance-Sheet Arrangements At September 30, 2017 , none of the Ameren Companies had off-balance-sheet financing arrangements, other than operating leases entered into in the ordinary course of business, letters of credit, and Ameren parent guarantee arrangements on behalf of its subsidiaries.

Other Income and Expenses

Other Income and Expenses	9 Months Ended
	Sep. 30, 2017
Other Nonoperating Income (Expense) [Abstract]
OTHER INCOME AND EXPENSES	OTHER INCOME AND EXPENSES The following table presents the components of “Other Income and Expenses” in the Ameren Companies’ statements of income for the three and nine months ended September 30, 2017 and 2016 : Three Months Nine Months 2017 2016 2017 2016 Ameren: (a) Miscellaneous income: Allowance for equity funds used during construction $ 6 $ 7 $ 16 $ 20 Interest income on industrial development revenue bonds 7 7 20 20 Interest income — 3 5 11 Other — 1 1 3 Total miscellaneous income $ 13 $ 18 $ 42 $ 54 Miscellaneous expense: Donations $ — $ 1 $ 7 $ 8 Other 2 7 9 13 Total miscellaneous expense $ 2 $ 8 $ 16 $ 21 Ameren Missouri: Miscellaneous income: Allowance for equity funds used during construction $ 6 $ 6 $ 15 $ 16 Interest income on industrial development revenue bonds 7 7 20 20 Interest income — 1 — 1 Other — — 1 1 Total miscellaneous income $ 13 $ 14 $ 36 $ 38 Miscellaneous expense: Donations $ — $ — $ 2 $ 2 Other 2 2 4 4 Total miscellaneous expense $ 2 $ 2 $ 6 $ 6 Ameren Illinois: Miscellaneous income: Allowance for equity funds used during construction $ — $ 1 $ 1 $ 4 Interest income 1 2 5 9 Other — 1 1 2 Total miscellaneous income $ 1 $ 4 $ 7 $ 15 Miscellaneous expense: Donations $ — $ 1 $ 5 $ 6 Other — 2 3 5 Total miscellaneous expense $ — $ 3 $ 8 $ 11 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Derivative Financial Instrument

Derivative Financial Instruments	9 Months Ended
	Sep. 30, 2017
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, power, and uranium, as well as the risk of changes in rail transportation surcharges through fuel oil hedges. Such price fluctuations may cause the following: • an unrealized appreciation or depreciation of our contracted commitments to purchase or sell when purchase or sale prices under the commitments are compared with current commodity prices; • market values of natural gas and uranium inventories that differ from the cost of those commodities in inventory; and • actual cash outlays for the purchase of these commodities that differ from anticipated cash outlays. The derivatives that we use to hedge these risks are governed by our risk management policies for forward contracts, futures, options, and swaps. Our net positions are continually assessed within our structured hedging programs to determine whether new or offsetting transactions are required. The goal of the hedging program is generally to mitigate financial risks while ensuring that sufficient volumes are available to meet our requirements. Contracts we enter into as part of our risk management program may be settled financially, settled by physical delivery, or net settled with the counterparty. The following table presents open gross commodity contract volumes by commodity type for derivative assets and liabilities as of September 30, 2017 , and December 31, 2016 . As of September 30, 2017 , these contracts extended through October 2019, March 2023, May 2032, and March 2020 for fuel oils, natural gas, power, and uranium, respectively. Quantity (in millions, except as indicated) 2017 2016 Commodity Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils (in gallons) (a) 30 (b) 30 30 (b) 30 Natural gas (in mmbtu) 24 145 169 25 129 154 Power (in megawatthours) 2 9 11 1 9 10 Uranium (pounds in thousands) 370 (b) 370 345 (b) 345 (a) Consists of ultra-low-sulfur diesel products. (b) Not applicable. All contracts considered to be derivative instruments are required to be recorded on the balance sheet at their fair values, unless the NPNS exception applies. See Note 7 – Fair Value Measurements for a discussion of our methods of assessing the fair value of derivative instruments. Many of our physical contracts, such as our purchased power contracts, qualify for the NPNS exception to derivative accounting rules. The revenue or expense on NPNS contracts is recognized at the contract price upon physical delivery. If we determine that a contract meets the definition of a derivative and is not eligible for the NPNS exception, we review the contract to determine whether the resulting gains or losses qualify for regulatory deferral. Derivative contracts that qualify for regulatory deferral are recorded at fair value, with changes in fair value recorded as regulatory assets or liabilities in the period in which the change occurs. We believe derivative losses and gains deferred as regulatory assets and liabilities are probable of recovery, or refund, through future rates charged to customers. Regulatory assets and liabilities are amortized to operating income as related losses and gains are reflected in rates charged to customers. Therefore, gains and losses on these derivatives have no effect on operating income. As of September 30, 2017 , and December 31, 2016 , all contracts that met the definition of a derivative and were not eligible for the NPNS exception received regulatory deferral. The following table presents the carrying value and balance sheet location of all derivative commodity contracts, none of which were designated as hedging instruments, as of September 30, 2017 , and December 31, 2016 : Balance Sheet Location Ameren Missouri Ameren Illinois Ameren 2017 Fuel oils Other current assets $ 2 $ — $ 2 Other assets 1 — 1 Natural gas Other current assets — 1 1 Other assets — 1 1 Power Other current assets 10 — 10 Other assets 1 — 1 Total assets (a) $ 14 $ 2 $ 16 Fuel oils Other current liabilities $ 2 $ — $ 2 Natural gas Other current liabilities 3 8 11 Other deferred credits and liabilities 4 6 10 Power Other current liabilities 1 13 14 Other deferred credits and liabilities — 179 179 Uranium Other deferred credits and liabilities — (b) — — (b) Total liabilities (c) $ 10 $ 206 $ 216 2016 Fuel oils Other current assets $ 2 $ — $ 2 Other assets 1 — 1 Natural gas Other current assets 1 11 12 Other assets 1 2 3 Power Other current assets 9 — 9 Total assets (a) $ 14 $ 13 $ 27 Fuel oils Other current liabilities $ 5 $ — $ 5 Natural gas Other current liabilities 1 3 4 Other deferred credits and liabilities 5 5 10 Power Other current liabilities 3 12 15 Other deferred credits and liabilities — 173 173 Uranium Other deferred credits and liabilities 4 — 4 Total liabilities (c) $ 18 $ 193 $ 211 (a) The cumulative amount of pretax net gains on all derivative instruments is deferred as a regulatory liability. (b) Beginning in 2017, as a result of rulebook amendments at the Chicago Mercantile Exchange, the fair value of uranium derivative liabilities are offset by certain settlement payments made to the exchange previously characterized as collateral and included within “Other assets” on Ameren’s and Ameren Missouri’s balance sheet. (c) The cumulative amount of pretax net losses on all derivative instruments is deferred as a regulatory asset. Derivative instruments are subject to various credit-related losses in the event of nonperformance by counterparties to the transaction. Exchange-traded contracts are supported by the financial and credit quality of the clearing members of the respective exchanges; these contracts have nominal credit risk. In all other transactions, we are exposed to credit risk. Our credit risk management program involves establishing credit limits and collateral requirements for counterparties, using master netting arrangements or similar agreements, and reporting daily exposure to senior management. We believe that entering into master netting arrangements or similar agreements mitigates the level of financial loss that could result from default by allowing net settlement of derivative assets and liabilities. These master netting arrangements allow the counterparties to net settle sale and purchase transactions. Further, collateral requirements are calculated at the master netting arrangement or similar agreement level by counterparty. The Ameren Companies elect to present the fair value amounts of derivative assets and derivative liabilities subject to an enforceable master netting arrangement or similar agreement gross on the balance sheet. However, if the gross amounts recognized on the balance sheet were netted with derivative instruments and cash collateral received or posted, the net amounts would not be materially different from the gross amounts at September 30, 2017 , and December 31, 2016 . Concentrations of Credit Risk In determining our concentrations of credit risk related to derivative instruments, we review our individual counterparties and categorize each counterparty into groupings according to the primary business in which each engages. We calculate maximum exposures based on the gross fair value of financial instruments, including NPNS and other accrual contracts. These exposures are calculated on a gross basis, which include affiliate exposure not eliminated at the consolidated Ameren level. As of September 30, 2017 , if counterparty groups were to fail completely to perform on contracts, the Ameren Companies’ maximum exposure would have been immaterial with or without consideration of the application of master netting arrangements or similar agreements and collateral held. Derivative Instruments with Credit Risk-Related Contingent Features Our commodity contracts contain collateral provisions tied to the Ameren Companies’ credit ratings. If our credit ratings were downgraded, or if a counterparty with reasonable grounds for uncertainty regarding our ability to satisfy an obligation requested adequate assurance of performance, additional collateral postings might be required. The following table presents, as of September 30, 2017 , the aggregate fair value of all derivative instruments with credit risk-related contingent features in a gross liability position, the cash collateral posted, and the aggregate amount of additional collateral that counterparties could require. The additional collateral required is the net liability position allowed under the master netting arrangements or similar agreements, assuming (1) the credit risk-related contingent features underlying these arrangements were triggered on September 30, 2017 , and (2) those counterparties with rights to do so requested collateral. Aggregate Fair Value of Derivative Liabilities (a) Cash Collateral Posted Potential Aggregate Amount of Additional Collateral Required (b) 2017 Ameren Missouri $ 59 $ 3 $ 48 Ameren Illinois 48 — 41 Ameren $ 107 $ 3 $ 89 (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	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
FAIR VALUE MEASUREMENTS	FAIR VALUE MEASUREMENTS Fair value is defined as the price that would be received for an asset or paid to transfer a liability (an exit price) in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. We use various methods to determine fair value, including market, income, and cost approaches. With these approaches, we adopt certain assumptions that market participants would use in pricing the asset or liability, including assumptions about market risk or the risks inherent in the inputs to the valuation. Inputs to valuation can be readily observable, market-corroborated, or unobservable. We use valuation techniques that maximize the use of observable inputs and minimize the use of unobservable inputs. Authoritative accounting guidance established a fair value hierarchy that prioritizes the inputs used to measure fair value. All financial assets and liabilities carried at fair value are classified and disclosed in one of 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. The following table describes the valuation techniques and unobservable inputs utilized by the Ameren Companies for the fair value of financial assets and liabilities classified as Level 3 in the fair value hierarchy for the periods ended September 30, 2017 and December 31, 2016 : Fair Value Weighted Average Assets Liabilities Valuation Technique(s) Unobservable Input Range Level 3 Derivative asset and liability – commodity contracts (a) : 2017 Fuel oils $ 1 $ (1 ) Option model Volatilities(%) (b) 24 – 32 26 Discounted cash flow Counterparty credit risk(%) (c)(d) 0.12 – 0.22 0.17 Ameren Missouri credit risk(%) (c)(d) 0.37 (e) Natural gas — (2 ) Discounted cash flow Nodal basis ($/mmbtu) (b) (1.10) – (0.10) (0.80) Counterparty credit risk (%) (c)(d) 0.34 – 6 0.73 Ameren Illinois credit risk (%) (c)(d) 0.37 (e) Power (g) $ 11 $ (193 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps ($/MWh) (h) 25 – 41 28 Estimated auction price for FTRs ($/MW) (b) (324) – 1,194 269 Nodal basis ($/MWh) (h) (3) – 0 (2) Fair Value Weighted Average Assets Liabilities Valuation Technique(s) Unobservable Input Range Counterparty credit risk (%) (c)(d) 0.28 (e) Ameren Illinois credit risk (%) (c)(d) 0.37 (e) Fundamental energy production model Estimated future natural gas prices ($/mmbtu) (b) 3 – 4 3 Escalation rate (%) (b)(i) 3 (e) Contract price allocation Estimated renewable energy credit costs ($/credit) (b) 5 – 7 6 2016 Fuel oils $ 1 $ — Option model Volatilities (%) (b) 24 – 66 28 Discounted cash flow Counterparty credit risk (%) (c)(d) 0.13 – 0.22 0.15 Ameren Missouri credit risk (%) (c)(d) 0.38 (e) Escalation rate (%) (b)(f) (2) – 2 0 Natural gas 1 (1 ) Option model Volatilities (%) (b) 31 – 66 36 Nodal basis ($/mmbtu) (b) (0.40) – (0.10) (0.20) Discounted cash flow Nodal basis ($/mmbtu) (b) (0.80) – 0 (0.50) Counterparty credit risk (%) (c)(d) 0.13 – 8 1 Ameren Illinois credit risk (%) (c)(d) 0.38 (e) Power (g) 9 (187 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps ($/MWh) (h) 26 – 44 29 Estimated auction price for FTRs ($/MW) (b) (71) – 5,270 125 Nodal basis ($/MWh) (h) (6) – 0 (2) Ameren Illinois credit risk (%) (c)(d) 0.38 (e) Fundamental energy production model Estimated future natural gas prices ($/mmbtu) (b) 3 – 4 3 Escalation rate (%) (b)(i) 5 (e) Contract price allocation Estimated renewable energy credit costs ($/credit) (b) 5 – 7 6 Uranium — (4 ) Option model Volatilities (%) (b) 24 (e) Discounted cash flow Average forward uranium pricing ($/pound) (b) 22 – 24 22 Ameren Missouri credit risk (%) (c)(d) 0.38 (e) (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement. (c) Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement. (d) Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances. (e) Not applicable. (f) Escalation rate applies to fuel oil prices 2019 and beyond. (g) Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2021 for September 30, 2017, and through 2020 for December 31, 2016. Valuations beyond 2021 for September 30, 2017, and 2020 for December 31, 2016 use fundamentally modeled pricing by month for peak and off-peak demand. (h) The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes due to their opposing positions. (i) Escalation rate applies to power prices in 2031 and beyond. We consider nonperformance risk in our valuation of derivative instruments by analyzing the credit standing of our counterparties and considering any counterparty credit enhancements (e.g., collateral). The guidance also requires that the fair value measurement of liabilities reflect the nonperformance risk of the reporting entity, as applicable. Therefore, we have factored the impact of our credit standing, as well as any potential credit enhancements, into the fair value measurement of both derivative assets and derivative liabilities. Included in our valuation, and based on current market conditions, is a valuation adjustment for counterparty default derived from market data such as the price of credit default swaps, bond yields, and credit ratings. No gains or losses related to valuation adjustments for counterparty default risk were recorded at Ameren, Ameren Missouri, or Ameren Illinois in the three and nine months ended September 30, 2017 or 2016 . At September 30, 2017 , and December 31, 2016 , the counterparty default risk valuation adjustment related to derivative contracts was immaterial for Ameren, Ameren Missouri, and Ameren Illinois. The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of September 30, 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 $ 2 $ — $ 1 $ 3 Natural gas 1 1 — 2 Power — — 11 11 Total derivative assets – commodity contracts $ 3 $ 1 $ 12 $ 16 Nuclear decommissioning trust fund: Cash and cash equivalents $ 2 $ — $ — $ 2 Equity securities: U.S. large capitalization 445 — — 445 Debt securities: U.S. treasury and agency securities — 119 — 119 Corporate bonds — 82 — 82 Other — 24 — 24 Total nuclear decommissioning trust fund $ 447 $ 225 $ — $ 672 Total Ameren $ 450 $ 226 $ 12 $ 688 Ameren Missouri Derivative assets – commodity contracts (a) : Fuel oils $ 2 $ — $ 1 $ 3 Power — — 11 11 Total derivative assets – commodity contracts $ 2 $ — $ 12 $ 14 Nuclear decommissioning trust fund: Cash and cash equivalents $ 2 $ — $ — $ 2 Equity securities: U.S. large capitalization 445 — — 445 Debt securities: U.S. treasury and agency securities — 119 — 119 Corporate bonds — 82 — 82 Other — 24 — 24 Total nuclear decommissioning trust fund $ 447 $ 225 $ — $ 672 Total Ameren Missouri $ 449 $ 225 $ 12 $ 686 Ameren Illinois Derivative assets – commodity contracts (a) : Natural gas $ 1 $ 1 $ — $ 2 Liabilities: Ameren Derivative liabilities – commodity contracts (a) : Fuel oils $ 1 $ — $ 1 $ 2 Natural gas — 19 2 21 Power — — 193 193 Total Ameren $ 1 $ 19 $ 196 $ 216 Ameren Missouri Derivative liabilities – commodity contracts (a) : Fuel oils $ 1 $ — $ 1 $ 2 Natural gas — 7 — 7 Power — — 1 1 Total Ameren Missouri $ 1 $ 7 $ 2 $ 10 Ameren Illinois Derivative liabilities – commodity contracts (a) : Natural gas $ — $ 12 $ 2 $ 14 Power — — 192 192 Total Ameren Illinois $ — $ 12 $ 194 $ 206 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of December 31, 2016 : Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Total Assets: Ameren Derivative assets – commodity contracts (a) : Fuel oils $ 2 $ — $ 1 $ 3 Natural gas 2 12 1 15 Power — — 9 9 Total derivative assets – commodity contracts $ 4 $ 12 $ 11 $ 27 Nuclear decommissioning trust fund: Cash and cash equivalents $ 1 $ — $ — $ 1 Equity securities: U.S. large capitalization 408 — — 408 Debt securities: U.S. treasury and agency securities — 112 — 112 Corporate bonds — 67 — 67 Other — 17 — 17 Total nuclear decommissioning trust fund $ 409 $ 196 $ — $ 605 (b) Total Ameren $ 413 $ 208 $ 11 $ 632 Ameren Missouri Derivative assets – commodity contracts (a) : Fuel oils $ 2 $ — $ 1 $ 3 Natural gas — 1 1 2 Power — — 9 9 Total derivative assets – commodity contracts $ 2 $ 1 $ 11 $ 14 Nuclear decommissioning trust fund: Cash and cash equivalents $ 1 $ — $ — $ 1 Equity securities: U.S. large capitalization 408 — — 408 Debt securities: U.S. treasury and agency securities — 112 — 112 Corporate bonds — 67 — 67 Other — 17 — 17 Total nuclear decommissioning trust fund $ 409 $ 196 $ — $ 605 (b) Total Ameren Missouri $ 411 $ 197 $ 11 $ 619 Ameren Illinois Derivative assets – commodity contracts (a) : Natural gas $ 2 $ 11 $ — $ 13 Liabilities: Ameren Derivative liabilities – commodity contracts (a) : Fuel oils $ 5 $ — $ — $ 5 Natural gas — 13 1 14 Power — 1 187 188 Uranium — — 4 4 Total Ameren $ 5 $ 14 $ 192 $ 211 Ameren Missouri Derivative liabilities – commodity contracts (a) : Fuel oils $ 5 $ — $ — $ 5 Natural gas — 6 — 6 Power — 1 2 3 Uranium — — 4 4 Total Ameren Missouri $ 5 $ 7 $ 6 $ 18 Ameren Illinois Derivative liabilities – commodity contracts (a) : Natural gas $ — $ 7 $ 1 $ 8 Power — — 185 185 Total Ameren Illinois $ — $ 7 $ 186 $ 193 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Balance excludes $2 million of receivables, payables, and accrued income, net. All costs related to financial assets and liabilities classified as Level 3 in the fair value hierarchy are expected to be recoverable through customer rates; therefore, there is no impact to net income resulting from changes in the fair value of these instruments. For the three and nine months ended September 30, 2017 and 2016, the balances and changes in the fair value of Level 3 financial assets and liabilities associated with fuel oils, natural gas, and uranium were immaterial. The following table summarizes the changes in the fair value of power financial assets and liabilities classified as Level 3 in the fair value hierarchy: Net derivative commodity contracts Ameren Missouri Ameren Illinois Ameren For the three months ended September 30, 2017 Beginning balance at July 1, 2017 $ 14 $ (192 ) $ (178 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (2 ) (3 ) (5 ) Sales 1 — 1 Settlements (3 ) 3 — Ending balance at September 30, 2017 $ 10 $ (192 ) $ (182 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2017 $ — $ (2 ) $ (2 ) For the three months ended September 30, 2016 Beginning balance at July 1, 2016 $ 14 $ (169 ) $ (155 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities — (6 ) (6 ) Settlements (5 ) 3 (2 ) Ending balance at September 30, 2016 $ 9 $ (172 ) $ (163 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2016 $ — $ (2 ) $ (2 ) For the nine months ended September 30, 2017 Beginning balance at January 1, 2017 $ 7 $ (185 ) $ (178 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (3 ) (14 ) (17 ) Purchases 15 — 15 Sales 1 — 1 Settlements (10 ) 7 (3 ) Ending balance at September 30, 2017 $ 10 $ (192 ) $ (182 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2017 $ — $ (15 ) $ (15 ) For the nine months ended September 30, 2016 Beginning balance at January 1, 2016 $ 16 $ (170 ) $ (154 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (4 ) (13 ) (17 ) Purchases 13 — 13 Settlements (16 ) 11 (5 ) Ending balance at September 30, 2016 $ 9 $ (172 ) $ (163 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2016 $ — $ (7 ) $ (7 ) Transfers into or out of Level 3 represent either (1) existing assets and liabilities that were previously categorized as a higher level, but were recategorized to Level 3 because the inputs to the model became unobservable during the period or (2) existing assets and liabilities that were previously classified as Level 3, but were recategorized to a higher level because the lowest significant input became observable during the period. For the three and nine months ended September 30, 2017 and 2016 , there were no material transfers between Level 1 and Level 2, Level 1 and Level 3, or Level 2 and Level 3 related to derivative commodity contracts. The Ameren Companies’ carrying amounts of cash and cash equivalents, accounts receivable, unbilled revenue, accounts payable, and other current financial instruments approximate fair value because of the short-term nature of these instruments. They are considered to be Level 1 in the fair value hierarchy. The Ameren Companies' short-term borrowings also approximate fair value because of their short-term nature. Short-term borrowings are considered to be Level 2 in the fair value hierarchy as they are valued based on market rates for similar market transactions. The estimated fair value of long-term debt and preferred stock is based on the quoted market prices for same or similar issuances for companies with similar credit profiles or on the current rates offered to the Ameren Companies for similar financial instruments, which fair value measurement is considered to be Level 2 in the fair value hierarchy. The following table presents the carrying amounts and estimated fair values of our long-term debt, capital lease obligations and preferred stock at September 30, 2017 , and December 31, 2016 : September 30, 2017 December 31, 2016 Carrying Amount Fair Value Carrying Amount Fair Value Ameren: Long-term debt and capital lease obligations (including current portion) $ 7,699 $ 8,234 $ 7,276 $ 7,772 Preferred stock (a) 142 131 142 131 Ameren Missouri: Long-term debt and capital lease obligations (including current portion) $ 3,967 $ 4,312 $ 3,994 $ 4,304 Preferred stock 80 79 80 79 Ameren Illinois: Long-term debt (including current portion) $ 2,590 $ 2,759 $ 2,588 $ 2,765 Preferred stock 62 52 62 52 (a) Preferred stock is recorded in “Noncontrolling Interests” on the consolidated balance sheet.

Related Party Transactions

Related Party Transactions	9 Months Ended
	Sep. 30, 2017
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 power purchases and sales, services received or rendered, and borrowings and lendings. Transactions between affiliates are reported as intercompany transactions on their financial statements but are eliminated in consolidation for Ameren’s financial statements. For a discussion of our material related party agreements, see Note 14 – Related Party Transactions under Part II, Item 8, of the Form 10-K and the money pool arrangements discussed in Note 3 – Short-term Debt and Liquidity of this report. Electric Power Supply Agreement In April 2017, 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 2017, Ameren Missouri and Ameren Illinois entered into an energy product agreement by which Ameren Missouri agreed to sell, and Ameren Illinois agreed to purchase, 85,600 megawatthours at an average price of $34 per megawatthour during the period of March 2019 through May 2020. The following table presents the impact on Ameren Missouri and Ameren Illinois of related party transactions for the three and nine months ended September 30, 2017 and 2016 : Three Months Nine Months Agreement Income Statement Line Item Ameren Missouri Ameren Illinois Ameren Missouri Ameren Illinois Ameren Missouri power supply Operating Revenues 2017 $ 4 $ (a) $ 21 $ (a) agreements with Ameren Illinois 2016 9 (a) 21 (a) Ameren Missouri and Ameren Illinois Operating Revenues 2017 7 1 20 3 rent and facility services 2016 5 1 18 3 Ameren Missouri and Ameren Illinois Operating Revenues 2017 (b) (b) (b) 1 miscellaneous support services 2016 1 (b) 1 (b) Total Operating Revenues 2017 $ 11 $ 1 $ 41 $ 4 2016 15 1 40 3 Ameren Illinois power supply Purchased Power 2017 $ (a) $ 4 $ (a) $ 21 agreements with Ameren Missouri 2016 (a) 9 (a) 21 Ameren Illinois transmission Purchased Power 2017 (a) (b) (a) 1 services with ATXI 2016 (a) 1 (a) 2 Total Purchased Power 2017 $ (a) $ 4 $ (a) $ 22 2016 (a) 10 (a) 23 Ameren Services support services Other Operations and Maintenance 2017 $ 34 $ 33 $ 103 $ 99 agreement 2016 30 29 96 90 Money pool borrowings (advances) Interest Charges/ Miscellaneous Income 2017 $ (b) $ (b) $ (b) $ (b) 2016 (b) (b) (b) (b) (a) Not applicable. (b) Amount less than $1 million.

Commitments And Contingencies

Commitments And Contingencies	9 Months Ended
	Sep. 30, 2017
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	COMMITMENTS AND CONTINGENCIES We are involved in legal, tax, and regulatory proceedings before various courts, regulatory commissions, authorities, and governmental agencies with respect to matters that arise in the ordinary course of business, some of which involve substantial amounts of money. We believe that the final disposition of these proceedings, except as otherwise disclosed in 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 14 – Related Party Transactions, and Note 15 – 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 and September 2017, Ameren Illinois conducted procurement events, administered by the IPA, to purchase energy products through May 2020. In the April 2017 procurement event, Ameren Illinois contracted to purchase 4,249,800 megawatthours of energy products for $128 million from June 2017 through May 2020. In the September 2017 procurement event, Ameren Illinois contracted to purchase approximately 1,950,200 megawatthours of energy products for $57 million from October 2017 through May 2020. The results of both procurement events are reflected in the below table. 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. The table below presents our estimated minimum fuel, purchased power, and other commitments for fuel at September 30, 2017 . Ameren’s and Ameren Illinois’ purchased power commitments include the Ameren Illinois agreements entered into as part of the IPA-administered power procurement process. Included in the Other column are minimum purchase commitments under contracts for equipment, design and construction, and meter reading services, among other agreements, at September 30, 2017 . Coal Natural Gas (a) Nuclear Fuel Purchased Power (b)(c) Methane Gas Other Total Ameren: (d) 2017 $ 162 $ 65 $ 19 $ 59 $ 1 $ 33 $ 339 2018 453 200 67 170 4 57 951 2019 356 148 27 63 4 39 637 2020 79 94 39 13 5 39 269 2021 27 36 45 2 5 26 141 Thereafter — 47 58 20 64 123 312 Total $ 1,077 $ 590 $ 255 $ 327 $ 83 $ 317 $ 2,649 Ameren Missouri: 2017 $ 162 $ 14 $ 19 $ — $ 1 $ 20 $ 216 2018 453 42 67 — 4 44 610 2019 356 34 27 — 4 25 446 2020 79 26 39 — 5 25 174 2021 27 13 45 — 5 26 116 Thereafter — 22 58 — 64 100 244 Total $ 1,077 $ 151 $ 255 $ — $ 83 $ 240 $ 1,806 Ameren Illinois: 2017 $ — $ 51 $ — $ 59 $ — $ 13 $ 123 2018 — 158 — 170 — 13 341 2019 — 114 — 63 — 14 191 2020 — 68 — 13 — 14 95 2021 — 23 — 2 — — 25 Thereafter — 25 — 20 — — 45 Total $ — $ 439 $ — $ 327 $ — $ 54 $ 820 (a) Includes amounts for generation and for distribution. (b) The purchased power amounts for Ameren and Ameren Illinois exclude agreements for renewable energy credits through 2032 with various renewable energy suppliers due to the contingent nature of the payment amounts. (c) The purchased power amounts for Ameren and Ameren Missouri exclude a 102-megawatt power purchase agreement with a wind farm operator, which expires in 2024, due to the contingent nature of the payment amounts. (d) Includes amounts for Ameren registrant and nonregistrant subsidiaries. Environmental Matters We are subject to various environmental laws and regulations enforced by federal, state, and local authorities. The development and operation of electric generation, transmission, and distribution facilities and natural gas storage, transmission, and distribution facilities can trigger compliance obligations with respect to diverse environmental laws and regulations. These laws and regulations address emissions, discharges into water, water usage, impacts to air, land, and water, and chemical and waste handling. Complex and lengthy processes are required to obtain and renew approvals, permits, and licenses for new, existing or modified facilities. Additionally, the use and handling of various chemicals or hazardous materials require release prevention plans and emergency response procedures. The EPA has promulgated environmental regulations that have a significant impact on the electric utility industry. Over time, compliance with these regulations could be costly for Ameren Missouri, which operates coal-fired power plants. As of December 31, 2016, Ameren Missouri’s fossil fuel-fired energy centers represented 18% and 34% of Ameren’s and Ameren Missouri’s rate base, respectively. Regulations impacting air emissions from the electric utility industry include the revised NSPS, the CSAPR, the MATS, and the revised National Ambient Air Quality Standards, which are subject to periodic review for certain pollutants. Collectively, these regulations cover a variety of pollutants such as SO 2 , particulate matter, NO X , mercury, toxic metals, and acid gases. Water intake and discharges from power plants are regulated under the Clean Water Act. Modifications to water intake structures and more stringent limitations or prohibitions against wastewater discharges at Ameren Missouri’s energy centers 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, resulting in significant capital expenditures. The individual or combined effects of existing environmental regulations could result in significant capital expenditures, increased operating costs, the closure or alteration of the operation of some of Ameren Missouri’s energy centers, or require further capital investment. Ameren and Ameren Missouri expect that 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 $425 million to $525 million in the aggregate from 2017 through 2021 in order to comply with existing environmental regulations. Additional environmental controls beyond 2021 could be required. This estimate of capital expenditures includes expenditures required for the CCR regulations, Clean Water Act rules applicable to cooling water intake structures at existing power plants, and effluent limitation guidelines applicable to steam electric generating units, all of which are discussed below. The EPA has proposed to repeal the Clean Power Plan, which would have regulated CO 2 emissions from power plants. The above capital expenditure amounts exclude estimated impacts from the Clean Power Plan. 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 substantively revise regulatory obligations, the precise compliance strategies that will be used and their ultimate cost, among other things. The following sections describe the more significant environmental laws and rules and environmental enforcement and remediation matters that affect or could affect our operations. The EPA has initiated an administrative review of several regulations and rulemaking activities, including the effluent limitation guidelines and the CCR rule, which could ultimately result in the revision of all or part of such rules. Clean Air Act Federal and state laws require significant reductions in SO 2 and NO x through either emission source reductions or the use and retirement of emission allowances. The first phase of the CSAPR emission reduction requirements became effective in 2015. The second phase of emission reduction requirements, which were revised by the EPA in 2016, became effective in 2017; additional emission reduction requirements may apply in subsequent years. To achieve compliance with the CSAPR, Ameren Missouri burns ultra-low-sulfur coal, operates two scrubbers at its Sioux energy center, and optimizes other existing pollution control equipment. Ameren Missouri did not make additional capital investments to comply with the 2017 CSAPR requirements. However, Ameren Missouri expects to incur additional costs to lower its emissions at one or more of its energy centers to comply with the CSAPR in future years. These higher costs are expected to be recovered from customers through the FAC or higher base rates. CO 2 Emissions Standards In 2015, the EPA issued the Clean Power Plan, which sets forth CO 2 emissions standards applicable to existing power plants. In October 2017, the EPA announced a proposal to repeal the Clean Power Plan and will seek public comment as to the scope of future regulations under the Clean Air Act. The United States Court of Appeals for the District of Columbia Circuit has stayed further action pending the EPA’s administrative review. We cannot predict the outcome of the EPA’s future rulemaking, the outcome of legal challenges relating to either the repeal of the Clean Power Plan or such future rulemakings, nor the resulting impact on our results of operations, financial position, or 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 will now proceed to the second phase to determine the actions required to remedy the violations found in the liability phase of the litigation. The EPA previously withdrew all claims for penalties and fines. 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. Implementation of the rule will occur during the permit renewal process of each energy center’s water discharge permit, which will occur between 2018 and 2023. Additionally, in 2015, the EPA issued a rule to revise the effluent limitation guidelines applicable to steam electric generating units. These guidelines established national standards for water discharges that are based on the effectiveness of available control technology. The EPA's 2015 rule prohibits effluent discharges of certain waste streams and imposes more stringent limitations on certain water discharges from power plants. In September 2017, the EPA published a rule that postponed the compliance dates by two years for the limitations applicable to two specific waste streams so that it could potentially revise those standards. Both the intake and effluent rules, if implemented as enacted, could have an adverse effect on Ameren’s and Ameren Missouri’s results of operations, financial position, and liquidity should such implementation require extensive modifications to the cooling water systems and water discharge systems at Ameren Missouri’s energy centers, and if such investments are not recovered on a timely basis in electric rates charged to Ameren Missouri’s customers. Ash Management In 2015, the EPA issued regulations regarding the management and disposal of CCR from coal-fired energy centers. These regulations affect CCR disposal and handling costs at Ameren Missouri's energy centers. They require closure of impoundments if performance criteria relating to groundwater impacts and location restrictions are not achieved. In September 2017, the EPA granted petitions filed on behalf of coal-fired electricity generators in which the EPA agreed to reconsider provisions of the CCR rules. Ameren and Ameren Missouri’s AROs associated with CCR storage facilities reflect the regulations issued in 2015. Ameren plans to close these CCR storage facilities between 2018 and 2024. Ameren Missouri's capital expenditure plan includes the cost of constructing landfills as part of its environmental compliance plan. Remediation The Ameren Companies are involved in a number of remediation actions to clean up sites impacted by the use or disposal of materials containing hazardous substances. Federal and state laws can require responsible parties to fund remediation regardless of their degree of fault, the legality of original disposal, or the ownership of a disposal site. Ameren Missouri and Ameren Illinois have each been identified by federal or state governments as a potentially responsible party at several contaminated sites. As of September 30, 2017 , Ameren Illinois owned or was otherwise responsible for 44 former MGP sites in Illinois, which are in various stages of investigation, evaluation, remediation, and closure. Ameren Illinois estimates it could substantially conclude remediation efforts by 2023. The ICC allows Ameren Illinois to recover remediation and litigation costs associated with its former MGP sites from its electric and natural gas utility customers through environmental adjustment rate riders. Costs are subject to annual review by the ICC. As of September 30, 2017 , Ameren Illinois estimated the obligation related to these former MGP sites at $184 million to $255 million . Ameren and Ameren Illinois recorded a liability of $184 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. From about 1926 until 1976, Ameren Missouri operated an energy center adjacent to Sauget Area 2. Ameren Missouri currently owns a parcel of property at Sauget Area 2 that was once used by others as a landfill. In 2013, the EPA issued its record of decision for Sauget Area 2, approving the investigation and the remediation actions recommended by the potentially responsible parties. Further negotiation among the potentially responsible parties will determine how to fund the implementation of the EPA-approved remedies. As of September 30, 2017 , Ameren Missouri estimated its obligation related to Sauget Area 2 at $1 million to $2.5 million . Ameren Missouri recorded a liability of $1 million to represent its estimated minimum obligation for this site, as no other amount within the range was a better estimate. Our operations or those of our predecessor companies involve the use of, disposal of, and, in appropriate circumstances, the cleanup of substances regulated under environmental laws. We are unable to determine whether such practices will result in future environmental commitments or will affect our results of operations, financial position, or liquidity. Ameren Missouri Municipal Taxes The cities of Creve Coeur and Winchester, Missouri, on behalf of themselves and other municipalities in Ameren Missouri’s service area, filed a class action lawsuit in 2011 against Ameren Missouri in the Circuit Court of St. Louis County, Missouri. The lawsuit alleges that Ameren Missouri failed to pay gross receipts taxes or license fees on certain revenues, including revenues from wholesale power and interchange sales. In the third quarter of 2017, the court issued an order preliminarily approving a settlement between Ameren Missouri and the plaintiffs, with final resolution of the case expected in the first quarter of 2018. Ameren and Ameren Missouri recorded immaterial liabilities on their respective balance sheets as of September 30, 2017 , and December 31, 2016, representing their estimate of the probable loss due as a result of this lawsuit. Ameren and Ameren Missouri believe there is a remote possibility that a liability relating to this lawsuit could be material to Ameren's and Ameren Missouri’s results of operations, financial position, and liquidity.

Callaway Energy Center

Callaway Energy Center	9 Months Ended
	Sep. 30, 2017
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 that Ameren Missouri and other utilities that own and operate those energy centers pay the federal government for disposing of the spent nuclear fuel at one mill, or one-tenth of one cent, for each kilowatthour generated and sold by those plants. The NWPA also requires the DOE to review the nuclear waste fee annually against the cost of the nuclear waste disposal program and to propose to the United States Congress any fee adjustment necessary to offset the costs of the program. As required by the NWPA, Ameren Missouri and other utilities have entered into standard contracts with the DOE. Consistent with the NWPA and its standard contract, which stated that the DOE would begin to dispose of spent nuclear fuel by 1998, Ameren Missouri had historically collected one mill from its electric customers for each kilowatthour of electricity that it generated and sold from its Callaway energy center. Because the federal government is not meeting its disposal obligation, the collection of this fee was suspended in May 2014. The DOE's delay in carrying out its obligation to dispose of spent nuclear fuel from the Callaway energy center is not expected to adversely affect the continued operations of the energy center. As a result of the DOE's failure to fulfill its contractual obligations, Ameren Missouri and other nuclear energy center owners sued the DOE to recover costs incurred for ongoing storage of their spent fuel. The lawsuit resulted in a settlement agreement that provides for annual reimbursement of additional spent fuel storage and related costs. Ameren Missouri received reimbursements from the DOE of $3 million and $24 million in October 2017 and September 2016, respectively. Ameren Missouri will continue to apply for reimbursement from the DOE for allowable costs associated with the ongoing storage of spent fuel. Decommissioning Electric utility rates charged to customers provide for the recovery of the Callaway energy center's decommissioning costs, which include decontamination, dismantling, and site restoration costs, over the expected life of the nuclear energy center. Amounts collected from customers are deposited into the external nuclear decommissioning trust fund to provide for the Callaway energy center’s decommissioning. It is assumed that the Callaway energy center site will be decommissioned through the immediate dismantlement method and removed from service. Ameren and Ameren Missouri have recorded an ARO for the Callaway energy center decommissioning costs at fair value, which represents the present value of estimated future cash outflows. Annual decommissioning costs of $7 million are included in the costs used to establish electric rates for Ameren Missouri's customers. Every three years, the MoPSC requires Ameren Missouri to file an updated cost study and funding analysis for decommissioning its Callaway energy center. An updated cost study and funding analysis was filed with the MoPSC in September 2017 and reflected within the ARO. Ameren Missouri’s filing supported no change in electric service rates for decommissioning costs. There is no deadline by which the MoPSC must issue an order regarding the filing. 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. Supplier of Fuel Assemblies Ameren Missouri received all necessary fuel assemblies for the fourth quarter 2017 refueling and maintenance outage. The Callaway energy center uses nuclear fuel assemblies fabricated by Westinghouse, which is the only NRC-licensed supplier authorized to provide fuel assemblies to the Callaway energy center. During the first quarter of 2017, Westinghouse filed voluntary petitions for a court-supervised restructuring process under Chapter 11 of the United States Bankruptcy Code. Westinghouse could petition the bankruptcy court to reject Ameren Missouri’s contracts as part of the restructuring process, and if the bankruptcy court agrees, this could result in Ameren Missouri not having access to the fuel assemblies necessary to refuel the Callaway energy center in future scheduled refueling and maintenance outages. At this time, Ameren and Ameren Missouri believe the restructuring proceeding will not affect Westinghouse’s performance under the terms of its existing contracts with Ameren Missouri, and therefore do not expect any material impact to Ameren Missouri’s operations as a result of this restructuring proceeding. However, Ameren and Ameren Missouri could incur material unexpected costs as a result of the Westinghouse bankruptcy, such as the loss of fuel inventory that is stored at Westinghouse’s facility and the cost of replacement power if nuclear fuel assemblies were not available for a future scheduled refueling and maintenance outage. A change of fuel suppliers or a change in the type of fuel assembly design that is currently licensed for use at the Callaway energy center could take an estimated three years of analysis and NRC licensing efforts to implement. Insurance The following table presents insurance coverage at Ameren Missouri’s Callaway energy center at September 30, 2017 . 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 2017. Type and Source of Coverage Maximum Coverages Maximum Assessments for Single Incidents Public liability and nuclear worker liability: American Nuclear Insurers $ 450 $ — Pool participation 12,986 (a) 127 (b) $ 13,436 (c) $ 127 Property damage: NEIL and EMANI $ 3,200 (d) $ 29 (e) Replacement power: NEIL $ 490 (f) $ 7 (e) (a) Provided through mandatory participation in an industrywide retrospective premium assessment program. (b) Retrospective premium under the Price-Anderson Act. This is subject to retrospective assessment with respect to a covered loss in excess of $450 million in the event of an incident at any licensed United States commercial reactor, payable at $19 million per year. (c) Limit of liability for each incident under the Price-Anderson liability provisions of the Atomic Energy Act of 1954, as amended. This limit is subject to change to account for the effects of inflation and changes in the number of licensed reactors. (d) NEIL provides $2.7 billion in property damage, stabilization, decontamination, and premature decommissioning insurance for radiation events and $2.3 billion in property damage insurance for nonradiation events. EMANI provides $490 million in property damage insurance for both radiation and nonradiation events. (e) All NEIL insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL. (f) Provides replacement power cost insurance in the event of a prolonged accidental outage. Weekly indemnity up to $4.5 million for 52 weeks, which commences after the first 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	9 Months Ended
	Sep. 30, 2017
Defined Benefit Pension Plans and Defined Benefit Postretirement Plans Disclosure [Abstract]
RETIREMENT BENEFITS	RETIREMENT BENEFITS The following table presents the components of the net periodic benefit cost (benefit), prior to capitalization, incurred for Ameren’s pension and postretirement benefit plans for the three and nine months ended September 30, 2017 and 2016 : Pension Benefits Postretirement Benefits Three Months Nine Months Three Months Nine Months 2017 2016 2017 2016 2017 2016 2017 2016 Service cost $ 24 $ 20 $ 70 $ 60 $ 6 $ 5 $ 16 $ 15 Interest cost 44 46 134 138 12 12 35 36 Expected return on plan assets (65 ) (63 ) (196 ) (189 ) (19 ) (18 ) (56 ) (54 ) Amortization of: Prior service benefit (1 ) — (1 ) — (2 ) (1 ) (4 ) (3 ) Actuarial loss (gain) 14 8 41 24 (2 ) (3 ) (5 ) (8 ) Net periodic benefit cost (benefit) $ 16 $ 11 $ 48 $ 33 $ (5 ) $ (5 ) $ (14 ) $ (14 ) Ameren Missouri and Ameren Illinois are responsible for their respective shares of Ameren’s pension and postretirement costs. The following table presents the pension costs and the postretirement benefit costs (benefit) incurred for the three and nine months ended September 30, 2017 and 2016 : Pension Benefits Postretirement Benefits Three Months Nine Months Three Months Nine Months 2017 2016 2017 2016 2017 2016 2017 2016 Ameren Missouri (a) $ 6 $ 6 $ 18 $ 19 $ (1 ) $ (1 ) $ (3 ) $ (3 ) Ameren Illinois 10 6 30 17 (3 ) (3 ) (10 ) (10 ) Other — (1 ) — (3 ) (1 ) (1 ) (1 ) (1 ) Ameren (a)(b) $ 16 $ 11 $ 48 $ 33 $ (5 ) $ (5 ) $ (14 ) $ (14 ) (a) Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. (b) Includes amounts for Ameren registrants and nonregistrant subsidiaries.

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2017
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 company activities and Ameren Services. Ameren Missouri has one segment. Ameren Illinois has three segments: Ameren Illinois Electric Distribution, Ameren Illinois Natural Gas, and Ameren Illinois Transmission. See Note 1 – Summary of Significant Accounting Policies for additional information regarding the operations of Ameren Missouri, Ameren Illinois, and ATXI. Segment operating revenues and a majority of operating expenses are directly recognized and incurred by Ameren Illinois to each Ameren Illinois segment. Common operating expenses, miscellaneous income and expenses, interest charges, and income tax expense are allocated by Ameren Illinois to each Ameren Illinois segment based on certain factors, which primarily relate to the nature of the cost. Additionally, Ameren Illinois Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. The transmission expense for Illinois customers who have elected to purchase their power from Ameren Illinois is recovered through a cost recovery mechanism with no net effect on Ameren Illinois Electric Distribution earnings, as costs are offset by corresponding revenues. Transmission revenues from these transactions are reflected at Ameren Transmission and Ameren Illinois Transmission. An intersegment elimination at Ameren and Ameren Illinois occurs to eliminate these transmission revenues and expenses. The following tables present revenues, net income attributable to common shareholders, and capital expenditures by segment at Ameren and Ameren Illinois for the three and nine months ended September 30, 2017 and 2016 . 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 2017 External revenues $ 1,104 $ 405 $ 111 $ 105 $ (2 ) $ — $ 1,723 Intersegment revenues 11 — 1 14 (a) — (26 ) — Net income attributable to Ameren common shareholders 234 31 2 38 (b) (17 ) — 288 Capital expenditures 178 112 71 173 (2 ) (7 ) 525 2016 External revenues $ 1,150 $ 502 $ 113 $ 94 $ — $ — $ 1,859 Intersegment revenues 15 1 1 14 (a) — (31 ) — Net income attributable to Ameren common shareholders 241 93 2 39 (b) (6 ) — 369 Capital expenditures 147 123 50 175 1 — 496 Nine Months 2017 External revenues $ 2,799 $ 1,176 $ 509 $ 293 $ (2 ) $ — $ 4,775 Intersegment revenues 41 3 1 33 (a) — (78 ) — Net income attributable to Ameren common shareholders 359 94 40 106 (b) (16 ) — 583 Capital expenditures 533 354 180 463 3 (10 ) 1,523 2016 External revenues $ 2,733 $ 1,210 $ 529 $ 247 $ 1 $ — $ 4,720 Intersegment revenues 40 3 1 36 (a) — (80 ) — Net income attributable to Ameren common shareholders 347 122 44 98 (b) 10 — 621 Capital expenditures 500 359 130 503 4 — 1,496 (a) Ameren Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above. (b) 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 Consolidated 2017 External revenues $ 405 $ 112 $ 58 $ — $ 575 Intersegment revenues — — 14 (a) (14 ) — Net income available to common shareholder 31 2 22 — 55 Capital expenditures 112 71 93 — 276 2016 External revenues $ 503 $ 114 $ 59 $ — $ 676 Intersegment revenues — — 14 (a) (14 ) — Net income available to common shareholder 93 2 24 — 119 Capital expenditures 123 50 68 — 241 Nine Months 2017 External revenues $ 1,179 $ 510 $ 165 $ — $ 1,854 Intersegment revenues — — 32 (a) (32 ) — Net income available to common shareholder 94 40 57 — 191 Capital expenditures 354 180 226 — 760 2016 External revenues $ 1,213 $ 530 $ 152 $ — $ 1,895 Intersegment revenues — — 35 (a) (35 ) — Net income available to common shareholder 122 44 57 — 223 Capital expenditures 359 130 194 — 683 (a) Ameren Illinois Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above.

Summary Of Significant Accoun21

Summary Of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
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. Also see the Glossary of Terms and Abbreviations at the front of this report and in the Form 10-K. As of September 30, 2017 and December 31, 2016, Ameren had unconsolidated variable interests as a limited partner in various equity method investments, totaling $14 million and $9 million , respectively, included in “Other assets” on Ameren’s consolidated balance sheet. Ameren is not the primary beneficiary of these investments because it does not have the power to direct matters that most significantly impact the activities of these variable interest entities. As of September 30, 2017, the maximum exposure to loss related to these variable interests is limited to the investment in these partnerships of $14 million plus associated outstanding funding commitments of $23 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 statement 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. See Note 2 – Rate and Regulatory Matters for information regarding the 2017 change in Ameren Illinois' method used to recognize interim period revenue in connection with the revenue decoupling provisions of the FEJA. These financial statements should be read in conjunction with the financial statements and the notes thereto included in the Form 10-K.
Revenue Recognition	The Ameren Companies record operating revenue for electric or natural gas service when it is delivered to customers. We accrue an estimate of electric and natural gas revenues for service rendered but unbilled at the end of each accounting period. For certain regulatory recovery mechanisms qualifying as alternative revenue programs, such as revenue requirement reconciliations, the Ameren Companies 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.
Excise Taxes	Ameren Missouri and Ameren Illinois collect certain excise taxes from customers 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 and are recorded gross in “Operating Revenues – Electric,” “Operating Revenues – Natural gas” and “Operating Expenses – Taxes other than income taxes” on the statement of income or the statement of income and comprehensive income. Excise taxes for electric service in Illinois are levied on the customer and therefore are not included in Ameren Illinois’ revenues and expenses.
Accounting and Reporting Developments	Below is a summary of updates related to our adoption of recently issued authoritative accounting standards. 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, financial instruments, and restricted cash. Revenue from Contracts with Customers In May 2014, the FASB issued authoritative guidance that changes the criteria for recognizing revenue from a contract with a customer. The underlying principle of the guidance is that an entity will recognize revenue for the transfer of promised goods or services to customers at an amount that the entity expects to be entitled to in exchange for those goods or services. The guidance requires additional disclosures to enable users of financial statements to understand the nature, amount, timing, and uncertainty of revenue and cash flows arising from contracts with customers, as well as separate presentation of alternative revenue programs on the income statement. Entities can apply the guidance to each reporting period presented (the full retrospective method) or by recording a cumulative effect adjustment to retained earnings in the period of initial adoption (the modified retrospective method). We have substantially completed the evaluation of our contracts and do not expect material changes to the amount or timing of revenue recognition. We will finalize our contract assessments by the end of 2017. We will apply the guidance using the full retrospective method and include disaggregated revenue disclosures by segment and customer class in the combined notes to the financial statements in the first quarter of 2018. Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost In March 2017, the FASB issued authoritative guidance that requires an entity to retrospectively report the service cost component of net benefit cost in the same line item(s) as other compensation costs arising from services rendered by employees during the period and to present the other components of net benefit cost in the income statement separately from the service cost component and outside of operating income. The guidance also requires that an entity only capitalize 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 will not affect our ability to continue to obtain recovery of net benefit costs through customer rates. See Note 11 – Retirement Benefits for the components of net benefit cost. This guidance will be effective for the Ameren Companies in the first quarter of 2018. We are currently assessing the impacts of this guidance on our results of operations, financial position, and disclosures.

Derivative Financial Instrume22

Derivative Financial Instruments Derivative Financial Instruments (Policies)	9 Months Ended
	Sep. 30, 2017
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 and uranium inventories that differ from the cost of those commodities in inventory; and • actual cash outlays for the purchase of these commodities that differ from anticipated cash outlays. The derivatives that we use to hedge these risks are governed by our risk management policies for forward contracts, futures, options, and swaps. Our net positions are continually assessed within our structured hedging programs to determine whether new or offsetting transactions are required. The goal of the hedging program is generally to mitigate financial risks while ensuring that sufficient volumes are available to meet our requirements. Contracts we enter into as part of our risk management program may be settled financially, settled by physical delivery, or net settled with the counterparty.

Summary Of Significant Accoun23

Summary Of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Asset Retirement Obligation Disclosure	The following table provides a reconciliation of the beginning and ending carrying amount of AROs for the nine months ended September 30, 2017 : Ameren Missouri Ameren Illinois (a) Ameren Balance at December 31, 2016 $ 644 (b) $ 6 $ 650 (b) Liabilities settled (4 ) (1 ) (5 ) Accretion (c) 20 (d) 20 Change in estimates (e) (18 ) (1 ) (19 ) Balance at September 30, 2017 $ 642 (b) $ 4 $ 646 (b) (a) Included in “Other deferred credits and liabilities” on the balance sheet. (b) Balance included $15 million in “Other current liabilities” on the balance sheet as of both December 31, 2016 and September 30, 2017, respectively. (c) Accretion expense was recorded as a decrease to regulatory liabilities. (d) Less than $1 million. (e) Ameren Missouri changed its fair value estimate primarily due to an extension of the remediation period of certain CCR storage facilities, an update to the decommissioning of the Callaway energy center to reflect the cost study and funding analysis filed with the MoPSC in 2017, and an increase in the discount rate assumption.
Summary Of Nonvested Shares Related To Long-Term Incentive Plan	A summary of nonvested performance share units at September 30, 2017 , and changes during the nine months ended September 30, 2017 , under the 2014 Incentive Plan are presented below: Performance Share Units Share Units Weighted-average Fair Value per Share Unit Nonvested at January 1, 2017 1,059,639 $ 48.04 Granted (a) 500,943 59.16 Forfeitures (48,661 ) 52.54 Vested (b) (27,446 ) 52.88 Nonvested at September 30, 2017 1,484,475 $ 51.55 (a) Performance share units granted to certain executive and nonexecutive officers and other eligible employees under the 2014 Incentive Plan. (b) Performance share units vested due to the attainment of retirement eligibility by certain employees. Actual shares issued for retirement-eligible employees vary depending on actual performance over the three -year measurement period.
Schedule of Excise Taxes	The following table presents excise taxes recorded in “Operating Revenues – Electric,” “Operating Revenues – Natural gas” and “Operating Expenses – Taxes other than income taxes” for the three and nine months ended September 30, 2017 and 2016 : Three Months Nine Months 2017 2016 2017 2016 Ameren Missouri $ 51 $ 52 $ 122 $ 122 Ameren Illinois 10 9 40 40 Ameren $ 61 $ 61 $ 162 $ 162

Short-Term Debt And Liquidity S

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Tables)	9 Months Ended
	Sep. 30, 2017
Debt Disclosure [Abstract]
Schedule of Short-term Debt	The following table presents commercial paper outstanding as of September 30, 2017 , and December 31, 2016 : 2017 2016 Ameren (parent) $ 277 $ 507 Ameren Missouri — — Ameren Illinois 169 51 Ameren Consolidated $ 446 $ 558 The following table summarizes the borrowing activity and relevant interest rates under Ameren’s (parent), Ameren Missouri’s, and Ameren Illinois’ commercial paper programs for the nine months ended September 30, 2017 and 2016: Ameren (parent) Ameren Missouri Ameren Illinois Ameren Consolidated 2017 Average daily commercial paper outstanding $ 669 $ 7 $ 78 $ 754 Weighted-average interest rate 1.27 % 1.20 % 1.28 % 1.27 % Peak commercial paper during period (a) $ 841 $ 64 $ 193 $ 948 Peak interest rate 1.50 % 1.41 % 1.50 % 1.50 % 2016 Average daily commercial paper outstanding $ 435 $ 80 $ 48 $ 563 Weighted-average interest rate 0.81 % 0.74 % 0.72 % 0.79 % Peak commercial paper during period (a) $ 574 $ 208 $ 195 $ 839 Peak interest rate 0.95 % 0.85 % 0.85 % 0.95 % (a) The timing of peak commercial paper issuances varies by company. Therefore, the sum of peak commercial paper issuances presented by company does not equal the Ameren Consolidated peak commercial paper issuances for the period.

Long-Term Debt And Equity Fin25

Long-Term Debt And Equity Financings Long Term Debt and Equity Financings (Tables)	9 Months Ended
	Sep. 30, 2017
Long-Term Debt And Equity Financings [Abstract]
Schedule of Maturities of Long-term Debt	The following table presents the principal maturities schedule for the notes: Payment Date Principal Payment August 2022 $ 49.5 August 2024 49.5 August 2027 49.5 August 2030 49.5 August 2032 49.5 August 2038 49.5 August 2043 76.5 August 2050 76.5 Total Principal Amount of Notes $ 450.0

Other Income and Expenses (Tabl

Other Income and Expenses (Tables)	9 Months Ended
	Sep. 30, 2017
Other Nonoperating Income (Expense) [Abstract]
Other Income And Expenses	The following table presents the components of “Other Income and Expenses” in the Ameren Companies’ statements of income for the three and nine months ended September 30, 2017 and 2016 : Three Months Nine Months 2017 2016 2017 2016 Ameren: (a) Miscellaneous income: Allowance for equity funds used during construction $ 6 $ 7 $ 16 $ 20 Interest income on industrial development revenue bonds 7 7 20 20 Interest income — 3 5 11 Other — 1 1 3 Total miscellaneous income $ 13 $ 18 $ 42 $ 54 Miscellaneous expense: Donations $ — $ 1 $ 7 $ 8 Other 2 7 9 13 Total miscellaneous expense $ 2 $ 8 $ 16 $ 21 Ameren Missouri: Miscellaneous income: Allowance for equity funds used during construction $ 6 $ 6 $ 15 $ 16 Interest income on industrial development revenue bonds 7 7 20 20 Interest income — 1 — 1 Other — — 1 1 Total miscellaneous income $ 13 $ 14 $ 36 $ 38 Miscellaneous expense: Donations $ — $ — $ 2 $ 2 Other 2 2 4 4 Total miscellaneous expense $ 2 $ 2 $ 6 $ 6 Ameren Illinois: Miscellaneous income: Allowance for equity funds used during construction $ — $ 1 $ 1 $ 4 Interest income 1 2 5 9 Other — 1 1 2 Total miscellaneous income $ 1 $ 4 $ 7 $ 15 Miscellaneous expense: Donations $ — $ 1 $ 5 $ 6 Other — 2 3 5 Total miscellaneous expense $ — $ 3 $ 8 $ 11 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Derivative Financial Instrume27

Derivative Financial Instruments (Tables)	9 Months Ended
	Sep. 30, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Open Gross Derivative Volumes By Commodity Type	The following table presents open gross commodity contract volumes by commodity type for derivative assets and liabilities as of September 30, 2017 , and December 31, 2016 . As of September 30, 2017 , these contracts extended through October 2019, March 2023, May 2032, and March 2020 for fuel oils, natural gas, power, and uranium, respectively. Quantity (in millions, except as indicated) 2017 2016 Commodity Ameren Missouri Ameren Illinois Ameren Ameren Missouri Ameren Illinois Ameren Fuel oils (in gallons) (a) 30 (b) 30 30 (b) 30 Natural gas (in mmbtu) 24 145 169 25 129 154 Power (in megawatthours) 2 9 11 1 9 10 Uranium (pounds in thousands) 370 (b) 370 345 (b) 345 (a) Consists of ultra-low-sulfur diesel products. (b) Not applicable.
Derivative Instruments Carrying Value	The following table presents the carrying value and balance sheet location of all derivative commodity contracts, none of which were designated as hedging instruments, as of September 30, 2017 , and December 31, 2016 : Balance Sheet Location Ameren Missouri Ameren Illinois Ameren 2017 Fuel oils Other current assets $ 2 $ — $ 2 Other assets 1 — 1 Natural gas Other current assets — 1 1 Other assets — 1 1 Power Other current assets 10 — 10 Other assets 1 — 1 Total assets (a) $ 14 $ 2 $ 16 Fuel oils Other current liabilities $ 2 $ — $ 2 Natural gas Other current liabilities 3 8 11 Other deferred credits and liabilities 4 6 10 Power Other current liabilities 1 13 14 Other deferred credits and liabilities — 179 179 Uranium Other deferred credits and liabilities — (b) — — (b) Total liabilities (c) $ 10 $ 206 $ 216 2016 Fuel oils Other current assets $ 2 $ — $ 2 Other assets 1 — 1 Natural gas Other current assets 1 11 12 Other assets 1 2 3 Power Other current assets 9 — 9 Total assets (a) $ 14 $ 13 $ 27 Fuel oils Other current liabilities $ 5 $ — $ 5 Natural gas Other current liabilities 1 3 4 Other deferred credits and liabilities 5 5 10 Power Other current liabilities 3 12 15 Other deferred credits and liabilities — 173 173 Uranium Other deferred credits and liabilities 4 — 4 Total liabilities (c) $ 18 $ 193 $ 211 (a) The cumulative amount of pretax net gains on all derivative instruments is deferred as a regulatory liability. (b) Beginning in 2017, as a result of rulebook amendments at the Chicago Mercantile Exchange, the fair value of uranium derivative liabilities are offset by certain settlement payments made to the exchange previously characterized as collateral and included within “Other assets” on Ameren’s and Ameren Missouri’s balance sheet. (c) The cumulative amount of pretax net losses on all derivative instruments is deferred as a regulatory asset.
Derivative Instruments With Credit Risk-Related Contingent Features	The following table presents, as of September 30, 2017 , the aggregate fair value of all derivative instruments with credit risk-related contingent features in a gross liability position, the cash collateral posted, and the aggregate amount of additional collateral that counterparties could require. The additional collateral required is the net liability position allowed under the master netting arrangements or similar agreements, assuming (1) the credit risk-related contingent features underlying these arrangements were triggered on September 30, 2017 , and (2) those counterparties with rights to do so requested collateral. Aggregate Fair Value of Derivative Liabilities (a) Cash Collateral Posted Potential Aggregate Amount of Additional Collateral Required (b) 2017 Ameren Missouri $ 59 $ 3 $ 48 Ameren Illinois 48 — 41 Ameren $ 107 $ 3 $ 89 (a) Before consideration of master netting arrangements or similar agreements and including NPNS and other accrual contract exposures. (b) As collateral requirements with certain counterparties are based on master netting arrangements or similar agreements, the aggregate amount of additional collateral required to be posted is determined after consideration of the effects of such arrangements.

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2017
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 classified as Level 3 in the fair value hierarchy for the periods ended September 30, 2017 and December 31, 2016 : Fair Value Weighted Average Assets Liabilities Valuation Technique(s) Unobservable Input Range Level 3 Derivative asset and liability – commodity contracts (a) : 2017 Fuel oils $ 1 $ (1 ) Option model Volatilities(%) (b) 24 – 32 26 Discounted cash flow Counterparty credit risk(%) (c)(d) 0.12 – 0.22 0.17 Ameren Missouri credit risk(%) (c)(d) 0.37 (e) Natural gas — (2 ) Discounted cash flow Nodal basis ($/mmbtu) (b) (1.10) – (0.10) (0.80) Counterparty credit risk (%) (c)(d) 0.34 – 6 0.73 Ameren Illinois credit risk (%) (c)(d) 0.37 (e) Power (g) $ 11 $ (193 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps ($/MWh) (h) 25 – 41 28 Estimated auction price for FTRs ($/MW) (b) (324) – 1,194 269 Nodal basis ($/MWh) (h) (3) – 0 (2) Fair Value Weighted Average Assets Liabilities Valuation Technique(s) Unobservable Input Range Counterparty credit risk (%) (c)(d) 0.28 (e) Ameren Illinois credit risk (%) (c)(d) 0.37 (e) Fundamental energy production model Estimated future natural gas prices ($/mmbtu) (b) 3 – 4 3 Escalation rate (%) (b)(i) 3 (e) Contract price allocation Estimated renewable energy credit costs ($/credit) (b) 5 – 7 6 2016 Fuel oils $ 1 $ — Option model Volatilities (%) (b) 24 – 66 28 Discounted cash flow Counterparty credit risk (%) (c)(d) 0.13 – 0.22 0.15 Ameren Missouri credit risk (%) (c)(d) 0.38 (e) Escalation rate (%) (b)(f) (2) – 2 0 Natural gas 1 (1 ) Option model Volatilities (%) (b) 31 – 66 36 Nodal basis ($/mmbtu) (b) (0.40) – (0.10) (0.20) Discounted cash flow Nodal basis ($/mmbtu) (b) (0.80) – 0 (0.50) Counterparty credit risk (%) (c)(d) 0.13 – 8 1 Ameren Illinois credit risk (%) (c)(d) 0.38 (e) Power (g) 9 (187 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps ($/MWh) (h) 26 – 44 29 Estimated auction price for FTRs ($/MW) (b) (71) – 5,270 125 Nodal basis ($/MWh) (h) (6) – 0 (2) Ameren Illinois credit risk (%) (c)(d) 0.38 (e) Fundamental energy production model Estimated future natural gas prices ($/mmbtu) (b) 3 – 4 3 Escalation rate (%) (b)(i) 5 (e) Contract price allocation Estimated renewable energy credit costs ($/credit) (b) 5 – 7 6 Uranium — (4 ) Option model Volatilities (%) (b) 24 (e) Discounted cash flow Average forward uranium pricing ($/pound) (b) 22 – 24 22 Ameren Missouri credit risk (%) (c)(d) 0.38 (e) (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement. (c) Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement. (d) Counterparty credit risk is applied only to counterparties with derivative asset balances. Ameren Missouri and Ameren Illinois credit risk is applied only to counterparties with derivative liability balances. (e) Not applicable. (f) Escalation rate applies to fuel oil prices 2019 and beyond. (g) Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2021 for September 30, 2017, and through 2020 for December 31, 2016. Valuations beyond 2021 for September 30, 2017, and 2020 for December 31, 2016 use fundamentally modeled pricing by month for peak and off-peak demand. (h) The balance at Ameren is comprised of Ameren Missouri and Ameren Illinois power contracts, which respond differently to unobservable input changes due to their opposing positions. (i) Escalation rate applies to power prices in 2031 and beyond.
Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis	The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of September 30, 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 $ 2 $ — $ 1 $ 3 Natural gas 1 1 — 2 Power — — 11 11 Total derivative assets – commodity contracts $ 3 $ 1 $ 12 $ 16 Nuclear decommissioning trust fund: Cash and cash equivalents $ 2 $ — $ — $ 2 Equity securities: U.S. large capitalization 445 — — 445 Debt securities: U.S. treasury and agency securities — 119 — 119 Corporate bonds — 82 — 82 Other — 24 — 24 Total nuclear decommissioning trust fund $ 447 $ 225 $ — $ 672 Total Ameren $ 450 $ 226 $ 12 $ 688 Ameren Missouri Derivative assets – commodity contracts (a) : Fuel oils $ 2 $ — $ 1 $ 3 Power — — 11 11 Total derivative assets – commodity contracts $ 2 $ — $ 12 $ 14 Nuclear decommissioning trust fund: Cash and cash equivalents $ 2 $ — $ — $ 2 Equity securities: U.S. large capitalization 445 — — 445 Debt securities: U.S. treasury and agency securities — 119 — 119 Corporate bonds — 82 — 82 Other — 24 — 24 Total nuclear decommissioning trust fund $ 447 $ 225 $ — $ 672 Total Ameren Missouri $ 449 $ 225 $ 12 $ 686 Ameren Illinois Derivative assets – commodity contracts (a) : Natural gas $ 1 $ 1 $ — $ 2 Liabilities: Ameren Derivative liabilities – commodity contracts (a) : Fuel oils $ 1 $ — $ 1 $ 2 Natural gas — 19 2 21 Power — — 193 193 Total Ameren $ 1 $ 19 $ 196 $ 216 Ameren Missouri Derivative liabilities – commodity contracts (a) : Fuel oils $ 1 $ — $ 1 $ 2 Natural gas — 7 — 7 Power — — 1 1 Total Ameren Missouri $ 1 $ 7 $ 2 $ 10 Ameren Illinois Derivative liabilities – commodity contracts (a) : Natural gas $ — $ 12 $ 2 $ 14 Power — — 192 192 Total Ameren Illinois $ — $ 12 $ 194 $ 206 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of December 31, 2016 : Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Total Assets: Ameren Derivative assets – commodity contracts (a) : Fuel oils $ 2 $ — $ 1 $ 3 Natural gas 2 12 1 15 Power — — 9 9 Total derivative assets – commodity contracts $ 4 $ 12 $ 11 $ 27 Nuclear decommissioning trust fund: Cash and cash equivalents $ 1 $ — $ — $ 1 Equity securities: U.S. large capitalization 408 — — 408 Debt securities: U.S. treasury and agency securities — 112 — 112 Corporate bonds — 67 — 67 Other — 17 — 17 Total nuclear decommissioning trust fund $ 409 $ 196 $ — $ 605 (b) Total Ameren $ 413 $ 208 $ 11 $ 632 Ameren Missouri Derivative assets – commodity contracts (a) : Fuel oils $ 2 $ — $ 1 $ 3 Natural gas — 1 1 2 Power — — 9 9 Total derivative assets – commodity contracts $ 2 $ 1 $ 11 $ 14 Nuclear decommissioning trust fund: Cash and cash equivalents $ 1 $ — $ — $ 1 Equity securities: U.S. large capitalization 408 — — 408 Debt securities: U.S. treasury and agency securities — 112 — 112 Corporate bonds — 67 — 67 Other — 17 — 17 Total nuclear decommissioning trust fund $ 409 $ 196 $ — $ 605 (b) Total Ameren Missouri $ 411 $ 197 $ 11 $ 619 Ameren Illinois Derivative assets – commodity contracts (a) : Natural gas $ 2 $ 11 $ — $ 13 Liabilities: Ameren Derivative liabilities – commodity contracts (a) : Fuel oils $ 5 $ — $ — $ 5 Natural gas — 13 1 14 Power — 1 187 188 Uranium — — 4 4 Total Ameren $ 5 $ 14 $ 192 $ 211 Ameren Missouri Derivative liabilities – commodity contracts (a) : Fuel oils $ 5 $ — $ — $ 5 Natural gas — 6 — 6 Power — 1 2 3 Uranium — — 4 4 Total Ameren Missouri $ 5 $ 7 $ 6 $ 18 Ameren Illinois Derivative liabilities – commodity contracts (a) : Natural gas $ — $ 7 $ 1 $ 8 Power — — 185 185 Total Ameren Illinois $ — $ 7 $ 186 $ 193 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Balance excludes $2 million of receivables, payables, and accrued income, net.
Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level Three In The Fair Value Hierarchy	The following table summarizes the changes in the fair value of power financial assets and liabilities classified as Level 3 in the fair value hierarchy: Net derivative commodity contracts Ameren Missouri Ameren Illinois Ameren For the three months ended September 30, 2017 Beginning balance at July 1, 2017 $ 14 $ (192 ) $ (178 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (2 ) (3 ) (5 ) Sales 1 — 1 Settlements (3 ) 3 — Ending balance at September 30, 2017 $ 10 $ (192 ) $ (182 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2017 $ — $ (2 ) $ (2 ) For the three months ended September 30, 2016 Beginning balance at July 1, 2016 $ 14 $ (169 ) $ (155 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities — (6 ) (6 ) Settlements (5 ) 3 (2 ) Ending balance at September 30, 2016 $ 9 $ (172 ) $ (163 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2016 $ — $ (2 ) $ (2 ) For the nine months ended September 30, 2017 Beginning balance at January 1, 2017 $ 7 $ (185 ) $ (178 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (3 ) (14 ) (17 ) Purchases 15 — 15 Sales 1 — 1 Settlements (10 ) 7 (3 ) Ending balance at September 30, 2017 $ 10 $ (192 ) $ (182 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2017 $ — $ (15 ) $ (15 ) For the nine months ended September 30, 2016 Beginning balance at January 1, 2016 $ 16 $ (170 ) $ (154 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (4 ) (13 ) (17 ) Purchases 13 — 13 Settlements (16 ) 11 (5 ) Ending balance at September 30, 2016 $ 9 $ (172 ) $ (163 ) Change in unrealized gains (losses) related to assets/liabilities held at September 30, 2016 $ — $ (7 ) $ (7 )
Schedule Of Carrying Amounts And Estimated Fair Values Of Long-Term Debt, Capital Lease Obligations And Preferred Stock	The following table presents the carrying amounts and estimated fair values of our long-term debt, capital lease obligations and preferred stock at September 30, 2017 , and December 31, 2016 : September 30, 2017 December 31, 2016 Carrying Amount Fair Value Carrying Amount Fair Value Ameren: Long-term debt and capital lease obligations (including current portion) $ 7,699 $ 8,234 $ 7,276 $ 7,772 Preferred stock (a) 142 131 142 131 Ameren Missouri: Long-term debt and capital lease obligations (including current portion) $ 3,967 $ 4,312 $ 3,994 $ 4,304 Preferred stock 80 79 80 79 Ameren Illinois: Long-term debt (including current portion) $ 2,590 $ 2,759 $ 2,588 $ 2,765 Preferred stock 62 52 62 52 (a) Preferred stock is recorded in “Noncontrolling Interests” on the consolidated balance sheet.

Related Party Transactions (Tab

Related Party Transactions (Tables)	9 Months Ended
	Sep. 30, 2017
Related Party Transactions [Abstract]
Schedule of Related Party Transactions	The following table presents the impact on Ameren Missouri and Ameren Illinois of related party transactions for the three and nine months ended September 30, 2017 and 2016 : Three Months Nine Months Agreement Income Statement Line Item Ameren Missouri Ameren Illinois Ameren Missouri Ameren Illinois Ameren Missouri power supply Operating Revenues 2017 $ 4 $ (a) $ 21 $ (a) agreements with Ameren Illinois 2016 9 (a) 21 (a) Ameren Missouri and Ameren Illinois Operating Revenues 2017 7 1 20 3 rent and facility services 2016 5 1 18 3 Ameren Missouri and Ameren Illinois Operating Revenues 2017 (b) (b) (b) 1 miscellaneous support services 2016 1 (b) 1 (b) Total Operating Revenues 2017 $ 11 $ 1 $ 41 $ 4 2016 15 1 40 3 Ameren Illinois power supply Purchased Power 2017 $ (a) $ 4 $ (a) $ 21 agreements with Ameren Missouri 2016 (a) 9 (a) 21 Ameren Illinois transmission Purchased Power 2017 (a) (b) (a) 1 services with ATXI 2016 (a) 1 (a) 2 Total Purchased Power 2017 $ (a) $ 4 $ (a) $ 22 2016 (a) 10 (a) 23 Ameren Services support services Other Operations and Maintenance 2017 $ 34 $ 33 $ 103 $ 99 agreement 2016 30 29 96 90 Money pool borrowings (advances) Interest Charges/ Miscellaneous Income 2017 $ (b) $ (b) $ (b) $ (b) 2016 (b) (b) (b) (b) (a) Not applicable. (b) Amount less than $1 million.

Commitments And Contingencies C

Commitments And Contingencies Commitments and Contingencies (Tables)	9 Months Ended
	Sep. 30, 2017
Commitments and Contingencies Disclosure [Abstract]
Long-term Purchase Commitment	The table below presents our estimated minimum fuel, purchased power, and other commitments for fuel at September 30, 2017 . Ameren’s and Ameren Illinois’ purchased power commitments include the Ameren Illinois agreements entered into as part of the IPA-administered power procurement process. Included in the Other column are minimum purchase commitments under contracts for equipment, design and construction, and meter reading services, among other agreements, at September 30, 2017 . Coal Natural Gas (a) Nuclear Fuel Purchased Power (b)(c) Methane Gas Other Total Ameren: (d) 2017 $ 162 $ 65 $ 19 $ 59 $ 1 $ 33 $ 339 2018 453 200 67 170 4 57 951 2019 356 148 27 63 4 39 637 2020 79 94 39 13 5 39 269 2021 27 36 45 2 5 26 141 Thereafter — 47 58 20 64 123 312 Total $ 1,077 $ 590 $ 255 $ 327 $ 83 $ 317 $ 2,649 Ameren Missouri: 2017 $ 162 $ 14 $ 19 $ — $ 1 $ 20 $ 216 2018 453 42 67 — 4 44 610 2019 356 34 27 — 4 25 446 2020 79 26 39 — 5 25 174 2021 27 13 45 — 5 26 116 Thereafter — 22 58 — 64 100 244 Total $ 1,077 $ 151 $ 255 $ — $ 83 $ 240 $ 1,806 Ameren Illinois: 2017 $ — $ 51 $ — $ 59 $ — $ 13 $ 123 2018 — 158 — 170 — 13 341 2019 — 114 — 63 — 14 191 2020 — 68 — 13 — 14 95 2021 — 23 — 2 — — 25 Thereafter — 25 — 20 — — 45 Total $ — $ 439 $ — $ 327 $ — $ 54 $ 820 (a) Includes amounts for generation and for distribution. (b) The purchased power amounts for Ameren and Ameren Illinois exclude agreements for renewable energy credits through 2032 with various renewable energy suppliers due to the contingent nature of the payment amounts. (c) The purchased power amounts for Ameren and Ameren Missouri exclude a 102-megawatt power purchase agreement with a wind farm operator, which expires in 2024, due to the contingent nature of the payment amounts. (d) Includes amounts for Ameren registrant and nonregistrant subsidiaries.

Callaway Energy Center (Tables)

Callaway Energy Center (Tables)	9 Months Ended
	Sep. 30, 2017
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 at September 30, 2017 . 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 2017. Type and Source of Coverage Maximum Coverages Maximum Assessments for Single Incidents Public liability and nuclear worker liability: American Nuclear Insurers $ 450 $ — Pool participation 12,986 (a) 127 (b) $ 13,436 (c) $ 127 Property damage: NEIL and EMANI $ 3,200 (d) $ 29 (e) Replacement power: NEIL $ 490 (f) $ 7 (e) (a) Provided through mandatory participation in an industrywide retrospective premium assessment program. (b) Retrospective premium under the Price-Anderson Act. This is subject to retrospective assessment with respect to a covered loss in excess of $450 million in the event of an incident at any licensed United States commercial reactor, payable at $19 million per year. (c) Limit of liability for each incident under the Price-Anderson liability provisions of the Atomic Energy Act of 1954, as amended. This limit is subject to change to account for the effects of inflation and changes in the number of licensed reactors. (d) NEIL provides $2.7 billion in property damage, stabilization, decontamination, and premature decommissioning insurance for radiation events and $2.3 billion in property damage insurance for nonradiation events. EMANI provides $490 million in property damage insurance for both radiation and nonradiation events. (e) All NEIL insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL. (f) Provides replacement power cost insurance in the event of a prolonged accidental outage. Weekly indemnity up to $4.5 million for 52 weeks, which commences after the first twelve weeks of an outage, plus up to $3.6 million per week for a minimum of 71 weeks thereafter for a total not exceeding the policy limit of $490 million . Nonradiation events are limited to $328 million .

Retirement Benefits (Tables)

Retirement Benefits (Tables)	9 Months Ended
	Sep. 30, 2017
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 (benefit), prior to capitalization, incurred for Ameren’s pension and postretirement benefit plans for the three and nine months ended September 30, 2017 and 2016 : Pension Benefits Postretirement Benefits Three Months Nine Months Three Months Nine Months 2017 2016 2017 2016 2017 2016 2017 2016 Service cost $ 24 $ 20 $ 70 $ 60 $ 6 $ 5 $ 16 $ 15 Interest cost 44 46 134 138 12 12 35 36 Expected return on plan assets (65 ) (63 ) (196 ) (189 ) (19 ) (18 ) (56 ) (54 ) Amortization of: Prior service benefit (1 ) — (1 ) — (2 ) (1 ) (4 ) (3 ) Actuarial loss (gain) 14 8 41 24 (2 ) (3 ) (5 ) (8 ) Net periodic benefit cost (benefit) $ 16 $ 11 $ 48 $ 33 $ (5 ) $ (5 ) $ (14 ) $ (14 )
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 pension costs and the postretirement benefit costs (benefit) incurred for the three and nine months ended September 30, 2017 and 2016 : Pension Benefits Postretirement Benefits Three Months Nine Months Three Months Nine Months 2017 2016 2017 2016 2017 2016 2017 2016 Ameren Missouri (a) $ 6 $ 6 $ 18 $ 19 $ (1 ) $ (1 ) $ (3 ) $ (3 ) Ameren Illinois 10 6 30 17 (3 ) (3 ) (10 ) (10 ) Other — (1 ) — (3 ) (1 ) (1 ) (1 ) (1 ) Ameren (a)(b) $ 16 $ 11 $ 48 $ 33 $ (5 ) $ (5 ) $ (14 ) $ (14 ) (a) Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. (b) Includes amounts for Ameren registrants and nonregistrant subsidiaries.

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2017
Segment Reporting [Abstract]
Schedule Of Segment Reporting Information By Segment	The following tables present revenues, net income attributable to common shareholders, and capital expenditures by segment at Ameren and Ameren Illinois for the three and nine months ended September 30, 2017 and 2016 . 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 2017 External revenues $ 1,104 $ 405 $ 111 $ 105 $ (2 ) $ — $ 1,723 Intersegment revenues 11 — 1 14 (a) — (26 ) — Net income attributable to Ameren common shareholders 234 31 2 38 (b) (17 ) — 288 Capital expenditures 178 112 71 173 (2 ) (7 ) 525 2016 External revenues $ 1,150 $ 502 $ 113 $ 94 $ — $ — $ 1,859 Intersegment revenues 15 1 1 14 (a) — (31 ) — Net income attributable to Ameren common shareholders 241 93 2 39 (b) (6 ) — 369 Capital expenditures 147 123 50 175 1 — 496 Nine Months 2017 External revenues $ 2,799 $ 1,176 $ 509 $ 293 $ (2 ) $ — $ 4,775 Intersegment revenues 41 3 1 33 (a) — (78 ) — Net income attributable to Ameren common shareholders 359 94 40 106 (b) (16 ) — 583 Capital expenditures 533 354 180 463 3 (10 ) 1,523 2016 External revenues $ 2,733 $ 1,210 $ 529 $ 247 $ 1 $ — $ 4,720 Intersegment revenues 40 3 1 36 (a) — (80 ) — Net income attributable to Ameren common shareholders 347 122 44 98 (b) 10 — 621 Capital expenditures 500 359 130 503 4 — 1,496 (a) Ameren Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above. (b) 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 Consolidated 2017 External revenues $ 405 $ 112 $ 58 $ — $ 575 Intersegment revenues — — 14 (a) (14 ) — Net income available to common shareholder 31 2 22 — 55 Capital expenditures 112 71 93 — 276 2016 External revenues $ 503 $ 114 $ 59 $ — $ 676 Intersegment revenues — — 14 (a) (14 ) — Net income available to common shareholder 93 2 24 — 119 Capital expenditures 123 50 68 — 241 Nine Months 2017 External revenues $ 1,179 $ 510 $ 165 $ — $ 1,854 Intersegment revenues — — 32 (a) (32 ) — Net income available to common shareholder 94 40 57 — 191 Capital expenditures 354 180 226 — 760 2016 External revenues $ 1,213 $ 530 $ 152 $ — $ 1,895 Intersegment revenues — — 35 (a) (35 ) — Net income available to common shareholder 122 44 57 — 223 Capital expenditures 359 130 194 — 683 (a) Ameren Illinois Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above.

Summary Of Significant Accoun34

Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Schedule of Asset Retirement Obligations) (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2017	Dec. 31, 2016
Asset Retirement Obligation [Line Items]
Balance	[1]	$ 646	$ 650
Liabilities settled		(5)
Accretion	[2]	20
Change of estimates	[3]	(19)
Other current liabilities		318	274
Union Electric Company
Asset Retirement Obligation [Line Items]
Balance	[1]	642	644
Liabilities settled		(4)
Accretion	[2]	20
Change of estimates	[3]	(18)
Other current liabilities		118	123
Ameren Illinois Company
Asset Retirement Obligation [Line Items]
Balance	[4]	4	6
Liabilities settled		(1)
Accretion	[2],[5]	1
Change of estimates	[3],[4]	(1)
Other current liabilities		153	109
Asset Retirement Obligation Balance
Asset Retirement Obligation [Line Items]
Other current liabilities		15	15
Asset Retirement Obligation Balance | Union Electric Company
Asset Retirement Obligation [Line Items]
Other current liabilities		$ 15	$ 15
[1]	Balance included $15 million in “Other current liabilities” on the balance sheet as of both December 31, 2016 and September 30, 2017, respectively.
[2]	Accretion expense was recorded as a decrease to regulatory liabilities.
[3]	Ameren Missouri changed its fair value estimate primarily due to an extension of the remediation period of certain CCR storage facilities, an update to the decommissioning of the Callaway energy center to reflect the cost study and funding analysis filed with the MoPSC in 2017, and an increase in the discount rate assumption.
[4]	Included in “Other deferred credits and liabilities” on the balance sheet.
[5]	Less than $1 million.

Summary Of Significant Accoun35

Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Summary of Nonvested Shares Related To Long-Term Incentive Plan) (Details) - $ / shares		9 Months Ended
		Sep. 30, 2017	Dec. 31, 2016
Weighted-average Fair Value per Unit at Grant Date
Closing common share price			$ 52.46
Performance Shares
Share Units
Share Units, Nonvested as of beginning of year		1,059,639
Share Units, Granted	[1]	500,943
Share Units, Forfeitures		(48,661)
Share Units, Vested	[2]	(27,446)
Share Units, Nonvested as of Current period end		1,484,475
Weighted-average Fair Value per Unit at Grant Date
Weighted-average Fair Value per Unit, Nonvested as of Beginning of year		$ 48.04
Weighted-average Fair Value per Unit, Granted	[1]	59.16
Weighted-average Fair Value per Unit, Forfeitures		52.54
Weighted-average Fair Value per Unit, Vested	[2]	52.88
Weighted-average Fair Value per Unit, Nonvested as of Current period end		$ 51.55
Performance period		3 years
Risk free interest rate period		3 years
Three-year risk-free rate		1.47%
Volatility rate, minimum		15.00%
Volatility rate, maximum		21.00%
[1]	Performance share units granted to certain executive and nonexecutive officers and other eligible employees under the 2014 Incentive Plan.
[2]	Performance share units vested due to the attainment of retirement eligibility by certain employees. Actual shares issued for retirement-eligible employees vary depending on actual performance over the three-year measurement period.

Summary Of Significant Accoun36

Summary Of Significant Accounting Policies (Schedule Of Excise Taxes) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accounting Policies [Line Items]
Excise tax expense	$ 61	$ 61	$ 162	$ 162
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	51	52	122	122
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$ 10	$ 9	$ 40	$ 40

Summary Of Significant Accoun37

Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Narrative) (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Basis Of Presentation And Significant Accounting Policies [Line Items]
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	2.1	0.3	1.4	0.4
Variable Interest Entity, Nonconsolidated, Carrying Amount, Assets and Liabilities, Net	$ 14		$ 14		$ 9
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount	23		$ 23
State and Local Jurisdiction [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
State Income Tax Statutory Rate			7.75%
Deferred Tax Liabilities, Net	97		$ 97
Other Affiliated Entities and Intercompany Eliminations | State and Local Jurisdiction [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Increase (Decrease) in Income Taxes			14
Ameren Illinois Company | State and Local Jurisdiction [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Increase (Decrease) in Income Taxes			1
Deferred Tax Liabilities, Net	$ 79		$ 79
Maximum | State and Local Jurisdiction [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
State Income Tax Statutory Rate			9.50%
Minimum | State and Local Jurisdiction [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
State Income Tax Statutory Rate			7.30%

Rate And Regulatory Matters (Na

Rate And Regulatory Matters (Narrative-Missouri) (Detail) - Electric Distribution - Union Electric Company $ in Millions	9 Months Ended
	Sep. 30, 2017USD ($)
Final Rate Order
Rate And Regulatory Matters [Line Items]
Revenue Requirement	$ 3,400
Public Utilities, Requested Rate Increase (Decrease), Amount	92
Components of Rate Increase - Net Energy Costs
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	54
Components of Rate Increase - Regulatory Amortizations
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 26

Rate And Regulatory Matters (39

Rate And Regulatory Matters (Narrative-Illinois) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Rate And Regulatory Matters [Line Items]
Regulatory assets	$ 1,509		$ 1,509		$ 1,437
Current regulatory assets	75		75		149
Regulatory liabilities	2,045		2,045		1,985
Electric	1,594	$ 1,725	4,183	$ 4,101
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets	921		921		816
Current regulatory assets	51		51		108
Regulatory liabilities	766		766		$ 768
Electric	463	562	1,343	1,365
Ameren Transmission Company of Illinois
Rate And Regulatory Matters [Line Items]
Estimated Capital Project Costs			85
Property, Plant and Equipment, Additions			32
IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Electric	76	$ 11	1	$ 24
2016 IEIMA Revenue Requirement Reconciliation | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Regulatory assets	24		24
2015 IEIMA Revenue Requirement Reconciliation | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Current regulatory assets	16		16
2017 IEIMA Revenue Requirement Reconciliation | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	$ 1		1
Pending Rate Case | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount			$ 17
FEJA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Equity Capital Structure, Percentage			50.00%
FEJA 2018 through 2021 | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
FEJA Energy Efficiency Investments			$ 99

Rate And Regulatory Matters (40

Rate And Regulatory Matters (Narrative-Federal) (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	$ 119	$ 110
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	$ 85	$ 78
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 | 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%
Payments for Legal Settlements	$ 21
Incentive adder to FERC allowed base return on common equity	0.50%
Midwest Independent Transmission System Operator, Inc | Final Rate Order | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Payments for Legal Settlements	$ 17
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	$ 41
Midwest Independent Transmission System Operator, Inc | Pending Ferc Case | Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	$ 24
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) $ in Billions	3 Months Ended		9 Months Ended
	Sep. 30, 2017USD ($)	Sep. 30, 2016	Sep. 30, 2017USD ($)	Sep. 30, 2016
Utilities
Line of Credit Facility [Line Items]
Short Term Debt, Weighted Average Interest Rate During Period	1.24%	0.53%	1.18%	0.54%
Credit Agreements
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1.7		$ 1.7
Actual debt-to-capital ratio	0.51		0.51
Missouri Credit Agreement | Union Electric Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.47		0.47
Illinois Credit Agreement | Ameren Illinois Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.46		0.46

Short-Term Debt And Liquidity42

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper outstanding) (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Commercial paper outstanding	$ 446	$ 558
Ameren (parent)
Short-term Debt [Line Items]
Commercial paper outstanding	277	507
Union Electric Company
Short-term Debt [Line Items]
Commercial paper outstanding	0	0
Ameren Illinois Company
Short-term Debt [Line Items]
Commercial paper outstanding	$ 169	$ 51

Short-Term Debt And Liquidity43

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 754	$ 563
Weighted-average interest rate		1.27%	0.79%
Peak commercial paper during period	[1]	$ 948	$ 839
Peak interest rate		1.50%	0.95%
Ameren (parent)
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 669	$ 435
Weighted-average interest rate		1.27%	0.81%
Peak commercial paper during period	[1]	$ 841	$ 574
Peak interest rate		1.50%	0.95%
Union Electric Company
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 7	$ 80
Weighted-average interest rate		1.20%	0.74%
Peak commercial paper during period	[1]	$ 64	$ 208
Peak interest rate		1.41%	0.85%
Ameren Illinois Company
Short-term Debt [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 78	$ 48
Weighted-average interest rate		1.28%	0.72%
Peak commercial paper during period	[1]	$ 193	$ 195
Peak interest rate		1.50%	0.85%
[1]	(a)The timing of peak commercial paper issuances varies by company. Therefore, the sum of peak commercial paper issuances presented by company does not equal the Ameren Consolidated peak commercial paper issuances for the period.

Long-Term debt and Equity Fin44

Long-Term debt and Equity Financings (Narrative) (Details)	9 Months Ended
	Sep. 30, 2017USD ($)
Union Electric Company | Senior Secured Notes, 2.95%, Due 2027 - $400 Issuance | Secured Debt
Long-Term Debt And Equity Financings [Line Items]
Total principal amount of notes	$ 400,000,000
State interest rate	2.95%
Proceeds from Issuance of Secured Debt	$ 396,000,000
Union Electric Company | Senior Secured Notes640 Due2017
Long-Term Debt And Equity Financings [Line Items]
State interest rate	6.40%
Repayments of other long-term debt	$ 425,000,000
Ameren Transmission Company of Illinois
Long-Term Debt And Equity Financings [Line Items]
Redemption price, percentage	100.00%
Ameren Transmission Company of Illinois | Senior Unsecured Notes, 3.43%, Due 2050 | Unsecured Debt
Long-Term Debt And Equity Financings [Line Items]
Total principal amount of notes	$ 300,000,000
State interest rate	3.43%
Proceeds from issuance of unsecured debt	$ 449,000,000
Ratio of indebtedness to net capital	0.70
Ratio of indebtedness to total assets	0.10
Maximum | Ameren Transmission Company of Illinois | Senior Unsecured Notes, 3.43%, Due 2050 | Unsecured Debt
Long-Term Debt And Equity Financings [Line Items]
Total principal amount of notes	$ 450,000,000
Minimum | Ameren Transmission Company of Illinois
Long-Term Debt And Equity Financings [Line Items]
Redemption price, percentage	5.00%
Minimum | Ameren Transmission Company of Illinois | Senior Unsecured Notes, 3.43%, Due 2050 | Unsecured Debt
Long-Term Debt And Equity Financings [Line Items]
Total principal amount of notes	$ 150,000,000

Long-Term Debt and Equity Fin45

Long-Term Debt and Equity Financings (Schedule of Maturities of Long-Term Debt) (Details) - Ameren Transmission Company of Illinois - Unsecured Debt	Sep. 30, 2017USD ($)
August 2,022
Debt Instrument [Line Items]
Repayments of principal in year five	$ 49,500,000
August 2,024
Debt Instrument [Line Items]
Repayments of principal after year five	49,500,000
August 2,027
Debt Instrument [Line Items]
Repayments of principal after year five	49,500,000
August 2,030
Debt Instrument [Line Items]
Repayments of principal after year five	49,500,000
August 2,032
Debt Instrument [Line Items]
Repayments of principal after year five	49,500,000
August 2,038
Debt Instrument [Line Items]
Repayments of principal after year five	49,500,000
August 2,043
Debt Instrument [Line Items]
Repayments of principal after year five	76,500,000
August 2,050
Debt Instrument [Line Items]
Repayments of principal after year five	76,500,000
Senior Unsecured Notes, 3.43%, Due 2050
Debt Instrument [Line Items]
Total principal amount of notes	300,000,000
Senior Unsecured Notes, 3.43%, Due 2050 | Maximum
Debt Instrument [Line Items]
Total principal amount of notes	$ 450,000,000

Other Income and Expenses (Deta

Other Income and Expenses (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	[1]	$ 6	$ 7	$ 16	$ 20
Interest income on industrial development revenue bonds	[1]	7	7	20	20
Interest income	[1]	0	3	5	11
Other	[1]	0	1	1	3
Total miscellaneous income	[1]	13	18	42	54
Donations	[1]	0	1	7	8
Other	[1]	2	7	9	13
Total miscellaneous expense	[1]	2	8	16	21
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		6	6	15	16
Interest income on industrial development revenue bonds		7	7	20	20
Interest income		0	1	0	1
Other		0	0	1	1
Total miscellaneous income		13	14	36	38
Donations		0	0	2	2
Other		2	2	4	4
Total miscellaneous expense		2	2	6	6
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		0	1	1	4
Interest income		1	2	5	9
Other		0	1	1	2
Total miscellaneous income		1	4	7	15
Donations		0	1	5	6
Other		0	2	3	5
Total miscellaneous expense		$ 0	$ 3	$ 8	$ 11
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Derivative Financial Instrume47

Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) lb in Thousands, gal in Millions, MWh in Millions, MMBTU in Millions		Sep. 30, 2017galMWhMMBTUlb	Dec. 31, 2016galMWhMMBTUlb
Fuel Oils
Derivative [Line Items]
Quantity | gal	[1]	30	30
Natural Gas
Derivative [Line Items]
Quantity | MMBTU		169	154
Power
Derivative [Line Items]
Quantity | MWh		11	10
Uranium
Derivative [Line Items]
Quantity | lb		370	345
Union Electric Company | Fuel Oils
Derivative [Line Items]
Quantity | gal	[1]	30	30
Union Electric Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU		24	25
Union Electric Company | Power
Derivative [Line Items]
Quantity | MWh		2	1
Union Electric Company | Uranium
Derivative [Line Items]
Quantity | lb		370	345
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU		145	129
Ameren Illinois Company | Power
Derivative [Line Items]
Quantity | MWh		9	9
[1]	Consists of ultra-low-sulfur diesel products.

Derivative Financial Instrume48

Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions		Sep. 30, 2017		Dec. 31, 2016
Derivative [Line Items]
Derivative assets	[1]	$ 16		$ 27
Derivative liabilities	[2]	216		211
Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets		2		2
Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets		1		1
Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		2		5
Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets		1		12
Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets		1		3
Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		11		4
Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		10		10
Power | Other Current Assets
Derivative [Line Items]
Derivative assets		10		9
Power | Other Assets
Derivative [Line Items]
Derivative assets		1
Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		14		15
Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		179		173
Uranium | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		0	[3]	4
Union Electric Company
Derivative [Line Items]
Derivative assets	[1]	14		14
Derivative liabilities	[2]	10		18
Union Electric Company | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets		2		2
Union Electric Company | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets		1		1
Union Electric Company | Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		2		5
Union Electric Company | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets		0		1
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		3		1
Union Electric Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		4		5
Union Electric Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets		10		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		3
Union Electric Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		0		0
Union Electric Company | Uranium | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		0	[3]	4
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	[1]	2		13
Derivative liabilities	[2]	206		193
Ameren Illinois Company | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets		0		0
Ameren Illinois Company | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets		0		0
Ameren Illinois Company | Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		0		0
Ameren Illinois Company | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets		1		11
Ameren Illinois Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets		1		2
Ameren Illinois Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		8		3
Ameren Illinois Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		6		5
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		13		12
Ameren Illinois Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		179		173
Ameren Illinois Company | Uranium | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		$ 0		$ 0
[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.
[3]	Beginning in 2017, as a result of rulebook amendments at the Chicago Mercantile Exchange, the fair value of uranium derivative liabilities are offset by certain settlement payments made to the exchange previously characterized as collateral and included within “Other assets” on Ameren’s and Ameren Missouri’s balance sheet.

Derivative Financial Instrume49

Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Detail) $ in Millions	Sep. 30, 2017USD ($)
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	$ 107	[1]
Cash Collateral Posted	3
Potential Aggregate Amount of Additional Collateral Required	89	[2]
Union Electric Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	59	[1]
Cash Collateral Posted	3
Potential Aggregate Amount of Additional Collateral Required	48	[2]
Ameren Illinois Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	48	[1]
Cash Collateral Posted	0
Potential Aggregate Amount of Additional Collateral Required	$ 41	[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 Valuation Process And Unobservable Inputs) (Detail) $ in Millions		9 Months Ended	12 Months Ended
		Sep. 30, 2017USD ($)$ / MMBTU$ / MWh$ / credit$ / MW	Dec. 31, 2016USD ($)$ / MMBTU$ / MWh$ / credit$ / lb$ / MW
Natural Gas
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	[1]	$ 0
Natural Gas | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	[2],[3]	0.34%	0.13%
Nodal basis | $ / MMBTU	[4]	(1.1)	(0.8)
Credit risk	[2],[3]	0.37%	0.38%
Natural Gas | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	[2],[3]	6.00%	8.00%
Nodal basis | $ / MMBTU	[4]	(0.1)	0
Credit risk	[2],[3]	0.37%	0.38%
Natural Gas | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	[2],[3]	0.73%	1.00%
Nodal basis | $ / MMBTU	[4]	(0.8)	(0.5)
Credit risk	[2],[3]	0.37%	0.38%
Natural Gas | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities	[4]		31.00%
Nodal basis | $ / MMBTU	[4]		(0.40)
Natural Gas | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities	[4]		66.00%
Nodal basis | $ / MMBTU	[4]		(0.1)
Natural Gas | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities	[4]		36.00%
Nodal basis | $ / MMBTU	[4]		(0.20)
Natural Gas | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	[1]	$ 0	$ 1
Natural Gas | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	[1]	$ (2)	$ (1)
Fuel Oils | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Escalation rate	[4],[5]		(2.00%)
Counterparty credit risk	[2],[3]	0.12%	0.13%
Credit risk	[2],[3]	0.37%	0.38%
Fuel Oils | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Escalation rate	[4],[5]		2.00%
Counterparty credit risk	[2],[3]	0.22%	0.22%
Credit risk	[2],[3]	0.37%	0.38%
Fuel Oils | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Escalation rate	[4],[5]		0.00%
Counterparty credit risk	[2],[3]	0.17%	0.15%
Credit risk	[2],[3]	0.37%	0.38%
Fuel Oils | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities	[4]	24.00%	24.00%
Fuel Oils | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities	[4]	32.00%	66.00%
Fuel Oils | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities	[4]	26.00%	28.00%
Fuel Oils | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	[1]	$ 1	$ 1
Fuel Oils | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	[1]	$ (1)	$ 0
Power | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	[2],[3]	0.28%
Nodal basis | $ / MWh	[6]	(3)	(6)
Credit risk	[2],[3]	0.37%	0.38%
Average forward peak and off-peak pricing | $ / MWh	[6]	25	26
Estimated auction price | $ / MW	[4]	(324)	(71)
Power | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	[2],[3]	0.28%
Nodal basis | $ / MWh	[6]	0	0
Credit risk	[2],[3]	0.37%	0.38%
Average forward peak and off-peak pricing | $ / MWh	[6]	41	44
Estimated auction price | $ / MW	[4]	1,194	5,270
Power | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	[2],[3]	0.28%
Nodal basis | $ / MWh	[6]	(2)	(2)
Credit risk	[2],[3]	0.37%	0.38%
Average forward peak and off-peak pricing | $ / MWh	[6]	28	29
Estimated auction price | $ / MW	[4]	269	125
Power | Fundamental Energy Production Model | Minimum
Fair Value Inputs [Abstract]
Escalation rate	[4],[7]	3.00%	5.00%
Estimated future gas prices | $ / MMBTU	[4]	3	3
Power | Fundamental Energy Production Model | Maximum
Fair Value Inputs [Abstract]
Escalation rate	[4],[7]	3.00%	5.00%
Estimated future gas prices | $ / MMBTU	[4]	4	4
Power | Fundamental Energy Production Model | Weighted Average
Fair Value Inputs [Abstract]
Escalation rate	[4],[7]	3.00%	5.00%
Estimated future gas prices | $ / MMBTU	[4]	3	3
Power | Contract Price Allocation | Minimum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit	[4]	5	5
Power | Contract Price Allocation | Maximum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit	[4]	7	7
Power | Contract Price Allocation | Weighted Average
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit	[4]	6	6
Power | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	[1],[8]	$ 11	$ 9
Power | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	[1],[8]	$ (193)	$ (187)
Uranium | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Credit risk	[2],[3]		0.38%
Average forward pricing | $ / lb	[4]		22
Uranium | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Credit risk	[2],[3]		0.38%
Average forward pricing | $ / lb	[4]		24
Uranium | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Credit risk	[2],[3]		0.38%
Average forward pricing | $ / lb	[4]		22
Uranium | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities	[4]		24.00%
Uranium | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities	[4]		24.00%
Uranium | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities	[4]		24.00%
Uranium | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	[1]		$ 0
Uranium | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	[1]		$ (4)
[1]	The derivative asset and liability balances are presented net of counterparty credit considerations.
[2]	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.
[3]	Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.
[4]	Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.
[5]	Escalation rate applies to fuel oil prices 2019 and beyond.
[6]	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.
[7]	Escalation rate applies to power prices in 2031 and beyond.
[8]	Power valuations use visible third-party pricing evaluated by month for peak and off-peak demand through 2021 for September 30, 2017, and through 2020 for December 31, 2016. Valuations beyond 2021 for September 30, 2017, and 2020 for December 31, 2016 use fundamentally modeled pricing by month for peak and off-peak demand.

Fair Value Measurements (Sche51

Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions		Sep. 30, 2017		Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		$ 672		$ 605	[1]
Assets fair value		688	[2]	632	[3]
Excluded receivables, payables, and accrued income, net				2
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		672		605	[1]
Assets fair value		686	[2]	619	[3]
Excluded receivables, payables, and accrued income, net		2		(1)
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		16	[2]	27	[3]
Derivative liabilities		216	[2]	211	[3]
Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		14	[2]	14	[3]
Derivative liabilities		10	[2]	18	[3]
Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		206	[2]	193	[3]
Natural Gas
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]	0
Cash and cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		2		1
Cash and cash equivalents | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		2		1
Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		445		408
Equity Securities | U.S. large capitalization | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		445		408
Debt Securities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		82		67
Debt Securities | Corporate bonds | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		82		67
Debt Securities | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		119		112
Debt Securities | US treasury and government securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		119		112
Debt Securities | Other Debt Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		24		17
Debt Securities | Other Debt Securities | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		24		17
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		447		409
Assets fair value		450	[2]	413	[3]
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		447		409
Assets fair value		449	[2]	411	[3]
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		3	[2]	4	[3]
Derivative liabilities		1	[2]	5	[3]
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		2	[2]	2	[3]
Derivative liabilities		1	[2]	5	[3]
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	[2]	0	[3]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Cash and cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		2		1
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Cash and cash equivalents | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		2		1
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		445		408
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		445		408
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		225		196
Assets fair value		226	[2]	208	[3]
Significant Other Observable Inputs (Level 2) | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		225		196
Assets fair value		225	[2]	197	[3]
Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[2]	12	[3]
Derivative liabilities		19	[2]	14	[3]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[2]	1	[3]
Derivative liabilities		7	[2]	7	[3]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		12	[2]	7	[3]
Significant Other Observable Inputs (Level 2) | Cash and cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Observable Inputs (Level 2) | Cash and cash equivalents | 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) | 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		82		67
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		82		67
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		119		112
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		119		112
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		24		17
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		24		17
Significant Other Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Assets fair value		12	[2]	11	[3]
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		12	[2]	11	[3]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		12	[2]	11	[3]
Derivative liabilities		196	[2]	192	[3]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		12	[2]	11	[3]
Derivative liabilities		2	[2]	6	[3]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		194	[2]	186	[3]
Significant Other Unobservable Inputs (Level 3) | Cash and cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Significant Other Unobservable Inputs (Level 3) | Cash and cash equivalents | 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) | 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
Uranium | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	[3]			4
Uranium | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	[3]			4
Uranium | 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 liabilities	[3]			0
Uranium | 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 liabilities	[3]			0
Uranium | Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	[3]			0
Uranium | Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	[3]			0
Uranium | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	[3]			4
Uranium | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	[3]			4
Fuel Oils | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		3	[2]	3	[3]
Derivative liabilities		2	[2]	5	[3]
Fuel Oils | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		3	[2]	3	[3]
Derivative liabilities		2	[2]	5	[3]
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		2	[2]	2	[3]
Derivative liabilities		1	[2]	5	[3]
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		2	[2]	2	[3]
Derivative liabilities		1	[2]	5	[3]
Fuel Oils | Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[2]	0	[3]
Derivative liabilities		0	[2]	0	[3]
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	[2]	0	[3]
Derivative liabilities		0	[2]	0	[3]
Fuel Oils | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[2]	1	[3]
Derivative liabilities		1	[2]	0	[3]
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		1	[2]	1	[3]
Derivative liabilities		1	[2]	0	[3]
Power | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		11	[2]	9	[3]
Derivative liabilities		193	[2]	188	[3]
Power | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		11	[2]	9	[3]
Derivative liabilities		1	[2]	3	[3]
Power | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		192	[2]	185	[3]
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	[2]	0	[3]
Derivative liabilities		0	[2]	0	[3]
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	[2]	0	[3]
Derivative liabilities		0	[2]	0	[3]
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	[2]	0	[3]
Power | Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[2]	0	[3]
Derivative liabilities		0	[2]	1	[3]
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	[2]	0	[3]
Derivative liabilities		0	[2]	1	[3]
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	[2]	0	[3]
Power | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		11	[2]	9	[3]
Derivative liabilities		193	[2]	187	[3]
Power | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		11	[2]	9	[3]
Derivative liabilities		1	[2]	2	[3]
Power | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		192	[2]	185	[3]
Natural Gas | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		2	[2]	15	[3]
Derivative liabilities		21	[2]	14	[3]
Natural Gas | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]			2
Derivative liabilities		7	[2]	6	[3]
Natural Gas | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		2	[2]	13	[3]
Derivative liabilities		14	[2]	8	[3]
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		1	[2]	2	[3]
Derivative liabilities		0	[2]	0	[3]
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	[3]			0
Derivative liabilities		0	[2]	0	[3]
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		1	[2]	2	[3]
Derivative liabilities		0	[2]	0	[3]
Natural Gas | Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[2]	12	[3]
Derivative liabilities		19	[2]	13	[3]
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	[3]			1
Derivative liabilities		7	[2]	6	[3]
Natural Gas | Significant Other Observable Inputs (Level 2) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[2]	11	[3]
Derivative liabilities		12	[2]	7	[3]
Natural Gas | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[3]			1
Derivative liabilities		2	[2]	1	[3]
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	[3]			1
Derivative liabilities		0	[2]	0	[3]
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		0	[2]	0	[3]
Derivative liabilities		$ 2	[2]	$ 1	[3]
[1]	Balance excludes $2 million of receivables, payables, and accrued income, net.
[2]	The derivative asset and liability balances are presented net of counterparty credit considerations.
[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 (Sche52

Fair Value Measurements (Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level Three In The Fair Value Hierarchy) (Detail) - Power - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	$ (182)	$ (163)	$ (182)	$ (163)	$ (178)	$ (178)	$ (155)	$ (154)
Included in regulatory assets/liabilities	(5)	(6)	(17)	(17)
Purchases			15	13
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	1		1
Settlements	0	(2)	(3)	(5)
Change in unrealized gains (losses) related to assets/liabilities held at period end	(2)	(2)	(15)	(7)
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	10	9	10	9	14	7	14	16
Included in regulatory assets/liabilities	(2)	0	(3)	(4)
Purchases			15	13
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	1		1
Settlements	(3)	(5)	(10)	(16)
Change in unrealized gains (losses) related to assets/liabilities held at period end	0	0	0	0
Ameren Illinois Company
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs	(192)	(172)	(192)	(172)	$ (192)	$ (185)	$ (169)	$ (170)
Included in regulatory assets/liabilities	(3)	(6)	(14)	(13)
Purchases			0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	0		0
Settlements	3	3	7	11
Change in unrealized gains (losses) related to assets/liabilities held at period end	$ (2)	$ (2)	$ (15)	$ (7)

Fair Value Measurements (Sche53

Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Long-Term Debt And Preferred Stock) (Detail) - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)	[1]	$ 8,234	$ 7,772
Preferred stock	[1]	131	131
Fair Value | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)		4,312	4,304
Preferred stock		79	79
Fair Value | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)		2,759	2,765
Preferred stock		52	52
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)	[1]	7,699	7,276
Preferred stock	[1]	142	142
Carrying Amount | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)		3,967	3,994
Preferred stock		80	80
Carrying Amount | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)		2,590	2,588
Preferred stock		$ 62	$ 62
[1]	Preferred stock is recorded in “Noncontrolling Interests” on the consolidated balance sheet.

Related Party Transactions (Sch

Related Party Transactions (Schedule of Related Party Transactions) (Detail) - USD ($) $ in Millions		3 Months Ended				9 Months Ended
		Sep. 30, 2017		Sep. 30, 2016		Sep. 30, 2017		Sep. 30, 2016
Related Party Transaction [Line Items]
Operating Revenues		$ 0		$ 0		$ 0		$ 0
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues		0		0		0
Purchased Power | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses		4		10		22		23
Ameren Missouri Power Supply Agreements with Ameren Illinois | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues		4		9		21		21
Ameren Missouri and Ameren Illinois Rent and Facility Services | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues		7		5		20		18
Ameren Missouri and Ameren Illinois Rent and Facility Services | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues		1		1		3		3
Ameren Missouri and Ameren Illinois miscellaneous support services | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues		1	[1]	1		1	[1]	1
Ameren Missouri and Ameren Illinois miscellaneous support services | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues		1	[1]	1	[1]	1		1	[1]
Total Related Party Operating Revenues [Member] | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues		11		15		41		40
Total Related Party Operating Revenues [Member] | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues		1		1		4		3
Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses		4		9		21		21
Ameren Illinois Transmission Agreements with ATXI | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses		1	[1]	1		1		2
Ameren Services Support Services Agreement | Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses		34		30		103		96
Ameren Services Support Services Agreement | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses		33		29		99		90
Related Party Money Pool Interest | Union Electric Company
Related Party Transaction [Line Items]
Interest Charges	[1]	1		1		1		1
Related Party Money Pool Interest | Ameren Illinois Company
Related Party Transaction [Line Items]
Interest Charges	[1]	$ 1		$ 1		$ 1		$ 1
[1]	Amount less than $1 million.

Related Party Transactions Narr

Related Party Transactions Narrative (Details) - April 2017 Procurement - Ameren Illinois Company - Ameren Illinois Power Supply Agreements with Ameren Missouri	9 Months Ended
	Sep. 30, 2017MWh$ / MWh
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power | MWh	85,600
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate | $ / MWh	34

Commitments And Contingencies (

Commitments And Contingencies (Other Obligations) (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2017USD ($)MWh
Long-term Purchase Commitment [Line Items]
2,017	$ 339	[1]
2,018	951	[1]
2,019	637	[1]
2,020	269	[1]
2,021	141	[1]
Thereafter	312	[1]
Total	2,649	[1]
Union Electric Company
Long-term Purchase Commitment [Line Items]
2,017	216
2,018	610
2,019	446
2,020	174
2,021	116
Thereafter	244
Total	1,806
Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
2,017	123
2,018	341
2,019	191
2,020	95
2,021	25
Thereafter	45
Total	820
Coal
Long-term Purchase Commitment [Line Items]
2,017	162	[1]
2,018	453	[1]
2,019	356	[1]
2,020	79	[1]
2,021	27	[1]
Thereafter	0	[1]
Total	1,077	[1]
Coal | Union Electric Company
Long-term Purchase Commitment [Line Items]
2,017	162
2,018	453
2,019	356
2,020	79
2,021	27
Thereafter	0
Total	1,077
Natural Gas
Long-term Purchase Commitment [Line Items]
2,017	65	[1],[2]
2,018	200	[1],[2]
2,019	148	[1],[2]
2,020	94	[1],[2]
2,021	36	[1],[2]
Thereafter	47	[1],[2]
Total	590	[1],[2]
Natural Gas | Union Electric Company
Long-term Purchase Commitment [Line Items]
2,017	14	[2]
2,018	42	[2]
2,019	34	[2]
2,020	26	[2]
2,021	13	[2]
Thereafter	22	[2]
Total	151	[2]
Natural Gas | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
2,017	51	[2]
2,018	158	[2]
2,019	114	[2]
2,020	68	[2]
2,021	23	[2]
Thereafter	25	[2]
Total	439	[2]
Nuclear Fuel
Long-term Purchase Commitment [Line Items]
2,017	19	[1]
2,018	67	[1]
2,019	27	[1]
2,020	39	[1]
2,021	45	[1]
Thereafter	58	[1]
Total	255	[1]
Nuclear Fuel | Union Electric Company
Long-term Purchase Commitment [Line Items]
2,017	19
2,018	67
2,019	27
2,020	39
2,021	45
Thereafter	58
Total	255
Purchased Power
Long-term Purchase Commitment [Line Items]
2,017	59	[1],[3],[4]
2,018	170	[1],[3],[4]
2,019	63	[1],[3],[4]
2,020	13	[1],[3],[4]
2,021	2	[1],[3],[4]
Thereafter	20	[1],[3],[4]
Total	327	[1],[3],[4]
Purchased Power | Union Electric Company
Long-term Purchase Commitment [Line Items]
2,017	0	[3],[4]
2,018	0	[3],[4]
2,019	0	[3],[4]
2,020	0	[3],[4]
2,021	0	[3],[4]
Thereafter	0	[3],[4]
Total	0	[3],[4]
Purchased Power | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
2,017	59	[3],[4]
2,018	170	[3],[4]
2,019	63	[3],[4]
2,020	13	[3],[4]
2,021	2	[3],[4]
Thereafter	20	[3],[4]
Total	327	[3],[4]
Methane Gas
Long-term Purchase Commitment [Line Items]
2,017	1	[1]
2,018	4	[1]
2,019	4	[1]
2,020	5	[1]
2,021	5	[1]
Thereafter	64	[1]
Total	83	[1]
Methane Gas | Union Electric Company
Long-term Purchase Commitment [Line Items]
2,017	1
2,018	4
2,019	4
2,020	5
2,021	5
Thereafter	64
Total	83
Other Purchased Fuel
Long-term Purchase Commitment [Line Items]
2,017	33	[1]
2,018	57	[1]
2,019	39	[1]
2,020	39	[1]
2,021	26	[1]
Thereafter	123	[1]
Total	317	[1]
Other Purchased Fuel | Union Electric Company
Long-term Purchase Commitment [Line Items]
2,017	20
2,018	44
2,019	25
2,020	25
2,021	26
Thereafter	100
Total	240
Other Purchased Fuel | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
2,017	13
2,018	13
2,019	14
2,020	14
2,021	0
Thereafter	0
Total	$ 54
April 2017 Procurement | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Amount of Megawatthours | MWh	4,249,800
Long-term Purchase Commitment, Amount	$ 128
September 2017 Procurement | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Amount of Megawatthours | MWh	1,950,200
Long-term Purchase Commitment, Amount	$ 57
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries.
[2]	(a)Includes amounts for generation and for distribution.
[3]	(b)The purchased power amounts for Ameren and Ameren Illinois exclude agreements for renewable energy credits through 2032 with various renewable energy suppliers due to the contingent nature of the payment amounts.
[4]	(c)The purchased power amounts for Ameren and Ameren Missouri exclude a 102-megawatt power purchase agreement with a wind farm operator, which expires in 2024, due to the contingent nature of the payment amounts.

Commitments And Contingencies57

Commitments And Contingencies (Environmental Matters) (Detail) $ in Millions	Sep. 30, 2017USD ($)scrubbersite	Dec. 31, 2016
Loss Contingencies [Line Items]
Percentage of Rate Base Related to Carbon Dioxide Energy Centers		18.00%
Minimum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 425
Maximum
Loss Contingencies [Line Items]
Estimated capital costs to comply with existing and known federal and state air emissions regulations	$ 525
Union Electric Company
Loss Contingencies [Line Items]
Percentage of Rate Base Related to Carbon Dioxide Energy Centers		34.00%
Number of Energy Center Scrubbers | scrubber	2
Manufactured Gas Plant
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 184
Manufactured Gas Plant | Ameren Illinois Company
Loss Contingencies [Line Items]
Number of remediation sites | site	44
Accrual for environmental loss contingencies	$ 184
Manufactured Gas Plant | Ameren Illinois Company | Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	184
Manufactured Gas Plant | Ameren Illinois Company | Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	255
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

Callaway Energy Center (Narrati

Callaway Energy Center (Narrative) (Detail) $ in Millions	3 Months Ended	9 Months Ended
	Dec. 31, 2017USD ($)	Sep. 30, 2017USD ($)mill	Sep. 30, 2016USD ($)
Nuclear Waste Matters [Line Items]
Litigation Settlement, Amount			$ 24
Nuclear Plant
Nuclear Waste Matters [Line Items]
Number of mills charged for NWF fee | mill		1
Annual decommissioning costs included in costs of service		$ 7
Subsequent Event
Nuclear Waste Matters [Line Items]
Litigation Settlement, Amount	$ 3

Callaway Energy Center (Insuran

Callaway Energy Center (Insurance Disclosure) (Detail) € in Millions	9 Months Ended
	Sep. 30, 2017USD ($)		Sep. 30, 2017EUR (€)
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	12,986,000,000	[1]
Maximum Assessments for Single Incidents	127,000,000	[2]
Public Liability
Nuclear Waste Matters [Line Items]
Maximum Coverages	13,436,000,000	[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	29,000,000	[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 (Components

Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		$ 24	$ 20	$ 70	$ 60
Interest cost		44	46	134	138
Expected return on plan assets		(65)	(63)	(196)	(189)
Prior service cost (benefit)		(1)	0	(1)	0
Actuarial loss		14	8	41	24
Net periodic benefit cost	[1],[2]	16	11	48	33
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		6	5	16	15
Interest cost		12	12	35	36
Expected return on plan assets		(19)	(18)	(56)	(54)
Prior service cost (benefit)		(2)	(1)	(4)	(3)
Actuarial loss		(2)	(3)	(5)	(8)
Net periodic benefit cost	[1],[2]	(5)	(5)	(14)	(14)
Union Electric Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	[1]	6	6	18	19
Union Electric Company | Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost	[1]	(1)	(1)	(3)	(3)
Ameren Illinois Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost		10	6	30	17
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		$ (3)	$ (3)	$ (10)	$ (10)
[1]	(a)Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates
[2]	(b)Includes amounts for Ameren registrants and nonregistrant subsidiaries.

Retirement Benefits (Summary Of

Retirement Benefits (Summary Of Benefit Plan Costs Incurred) (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost	[1],[2]	$ 16	$ 11	$ 48	$ 33
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	[1]	6	6	18	19
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		10	6	30	17
Pension Plan | Other Affiliated Entities and Intercompany Eliminations
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost		0	(1)	0	(3)
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	[1],[2]	(5)	(5)	(14)	(14)
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	[1]	(1)	(1)	(3)	(3)
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		(3)	(3)	(10)	(10)
Other Postretirement Benefit Plan, Defined Benefit | Other Affiliated Entities and Intercompany Eliminations
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost		$ (1)	$ (1)	$ (1)	$ (1)
[1]	(a)Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates
[2]	(b)Includes amounts for Ameren registrants and nonregistrant subsidiaries.

Segment Information (Schedule O

Segment Information (Schedule Of Segment Reporting Information By Segment) (Detail) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2017USD ($)segment	Sep. 30, 2016USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment				4
External revenues		$ 1,723	$ 1,859	$ 4,775	$ 4,720
Intersegment revenues		0	0	0	0
Net Income Attributable to Ameren Common Shareholders		288	369	583	621
Capital expenditures		525	496	$ 1,523	1,496
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		575	676	$ 1,854	1,895
Intersegment revenues		0	0	0
Net Income Available to Common Shareholder		55	119	191	223
Capital expenditures		276	241	760	683
Intersegment Eliminations
Segment Reporting Information [Line Items]
External revenues		0	0	0	0
Intersegment revenues		(26)	(31)	(78)	(80)
Net Income Available to Common Shareholder		0	0	0	0
Capital expenditures		(7)	0	(10)	0
Intersegment Eliminations | Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues		0	0	0
Intersegment revenues		(14)	(14)	(32)	(35)
Net Income Available to Common Shareholder		0	0	0
Capital expenditures		0	0	0
Operating Segments | Union Electric Company
Segment Reporting Information [Line Items]
External revenues		1,104	1,150	2,799	2,733
Intersegment revenues		11	15	41	40
Net Income Available to Common Shareholder		234	241	359	347
Capital expenditures		178	147	533	500
Operating Segments | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues		405	502	1,176	1,210
Intersegment revenues		0	1	3	3
Net Income Available to Common Shareholder		31	93	94	122
Capital expenditures		112	123	354	359
Operating Segments | Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues		111	113	509	529
Intersegment revenues		1	1	1	1
Net Income Available to Common Shareholder		2	2	40	44
Capital expenditures		71	50	180	130
Operating Segments | Ameren Transmission
Segment Reporting Information [Line Items]
External revenues		105	94	293	247
Intersegment revenues	[1]	14	14	33	36
Net Income Available to Common Shareholder	[2]	38	39	106	98
Capital expenditures		173	175	463	503
Operating Segments | Other
Segment Reporting Information [Line Items]
External revenues		(2)	0	(2)	1
Intersegment revenues		0	0	0	0
Net Income Available to Common Shareholder		(17)	(6)	(16)	10
Capital expenditures		(2)	1	3	4
Operating Segments | Ameren Illinois Company | Ameren Illinois Electric Distribution
Segment Reporting Information [Line Items]
External revenues		405	503	1,179	1,213
Intersegment revenues		0	0	0
Net Income Available to Common Shareholder		31	93	94	122
Capital expenditures		112	123	354	359
Operating Segments | Ameren Illinois Company | Ameren Illinois Gas
Segment Reporting Information [Line Items]
External revenues		112	114	510	530
Intersegment revenues		0	0	0
Net Income Available to Common Shareholder		2	2	40	44
Capital expenditures		71	50	180	130
Operating Segments | Ameren Illinois Company | Ameren Illinois Transmission
Segment Reporting Information [Line Items]
External revenues		58	59	165	152
Intersegment revenues	[3]	14	14	32	35
Net Income Available to Common Shareholder		22	24	57	57
Capital expenditures		$ 93	$ 68	$ 226	$ 194
[1]	(a)Ameren Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above.
[2]	(b)Ameren Transmission earnings include an allocation of financing costs from Ameren (parent).
[3]	(a)Ameren Illinois Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above.