Ameren Illinois (AILIN) 10-Q 5 May 17 2017 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2017	Apr. 28, 2017
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q1
Trading Symbol	AEE
Entity Registrant Name	AMEREN CORP
Entity Central Index Key	1,002,910
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		242,634,798
Union Electric Company
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q1
Entity Registrant Name	UNION ELECTRIC CO
Entity Central Index Key	100,826
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		102,123,834
Ameren Illinois Company
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q1
Entity Registrant Name	AMEREN ILLINOIS CO
Entity Central Index Key	18,654
Current Fiscal Year End Date	--12-31
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		25,452,373

Consolidated Statement of Incom

Consolidated Statement of Income (Loss) - USD ($) shares in Millions, $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Operating Revenues:
Electric		$ 1,206	$ 1,102
Gas		308	332
Total operating revenues		1,514	1,434
Operating Expenses:
Fuel		206	203
Purchased power		180	138
Gas purchased for resale		130	152
Other operations and maintenance		405	400
Depreciation and amortization		221	207
Taxes other than income taxes		118	114
Total operating expenses		1,260	1,214
Operating Income		254	220
Other Income and Expenses:
Miscellaneous income	[1]	15	20
Miscellaneous expense	[1]	9	7
Total other income (expense)		6	13
Interest Charges		99	95
Income Before Income Taxes		161	138
Income Taxes		57	31
Net income		104	107
Pension and other postretirement benefit plan activity, net of income taxes (benefit)		0	(2)
Comprehensive Income		102	103
Less: Net Income Attributable to Noncontrolling Interests		2	2
Net Income Attributable to Ameren Common Shareholders		$ 102	$ 105
Earnings Per Share, Basic and Diluted [Abstract]
Earnings Per Share, Basic and Diluted		$ 0.42	$ 0.43
Dividends per Common Share		$ 0.44	$ 0.425
Average Common Shares Outstanding - Basic		242.6	242.6
Union Electric Company
Operating Revenues:
Electric		$ 746	$ 694
Gas		44	47
Total operating revenues		790	741
Operating Expenses:
Fuel		206	203
Purchased power		91	42
Gas purchased for resale		20	21
Other operations and maintenance		212	212
Depreciation and amortization		133	127
Taxes other than income taxes		75	73
Total operating expenses		737	678
Operating Income		53	63
Other Income and Expenses:
Miscellaneous income		12	15
Miscellaneous expense		2	2
Total other income (expense)		10	13
Interest Charges		54	52
Income Before Income Taxes		9	24
Income Taxes		3	9
Net income		6	15
Other Comprehensive Income		0	0
Comprehensive Income		6	15
Earnings Per Share, Basic and Diluted [Abstract]
Preferred Stock Dividends		1	1
Net Income Available to Common Stockholder		5	14
Ameren Illinois Company
Operating Revenues:
Electric		439	392
Gas		264	285
Total operating revenues		703	677
Operating Expenses:
Purchased power		101	104
Gas purchased for resale		110	131
Other operations and maintenance		197	194
Depreciation and amortization		83	77
Taxes other than income taxes		40	38
Total operating expenses		531	544
Operating Income		172	133
Other Income and Expenses:
Miscellaneous income		3	5
Miscellaneous expense		6	5
Total other income (expense)		(3)	0
Interest Charges		37	35
Income Before Income Taxes		132	98
Income Taxes		52	38
Net income		80	60
Pension and other postretirement benefit plan activity, net of income taxes (benefit)		0	(1)
Comprehensive Income		80	59
Earnings Per Share, Basic and Diluted [Abstract]
Preferred Stock Dividends		1	1
Net Income Available to Common Stockholder		$ 79	$ 59
[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
	Mar. 31, 2017	Mar. 31, 2016
Pension and other postretirement benefit plan activity, tax expense (benefit)	$ 0	$ 1
Ameren Illinois Company
Pension and other postretirement benefit plan activity, tax expense (benefit)	$ 0	$ (1)

Consolidated Statement of Compr

Consolidated Statement of Comprehensive Income (Loss) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 104	$ 107
Other Comprehensive Loss, Net of Taxes
Pension and other postretirement benefit plan activity, net of income taxes (benefit)	0	(2)
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	104	105
Comprehensive Income (Loss), Net of Tax, Attributable to Noncontrolling Interest	2	2
Comprehensive Income Attributable to Ameren Common Shareholders	$ 102	$ 103

Consolidated Statement of Comp5

Consolidated Statement of Comprehensive Income (Loss) Consolidated Statement of Income (Parenthetical) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Statement of Comprehensive Income [Abstract]
Pension and other postretirement benefit plan activity, tax expense (benefit)	$ 0	$ 1

Consolidated Balance Sheet

Consolidated Balance Sheet - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Current Assets:
Cash and cash equivalents	$ 8	$ 9
Accounts receivable - trade (less allowance for doubtful accounts)	457	437
Unbilled revenue	228	295
Miscellaneous accounts and notes receivable	67	63
Materials and supplies	467	527
Current regulatory assets	118	149
Other current assets	105	113
Total current assets	1,450	1,593
Property and Plant, Net	20,298	20,113
Investments and Other Assets:
Nuclear decommissioning trust fund	635	607
Goodwill	411	411
Regulatory assets	1,485	1,437
Other assets	532	538
Total investments and other assets	3,063	2,993
TOTAL ASSETS	24,811	24,699
Current Liabilities:
Current maturities of long-term debt	681	681
Short-term Debt	914	558
Accounts and wages payable	460	805
Taxes accrued	77	46
Interest accrued	100	93
Customer deposits	106	107
Current regulatory liabilities	144	110
Other current liabilities	280	274
Total current liabilities	2,762	2,674
Long-term Debt, Net	6,597	6,595
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	4,321	4,264
Accumulated deferred investment tax credits	53	55
Regulatory liabilities	1,982	1,985
Asset retirement obligations	641	635
Pension and other postretirement benefits	768	769
Other deferred credits and liabilities	481	477
Total deferred credits and other liabilities	8,246	8,185
Commitments and Contingencies
Stockholders' Equity:
Common Stock	2	2
Other paid-in capital	5,522	5,556
Retained earnings	1,563	1,568
Accumulated other comprehensive income (loss)	(23)	(23)
Stockholder's equity	7,064	7,103
Noncontrolling Interest	142	142
Total equity	7,206	7,245
TOTAL LIABILITIES AND EQUITY	24,811	24,699
Union Electric Company
Current Assets:
Cash and cash equivalents	0	0
Advances to money pool	0	161
Accounts receivable - trade (less allowance for doubtful accounts)	175	187
Accounts receivable - affiliates	15	12
Unbilled revenue	126	154
Miscellaneous accounts and notes receivable	23	14
Materials and supplies	381	392
Current regulatory assets	23	35
Other current assets	45	49
Total current assets	788	1,004
Property and Plant, Net	11,471	11,478
Investments and Other Assets:
Nuclear decommissioning trust fund	635	607
Regulatory assets	606	619
Other assets	314	327
Total investments and other assets	1,555	1,553
TOTAL ASSETS	13,814	14,035
Current Liabilities:
Current maturities of long-term debt	431	431
Short-term Debt	36	0
Accounts and wages payable	191	444
Accounts payable - affiliates	54	68
Taxes accrued	63	30
Interest accrued	53	54
Current regulatory liabilities	8	12
Other current liabilities	128	123
Total current liabilities	964	1,162
Long-term Debt, Net	3,564	3,563
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	3,017	3,013
Accumulated deferred investment tax credits	51	53
Regulatory liabilities	1,240	1,215
Asset retirement obligations	635	629
Pension and other postretirement benefits	292	291
Other deferred credits and liabilities	16	19
Total deferred credits and other liabilities	5,251	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	1,616	1,671
Stockholder's equity	4,035	4,090
TOTAL LIABILITIES AND EQUITY	13,814	14,035
Ameren Illinois Company
Current Assets:
Cash and cash equivalents	0	0
Accounts receivable - trade (less allowance for doubtful accounts)	270	242
Accounts receivable - affiliates	10	10
Unbilled revenue	102	141
Miscellaneous accounts and notes receivable	17	22
Materials and supplies	86	135
Current regulatory assets	91	108
Other current assets	16	25
Total current assets	592	683
Property and Plant, Net	7,596	7,469
Investments and Other Assets:
Goodwill	411	411
Regulatory assets	875	816
Other assets	95	95
Total investments and other assets	1,381	1,322
TOTAL ASSETS	9,569	9,474
Current Liabilities:
Current maturities of long-term debt	250	250
Short-term Debt	68	51
Accounts and wages payable	198	264
Accounts payable - affiliates	57	63
Taxes accrued	13	16
Interest accrued	42	33
Customer deposits	68	69
Mark-to-market derivative liabilities	18	15
Current environmental remediation	42	38
Current regulatory liabilities	118	78
Other current liabilities	88	94
Total current liabilities	962	971
Long-term Debt, Net	2,338	2,338
Deferred Credits and Other Liabilities:
Accumulated deferred income taxes, net	1,682	1,631
Accumulated deferred investment tax credits	2	2
Regulatory liabilities	739	768
Pension and other postretirement benefits	345	346
Environmental remediation	152	162
Other deferred credits and liabilities	236	222
Total deferred credits and other liabilities	3,156	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	1,046	967
Stockholder's equity	3,113	3,034
TOTAL LIABILITIES AND EQUITY	$ 9,569	$ 9,474

Consolidated Balance Sheet (Par

Consolidated Balance Sheet (Parenthetical) - USD ($) shares in Millions, $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Accounts receivable - trade allowance for doubtful accounts	$ 19	$ 19
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	400	400
Common stock, shares outstanding	242.6	242.6
Union Electric Company
Accounts receivable - trade allowance for doubtful accounts	$ 6	$ 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	$ 13	$ 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		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Cash Flows From Operating Activities:
Net income		$ 104	$ 107
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization		217	210
Amortization of nuclear fuel		24	24
Amortization of debt issuance costs and premium/discounts		6	6
Deferred income taxes and investment tax credits, net		51	42
Allowance for equity funds used during construction	[1]	(6)	(8)
Share-based compensation costs		4	6
Other		(4)	(3)
Changes in assets and liabilities:
Receivables		44	55
Inventories		60	55
Accounts and wages payable		(231)	(246)
Taxes accrued		36	30
Regulatory assets and liabilities		7	81
Assets, other		7	7
Liabilities, other		3	(26)
Pension and other postretirement benefits		9	9
Net cash provided by operating activities		331	349
Cash Flows From Investing Activities:
Capital expenditures		(504)	(496)
Nuclear fuel expenditures		(27)	(21)
Purchases of securities - nuclear decommissioning trust fund		(64)	(130)
Sales and maturities of securities - nuclear decommissioning trust fund		58	125
Other		(2)	12
Net cash used in investing activities		(539)	(510)
Cash Flows From Financing Activities:
Dividends on common stock		(107)	(103)
Dividends paid to noncontrolling interest holders		(2)	(2)
Short-term debt, net		356	280
Maturities of Long-term Debt		0	(260)
Share-based payments		(39)	(32)
Other		(1)	(1)
Net cash provided by (used in) financing activities		207	(118)
Net change in cash and cash equivalents		(1)	(279)
Cash and cash equivalents at beginning of period		9	292
Cash and cash equivalents at end of period		8	13
Union Electric Company
Cash Flows From Operating Activities:
Net income		6	15
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization		127	130
Amortization of nuclear fuel		24	24
Amortization of debt issuance costs and premium/discounts		2	2
Deferred income taxes and investment tax credits, net		2	9
Allowance for equity funds used during construction		(5)	(7)
Other		1	0
Changes in assets and liabilities:
Receivables		28	81
Inventories		11	(2)
Accounts and wages payable		(186)	(172)
Taxes accrued		35	31
Regulatory assets and liabilities		29	45
Assets, other		11	5
Liabilities, other		4	3
Pension and other postretirement benefits		4	5
Net cash provided by operating activities		93	169
Cash Flows From Investing Activities:
Capital expenditures		(196)	(178)
Nuclear fuel expenditures		(27)	(21)
Purchases of securities - nuclear decommissioning trust fund		(64)	(130)
Sales and maturities of securities - nuclear decommissioning trust fund		58	125
Money pool advances, net		161	36
Other		0	(2)
Net cash used in investing activities		(68)	(170)
Cash Flows From Financing Activities:
Dividends on common stock		(60)	(140)
Dividends on preferred stock		(1)	(1)
Short-term debt, net		36	165
Maturities of Long-term Debt		0	(260)
Capital contribution from parent		0	38
Net cash provided by (used in) financing activities		(25)	(198)
Net change in cash and cash equivalents		0	(199)
Cash and cash equivalents at beginning of period		0	199
Cash and cash equivalents at end of period		0	0
Ameren Illinois Company
Cash Flows From Operating Activities:
Net income		80	60
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization		83	77
Amortization of debt issuance costs and premium/discounts		3	4
Deferred income taxes and investment tax credits, net		51	37
Allowance for equity funds used during construction		(1)	(1)
Other		0	(3)
Changes in assets and liabilities:
Receivables		16	(22)
Inventories		49	57
Accounts and wages payable		(51)	(33)
Taxes accrued		(2)	(3)
Regulatory assets and liabilities		(19)	32
Assets, other		2	7
Liabilities, other		(5)	12
Pension and other postretirement benefits		5	4
Net cash provided by operating activities		212	229
Cash Flows From Investing Activities:
Capital expenditures		(227)	(211)
Money pool advances, net		0	(58)
Net cash used in investing activities		(227)	(269)
Cash Flows From Financing Activities:
Dividends on common stock		0	(30)
Dividends on preferred stock		(1)	(1)
Short-term debt, net		17	0
Other		(1)	0
Net cash provided by (used in) financing activities		15	(31)
Net change in cash and cash equivalents		0	(71)
Cash and cash equivalents at beginning of period		0	71
Cash and cash equivalents at end of period		$ 0	$ 0
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Summary Of Significant Accounti

Summary Of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2017
Accounting Policies [Abstract]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES General Ameren, headquartered in St. Louis, Missouri, is a public utility holding company under PUHCA 2005. Ameren’s primary assets are its equity interests in its subsidiaries, including Ameren Missouri, Ameren Illinois, and ATXI. Ameren’s subsidiaries are separate, independent legal entities with separate businesses, assets, and liabilities. Dividends on Ameren’s common stock and the payment of expenses by Ameren depend on distributions made to it by its subsidiaries. Ameren’s principal subsidiaries are listed below. Ameren also has various other subsidiaries that conduct other activities, such as the provision of shared services. Ameren is also evaluating 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. 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 December 31, 2016 balance sheet presentation of discontinued operations has been reclassified in this report to “Other current assets” for assets of discontinued operations and to “Other current liabilities” for liabilities of discontinued operations. See Note 1 – Summary of Significant Accounting Policies under Part II, Item 8, of the Form 10-K for additional information. Asset Retirement Obligations AROs at Ameren, Ameren Missouri, and Ameren Illinois increased during the three months ended March 31, 2017 , to reflect the accretion of the estimated obligation due to the passage of time, partially offset by immaterial settlements. Share-based Compensation A summary of nonvested performance share units at March 31, 2017 , and changes during the three months ended March 31, 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) 496,068 59.16 Forfeitures (3,192 ) 49.13 Vested (b) (3,779 ) 52.88 Nonvested at March 31, 2017 1,548,736 $ 51.59 (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. 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 – 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 – Gas” and “Operating Expenses – Taxes other than income taxes” for the three months ended March 31, 2017 and 2016 : Three Months 2017 2016 Ameren Missouri $ 31 $ 30 Ameren Illinois 19 20 Ameren $ 50 $ 50 Earnings Per Share There were no material differences between Ameren’s basic and diluted earnings per share amounts for the three months ended March 31, 2017 and 2016 . The assumed settlement of dilutive performance share units had an immaterial impact on earnings per share. There were no potentially dilutive securities excluded from the earnings per diluted share calculations for the three months ended March 31, 2017 and 2016 . 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, restricted cash, and the consolidation analysis for variable interest entities and voting interest entities. 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. 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. During the first quarter of 2017, we determined that contributions in aid of construction and similar arrangements are not within the scope of this guidance. Therefore, our accounting for such arrangements will not change upon adoption of this guidance. In addition, we determined that the calculation of revenue and bad debt expense for tariff sales contracts will not materially change as a result of the collectibility criterion included within the guidance. We plan to complete our assessment of the impacts of this guidance on our results of operations, financial position, disclosures, and determine our transition method, in the next few months prior to our adoption in the first quarter of 2018. Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost In February 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. The adoption of this guidance in the first quarter of 2018 may result in the recognition of new regulatory assets related to the recovery of, and return on, the non-service cost components of net benefit cost and related allowance for funds used during construction balances. 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	3 Months Ended
	Mar. 31, 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 included 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 decrease 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. In April 2016, the MoPSC granted ATXI a certificate of convenience and necessity for the Mark Twain project conditioned upon ATXI obtaining county assents for road crossings. None of the five county commissions have approved ATXI’s requests for the assents. In October 2016, ATXI filed suit in the circuit courts for each of the five counties to obtain the assents for the original project route. A decision in each of the five lawsuits is expected in late 2017. In March 2017, the MoPSC’s April 2016 order was vacated by the Missouri Court of Appeals, Western District, which ruled that the MoPSC could not lawfully grant a certificate of convenience and necessity conditioned upon ATXI obtaining the assents. ATXI is evaluating whether to appeal the March 2017 Court of Appeals decision to the Missouri Supreme Court. In April 2017, ATXI reached agreements in principle with a cooperative electric company in northeast Missouri and with Ameren Missouri to locate portions of the Mark Twain project on existing transmission line corridors, resulting in a proposed alternative project route. ATXI will finalize the proposed alternative project route and then request assents for road crossings from the five affected counties. If all five county commissions provide assents for the proposed alternative project route, ATXI will then seek MoPSC approval. ATXI plans to complete the project in 2019; however, delays in obtaining the assents and approval from the MoPSC could delay completion. Illinois IEIMA & FEJA Under the provisions of Illinois law, Ameren Illinois’ electric distribution service rates are subject to an annual revenue requirement reconciliation to its actual recoverable costs and allowed return on equity. This revenue requirement reconciliation 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 March 31, 2017 , Ameren Illinois had recorded regulatory assets of $24 million and $54 million to reflect its 2016 revenue requirement reconciliation adjustment, which was included in the April 2017 formula rate update discussed below, and the approved 2015 revenue requirement reconciliation adjustment, with interest, respectively. For the three months ended March 31, 2017 , Ameren Illinois recorded a regulatory asset of $46 million to reflect the difference between Ameren Illinois’ estimate of its revenue requirement and the revenue requirement reflected in customer rates. 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. Pending ICC approval, this update filing will result in a $16 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, an increase to include the 2016 revenue requirement reconciliation adjustment, and 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. The revenue requirement decrease in the April 2017 update filing is a result of the 2015 revenue requirement reconciliation adjustment being larger than the 2016 revenue requirement reconciliation adjustment. 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 will now recognize revenue consistent with the timing of actual incurred electric distribution recoverable costs and recognize revenue associated with the expected return on its rate base ratably over the year. Ameren Illinois recognized $46 million and $11 million of electric distribution revenue to reflect the difference between the estimate of its revenue requirement and the revenue requirement reflected in customer rates for the three months ended March 31, 2017 and March 31, 2016, respectively. 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 the 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. During the first quarter 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. As 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. The February 2015 complaint case seeks a further reduction in the allowed base return on common equity for the 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 the 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, which will prospectively replace the current 10.82% allowed return on equity, is uncertain for two reasons. First, at this time the FERC lacks a quorum of three commissioners. Second, the United States Court of Appeals for the District of Columbia recently vacated and remanded to the FERC an order in a separate case in which the FERC established the allowed base return on common equity methodology used in the two MISO complaint cases described above. Ameren is unable to predict the impact of the outcome of the United States Court of Appeals for the District of Columbia Circuit’s remand on the MISO FERC complaint cases at this time. As of March 31, 2017 , Ameren and Ameren Illinois 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	3 Months Ended
	Mar. 31, 2017
Line of Credit Facility [Abstract]
SHORT-TERM DEBT AND LIQUIDITY	SHORT-TERM DEBT AND LIQUIDITY The liquidity needs of the Ameren Companies are typically supported through the use of available cash, drawings under committed credit agreements, commercial paper issuances, or, in the case of Ameren Missouri and Ameren Illinois, short-term 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 three months ended March 31, 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 March 31, 2017 , was $1.2 billion . The Ameren Companies were in compliance with the covenants in their credit agreements as of March 31, 2017. As of March 31, 2017 , the ratios of consolidated indebtedness to consolidated total capitalization, calculated in accordance with the provisions of the Credit Agreements, were 53% , 48% , and 46% for Ameren, Ameren Missouri, and Ameren Illinois, respectively. Commercial Paper The following table presents commercial paper outstanding as of March 31, 2017 , and December 31, 2016 : 2017 2016 Ameren (parent) $ 810 $ 507 Ameren Missouri 36 — Ameren Illinois 68 51 Ameren Consolidated $ 914 $ 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 three months ended March 31, 2017 and 2016: Ameren (parent) Ameren Missouri Ameren Illinois Ameren Consolidated 2017 Average daily commercial paper outstanding $ 682 $ 4 $ 50 $ 736 Weighted-average interest rate 1.07 % 0.93 % 0.95 % 1.06 % Peak commercial paper during period (a) $ 810 $ 45 $ 74 $ 914 Peak interest rate 1.30 % 1.15 % 1.15 % 1.30 % 2016 Average daily commercial paper outstanding $ 349 $ 68 $ — $ 417 Weighted-average interest rate 0.82 % 0.80 % — % 0.81 % Peak commercial paper during period (a) $ 482 $ 208 $ — $ 581 Peak interest rate 0.95 % 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 certain short-term cash and working capital requirements. The average interest rate for borrowing under the utility money pool for the three months ended March 31, 2017 and 2016 , was 1.01% and 0.47% , respectively. See Note 8 – Related Party Transactions for the amount of interest income and expense from the money pool arrangements recorded by the Ameren Companies for the three months ended March 31, 2017 and 2016 .

Long-Term Debt And Equity Finan

Long-Term Debt And Equity Financings	3 Months Ended
	Mar. 31, 2017
Long-Term Debt And Equity Financings [Abstract]
LONG-TERM DEBT AND EQUITY FINANCINGS	LONG-TERM DEBT AND EQUITY FINANCINGS 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. The Ameren Companies were in compliance with those indenture provisions and other covenants as of March 31, 2017 . Off-Balance-Sheet Arrangements At March 31, 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. None of the Ameren Companies expect to engage in any significant off-balance-sheet financing arrangements in the near future.

Other Income and Expenses

Other Income and Expenses	3 Months Ended
	Mar. 31, 2017
Other Nonoperating Income (Expense) [Abstract]
OTHER INCOME AND EXPENSES	OTHER INCOME AND EXPENSES The following table presents the components of “Other Income and Expenses” in the Ameren Companies’ statements of income for the three months ended March 31, 2017 and 2016 : Three Months 2017 2016 Ameren: (a) Miscellaneous income: Three Months 2017 2016 Allowance for equity funds used during construction $ 6 $ 8 Interest income on industrial development revenue bonds 7 7 Interest income 2 4 Other — 1 Total miscellaneous income $ 15 $ 20 Miscellaneous expense: Donations $ 5 $ 5 Other 4 2 Total miscellaneous expense $ 9 $ 7 Ameren Missouri: Miscellaneous income: Allowance for equity funds used during construction $ 5 $ 7 Interest income on industrial development revenue bonds 7 7 Other — 1 Total miscellaneous income $ 12 $ 15 Miscellaneous expense: Donations $ — $ 1 Other 2 1 Total miscellaneous expense $ 2 $ 2 Ameren Illinois: Miscellaneous income: Allowance for equity funds used during construction $ 1 $ 1 Interest income 2 4 Total miscellaneous income $ 3 $ 5 Miscellaneous expense: Donations $ 4 $ 4 Other 2 1 Total miscellaneous expense $ 6 $ 5 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Derivative Financial Instrument

Derivative Financial Instruments	3 Months Ended
	Mar. 31, 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 March 31, 2017 and December 31, 2016 . As of March 31, 2017 , these contracts extended through October 2019, March 2023, May 2032, and February 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) 36 (b) 36 30 (b) 30 Natural gas (in mmbtu) 28 149 177 25 129 154 Power (in megawatthours) 1 9 10 1 9 10 Uranium (pounds in thousands) 345 (b) 345 345 (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 March 31, 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 March 31, 2017 and December 31, 2016 : Balance Sheet Location Ameren Missouri Ameren Illinois Ameren 2017 Fuel oils Other current assets $ 1 $ — $ 1 Other assets 1 — 1 Natural gas Other current assets — 5 5 Power Other current assets 5 — 5 Total assets (a) $ 7 $ 5 $ 12 Fuel oils Other current liabilities $ 5 $ — $ 5 Natural gas MTM derivative liabilities (b) 6 (b) Other current liabilities 2 — 8 Other deferred credits and liabilities 6 9 15 Power MTM derivative liabilities (b) 12 (b) Other current liabilities 1 — 13 Other deferred credits and liabilities — 182 182 Uranium Other deferred credits and liabilities — (c) — — (c) Total liabilities (d) $ 14 $ 209 $ 223 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 MTM derivative liabilities (b) 3 (b) Other current liabilities 1 — 4 Other deferred credits and liabilities 5 5 10 Power MTM derivative liabilities (b) 12 (b) Other current liabilities 3 — 15 Other deferred credits and liabilities — 173 173 Uranium Other deferred credits and liabilities 4 — 4 Total liabilities (d) $ 18 $ 193 $ 211 (a) The cumulative amount of pretax net gains on all derivative instruments is deferred as a regulatory liability. (b) Balance sheet line item not applicable to registrant. (c) 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. (d) The cumulative amount of pretax net losses on all derivative instruments is deferred as a regulatory asset. Derivative instruments are subject to various credit-related losses in the event of nonperformance by counterparties to the transaction. Exchange-traded contracts are supported by the financial and credit quality of the clearing members of the respective exchanges; these contracts have nominal credit risk. In all other transactions, we are exposed to credit risk. Our credit risk management program involves establishing credit limits and collateral requirements for counterparties, using master netting arrangements or similar agreements, and reporting daily exposure to senior management. We believe that entering into master netting arrangements or similar agreements mitigates the level of financial loss that could result from default by allowing net settlement of derivative assets and liabilities. These master netting arrangements allow the counterparties to net settle sale and purchase transactions. Further, collateral requirements are calculated at the master netting arrangement or similar agreement level by counterparty. The Ameren Companies elect to present the fair value amounts of derivative assets and derivative liabilities subject to an enforceable master netting arrangement or similar agreement gross on the balance sheet. However, if the gross amounts recognized on the balance sheet were netted with derivative instruments and cash collateral received or posted, the net amounts would not be materially different from the gross amounts at March 31, 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 March 31, 2017 , if counterparty groups were to fail completely to perform on contracts, the Ameren Companies’ maximum exposure would have been immaterial with or without consideration of the application of master netting arrangements or similar agreements and collateral held. Derivative Instruments with Credit Risk-Related Contingent Features Our commodity contracts contain collateral provisions tied to the Ameren Companies’ credit ratings. If our credit ratings were downgraded, or if a counterparty with reasonable grounds for uncertainty regarding our ability to satisfy an obligation requested adequate assurance of performance, additional collateral postings might be required. The following table presents, as of March 31, 2017 , the aggregate fair value of all derivative instruments with credit risk-related contingent features in a gross liability position, the cash collateral posted, and the aggregate amount of additional collateral that counterparties could require. The additional collateral required is the net liability position allowed under the master netting arrangements or similar agreements, assuming (1) the credit risk-related contingent features underlying these arrangements were triggered on March 31, 2017 , and (2) those counterparties with rights to do so requested collateral. Aggregate Fair Value of Derivative Liabilities (a) Cash Collateral Posted Potential Aggregate Amount of Additional Collateral Required (b) 2017 Ameren Missouri $ 58 $ 8 $ 44 Ameren Illinois 38 — 30 Ameren $ 96 $ 8 $ 74 (a) Before consideration of master netting arrangements or similar agreements and including NPNS and other accrual contract exposures. (b) As collateral requirements with certain counterparties are based on master netting arrangements or similar agreements, the aggregate amount of additional collateral required to be posted is determined after consideration of the effects of such arrangements.

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 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 March 31, 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) 23 – 35 27 Discounted cash flow Counterparty credit risk(%) (c)(d) 0.12 – 0.22 0.15 Fair Value Weighted Average Assets Liabilities Valuation Technique(s) Unobservable Input Range Ameren Missouri credit risk(%) (c)(d) 0.37 (e) Escalation rate (%) (b)(f) 0 – 1 0 Natural gas — (2 ) Discounted cash flow Nodal basis ($/mmbtu) (b) (0.80) – (0.10) (0.70) Counterparty credit risk (%) (c)(d) 0.45 – 6 0.70 Ameren Illinois credit risk (%) (c)(d) 0.37 (e) Power (g) 5 (195 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps ($/MWh) (h) 25 – 44 29 Estimated auction price for FTRs ($/MW) (b) (89) – 1,568 164 Nodal basis ($/MWh) (h) (4) – 0 (2) Ameren Illinois credit risk (%) (c)(d) 0.37 (e) Fundamental energy production model Estimated future 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 ($/MWh) (h) (6) – 0 (2) Ameren Illinois credit risk (%) (c)(d) 0.38 (e) Fundamental energy production model Estimated future gas prices ($/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 March 31, 2017 and through 2020 for December 31, 2016. Valuations beyond 2021 for March 31, 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 first quarter of 2017 or 2016 . At March 31, 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 March 31, 2017 : Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Total Assets: Ameren Derivative assets – commodity contracts (a) : Fuel oils $ 1 $ — $ 1 $ 2 Natural gas 1 4 — 5 Power — — 5 5 Total derivative assets – commodity contracts $ 2 $ 4 $ 6 $ 12 Nuclear decommissioning trust fund: Cash and cash equivalents $ 1 $ — $ — $ 1 Equity securities: U.S. large capitalization 420 — — 420 Debt securities: U.S. treasury and agency securities — 118 — 118 Corporate bonds — 70 — 70 Other — 24 — 24 Total nuclear decommissioning trust fund $ 421 $ 212 $ — $ 633 (b) Total Ameren $ 423 $ 216 $ 6 $ 645 Ameren Missouri Derivative assets – commodity contracts (a) : Fuel oils $ 1 $ — $ 1 $ 2 Power — — 5 5 Total derivative assets – commodity contracts $ 1 $ — $ 6 $ 7 Nuclear decommissioning trust fund: Cash and cash equivalents $ 1 $ — $ — $ 1 Equity securities: U.S. large capitalization 420 — — 420 Debt securities: U.S. treasury and agency securities — 118 — 118 Corporate bonds — 70 — 70 Other — 24 — 24 Total nuclear decommissioning trust fund $ 421 $ 212 $ — $ 633 (b) Total Ameren Missouri $ 422 $ 212 $ 6 $ 640 Ameren Illinois Derivative assets – commodity contracts (a) : Natural gas $ 1 $ 4 $ — $ 5 Liabilities: Ameren Derivative liabilities – commodity contracts (a) : Fuel oils $ 4 $ — $ 1 $ 5 Natural gas — 21 2 23 Power — — 195 195 Total Ameren $ 4 $ 21 $ 198 $ 223 Ameren Missouri Derivative liabilities – commodity contracts (a) : Fuel oils $ 4 $ — $ 1 $ 5 Natural gas — 8 — 8 Power — — 1 1 Total Ameren Missouri $ 4 $ 8 $ 2 $ 14 Ameren Illinois Derivative liabilities – commodity contracts (a) : Natural gas $ — $ 13 $ 2 $ 15 Power — — 194 194 Total Ameren Illinois $ — $ 13 $ 196 $ 209 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Balance excludes $ 2 million of receivables, payables, and accrued income, net. The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of December 31, 2016 : Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Total Assets: Ameren Derivative assets – commodity contracts (a) : Fuel oils $ 2 $ — $ 1 $ 3 Natural gas 2 12 1 15 Power — — 9 9 Total derivative assets – commodity contracts $ 4 $ 12 $ 11 $ 27 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 months ended March 31, 2017 and 2016, the balances and changes in the fair value of Level 3 financial assets and liabilities associated with fuel oils, natural gas, and uranium were immaterial. The following table summarizes the changes in the fair value of power financial assets and liabilities classified as Level 3 in the fair value hierarchy: Net derivative commodity contracts Ameren Missouri Ameren Illinois Ameren For the three months ended March 31, 2017 Beginning balance at January 1, 2017 $ 7 $ (185 ) $ (178 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities — (10 ) (10 ) Settlements (3 ) 1 (2 ) Ending balance at March 31, 2017 $ 4 $ (194 ) $ (190 ) Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2017 $ — $ (11 ) $ (11 ) For the three months ended March 31, 2016 Beginning balance at January 1, 2016 $ 16 $ (170 ) $ (154 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (3 ) (21 ) (24 ) Settlements (7 ) 4 (3 ) Ending balance at March 31, 2016 $ 6 $ (187 ) $ (181 ) Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2016 $ — $ (19 ) $ (19 ) Transfers into or out of Level 3 represent either (1) existing assets and liabilities that were previously categorized as a higher level, but were recategorized to Level 3 because the inputs to the model became unobservable during the period or (2) existing assets and liabilities that were previously classified as Level 3, but were recategorized to a higher level because the lowest significant input became observable during the period. For the three months ended March 31, 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 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 March 31, 2017 and December 31, 2016 : March 31, 2017 December 31, 2016 Carrying Amount Fair Value Carrying Amount Fair Value Ameren: Long-term debt and capital lease obligations (including current portion) $ 7,278 $ 7,762 $ 7,276 $ 7,772 Preferred stock (a) 142 129 142 131 Ameren Missouri: Long-term debt and capital lease obligations (including current portion) $ 3,995 $ 4,312 $ 3,994 $ 4,304 Preferred stock 80 77 80 79 Ameren Illinois: Long-term debt (including current portion) $ 2,588 $ 2,742 $ 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	3 Months Ended
	Mar. 31, 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 $33.64 per megawatthour during the period of March 1, 2019, through May 31, 2020. The following table presents the impact on Ameren Missouri and Ameren Illinois of related party transactions for the three months ended March 31, 2017 and 2016 : Three Months Agreement Income Statement Line Item Ameren Missouri Ameren Illinois Ameren Missouri power supply Operating Revenues 2017 $ 11 $ (a) agreements with Ameren Illinois 2016 9 (a) Ameren Missouri and Ameren Illinois Operating Revenues 2017 7 1 rent and facility services 2016 6 1 Total Operating Revenues 2017 $ 18 $ 1 2016 15 1 Ameren Illinois power supply Purchased Power 2017 $ (a) $ 11 agreements with Ameren Missouri 2016 (a) 9 Ameren Illinois transmission Purchased Power 2017 (a) (b) services with ATXI 2016 (a) (b) Total Purchased Power 2017 $ (a) $ 11 2016 (a) 9 Ameren Services support services Other Operations and Maintenance 2017 $ 35 $ 32 agreement 2016 34 31 Money pool borrowings (advances) Interest Charges/ Miscellaneous Income 2017 $ (b) $ (b) 2016 (b) (b) (a) Not applicable. (b) Amount less than $1 million.

Commitments And Contingencies

Commitments And Contingencies	3 Months Ended
	Mar. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
COMMITMENTS AND CONTINGENCIES	COMMITMENTS AND CONTINGENCIES We are involved in legal, tax and regulatory proceedings before various courts, regulatory commissions, authorities and governmental agencies with respect to matters that arise in the ordinary course of business, some of which involve substantial amounts of money. We believe that the final disposition of these proceedings, except as otherwise disclosed in the notes to our financial statements in this report and in our 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 2 – Rate and Regulatory Matters, Note 8 – Related Party Transactions, and Note 10 – Callaway Energy Center of this report. Other Obligations In order 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. Additionally, Ameren Missouri and Ameren Illinois have entered into various long-term commitments for purchased power and natural gas for distribution. At March 31, 2017 , total obligations related to commitments for coal, natural gas, nuclear fuel, purchased power, methane gas, equipment, and meter reading services, among other agreements, at Ameren, Ameren Missouri, and Ameren Illinois were $3,824 million , $2,340 million , and $1,418 million , respectively. For additional information regarding our obligations and commitments at December 31, 2016, see Note 15 – Commitments and Contingencies under Part II, Item 8 of the Form 10-K. In April 2017, Ameren Illinois conducted a procurement event, administered by the IPA, to purchase energy products through May 31, 2020 . In the April 2017 procurement event, Ameren Illinois contracted to purchase 4,249,800 megawatthours of energy products for $128 million from June 1, 2017, through May 31, 2020. See Note 8 – Related Party Transactions for additional information regarding energy product agreements between Ameren Missouri and Ameren Illinois as a result of this procurement event. Environmental Matters We are subject to various environmental laws and regulations enforced by federal, state, and local authorities. The development and operation of electric generation, transmission, and distribution facilities and natural gas storage, transmission, and distribution facilities, can trigger compliance obligations with respect to diverse environmental laws and regulations. These laws and regulations address emissions, discharges to water, water usage, impacts to air, land, and water, and chemical and waste handling. Complex and lengthy processes are required to obtain and renew approvals, permits, and licenses for new, existing or modified facilities. Additionally, the use and handling of various chemicals or hazardous materials require release prevention plans and emergency response procedures. The EPA has promulgated environmental regulations that have a significant impact on the electric utility industry. Over time, compliance with these regulations could be costly for Ameren Missouri, which operates coal-fired power plants. As of December 31, 2016, Ameren Missouri’s fossil-fueled energy centers represented 18% and 34% of Ameren’s and Ameren Missouri’s rate base, respectively. Recent regulations impacting air emissions from electric utility industry include revised NSPS, the CSAPR, the MATS and 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. Regulation of CO 2 emissions from existing power plants through the Clean Power Plan have been stayed by the United States Supreme Court. In addition, the EPA recently announced it is re-evaluating the legal and policy basis for the Clean Power Plan. Water intake and discharges from power plants are regulated under the Clean Water Act and could require significant capital expenditures, such as modifications to water intake structures at Ameren Missouri’s energy centers. The management and disposal of coal ash is regulated under the CCR Rule, which will require significant capital expenditures, such as the closure of surface impoundments and the installations of dry ash handling systems at several of our energy centers. The EPA has initiated an administrative review of several regulations and rulemaking activities, including the Clean Power Plan, the MATS, and the effluent limitation guidelines, which could ultimately result in the revision of all or part of such rules. However, the individual or combined effects of existing environmental regulations could result in significant capital expenditures and increased operating costs for Ameren and Ameren Missouri. Compliance with existing environmental laws and regulations could be prohibitively expensive, result in 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 environmental regulations for air emissions 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. Ameren Missouri may be required to install additional environmental controls beyond 2021. This estimate of capital expenditures includes expenditures required for the CCR regulations, the Clean Water Act rule applicable to cooling water intake structures at existing power plants, and the Clean Water Act effluent limitation guidelines applicable to steam electric generating units, all of which are discussed below. This estimate does not include the potential impacts of the Clean Power Plan discussed below. The actual amount of capital expenditures required to comply with existing environmental regulations may vary substantially from the above estimate because of uncertainty as to whether the EPA will 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 new environmental laws and rules and environmental enforcement and remediation matters that affect or could affect our operations. 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 regulations that set CO 2 emissions standards for new power plants. These new standards establish separate emissions limits for new natural-gas-fired combined cycle plants and new coal-fired plants. The Clean Power Plan sets forth CO 2 emissions standards applicable to existing power plants. The rule was stayed by the United States Supreme Court in February 2016, pending the outcome of various legal challenges. In April 2017, the EPA announced that it is reviewing and, if appropriate, will initiate proceedings to suspend, revise, or rescind the Clean Power Plan. The District of Columbia Circuit Court of Appeals has stayed further action on the litigation that resulted from the Supreme Court’s February 2016 stay of the Clean Power Plan pending the EPA’s administrative review. The Clean Power Plan would require significant reductions in CO 2 emissions from power plants by 2030 including interim compliance periods commencing in 2022. The EPA has advised all states to discontinue implementation planning. We cannot predict the outcome of the EPA’s administrative review, 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. At the conclusion of both phases of the litigation, Ameren Missouri intends to appeal the liability ruling to the United States Circuit Court of Appeals for the Eighth Circuit. A decision by the district court regarding the remedy phase of the litigation could occur in 2019. 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 April 2017, the EPA announced that it would review and reconsider the effluent limitation guidelines and administratively stayed all compliance deadlines. 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. 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 2023. 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 March 31, 2017 , Ameren Illinois owned or was otherwise responsible for 44 former MGP sites in Illinois, which are in various stages of investigation, evaluation, remediation, and closure. Ameren Illinois estimates it could substantially conclude remediation efforts by 2023. The ICC allows Ameren Illinois to recover 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 March 31, 2017 , Ameren Illinois estimated the obligation related to these former MGP sites at $193 million to $258 million . Ameren and Ameren Illinois recorded a liability of $193 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 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 March 31, 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 collect and pay gross receipts taxes or license fees on certain revenues, including revenues from wholesale power and interchange sales. Ameren and Ameren Missouri recorded immaterial liabilities on their respective balance sheets as of March 31, 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. Ameren Missouri believes its defenses are meritorious and is defending itself vigorously. However, there can be no assurances that Ameren Missouri will be successful in its efforts. A 2018 trial has been set and an order is expected later that year.

Callaway Energy Center

Callaway Energy Center	3 Months Ended
	Mar. 31, 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. For the three months ended March 31, 2017 and March 31, 2016, Ameren Missouri did not receive any such reimbursements. Ameren Missouri will continue to apply for reimbursement from the DOE for allowable costs associated with the ongoing storage of spent fuel. 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. In April 2016, the MoPSC approved no change in the annual decommissioning costs used to establish electric rates. 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 The next scheduled refueling and maintenance outage at Ameren Missouri’s Callaway energy center will be in fall 2017. The Callaway energy center uses nuclear fuel assemblies fabricated by Westinghouse. Westinghouse is currently 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 operate the Callaway energy center. 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, including with respect to the delivery of fuel assembles for the upcoming refueling and maintenance outage, 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 cost of replacement power, the loss of fuel inventory that is stored at Westinghouse’s facility, and payments made for fuel fabrication. 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 March 31, 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 for nonradiation events. EMANI provides $490 million for both radiation and nonradiation events. (e) All NEIL insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL. (f) Provides replacement power cost insurance in the event of a prolonged accidental outage. Weekly indemnity up to $4.5 million for 52 weeks, which commences after the first twelve weeks of an outage, plus up to $3.6 million per week for a minimum of 71 weeks thereafter for a total not exceeding the policy limit of $490 million . Nonradiation events are limited to $328 million . The Price-Anderson Act is a federal law that limits the liability for claims from an incident involving any licensed United States commercial nuclear energy center. The limit is based on the number of licensed reactors. The limit of liability and the maximum potential annual payments are adjusted at least every five years for inflation to reflect changes in the Consumer Price Index. The most recent five-year inflationary adjustment became effective in September 2013. Owners of a nuclear reactor cover this exposure through a combination of private insurance and mandatory participation in a financial protection pool, as established by the Price-Anderson Act. Losses resulting from terrorist attacks on nuclear facilities are subject to industrywide aggregates, such that terrorist acts against one or more commercial nuclear power plants insured by NEIL or EMANI within a stated time period would be treated as a single event, and the owners of the nuclear power plants would share one full limit of liability. NEIL policies have an aggregate limit of $3.2 billion within a 12-month period for radiation events, or $1.8 billion for events not involving radiation contamination. The EMANI policies have an aggregate limit of €600 million for radiation and nonradiation events within a period of 72 hours. If losses from a nuclear incident at the Callaway energy center exceed the limits of, or are not covered by insurance, or if coverage is unavailable, Ameren Missouri is at risk for any uninsured losses. If a serious nuclear incident were to occur, it could have a material adverse effect on Ameren’s and Ameren Missouri’s results of operations, financial position, or liquidity.

Retirement Benefits

Retirement Benefits	3 Months Ended
	Mar. 31, 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) incurred for Ameren’s pension and postretirement benefit plans for the three months ended March 31, 2017 and 2016 : Pension Benefits Postretirement Benefits Three Months Three Months 2017 2016 2017 2016 Service cost $ 23 $ 20 $ 5 $ 5 Interest cost 45 47 12 12 Expected return on plan assets (66 ) (63 ) (19 ) (18 ) Amortization of: Prior service benefit — — (1 ) (1 ) Actuarial loss (gain) 14 9 (2 ) (3 ) Net periodic benefit cost (benefit) $ 16 $ 13 $ (5 ) $ (5 ) 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 months ended March 31, 2017 and 2016 : Pension Benefits Postretirement Benefits Three Months Three Months 2017 2016 2017 2016 Ameren Missouri (a) $ 6 $ 8 $ (1 ) $ (1 ) Ameren Illinois 10 5 (4 ) (4 ) Ameren (a)(b) $ 16 $ 13 $ (5 ) $ (5 ) (a) Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. (b) Includes amounts for Ameren registrants and nonregistrant subsidiaries.

Segment Information

Segment Information	3 Months Ended
	Mar. 31, 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, and associated Ameren (parent) interest charges. 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 and Ameren Illinois. 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 information about the reported revenues and specified items reflected in net income attributable to common shareholders and capital expenditures at Ameren and Ameren Illinois for the three months ended March 31, 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 $ 772 $ 384 $ 264 $ 96 $ (2 ) $ — $ 1,514 Intersegment revenues 18 1 — 6 (a) — (25 ) — Net income attributable to Ameren common shareholders 5 30 33 34 — — 102 Capital expenditures 196 120 51 134 4 (b) (1 ) 504 2016 External revenues $ 726 $ 351 $ 285 $ 72 $ — $ — $ 1,434 Intersegment revenues 15 1 — 11 (a) — (27 ) — Net income attributable to Ameren common shareholders 14 11 35 27 18 — 105 Capital expenditures 178 117 35 164 2 (b) — 496 (a) Ameren Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above. (b) Includes the elimination of intercompany transfers. Ameren Illinois Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Consolidated 2017 External revenues $ 385 $ 264 $ 54 $ — $ 703 Intersegment revenues — — 6 (a) (6 ) — Net income available to common shareholder 30 33 16 — 79 Capital expenditures 120 51 56 — 227 2016 External revenues $ 352 $ 285 $ 40 $ — $ 677 Intersegment revenues — — 11 (a) (11 ) — Net income available to common shareholder 11 35 13 — 59 Capital expenditures 117 35 59 — 211 (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)	3 Months Ended
	Mar. 31, 2017
Accounting Policies [Abstract]
Revenue Recognition, Policy [Policy Text Block]	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.
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. 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.
Excise Taxes	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 – 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	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, restricted cash, and the consolidation analysis for variable interest entities and voting interest entities. 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. 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. During the first quarter of 2017, we determined that contributions in aid of construction and similar arrangements are not within the scope of this guidance. Therefore, our accounting for such arrangements will not change upon adoption of this guidance. In addition, we determined that the calculation of revenue and bad debt expense for tariff sales contracts will not materially change as a result of the collectibility criterion included within the guidance. We plan to complete our assessment of the impacts of this guidance on our results of operations, financial position, disclosures, and determine our transition method, in the next few months prior to our adoption in the first quarter of 2018. Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost In February 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. The adoption of this guidance in the first quarter of 2018 may result in the recognition of new regulatory assets related to the recovery of, and return on, the non-service cost components of net benefit cost and related allowance for funds used during construction balances. 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)	3 Months Ended
	Mar. 31, 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)	3 Months Ended
	Mar. 31, 2017
Accounting Policies [Abstract]
Summary Of Nonvested Shares Related To Long-Term Incentive Plan	A summary of nonvested performance share units at March 31, 2017 , and changes during the three months ended March 31, 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) 496,068 59.16 Forfeitures (3,192 ) 49.13 Vested (b) (3,779 ) 52.88 Nonvested at March 31, 2017 1,548,736 $ 51.59 (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 – Gas” and “Operating Expenses – Taxes other than income taxes” for the three months ended March 31, 2017 and 2016 : Three Months 2017 2016 Ameren Missouri $ 31 $ 30 Ameren Illinois 19 20 Ameren $ 50 $ 50

Short-Term Debt And Liquidity S

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Tables)	3 Months Ended
	Mar. 31, 2017
Schedule of Commercial paper [Abstract]
Schedule of Short-term Debt	The following table presents commercial paper outstanding as of March 31, 2017 , and December 31, 2016 : 2017 2016 Ameren (parent) $ 810 $ 507 Ameren Missouri 36 — Ameren Illinois 68 51 Ameren Consolidated $ 914 $ 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 three months ended March 31, 2017 and 2016: Ameren (parent) Ameren Missouri Ameren Illinois Ameren Consolidated 2017 Average daily commercial paper outstanding $ 682 $ 4 $ 50 $ 736 Weighted-average interest rate 1.07 % 0.93 % 0.95 % 1.06 % Peak commercial paper during period (a) $ 810 $ 45 $ 74 $ 914 Peak interest rate 1.30 % 1.15 % 1.15 % 1.30 % 2016 Average daily commercial paper outstanding $ 349 $ 68 $ — $ 417 Weighted-average interest rate 0.82 % 0.80 % — % 0.81 % Peak commercial paper during period (a) $ 482 $ 208 $ — $ 581 Peak interest rate 0.95 % 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.

Other Income and Expenses (Tabl

Other Income and Expenses (Tables)	3 Months Ended
	Mar. 31, 2017
Other Nonoperating Income (Expense) [Abstract]
Other Income And Expenses	The following table presents the components of “Other Income and Expenses” in the Ameren Companies’ statements of income for the three months ended March 31, 2017 and 2016 : Three Months 2017 2016 Ameren: (a) Miscellaneous income: Three Months 2017 2016 Allowance for equity funds used during construction $ 6 $ 8 Interest income on industrial development revenue bonds 7 7 Interest income 2 4 Other — 1 Total miscellaneous income $ 15 $ 20 Miscellaneous expense: Donations $ 5 $ 5 Other 4 2 Total miscellaneous expense $ 9 $ 7 Ameren Missouri: Miscellaneous income: Allowance for equity funds used during construction $ 5 $ 7 Interest income on industrial development revenue bonds 7 7 Other — 1 Total miscellaneous income $ 12 $ 15 Miscellaneous expense: Donations $ — $ 1 Other 2 1 Total miscellaneous expense $ 2 $ 2 Ameren Illinois: Miscellaneous income: Allowance for equity funds used during construction $ 1 $ 1 Interest income 2 4 Total miscellaneous income $ 3 $ 5 Miscellaneous expense: Donations $ 4 $ 4 Other 2 1 Total miscellaneous expense $ 6 $ 5 (a) Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Derivative Financial Instrume26

Derivative Financial Instruments (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 2017 and December 31, 2016 . As of March 31, 2017 , these contracts extended through October 2019, March 2023, May 2032, and February 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) 36 (b) 36 30 (b) 30 Natural gas (in mmbtu) 28 149 177 25 129 154 Power (in megawatthours) 1 9 10 1 9 10 Uranium (pounds in thousands) 345 (b) 345 345 (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 March 31, 2017 and December 31, 2016 : Balance Sheet Location Ameren Missouri Ameren Illinois Ameren 2017 Fuel oils Other current assets $ 1 $ — $ 1 Other assets 1 — 1 Natural gas Other current assets — 5 5 Power Other current assets 5 — 5 Total assets (a) $ 7 $ 5 $ 12 Fuel oils Other current liabilities $ 5 $ — $ 5 Natural gas MTM derivative liabilities (b) 6 (b) Other current liabilities 2 — 8 Other deferred credits and liabilities 6 9 15 Power MTM derivative liabilities (b) 12 (b) Other current liabilities 1 — 13 Other deferred credits and liabilities — 182 182 Uranium Other deferred credits and liabilities — (c) — — (c) Total liabilities (d) $ 14 $ 209 $ 223 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 MTM derivative liabilities (b) 3 (b) Other current liabilities 1 — 4 Other deferred credits and liabilities 5 5 10 Power MTM derivative liabilities (b) 12 (b) Other current liabilities 3 — 15 Other deferred credits and liabilities — 173 173 Uranium Other deferred credits and liabilities 4 — 4 Total liabilities (d) $ 18 $ 193 $ 211 (a) The cumulative amount of pretax net gains on all derivative instruments is deferred as a regulatory liability. (b) Balance sheet line item not applicable to registrant. (c) 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. (d) The cumulative amount of pretax net losses on all derivative instruments is deferred as a regulatory asset.
Derivative Instruments With Credit Risk-Related Contingent Features	The following table presents, as of March 31, 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 March 31, 2017 , and (2) those counterparties with rights to do so requested collateral. Aggregate Fair Value of Derivative Liabilities (a) Cash Collateral Posted Potential Aggregate Amount of Additional Collateral Required (b) 2017 Ameren Missouri $ 58 $ 8 $ 44 Ameren Illinois 38 — 30 Ameren $ 96 $ 8 $ 74 (a) Before consideration of master netting arrangements or similar agreements and including NPNS and other accrual contract exposures. (b) As collateral requirements with certain counterparties are based on master netting arrangements or similar agreements, the aggregate amount of additional collateral required to be posted is determined after consideration of the effects of such arrangements.

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 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) 23 – 35 27 Discounted cash flow Counterparty credit risk(%) (c)(d) 0.12 – 0.22 0.15 Fair Value Weighted Average Assets Liabilities Valuation Technique(s) Unobservable Input Range Ameren Missouri credit risk(%) (c)(d) 0.37 (e) Escalation rate (%) (b)(f) 0 – 1 0 Natural gas — (2 ) Discounted cash flow Nodal basis ($/mmbtu) (b) (0.80) – (0.10) (0.70) Counterparty credit risk (%) (c)(d) 0.45 – 6 0.70 Ameren Illinois credit risk (%) (c)(d) 0.37 (e) Power (g) 5 (195 ) Discounted cash flow Average forward peak and off-peak pricing – forwards/swaps ($/MWh) (h) 25 – 44 29 Estimated auction price for FTRs ($/MW) (b) (89) – 1,568 164 Nodal basis ($/MWh) (h) (4) – 0 (2) Ameren Illinois credit risk (%) (c)(d) 0.37 (e) Fundamental energy production model Estimated future 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 ($/MWh) (h) (6) – 0 (2) Ameren Illinois credit risk (%) (c)(d) 0.38 (e) Fundamental energy production model Estimated future gas prices ($/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 March 31, 2017 and through 2020 for December 31, 2016. Valuations beyond 2021 for March 31, 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 March 31, 2017 : Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Total Assets: Ameren Derivative assets – commodity contracts (a) : Fuel oils $ 1 $ — $ 1 $ 2 Natural gas 1 4 — 5 Power — — 5 5 Total derivative assets – commodity contracts $ 2 $ 4 $ 6 $ 12 Nuclear decommissioning trust fund: Cash and cash equivalents $ 1 $ — $ — $ 1 Equity securities: U.S. large capitalization 420 — — 420 Debt securities: U.S. treasury and agency securities — 118 — 118 Corporate bonds — 70 — 70 Other — 24 — 24 Total nuclear decommissioning trust fund $ 421 $ 212 $ — $ 633 (b) Total Ameren $ 423 $ 216 $ 6 $ 645 Ameren Missouri Derivative assets – commodity contracts (a) : Fuel oils $ 1 $ — $ 1 $ 2 Power — — 5 5 Total derivative assets – commodity contracts $ 1 $ — $ 6 $ 7 Nuclear decommissioning trust fund: Cash and cash equivalents $ 1 $ — $ — $ 1 Equity securities: U.S. large capitalization 420 — — 420 Debt securities: U.S. treasury and agency securities — 118 — 118 Corporate bonds — 70 — 70 Other — 24 — 24 Total nuclear decommissioning trust fund $ 421 $ 212 $ — $ 633 (b) Total Ameren Missouri $ 422 $ 212 $ 6 $ 640 Ameren Illinois Derivative assets – commodity contracts (a) : Natural gas $ 1 $ 4 $ — $ 5 Liabilities: Ameren Derivative liabilities – commodity contracts (a) : Fuel oils $ 4 $ — $ 1 $ 5 Natural gas — 21 2 23 Power — — 195 195 Total Ameren $ 4 $ 21 $ 198 $ 223 Ameren Missouri Derivative liabilities – commodity contracts (a) : Fuel oils $ 4 $ — $ 1 $ 5 Natural gas — 8 — 8 Power — — 1 1 Total Ameren Missouri $ 4 $ 8 $ 2 $ 14 Ameren Illinois Derivative liabilities – commodity contracts (a) : Natural gas $ — $ 13 $ 2 $ 15 Power — — 194 194 Total Ameren Illinois $ — $ 13 $ 196 $ 209 (a) The derivative asset and liability balances are presented net of counterparty credit considerations. (b) Balance excludes $ 2 million of receivables, payables, and accrued income, net. The following table sets forth, by level within the fair value hierarchy, our assets and liabilities measured at fair value on a recurring basis as of December 31, 2016 : Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) Significant Other Observable Inputs (Level 2) Significant Other Unobservable Inputs (Level 3) Total Assets: Ameren Derivative assets – commodity contracts (a) : Fuel oils $ 2 $ — $ 1 $ 3 Natural gas 2 12 1 15 Power — — 9 9 Total derivative assets – commodity contracts $ 4 $ 12 $ 11 $ 27 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 March 31, 2017 Beginning balance at January 1, 2017 $ 7 $ (185 ) $ (178 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities — (10 ) (10 ) Settlements (3 ) 1 (2 ) Ending balance at March 31, 2017 $ 4 $ (194 ) $ (190 ) Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2017 $ — $ (11 ) $ (11 ) For the three months ended March 31, 2016 Beginning balance at January 1, 2016 $ 16 $ (170 ) $ (154 ) Realized and unrealized gains (losses) included in regulatory assets/liabilities (3 ) (21 ) (24 ) Settlements (7 ) 4 (3 ) Ending balance at March 31, 2016 $ 6 $ (187 ) $ (181 ) Change in unrealized gains (losses) related to assets/liabilities held at March 31, 2016 $ — $ (19 ) $ (19 )
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 March 31, 2017 and December 31, 2016 : March 31, 2017 December 31, 2016 Carrying Amount Fair Value Carrying Amount Fair Value Ameren: Long-term debt and capital lease obligations (including current portion) $ 7,278 $ 7,762 $ 7,276 $ 7,772 Preferred stock (a) 142 129 142 131 Ameren Missouri: Long-term debt and capital lease obligations (including current portion) $ 3,995 $ 4,312 $ 3,994 $ 4,304 Preferred stock 80 77 80 79 Ameren Illinois: Long-term debt (including current portion) $ 2,588 $ 2,742 $ 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)	3 Months Ended
	Mar. 31, 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 months ended March 31, 2017 and 2016 : Three Months Agreement Income Statement Line Item Ameren Missouri Ameren Illinois Ameren Missouri power supply Operating Revenues 2017 $ 11 $ (a) agreements with Ameren Illinois 2016 9 (a) Ameren Missouri and Ameren Illinois Operating Revenues 2017 7 1 rent and facility services 2016 6 1 Total Operating Revenues 2017 $ 18 $ 1 2016 15 1 Ameren Illinois power supply Purchased Power 2017 $ (a) $ 11 agreements with Ameren Missouri 2016 (a) 9 Ameren Illinois transmission Purchased Power 2017 (a) (b) services with ATXI 2016 (a) (b) Total Purchased Power 2017 $ (a) $ 11 2016 (a) 9 Ameren Services support services Other Operations and Maintenance 2017 $ 35 $ 32 agreement 2016 34 31 Money pool borrowings (advances) Interest Charges/ Miscellaneous Income 2017 $ (b) $ (b) 2016 (b) (b) (a) Not applicable. (b) Amount less than $1 million.

Callaway Energy Center (Tables)

Callaway Energy Center (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 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 for nonradiation events. EMANI provides $490 million for both radiation and nonradiation events. (e) All NEIL insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL. (f) Provides replacement power cost insurance in the event of a prolonged accidental outage. Weekly indemnity up to $4.5 million for 52 weeks, which commences after the first twelve weeks of an outage, plus up to $3.6 million per week for a minimum of 71 weeks thereafter for a total not exceeding the policy limit of $490 million . Nonradiation events are limited to $328 million .

Retirement Benefits (Tables)

Retirement Benefits (Tables)	3 Months Ended
	Mar. 31, 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) incurred for Ameren’s pension and postretirement benefit plans for the three months ended March 31, 2017 and 2016 : Pension Benefits Postretirement Benefits Three Months Three Months 2017 2016 2017 2016 Service cost $ 23 $ 20 $ 5 $ 5 Interest cost 45 47 12 12 Expected return on plan assets (66 ) (63 ) (19 ) (18 ) Amortization of: Prior service benefit — — (1 ) (1 ) Actuarial loss (gain) 14 9 (2 ) (3 ) Net periodic benefit cost (benefit) $ 16 $ 13 $ (5 ) $ (5 )
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 months ended March 31, 2017 and 2016 : Pension Benefits Postretirement Benefits Three Months Three Months 2017 2016 2017 2016 Ameren Missouri (a) $ 6 $ 8 $ (1 ) $ (1 ) Ameren Illinois 10 5 (4 ) (4 ) Ameren (a)(b) $ 16 $ 13 $ (5 ) $ (5 ) (a) Does not include the impact of the regulatory tracking mechanism for the difference between the level of pension and postretirement benefit costs incurred by Ameren Missouri under GAAP and the level of such costs included in rates. (b) Includes amounts for Ameren registrants and nonregistrant subsidiaries.

Segment Information (Tables)

Segment Information (Tables)	3 Months Ended
	Mar. 31, 2017
Segment Reporting [Abstract]
Schedule Of Segment Reporting Information By Segment	The following tables present information about the reported revenues and specified items reflected in net income attributable to common shareholders and capital expenditures at Ameren and Ameren Illinois for the three months ended March 31, 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 $ 772 $ 384 $ 264 $ 96 $ (2 ) $ — $ 1,514 Intersegment revenues 18 1 — 6 (a) — (25 ) — Net income attributable to Ameren common shareholders 5 30 33 34 — — 102 Capital expenditures 196 120 51 134 4 (b) (1 ) 504 2016 External revenues $ 726 $ 351 $ 285 $ 72 $ — $ — $ 1,434 Intersegment revenues 15 1 — 11 (a) — (27 ) — Net income attributable to Ameren common shareholders 14 11 35 27 18 — 105 Capital expenditures 178 117 35 164 2 (b) — 496 (a) Ameren Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above. (b) Includes the elimination of intercompany transfers. Ameren Illinois Ameren Illinois Electric Distribution Ameren Illinois Natural Gas Ameren Illinois Transmission Intersegment Eliminations Consolidated 2017 External revenues $ 385 $ 264 $ 54 $ — $ 703 Intersegment revenues — — 6 (a) (6 ) — Net income available to common shareholder 30 33 16 — 79 Capital expenditures 120 51 56 — 227 2016 External revenues $ 352 $ 285 $ 40 $ — $ 677 Intersegment revenues — — 11 (a) (11 ) — Net income available to common shareholder 11 35 13 — 59 Capital expenditures 117 35 59 — 211 (a) Ameren Illinois Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above.

Summary Of Significant Accoun32

Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Summary of Nonvested Shares Related To Long-Term Incentive Plan) (Details) - $ / shares		3 Months Ended
		Mar. 31, 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]	496,068
Share Units, Forfeitures		(3,192)
Share Units, Vested	[2]	(3,779)
Share Units, Nonvested as of Current period end		1,548,736
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, Grants	[1]	59.16
Weighted-average Fair Value per Unit, Forfeitures		49.13
Weighted-average Fair Value per Unit, Vested	[2]	52.88
Weighted-average Fair Value per Unit, Nonvested as of Current period end		$ 51.59
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]	(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.

Summary Of Significant Accoun33

Summary Of Significant Accounting Policies (Schedule Of Excise Taxes) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Accounting Policies [Line Items]
Excise tax expense	$ 50	$ 50
Union Electric Company
Accounting Policies [Line Items]
Excise tax expense	31	30
Ameren Illinois Company
Accounting Policies [Line Items]
Excise tax expense	$ 19	$ 20

Rate And Regulatory Matters (Na

Rate And Regulatory Matters (Narrative-Missouri) (Detail) - Electric Distribution - Union Electric Company $ in Millions	3 Months Ended
	Mar. 31, 2017USD ($)
Final Rate Order [Member]
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 [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	54
Components of Rate Increase - Regulatory Amortizations [Member]
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 26

Rate And Regulatory Matters (35

Rate And Regulatory Matters (Narrative-Illinois) (Detail) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2017	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Rate And Regulatory Matters [Line Items]
Regulatory assets		$ 1,485		$ 1,437
Current regulatory assets		118		149
Electric		1,206	$ 1,102
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Regulatory assets		875		816
Current regulatory assets		91		$ 108
Electric		439	392
IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Electric		46	$ 11
2016 IEIMA Revenue Requirement Reconciliation [Member] | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Regulatory assets		24
2015 IEIMA Revenue Requirement Reconciliation | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Current regulatory assets		54
2014 IEIMA Revenue Requirement Reconciliation | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Regulatory assets		$ 46
FEJA [Member] | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Equity Capital Structure, Percentage		50.00%
Subsequent Event | Pending Rate Case | IEIMA | Ameren Illinois Company | Electric Distribution
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 16

Rate And Regulatory Matters (36

Rate And Regulatory Matters (Narrative-Federal) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	$ 144	$ 110
Ameren Illinois Company
Rate And Regulatory Matters [Line Items]
Current regulatory liabilities	$ 118	$ 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 [Member]
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 [Member] | 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 [Member]
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
	Mar. 31, 2017USD ($)	Mar. 31, 2016
Utilities
Line of Credit Facility [Line Items]
Short Term Debt, Weighted Average Interest Rate During Period	1.01%	0.47%
Credit Agreements 2012
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1.2
Actual debt-to-capital ratio	0.53
Missouri Credit Agreement 2012 | Union Electric Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.48
Illinois Credit Agreement 2012 | Ameren Illinois Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.46

Short-Term Debt And Liquidity38

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper outstanding) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Commercial paper outstanding	$ 914	$ 558
Ameren (parent)
Line of Credit Facility [Line Items]
Commercial paper outstanding	810	507
Union Electric Company
Line of Credit Facility [Line Items]
Commercial paper outstanding	36	0
Ameren Illinois Company
Line of Credit Facility [Line Items]
Commercial paper outstanding	$ 68	$ 51

Short-Term Debt And Liquidity39

Short-Term Debt And Liquidity Short-Term Debt and Liquidity (Commercial Paper) (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 736	$ 417
Weighted Average Interest Rate		1.06%	0.81%
Peak Short Term Borrowings	[1]	$ 914	$ 581
Peak Short Term Borrowings Interest Rate		1.30%	0.95%
Ameren (parent)
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 682	$ 349
Weighted Average Interest Rate		1.07%	0.82%
Peak Short Term Borrowings	[1]	$ 810	$ 482
Peak Short Term Borrowings Interest Rate		1.30%	0.95%
Union Electric Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 4	$ 68
Weighted Average Interest Rate		0.93%	0.80%
Peak Short Term Borrowings	[1]	$ 45	$ 208
Peak Short Term Borrowings Interest Rate		1.15%	0.85%
Ameren Illinois Company
Line of Credit Facility [Line Items]
Average Daily Commercial Paper Borrowings Outstanding		$ 50	$ 0
Weighted Average Interest Rate		0.95%	0.00%
Peak Short Term Borrowings	[1]	$ 74	$ 0
Peak Short Term Borrowings Interest Rate		1.15%	0.00%
[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.

Other Income and Expenses (Deta

Other Income and Expenses (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction	[1]	$ 6	$ 8
Interest income on industrial development revenue bonds	[1]	7	7
Interest income	[1]	2	4
Other	[1]	0	1
Total miscellaneous income	[1]	15	20
Donations	[1]	5	5
Other	[1]	4	2
Total miscellaneous expense	[1]	9	7
Union Electric Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		5	7
Interest income on industrial development revenue bonds		7	7
Other		0	1
Total miscellaneous income		12	15
Donations		0	1
Other		2	1
Total miscellaneous expense		2	2
Ameren Illinois Company
Other Nonoperating Income (Expense) [Line Items]
Allowance for equity funds used during construction		1	1
Interest income		2	4
Total miscellaneous income		3	5
Donations		4	4
Other		2	1
Total miscellaneous expense		$ 6	$ 5
[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

Derivative Financial Instrume41

Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) lb in Thousands, gal in Millions, MWh in Millions, MMBTU in Millions		Mar. 31, 2017galMWhMMBTUlb	Dec. 31, 2016galMWhMMBTUlb
Fuel Oils
Derivative [Line Items]
Quantity | gal	[1]	36	30
Natural Gas
Derivative [Line Items]
Quantity | MMBTU		177	154
Power
Derivative [Line Items]
Quantity | MWh		10	10
Uranium
Derivative [Line Items]
Quantity | lb		345	345
Union Electric Company | Fuel Oils
Derivative [Line Items]
Quantity | gal	[1]	36	30
Union Electric Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU		28	25
Union Electric Company | Power
Derivative [Line Items]
Quantity | MWh		1	1
Union Electric Company | Uranium
Derivative [Line Items]
Quantity | lb		345	345
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU		149	129
Ameren Illinois Company | Power
Derivative [Line Items]
Quantity | MWh		9	9
[1]	Consists of ultra-low-sulfur diesel products.

Derivative Financial Instrume42

Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions		Mar. 31, 2017		Dec. 31, 2016
Derivative [Line Items]
Derivative assets	[1]	$ 12		$ 27
Derivative liabilities	[2]	223		211
Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets		1		2
Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets		1		1
Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		5		5
Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets		5		12
Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets				3
Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		8		4
Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		15		10
Power | Other Current Assets
Derivative [Line Items]
Derivative assets		5		9
Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		13		15
Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		182		173
Uranium | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		0	[3]	4
Union Electric Company
Derivative [Line Items]
Derivative assets	[1]	7		14
Derivative liabilities	[2]	14		18
Union Electric Company | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets		1		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		5		5
Union Electric Company | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets				1
Union Electric Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets				1
Union Electric Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		2		1
Union Electric Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		6		5
Union Electric Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets		5		9
Union Electric Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		1		3
Union Electric Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		0		0
Union Electric Company | Uranium | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		0	[3]	4
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	[1]	5		13
Derivative liabilities	[2]	209		193
Ameren Illinois Company | Fuel Oils | Other Current 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		5		11
Ameren Illinois Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets				2
Ameren Illinois Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		0		0
Ameren Illinois Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		9		5
Ameren Illinois Company | Natural Gas | Mark To Market Derivative Liabilities
Derivative [Line Items]
Derivative liabilities		6		3
Ameren Illinois Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets		0		0
Ameren Illinois Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		0		0
Ameren Illinois Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		182		173
Ameren Illinois Company | Power | Mark To Market Derivative Liabilities
Derivative [Line Items]
Derivative liabilities		12		12
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]	{F|ahBzfndlYmZpbGluZ3MtaHJkcmoLEgZYTUxEb2MiXlhCUkxEb2NHZW5JbmZvOjRjZDVlNzcwZmZkOTQxZjliMGIxYmE1N2RmYzJmN2QxfFRleHRTZWxlY3Rpb246QThFRjZENDY4MDM0NTAxRkExNkVGNzQzMEE3QjMyNDIM}

Derivative Financial Instrume43

Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Detail) $ in Millions	Mar. 31, 2017USD ($)
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	$ 96	[1]
Cash Collateral Posted	8
Potential Aggregate Amount of Additional Collateral Required	74	[2]
Union Electric Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	58	[1]
Cash Collateral Posted	8
Potential Aggregate Amount of Additional Collateral Required	44	[2]
Ameren Illinois Company
Derivative [Line Items]
Aggregate Fair Value of Derivative Liabilities	38	[1]
Cash Collateral Posted	0
Potential Aggregate Amount of Additional Collateral Required	$ 30	[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		3 Months Ended	12 Months Ended
		Mar. 31, 2017USD ($)$ / credit$ / MMBTU$ / MWh$ / MW	Dec. 31, 2016USD ($)$ / credit$ / MMBTU$ / lb$ / MWh$ / MW
Natural Gas | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Counterparty credit risk	[1],[2]	0.45%	0.13%
Nodal basis	[3]	(0.80)	(0.8)
Credit risk	[1],[2]	0.37%	0.38%
Natural Gas | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Counterparty credit risk	[1],[2]	6.00%	8.00%
Nodal basis	[3]	(0.1)	0
Credit risk	[1],[2]	0.37%	0.38%
Natural Gas | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	[1],[2]	0.70%	1.00%
Nodal basis	[3]	(0.70)	(0.5)
Credit risk	[1],[2]	0.37%	0.38%
Natural Gas | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities	[3]		31.00%
Nodal basis	[3]		(0.40)
Natural Gas | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities	[3]		66.00%
Nodal basis	[3]		(0.1)
Natural Gas | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities	[3]		36.00%
Nodal basis	[3]		(0.20)
Natural Gas | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	[4]	$ 0	$ 1
Natural Gas | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	[4]	$ (2)	$ (1)
Fuel Oils | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Escalation rate	[3],[5]		(2.00%)
Counterparty credit risk	[1],[2]	0.12%	0.13%
Credit risk	[1],[2]	0.37%	0.38%
Fuel Oils | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Escalation rate	[3],[5]	1.00%	2.00%
Counterparty credit risk	[1],[2]	0.22%	0.22%
Credit risk	[1],[2]	0.37%	0.38%
Fuel Oils | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Counterparty credit risk	[1],[2]	0.15%	0.15%
Credit risk	[1],[2]	0.37%	0.38%
Fuel Oils | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities	[3]	23.00%	24.00%
Fuel Oils | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities	[3]	35.00%	66.00%
Fuel Oils | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities	[3]	27.00%	28.00%
Fuel Oils | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	[4]	$ 1	$ 1
Fuel Oils | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	[4]	$ (1)
Power | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Nodal basis | $ / MWh	[6]	(4)	(6)
Credit risk	[1],[2]	0.37%	0.38%
Average forward peak and off-peak pricing | $ / MWh	[6]	25	26
Estimated auction price | $ / MW	[3]	(89)	(71)
Power | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Nodal basis | $ / MWh	[6]	0	0
Credit risk	[1],[2]	0.37%	0.38%
Average forward peak and off-peak pricing | $ / MWh	[6]	44	44
Estimated auction price | $ / MW	[3]	1,568	5,270
Power | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Nodal basis | $ / MWh	[6]	(2)	(2)
Credit risk	[1],[2]	0.37%	0.38%
Average forward peak and off-peak pricing | $ / MWh	[6]	29	29
Estimated auction price | $ / MW	[3]	164	125
Power | Fundamental Energy Production Model | Minimum
Fair Value Inputs [Abstract]
Escalation rate	[3],[7]	3.00%	3.00%
Estimated future gas prices	[3]	3	3
Power | Fundamental Energy Production Model | Maximum
Fair Value Inputs [Abstract]
Escalation rate	[3],[7]	3.00%	3.00%
Estimated future gas prices	[3]	4	4
Power | Fundamental Energy Production Model | Weighted Average
Fair Value Inputs [Abstract]
Escalation rate	[3],[7]	3.00%	3.00%
Estimated future gas prices	[3]	3	3
Power | Contract Price Allocation | Minimum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit	[3]	5	5
Power | Contract Price Allocation | Maximum
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit	[3]	7	7
Power | Contract Price Allocation | Weighted Average
Fair Value Inputs [Abstract]
Estimated renewable energy credit costs | $ / credit	[3]	6	6
Power | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	[4],[8]	$ 5	$ 9
Power | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	[4],[8]	$ (195)	$ (187)
Uranium | Discounted Cash Flow | Minimum
Fair Value Inputs [Abstract]
Credit risk	[1],[2]		0.38%
Average forward pricing | $ / lb	[3]		22
Uranium | Discounted Cash Flow | Maximum
Fair Value Inputs [Abstract]
Credit risk	[1],[2]		0.38%
Average forward pricing | $ / lb	[3]		24
Uranium | Discounted Cash Flow | Weighted Average
Fair Value Inputs [Abstract]
Credit risk	[1],[2]		0.38%
Average forward pricing | $ / lb	[3]		22
Uranium | Option Model | Minimum
Fair Value Inputs [Abstract]
Volatilities	[3]		24.00%
Uranium | Option Model | Maximum
Fair Value Inputs [Abstract]
Volatilities	[3]		24.00%
Uranium | Option Model | Weighted Average
Fair Value Inputs [Abstract]
Volatilities	[3]		24.00%
Uranium | Derivative Assets
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative assets | $	[4]		$ 0
Uranium | Derivative Liabilities
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]
Derivative liabilities | $	[4]		$ (4)
[1]	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.
[2]	Generally, significant increases (decreases) in this input in isolation would result in a significantly lower (higher) fair value measurement.
[3]	Generally, significant increases (decreases) in this input in isolation would result in a significantly higher (lower) fair value measurement.
[4]	The derivative asset and liability balances are presented net of counterparty credit considerations.
[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 March 31, 2017 and through 2020 for December 31, 2016. Valuations beyond 2021 for March 31, 2017 and 2020 for December 31, 2016 use fundamentally modeled pricing by month for peak and off-peak demand.

Fair Value Measurements (Sche45

Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) - USD ($) $ in Millions		Mar. 31, 2017		Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		$ 633	[1]	$ 605	[2]
Assets fair value		645	[3]	632	[4]
Excluded receivables, payables, and accrued income, net		2		2
Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		633	[1]	605	[2]
Assets fair value		640	[3]	619	[4]
Excluded receivables, payables, and accrued income, net		2		(1)
Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		12	[3]	27	[4]
Derivative liabilities		223	[3]	211	[4]
Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		7	[3]	14	[4]
Derivative liabilities		14	[3]	18	[4]
Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		209	[3]	193	[4]
Cash and cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		1		1
Cash and cash equivalents | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		1		1
Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		420		408
Equity Securities | U.S. large capitalization | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		420		408
Debt Securities | Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		70		67
Debt Securities | Corporate bonds | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		70		67
Debt Securities | US treasury and government securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		118		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		118		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		421		409
Assets fair value		423	[3]	413	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		421		409
Assets fair value		422	[3]	411	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		2	[3]	4	[4]
Derivative liabilities		4	[3]	5	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	2	[4]
Derivative liabilities		4	[3]	5	[4]
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0	[3]	0	[4]
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		1		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		1		1
Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Equity Securities | U.S. large capitalization
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		420		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		420		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		212		196
Assets fair value		216	[3]	208	[4]
Significant Other Observable Inputs (Level 2) | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		212		196
Assets fair value		212	[3]	197	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		4	[3]	12	[4]
Derivative liabilities		21	[3]	14	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	1	[4]
Derivative liabilities		8	[3]	7	[4]
Significant Other Observable Inputs (Level 2) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		13	[3]	7	[4]
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		70		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		70		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		118		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		118		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		6	[3]	11	[4]
Significant Other Unobservable Inputs (Level 3) | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Nuclear Decommissioning Trust Fund		0		0
Assets fair value		6	[3]	11	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		6	[3]	11	[4]
Derivative liabilities		198	[3]	192	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		6	[3]	11	[4]
Derivative liabilities		2	[3]	6	[4]
Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		196	[3]	186	[4]
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	[4]			4
Uranium | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	[4]			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	[4]			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	[4]			0
Uranium | Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	[4]			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	[4]			0
Uranium | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities	[4]			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	[4]			4
Fuel Oils | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		2	[3]	3	[4]
Derivative liabilities		5	[3]	5	[4]
Fuel Oils | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		2	[3]	3	[4]
Derivative liabilities		5	[3]	5	[4]
Fuel Oils | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	2	[4]
Derivative liabilities		4	[3]	5	[4]
Fuel Oils | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	2	[4]
Derivative liabilities		4	[3]	5	[4]
Fuel Oils | Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Derivative liabilities		0	[3]	0	[4]
Fuel Oils | Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Derivative liabilities		0	[3]	0	[4]
Fuel Oils | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	1	[4]
Derivative liabilities		1	[3]	0	[4]
Fuel Oils | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	1	[4]
Derivative liabilities		1	[3]	0	[4]
Power | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		5	[3]	9	[4]
Derivative liabilities		195	[3]	188	[4]
Power | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		5	[3]	9	[4]
Derivative liabilities		1	[3]	3	[4]
Power | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		194	[3]	185	[4]
Power | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Derivative liabilities		0	[3]	0	[4]
Power | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Derivative liabilities		0	[3]	0	[4]
Power | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0	[3]	0	[4]
Power | Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Derivative liabilities		0	[3]	1	[4]
Power | Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		0	[3]	0	[4]
Derivative liabilities		0	[3]	1	[4]
Power | Significant Other Observable Inputs (Level 2) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		0	[3]	0
Power | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		5	[3]	9	[4]
Derivative liabilities		195	[3]	187	[4]
Power | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		5	[3]	9	[4]
Derivative liabilities		1	[3]	2	[4]
Power | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative liabilities		194	[3]	185	[4]
Natural Gas | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		5	[3]	15	[4]
Derivative liabilities		23	[3]	14	[4]
Natural Gas | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			2
Derivative liabilities		8	[3]	6	[4]
Natural Gas | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		5	[3]	13	[4]
Derivative liabilities		15	[3]	8	[4]
Natural Gas | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	2	[4]
Derivative liabilities		0	[3]	0	[4]
Natural Gas | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			0
Derivative liabilities		0	[3]	0	[4]
Natural Gas | Quoted Prices In Active Markets For Identical Assets or Liabilities (Level 1) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		1	[3]	2	[4]
Derivative liabilities		0	[3]	0	[4]
Natural Gas | Significant Other Observable Inputs (Level 2) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		4	[3]	12	[4]
Derivative liabilities		21	[3]	13	[4]
Natural Gas | Significant Other Observable Inputs (Level 2) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			1
Derivative liabilities		8	[3]	6	[4]
Natural Gas | Significant Other Observable Inputs (Level 2) | Commodity Contract | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets		4	[3]	11	[4]
Derivative liabilities		13	[3]	7	[4]
Natural Gas | Significant Other Unobservable Inputs (Level 3) | Commodity Contract
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			1
Derivative liabilities		2	[3]	1	[4]
Natural Gas | Significant Other Unobservable Inputs (Level 3) | Commodity Contract | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative assets	[4]			1
Derivative liabilities	[3]	0
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	[3]	0	[4]
Derivative liabilities		$ 2	[3]	$ 1	[4]
[1]	Balance excludes $2 million of receivables, payables, and accrued income, net.
[2]	Balance excludes $2 million of receivables, payables, and accrued income, net.
[3]	The derivative asset and liability balances are presented net of counterparty credit considerations.
[4]	The derivative asset and liability balances are presented net of counterparty credit considerations.

Fair Value Measurements (Sche46

Fair Value Measurements (Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level Three In The Fair Value Hierarchy) (Detail) - Power - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	$ (190)	$ (181)	$ (178)	$ (154)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Included in regulatory assets/liabilities	(10)	(24)
Settlements	(2)	(3)
Change in unrealized gains (losses) related to assets/liabilities held at period end	(11)	(19)
Union Electric Company
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	7	16
Included in regulatory assets/liabilities	0	(3)
Settlements	(3)	(7)
Ending balance	4	6
Change in unrealized gains (losses) related to assets/liabilities held at period end	0	0
Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value	(194)	(187)	$ (185)	$ (170)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Included in regulatory assets/liabilities	(10)	(21)
Settlements	1	4
Change in unrealized gains (losses) related to assets/liabilities held at period end	$ (11)	$ (19)

Fair Value Measurements (Sche47

Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Long-Term Debt And Preferred Stock) (Detail) - USD ($) $ in Millions		Mar. 31, 2017	Dec. 31, 2016
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)	[1]	$ 7,762	$ 7,772
Preferred stock	[1]	129	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		77	79
Fair Value | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)		2,742	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]	7,278	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)		3,995	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)		2,588	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
		Mar. 31, 2017	Mar. 31, 2016
Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues		$ 0	$ 0
Purchased Power | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses		11	9
Ameren Missouri Power Supply Agreements with Ameren Illinois | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues		11	9
Ameren Missouri and Ameren Illinois Rent and Facility Services | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues		7	6
Ameren Missouri and Ameren Illinois Rent and Facility Services | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues		1	1
Total Related Party Operating Revenues [Member] | Union Electric Company
Related Party Transaction [Line Items]
Operating Revenues		18	15
Total Related Party Operating Revenues [Member] | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues		1	1
Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses		11	9
Ameren Illinois Transmission Agreements with ATXI | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Revenues	[1]	1	1
Ameren Services Support Services Agreement | Union Electric Company
Related Party Transaction [Line Items]
Operating Expenses		35	34
Ameren Services Support Services Agreement | Ameren Illinois Company
Related Party Transaction [Line Items]
Operating Expenses		32	31
Related Party Money Pool Interest | Union Electric Company
Related Party Transaction [Line Items]
Interest Charges	[1]	1	1
Related Party Money Pool Interest | Ameren Illinois Company
Related Party Transaction [Line Items]
Interest Charges	[1]	$ 1	$ 1
[1]	Amount less than $1 million.

Related Party Transactions Narr

Related Party Transactions Narrative (Details) - April 2017 Procurement [Member] - Ameren Illinois Company - Ameren Illinois Power Supply Agreements with Ameren Missouri	3 Months Ended
	Mar. 31, 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	33.64

Commitments And Contingencies (

Commitments And Contingencies (Other Obligations) (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2017USD ($)MWh
Long-term Purchase Commitment [Line Items]
Total Other Obligations	$ 3,824
Union Electric Company
Long-term Purchase Commitment [Line Items]
Total Other Obligations	2,340
Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Total Other Obligations	$ 1,418
April 2017 Procurement [Member] | Ameren Illinois Company
Long-term Purchase Commitment [Line Items]
Amount of Megawatthours | MWh	4,249,800
Long-term Purchase Commitment, Amount	$ 128

Commitments And Contingencies51

Commitments And Contingencies (Environmental Matters) (Detail) $ in Millions	Mar. 31, 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	$ 193
Manufactured Gas Plant | Ameren Illinois Company
Loss Contingencies [Line Items]
Number of remediation sites | site	44
Accrual for environmental loss contingencies	$ 193
Manufactured Gas Plant | Ameren Illinois Company | Minimum
Loss Contingencies [Line Items]
Estimate of possible loss	193
Manufactured Gas Plant | Ameren Illinois Company | Maximum
Loss Contingencies [Line Items]
Estimate of possible loss	258
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) - Nuclear Plant $ in Millions	3 Months Ended
	Mar. 31, 2017USD ($)mill
Nuclear Waste Matters [Line Items]
Number of mills charged for NWF fee | mill	1
Annual decommissioning costs included in costs of service | $	$ 7

Callaway Energy Center (Insuran

Callaway Energy Center (Insurance Disclosure) (Detail) € in Millions	3 Months Ended
	Mar. 31, 2017USD ($)		Mar. 31, 2017EUR (€)
Nuclear Waste Matters [Line Items]
Number of weeks of coverage after the first eight weeks of an outage	1 year		1 year
Threshold 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]
Property Damage - Nuclear Electric Insurance Ltd
Nuclear Waste Matters [Line Items]
Maximum Coverages	3,200,000,000	[3]
Maximum Assessments for Single Incidents	29,000,000	[4]
Replacement Power - Nuclear Electric Insurance Ltd
Nuclear Waste Matters [Line Items]
Maximum Coverages	490,000,000	[5]
Maximum Assessments for Single Incidents	7,000,000	[4]
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
Public Liability
Nuclear Waste Matters [Line Items]
Maximum Coverages	13,436,000,000	[6]
Maximum Assessments for Single Incidents	$ 127,000,000
[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]	NEIL provides $2.7 billion in property damage, stabilization, decontamination, and premature decommissioning insurance for radiation events and $2.3 billion in property damage for nonradiation events. EMANI provides $490 million for both radiation and nonradiation events.
[4]	All NEIL insured plants could be subject to assessments should losses exceed the accumulated funds from NEIL.
[5]	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.
[6]	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.

Retirement Benefits (Components

Retirement Benefits (Components Of Net Periodic Benefit Cost) (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		$ 23	$ 20
Interest cost		45	47
Expected return on plan assets		(66)	(63)
Prior service cost (benefit)		0	0
Actuarial loss		14	9
Net periodic benefit cost	[1],[2]	16	13
Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost		5	5
Interest cost		12	12
Expected return on plan assets		(19)	(18)
Prior service cost (benefit)		(1)	(1)
Actuarial loss		(2)	(3)
Net periodic benefit cost	[1],[2]	(5)	(5)
Union Electric Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost		6	8
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)
Ameren Illinois Company | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost		10	5
Ameren Illinois Company | Other Postretirement Benefit Plan, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Net periodic benefit cost		$ (4)	$ (4)
[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
		Mar. 31, 2017	Mar. 31, 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	$ 13
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		6	8
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	5
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)
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)
Other Postretirement Benefit Plan, Defined Benefit | Ameren Illinois Company
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost		$ (4)	$ (4)
[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
		Mar. 31, 2017USD ($)segment	Mar. 31, 2016USD ($)
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment		4
External revenues		$ 1,514	$ 1,434
Net Income Attributable to Ameren Common Shareholders		102	105
Capital expenditures		$ 504	496
Union Electric Company
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment		1
Consolidated
Segment Reporting Information [Line Items]
External revenues		$ 1,514	1,434
Intersegment revenues		0	0
Net Income Attributable to Ameren Common Shareholders		102	105
Capital expenditures		$ 504	496
Ameren Illinois Company
Segment Reporting Information [Line Items]
Number of Reportable Segments | segment		3
Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues		$ 703	677
Intersegment revenues		0	0
Net Income Available to Common Shareholder		79	59
Capital expenditures		227	211
Intersegment Eliminations
Segment Reporting Information [Line Items]
External revenues		0	0
Intersegment revenues		(25)	(27)
Net Income Available to Common Shareholder		0	0
Capital expenditures		(1)	0
Intersegment Eliminations | Ameren Illinois Company
Segment Reporting Information [Line Items]
External revenues		0	0
Intersegment revenues		(6)	(11)
Net Income Available to Common Shareholder		0	0
Capital expenditures		0	0
Operating Segments [Member] | Union Electric Company
Segment Reporting Information [Line Items]
External revenues		772	726
Intersegment revenues		18	15
Net Income Available to Common Shareholder		5	14
Capital expenditures		196	178
Operating Segments [Member] | Ameren Illinois Electric Distribution [Member]
Segment Reporting Information [Line Items]
External revenues		384	351
Intersegment revenues		1	1
Net Income Available to Common Shareholder		30	11
Capital expenditures		120	117
Operating Segments [Member] | Ameren Illinois Gas [Member]
Segment Reporting Information [Line Items]
External revenues		264	285
Intersegment revenues		0	0
Net Income Available to Common Shareholder		33	35
Capital expenditures		51	35
Operating Segments [Member] | Ameren Transmission [Member]
Segment Reporting Information [Line Items]
External revenues		96	72
Intersegment revenues	[1]	6	11
Net Income Available to Common Shareholder		34	27
Capital expenditures		134	164
Operating Segments [Member] | Other
Segment Reporting Information [Line Items]
External revenues		(2)	0
Intersegment revenues		0	0
Net Income Available to Common Shareholder		0	18
Capital expenditures	[2]	4	2
Operating Segments [Member] | Ameren Illinois Company | Ameren Illinois Electric Distribution [Member]
Segment Reporting Information [Line Items]
External revenues		385	352
Intersegment revenues		0	0
Net Income Available to Common Shareholder		30	11
Capital expenditures		120	117
Operating Segments [Member] | Ameren Illinois Company | Ameren Illinois Gas [Member]
Segment Reporting Information [Line Items]
External revenues		264	285
Intersegment revenues		0	0
Net Income Available to Common Shareholder		33	35
Capital expenditures		51	35
Operating Segments [Member] | Ameren Illinois Company | Ameren Illinois Transmission [Member]
Segment Reporting Information [Line Items]
External revenues		54	40
Intersegment revenues	[3]	6	11
Net Income Available to Common Shareholder		16	13
Capital expenditures		$ 56	$ 59
[1]	(a)Ameren Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above.
[2]	(b)Includes the elimination of intercompany transfers.
[3]	(a)Ameren Illinois Transmission earns revenue from transmission service provided to Ameren Illinois Electric Distribution. See discussion of transactions above.