Retirement Benefits (Fair Value Of Plan Assets Utilizing Fair Value Hierarchy) (Details) - USD ($) $ in Millions		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	$ 3,813	$ 3,653	$ 3,794
Alternative Investments, Fair Value Disclosure	[2]	1,995	1,749
Pension Benefits | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		494	396
Pension Benefits | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1,434	1,603
Pension Benefits | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		33	20
Alternative Investments, Fair Value Disclosure	[2]	33	20
Pension Benefits | Cash And Cash Equivalents | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Cash And Cash Equivalents | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Cash And Cash Equivalents | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | U.S. large capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1,352	1,296
Alternative Investments, Fair Value Disclosure	[2]	1,352	1,296
Pension Benefits | U.S. large capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | U.S. large capitalization | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | U.S. large capitalization | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		361	268
Alternative Investments, Fair Value Disclosure	[2]	0	0
Pension Benefits | U.S. small and mid-capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		361	268
Pension Benefits | U.S. small and mid-capitalization | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | U.S. small and mid-capitalization | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | International and emerging markets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		522	491
Alternative Investments, Fair Value Disclosure	[2]	389	243
Pension Benefits | International and emerging markets | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		133	122
Pension Benefits | International and emerging markets | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	126
Pension Benefits | International and emerging markets | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		630	631
Alternative Investments, Fair Value Disclosure	[2]	13	14
Pension Benefits | Corporate Bonds | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		617	617
Pension Benefits | Corporate Bonds | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		95	104
Alternative Investments, Fair Value Disclosure	[2]	0	0
Pension Benefits | Municipal Bonds | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Municipal Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		95	104
Pension Benefits | Municipal Bonds | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | U.S. treasury and agency securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		701	757
Alternative Investments, Fair Value Disclosure	[2]	0	0
Pension Benefits | U.S. treasury and agency securities | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	6
Pension Benefits | U.S. treasury and agency securities | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		701	751
Pension Benefits | U.S. treasury and agency securities | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		21	5
Alternative Investments, Fair Value Disclosure	[2]	0	0
Pension Benefits | Other | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Other | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		21	5
Pension Benefits | Other | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Real estate
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		202	168
Alternative Investments, Fair Value Disclosure	[2]	202	168
Pension Benefits | Real estate | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Real estate | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Real estate | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Private equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		6	8
Alternative Investments, Fair Value Disclosure	[2]	6	8
Pension Benefits | Private equity | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Private equity | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Private equity | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Pension Benefits | Medical benefit assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[3]	(132)	(123)
Pension Benefits | Net receivables
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[4]	22	28
Pension Benefits | Includes Medical Benefit Component Under Section401 H And Excludes Receivables Related To Pending Security Sales [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		3,923	3,748
Pension Benefits | Excludes Medical Benefit Component Under Section401 H And Includes Receivables Related To Pending Security Sales [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		3,813	3,653
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	1,101	1,071	$ 1,109
Alternative Investments, Fair Value Disclosure	[2]	212	189
Postretirement Benefits | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		456	431
Postretirement Benefits | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		326	356
Postretirement Benefits | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		53	61
Alternative Investments, Fair Value Disclosure	[2]	0	0
Postretirement Benefits | Cash And Cash Equivalents | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		53	61
Postretirement Benefits | Cash And Cash Equivalents | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | Cash And Cash Equivalents | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | U.S. large capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		392	370
Alternative Investments, Fair Value Disclosure	[2]	101	98
Postretirement Benefits | U.S. large capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		291	272
Postretirement Benefits | U.S. large capitalization | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | U.S. large capitalization | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		72	65
Alternative Investments, Fair Value Disclosure	[2]	0	0
Postretirement Benefits | U.S. small and mid-capitalization | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		72	65
Postretirement Benefits | U.S. small and mid-capitalization | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | U.S. small and mid-capitalization | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		132	126
Alternative Investments, Fair Value Disclosure	[2]	92	55
Postretirement Benefits | International | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		40	33
Postretirement Benefits | International | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	38
Postretirement Benefits | International | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | Other Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		7	7
Alternative Investments, Fair Value Disclosure	[2]	0	0
Postretirement Benefits | Other Equity | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | Other Equity | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		7	7
Postretirement Benefits | Other Equity | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		141	138
Alternative Investments, Fair Value Disclosure	[2]	0	0
Postretirement Benefits | Corporate Bonds | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		141	138
Postretirement Benefits | Corporate Bonds | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		110	114
Alternative Investments, Fair Value Disclosure	[2]	0	0
Postretirement Benefits | Municipal Bonds | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | Municipal Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		110	114
Postretirement Benefits | Municipal Bonds | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | U.S. treasury and agency securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		68	55
Alternative Investments, Fair Value Disclosure	[2]	0	0
Postretirement Benefits | U.S. treasury and agency securities | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | U.S. treasury and agency securities | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		68	55
Postretirement Benefits | U.S. treasury and agency securities | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		19	40
Alternative Investments, Fair Value Disclosure	[2]	19	36
Postretirement Benefits | Other | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | Other | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	4
Postretirement Benefits | Other | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		0	0
Postretirement Benefits | Medical benefit assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[3]	132	123
Postretirement Benefits | Net payables
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[5]	(25)	(28)
Postretirement Benefits | Excludes Medical Benefit Component Under Section401 H And Excludes Payables Related To Pending Security Sales [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		994	976
Postretirement Benefits | Includes Medical Benefit Component Under Section401 H And Excludes Payables Related To Pending Security Sales [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 1,101	$ 1,071

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries.

[2] Reflects the adoption of the new authoritative accounting guidance related to investments measured at the NAV practical expedient. See Note 1 - Summary of Significant Accounting Policies for additional information.

[3] Medical benefit (health and welfare) component for accounts maintained in accordance with Section 401(h) of the Internal Revenue Code to fund a portion of the postretirement obligation.

[4] Receivables related to pending security sales, offset by payables related to pending security purchases.

[5] Payables related to pending security purchases, offset by interest receivables and receivables related to pending security sales.