Retirement Benefits (Fair Value Of Plan Assets Utilizing Fair Value Hierarchy) (Details) - USD ($) $ in Millions		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Real estate
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		$ 168	$ 147	$ 131
Private equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		8	13	15
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	3,653	3,794	3,461
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		396	411
Pension Benefits | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		3,176	3,327
Pension Benefits | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		176	160
Pension Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		20	38
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		20	38
Pension Benefits | U.S. large capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		1,296	1,331
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		1,296	1,331
Pension Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		268	270
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		268	270
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 | International and emerging markets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		491	494
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		122	134
Pension Benefits | International and emerging markets | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		369	360
Pension Benefits | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		631	1,026
Pension Benefits | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		631	1,026
Pension Benefits | Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		104	175
Pension Benefits | Municipal Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		104	175
Pension Benefits | U.S. treasury and agency securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		757	372
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		6	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		751	366
Pension Benefits | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		5	31
Pension Benefits | Other | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		5	31
Pension Benefits | Real estate
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		168	147
Pension Benefits | Real estate | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		168	147
Pension Benefits | Private equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		8	13
Pension Benefits | Private equity | Significant Other Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		8	13
Pension Benefits | Derivative assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets			1
Pension Benefits | Derivative assets | Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets			1
Pension Benefits | Medical benefit assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[2]	(123)	(125)
Pension Benefits | Net receivables
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[3]	28	21
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		3,748	3,898
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		3,653	3,794
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[1]	1,071	1,109	$ 1,074
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		431	487
Postretirement Benefits | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		545	551
Postretirement Benefits | Cash And Cash Equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		61	89
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		61	89
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 | U.S. large capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		370	392
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		272	291
Postretirement Benefits | U.S. large capitalization | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		98	101
Postretirement Benefits | U.S. small and mid-capitalization
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		65	70
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		65	70
Postretirement Benefits | International
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		126	131
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		33	37
Postretirement Benefits | International | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		93	94
Postretirement Benefits | Other Equity
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		7	7
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 | Corporate Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		138	105
Postretirement Benefits | Corporate Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		138	105
Postretirement Benefits | Municipal Bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		114	111
Postretirement Benefits | Municipal Bonds | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		114	111
Postretirement Benefits | U.S. treasury and agency securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		55	89
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		55	89
Postretirement Benefits | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		40	44
Postretirement Benefits | Other | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets		40	44
Postretirement Benefits | Medical benefit assets
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[2]	123	125
Postretirement Benefits | Net payables
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	[4]	(28)	(54)
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		976	1,038
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		$ 1,071	$ 1,109

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

[2] 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.

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

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