Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Excluded receivables, payables, and accrued income, net	$ (1)		$ 1
Cash And Cash Equivalents [Member]
Nuclear Decommissioning Trust Fund	3	[1],[2]	1	[1],[2]
Cash And Cash Equivalents [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
Nuclear Decommissioning Trust Fund	3	[1],[2]	1	[1],[2]
Commodity Contracts [Member] | Fuel Oils [Member]
Derivative assets	37	[1],[3]	64	[1],[3]
Derivative liabilities	2	[1],[3]	13	[1],[3]
Commodity Contracts [Member] | Fuel Oils [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
Derivative assets	33	[1],[3]
Derivative liabilities	2	[1],[3]
Commodity Contracts [Member] | Fuel Oils [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
Derivative assets	4	[1],[3]	64	[1],[3]
Derivative liabilities			13	[1],[3]
Commodity Contracts [Member] | Natural Gas [Member]
Derivative assets	6	[1],[3]	5	[1],[3]
Derivative liabilities	198	[1],[3]	171	[1],[3]
Commodity Contracts [Member] | Natural Gas [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
Derivative assets	4	[1],[3]	3	[1],[3]
Derivative liabilities	22	[1],[3]	21	[1],[3]
Commodity Contracts [Member] | Natural Gas [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
Derivative assets	2	[1],[3]	2	[1],[3]
Derivative liabilities	176	[1],[3]	150	[1],[3]
Commodity Contracts [Member] | Power [Member]
Derivative assets	195	[1],[3]	103	[1],[3]
Derivative liabilities	80	[1],[3]	69	[1],[3]
Commodity Contracts [Member] | Power [Member] | Significant Other Observable Inputs (Level 2) [Member]
Derivative assets	2	[1],[3]	17	[1],[3]
Derivative liabilities	2	[1],[3]	19	[1],[3]
Commodity Contracts [Member] | Power [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
Derivative assets	193	[1],[3]	86	[1],[3]
Derivative liabilities	78	[1],[3]	50	[1],[3]
Commodity Contracts [Member] | Uranium [Member]
Derivative assets			2	[1],[3]
Derivative liabilities	1	[1],[3]
Commodity Contracts [Member] | Uranium [Member] | Significant Other Unobservable Inputs (Level 3) [Member]
Derivative assets			2	[1],[3]
Derivative liabilities	1	[1],[3]
Equity Securities [Member] | U.S. Large Capitalization [Member]
Nuclear Decommissioning Trust Fund	234	[1],[2]	228	[1]
Equity Securities [Member] | U.S. Large Capitalization [Member] | Quoted Prices In Active Markets For Identical Assets Or Liabilities (Level 1) [Member]
Nuclear Decommissioning Trust Fund	234	[1],[2]	228	[1]
Debt Securities [Member] | Corporate Bonds [Member]
Nuclear Decommissioning Trust Fund	44	[1]	40	[1]
Debt Securities [Member] | Corporate Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member]
Nuclear Decommissioning Trust Fund	44	[1]	40	[1]
Debt Securities [Member] | Municipal Bonds [Member]
Nuclear Decommissioning Trust Fund	1	[1]	2	[1]
Debt Securities [Member] | Municipal Bonds [Member] | Significant Other Observable Inputs (Level 2) [Member]
Nuclear Decommissioning Trust Fund	1	[1]	2	[1]
Debt Securities [Member] | U.S. Treasury And Agency Securities [Member]
Nuclear Decommissioning Trust Fund	65	[1]	50	[1]
Debt Securities [Member] | U.S. Treasury And Agency Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Nuclear Decommissioning Trust Fund	65	[1]	50	[1]
Debt Securities [Member] | Asset-Backed Securities [Member]
Nuclear Decommissioning Trust Fund	10	[1]	14	[1]
Debt Securities [Member] | Asset-Backed Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Nuclear Decommissioning Trust Fund	10	[1]	14	[1]
Debt Securities [Member] | Other Debt Securities [Member]
Nuclear Decommissioning Trust Fund	1	[1]	1	[1]
Debt Securities [Member] | Other Debt Securities [Member] | Significant Other Observable Inputs (Level 2) [Member]
Nuclear Decommissioning Trust Fund	$ 1	[1]	$ 1	[1]

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

[2] Balance excludes $(1) million of receivables, payables, and accrued income, net.

[3] The derivative asset and liability balances are presented net of counterparty credit considerations.