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	Mar. 31, 2012		Dec. 31, 2011
Excluded receivables, payables, and accrued income, net	$ (1)		$ (1)
Cash and Cash Equivalents [Member]
Nuclear Decommissioning Trust Fund	3	[1],[2]	3	[1],[2]
Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member]
Nuclear Decommissioning Trust Fund	3	[1],[2]	3	[1],[2]
Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 2 [Member]
Nuclear Decommissioning Trust Fund				[1],[2]
Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 3 [Member]
Nuclear Decommissioning Trust Fund				[1],[2]
Commodity Contracts [Member] | Coal [Member]
Derivative liabilities	4	[1],[3]
Commodity Contracts [Member] | Coal [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative liabilities	4	[1],[3]
Commodity Contracts [Member] | Fuel Oils [Member]
Derivative assets	45	[1],[3]	37	[1],[3]
Derivative liabilities	1	[1],[3]	2	[1],[3]
Commodity Contracts [Member] | Fuel Oils [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative assets	36	[1],[3]	33	[1],[3]
Derivative liabilities	1	[1],[3]	2	[1],[3]
Commodity Contracts [Member] | Fuel Oils [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative assets				[1],[3]
Derivative liabilities				[1],[3]
Commodity Contracts [Member] | Fuel Oils [Member] | Fair Value, Inputs, Level 3 [Member]
Derivative assets	9	[1],[3]	4	[1],[3]
Derivative liabilities				[1],[3]
Commodity Contracts [Member] | Natural Gas [Member]
Derivative assets	6	[1],[3]	6	[1],[3]
Derivative liabilities	215	[1],[3]	198	[1],[3]
Commodity Contracts [Member] | Natural Gas [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative assets	4	[1],[3]	4	[1],[3]
Derivative liabilities	19	[1],[3]	22	[1],[3]
Commodity Contracts [Member] | Natural Gas [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative assets	2	[1],[3]		[1],[3]
Derivative liabilities	196	[1],[3]		[1],[3]
Commodity Contracts [Member] | Natural Gas [Member] | Fair Value, Inputs, Level 3 [Member]
Derivative assets			2	[1],[3]
Derivative liabilities			176	[1],[3]
Commodity Contracts [Member] | Power [Member]
Derivative assets	185	[1],[3]	195	[1],[3]
Derivative liabilities	210	[1],[3]	80	[1],[3]
Commodity Contracts [Member] | Power [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative assets				[1],[3]
Derivative liabilities				[1],[3]
Commodity Contracts [Member] | Power [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative assets	15	[1],[3]	2	[1],[3]
Derivative liabilities	16	[1],[3]	2	[1],[3]
Commodity Contracts [Member] | Power [Member] | Fair Value, Inputs, Level 3 [Member]
Derivative assets	170	[1],[3]	193	[1],[3]
Derivative liabilities	194	[1],[3]	78	[1],[3]
Commodity Contracts [Member] | Uranium [Member]
Derivative liabilities	1	[1],[3]	1	[1],[3]
Commodity Contracts [Member] | Uranium [Member] | Fair Value, Inputs, Level 1 [Member]
Derivative liabilities				[1],[3]
Commodity Contracts [Member] | Uranium [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative liabilities				[1],[3]
Commodity Contracts [Member] | Uranium [Member] | Fair Value, Inputs, Level 3 [Member]
Derivative liabilities	1	[1],[3]	1	[1],[3]
Equity Securities [Member] | U.S. Large Capitalization [Member]
Nuclear Decommissioning Trust Fund	264	[1],[2]	234	[1],[2]
Equity Securities [Member] | U.S. Large Capitalization [Member] | Fair Value, Inputs, Level 1 [Member]
Nuclear Decommissioning Trust Fund	264	[1],[2]	234	[1],[2]
Equity Securities [Member] | U.S. Large Capitalization [Member] | Fair Value, Inputs, Level 2 [Member]
Nuclear Decommissioning Trust Fund				[1],[2]
Equity Securities [Member] | U.S. Large Capitalization [Member] | Fair Value, Inputs, Level 3 [Member]
Nuclear Decommissioning Trust Fund				[1],[2]
Debt Securities [Member] | Corporate Debt Securities [Member]
Nuclear Decommissioning Trust Fund	46	[1],[2]	44	[1],[2]
Debt Securities [Member] | Corporate Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Nuclear Decommissioning Trust Fund				[1],[2]
Debt Securities [Member] | Corporate Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Nuclear Decommissioning Trust Fund	46	[1],[2]	44	[1],[2]
Debt Securities [Member] | Corporate Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Nuclear Decommissioning Trust Fund				[1],[2]
Debt Securities [Member] | Municipal Bonds [Member]
Nuclear Decommissioning Trust Fund	1	[1],[2]	1	[1],[2]
Debt Securities [Member] | Municipal Bonds [Member] | Fair Value, Inputs, Level 1 [Member]
Nuclear Decommissioning Trust Fund				[1],[2]
Debt Securities [Member] | Municipal Bonds [Member] | Fair Value, Inputs, Level 2 [Member]
Nuclear Decommissioning Trust Fund	1	[1],[2]	1	[1],[2]
Debt Securities [Member] | Municipal Bonds [Member] | Fair Value, Inputs, Level 3 [Member]
Nuclear Decommissioning Trust Fund				[1],[2]
Debt Securities [Member] | U.S. Treasury And Agency Securities [Member]
Nuclear Decommissioning Trust Fund	66	[1],[2]	65	[1],[2]
Debt Securities [Member] | U.S. Treasury And Agency Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Nuclear Decommissioning Trust Fund				[1],[2]
Debt Securities [Member] | U.S. Treasury And Agency Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Nuclear Decommissioning Trust Fund	66	[1],[2]	65	[1],[2]
Debt Securities [Member] | U.S. Treasury And Agency Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Nuclear Decommissioning Trust Fund				[1],[2]
Debt Securities [Member] | Asset-backed Securities [Member]
Nuclear Decommissioning Trust Fund	10	[1],[2]	10	[1],[2]
Debt Securities [Member] | Asset-backed Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Nuclear Decommissioning Trust Fund				[1],[2]
Debt Securities [Member] | Asset-backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Nuclear Decommissioning Trust Fund	10	[1],[2]	10	[1],[2]
Debt Securities [Member] | Asset-backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Nuclear Decommissioning Trust Fund				[1],[2]
Debt Securities [Member] | Other Debt Securities [Member]
Nuclear Decommissioning Trust Fund	1	[1],[2]	1	[1],[2]
Debt Securities [Member] | Other Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Nuclear Decommissioning Trust Fund				[1],[2]
Debt Securities [Member] | Other Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Nuclear Decommissioning Trust Fund	1	[1],[2]	1	[1],[2]
Debt Securities [Member] | Other Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Nuclear Decommissioning Trust Fund				[1],[2]

[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.