Fair Value Measurements (Schedule Of Fair Value Hierarchy Of Assets And Liabilities Measured At Fair Value On Recurring Basis) (Details) (USD $) In Millions	Sep. 30, 2011		Dec. 31, 2010
Excluded receivables, payables, and accrued income, net	$ 1		$ 1
Heating Oil [Member] | Fair Value, Inputs, Level 3 [Member] | Commodity Contract [Member]
Derivative assets	37	[1],[2]	64	[1],[2]
Derivative liabilities	7	[1],[2]	13
Heating Oil [Member] | Commodity Contract [Member]
Derivative assets	37	[1],[2]	64	[1],[2]
Derivative liabilities	7	[1],[2]	13
Natural Gas [Member] | Fair Value, Inputs, Level 1 [Member] | Commodity Contract [Member]
Derivative assets	4	[1],[2]	3	[1],[2]
Derivative liabilities	20	[1],[2]	21
Natural Gas [Member] | Fair Value, Inputs, Level 3 [Member] | Commodity Contract [Member]
Derivative assets	2	[1],[2]	2	[1],[2]
Derivative liabilities	130	[1],[2]	150
Natural Gas [Member] | Commodity Contract [Member]
Derivative assets	6	[1],[2]	5	[1],[2]
Derivative liabilities	150	[1],[2]	171
Power [Member] | Fair Value, Inputs, Level 2 [Member] | Commodity Contract [Member]
Derivative assets	7	[1],[2]	17	[1],[2]
Derivative liabilities	5	[1],[2]	19
Power [Member] | Fair Value, Inputs, Level 3 [Member] | Commodity Contract [Member]
Derivative assets	160	[1],[2]	86	[1],[2]
Derivative liabilities	46	[1],[2]	50
Power [Member] | Commodity Contract [Member]
Derivative assets	167	[1],[2]	103	[1],[2]
Derivative liabilities	51	[1],[2]	69
Uranium [Member] | Fair Value, Inputs, Level 3 [Member] | Commodity Contract [Member]
Derivative assets			2	[1],[2]
Derivative liabilities	1	[1],[2]
Uranium [Member] | Commodity Contract [Member]
Derivative assets			2	[1],[2]
Derivative liabilities	1	[1],[2]
Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member] | U.S. Large Capitalization [Member]
Nuclear Decommissioning Trust Fund	200	[1],[3]	228	[3]
Fair Value, Inputs, Level 1 [Member] | Cash And Cash Equivalents [Member]
Nuclear Decommissioning Trust Fund	2	[1],[3]	1	[1],[3]
Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Municipal Bonds [Member]
Nuclear Decommissioning Trust Fund	2	[1],[3]	2	[1],[3]
Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Other Debt Obligations [Member]
Nuclear Decommissioning Trust Fund	1	[1],[3]	1	[1],[3]
Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | U.S. Treasury And Agency Securities [Member]
Nuclear Decommissioning Trust Fund	82	[1],[3]	50	[1],[3]
Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Corporate Bonds [Member]
Nuclear Decommissioning Trust Fund	36	[1],[3]	40	[1],[3]
Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Asset-Backed Securities [Member]
Nuclear Decommissioning Trust Fund	6	[1],[3]	14	[1],[3]
Equity Securities [Member] | U.S. Large Capitalization [Member]
Nuclear Decommissioning Trust Fund	200	[1],[3]	228	[3]
Debt Securities [Member] | Municipal Bonds [Member]
Nuclear Decommissioning Trust Fund	2	[1],[3]	2	[1],[3]
Debt Securities [Member] | Other Debt Obligations [Member]
Nuclear Decommissioning Trust Fund	1	[1],[3]	1	[1],[3]
Debt Securities [Member] | U.S. Treasury And Agency Securities [Member]
Nuclear Decommissioning Trust Fund	82	[1],[3]	50	[1],[3]
Debt Securities [Member] | Corporate Bonds [Member]
Nuclear Decommissioning Trust Fund	36	[1],[3]	40	[1],[3]
Debt Securities [Member] | Asset-Backed Securities [Member]
Nuclear Decommissioning Trust Fund	6	[1],[3]	14	[1],[3]
Cash And Cash Equivalents [Member]
Nuclear Decommissioning Trust Fund	$ 2	[1],[3]	$ 1	[1],[3]

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

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

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