Fair Value Measurements (Schedule of Fair Value Hierarchy of Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) (USD $) In Millions	Jun. 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	72	[1],[2]	64	[1],[2]
Derivative liabilities	4	[1],[2]	13	[1],[2]
Heating Oil [Member] | Commodity Contract [Member]
Derivative assets	72	[1],[2]	64	[1],[2]
Derivative liabilities	4	[1],[2]	13	[1],[2]
Natural Gas [Member] | Fair Value, Inputs, Level 1 [Member] | Commodity Contract [Member]
Derivative assets	3	[1],[2]	3	[1],[2]
Derivative liabilities	18	[1],[2]	21	[1],[2]
Natural Gas [Member] | Fair Value, Inputs, Level 3 [Member] | Commodity Contract [Member]
Derivative assets	3	[1],[2]	2	[1],[2]
Derivative liabilities	120	[1],[2]	150	[1],[2]
Natural Gas [Member] | Commodity Contract [Member]
Derivative assets	6	[1],[2]	5	[1],[2]
Derivative liabilities	138	[1],[2]	171	[1],[2]
Power [Member] | Fair Value, Inputs, Level 2 [Member] | Commodity Contract [Member]
Derivative assets	18	[1],[2]	17	[1],[2]
Derivative liabilities	16	[1],[2]	19	[1],[2]
Power [Member] | Fair Value, Inputs, Level 3 [Member] | Commodity Contract [Member]
Derivative assets	174	[1],[2]	86	[1],[2]
Derivative liabilities	57	[1],[2]	50	[1],[2]
Power [Member] | Commodity Contract [Member]
Derivative assets	192	[1],[2]	103	[1],[2]
Derivative liabilities	73	[1],[2]	69	[1],[2]
Uranium [Member] | Fair Value, Inputs, Level 3 [Member] | Commodity Contract [Member]
Derivative assets			2	[1],[2]
Derivative liabilities	2	[1],[2]
Uranium [Member] | Commodity Contract [Member]
Derivative assets			2	[1],[2]
Derivative liabilities	2	[1],[2]
Fair Value, Inputs, Level 1 [Member] | Equity Securities [Member] | US Large Capitalization [Member]
Nuclear Decommissioning Trust Fund	234	[1],[3]	228	[1],[3]
Fair Value, Inputs, Level 1 [Member] | Cash and Cash Equivalents [Member]
Nuclear Decommissioning Trust Fund	1	[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] | US Treasury and Government [Member]
Nuclear Decommissioning Trust Fund	59	[1],[3]	50	[1],[3]
Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Corporate Debt Securities [Member]
Nuclear Decommissioning Trust Fund	45	[1],[3]	40	[1],[3]
Fair Value, Inputs, Level 2 [Member] | Debt Securities [Member] | Asset-backed Securities [Member]
Nuclear Decommissioning Trust Fund	13	[1],[3]	14	[1],[3]
Equity Securities [Member] | US Large Capitalization [Member]
Nuclear Decommissioning Trust Fund	234	[1],[3]	228	[1],[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] | US Treasury and Government [Member]
Nuclear Decommissioning Trust Fund	59	[1],[3]	50	[1],[3]
Debt Securities [Member] | Corporate Debt Securities [Member]
Nuclear Decommissioning Trust Fund	45	[1],[3]	40	[1],[3]
Debt Securities [Member] | Asset-backed Securities [Member]
Nuclear Decommissioning Trust Fund	13	[1],[3]	14	[1],[3]
Cash and Cash Equivalents [Member]
Nuclear Decommissioning Trust Fund	$ 1	[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.