Derivative Financial Instruments (Derivative Instruments With Credit Risk-Related Contingent Features) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Derivative Financial Instruments [Abstract]
Aggregate Fair Value of Derivative Liabilities	$ 456	[1]	$ 431	[1]
Cash Collateral Posted	116		134
Potential Aggregate Amount of Additional Collateral Required	$ 332		$ 274

[1] Primarily composed of Marketing Company's exposure to Ameren Illinois related to financial contracts. The exposure is not eliminated at the consolidated Ameren level for purposes of this disclosure, as it is calculated without regard to the offsetting affiliate counterparty's liability position. See Note 14 - Related Party Transactions for additional information on these financial contracts.