Derivative Financial Instruments (Cash Collateral Held from Counterparties) (Details) (USD $) In Millions	Sep. 30, 2011		Dec. 31, 2010
Concentration Risk [Line Items]
Cash collateral held from counterparties		[1]	$ 1	[1]
Retail Companies [Member]
Concentration Risk [Line Items]
Cash collateral held from counterparties		[1]	$ 1	[1]

[1] Represents amounts held by Marketing Company. As of September 30, 2011, and December 31, 2010, Ameren registrant subsidiaries held no cash collateral.