Derivative Financial Instruments (Cash Collateral Held from Counterparties) (Details) (USD $) In Millions	Jun. 30, 2011		Dec. 31, 2010
Cash collateral held from counterparties		[1]	$ 1	[1]
Affiliates [Member]
Cash collateral held from counterparties		[1]		[1]
Coal Producers [Member]
Cash collateral held from counterparties		[1]		[1]
Commodity Marketing Companies [Member]
Cash collateral held from counterparties		[1]		[1]
Electric Utilities [Member]
Cash collateral held from counterparties		[1]		[1]
Financial Companies [Member]
Cash collateral held from counterparties		[1]		[1]
Municipalities/Cooperatives [Member]
Cash collateral held from counterparties		[1]		[1]
Oil and Gas Companies [Member]
Cash collateral held from counterparties		[1]		[1]
Retail Companies [Member]
Cash collateral held from counterparties		[1]	$ 1	[1]

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