Derivative Financial Instruments (Potential Loss on Counterparty Exposures) (Details) (USD $) In Millions	Jun. 30, 2011		Dec. 31, 2010
Potential loss on counterparty exposures related to derivative contracts	$ 729		$ 1,094
Affiliates [Member]
Potential loss on counterparty exposures related to derivative contracts	317	[1]	404	[1]
Coal Producers [Member]
Potential loss on counterparty exposures related to derivative contracts	18		10
Commodity Marketing Companies [Member]
Potential loss on counterparty exposures related to derivative contracts	35		11
Electric Utilities [Member]
Potential loss on counterparty exposures related to derivative contracts	8		9
Financial Companies [Member]
Potential loss on counterparty exposures related to derivative contracts	69		59
Municipalities/Cooperatives [Member]
Potential loss on counterparty exposures related to derivative contracts	212		523
Oil and Gas Companies [Member]
Potential loss on counterparty exposures related to derivative contracts	3		7
Retail Companies [Member]
Potential loss on counterparty exposures related to derivative contracts	$ 67		$ 71

[1] Primarily comprised of Marketing Company's exposure to Ameren Illinois related to financial contracts. The exposure is not eliminated at the consolidated Ameren level as it is calculated without regard to the offsetting affiliate counterparty's liability position. See Note 14 - Related Party Transactions under Part II, Item 8, of the Form 10-K for additional information on these financial contracts.