Derivative Financial Instruments (Potential Loss On Counterparty Exposures) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$ 750		$ 1,094
Affiliates [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	274	[1]	404	[1]
Coal Producers [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	35		10
Commodity Marketing Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	88		11
Electric Utilities [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	9		9
Financial Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	65		59
Municipalities/Cooperatives [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	191		523
Oil And Gas Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	2		7
Retail Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$ 86		$ 71

[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.