Derivative Financial Instruments (Potential Loss On Counterparty Exposures) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012		Dec. 31, 2011
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$ 953		$ 750
Affiliates [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	267	[1]	274	[1]
Coal Producers [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts			35
Commodity Marketing Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	4		88
Electric Utilities [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	13		9
Financial Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	98		65
Municipalities/Cooperatives [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	459		191
Oil And Gas Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	2		2
Retail Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$ 110		$ 86

[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 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 in the Form 10-K for additional information on these financial contracts.