Derivative Financial Instruments (Maximum Exposure If Counterparties Fail To Perform On Contracts) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2012		Dec. 31, 2011
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	$ 1,004		$ 790
Affiliates [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	267	[1]	276	[1]
Coal Producers [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts			37
Commodity Marketing Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	5		89
Electric Utilities [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	30		16
Financial Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	118		84
Municipalities/Cooperatives [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	465		198
Oil And Gas Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	8		3
Retail Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	$ 111		$ 87

[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.