Derivative Financial Instruments (Maximum Exposure If Counterparties Fail To Perform On Contracts) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	$ 790		$ 1,182
Affiliates [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	276	[1]	410	[1]
Coal Producers [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	37		30
Commodity Marketing Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	89		16
Electric Utilities [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	16		22
Financial Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	84		72
Municipalities/Cooperatives [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	198		550
Oil And Gas Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	3		10
Retail Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	$ 87		$ 72

[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.