Derivative Financial Instruments (Maximum Exposure If Counterparties Fail To Perform On Contracts) (Details) (USD $) In Millions	9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	$ 1,027		$ 1,182
Affiliates [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	272	[1]	410	[1]
Coal Producers [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	198		30
Commodity Marketing Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	129		16
Electric Utilities [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	8		22
Financial Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	66		72
Municipalities/Cooperatives [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	291		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	$ 60		$ 72

[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.