Derivative Financial Instruments (Potential Loss On Counterparty Exposures) (Details) (USD $) In Millions	Sep. 30, 2011		Sep. 30, 2010
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$ 858		$ 1,094
Affiliates [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	272	[1]	404	[1]
Coal Producers [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	179		10
Commodity Marketing Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	126		11
Electric Utilities [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	5		9
Financial Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	41		59
Municipalities/Cooperatives [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	174		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	$ 59		$ 71

[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.