Derivative Financial Instruments (Maximum Exposure If Counterparties Fail To Perform on Contracts) (Details) (USD $) In Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2011		Dec. 31, 2010
Maximum exposure to counterparties related to derivative contracts	$ 817		$ 1,182
Affiliates [Member]
Maximum exposure to counterparties related to derivative contracts	333	[1]	410	[1]
Coal Producers [Member]
Maximum exposure to counterparties related to derivative contracts	41		30
Commodity Marketing Companies [Member]
Maximum exposure to counterparties related to derivative contracts	39		16
Electric Utilities [Member]
Maximum exposure to counterparties related to derivative contracts	16		22
Financial Companies [Member]
Maximum exposure to counterparties related to derivative contracts	91		72
Municipalities/Cooperatives [Member]
Maximum exposure to counterparties related to derivative contracts	224		550
Oil and Gas Companies [Member]
Maximum exposure to counterparties related to derivative contracts	5		10
Retail Companies [Member]
Maximum exposure to counterparties related to derivative contracts	$ 68		$ 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.