Income Taxes (Schedule Of Components Of Income Tax Expense (Benefit)) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Income Taxes [Abstract]
Current Federal taxes	$ (27)	[1]	$ 13	[1]	$ (73)	[1]
Current State taxes	(5)	[1]	10	[1]	3	[1]
Deferred Federal taxes	273	[1]	274	[1]	337	[1]
Deferred State taxes	76	[1]	36	[1]	74	[1]
Deferred investment tax credits, amortization	(7)	[1]	(8)	[1]	(9)	[1]
Total income tax expense	$ 310	[1]	$ 325	[1]	$ 332	[1]

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.