Other Comprehensive Income (Schedule Of Comprehensive Income) (Details) (USD $) In Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010		Sep. 30, 2011		Sep. 30, 2010
Other Comprehensive Income [Abstract]
Net income (loss)	$ 287	[1]	$ (164)	[1]	$ 500	[1]	$ 97	[1]
Unrealized net gain (loss) on derivative hedging instruments, net of taxes (benefit)	(2)	[1]	14	[1]	(8)	[1]	31	[1]
Reclassification adjustments for derivative (gain) loss included in net income, net of taxes	(1)	[1]	(14)	[1]	2	[1]	(34)	[1]
Pension and other postretirement activity, net of income taxes (benefit)	(1)	[1]		[1]	(2)	[1]	6	[1]
Total comprehensive income (loss), net of taxes	283	[1]	(164)	[1]	492	[1]	100	[1]
Less: Net income attributable to noncontrolling interests, net of taxes	2	[1]	3	[1]	6	[1]	10	[1]
Total comprehensive income (loss) attributable to Ameren Corporation, net of taxes	$ 281	[1]	$ (167)	[1]	$ 486	[1]	$ 90	[1]

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