Other Comprehensive Income (Schedule of Comprehensive Income) (Details) (USD $) In Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010
Other Comprehensive Income
Net income	$ 139	[1]	$ 155	[1]	$ 213	[1]	$ 261	[1]
Unrealized net gain (loss) on derivative hedging instruments, net of taxes (benefit)	(8)	[1]	(11)	[1]	(6)	[1]	17	[1]
Reclassification adjustments for derivative (gain) loss included in net income, net of taxes	7	[1]	(5)	[1]	3	[1]	(20)	[1]
Pension and other postretirement activity, net of income taxes (benefit)		[1]	7	[1]	(1)	[1]	6	[1]
Total comprehensive income, net of taxes	138	[1]	146	[1]	209	[1]	264	[1]
Less: Net income attributable to noncontrolling interests, net of taxes	1	[1]	3	[1]	4	[1]	7	[1]
Total comprehensive income attributable to Ameren Corporation, net of taxes	$ 137	[1]	$ 143	[1]	$ 205	[1]	$ 257	[1]

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