Consolidated Statement Of Comprehensive Income (Parenthetical) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Unrealized net gain (loss) on derivative hedging instruments, tax (benefit)	$ 1	$ (1)	$ 78
Reclassification adjustments for derivative (gain) included in net income, tax	(3)	5	82
Reclassification adjustment due to implementation of FAC, taxes			18
Pension and other postretirement activity, tax (benefit)	(32)	6	22
Union Electric Company [Member]
Unrealized net gain (loss) on derivative hedging instruments, tax (benefit)			11
Reclassification adjustments for derivative (gain) included in net income, tax			8
Reclassification adjustment due to implementation of FAC, taxes			18
Ameren Illinois Company [Member]
Pension and other postretirement activity, tax (benefit)	(2)	(2)	(2)
Ameren Energy Generating Company [Member]
Pension and other postretirement activity, tax (benefit)	$ (24)	$ 5	$ 12