Consolidated Statement of Comprehensive Income (Loss) (Parenthetical) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Unrealized net gain (loss) on derivative hedging instruments, tax (benefit)	$ 2	$ (5)	$ 9	$ (4)
Reclassification adjustments for derivative (gains) losses included in net income, tax (benefit)	(1)	(4)	(2)	(2)
Pension and other postretirement benefit plan activity, tax (benefit)	1	(1)	1	(2)
Ameren Illinois Company [Member]
Pension and other postretirement benefit plan activity, tax (benefit)	(1)	(1)	(1)	(1)
Ameren Energy Generating Company [Member]
Pension and other postretirement benefit plan activity, tax (benefit)	$ 3	$ 1	$ 3	$ 1