Consolidated Statement of Income (Loss) (Parenthetical) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Pension and other postretirement benefit plan activity, tax expense (benefit)	$ 8	$ 0	$ 8	$ 0
Ameren Illinois Company [Member]
Pension and other postretirement benefit plan activity, tax expense (benefit)	$ 0	$ (1)	$ (1)	$ (1)