CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) (Parenthetical) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net gain (loss) adjustment, tax expense (benefit)	$ 4	$ (10)	$ (10)
Unrealized holding losses arising during period, income tax expense (benefit)	(6)	(7)	37
Reclassification adjustments included in net loss, income tax benefit	16	25	96
Midwest Generation LLC
Net gain (loss) adjustment, tax expense (benefit)	0	(8)	(4)
Unrealized holding losses arising during period, income tax expense (benefit)	3	15	29
Reclassification adjustments included in net loss, income tax benefit	$ 17	$ 16	$ 58