CONSOLIDATED STATEMENTS OF COMPREHENSIVE 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
Unrealized holding gains (losses) arising during period, income tax expense (benefit)	$ 13	$ (19)	$ 14	$ (2)
Reclassification adjustments included in net loss, income tax expense	(2)	6	(1)	13
Midwest Generation LLC
Unrealized holding gains (losses) arising during period, income tax expense (benefit)	2	(4)	(1)	8
Reclassification adjustments included in net loss, income tax expense	$ (1)	$ 6	$ 0	$ 12