CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) (Parenthetical) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Unrealized holding gains (losses) arising during period, income tax expense (benefit)	$ (1)	$ (11)	$ 13	$ (13)
Reclassification adjustments included in net loss, income tax expense	(1)	(1)	(2)	12
Midwest Generation LLC
Unrealized holding gains (losses) arising during period, income tax expense (benefit)	0	(5)	(1)	3
Reclassification adjustments included in net loss, income tax expense	$ (2)	$ 1	$ (2)	$ 13