Accumulated Other Comprehensive Income (Schedule of AOCI Rollforward) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended						6 Months Ended
	Jun. 30, 2013		Jun. 30, 2013 Commodity contracts	Dec. 31, 2012 Commodity contracts	Jun. 30, 2013 Interest rate contracts	Dec. 31, 2012 Interest rate contracts	Jun. 30, 2013 Electricity contracts	Jun. 30, 2013 Unrealized gains and losses on cash flow hedges		Jun. 30, 2013 Unrecognized Losses and Prior Service Adjustments, Net		Jun. 30, 2013 Valuation Allowance on Deferred Tax Asset		Jun. 30, 2013 Midwest Generation LLC	Jun. 30, 2013 Midwest Generation LLC Unrealized gains and losses on cash flow hedges	Jun. 30, 2013 Midwest Generation LLC Unrecognized Losses and Prior Service Adjustments, Net		Jun. 30, 2013 Midwest Generation LLC Valuation Allowance on Deferred Tax Asset
Increase (Decrease) in Accumulated Other Comprehensive Income
Balance, at the beginning of period	$ (138)							$ (76)		$ (56)	[1]	$ (6)		$ (50)	$ (1)	$ (37)	[1]	$ (12)
OCI before reclassifications	25							27		(2)		0		(2)	(2)	0		0
Amount reclassified from AOCI	4							2		2		0		2	1	1		0
Balance, at the end of period	(109)	[2]						(47)	[2]	(56)	[1]	(6)	[2]	(50)	(2)	(36)	[1]	(12)
Unrealized derivative gains (losses), after-tax recorded in accumulated other comprehensive income (loss)			(2)	(1)	(45)	(75)
Unrealized (losses) on cash flow hedges, net of tax, expected to be reclassified into earnings during the next 12 months							$ (2)

[1] For further detail, see Note 8—Compensation and Benefit Plans.

[2] EME and Midwest Generation both expect to reclassify unrealized losses on cash flow hedges into earnings in the next 12 months. For further explanation, see "—Unrealized Losses on Cash Flow Hedges."