Discontinued Operations (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended				3 Months Ended		12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Sep. 30, 2012 Homer City	Dec. 31, 2011 Homer City	Dec. 14, 2012 Homer City	Oct. 03, 2012 Homer City Plan Support Agreement
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Outstanding principal amount (as a percent)																76.00%
Forbearance of senior notes															$ 47
Waiver of equity rent payment															65
Impairment charges					704									1,032
Write down of assets held for sale to net realizable value													113		89
Write down of assets held for sale to net realizable value, net of tax					424								68	623	53
Summarized results of discontinued operations:
Total operating revenues									395	527	636
Total operating expenses									(496)	(538)	(522)
Asset impairment and other charges									(89)	(1,032)	(1)
Other income (expense)									5	0	13
Income (loss) before income taxes									(185)	(1,043)	126
Provision (benefit) for income taxes									(73)	(411)	44
Income (loss) from operations of discontinued operations	17	(76)	(29)	(24)	(629)	15	(6)	(12)	(112)	(632)	82
Major components of asset and liabilities of discontinued operations:
Cash and cash equivalents	2				79				2	79	127	116
Other current assets	7				128				7	128
Carrying value adjustment	(9)				0				(9)	0
Total current assets	0				207				0	207
Other long-term assets	0				45				0	45
Assets of discontinued operations	0				252				0	252
Total current liabilities	0				27				0	27
Other long-term liabilities	0				9				0	9
Liabilities of discontinued operations	$ 0				$ 36				$ 0	$ 36