Discontinued Operations (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended				3 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Dec. 31, 2010	Sep. 30, 2012 Homer City	Dec. 31, 2011 Homer City	Oct. 03, 2012 Homer City Plan Support Agreement Subsequent Event
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Principal outstanding (as a percent)									76.00%
Impairment and other charges							$ 113	$ 1,030
Other charges to write down assets held for sale to net realizable value, net of tax							68
Summarized results of discontinued operations:
Total operating revenues	121	159	303	409
Total operating expenses	(138)	(134)	(385)	(409)
Asset impairment and other charges	(113)	0	(134)	0
Other income (expense)	5	2	7	(1)
Income (loss) before income taxes	(125)	27	(209)	(1)
Provision (benefit) for income taxes	(49)	12	(80)	2
Income (loss) from operations of discontinued operations	(76)	15	(129)	(3)
Major components of asset and liabilities of discontinued operations:
Cash and cash equivalents	55	104	55	104	79	128
Other current assets	114		114		128
Carrying value adjustment	(108)		(108)		0
Total current assets	61		61		207
Other long-term assets	0		0		45
Assets of discontinued operations	61		61		252
Total current liabilities	61		61		27
Other long-term liabilities	0		0		9
Liabilities of discontinued operations	$ 61		$ 61		$ 36