Discontinued Operations (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale of hydropower generation plants through assumption of debt by buyer	$ 700	$ 0
Maine Hydro [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Capacity associated with discontinued operations (in megawatts)	351
Discontinued Operation, Gain (Loss) from Disposal of Discontinued Operation, before Income Tax	372
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax	231
Maine fossil [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Capacity associated with discontinued operations (in megawatts)	796
Discontinued Operation, Gain (Loss) from Disposal of Discontinued Operation, before Income Tax	(67)
Discontinued Operation, Gain (Loss) on Disposal of Discontinued Operation, Net of Tax	$ (43)