Dispositions and Held-for-Sale Businesses Dispositions and Held-for-Sale Businesses (Details) $ in Millions		3 Months Ended		9 Months Ended
	Jul. 01, 2015 USD ($) MW	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Aug. 31, 2015 MW	Aug. 22, 2014 MW
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from sale of ownership interest				$ (461)	$ 0
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | UK Wind Projects [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal Group Not Discontinued Operation Ownership Interest Sold							100.00%
Disposal Group Not Discontinued Operation Generation Capacity | MW							88
Proceeds from sale of ownership interest			$ 161
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal			78
Pre-tax loss of disposed businesses, Excluding gain on disposal, included in continuing operations			19		18
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Armenia Mountain [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Disposal Group, Including Discontinued Operation, Gross Profit (Loss)			(1)	6	4
Net proceeds received from sale of businesses	$ 64
Generation Capacity Mega Watts | MW	101
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal	$ 22
Disposal Group, Held-for-sale, Not Discontinued Operations [Member] | Kelanitissa [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale of ownership in subsidiary percent						90.00%
Generation Capacity Mega Watts | MW						168
Disposal Group, Not Discontinued Operation, Gain (Loss) on Disposal		$ (2)	$ (1)	$ (7)	$ 1