Business Acquisitions and Dispositions Business Acquisitions and Dispositions - 2015 & 2014 Dispositions (Details) $ in Millions			9 Months Ended	12 Months Ended
	Dec. 03, 2014 USD ($)	Jul. 07, 2014 USD ($)	Sep. 30, 2014 USD ($) MW	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2016	Feb. 01, 2016 USD ($)	Dec. 02, 2014 MW	Jul. 03, 2014
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Long-term Debt				$ 19,406	$ 19,620
Debt Instrument, Description of Variable Rate Basis				3 month LIBOR
Petra Nova Parish Holdings [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity Method Investment, Ownership Percentage							50.00%
Power Generation Capacity, Megawatts | MW			75
Altenex [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Equity Method Investment, Ownership Percentage					32.00%
Sabine CoGen, LP [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Sale of Equity Method Investments	$ 35
Equity Method Investment, Realized Gain (Loss) on Disposal						$ 18
Percentage of Ownership Sold of Subsidiary									50.00%
Power Generation Capacity, Megawatts | MW									105
Altenex [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from Sale of Equity Method Investments					$ 26
Equity Method Investment, Realized Gain (Loss) on Disposal					$ 14
Petra Nova Parish Holdings [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Percentage of Ownership Sold of Subsidiary										50.00%
Capital Contribution to Equity Method Investment		$ 35
Proceeds from Divestiture of Interest in Subsidiaries and Affiliates		$ 76
Capital Contributions From Partners in Equity Method Investment			$ 300
Percentage of Ownership			50.00%
Department of Energy [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Amount guaranteed to borrow by US DOE			$ 167
Additional US DOE funding								$ 23
JBIC and Mizuho [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Long-term Debt			250
NEXI Covered Loan [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Long-term Debt			$ 75
Debt Instrument, Description of Variable Rate Basis			LIBOR
Debt Instrument, Basis Spread on Variable Rate			1.75%
JBIC [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Long-term Debt			$ 175
Debt Instrument, Description of Variable Rate Basis			LIBOR
Debt Instrument, Basis Spread on Variable Rate			0.50%
Debt Instrument, Incremental Escalation On Basis Spread, Percentage			1.50%
Design and Engineering Phase [Member] | Department of Energy [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Long-term Debt			$ 7
Construction Phase [Member] | Department of Energy [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Long-term Debt			$ 106