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Debt and Capital Leases - Recourse Debt, Senior Credit Facility, Tax Exempt Bonds (Details) $ in Millions			12 Months Ended
	Jan. 24, 2017	Jun. 30, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Debt Instrument
Long-term debt			$ 16,633	$ 16,704
Net (loss)/gain on debt extinguishment			(53)	(142)	$ 10
Senior Credit Facility
Debt Instrument
Securitization threshold		66.00%
Recourse Debt
Debt Instrument
Long-term debt			7,182	7,795
Recourse Debt | Term loan facility, due 2023
Debt Instrument
Long-term debt		$ 1,900	1,872	1,891
Percent of face value		99.50%
Periodic payment, percentage of principal		0.0025
Net (loss)/gain on debt extinguishment		$ (21)
Recourse Debt | Indian River Power tax exempt bonds, due 2040
Debt Instrument
Long-term debt			$ 57	57
Interest rate, stated percentage			6.00%
Recourse Debt | Indian River Power LLC, tax exempt bonds, due 2045
Debt Instrument
Long-term debt			$ 190	190
Interest rate, stated percentage			5.375%
Recourse Debt | Dunkirk Power LLC, tax exempt bonds, due 2042
Debt Instrument
Long-term debt			$ 59	59
Interest rate, stated percentage			5.875%
Recourse Debt | City of Texas City, tax exempt bonds, due 2045
Debt Instrument
Long-term debt			$ 32	22
Interest rate, stated percentage			4.125%
Recourse Debt | Fort Bend County, tax exempt bonds, due 2038
Debt Instrument
Long-term debt			$ 54	54
Interest rate, stated percentage			4.75%
Recourse Debt | Fort Bend County, tax exempt bonds, due 2042
Debt Instrument
Long-term debt			$ 73	73
Interest rate, stated percentage			4.75%
Recourse Debt | Tax-exempt bonds
Debt Instrument
Long-term debt			$ 465	$ 455
Recourse Debt | LIBOR | Term loan facility, due 2023
Debt Instrument
Basis spread on variable rate (as a percent)	2.25%	2.75%	2.25%
Recourse Debt | LIBOR floor | Term loan facility, due 2023
Debt Instrument
Basis spread on variable rate (as a percent)		0.75%
Revolving Credit Facility | 2016 Tranche A Revolving Credit Facility due 2018
Debt Instrument
Revolving credit facility		$ 289
Revolving Credit Facility | 2016 Tranche B Revolving Credit Facility due 2021
Debt Instrument
Basis spread on variable rate (as a percent)		2.25%
Revolving credit facility		$ 2,200

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Debt Instrument, Periodic Payment, Percentage of Principal

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Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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