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Debt and Capital Leases - Senior Credit Facility and Tax Exempt Bonds (Details)					12 Months Ended
	Nov. 05, 2018 USD ($)	Mar. 21, 2018	Jan. 24, 2017	Jun. 30, 2016 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Long-term debt					$ 6,691,000,000	$ 9,494,000,000
Loss on debt extinguishment					44,000,000	49,000,000	$ 142,000,000
Recourse Debt
Debt Instrument [Line Items]
Long-term debt					6,523,000,000	7,182,000,000
Recourse Debt | Term loan facility, due 2023
Debt Instrument [Line Items]
Long-term debt	$ 260,000,000			$ 1,900,000,000	$ 1,698,000,000	1,872,000,000
Percent of face value				99.50%
Periodic payment, percentage of principal				0.0025
Prepaid amount	155,000,000
Loss on debt extinguishment	3,000,000
Write off of deferred debt issuance cost	2,000,000
Recourse Debt | Term loan facility, due 2023 | LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)		1.75%	2.25%	2.75%	1.75%
Decrease in interest rate margin		50.00%	50.00%
Recourse Debt | Term loan facility, due 2023 | LIBOR floor
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)		0.00%	0.75%	0.75%
Recourse Debt | Indian River Power, tax exempt bonds, due 2040
Debt Instrument [Line Items]
Long-term debt					$ 57,000,000	57,000,000
Interest rate, stated percentage					6.00%
Recourse Debt | Indian River Power LLC, tax exempt bonds, due 2045
Debt Instrument [Line Items]
Long-term debt					$ 190,000,000	190,000,000
Interest rate, stated percentage					5.375%
Recourse Debt | Dunkirk Power LLC, tax exempt bonds, due 2042
Debt Instrument [Line Items]
Long-term debt					$ 59,000,000	59,000,000
Interest rate, stated percentage					5.875%
Recourse Debt | City of Texas City, tax exempt bonds, due 2045
Debt Instrument [Line Items]
Long-term debt					$ 33,000,000	32,000,000
Interest rate, stated percentage					4.125%
Recourse Debt | Fort Bend County, tax exempt bonds, due 2038
Debt Instrument [Line Items]
Long-term debt					$ 54,000,000	54,000,000
Interest rate, stated percentage					4.75%
Recourse Debt | Fort Bend County, tax exempt bonds, due 2042
Debt Instrument [Line Items]
Long-term debt					$ 73,000,000	73,000,000
Interest rate, stated percentage					4.75%
Recourse Debt | Tax-exempt bonds
Debt Instrument [Line Items]
Long-term debt					$ 466,000,000	$ 465,000,000
Senior Credit Facility | Term loan facility, due 2023
Debt Instrument [Line Items]
Long-term debt	$ 105,000,000
Revolving Credit Facility | 2016 Tranche A Revolving Credit Facility due 2018
Debt Instrument [Line Items]
Revolving credit facility				$ 289,000,000
Revolving Credit Facility | 2016 Tranche B Revolving Credit Facility due 2021
Debt Instrument [Line Items]
Basis spread on variable rate (as a percent)				2.25%
Revolving credit facility				$ 2,200,000,000.0

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