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Impairments (Details) - USD ($)		3 Months Ended		9 Months Ended
Impairments (Details) - USD ($)	Jun. 29, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Impairment losses					$ 16,000,000
Impairment losses on cost investments and intangible assets				$ 7,000,000
Carrying amount		$ 405,000,000		405,000,000		$ 412,000,000
Petra Nova
Schedule of Equity Method Investments [Line Items]
Investment in cash		95,000,000
Letters of credit issued		12,000,000		12,000,000
Guarantor obligations (up to)		124,000,000		$ 124,000,000
Impairment losses		$ 101,000,000
Keystone
Schedule of Equity Method Investments [Line Items]
Impairment losses	$ 14,000,000
Interest in generating stations	3.70%
Conemaugh
Schedule of Equity Method Investments [Line Items]
Impairment losses	$ 14,000,000
Dunkirk
Schedule of Equity Method Investments [Line Items]
Impairment losses			$ 46,000,000
Carrying amount			$ 0		$ 0