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Impairments (Details) - USD ($)		3 Months Ended
Impairments (Details) - USD ($)	Jun. 29, 2018	Jun. 30, 2018	Jun. 30, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Carrying amount			$ 383,000,000	$ 412,000,000
Keystone
Schedule of Equity Method Investments [Line Items]
Interest in generating stations	3.70%
Impairment losses	$ 14,000,000
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