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Impairments (Details) - USD ($)		3 Months Ended				9 Months Ended
Impairments (Details) - USD ($)	Jun. 29, 2018	Sep. 30, 2018	Jun. 30, 2018	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Impaired Long-Lived Assets Held and Used [Line Items]
Carrying amount of related assets		$ 452,000,000				$ 452,000,000		$ 182,000,000
Impairment losses		$ 0		$ 0		$ 74,000,000	$ 60,000,000
Construction in progress					$ 0
Renewables
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment losses							$ 19,000,000
Construction in Progress
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment losses					$ 41,000,000
Keystone
Impaired Long-Lived Assets Held and Used [Line Items]
Ownership interest	3.70%
Impairment losses to reduce carrying amounts	$ 14,000,000
Conemaugh
Impaired Long-Lived Assets Held and Used [Line Items]
Ownership interest	3.70%
Impairment losses to reduce carrying amounts	$ 14,000,000
Dunkirk
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment losses to reduce carrying amounts			$ 46,000,000
Carrying amount of related assets			$ 0