Acquisitions, Discontinued Operations and Dispositions - Sale of Assets to NRG Yield, Inc. Prior to Discontinued Operations (Details) $ in Millions	Jun. 30, 2019 MW	Jan. 31, 2019 USD ($)	Jun. 19, 2018 USD ($)	Mar. 30, 2018 USD ($) MW
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Generation capacity (in MW) | MW	23,000
UPMC Thermal Project
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash consideration		$ 3	$ 84
Buckthorn Renewables, LLC
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash consideration				$ 42
Percentage of ownership sold				100.00%
Generation capacity (in MW) | MW				154
Debt assumed				$ 183