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Related Party Transactions - Credit Agreement with GenOn (Details) - USD ($)
Jun. 30, 2018
Jun. 19, 2018
Dec. 31, 2017
Related Party Transaction [Line Items]      
Amount $ 16,092,000,000 $ 120,000,000 $ 16,633,000,000
Cash collateral paid in support of energy risk management activities 224,000,000   171,000,000
GenOn      
Related Party Transaction [Line Items]      
Cash collateral paid in support of energy risk management activities 24,000,000   32,000,000
GenOn | Revolving Credit Facility | Intercompany Credit Agreement      
Related Party Transaction [Line Items]      
Credit facility maximum borrowing capacity 500,000,000    
Letters of credit outstanding under revolver 45,000,000   92,000,000
Amount $ 151,000,000   $ 125,000,000