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Related Party Transactions - Credit Agreement with GenOn and Commercial Operations Agreement (Details) - USD ($)
Dec. 31, 2017
Dec. 31, 2016
Related Party Transaction [Line Items]    
Long-term debt $ 16,633,000,000 $ 16,704,000,000
Cash collateral posted in support of energy risk management activities 171,000,000 150,000,000
GenOn    
Related Party Transaction [Line Items]    
Cash collateral posted in support of energy risk management activities 32,000,000 79,000,000
Revolving Credit Facility | Intercompany Credit Agreement | GenOn    
Related Party Transaction [Line Items]    
Revolving credit facility 500,000,000  
Letters of credit outstanding under revolver 92,000,000 272,000,000
Long-term debt 125,000,000 $ 0
Revolving Credit Facility | Restructuring Support Agreement, Letter of Credit Credit Facility | GenOn    
Related Party Transaction [Line Items]    
Revolving credit facility $ 330,000,000  
Line of credit facility cash collateralized percentage required 103.00%