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Related Party Transactions - Credit Agreement with GenOn and Commercial Operations Agreement (Details) - Revolving Credit Facility - Intercompany Credit Agreement - GenOn - USD ($)
Dec. 31, 2019
Dec. 31, 2017
Related Party Transaction [Line Items]    
Revolving credit facility $ 500,000,000  
Letters of credit outstanding under revolver $ 14,000,000 $ 92,000,000