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Fair Value of Financial Instruments (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic) - Derivative FV (Details)
Sep. 30, 2017
USD ($)
$ / MWh
Dec. 31, 2016
USD ($)
$ / T
$ / MWh
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Contracts valued with prices provided by models and other valuation techniques, percent of derivative assets 0.00%  
Contracts valued with prices provided by models and other valuation techniques, percent of derivative liability 12.00%  
Credit reserve balance $ 0 $ 1,000,000
Cash collateral paid 72,000,000  
NRG    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cash collateral paid 25,000,000  
Commodity contracts | Fair Value, Measurements, Recurring    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative assets 56,000,000 124,000,000
Commodity contracts | Fair Value, Measurements, Recurring | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative assets 0 2,000,000
Derivative liabilities 5,000,000 3,000,000
Power Contracts | Commodity contracts | Fair Value, Measurements, Recurring | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative assets   1,000,000
Derivative liabilities   $ 0
Power Contracts | Commodity contracts | Fair Value, Measurements, Recurring | Low | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Forward market price | $ / MWh   29
Power Contracts | Commodity contracts | Fair Value, Measurements, Recurring | High | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Forward market price | $ / MWh   59
Power Contracts | Commodity contracts | Fair Value, Measurements, Recurring | Weighted Average | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Forward market price | $ / MWh   43
Coal Contracts | Commodity contracts | Fair Value, Measurements, Recurring | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative assets   $ 0
Derivative liabilities   $ 1,000,000
Coal Contracts | Commodity contracts | Fair Value, Measurements, Recurring | Low | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Forward market price | $ / T   42
Coal Contracts | Commodity contracts | Fair Value, Measurements, Recurring | High | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Forward market price | $ / T   51
Coal Contracts | Commodity contracts | Fair Value, Measurements, Recurring | Weighted Average | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Forward market price | $ / T   45
FTRs | Commodity contracts | Fair Value, Measurements, Recurring | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative assets 0 $ 1,000,000
Derivative liabilities $ 5,000,000 $ 2,000,000
FTRs | Commodity contracts | Fair Value, Measurements, Recurring | Low | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Auction prices | $ / MWh (3) (2)
FTRs | Commodity contracts | Fair Value, Measurements, Recurring | High | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Auction prices | $ / MWh 1 3
FTRs | Commodity contracts | Fair Value, Measurements, Recurring | Weighted Average | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Auction prices | $ / MWh 0 0
GenOn Americas Generation    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Contracts valued with prices provided by models and other valuation techniques, percent of derivative assets 5.00%  
Contracts valued with prices provided by models and other valuation techniques, percent of derivative liability 6.00%  
Credit reserve balance $ 0 $ 1,000,000
Cash collateral paid 70,000,000  
GenOn Americas Generation | NRG    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Cash collateral paid 25,000,000  
GenOn Americas Generation | Commodity contracts | Fair Value, Measurements, Recurring    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative assets 94,000,000 214,000,000
GenOn Americas Generation | Commodity contracts | Fair Value, Measurements, Recurring | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative assets 5,000,000 5,000,000
Derivative liabilities 6,000,000 5,000,000
GenOn Americas Generation | Power Contracts | Commodity contracts | Fair Value, Measurements, Recurring | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative assets   1,000,000
Derivative liabilities   $ 0
GenOn Americas Generation | Power Contracts | Commodity contracts | Fair Value, Measurements, Recurring | Low | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Forward market price | $ / MWh   29
GenOn Americas Generation | Power Contracts | Commodity contracts | Fair Value, Measurements, Recurring | High | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Forward market price | $ / MWh   59
GenOn Americas Generation | Power Contracts | Commodity contracts | Fair Value, Measurements, Recurring | Weighted Average | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Forward market price | $ / MWh   43
GenOn Americas Generation | Coal Contracts | Commodity contracts | Fair Value, Measurements, Recurring | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative assets   $ 1,000,000
Derivative liabilities   $ 1,000,000
GenOn Americas Generation | Coal Contracts | Commodity contracts | Fair Value, Measurements, Recurring | Low | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Forward market price | $ / T   42
GenOn Americas Generation | Coal Contracts | Commodity contracts | Fair Value, Measurements, Recurring | High | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Forward market price | $ / T   51
GenOn Americas Generation | Coal Contracts | Commodity contracts | Fair Value, Measurements, Recurring | Weighted Average | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Forward market price | $ / T   45
GenOn Americas Generation | FTRs | Commodity contracts | Fair Value, Measurements, Recurring | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative assets 5,000,000 $ 3,000,000
Derivative liabilities $ 6,000,000 $ 4,000,000
GenOn Americas Generation | FTRs | Commodity contracts | Fair Value, Measurements, Recurring | Low | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Auction prices | $ / MWh (3) (2)
GenOn Americas Generation | FTRs | Commodity contracts | Fair Value, Measurements, Recurring | High | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Auction prices | $ / MWh 1 3
GenOn Americas Generation | FTRs | Commodity contracts | Fair Value, Measurements, Recurring | Weighted Average | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Auction prices | $ / MWh 0 0
GenOn Mid-Atlantic    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Contracts valued with prices provided by models and other valuation techniques, percent of derivative assets 0.00%  
Contracts valued with prices provided by models and other valuation techniques, percent of derivative liability 7.00%  
Cash collateral paid $ 0  
Cash collateral received in support of energy risk management activities 0  
GenOn Mid-Atlantic | Commodity contracts | Fair Value, Measurements, Recurring    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative assets 12,000,000 $ 48,000,000
GenOn Mid-Atlantic | Commodity contracts | Fair Value, Measurements, Recurring | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative assets 0 1,000,000
Derivative liabilities 1,000,000 1,000,000
GenOn Mid-Atlantic | Power Contracts | Commodity contracts | Fair Value, Measurements, Recurring | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative assets 0 1,000,000
Derivative liabilities 0 $ 0
GenOn Mid-Atlantic | Power Contracts | Commodity contracts | Fair Value, Measurements, Recurring | Low | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Forward market price | $ / MWh   29
GenOn Mid-Atlantic | Power Contracts | Commodity contracts | Fair Value, Measurements, Recurring | High | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Forward market price | $ / MWh   59
GenOn Mid-Atlantic | Power Contracts | Commodity contracts | Fair Value, Measurements, Recurring | Weighted Average | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Forward market price | $ / MWh   43
GenOn Mid-Atlantic | FTRs | Commodity contracts | Fair Value, Measurements, Recurring | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Derivative assets 0 $ 0
Derivative liabilities $ 1,000,000 $ 1,000,000
GenOn Mid-Atlantic | FTRs | Commodity contracts | Fair Value, Measurements, Recurring | Low | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Auction prices | $ / MWh 0 0
GenOn Mid-Atlantic | FTRs | Commodity contracts | Fair Value, Measurements, Recurring | High | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Auction prices | $ / MWh 1 1
GenOn Mid-Atlantic | FTRs | Commodity contracts | Fair Value, Measurements, Recurring | Weighted Average | Level 3    
Fair Value Inputs, Assets, Quantitative Information [Line Items]    
Auction prices | $ / MWh 0 0