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Stock-Based Compensation - Deferred Stock Units, DSUs (Details) - DSUs - USD ($)
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Units      
Balance outstanding at the beginning of the period (in shares) 427,148    
Granted (in shares) 61,645    
Converted to Common Stock (in shares) (156,878)    
Balance outstanding at the end of the period (in shares) 331,915.000000 427,148  
Weighted Average Grant Date Fair Value per Unit      
Balance outstanding at the beginning of the period, (in usd per share) $ 21.54    
Granted (in usd per share) 33.43 $ 16.76 $ 16.85
Vested (in usd per share) 23.59    
Balance outstanding at the end of the period (in usd per share) $ 22.94 $ 21.54  
Aggregate intrinsic value for DSUs outstanding $ 13,000,000 $ 12,000,000 $ 6,000,000
Aggregate intrinsic values for DSUs converted to common stock during the period $ 0 $ 4,000,000 $ 1,000,000
GenOn Employees      
Units      
Granted (in shares) 0