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Stock-Based Compensation - Deferred Stock Units, DSUs (Details) - DSUs - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Units      
Balance outstanding at the beginning of the period (in shares) 453,674    
Granted (in shares) 120,251    
Converted to Common Stock (in shares) (146,777)    
Balance outstanding at the end of the period (in shares) 427,148.000000 453,674  
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) 16.76 $ 16.85 $ 25.14
Converted to Common Stock (in usd per share) 17.62    
Balance outstanding at the end of the period (in usd per share) $ 21.54 $ 21.54  
Aggregate intrinsic value for DSUs outstanding $ 12 $ 6 $ 5
Aggregate intrinsic values for DSUs converted to common stock during the period $ 4 $ 1 $ 0
GenOn Employees      
Units      
Granted (in shares) 0