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Earnings /Loss Per Share (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 3 Months Ended 12 Months Ended
Jun. 13, 2016
May 24, 2016
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]                          
Redemption of shares (as percentage) 100.00% 100.00%                      
Basic and diluted loss per share attributable to NRG common stockholders                          
Net loss attributable to NRG Energy, Inc.     $ (1,535) $ 171 $ (626) $ (163) $ (987) $ 402 $ (271) $ 82 $ (2,153) $ (774) $ (6,382)
Dividends for preferred shares                     0 5 20
Gain on redemption of preferred shares                     0 (78) 0
(Loss)/Income Available for Common Stockholders     $ (1,535) $ 171 $ (626) $ (163) $ (987) $ 402 $ (193) $ 77 $ (2,153) $ (701) $ (6,402)
Weighted average number of common shares outstanding (in shares)                     317 316 329
Loss per weighted average common share — basic and diluted (in USD PER SHARE)                     $ (6.79) $ (2.22) $ (19.46)
Convertible Preferred Stock                          
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]                          
Preferred stock, dividend rate (as percent)   2.822%                 2.822% 2.822% 2.822%
Basic and diluted loss per share attributable to NRG common stockholders                          
Gain on redemption of preferred shares $ (78)