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Earnings Per Share (Details) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Basic income per share attributable to NRG, Inc;      
Net Income/(Loss) Attributable to NRG Energy, Inc. $ 510 $ 4,438 $ 268
Weighted average number of common shares outstanding — basic (in shares) 245,000,000 262,000,000 304,000,000
Income/(Loss) per weighted average common share — basic (in usd per share) $ 2.08 $ 16.94 $ 0.88
Incremental shares attributable to the issuance of equity compensation (treasury stock method) (in shares) 1,000,000 2,000,000 4,000,000
Weighted average number of common shares outstanding — diluted (in shares) 246,000,000 264,000,000 308,000,000
Income per weighted average common share — diluted (in usd per share) $ 2.07 $ 16.81 $ 0.87