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Earnings Per Share - Reconciliation of Basic and Diluted EPS (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Sep. 30, 2020
Sep. 30, 2019
Basic income per share:                
Net income attributable to NRG Energy, Inc. $ 249 $ 313 $ 121 $ 372 $ 201 $ 482 $ 683 $ 1,055
Weighted average number of common shares outstanding — basic (in shares) 244     254     246 266
Income per weighted average common share — basic (in usd per share) $ 1.02     $ 1.46     $ 2.78 $ 3.97
Diluted income per share:                
Net income available to common shareholders $ 249     $ 372     $ 683 $ 1,055
Weighted average number of common shares outstanding — basic (in shares) 244     254     246 266
Incremental shares attributable to the issuance of equity compensation (treasury stock method) (in shares) 1     2     1 2
Weighted average number of common shares outstanding- dilutive (in shares) 245     256     247 268
Income per weighted average common share — diluted (in usd per share) $ 1.02     $ 1.45     $ 2.77 $ 3.94