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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 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Basic income per share attributable to NRG Energy, Inc;        
Net income attributable to NRG Energy, Inc. common stockholders $ 201 $ 72 $ 683 $ 351
Weighted average number of common shares outstanding — basic (in shares) 265 310 272 314
Income per weighted average common share — basic (in usd per share) $ 0.76 $ 0.23 $ 2.51 $ 1.12
Diluted income per share attributable to NRG Energy, Inc;        
Net income attributable to NRG Energy, Inc. available to common shareholders $ 201 $ 72 $ 683 $ 351
Weighted average number of common shares outstanding - diluted (in shares) 265 310 272 314
Incremental shares attributable to the issuance of equity compensation (treasury stock method) (in shares) 2 4 2 4
Total dilutive shares (in shares) 267 314 274 318
Income per weighted average common share — diluted (in usd per share) $ 0.75 $ 0.23 $ 2.49 $ 1.10