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Summary of Significant Accounting Policies - Net Income/(Loss) Attributable to Continuing and Discontinued Operations (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Accounting Policies [Abstract]      
Income from continuing operations, net of income tax $ 510 $ 4,117 $ 465
Income/(loss) from discontinued operations, net of income tax 0 321 (197)
Net Income Attributable to NRG Energy, Inc. $ 510 $ 4,438 $ 268