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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
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Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Accounting Policies [Abstract] |  |  |
Income/(loss) from continuing operations, net of income tax | $ 4,117 | $ 465 | $ (977)
Income/(loss) from discontinued operations, net of income tax | 321 | (197) | (1,176)
Net Income/(Loss) Attributable to NRG Energy, Inc. | $ 4,438 | $ 268 | $ (2,153)

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