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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, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Accounting Policies [Abstract] |  |  |
Income/(loss) from continuing operations, net of income tax | $ 465 | $ (977) | $ (733)
Loss from discontinued operations, net of income tax | (197) | (1,176) | (41)
Net Income/(Loss) Attributable to NRG Energy, Inc. | $ 268 | $ (2,153) | $ (774)

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