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Summary of Significant Accounting Policies - Net (Loss)/Income Attributable to NRG Energy, Inc. After Removing Net Loss Attributable to Noncontrolling Interest (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2019 | Mar. 31, 2018
Accounting Policies [Abstract] |  |
Income from continuing operations, net of income tax | $ 94 | $ 245
Income from discontinued operations, net of income tax | 388 | 34
Net income attributable to NRG Energy, Inc. | $ 482 | $ 279

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