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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
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