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		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Accounting Policies [Abstract]
Income from continuing operations, net of income tax	$ 374	$ 274	$ 656	$ 547
(Loss)/income from discontinued operations, net of income tax	(2)	(346)	399	(267)
Net income/(loss) attributable to NRG Energy, Inc.	$ 372	$ (72)	$ 1,055	$ 280