Accounting for Derivative Instruments and Hedging Activities - Effect on the Company's of Accumulated OCI Balance Attributable to Cash Flow Hedge Derivatives, Net (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effect of ASC 815 on NRG's Accumulated OCI Balance Attributable to Cash Flow Hedge Derivatives
Accumulated OCI beginning balance	$ (101)
Reclassified from accumulated OCI to income:
Due to realization of previously deferred amounts	21
Mark-to-market of cash flow hedge accounting contracts	14
Sale of NRG Yield and Renewables	25	$ 0	$ 0
Accumulated OCI ending balance, net of $0, $8 and $16 tax	(66)	(101)
Accumulated OCI income (loss), tax	0	(8)	(16)
2018
Effect of ASC 815 on NRG's Accumulated OCI Balance Attributable to Cash Flow Hedge Derivatives
Accumulated OCI beginning balance	(54)	(66)
Reclassified from accumulated OCI to income:
Due to realization of previously deferred amounts	8	12
Mark-to-market of cash flow hedge accounting contracts	21	0
Accumulated OCI ending balance, net of $0, $8 and $16 tax	$ 0	$ (54)	$ (66)