Earnings Per Share - Reconciliation of Basic and Diluted EPS (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Basic income per share attributable to NRG Energy, Inc;
Net income/(loss) attributable to NRG Energy, Inc. common stockholders	$ 372	$ (72)	$ 1,055	$ 280
Weighted average number of common shares outstanding — basic (in shares)	254	299	266	309
Income/(loss) per weighted average common share — basic (in usd per share)	$ 1.46	$ (0.24)	$ 3.97	$ 0.91
Diluted income per share attributable to NRG Energy, Inc;
Net income/(loss) attributable to NRG Energy, Inc. available to common shareholders	$ 372	$ (72)	$ 1,055	$ 280
Weighted average number of common shares outstanding - diluted (in shares)	254	299	266	309
Incremental shares attributable to the issuance of equity compensation (treasury stock method) (in shares)	2	0	2	4
Total dilutive shares (in shares)	256	299	268	313
Income/(loss) per weighted average common share — diluted (in usd per share)	$ 1.45	$ (0.24)	$ 3.94	$ 0.89