Income Per Share - Reconciliation of Basic and Diluted EPS (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Basic income per share attributable to NRG Energy, Inc;
Net income attributable to NRG Energy, Inc. common stockholders	$ 2,187	$ 510	$ 4,438
Weighted average number of common shares outstanding — basic (in shares)	245,000,000	245,000,000	262,000,000
Diluted income per share attributable to NRG Energy, Inc;	$ 8.93	$ 2.08	$ 16.94
Diluted income per share attributable to NRG Energy, Inc;
Net income attributable to NRG Energy, Inc. common stockholders	$ 2,187	$ 510	$ 4,438
Weighted average number of common shares outstanding — basic (in shares)	245,000,000	245,000,000	262,000,000
Incremental shares attributable to the issuance of equity compensation (treasury stock method) (in shares)	0	1,000,000	2,000,000
Weighted average number of common shares outstanding- dilutive (in shares)	245,000,000	246,000,000	264,000,000
Income per weighted average common share — diluted (in usd per share)	$ 8.93	$ 2.07	$ 16.81