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Earnings /Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended								12 Months Ended
	Jun. 13, 2016	May 24, 2016	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Redemption of shares (as percentage)	100.00%	100.00%
Basic income/(loss) per share attributable to NRG common stockholders
Net Income/(Loss) Attributable to NRG Energy, Inc.											$ 268	$ (2,153)	$ (774)
Dividends for preferred shares											0	0	5
Gain on redemption of preferred shares											0	0	(78)
Income/(Loss) Available for Common Stockholders			$ (11)	$ (72)	$ 72	$ 279	$ (1,535)	$ 171	$ (626)	$ (163)	$ 268	$ (2,153)	$ (701)
Weighted average number of common shares outstanding — basic (in shares)			289,000,000	299,000,000	310,000,000	318,000,000	317,000,000	317,000,000	316,000,000	316,000,000	304,000,000	317,000,000	316,000,000
Income/(Loss) per weighted average common share — basic (in usd per share)			$ (0.04)	$ (0.24)	$ 0.23	$ 0.88	$ (4.84)	$ 0.54	$ (1.98)	$ (0.52)	$ 0.88	$ (6.79)	$ (2.22)
Incremental shares attributable to the issuance of equity compensation (treasury stock method) (in shares)											4,000,000	0	0
Weighted average number of common shares outstanding — diluted (in shares)			289,000,000	299,000,000	314,000,000	322,000,000	317,000,000	322,000,000	316,000,000	316,000,000	308,000,000	317,000,000	316,000,000
Income (Loss) per weighted average common share — diluted (in usd per share)			$ (0.04)	$ (0.24)	$ 0.23	$ 0.87	$ (4.84)	$ 0.53	$ (1.98)	$ (0.52)	$ 0.87	$ (6.79)	$ (2.22)
Convertible Preferred Stock
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Preferred stock, dividend rate (as percent)		2.822%									2.822%	2.822%	2.822%
Basic income/(loss) per share attributable to NRG common stockholders
Gain on redemption of preferred shares	$ (78)

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