Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 3,011	$ 4,434	$ 3,400	$ 3,829	$ 4,192	$ 4,569	$ 3,621	$ 3,486	$ 14,674	[1]	$ 15,868	[2]	$ 11,295	[3]
Operating Income (Loss)	(4,727)	379	232	76	453	549	89	180	(4,040)		1,271		343
Net (Loss)/Income	(6,358)	67	(9)	(136)	97	182	(80)	(67)	(6,436)		132		(352)
Less: Net (loss)/income attributable to noncontrolling interests and redeemable noncontrolling interests	(44)	1	5	(16)	(22)	14	17	(11)	(54)		(2)		34
Net (loss)/income attributable to NRG Energy, Inc.	(6,314)	66	(14)	(120)	119	168	(97)	(56)	(6,382)		134		(386)
Net Income (Loss) Available to Common Stockholders, Basic	$ (6,319)	$ 61	$ (19)	$ (125)	$ 70	$ 166	$ (100)	$ (58)	$ (6,402)		$ 78		$ (395)
Weighted average number of common shares outstanding — basic	315	331	333	336	338	338	337	324	329		334		323
Net (Loss)/Income per Weighted Average Common Share — Basic	$ (20.08)	$ 0.18	$ (0.06)	$ (0.37)	$ 0.21	$ 0.49	$ (0.30)	$ (0.18)	$ (19.46)		$ 0.23		$ (1.22)
Weighted average number of common shares outstanding — diluted	315	332	333	336	342	343	337	324	329		339		323
Net (Loss)/Income per Weighted Average Common Share — Diluted	$ (20.08)	$ 0.18	$ (0.06)	$ (0.37)	$ 0.20	$ 0.48	$ (0.30)	$ (0.18)	$ (19.46)		$ 0.23		$ (1.22)

[1] (a) Operating revenues include inter-segment sales and net derivative gains and losses of:$947 $6 $1 $23 $29 $212 $— $1,218

[2] (c) Operating revenues include inter-segment sales and net derivative gains and losses of:$1,820 $7 $— $25 $12 $85 $— $1,949

[3] (f) Operating revenues include inter-segment sales and net derivative gains and losses of:$2,055 $5 $— $14 $7 $227 $— $2,308