Condensed Consolidating Financial Information - CONSOLIDATING STATEMENTS OF OPERATIONS (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Operating Revenues
Total operating revenues	$ 2,809			$ 2,996			$ 7,066	$ 7,626
Operating Costs and Expenses
Cost of operations	2,034			2,153			4,925	5,649
Depreciation and amortization	99			91			318	261
Impairment losses	29			0			29	1
Selling, general and administrative costs	253			210			670	615
Reorganization costs	0			1			3	16
Development costs	1			1			6	5
Total operating costs and expenses	2,416			2,456			5,951	6,547
Gain on sale of assets	0			0			6	2
Operating Income/(Loss)	393			540			1,121	1,081
Other Income/(Expense)
Equity in earnings of consolidated subsidiaries	0			0			0	0
Equity in earnings of unconsolidated affiliates	36			29			37	8
Impairment losses on investments	0			(107)			(18)	(107)
Other income, net	11			17			52	49
Loss on debt extinguishment, net	0			0			(1)	(47)
Interest expense	(99)			(99)			(292)	(318)
Total other income/(expense)	(52)			(160)			(222)	(415)
Income from Continuing Operations Before Income Taxes	341			380			899	666
Income tax expense	92			6			216	9
Income from Continuing Operations	249			374			683	657
(Loss)/income from discontinued operations, net of income tax	0			(2)			0	399
Net Income	249			372			683	1,056
Less: Net income attributable to redeemable noncontrolling interests	0			0			0	1
Net Income Attributable to NRG Energy, Inc.	249	$ 313	$ 121	372	$ 201	$ 482	683	1,055
Eliminations
Operating Revenues
Total operating revenues	(1)			(2)			(10)	(8)
Operating Costs and Expenses
Cost of operations	(1)			(2)			(10)	(8)
Depreciation and amortization	0			0			0	0
Impairment losses	0						0	0
Selling, general and administrative costs	0			0			0	0
Reorganization costs				0			0	0
Development costs	0			0			0	0
Total operating costs and expenses	(1)			(2)			(10)	(8)
Gain on sale of assets							0	0
Operating Income/(Loss)	0			0			0	0
Other Income/(Expense)
Equity in earnings of consolidated subsidiaries	(540)			(557)			(1,391)	(1,298)
Equity in earnings of unconsolidated affiliates	0			0			0	0
Impairment losses on investments				0			0	0
Other income, net	0			0			0	0
Loss on debt extinguishment, net							0	0
Interest expense	0			0			0	0
Total other income/(expense)	(540)			(557)			(1,391)	(1,298)
Income from Continuing Operations Before Income Taxes	(540)			(557)			(1,391)	(1,298)
Income tax expense	0			0			0	0
Income from Continuing Operations				(557)				(1,298)
(Loss)/income from discontinued operations, net of income tax				0				0
Net Income	(540)			(557)			(1,391)	(1,298)
Less: Net income attributable to redeemable noncontrolling interests								0
Net Income Attributable to NRG Energy, Inc.	(540)			(557)			(1,391)	(1,298)
Guarantor Subsidiaries
Operating Revenues
Total operating revenues	2,513			2,473			6,346	6,382
Operating Costs and Expenses
Cost of operations	1,796			1,728			4,357	4,676
Depreciation and amortization	72			52			232	157
Impairment losses	0						0	1
Selling, general and administrative costs	154			132			431	366
Reorganization costs				0			0	0
Development costs	0			0			0	0
Total operating costs and expenses	2,022			1,912			5,020	5,200
Gain on sale of assets							0	1
Operating Income/(Loss)	491			561			1,326	1,183
Other Income/(Expense)
Equity in earnings of consolidated subsidiaries	0			20			6	32
Equity in earnings of unconsolidated affiliates	0			0			0	0
Impairment losses on investments				0			0	0
Other income, net	4			11			14	19
Loss on debt extinguishment, net							0	0
Interest expense	(3)			(4)			(12)	(11)
Total other income/(expense)	1			27			8	40
Income from Continuing Operations Before Income Taxes	492			588			1,334	1,223
Income tax expense	0			0			0	0
Income from Continuing Operations				588				1,223
(Loss)/income from discontinued operations, net of income tax				0				9
Net Income	492			588			1,334	1,232
Less: Net income attributable to redeemable noncontrolling interests								0
Net Income Attributable to NRG Energy, Inc.	492			588			1,334	1,232
Non-Guarantor Subsidiaries
Operating Revenues
Total operating revenues	297			525			730	1,252
Operating Costs and Expenses
Cost of operations	230			421			586	956
Depreciation and amortization	21			32			60	81
Impairment losses	29						29	0
Selling, general and administrative costs	7			28			19	56
Reorganization costs				0			0	0
Development costs	(1)			0			0	1
Total operating costs and expenses	286			481			694	1,094
Gain on sale of assets							1	1
Operating Income/(Loss)	11			44			37	159
Other Income/(Expense)
Equity in earnings of consolidated subsidiaries	0			0			0	0
Equity in earnings of unconsolidated affiliates	36			29			37	8
Impairment losses on investments				(101)			(18)	(101)
Other income, net	2			1			6	10
Loss on debt extinguishment, net							0	0
Interest expense	0			(3)			(3)	(12)
Total other income/(expense)	38			(74)			22	(95)
Income from Continuing Operations Before Income Taxes	49			(30)			59	64
Income tax expense	1			1			2	2
Income from Continuing Operations				(31)				62
(Loss)/income from discontinued operations, net of income tax				0				5
Net Income	48			(31)			57	67
Less: Net income attributable to redeemable noncontrolling interests								1
Net Income Attributable to NRG Energy, Inc.	48			(31)			57	66
NRG Energy, Inc. (Note Issuer)
Operating Revenues
Total operating revenues	0			0			0	0
Operating Costs and Expenses
Cost of operations	9			6			(8)	25
Depreciation and amortization	6			7			26	23
Impairment losses	0						0	0
Selling, general and administrative costs	92			50			220	193
Reorganization costs				1			3	16
Development costs	2			1			6	4
Total operating costs and expenses	109			65			247	261
Gain on sale of assets							5	0
Operating Income/(Loss)	(109)			(65)			(242)	(261)
Other Income/(Expense)
Equity in earnings of consolidated subsidiaries	540			537			1,385	1,266
Equity in earnings of unconsolidated affiliates	0			0			0	0
Impairment losses on investments				(6)			0	(6)
Other income, net	5			5			32	20
Loss on debt extinguishment, net							(1)	(47)
Interest expense	(96)			(92)			(277)	(295)
Total other income/(expense)	449			444			1,139	938
Income from Continuing Operations Before Income Taxes	340			379			897	677
Income tax expense	91			5			214	7
Income from Continuing Operations				374				670
(Loss)/income from discontinued operations, net of income tax				(2)				385
Net Income	249			372			683	1,055
Less: Net income attributable to redeemable noncontrolling interests								0
Net Income Attributable to NRG Energy, Inc.	$ 249			$ 372			$ 683	$ 1,055