Condensed Consolidating Financial Information - CONSOLIDATING BALANCE SHEETS (Details) - USD ($) $ in Millions	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Current Assets
Cash and cash equivalents	$ 243			$ 563
Funds deposited by counterparties	30			33
Restricted cash	4			17
Accounts receivable, net	1,376			1,024
Inventory	364			412
Derivative instruments	735			764
Cash collateral paid in support of energy risk management activities	164			287
Prepayments and other current assets	271			302
Current assets - held-for-sale	0			1
Current assets - discontinued operations	0			197
Total current assets	3,187			3,600
Property, plant and equipment, net	2,615			3,048
Other Assets
Investment in subsidiaries	0			0
Equity investments in affiliates	405			412
Operating lease right-of-use assets, net	482
Goodwill	591			573
Intangible assets, net	828			591
Nuclear decommissioning trust fund	756			663
Derivative instruments	358			317
Deferred income taxes	53			46
Other non-current assets	252			289
Non-current assets - held-for-sale	0			77
Non-current assets - discontinued operations	0			1,012
Total other assets	3,725			3,980
Total Assets	9,527			10,628
Current Liabilities
Current portion of long-term debt	302			72
Current portion of operating lease liabilities	73
Accounts payable	866			863
Derivative instruments	668			673
Cash collateral received in support of energy risk management activities	30			33
Accrued expenses and other current liabilities	625			680
Current liabilities - held-for-sale	0			5
Current liabilities - discontinued operations	0			72
Total current liabilities	2,564			2,398
Other Liabilities
Long-term debt	5,798			6,449
Non-current operating lease liabilities	500
Nuclear decommissioning reserve	294			282
Nuclear decommissioning trust liability	453			371
Derivative instruments	364			304
Deferred income taxes	70			65
Other non-current liabilities	1,036			1,274
Non-current liabilities - held-for-sale	0			65
Non-current liabilities - discontinued operations	0			635
Total other liabilities	8,515			9,445
Total Liabilities	11,079			11,843
Redeemable noncontrolling interest in subsidiaries	19			19
Total Stockholders' Equity	(1,571)	$ (1,648)	$ (1,538)	(1,234)	$ (936)	$ 2,071	$ 2,258	$ 1,968
Total Liabilities and Stockholders' Equity	9,527			10,628
Eliminations
Current Assets
Cash and cash equivalents	0			0
Funds deposited by counterparties	0			0
Restricted cash	0			0
Accounts receivable, net	(1,293)			(754)
Inventory	0			0
Derivative instruments	(33)			(81)
Cash collateral paid in support of energy risk management activities	0			0
Prepayments and other current assets	0			0
Current assets - held-for-sale				0
Current assets - discontinued operations				0
Total current assets	(1,326)			(835)
Property, plant and equipment, net	0			0
Other Assets
Investment in subsidiaries	(5,463)			(5,153)
Equity investments in affiliates	0			0
Operating lease right-of-use assets, net	0
Goodwill	0			0
Intangible assets, net	0			0
Nuclear decommissioning trust fund	0			0
Derivative instruments	(12)			(5)
Deferred income taxes	0			0
Other non-current assets	0			(12)
Non-current assets - held-for-sale				0
Non-current assets - discontinued operations				0
Total other assets	(5,475)			(5,170)
Total Assets	(6,801)			(6,005)
Current Liabilities
Current portion of long-term debt	0			0
Current portion of operating lease liabilities	0
Accounts payable	(1,293)			(754)
Derivative instruments	(33)			(81)
Cash collateral received in support of energy risk management activities	0			0
Accrued expenses and other current liabilities	0			0
Current liabilities - held-for-sale				0
Current liabilities - discontinued operations				0
Total current liabilities	(1,326)			(835)
Other Liabilities
Long-term debt	0			(12)
Non-current operating lease liabilities	0
Nuclear decommissioning reserve	0			0
Nuclear decommissioning trust liability	0			0
Derivative instruments	(12)			(5)
Deferred income taxes	0			0
Other non-current liabilities	0			0
Non-current liabilities - held-for-sale				0
Non-current liabilities - discontinued operations				0
Total other liabilities	(12)			(17)
Total Liabilities	(1,338)			(852)
Redeemable noncontrolling interest in subsidiaries	0			0
Total Stockholders' Equity	(5,463)			(5,153)
Total Liabilities and Stockholders' Equity	(6,801)			(6,005)
Guarantor Subsidiaries
Current Assets
Cash and cash equivalents	0			55
Funds deposited by counterparties	30			33
Restricted cash	2			7
Accounts receivable, net	2,049			1,354
Inventory	248			278
Derivative instruments	740			779
Cash collateral paid in support of energy risk management activities	146			275
Prepayments and other current assets	178			180
Current assets - held-for-sale				0
Current assets - discontinued operations				177
Total current assets	3,393			3,138
Property, plant and equipment, net	1,494			1,938
Other Assets
Investment in subsidiaries	786			446
Equity investments in affiliates	0			0
Operating lease right-of-use assets, net	85
Goodwill	359			359
Intangible assets, net	390			422
Nuclear decommissioning trust fund	756			663
Derivative instruments	356			296
Deferred income taxes	0			6
Other non-current assets	151			133
Non-current assets - held-for-sale				0
Non-current assets - discontinued operations				405
Total other assets	2,883			2,730
Total Assets	7,770			7,806
Current Liabilities
Current portion of long-term debt	0			0
Current portion of operating lease liabilities	21
Accounts payable	1,315			1,368
Derivative instruments	683			713
Cash collateral received in support of energy risk management activities	30			33
Accrued expenses and other current liabilities	271			291
Current liabilities - held-for-sale				0
Current liabilities - discontinued operations				24
Total current liabilities	2,320			2,429
Other Liabilities
Long-term debt	245			244
Non-current operating lease liabilities	70
Nuclear decommissioning reserve	294			282
Nuclear decommissioning trust liability	453			371
Derivative instruments	374			306
Deferred income taxes	(10)			112
Other non-current liabilities	412			402
Non-current liabilities - held-for-sale				0
Non-current liabilities - discontinued operations				58
Total other liabilities	1,838			1,775
Total Liabilities	4,158			4,204
Redeemable noncontrolling interest in subsidiaries	0			0
Total Stockholders' Equity	3,612			3,602
Total Liabilities and Stockholders' Equity	7,770			7,806
Non-Guarantor Subsidiaries
Current Assets
Cash and cash equivalents	26			28
Funds deposited by counterparties	0			0
Restricted cash	1			10
Accounts receivable, net	194			115
Inventory	116			134
Derivative instruments	28			50
Cash collateral paid in support of energy risk management activities	18			12
Prepayments and other current assets	15			32
Current assets - held-for-sale				1
Current assets - discontinued operations				20
Total current assets	398			402
Property, plant and equipment, net	965			957
Other Assets
Investment in subsidiaries	0			0
Equity investments in affiliates	405			412
Operating lease right-of-use assets, net	271
Goodwill	232			214
Intangible assets, net	438			169
Nuclear decommissioning trust fund	0			0
Derivative instruments	14			4
Deferred income taxes	53			(143)
Other non-current assets	30			71
Non-current assets - held-for-sale				77
Non-current assets - discontinued operations				607
Total other assets	1,443			1,411
Total Assets	2,806			2,770
Current Liabilities
Current portion of long-term debt	87			55
Current portion of operating lease liabilities	31
Accounts payable	146			(185)
Derivative instruments	18			41
Cash collateral received in support of energy risk management activities	0			0
Accrued expenses and other current liabilities	51			36
Current liabilities - held-for-sale				5
Current liabilities - discontinued operations				48
Total current liabilities	333			0
Other Liabilities
Long-term debt	85			192
Non-current operating lease liabilities	307
Nuclear decommissioning reserve	0			0
Nuclear decommissioning trust liability	0			0
Derivative instruments	2			3
Deferred income taxes	67			61
Other non-current liabilities	160			320
Non-current liabilities - held-for-sale				65
Non-current liabilities - discontinued operations				577
Total other liabilities	621			1,218
Total Liabilities	954			1,218
Redeemable noncontrolling interest in subsidiaries	19			19
Total Stockholders' Equity	1,833			1,533
Total Liabilities and Stockholders' Equity	2,806			2,770
NRG Energy, Inc. (Note Issuer)
Current Assets
Cash and cash equivalents	217			480
Funds deposited by counterparties	0			0
Restricted cash	1			0
Accounts receivable, net	426			309
Inventory	0			0
Derivative instruments	0			16
Cash collateral paid in support of energy risk management activities	0			0
Prepayments and other current assets	78			90
Current assets - held-for-sale				0
Current assets - discontinued operations				0
Total current assets	722			895
Property, plant and equipment, net	156			153
Other Assets
Investment in subsidiaries	4,677			4,707
Equity investments in affiliates	0			0
Operating lease right-of-use assets, net	126
Goodwill	0			0
Intangible assets, net	0			0
Nuclear decommissioning trust fund	0			0
Derivative instruments	0			22
Deferred income taxes	0			183
Other non-current assets	71			97
Non-current assets - held-for-sale				0
Non-current assets - discontinued operations				0
Total other assets	4,874			5,009
Total Assets	5,752			6,057
Current Liabilities
Current portion of long-term debt	215			17
Current portion of operating lease liabilities	21
Accounts payable	698			434
Derivative instruments	0			0
Cash collateral received in support of energy risk management activities	0			0
Accrued expenses and other current liabilities	303			353
Current liabilities - held-for-sale				0
Current liabilities - discontinued operations				0
Total current liabilities	1,237			804
Other Liabilities
Long-term debt	5,468			6,025
Non-current operating lease liabilities	123
Nuclear decommissioning reserve	0			0
Nuclear decommissioning trust liability	0			0
Derivative instruments	0			0
Deferred income taxes	13			(108)
Other non-current liabilities	464			552
Non-current liabilities - held-for-sale				0
Non-current liabilities - discontinued operations				0
Total other liabilities	6,068			6,469
Total Liabilities	7,305			7,273
Redeemable noncontrolling interest in subsidiaries	0			0
Total Stockholders' Equity	(1,553)			(1,216)
Total Liabilities and Stockholders' Equity	$ 5,752			$ 6,057