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Accounting for Derivative Instruments and Hedging Activities - Effect of ASC 815 on Accumulated OCI Balance Attributable to Hedge Derivatives, Net (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effect of ASC 815 on NRG's Accumulated OCI Balance Attributable to Cash Flow Hedge Derivatives
Accumulated OCI beginning balance	$ (66)	$ (101)	$ (68)
Reclassified from accumulated OCI to income:
Due to realization of previously deferred amounts	12	21	15
Mark-to-market of cash flow hedge accounting contracts	0	14	(48)
Accumulated OCI ending balance, net of tax of $8, $16 and $16 in 2017, 2016 and 2015 respectively	(54)	(66)	(101)
Accumulated OCI ending balance, tax	8	16	16
Losses expected to be realized from other comprehensive loss during the next 12 months, net of $2 tax	(12)
Losses expected to be realized from other comprehensive loss during the next 12 months, tax	2
Interest Rate
Effect of ASC 815 on NRG's Accumulated OCI Balance Attributable to Cash Flow Hedge Derivatives
Accumulated OCI beginning balance	(66)	(101)	(67)
Reclassified from accumulated OCI to income:
Due to realization of previously deferred amounts	12	21	14
Mark-to-market of cash flow hedge accounting contracts	0	14	(48)
Accumulated OCI ending balance, net of tax of $8, $16 and $16 in 2017, 2016 and 2015 respectively	(54)	(66)	(101)
Losses expected to be realized from other comprehensive loss during the next 12 months, net of $2 tax	$ (12)
Energy Commodities
Effect of ASC 815 on NRG's Accumulated OCI Balance Attributable to Cash Flow Hedge Derivatives
Accumulated OCI beginning balance		$ 0	(1)
Reclassified from accumulated OCI to income:
Due to realization of previously deferred amounts			1
Mark-to-market of cash flow hedge accounting contracts			0
Accumulated OCI ending balance, net of tax of $8, $16 and $16 in 2017, 2016 and 2015 respectively			$ 0

X

- Definition

Accumulated Other Comprehensive Income (Loss), Cumulative Change in Gain (Loss) from Cash Flow Hedges, Tax Amount

+ References

No definition available.

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- Definition

The estimated tax impact of existing gains or losses on cash flow hedges, at the reporting date, which are expected to be reclassified to earnings within the next 12 months.

+ References

No definition available.

+ Details

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- Definition

Accumulated change, net of tax, in accumulated gains and losses from derivative instruments designated and qualifying as the effective portion of cash flow hedges. Includes an entity's share of an equity investee's Increase or Decrease in deferred hedging gains or losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 10A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669646-108580

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e637-108580

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14A
-URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 14
-URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e681-108580

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=35736893&loc=d3e80784-113994

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- Definition

The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=35736893&loc=d3e80748-113994

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- Definition

The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4D
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624177-113959

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- Definition

The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959

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