Accounting for Derivative Instruments and Hedging Activities - Pre-tax effects of economic hedges not designated a cash flow hedges (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Unrealized mark-to-market results
Reversal of previously recognized unrealized losses on settled positions related to economic hedges	$ 30	$ 11	$ 39	$ 30
Reversal of acquired loss/(gain) positions related to economic hedges	3	1	4	(1)
Net unrealized gains on open positions related to economic hedges	54	9	88	12
Total unrealized mark-to-market gains for economic hedging activities	87	21	131	41
Reversal of previously recognized unrealized (gains) on settled positions related to trading activity	(5)	(1)	(7)	(7)
Net unrealized gains on open positions related to trading activity	4	13	17	26
Total unrealized mark-to-market (losses)/gains for trading activity	(1)	12	10	19
Total unrealized gains	86	33	141	60
Credit Risk Related Contingent Features
Unrealized gains from open economic hedge positions			88	12
Collateral due on net liability position that has not been called by a certain marginable agreement counterparty	8		8
Adequate Assurance Clauses
Credit Risk Related Contingent Features
Derivative net liability position, collateral required for contracts with credit rating contingent feature	33		33
Commodity contracts
Unrealized mark-to-market results
Total unrealized gains	86	33	141	60
Commodity contracts \| Operating revenues
Unrealized mark-to-market results
Total unrealized gains	42	253	49	280
Commodity contracts \| Cost of operations
Unrealized mark-to-market results
Total unrealized gains	44	(220)	92	(220)
Interest rate contracts
Unrealized mark-to-market results
Total unrealized gains	$ 0	$ (29)	$ 0	$ (38)

-- None. No documentation exists for this element. --

No definition available.

Name:	nrg_CreditRiskRelatedContingentFeaturesAbstract
Namespace Prefix:	nrg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

This element represents the amount recorded in the entity's statement of operations for net unrealized gains (losses) on open positions related to trading activities.

+ References

No definition available.

+ Details

Name:	nrg_DerivativeUnrealizedGainsOnOpenPositionsRelatedToTradingActivityNet
Namespace Prefix:	nrg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount reversed into entity's statement of operations for previously recognized unrealized (gains) losses on settled positions related to acquired economic hedges.

+ References

No definition available.

+ Details

Name:	nrg_ReversalPreviouslyRecognizedUnrealizedGainsLossesAcquiredasPartofAcquisitionEconomicHedges
Namespace Prefix:	nrg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount reversed into entity's statement of operations for previously recognized unrealized (gains) losses on settled positions related to economic hedges.

+ References

No definition available.

+ Details

Name:	nrg_ReversalPreviouslyRecognizedUnrealizedGainsLossesSettledPositionsEconomicHedges
Namespace Prefix:	nrg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount reversed into entity's statement of operations for previously recognized unrealized (gains) losses on settled positions related to trading activities.

+ References

No definition available.

+ Details

Name:	nrg_ReversalPreviouslyUnrecognizedUnrealizedGainsLossesSettledPositionsTradingActivity
Namespace Prefix:	nrg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Unrealized Gain (Loss) on Derivatives, Mark To Market, Economic Hedging Activities

+ References

No definition available.

+ Details

Name:	nrg_UnrealizedGainLossOnDerivativesMarkToMarketEconomicHedgingActivities
Namespace Prefix:	nrg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Unrealized Gain (Loss) on Derivatives, Mark To Market, Trading Activities

+ References

No definition available.

+ Details

Name:	nrg_UnrealizedGainLossonDerivativesMarkToMarketTradingActivities
Namespace Prefix:	nrg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

-- None. No documentation exists for this element. --

+ References

No definition available.

+ Details

Name:	nrg_UnrealizedMarkToMarketResultsAbstract
Namespace Prefix:	nrg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624258-113959

+ Details

Name:	us-gaap_CollateralAlreadyPostedAggregateFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4CC
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL109998890-113959

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624171-113959

+ Details

Name:	us-gaap_DerivativeInstrumentsNotDesignatedAsHedgingInstrumentsGainLossNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624258-113959

+ Details

Name:	us-gaap_DerivativeNetLiabilityPositionAggregateFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of unrealized gain (loss) related to derivatives designated as cash flow hedging instruments, recorded in results of operations to the extent that the cash flow hedge is determined to be ineffective.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=121590274&loc=SL5624171-113959

+ Details

Name:	us-gaap_UnrealizedGainLossOnCashFlowHedgingInstruments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3602-108585

+ Details

Name:	us-gaap_UnrealizedGainLossOnDerivatives
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_CreditDerivativesByContractTypeAxis=nrg_AdequateAssuranceClausesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_CommodityContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_SalesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_CostOfSalesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: