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Leases - 2018 Operating Lease Commitments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2014
Other Operating Leases
Long-term Purchase Commitment [Line Items]
Lease expense	$ 66	$ 69
Midwest Generation, LLC
Long-term Purchase Commitment [Line Items]
Leased interest	100.00%
Out-of-market contracts, net			$ 159
Lease expense	$ 14

X

- Definition

Leased Interest

+ References

No definition available.

+ Details

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Namespace Prefix:	nrg_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_LongTermPurchaseCommitmentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

This element represents a liability associated with the acquisition of an off-market lease when the terms of the lease are unfavorable to the market terms for the lease at the date of acquisition.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.24)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

X

- Definition

Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/otherTransitionRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41499-112717

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
Period Type:	duration

X

- Details

Name:	us-gaap_LeaseArrangementTypeAxis=nrg_OtherLeasedPropertiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	dei_LegalEntityAxis=nrg_MidwestGenerationLLCMember
Namespace Prefix:
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Balance Type:
Period Type: