Summary of Significant Accounting Policies (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic) - Narrative (Details) (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in cash flows provided by operating activities	$ 53	$ (190)
GenOn Americas Generation, LLC
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in cash flows provided by operating activities	$ (10)	(153)
ASU 2016-18
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in cash flows provided by operating activities		41
ASU 2016-18 \| GenOn Americas Generation, LLC
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Decrease in cash flows provided by operating activities		$ 41

Amount of cash inflow (outflow) from operating activities, including discontinued operations. Operating activity cash flows include transactions, adjustments, and changes in value not defined as investing or financing activities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 24
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3521-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3536-108585

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Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration