Chapter 11 Cases (GenOn and GenOn Americas Generation) - Narrative (Details)		1 Months Ended	4 Months Ended	9 Months Ended
	Jun. 13, 2017 USD ($)	Jun. 15, 2017 USD ($)	Sep. 30, 2017 USD ($) extension	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	May 08, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 13, 2016
Debt Instrument [Line Items]
Pension liability			$ 131,000,000	$ 131,000,000
Total amount of debt			2,616,000,000	2,616,000,000
Fees incurred in connection with 2022 Notes offering				18,000,000	$ 0
Financing costs			46,000,000	94,000,000	0
Cash payments for reorganization items				32,000,000
GenOn Energy
Debt Instrument [Line Items]
Total amount of debt			1,921,000,000	1,921,000,000			$ 0
GenOn Americas Generation
Debt Instrument [Line Items]
Pension liability			0	0
Total amount of debt			695,000,000	695,000,000			$ 0
Financing costs				16,000,000	$ 0
Intercompany Revolver, GenOn Senior Notes, and GenOn Americas Generation Senior Notes [Member]
Debt Instrument [Line Items]
Total amount of debt			2,600,000,000	2,600,000,000
Pension plan
Debt Instrument [Line Items]
Pension liability			106,000,000	$ 106,000,000
Senior Notes
Debt Instrument [Line Items]
Interest expense not recorded due to bankruptcy			49,000,000
Senior Notes | GenOn Americas Generation
Debt Instrument [Line Items]
Interest expense not recorded due to bankruptcy			18,000,000
Senior Notes | Senior Secured First Lien Notes, due 2022
Debt Instrument [Line Items]
Percent of aggregate principal amount of 2022 Notes (as percent)		4.00%
Interest rate (as percent)						10.50%
Face amount						$ 550,000,000
Percent of aggregate principal amount of 2022 Notes plus accrued interest		$ 28,000,000
Fees incurred in connection with 2022 Notes offering			$ 18,000,000
Senior Notes | GenOn Senior Notes, due 2017 | GenOn Energy
Debt Instrument [Line Items]
Interest rate (as percent)			7.875%	7.875%				7.875%
Senior Notes | GenOn Senior Notes, due 2018 | GenOn Energy
Debt Instrument [Line Items]
Interest rate (as percent)			9.50%	9.50%				9.50%
Senior Notes | GenOn Senior Notes, due 2020 | GenOn Energy
Debt Instrument [Line Items]
Interest rate (as percent)			9.875%	9.875%				9.875%
Senior Notes | GenOn Americas Generation Senior Notes, due 2021 | GenOn Americas Generation
Debt Instrument [Line Items]
Interest rate (as percent)			8.50%	8.50%				8.50%
Senior Notes | GenOn Americas Generation Senior Notes, due 2031 | GenOn Americas Generation
Debt Instrument [Line Items]
Interest rate (as percent)			9.125%	9.125%				9.125%
Senior Notes | New Secured Notes
Debt Instrument [Line Items]
Amount of aggregate principal in debt financing sought			$ 900,000,000	$ 900,000,000
Backstop fee	$ 45,000,000
Backstop commitment fee (as percent)	5.00%
Term				5 years
Principal Owner | Letter of Credit | Intercompany Credit Agreement
Debt Instrument [Line Items]
Amount of borrowings			125,000,000	$ 125,000,000
NRG
Debt Instrument [Line Items]
Settlement cash consideration			261,300,000
Cancellation of equity interests, cash payment			75,000,000
2017 pension contributions				13,000,000
Transition services, annualized rate			$ 84,000,000
Transition services, period of services provided at no charge if settlement is approved			2 months
Transition services, number of optional extensions for services if settlement is approved | extension			2
Transition services, Period of optional extensions for services if settlement is approved			1 month
NRG | Senior Secured First Lien Notes, due 2022
Debt Instrument [Line Items]
Credit against amount owed under shared services agreement upon emergence from bankruptcy			$ 28,000,000	28,000,000
NRG | Senior Notes | Senior Secured First Lien Notes, due 2022
Debt Instrument [Line Items]
Percent of aggregate principal amount of 2022 Notes (as percent)			4.00%
NRG | Principal Owner | Restructuring Support Agreement, Credit Against Amounts Owed Prior to Petition Date Under Services Agreement
Debt Instrument [Line Items]
Receivable from related party			$ 28,000,000	$ 28,000,000