Debt and Capital Leases (GenOn, GenOn Americas Generation, and GenOn Mid-Atlantic) (Details) - USD ($)						9 Months Ended
		Oct. 27, 2017	Jul. 14, 2017	Jun. 28, 2017	May 05, 2017	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2017	Feb. 28, 2017	Jan. 27, 2017	Dec. 31, 2016	Dec. 13, 2016
Debt Instrument [Line Items]
Debt and capital leases						$ 2,000,000					$ 2,754,000,000
Less: Liabilities subject to compromise						(2,616,000,000)
Less: current maturities						1,000,000					704,000,000
Total long-term debt and capital leases						1,000,000					2,050,000,000
Line of credit payment						130,000,000	$ 0
Prepaid rent — non-current						352,000,000					245,000,000
Line of Credit | Letter of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity			$ 300,000,000
Percent of letters of credit provided required to be cash collateralized (as percent)			101.00%
Letters of credit						6,000,000
Intercompany Credit Agreement | NRG Energy
Debt Instrument [Line Items]
Maximum borrowing capacity						500,000,000
Intercompany Credit Agreement | Letter of Credit | NRG Energy
Debt Instrument [Line Items]
Letters of credit						103,000,000					272,000,000
GenOn Americas Generation
Debt Instrument [Line Items]
Debt and capital leases						0					745,000,000
Premiums	[1]					0					50,000,000
Less: Liabilities subject to compromise						(695,000,000)					0
Total long-term debt and capital leases						0					745,000,000
Line of credit payment						130,000,000	0
Payment for intercompany revolver						125,000,000	0
Prepaid rent — non-current						278,000,000					204,000,000
GenOn Americas Generation | Senior Notes | GenOn Americas Generation Senior Notes, due 2021
Debt Instrument [Line Items]
Debt and capital leases						$ 366,000,000					366,000,000
Interest rate (as percent)						8.50%						8.50%
GenOn Americas Generation | Senior Notes | GenOn Americas Generation Senior Notes, due 2031
Debt Instrument [Line Items]
Debt and capital leases						$ 329,000,000					329,000,000
Interest rate (as percent)						9.125%						9.125%
GenOn Americas Generation | Intercompany Credit Agreement | Letter of Credit | NRG Energy
Debt Instrument [Line Items]
Letters of credit						$ 21,000,000					199,000,000
GenOn Energy
Debt Instrument [Line Items]
Debt and capital leases						2,000,000					2,009,000,000
Premiums	[1]					0					81,000,000
Less: Liabilities subject to compromise						(1,921,000,000)					0
GenOn Energy | Senior Notes | GenOn Senior Notes, due 2017
Debt Instrument [Line Items]
Debt and capital leases						$ 691,000,000					691,000,000
Interest rate (as percent)						7.875%						7.875%
GenOn Energy | Senior Notes | GenOn Senior Notes, due 2018
Debt Instrument [Line Items]
Debt and capital leases						$ 649,000,000					649,000,000
Interest rate (as percent)						9.50%						9.50%
GenOn Energy | Senior Notes | GenOn Senior Notes, due 2020
Debt Instrument [Line Items]
Debt and capital leases						$ 490,000,000					490,000,000
Interest rate (as percent)						9.875%						9.875%
GenOn Energy | Other liabilities
Debt Instrument [Line Items]
Debt and capital leases	[2]					$ 91,000,000					96,000,000
GenOn Energy | GenOn capital lease
Debt Instrument [Line Items]
Debt and capital leases						2,000,000					2,000,000
GenOn Mid-Atlantic
Debt Instrument [Line Items]
Line of credit payment						130,000,000	0
Fees										$ 1,000,000
Payment for intercompany revolver						125,000,000	$ 0
Prepaid rent — non-current						280,000,000					204,000,000
GenOn Mid-Atlantic | Intercompany Credit Agreement | Subsequent event
Debt Instrument [Line Items]
Written notice period for termination of letters of credit		60 days
Period prior to termination date of letters of credit to provide alternative qualifying credit support		30 days
GenOn Mid-Atlantic | Intercompany Credit Agreement | Letter of Credit
Debt Instrument [Line Items]
Payment for intercompany revolver				$ 125,000,000
Payment to owner lessors				$ 2,700,000
Prepaid rent — non-current								$ 125,000,000
GenOn Mid-Atlantic | Intercompany Credit Agreement | Letter of Credit | NRG Energy
Debt Instrument [Line Items]
Letters of credit						$ 1,000,000					$ 128,000,000
Amount of borrowings									$ 125,000,000
GenOn Mid-Atlantic | Intercompany Credit Agreement | Letter of Credit | GenOn Energy
Debt Instrument [Line Items]
Payment for intercompany revolver					$ 125,000,000

[1] Premiums were written-off at Petition Date in accordance with ASC 852, Reorganizations.

[2] Debt financing liabilities associated with the Long Term Service Agreements for the Choctaw and Hunterstown facilities.