Accounting for Derivative Instruments and Hedging Activities (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic) - Mark to Market (Details) - USD ($)	1 Months Ended	3 Months Ended				9 Months Ended
	Jul. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Unrealized mark-to-market results
Reversal of previously recognized unrealized losses/(gains) on settled positions related to economic hedges		$ 9,000,000		$ (29,000,000)		$ 7,000,000	$ (145,000,000)
Net unrealized (losses)/gains on open positions related to economic hedges		(28,000,000)		(68,000,000)		9,000,000	(43,000,000)
Total impact to statements of operations		(19,000,000)		(97,000,000)		16,000,000	(188,000,000)
Impact of derivative instruments to statement of operations
Total impact to statements of operations		(19,000,000)		(97,000,000)		16,000,000	(188,000,000)
Exchange clearing account, initial collateral posted	$ 20,000,000
Credit Risk Related Contingent Features
Collateral required for contracts with adequate assurance clauses in net liability positions		1,000,000				1,000,000
Collateral required for contracts with credit rating contingent features in net liability position		0				0
Collateral due on net liability position that has not been called by a certain marginable agreement counterparty		0				0
GenOn Americas Generation
Unrealized mark-to-market results
Reversal of previously recognized unrealized losses/(gains) on settled positions related to economic hedges		11,000,000		(46,000,000)		2,000,000	(165,000,000)
Net unrealized (losses)/gains on open positions related to economic hedges		(13,000,000)		6,000,000		6,000,000	(23,000,000)
Total impact to statements of operations		(2,000,000)		(40,000,000)		8,000,000	(188,000,000)
Impact of derivative instruments to statement of operations
Total impact to statements of operations		(2,000,000)		(40,000,000)		8,000,000	(188,000,000)
Credit Risk Related Contingent Features
Collateral required for contracts with adequate assurance clauses in net liability positions		1,000,000				1,000,000
Collateral required for contracts with credit rating contingent features in net liability position		0				0
Collateral due on net liability position that has not been called by a certain marginable agreement counterparty		0				0
GenOn Mid-Atlantic
Unrealized mark-to-market results
Reversal of previously recognized unrealized losses/(gains) on settled positions related to economic hedges		7,000,000		(47,000,000)		(3,000,000)	(146,000,000)
Net unrealized (losses)/gains on open positions related to economic hedges		(12,000,000)		4,000,000		1,000,000	(18,000,000)
Total impact to statements of operations		(5,000,000)		(43,000,000)		(2,000,000)	(164,000,000)
Impact of derivative instruments to statement of operations
Total impact to statements of operations		(5,000,000)		(43,000,000)		(2,000,000)	(164,000,000)
Revenue from operations — energy commodities
Unrealized mark-to-market results
Total impact to statements of operations		(23,000,000)		(142,000,000)		7,000,000	(233,000,000)
Impact of derivative instruments to statement of operations
Total impact to statements of operations		(23,000,000)		(142,000,000)		7,000,000	(233,000,000)
Revenue from operations — energy commodities | GenOn Americas Generation
Unrealized mark-to-market results
Total impact to statements of operations		(6,000,000)		(75,000,000)		(1,000,000)	(228,000,000)
Impact of derivative instruments to statement of operations
Total impact to statements of operations		(6,000,000)		(75,000,000)		(1,000,000)	(228,000,000)
Revenue from operations — energy commodities | GenOn Mid-Atlantic
Unrealized mark-to-market results
Total impact to statements of operations		(10,000,000)		(83,000,000)		(2,000,000)	(203,000,000)
Impact of derivative instruments to statement of operations
Total impact to statements of operations		(10,000,000)		(83,000,000)		(2,000,000)	(203,000,000)
Cost of operations
Unrealized mark-to-market results
Total impact to statements of operations		4,000,000		45,000,000		9,000,000	45,000,000
Impact of derivative instruments to statement of operations
Total impact to statements of operations		4,000,000		45,000,000		9,000,000	45,000,000
Cost of operations | GenOn Americas Generation
Unrealized mark-to-market results
Total impact to statements of operations		4,000,000		35,000,000		9,000,000	40,000,000
Impact of derivative instruments to statement of operations
Total impact to statements of operations		4,000,000		35,000,000		9,000,000	40,000,000
Cost of operations | GenOn Mid-Atlantic
Unrealized mark-to-market results
Total impact to statements of operations		5,000,000		40,000,000		0	39,000,000
Impact of derivative instruments to statement of operations
Total impact to statements of operations		$ 5,000,000		40,000,000		$ 0	$ 39,000,000
Realized Gain
Impact of derivative instruments to statement of operations
Amount realized			$ 5,000,000	98,000,000	$ 38,000,000
Realized Gain | GenOn Americas Generation
Impact of derivative instruments to statement of operations
Amount realized			5,000,000	50,000,000	35,000,000
Realized Gain | GenOn Mid-Atlantic
Impact of derivative instruments to statement of operations
Amount realized			5,000,000	50,000,000	35,000,000
Realized Gain | 2016
Impact of derivative instruments to statement of operations
Amount realized				82,000,000	18,000,000
Realized Gain | 2016 | GenOn Americas Generation
Impact of derivative instruments to statement of operations
Amount realized					16,000,000
Realized Gain | 2016 | GenOn Mid-Atlantic
Impact of derivative instruments to statement of operations
Amount realized					16,000,000
Realized Gain | 2017
Impact of derivative instruments to statement of operations
Amount realized			4,000,000	13,000,000	19,000,000
Realized Gain | 2017 | GenOn Americas Generation
Impact of derivative instruments to statement of operations
Amount realized			4,000,000	46,000,000	19,000,000
Realized Gain | 2017 | GenOn Mid-Atlantic
Impact of derivative instruments to statement of operations
Amount realized			4,000,000	46,000,000	19,000,000
Realized Gain | 2018
Impact of derivative instruments to statement of operations
Amount realized			1,000,000	3,000,000	$ 1,000,000
Realized Gain | 2018 | GenOn Americas Generation
Impact of derivative instruments to statement of operations
Amount realized			1,000,000	4,000,000
Realized Gain | 2018 | GenOn Mid-Atlantic
Impact of derivative instruments to statement of operations
Amount realized			$ 1,000,000	$ 4,000,000