Accounting for Derivative Instruments and Hedging Activities (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic) - Offsetting Derivatives (Details) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Derivative liabilities
Cash Collateral (Held) / Posted	$ 72,000,000
Commodity contracts
Total derivative instruments
Gross Amounts of Recognized Assets / Liabilities	15,000,000	$ 2,000,000
Derivative Instruments	0	0
Cash Collateral (Held) / Posted	(4,000,000)	(13,000,000)
Net Amount	19,000,000	15,000,000
GenOn Americas Generation
Derivative liabilities
Cash Collateral (Held) / Posted	70,000,000
GenOn Americas Generation | Commodity contracts
Total derivative instruments
Gross Amounts of Recognized Assets / Liabilities	0	(3,000,000)
Derivative Instruments	0	0
Cash Collateral (Held) / Posted	(4,000,000)	(13,000,000)
Net Amount	4,000,000	10,000,000
GenOn Mid-Atlantic
Derivative assets
Cash Collateral (Held) / Posted	0
Derivative liabilities
Cash Collateral (Held) / Posted	0
GenOn Mid-Atlantic | Commodity contracts
Total derivative instruments
Gross Amounts of Recognized Assets / Liabilities	(2,000,000)	2,000,000
Derivative Instruments	0	0
Cash Collateral (Held) / Posted	0	0
Net Amount	(2,000,000)	2,000,000
Non-affiliated Entity
Derivative liabilities
Cash Collateral (Held) / Posted	47,000,000	53,000,000
Non-affiliated Entity | Commodity contracts
Derivative assets
Gross Amounts of Recognized Assets / Liabilities	45,000,000	70,000,000
Derivative Instruments	(15,000,000)	(39,000,000)
Cash Collateral (Held) / Posted	0	0
Net Amount	30,000,000	31,000,000
Derivative liabilities
Gross Amounts of Recognized Assets / Liabilities	(28,000,000)	(56,000,000)
Derivative Instruments	15,000,000	39,000,000
Cash Collateral (Held) / Posted	2,000,000	1,000,000
Net Amount	(11,000,000)	(16,000,000)
Non-affiliated Entity | GenOn Americas Generation
Derivative liabilities
Cash Collateral (Held) / Posted	45,000,000	51,000,000
Non-affiliated Entity | GenOn Americas Generation | Commodity contracts
Derivative assets
Gross Amounts of Recognized Assets / Liabilities	45,000,000	70,000,000
Derivative Instruments	(15,000,000)	(39,000,000)
Cash Collateral (Held) / Posted	0	0
Net Amount	30,000,000	31,000,000
Derivative liabilities
Gross Amounts of Recognized Assets / Liabilities	(28,000,000)	(56,000,000)
Derivative Instruments	15,000,000	39,000,000
Cash Collateral (Held) / Posted	2,000,000	1,000,000
Net Amount	(11,000,000)	(16,000,000)
Affiliated Entity
Derivative liabilities
Cash Collateral (Held) / Posted	25,000,000	79,000,000
Affiliated Entity | Commodity contracts
Derivative assets
Gross Amounts of Recognized Assets / Liabilities	11,000,000	54,000,000
Derivative Instruments	(11,000,000)	(54,000,000)
Cash Collateral (Held) / Posted	0	0
Net Amount	0	0
Derivative liabilities
Gross Amounts of Recognized Assets / Liabilities	(13,000,000)	(66,000,000)
Derivative Instruments	11,000,000	54,000,000
Cash Collateral (Held) / Posted	2,000,000	12,000,000
Net Amount	0	0
Affiliated Entity | GenOn Americas Generation
Derivative liabilities
Cash Collateral (Held) / Posted	25,000,000	79,000,000
Affiliated Entity | GenOn Americas Generation | Commodity contracts
Derivative assets
Gross Amounts of Recognized Assets / Liabilities	49,000,000	144,000,000
Derivative Instruments	(49,000,000)	(144,000,000)
Cash Collateral (Held) / Posted	0	0
Net Amount	0	0
Derivative liabilities
Gross Amounts of Recognized Assets / Liabilities	(66,000,000)	(161,000,000)
Derivative Instruments	49,000,000	144,000,000
Cash Collateral (Held) / Posted	2,000,000	12,000,000
Net Amount	(15,000,000)	(5,000,000)
Affiliated Entity | GenOn Mid-Atlantic | Commodity contracts
Derivative assets
Gross Amounts of Recognized Assets / Liabilities	12,000,000	48,000,000
Derivative Instruments	(12,000,000)	(46,000,000)
Cash Collateral (Held) / Posted	0	0
Net Amount	0	2,000,000
Derivative liabilities
Gross Amounts of Recognized Assets / Liabilities	(14,000,000)	(46,000,000)
Derivative Instruments	12,000,000	46,000,000
Cash Collateral (Held) / Posted	0	0
Net Amount	$ (2,000,000)	$ 0