Accounting for Derivative Instruments and Hedging Activities (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic) - FV of Derivatives (Details) - Derivatives Not Designated as Cash Flow Hedges - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Derivative
Derivative assets	$ 56	$ 124
Derivative liabilities	41	122
GenOn Americas Generation
Derivative
Derivative assets	94	214
Derivative liabilities	94	217
GenOn Mid-Atlantic
Derivative
Derivative assets	12	48
Derivative liabilities	14	46
Commodity contracts current
Derivative
Derivative assets	39	108
Derivative liabilities	35	105
Commodity contracts current | GenOn Americas Generation
Derivative
Derivative assets	69	180
Derivative liabilities	71	185
Commodity contracts current | GenOn Mid-Atlantic
Derivative
Derivative assets	10	44
Derivative liabilities	14	44
Commodity contracts long-term
Derivative
Derivative assets	17	16
Derivative liabilities	6	17
Commodity contracts long-term | GenOn Americas Generation
Derivative
Derivative assets	25	34
Derivative liabilities	23	32
Commodity contracts long-term | GenOn Mid-Atlantic
Derivative
Derivative assets	2	4
Derivative liabilities	$ 0	$ 2