Accounting for Derivative Instruments and Hedging Activities (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic) - Underlying Derivative (Details) T in Millions, MWh in Millions, MMBTU in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017 MWh MMBTU T	Dec. 31, 2016 MWh MMBTU T
Long | Coal | Short Ton
Derivative
Total Volume, mass | T	2	5
Long | Coal | Short Ton | GenOn Americas Generation
Derivative
Total Volume, mass | T	1	4
Long | Coal | Short Ton | GenOn Mid-Atlantic
Derivative
Total Volume, mass | T	1	4
Long | Natural Gas | MMBtu
Derivative
Net notional volume, buy (sell) | MMBTU	72	138
Long | Natural Gas | MMBtu | GenOn Americas Generation
Derivative
Net notional volume, buy (sell) | MMBTU	13	30
Long | Natural Gas | MMBtu | GenOn Mid-Atlantic
Derivative
Net notional volume, buy (sell) | MMBTU	13	23
Short | Power | MWh
Derivative
Net notional volume, buy (sell) | MWh	12	35
Short | Power | MWh | GenOn Americas Generation
Derivative
Net notional volume, buy (sell) | MWh	3	12
Short | Power | MWh | GenOn Mid-Atlantic
Derivative
Net notional volume, buy (sell) | MWh	3	11