Fair Value of Financial Instruments (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic) - Credit Risk (Details)	9 Months Ended
	Sep. 30, 2017 USD ($)
Concentration of credit risk
Counterparty credit exposure	$ 47,000,000
Collateral held (cash and letters of credit) against counterparty credit exposure	0
Counterparty credit exposure, net	$ 47,000,000
Counterparty credit exposure before collateral expected to roll off by the end of 2014 (as a percent)	94.00%
Net exposure (as a percent)	100.00%	[1],[2]
Counterparty credit risk exposure percentage (more than)	10.00%
Aggregate counterparty credit risk exposure for counterparties representing exposure above threshold percentage	$ 41,000,000
Investment grade
Concentration of credit risk
Net exposure (as a percent)	96.00%	[1],[2]
Non-Investment grade/Non-rated
Concentration of credit risk
Net exposure (as a percent)	4.00%	[1],[2]
Utilities, energy merchants, marketers and other
Concentration of credit risk
Net exposure (as a percent)	100.00%	[1],[2]
GenOn Americas Generation
Concentration of credit risk
Counterparty credit exposure	$ 47,000,000
Collateral held (cash and letters of credit) against counterparty credit exposure	0
Counterparty credit exposure, net	$ 47,000,000
Counterparty credit exposure before collateral expected to roll off by the end of 2014 (as a percent)	94.00%
Net exposure (as a percent)	100.00%	[1],[2]
Aggregate counterparty credit risk exposure for counterparties representing exposure above threshold percentage	$ 41,000,000
GenOn Americas Generation | Investment grade
Concentration of credit risk
Net exposure (as a percent)	96.00%	[1],[2]
GenOn Americas Generation | Non-Investment grade/Non-rated
Concentration of credit risk
Net exposure (as a percent)	4.00%	[1],[2]
GenOn Americas Generation | Utilities, energy merchants, marketers and other
Concentration of credit risk
Net exposure (as a percent)	100.00%	[1],[2]
GenOn Mid-Atlantic
Concentration of credit risk
Counterparty credit exposure	$ 0
Net exposure (as a percent)	0.00%	[1],[2]
Aggregate counterparty credit risk exposure for counterparties representing exposure above threshold percentage	$ 0
GenOn Mid-Atlantic | Investment grade
Concentration of credit risk
Net exposure (as a percent)	0.00%	[1],[2]
GenOn Mid-Atlantic | Non-Investment grade/Non-rated
Concentration of credit risk
Net exposure (as a percent)	0.00%	[1],[2]
GenOn Mid-Atlantic | Utilities, energy merchants, marketers and other
Concentration of credit risk
Net exposure (as a percent)	0.00%	[1],[2]

[1] Counterparty credit exposure excludes transportation contracts because of the unavailability of market prices.

[2] The figures in the tables above exclude potential counterparty credit exposure related to RTOs, ISOs, registered commodity exchanges and certain long term contracts.