Income Taxes (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income tax provision
Income before income taxes	$ 72,000,000	$ 284,000,000	$ 49,000,000	$ 268,000,000
Income tax expense	$ 0	$ 21,000,000	$ 7,000,000	$ 20,000,000
Effective tax rate	0.00%	7.40%	14.30%	7.50%
Statutory tax rate (as a percent)	35.00%	35.00%	35.00%	35.00%
GenOn Americas Generation
Income tax provision
Income before income taxes	$ 32,000,000	$ 132,000,000	$ 48,000,000	$ 149,000,000
Income tax expense	0	0	0	0
GenOn Americas Generation | Pro Forma
Income tax provision
Income tax expense	0	0	0	0
GenOn Mid-Atlantic
Income tax provision
Income before income taxes	(6,000,000)	51,000,000	(33,000,000)	57,000,000
Income tax expense	0	0	0	0
GenOn Mid-Atlantic | Pro Forma
Income tax provision
Income tax expense	$ 0	$ 23,000,000	$ 0	$ 25,000,000