Fair Value of Financial Instruments (GenOn, GenOn Americas Generation and GenOn Mid-Atlantic) - CV and FV of Debt (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Carrying Amount | GenOn Americas Generation
Fair Value of Financial Instruments
Long-term debt, including current portion		$ 745
Fair Value | Level 2 | GenOn Americas Generation
Fair Value of Financial Instruments
Long-term debt, including current portion		570
Non-affiliated Entity | Carrying Amount
Fair Value of Financial Instruments
Long-term debt, including current portion		2,752
Non-affiliated Entity | Fair Value
Fair Value of Financial Instruments
Long-term debt, including current portion		1,946
Non-affiliated Entity | Fair Value | Level 2
Fair Value of Financial Instruments
Long-term debt, including current portion		1,850
Non-affiliated Entity | Fair Value | Level 3
Fair Value of Financial Instruments
Long-term debt, including current portion		$ 96
Affiliated Entity | Carrying Amount
Fair Value of Financial Instruments
Long-term debt, including current portion	$ 125
Affiliated Entity | Fair Value | Level 3
Fair Value of Financial Instruments
Long-term debt, including current portion	$ 125