Stock-Based Compensation - Restricted Stock Units (RSUs (Details) - RSUs - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock-Based Compensation
Award vesting period	3 years
Units
Non-vested at beginning of period (in shares)	2,377,813
Granted (in shares)	447,309
Forfeited (in shares)	(315,569)
Vested (in shares)	(1,051,471)
Non-vested at end of period (in shares)	1,458,082.000000	2,377,813
Weighted Average Grant Date Fair Value per Unit
Non-vested at beginning of period (in usd per share)	$ 14.63
Granted (in usd per share)	28.90	$ 12.44	$ 11.54
Forfeited (in usd per share)	18.93
Vested (in usd per share)	17.67
Non-vested at end of period (in usd per share)	$ 16.16	$ 14.63
Vested in period, fair value	$ 42	$ 19	$ 11
GenOn Employees
Units
Non-vested at end of period (in shares)	7,319

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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