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Stock-Based Compensation - Non-Qualified Stock Options (Details) - NQSOs - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock-Based Compensation
Maximum contractual term for outstanding options (in years)	10 years
Shares granted during the period	0	0	0
NQSO activity and changes
Outstanding at the beginning of the period (in shares)	1,285,858
Forfeited (in shares)	(36,866)
Exercised (in shares)	(969,058)		0
Outstanding at the end of the period (in shares)	279,934.000000	1,285,858
Exercisable at the end of the period (in shares)	279,934
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Outstanding (in usd per share)	$ 25.49
Forfeited (in usd per share)	43.64
Exercised (in usd per share)	24.93
Outstanding (in usd per share)	25.04	$ 25.49
Exercisable (in usd per share)	$ 25.04
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures
Weighted Average Remaining Contractual Term, Outstanding	2 years	3 years
Weighted Average Remaining Contractual Term, Exercisable	2 years
Aggregate Intrinsic Value, beginning of period	$ 6
Aggregate Intrinsic Value, end of period	4	$ 6
Aggregate Intrinsic Value, Exercisable	$ 4
GenOn Employees
NQSO activity and changes
Outstanding at the end of the period (in shares)	26,430
Exercisable at the end of the period (in shares)	26,430,000,000

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Share-based Compensation Arrangement by Share-based Payment Award, Maximum Contractual Term

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Gross number of share options (or share units) granted during the period.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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