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Stock-Based Compensation - Non-Qualified Stock Options (Details) - NQSOs - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock-Based Compensation
Award vesting period	3 years
Shares granted during the period	0	0	0
Maximum contractual term for outstanding options (in years)	10 years
NQSO activity and changes
Outstanding at the beginning of the period (in shares)	1,522,919
Forfeited (in shares)	(50,001)
Exercised (in shares)	(187,060)	0
Outstanding at the end of the period (in shares)	1,285,858.000000	1,522,919
Exercisable at the end of the period (in shares)	1,285,858
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Outstanding (in usd per share)	$ 25.03
Forfeited (in usd per share)	29.35
Exercised (in usd per share)	20.71
Outstanding (in usd per share)	25.49	$ 25.03
Exercisable (in usd per share)	$ 25.49
Weighted Average Remaining Contractual Term, Outstanding	3 years	3 years
Weighted Average Remaining Contractual Term, Exercisable	3 years
Aggregate Intrinsic Value, Outstanding	$ 6	$ 0
Aggregate Intrinsic Value, Exercisable	6
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures
Total intrinsic value of options exercised	1	0	$ 2
Cash received from options exercised	$ 4	$ 0	$ 9
GenOn Employees
NQSO activity and changes
Exercised (in shares)	(51,207,000,000)
Outstanding at the end of the period (in shares)	51,207
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]
Shares vested (in shares)	51,207,000,000

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Amount of cash inflow from exercise of stock options granted under share-based compensation arrangement.

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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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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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