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Stock-Based Compensation - Supplemental Information (Details) - USD ($) $ in Millions	5 Months Ended	12 Months Ended
	Jun. 13, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock-Based Compensation
Minimum tax withholdings		$ 5.0	$ 5.0	$ 21.0
Compensation Expense		44.0	23.0	40.0
Tax detriment recognized		(5.0)	(4.0)	(12.0)
Unrecognized Total Cost		37.0
GenOn
Stock-Based Compensation
Compensation Expense	$ 1.0	1.0	1.0	1.0
NQSOs
Stock-Based Compensation
Compensation Expense		0.0	0.0	0.0
Unrecognized Total Cost		$ 0.0
Weighted Average Recognition Period Remaining (In years)		0 years
Award vesting period		3 years
RSUs
Stock-Based Compensation
Compensation Expense		$ 17.0	13.0	22.0
Unrecognized Total Cost		$ 13.0
Weighted Average Recognition Period Remaining (In years)		1 year 4 months 13 days
Award vesting period		3 years
DSUs
Stock-Based Compensation
Compensation Expense		$ 2.0	2.0	2.0
Unrecognized Total Cost		$ 0.0
Weighted Average Recognition Period Remaining (In years)		0 years
MSUs
Stock-Based Compensation
Compensation Expense		$ 6.0	3.0	16.0
Unrecognized Total Cost		$ 4.0
Weighted Average Recognition Period Remaining (In years)		9 months 26 days
RPSUs
Stock-Based Compensation
Compensation Expense		$ 4.0	0.0	0.0
Unrecognized Total Cost		$ 6.0
Weighted Average Recognition Period Remaining (In years)		1 year 11 months 27 days
PRSUs
Stock-Based Compensation
Compensation Expense		$ 15.0	$ 5.0	$ 0.0
Unrecognized Total Cost		$ 14.0
Weighted Average Recognition Period Remaining (In years)		1 year 6 months 4 days
Phantom Share Units (PSUs)
Stock-Based Compensation
Award vesting period		3 years

X

- Definition

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Unrecognized cost of unvested share-based compensation awards.

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- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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- Definition

The total recognized tax benefit related to compensation cost for equity-based payment arrangements recognized in income during the period.

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Amount of cash outflow to satisfy an employee's income tax withholding obligation as part of a net-share settlement of a share-based award.

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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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