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Related Party Transactions - Service Agreement and Transition Service Agreement with GeOn (Details) - USD ($) $ in Millions			1 Months Ended	7 Months Ended	12 Months Ended
	Jun. 14, 2017	Jun. 12, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Other income - affiliate			$ 84.0		$ 87.0	$ 193.0	$ 193.0
Services Agreement | GenOn
Related Party Transaction [Line Items]
Service fees					193.0
Reserve against affiliate receivable			12.0	$ 12.0	12.0
Services Agreement | Restructuring Support Agreement
Related Party Transaction [Line Items]
Shared services annualized rate		$ 84.0	84.0	84.0	84.0
Monthly cost of shared services	$ 5.0		7.0	$ 5.0
Credit applied		$ 28.0	3.5		28.0
Transition Services Agreement | Restructuring Support Agreement
Related Party Transaction [Line Items]
Credit applied			$ 28.0
Transition Services Agreement | Restructuring Support Agreement | GenOn
Related Party Transaction [Line Items]
Other income - affiliate					87.0
Shared service, selling, general and administrative					$ 42.0

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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