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Income Taxes (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Valuation Allowance [Line Items]          
Change in valuation allowance $ 15.2 $ 3.0 $ 27.0 $ 17.2  
Deferred federal, state and local tax benefit 18.2 $ 3.6 31.7 $ 16.8  
Valuation allowance $ 424.9   424.9   $ 264.3
Formation of Blackstone Joint Venture [Member]          
Valuation Allowance [Line Items]          
Change in valuation allowance     85.0    
Adoption Of Accounting Standards Update 2016-09 [Member]          
Valuation Allowance [Line Items]          
Change in valuation allowance     $ 48.5