Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Valuation Allowance [Line Items]
Change in valuation allowance					$ 86.6
Deferred federal, state and local tax benefit	$ 15.4	$ 91.3		$ 71.3	123.0
Valuation allowance	388.3			388.3		$ 336.1
Formation of the Blackstone Venture
Valuation Allowance [Line Items]
Change in valuation allowance			$ 85.0		85.0
Adoption Of Accounting Standards Update 2016-09 [Member]
Valuation Allowance [Line Items]
Change in valuation allowance					48.5
Federal and State [Member]
Valuation Allowance [Line Items]
Change in valuation allowance	$ 2.7	$ 59.6		$ 52.2	$ 86.6