Income Taxes - Unrecognized Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Beginning Balance	$ 17.2	$ 12.3
Additions for Tax Positions Taken in the Current Year	9.9	0.0
Additions for Tax Positions Taken in Prior Years	6.2	6.6
Reductions for Tax Positions Taken in Prior Years	(5.4)	(1.2)
Reductions Resulting from Expiration of Statutes	(0.2)	(0.5)
Ending Balance	$ 27.7	$ 17.2