Income Taxes - Unrecognized Tax Positions - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Contingency [Line Items]
Unrecognized tax benefits	$ 372.4	$ 367.9	$ 162.9
Unrecognized tax benefits that would impact effective tax rate	365.0	360.0
Unrecognized tax benefits, income tax penalties and interest expense	8.3	3.4
Increase in unrecognized tax benefits is reasonably possible	$ 51.5
Business Acquisition, Acquiree [Member]
Income Tax Contingency [Line Items]
Unrecognized tax benefits		$ 216.0