Selected Quarterly Financial Data (Unaudited) - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Estimated one-time tax provision		$ 44.0		$ 44.0
Provision for transition tax		40.3	$ 37.2	40.3
Tax provision charge related to re-measurement of net deferred tax assets		$ 3.7		$ 3.7
Measurement period adjustment to reduce transition tax	$ 3.1		$ 3.1