Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets Sold under Agreements to Repurchase [Line Items]
Gain/Loss on remeasurement of contingent consideration	$ 0.7	$ 1.4	$ (1.9)
Senior Notes | 6% senior notes due 2022
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate (as a percent)	6.00%
Debt maturity period	2022