Inventories - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Inventory Disclosure [Abstract]
Amounts credited to the allowance for obsolete inventory	$ (0.2)	$ (1.2)	$ (0.7)
Items charged to the allowance for inventory write-offs	$ 0.6	$ 0.2	$ 0.2