Inventory (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Inventory Disclosure [Abstract]
Inventory, finished goods, trial and evaluation, gross	$ 1,600	$ 1,400
Raw materials	19,003	20,119
Finished goods	24,101	25,346
Total inventory	$ 43,104	$ 45,465