Changes in Initial Product Warranty Accrual and Expenses Under Initial and Extended Warranties (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Standard Product Warranty Accrual
Initial warranty accrual, beginning balance	$ 1,706	$ 2,188	$ 1,449
Provision for estimated warranty cost	492	348	1,715
Warranty expenditures	(1,008)	(830)	(976)
Initial warranty accrual, ending balance	1,190	1,706	2,188
Less warranty accrual, long-term	70	773	843
Total warranty accrual, current portion	$ 1,120	$ 933	$ 1,345