Changes in Initial Product Warranty Accrual and Expenses Under Initial and Extended Warranties (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Movement in Standard Product Warranty Accrual
Initial warranty accrual, beginning balance	$ 1,190	$ 1,706
Provision for estimated warranty cost	246	(23)
Warranty expenditures	(222)	(223)
Initial warranty accrual, ending balance	1,214	1,460
Less warranty accrual, long-term	124	487	$ 70
Total warranty accrual, current portion	$ 1,090	$ 973	$ 1,120