Changes in Initial Product Warranty Accrual and Expenses Under Initial and Extended Warranties (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Movement in Standard Product Warranty Accrual
Initial warranty accrual, beginning balance	$ 1,359	$ 1,285	$ 1,190	$ 1,706
Provision for estimated warranty cost	203	190	827	192
Warranty expenditures	(182)	(325)	(637)	(748)
Initial warranty accrual, ending balance	1,380	1,150	1,380	1,150
Less warranty accrual, long-term	375	107	375	107	$ 70
Total warranty accrual, current portion	$ 1,005	$ 1,043	$ 1,005	$ 1,043	$ 1,120