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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Movement in Standard Product Warranty Accrual
Initial warranty accrual, beginning balance	$ 1,285	$ 1,919	$ 1,706	$ 2,188
Provision for estimated warranty cost	190	282	192	379
Warranty expenditures	(325)	(224)	(748)	(590)
Initial warranty accrual, ending balance	1,150	1,977	1,150	1,977
Less warranty accrual, long-term	107	955	107	955	$ 773
Total warranty accrual, current portion	$ 1,043	$ 1,022	$ 1,043	$ 1,022	$ 933