Schedule II - Valuation and Qualifying Accounts (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Doubtful Accounts
Movement in Valuation Allowances and Reserves
Balance at Beginning of Period	$ 443	$ 322	$ 251
Charged to Costs and Expenses	592	205	86
Charged to Other Accounts	0	0	0
Deductions	(306)	(84)	(15)
Balance at End of Period	729	443	322
Warranty Reserve
Movement in Valuation Allowances and Reserves
Balance at Beginning of Period	780	314	675
Charged to Costs and Expenses	109	621	(62)
Charged to Other Accounts	0	0	0
Deductions	(245)	(155)	(299)
Balance at End of Period	$ 644	$ 780	$ 314