Property and Equipment - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Accumulated depreciation	$ 17,248	$ 14,816
Depreciation and amortization	3,300	2,900	$ 2,300
Lease and loaner instruments
Property, Plant and Equipment [Line Items]
Accumulated depreciation	$ 1,900	$ 1,500