Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Leased Assets [Line Items]
Total rent expense under operating leases	$ 999	$ 933	$ 862
Current deferred rent	91	44
Noncurrent deferred rent	59	99
Purchase obligations	431
Annual purchases, raw materials	1,100	943	1,200
Annual purchases	228	$ 139	$ 106
Clinical Trial Component
Operating Leased Assets [Line Items]
Purchase obligations	143
Raw Materials
Operating Leased Assets [Line Items]
Purchase obligations	$ 288