Income Taxes - Summary of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Noncurrent deferred tax assets:
Stock-based compensation	$ 2,216	$ 2,300
Inventory	375	204
Accrued expenses and other	637	564
Research credit carryforward	5,540	4,970
Fixed assets	450	557
Capitalized start-up costs and other intangibles	2,130	3,586
Net operating loss carryforwards	64,300	82,298
Deferred Tax Assets, Gross	75,648	94,479
Valuation allowance	(71,520)	(91,885)
Net noncurrent deferred tax asset	4,128	2,594
Noncurrent deferred tax liabilities
Fixed assets	(334)	(292)
Purchase accounting intangibles	(12,183)	(12,699)
Net noncurrent deferred tax liability	(12,517)	(12,991)
Net deferred tax asset (liability)	$ (8,389)	$ (10,397)