Income Taxes - Summary of Company's Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Net operating loss carryforwards	$ 49,593	$ 41,807
Unearned revenue	1,501	1,421
Stock-based compensation	2,861	2,485
Accrued payroll and benefits	215	226
Research and development credits	143	106
Capital loss carryover	13	16
Fixed asset basis difference	191	192
Inventory reserve	1,110	912
Charitable contributions	5	7
Total deferred tax assets	55,632	47,172
Less valuation allowance	(55,632)	(47,172)
Net deferred tax assets	$ 0	$ 0