Income Taxes - Components of Deferred Income Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred income tax assets:
Net operating loss carryforward	$ 3,691	$ 2,405
Deferred revenue	9,442	11,537
Deferred compensation	1,109	1,695
Inventory reserve	702	1,126
Non-qualified and non-employee stock option expense	3,704	4,410
Capitalized research and development	485	1,991
Research and development tax credit carryforward	3,817	2,722
Reserves, accruals, and other	1,921	1,239
Total deferred income tax assets	24,871	27,125
Deferred income tax liabilities:
Depreciation	(2,027)	(2,364)
Amortization	(1,398)	(1,473)
Other	(256)	(294)
Total deferred income tax liabilities	(3,681)	(4,131)
Net deferred income tax assets before valuation allowance	21,190	22,994
Valuation allowance	(5,435)	(3,479)
Net deferred income tax assets	$ 15,755	$ 19,515