Income Taxes - Components of Deferred Income Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred income tax assets:
Net operating loss carryforward	$ 2,347	$ 3,691
Deferred revenue	13,304	9,442
Deferred compensation	858	1,109
Inventory reserve	1,294	702
Non-qualified and non-employee stock option expense	3,758	3,704
Capitalized research and development	0	485
Research and development tax credit carryforward	5,193	3,817
Reserves, accruals, and other	3,094	1,921
Total deferred income tax assets	30,260	24,871
Deferred income tax liabilities:
Depreciation	(2,195)	(2,027)
Amortization	(57)	(1,398)
Other	(1,232)	(256)
Total deferred income tax liabilities	(3,484)	(3,681)
Net deferred income tax assets before valuation allowance	26,776	21,190
Valuation allowance	(7,429)	(5,435)
Net deferred income tax assets	19,347	15,755
Deferred Tax Assets, Amortization	$ 412	$ 0