Income Taxes - Net Deferred Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Inventory reserves	$ 1,104	$ 1,396
Capitalized research costs	23	44
Capitalized inventory costs	609	704
Net operating losses	999	485
Acquired intangible assets	287	136
Accrued liabilities	1,239	4,739
Income tax credits	8,861	12,509
Stock-based compensation	2,712	3,376
Amortization of intangible assets	526	0
Total deferred tax assets	16,360	23,389
Deferred tax liabilities:
Depreciation	(944)	(2,924)
Allowance for doubtful accounts	(444)	(241)
Amortization of intangible assets	0	(780)
Other	(2,680)	(1,479)
Total deferred tax liabilities	(4,068)	(5,424)
Net deferred tax assets before valuation allowance	12,292	17,965
Less: Valuation allowance	(8,802)	(8,635)
Net deferred tax assets	$ 3,490	$ 9,330