Income Taxes - Deferred tax assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating loss carryforwards	$ 22,586	$ 29,469
Inventory	1,520	1,187
Accruals	401	902
Depreciation and amortization	60	105
Other	1,228	1,312
Gross deferred tax assets	25,795	32,975
Valuation allowance	(25,721)	(32,850)
Deferred tax assets	74	125
Deferred tax liabilities:
Prepaid expenses	(74)	(125)
Deferred tax liabilities	(74)	(125)
Increase in valuation allowance	$ (7,100)	$ 5,300