Income Taxes - Deferred tax assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Net operating loss carryforwards	$ 29,469	$ 23,526
Inventory	1,187	1,309
Accruals	902	1,693
Depreciation and amortization	105	290
Other	1,312	752
Gross deferred tax assets	32,975	27,570
Valuation allowance	(32,850)	(27,554)
Deferred tax assets	125	16
Deferred tax liabilities:
Prepaid expenses	(125)	(16)
Deferred tax liabilities	(125)	(16)
Increase in valuation allowance	$ 5,300	$ 6,200