Income Taxes - Deferred Tax Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of income taxes computed using the federal statutory rate to the taxes reported in consolidated statements of operations
Loss before income taxes	$ 8,137	$ 17,286	$ 19,566
Federal statutory rate	21.00%	34.00%	34.00%
Taxes computed at federal statutory rate	$ (1,709)	$ (5,877)	$ (6,652)
State taxes (net of federal benefit)	(385)	(1,106)	(1,016)
Nondeductible stock compensation	(605)	563	549
Federal and State Rate Change	839	11,749	(614)
Other	172	116	56
Change in valuation allowance	1,688	(5,445)	7,677
Reported income taxes	0	0	$ 0
Deferred tax assets
Net operating loss carryforwards	10,969	10,487
Employee benefits and stock compensation	2,798	2,128
Research and development costs	9,067	8,275
Fixed assets	418	508
Inventory reserve	2,339	2,568
Other, net	345	282
Total deferred tax assets	25,936	24,248
Valuation allowance	(25,936)	(24,248)
Net deferred tax assets	$ 0	$ 0