Income Taxes - Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets
Net operating loss carryforward	$ 12,170	$ 12,286
Intangible assets	29	38
Accrued expense	333	335
Stock-based compensation	386	283
Other	75	76
Total deferred tax assets	19,389	16,713
Deferred tax liabilities
Depreciable assets	(63)	(60)
Total deferred tax liabilities	(63)	(60)
Less: Valuation allowance	(19,326)	(16,653)
Deferred tax assets, net	0	0
Federal
Deferred tax assets
Tax credits	5,572	3,291
State
Deferred tax assets
Tax credits	$ 824	$ 404