Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Net operating losses	$ 31,120	$ 27,827	$ 36,626
R&D tax credits	1,916	1,368	1,093
R&D expenditures, capitalized for tax	2,549	2,146	2,682
Interest expense carryforward	638	0	0
Accruals and other	1,688	753	1,065
Depreciation and amortization	31	4	0
Other comprehensive loss	16	0	0
Total deferred tax assets	37,958	32,098	41,466
Deferred tax liabilities:
Depreciation and amortization	0	0	(4)
Other	(101)	0	0
Total deferred tax liabilities	(101)	0	(4)
Net deferred tax assets	37,857	32,098	41,462
Valuation allowance	(37,857)	(32,098)	(41,462)
Deferred tax assets, net	$ 0	$ 0	$ 0