Income Taxes - Net Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax liabilities:
Convertible notes	$ 2,842	$ 2,821
Total deferred tax liabilities	2,842	2,821
Deferred tax assets:
Loss carryforwards (federal and state)	64,285	60,428
Intangible assets	57	68
Property and equipment	839	606
License fees	4,645	4,419
Accrued expenses and other	354	401
Stock-based compensation	2,838	2,753
Research and development tax credits	1,945	1,436
Total deferred tax assets before valuation allowance	74,963	70,111
Less: valuation allowance	(72,121)	(67,290)
Total deferred tax assets	2,842	2,821
Net deferred tax assets	$ 0	$ 0