Income Taxes - Net Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax liabilities:
Convertible notes	$ 2,821	$ 4,263
Total deferred tax liabilities	2,821	4,263
Deferred tax assets:
Loss carryforwards	60,428	85,263
Intangible assets	68	117
Capital loss carryforward	0	852
Property and equipment	606	1,067
License fees	4,419	7,776
Accrued expenses and other	401	549
Stock-based compensation	2,753	4,059
Credits	1,436	418
Total deferred tax assets before valuation allowance	70,111	100,101
Less: valuation allowance	(67,290)	(95,838)
Total deferred tax assets	2,821	4,263
Net deferred tax assets	$ 0	$ 0