Income Taxes - Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Accrued expenses	$ 182	$ 215
Stock-based compensation	1,309	1,664
Other	49	39
Current deferred tax assets	1,540	1,918
Net operating loss carryforwards	40,504	53,173
Research and development credits	3,230	4,185
Noncurrent deferred tax assets (liabilities)	43,734	57,358
Total deferred tax assets	45,274	59,276
Valuation allowances	(45,274)	(59,276)
Net deferred tax assets	$ 0	$ 0