Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Accrued expenses	$ 215	$ 783
Stock-based compensation	1,664	1,507
Other	39	19
Current deferred tax assets	1,918	2,309
Net operating loss carryforwards	53,173	42,358
Research and development credits	4,185	3,836
Noncurrent deferred tax assets (liabilities)	57,358	46,194
Total deferred tax assets	59,276	48,503
Valuation allowances	(59,276)	(48,503)
Net deferred tax assets	$ 0	$ 0