Income Taxes - Deferred Tax Asset (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 39,971	$ 52,154
Research and development credit	13,392	5,582
Depreciation and amortization	1,241	2,023
Other temporary differences	4,477	6,154
Gross deferred tax assets	59,081	65,913
Deferred tax asset valuation allowance	(59,081)	(65,913)
Net deferred tax assets	$ 0	$ 0