Income Taxes - Reconciliation of Expected Statutory Federal Income Tax Provision to Actual Income Tax Provision (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Expected income tax benefit at federal statutory tax rate	$ (24,515)	$ (27,826)	$ (18,960)
State income taxes, net of federal benefit	1	(1,152)	(1,580)
Tax credits	(5,151)	(5,447)	(5,039)
Change in fair value of convertible note payable	0	0	616
Change in valuation allowance	(8,705)	31,990	23,216
Prior year true-up	1	(1,320)	110
Stock compensation	1,135	2,458	1,161
Reserve for uncertain tax positions	311	1,314	254
Tax Cuts and Jobs Act rate change	36,249	0	0
Other	477	(21)	204
Income tax (benefit)	$ (197)	$ (4)	$ (18)