Income Taxes - Reconciliation of Expected Statutory Federal Income Tax Provision to Actual Income Tax Provision (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Expected income tax benefit at federal statutory tax rate	$ (27,826)	$ (18,960)	$ (19,271)
State income taxes, net of federal benefit	(1,152)	(1,580)	(2,348)
Tax credits	(5,447)	(5,039)	(3,307)
Change in fair value of convertible note payable	0	616	4,120
Change in valuation allowance	31,990	23,216	20,047
Prior year true-up	(1,320)	110	(92)
Stock compensation	2,458	1,161	902
Reserve for uncertain tax positions	1,314	254	0
Other	(21)	204	(50)
Income tax (benefit) expense	$ (4)	$ (18)	$ 1