Income Taxes - Reconciliation of Expected Statutory Federal Income Tax Provision to Actual Income Tax Provision (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Expected income tax benefit at federal statutory tax rate	$ (18,960)	$ (19,271)	$ (6,355)
State income taxes, net of federal benefit	(1,580)	(2,348)	(1,090)
Tax credits	(5,039)	(3,307)	(2,406)
Change in fair value of convertible note payable	616	4,120	456
Change in valuation allowance	23,216	20,047	9,026
Prior year true-up	110	(92)	(235)
Stock compensation	1,161	902	667
Reserve for uncertain tax positions	254	0	0
Other	204	(50)	(86)
Income tax (benefit) expense	$ (18)	$ 1	$ (23)