Significant Accounting Policies - Summary of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Payables And Accruals [Abstract]
Accrued compensation costs	$ 3,103	$ 2,233
Deferred tax liability - short term	1,062
Stock option early exercise liability	940	1,273
Accrued research and development costs	654	569
Tenant improvement liabilities - short term	274
Accrued patent expense reimbursement	224	126
Accrued legal expenses	241
Accrued related party costs	81	3
Accrued other expense	971	201
Total accrued expenses and other current liabilities	$ 7,550	$ 4,405