Other Balance Sheet Accounts - Summary of Accounts Payable and Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Employee compensation	$ 2,654	$ 2,518
Clinical trial accruals	1,017	446
Accounts payable	1,213	326
Deferred rent	0	223
Other accrued liabilities	333	614
Total accounts payable and accrued expenses	$ 5,217	$ 4,127