Accounts Payable and Accrued Expenses - Schedule of Accounts Payable and Accrued Expenses (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accounts payable	$ 3,692	$ 3,829
Accrued research projects	6,941	2,356
Accrued professional fees	356	384
Accrued compensation and benefits	1,478	1,864
Accrued other	86	73
Total	$ 12,553	$ 8,506