Accounts Payable and Accrued Expenses - Schedule of Accounts Payable and Accrued Expenses (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accounts payable	$ 2,638	$ 3,829
Accrued research projects	8,978	2,356
Accrued professional fees	253	384
Accrued compensation and benefits	2,011	1,864
Accrued other	123	73
Total	$ 14,003	$ 8,506