Accounts Payable and Accrued Expenses - Schedule of Accounts Payable and Accrued Expenses (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accounts payable	$ 1,156	$ 3,829
Accrued research projects	4,387	2,356
Accrued professional fees	257	384
Accrued compensation and benefits	954	1,864
Accrued other	121	73
Total	$ 6,875	$ 8,506