Accounts Payable and Accrued Expenses - Schedule of Accounts Payable and Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Accounts payable	$ 3,829	$ 4,738
Accrued research projects	2,356	4,352
Accrued professional fees	384	163
Accrued compensation and benefits	1,864	1,514
Accrued other	73	766
Total	$ 8,506	$ 11,533