Accrued Expenses (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Accrued Liabilities, Current [Abstract]
Accrued compensation	$ 3,694	$ 4,397
Accrued research and development expenses	8,467	3,414
Accrued professional expenses	1,270	1,049
Other accrued expenses	279	503
Total	$ 13,710	$ 9,363