Supplemental Disclosures - Schedules of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Accrued personnel costs	$ 3,896	$ 3,338
Accrued external research and development expenses	2,133	3,005
Accrued external general and administrative expenses	943	672
Accrued purchases of property and equipment	50	844
Other accrued expenses and current liabilities	492	267
Accrued expenses and other current liabilities	$ 7,514	$ 8,126