Supplemental Disclosures - Schedules of Accrued Expenses and Other Current Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Payables And Accruals [Abstract]
Accrued personnel costs	$ 5,789	$ 3,338
Accrued external general and administrative expenses	1,078	672
Accrued external research and development expenses	2,072	3,005
Accrued purchases of property and equipment	430	844
Other accrued expenses and current liabilities	236	267
Accrued expenses and other current liabilities	$ 9,605	$ 8,126