Prepaid Expenses and Other Current Assets - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Prepaid Expense And Other Assets Current [Abstract]
Tax incentive refund	$ 1,549	$ 1,325
Prepaid research and development costs	4,090	981
Prepaid insurance	1,626	316
Interest income receivable	163	220
Prepaid rent	83	81
Other current assets	329	795
Total	$ 7,840	$ 3,718