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Accrued Expenses and Other Current Liabilities - Schedule of Accrued Expenses and Other Current Liabilities (Detail) - USD ($)
$ in Thousands
Sep. 30, 2019
Dec. 31, 2018
Payables And Accruals [Abstract]    
Accrued external research and development expenses $ 8,364 $ 4,541
Accrued payroll and related expenses 2,315 2,379
Accrued professional fees 636 917
Accrued other 554 426
Accrued expenses and other current liabilities $ 11,869 $ 8,263