Accrued Expenses and Other Liabilities - Schedule of Accrued Expenses and Other Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Payables And Accruals [Abstract]
Accrued compensation and benefits	$ 3,738	$ 3,370
Accrued research and development expenses	3,012	1,059
Accrued professional fees	578	195
Accrued unvested common stock subject to repurchase	48	112
Accrued other	442	304
Total accrued expenses and other liabilities	$ 7,818	$ 5,040