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]
Research and development	$ 2,799	$ 1,216
Payroll and other employee-related expenses	10,635	8,207
Professional services fees	4,666	2,544
Accrued interest	3,052	1,195
Product revenue reserves	2,466	616
Accrual for employee stock purchase plan	591	251
Other	210	281
Total accrued expenses and other current liabilities	$ 24,419	$ 14,310