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Co-Promotion Agreement - Additional Information (Detail) - USD ($)
$ in Thousands
 9 Months Ended 12 Months Ended
Sep. 30, 2020
Dec. 31, 2018
Dec. 31, 2019
Co Promotion Agreement [Line Items]      
Accrued expenses and other current liabilities $ 189,650   $ 139,826
Co-Promotion Agreement | Kowa Pharmaceuticals America, Inc.      
Co Promotion Agreement [Line Items]      
Agreement amended year 2017    
Promotion fee as a percentage of gross margin   18.50%  
Maximum co-promotion tail payments receive period 3 years    
Co-promotion tail payments receivable   $ 17,800  
Accrued co- promotion tail payments   $ 16,600  
Accrued expenses and other liabilities $ 5,200   10,000
Accrued expenses and other current liabilities $ 3,900   $ 6,500