Quarterly report pursuant to Section 13 or 15(d)

Acquisitions - Summary of Amounts Allocated to Acquired Assets and Assumed Liabilities (Detail)

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Acquisitions - Summary of Amounts Allocated to Acquired Assets and Assumed Liabilities (Detail) - USD ($)
$ in Thousands
9 Months Ended
Sep. 30, 2020
Dec. 31, 2019
Dec. 31, 2018
Business Acquisition [Line Items]      
Goodwill $ 63,675 $ 63,675 $ 25,168
Cortina Asset Management, LLC      
Business Acquisition [Line Items]      
Goodwill 36,324    
Customer relationships (15 years) 21,500    
Aggregate purchase amount $ 57,824