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Fair value measurements - Additional Information (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2024
Dec. 31, 2023
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Securities owned, at fair value $ 1,044,089 $ 795,312
Valuation adjustment $ 200  
Forward or delayed delivery of the underlying instrument with settlement 180 days  
Level 2    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Securities owned, at fair value $ 85,124 $ 64,683
Other Assets | Level 2    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Investments 7,200  
Auction rate securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Securities owned, at fair value $ 2,700