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Revenues from Contracts with Customers - Summary of Revenues from Contracts with Customers and Other Sources of Revenues (Detail) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Revenues From Contracts With Customers [Line Items]        
Revenues from contracts with customers: $ 418,787 $ 412,732 $ 883,677 $ 897,465
Principal transactions 81,997 99,453 180,760 212,764
Interest and dividend income 11,455 6,536 14,493 10,697
Other revenues 211 367 1,096 1,329
Total revenues 512,450 519,088 1,080,026 1,122,255
Commissions        
Revenues From Contracts With Customers [Line Items]        
Revenues from contracts with customers: 389,768 382,640 824,988 838,495
Data, software, and post-trade        
Revenues From Contracts With Customers [Line Items]        
Revenues from contracts with customers: 21,602 20,139 43,588 39,537
Fees from related parties        
Revenues From Contracts With Customers [Line Items]        
Revenues from contracts with customers: 4,245 6,562 8,030 12,083
Other revenues        
Revenues From Contracts With Customers [Line Items]        
Revenues from contracts with customers: 3,172 3,391 7,071 7,350
Total revenues $ 40,685 $ 36,995 $ 74,278 $ 70,996