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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 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Revenues From Contracts With Customers [Line Items]        
Revenues from contracts with customers $ 386,441 $ 440,499 $ 1,283,906 $ 1,348,622
Principal transactions 65,182 75,536 277,946 250,198
Interest and dividend income 2,418 3,976 13,115 15,454
Other revenues 998 1,126 2,327 2,801
Total revenues 455,039 521,137 1,577,294 1,617,075
Commissions [Member]        
Revenues From Contracts With Customers [Line Items]        
Revenues from contracts with customers 352,027 409,765 1,190,522 1,262,921
Data, Software And Post-Trade [Member]        
Revenues From Contracts With Customers [Line Items]        
Revenues from contracts with customers 21,523 18,364 61,060 55,015
Fees From Related Parties [Member]        
Revenues From Contracts With Customers [Line Items]        
Revenues from contracts with customers 8,814 8,208 20,897 21,224
Other Revenues [Member]        
Revenues From Contracts With Customers [Line Items]        
Revenues from contracts with customers 4,077 4,162 11,427 9,462
Total revenues $ 37,830 $ 35,836 $ 108,826 $ 103,956