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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 12 Months Ended
Dec. 31, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Revenues From Contracts With Customers [Line Items]                      
Revenues from contracts with customers                 $ 1,690,290 $ 1,761,232  
Principal transactions $ 73,687 $ 65,182 $ 99,453 $ 113,311 $ 71,725 $ 75,536 $ 90,432 $ 84,230 351,633 321,923 $ 313,053
Interest and dividend income                 12,332 18,319 14,404
Other revenues                 2,465 2,757  
Total revenues $ 479,426 $ 455,039 $ 519,088 $ 603,167 $ 487,156 $ 521,137 $ 551,187 $ 544,751 2,056,720 2,104,231 1,937,810
Commissions [Member]                      
Revenues From Contracts With Customers [Line Items]                      
Revenues from contracts with customers                 1,567,668 1,645,818  
Data, Software And Post-Trade [Member]                      
Revenues From Contracts With Customers [Line Items]                      
Revenues from contracts with customers                 81,920 73,166  
Fees From Related Parties [Member]                      
Revenues From Contracts With Customers [Line Items]                      
Revenues from contracts with customers                 25,754 29,442  
Other Revenues [Member]                      
Revenues From Contracts With Customers [Line Items]                      
Revenues from contracts with customers                 14,948 12,806  
Total revenues                 $ 137,419 $ 136,490 $ 113,235