Revenues from Contracts with Customers - Summary of Revenues from Contracts with Customers and Other Sources of Revenues (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers	$ 1,663,537	$ 1,690,290	$ 1,761,232
Principal transactions	327,761	351,633	321,923
Interest and dividend income	21,977	12,332	18,319
Other revenues	2,089	2,506	3,132
Total revenues	2,015,364	2,056,761	2,104,606
Commissions
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers	1,541,900	1,567,668	1,645,818
Data, software, and post-trade
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers	89,963	81,920	73,166
Fees from related parties
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers	14,856	25,754	29,442
Other revenues
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers	16,818	14,948	12,806
Total revenues	$ 145,703	$ 137,460	$ 136,865