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, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers:	$ 331,433	$ 395,960	$ 1,057,901	$ 1,279,638
Principal transactions	79,568	73,997	283,338	254,757
Interest and dividend income	4,110	3,042	15,506	17,535
Other revenues	1,456	748	2,042	1,843
Total revenues	416,567	473,747	1,358,787	1,553,773
Commissions
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers:	299,430	367,016	965,636	1,192,004
Data, software, and post-trade
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers:	23,808	22,238	71,326	65,826
Fees from related parties
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers:	3,896	3,470	10,838	11,500
Other revenues
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers:	4,299	3,236	10,101	10,308
Total revenues	$ 37,569	$ 32,734	$ 109,813	$ 107,012