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, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers:	$ 339,067	$ 418,787	$ 726,467	$ 883,677
Principal transactions	88,169	81,997	203,770	180,760
Interest and dividend income	8,961	11,455	11,396	14,493
Other revenues (losses), net	(441)	211	587	1,096
Total revenues	435,756	512,450	942,220	1,080,026
Commissions
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers:	309,542	389,768	666,206	824,988
Data, software, and post-trade
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers:	23,391	21,602	47,518	43,588
Fees from related parties
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers:	3,625	4,245	6,942	8,030
Other revenues
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers:	2,509	3,172	5,801	7,071
Total revenues	$ 38,045	$ 40,685	$ 72,244	$ 74,278