Revenues from Contracts with Customers - Summary of Revenues from Contracts with Customers and Other Sources of Revenues (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers	$ 464,889	$ 484,731
Principal transactions	98,763	113,311
Interest and dividend income	3,038	4,161
Other revenues	886	964
Total revenues	567,576	603,167
Commissions [Member]
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers	435,220	455,855
Data, Software And Post-Trade [Member]
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers	21,986	19,398
Fees From Related Parties [Member]
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers	3,785	5,521
Other Revenues [Member]
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers	3,898	3,957
Total revenues	$ 33,593	$ 34,001