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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers	$ 412,732	$ 452,389	$ 897,465	$ 908,123
Principal transactions	99,453	90,432	212,764	174,662
Interest and dividend income	6,536	7,813	10,697	11,478
Other revenues	367	553	1,329	1,675
Total revenues	519,088	551,187	1,122,255	1,095,938
Commissions [Member]
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers	382,640	422,974	838,495	853,156
Data, Software And Post-Trade [Member]
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers	20,139	18,741	39,537	36,651
Fees From Related Parties [Member]
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers	6,562	7,221	12,083	13,016
Other Revenues [Member]
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers	3,391	3,453	7,350	5,300
Total revenues	$ 36,995	$ 37,781	$ 70,996	$ 68,120