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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	$ 439,291	$ 375,367	$ 1,344,538	$ 1,204,413
Principal transactions	75,536	73,360	250,198	250,266
Interest income	3,976	2,870	15,454	10,485
Other revenues	2,334	4,045	6,885	6,295
Total revenues	521,137	455,642	1,617,075	1,471,459
Commissions [Member]
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	409,082	351,969	1,259,571	1,138,313
Data, Software And Post-Trade [Member]
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	18,364	16,547	55,015	47,016
Fees From Related Parties [Member]
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	8,208	6,821	21,224	19,054
Other Revenues [Member]
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	3,637	30	8,728	30
Total revenues	$ 36,519	$ 30,313	$ 107,306	$ 82,880