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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	$ 449,247	$ 399,791	$ 905,247	$ 829,046
Principal transactions	90,432	84,988	174,662	176,906
Interest income	7,813	4,940	11,478	7,615
Other revenues	3,695	1,324	4,551	2,250
Total revenues	551,187	491,043	1,095,938	1,015,817
Commissions [Member]
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	420,969	378,487	850,489	786,344
Data, Software And Post-Trade [Member]
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	18,741	15,370	36,651	30,469
Fees From Related Parties [Member]
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	7,221	5,934	13,016	12,233
Other Revenues [Member]
Revenue From Contracts With Customers [Line Items]
Revenue from contracts with customers	2,316		5,091
Total revenues	$ 39,786	$ 27,568	$ 70,787	$ 52,567