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, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers:	$ 395,960	$ 386,441	$ 1,279,638	$ 1,283,906
Principal transactions	73,997	65,182	254,757	277,946
Interest and dividend income	3,042	2,418	17,535	13,115
Other revenues	748	1,001	1,843	2,368
Total revenues	473,747	455,042	1,553,773	1,577,335
Commissions
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers:	367,016	352,027	1,192,004	1,190,522
Data, software, and post-trade
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers:	22,238	21,523	65,826	61,060
Fees from related parties
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers:	3,470	8,814	11,500	20,897
Other revenues
Revenues From Contracts With Customers [Line Items]
Revenues from contracts with customers:	3,236	4,077	10,308	11,427
Total revenues	$ 32,734	$ 37,833	$ 107,012	$ 108,867