Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Commission revenue by fee type
Revenues	$ 162,093	$ 164,006	$ 533,891	$ 517,779
Commissions [Member]
Commission revenue by fee type
Disclosed trading	75,044	80,354	258,443	260,233
Matched principal trading	34,356	40,565	120,557	128,296
Total variable transaction fees	112,148	123,183	387,645	398,380
Distribution fees and unused minimum fees	30,678	27,403	87,450	80,252
Revenues	142,826	150,586	475,095	478,632
Commissions [Member] | U.S. Treasuries [Member]
Commission revenue by fee type
Matched principal trading	$ 2,748	$ 2,264	$ 8,645	$ 9,851