Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Commission revenue by fee type
Revenues	$ 172,112	$ 162,093	$ 540,398	$ 533,891
Commissions [Member]
Commission revenue by fee type
Disclosed trading	74,767	75,044	246,520	258,443
Matched principal trading	42,845	34,356	128,460	120,557
Total variable transaction fees	121,772	112,148	388,467	387,645
Distribution fees and unused minimum fees	31,392	30,678	94,273	87,450
Revenues	153,164	142,826	482,740	475,095
Commissions [Member] | U.S. Treasuries [Member]
Commission revenue by fee type
Matched principal trading	$ 4,160	$ 2,748	$ 13,487	$ 8,645