Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Commission revenue by fee type
Revenues	$ 197,660	$ 179,846	$ 407,978	$ 383,015
Commissions [Member]
Commission revenue by fee type
Disclosed trading	86,778	78,424	181,556	166,552
Open Trading - matched principal trading	42,439	41,606	90,619	95,842
Other Variable Transaction	5,076	0	9,925	0
Total variable transaction fees	138,440	123,257	289,959	270,485
Distribution fees and unused minimum fees	33,239	35,329	66,593	70,092
Revenues	171,679	158,586	356,552	340,577
Commissions [Member] | US Government Bonds [Member]
Commission revenue by fee type
Open Trading - matched principal trading	$ 4,147	$ 3,227	$ 7,859	$ 8,091