Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Commission revenue by fee type
Revenues	$ 219,462	$ 197,660	$ 428,038	$ 407,978
Commissions [Member]
Commission revenue by fee type
Disclosed trading	98,088	86,778	193,543	181,556
Open Trading - matched principal trading	47,319	42,439	90,371	90,619
Other Variable Transaction	7,337	5,076	12,292	9,925
Total variable transaction fees	158,073	138,440	306,064	289,959
Distribution fees and unused minimum fees	33,697	33,239	67,049	66,593
Revenues	191,770	171,679	373,113	356,552
Commissions [Member] | US Government Bonds [Member]
Commission revenue by fee type
Open Trading - matched principal trading	$ 5,329	$ 4,147	$ 9,858	$ 7,859