Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Commission revenue by fee type
Revenues	$ 208,576	$ 210,318
Commissions [Member]
Commission revenue by fee type
Disclosed trading	95,455	94,778
Open Trading - matched principal trading	43,052	48,180
Other Variable Transaction	4,955	4,849
Total variable transaction fees	147,991	151,519
Distribution fees and unused minimum fees	33,352	33,354
Revenues	181,343	184,873
Commissions [Member] | US Government Bonds [Member]
Commission revenue by fee type
Open Trading - matched principal trading	$ 4,529	$ 3,712