Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Commission revenue by fee type
Revenues	$ 210,318	$ 203,169
Commissions [Member]
Commission revenue by fee type
Disclosed trading	94,778	88,128
Matched principal trading	48,180	54,236
Other Variable Transaction	4,849	0
Total variable transaction fees	151,519	147,228
Distribution fees and unused minimum fees	33,354	34,763
Revenues	184,873	181,991
Commissions [Member] | US Government Bonds [Member]
Commission revenue by fee type
Matched principal trading	$ 3,712	$ 4,864