Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Commission revenue by fee type
Total commissions	$ 203,169	$ 186,057
Commissions [Member]
Commission revenue by fee type
Disclosed trading	88,128	87,067
Matched principal trading	54,236	42,991
Total variable transaction fees	147,228	134,873
Distribution fees and unused minimum fees	34,763	31,240
Total commissions	181,991	166,113
Commissions [Member] | US Government Bonds [Member]
Commission revenue by fee type
Matched principal trading	$ 4,864	$ 4,815