Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Commission revenue by fee type
Total commissions	$ 179,846	$ 182,229	$ 383,015	$ 368,286
Commissions [Member]
Commission revenue by fee type
Disclosed trading	78,424	84,686	166,552	171,753
Matched principal trading	41,606	42,624	95,842	85,615
Total variable transaction fees	123,257	131,822	270,485	266,695
Distribution fees and unused minimum fees	35,329	31,641	70,092	62,881
Total commissions	158,586	163,463	340,577	329,576
Commissions [Member] | US Government Bonds [Member]
Commission revenue by fee type
Matched principal trading	$ 3,227	$ 4,512	$ 8,091	$ 9,327