Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Commission revenue by fee type
Revenues	$ 206,715	$ 172,284	$ 614,693	$ 555,299
Commissions [Member]
Commission revenue by fee type
Disclosed trading	91,273	73,763	272,828	240,315
Open Trading - matched principal trading	45,137	36,682	135,756	132,524
Other Variable Transaction	4,802	0	14,728	0
Total variable transaction fees	146,737	114,274	436,696	384,759
Distribution fees and unused minimum fees	33,655	36,222	100,248	106,314
Revenues	180,392	150,496	536,944	491,073
Commissions [Member] | US Government Bonds [Member]
Commission revenue by fee type
Open Trading - matched principal trading	$ 5,525	$ 3,829	$ 13,384	$ 11,920