Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Commission revenue by fee type
Revenues	$ 208,821	$ 206,715	$ 636,859	$ 614,693
Commissions [Member]
Commission revenue by fee type
Disclosed trading	92,640	91,273	286,183	272,828
Open Trading - matched principal trading	40,590	45,137	130,961	135,756
Other Variable Transaction	8,590	4,802	20,882	14,728
Total variable transaction fees	145,949	146,737	452,013	436,696
Distribution fees and unused minimum fees	34,222	33,655	101,271	100,248
Revenues	180,171	180,392	553,284	536,944
Commissions [Member] | US Government Bonds [Member]
Commission revenue by fee type
Open Trading - matched principal trading	$ 4,129	$ 5,525	$ 13,987	$ 13,384