Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Commission revenue by fee type
Revenues	$ 218,415	$ 219,462	$ 451,795	$ 428,038
Commissions [Member]
Commission revenue by fee type
Disclosed trading	93,066	98,088	197,790	193,543
Open Trading - matched principal trading	44,924	47,319	94,154	90,371
Other Variable Transaction	10,309	7,337	21,006	12,292
Total variable transaction fees	152,698	158,073	322,664	306,064
Distribution fees and unused minimum fees	34,197	33,697	67,702	67,049
Revenues	186,895	191,770	390,366	373,113
Commissions [Member] | US Government Bonds [Member]
Commission revenue by fee type
Open Trading - matched principal trading	$ 4,399	$ 5,329	$ 9,714	$ 9,858