Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Commission revenue by fee type
Total commissions	$ 172,284	$ 172,112	$ 555,299	$ 540,398
Commissions [Member]
Commission revenue by fee type
Disclosed trading	73,763	74,767	240,315	246,520
Matched principal trading	36,682	42,845	132,524	128,460
Total variable transaction fees	114,274	121,772	384,759	388,467
Distribution fees and unused minimum fees	36,222	31,392	106,314	94,273
Total commissions	150,496	153,164	491,073	482,740
Commissions [Member] | US Government Bonds [Member]
Commission revenue by fee type
Matched principal trading	$ 3,829	$ 4,160	$ 11,920	$ 13,487