Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Commission revenue by fee type
Revenues	$ 186,057	$ 195,464
Commissions [Member]
Commission revenue by fee type
Disclosed trading	87,067	98,818
Matched principal trading	42,991	46,320
Total variable transaction fees	134,873	148,398
Distribution fees and unused minimum fees	31,240	27,440
Revenues	166,113	175,838
Commissions [Member] | U.S. Treasuries [Member]
Commission revenue by fee type
Matched principal trading	$ 4,815	$ 3,260