Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Commission revenue by fee type
Revenues	$ 195,464	$ 168,978
Commissions [Member]
Commission revenue by fee type
Disclosed trading	98,818	84,245
Matched principal trading	46,320	40,358
Total variable transaction fees	148,398	129,166
Distribution fees and unused minimum fees	27,440	26,788
Revenues	175,838	155,954
Commissions [Member] | U.S. Treasuries [Member]
Commission revenue by fee type
Matched principal trading	$ 3,260	$ 4,563