Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Commission revenue by fee type
Revenues	$ 182,229	$ 176,334	$ 368,286	$ 371,798
Commissions [Member]
Commission revenue by fee type
Disclosed trading	84,686	84,581	171,753	183,399
Matched principal trading	42,624	39,881	85,615	86,201
Total variable transaction fees	131,822	127,099	266,695	275,497
Distribution fees and unused minimum fees	31,641	29,332	62,881	56,772
Revenues	163,463	156,431	329,576	332,269
Commissions [Member] | U.S. Treasuries [Member]
Commission revenue by fee type
Matched principal trading	$ 4,512	$ 2,637	$ 9,327	$ 5,897