Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Commission revenue by fee type
Revenues	$ 176,334	$ 184,795	$ 371,798	$ 353,773
Commissions [Member]
Commission revenue by fee type
Disclosed trading	84,581	95,634	183,399	179,879
Matched principal trading	39,881	47,373	86,201	87,731
Total variable transaction fees	127,099	146,031	275,497	275,197
Distribution fees and unused minimum fees	29,332	26,061	56,772	52,849
Revenues	156,431	172,092	332,269	328,046
Commissions [Member] | U.S. Treasuries [Member]
Commission revenue by fee type
Matched principal trading	$ 2,637	$ 3,024	$ 5,897	$ 7,587