Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Commission revenue by fee type
Revenues	$ 184,795	$ 125,490	$ 353,773	$ 249,981
Commissions [Member]
Commission revenue by fee type
Disclosed trading	95,634	67,308	179,879	133,364
Matched principal trading	47,373	23,454	87,731	46,490
Total variable transaction fees	146,031	90,762	275,197	179,854
Distribution fees and unused minimum fees	26,061	23,362	52,849	47,030
Revenues	172,092	$ 114,124	328,046	$ 226,884
Commissions [Member] | U.S. Treasuries [Member]
Commission revenue by fee type
Matched principal trading	$ 3,024		$ 7,587