Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Commission revenue by fee type
Revenues	$ 698,951	$ 689,125	$ 511,352
Commissions [Member]
Commission revenue by fee type
Disclosed trading	333,712	343,427	266,916
Matched principal trading	155,465	170,537	98,080
Total variable transaction fees	501,577	526,337	367,180
Distribution fees and unused minimum fees	119,431	108,108	96,676
Revenues	621,008	634,445	463,856
Commissions [Member] | U.S. Treasuries [Member]
Commission revenue by fee type
Matched principal trading	$ 12,400	$ 12,372	$ 2,184