Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Commission revenue by fee type
Revenues	$ 689,125	$ 511,352	$ 435,565
Commissions [Member]
Commission revenue by fee type
Disclosed trading	343,427	266,916	228,004
Matched principal trading	170,537	98,080	65,932
Total variable transaction fees	526,337	367,180	293,936
Distribution fees and unused minimum fees	108,108	96,676	96,898
Revenues	634,445	463,856	$ 390,834
Commissions [Member] | U.S. Treasuries [Member]
Commission revenue by fee type
Matched principal trading	$ 12,372	$ 2,184