Significant Accounting Policies - Summary of Commission Revenue by Fee Type (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Commission revenue by fee type
Revenues	$ 164,006	$ 131,597	$ 517,779	$ 381,578
Commissions [Member]
Commission revenue by fee type
Disclosed trading	80,354	69,541	260,233	202,905
Matched principal trading	40,565	26,444	128,296	72,934
Total variable transaction fees	123,183	95,985	398,380	275,839
Distribution fees and unused minimum fees	27,403	23,884	80,252	70,914
Revenues	150,586	$ 119,869	478,632	$ 346,753
Commissions [Member] | U.S. Treasuries [Member]
Commission revenue by fee type
Matched principal trading	$ 2,264		$ 9,851