Segment, Geographic and Product Information - Product Information Regarding Revenues (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Total revenues	$ 473,747	$ 455,042	$ 1,553,773	$ 1,577,335
Rates
Segment Reporting Information [Line Items]
Total revenues	128,508	119,325	426,775	419,599
Credit
Segment Reporting Information [Line Items]
Total revenues	58,983	68,053	221,639	261,022
FX
Segment Reporting Information [Line Items]
Total revenues	72,976	73,281	229,216	242,040
Energy and commodities
Segment Reporting Information [Line Items]
Total revenues	74,328	65,871	224,931	220,935
Equity derivatives and cash equities
Segment Reporting Information [Line Items]
Total revenues	54,715	47,410	186,002	190,984
Insurance
Segment Reporting Information [Line Items]
Total revenues	51,503	43,269	158,198	133,888
Brokerage
Segment Reporting Information [Line Items]
Total revenues	441,013	417,209	1,446,761	1,468,468
All other revenues
Segment Reporting Information [Line Items]
Total revenues	$ 32,734	$ 37,833	$ 107,012	$ 108,867