Segment, Geographic and Product Information - Product Information Regarding Revenues (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Total revenues	$ 567,576	$ 603,167
Rates [Member]
Segment Reporting Information [Line Items]
Total revenues	161,793	167,240
Credit [Member]
Segment Reporting Information [Line Items]
Total revenues	90,047	97,189
FX [Member]
Segment Reporting Information [Line Items]
Total revenues	83,433	94,366
Energy and Commodities [Member]
Segment Reporting Information [Line Items]
Total revenues	75,868	83,738
Equities Derivatives and Cash Equities [Member]
Segment Reporting Information [Line Items]
Total revenues	70,462	81,797
Insurance [Member]
Segment Reporting Information [Line Items]
Total revenues	52,380	44,836
Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues	533,983	569,166
All Other Revenues [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 33,593	$ 34,001