Segment, Geographic and Product Information - Product Information Regarding Revenues (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Total revenues	$ 512,450	$ 519,088	$ 1,080,026	$ 1,122,255
Rates
Segment Reporting Information [Line Items]
Total revenues	136,474	133,034	298,267	300,274
Credit
Segment Reporting Information [Line Items]
Total revenues	72,609	95,780	162,656	192,969
FX
Segment Reporting Information [Line Items]
Total revenues	72,807	74,393	156,240	168,759
Energy and commodities
Segment Reporting Information [Line Items]
Total revenues	74,735	71,326	150,603	155,064
Equities derivatives and cash equities
Segment Reporting Information [Line Items]
Total revenues	60,825	61,777	131,287	143,574
Insurance
Segment Reporting Information [Line Items]
Total revenues	54,315	45,783	106,695	90,619
Brokerage
Segment Reporting Information [Line Items]
Total revenues	471,765	482,093	1,005,748	1,051,259
All other revenues
Segment Reporting Information [Line Items]
Total revenues	$ 40,685	$ 36,995	$ 74,278	$ 70,996