Segment, Geographic and Product Information - Product Information Regarding Revenues (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Total revenues	$ 551,187	$ 491,043	$ 1,095,938	$ 1,015,817
Rates [Member]
Segment Reporting Information [Line Items]
Total revenues	152,959	141,400	308,570	298,967
Credit [Member]
Segment Reporting Information [Line Items]
Total revenues	78,166	75,526	163,893	157,576
Foreign Exchange [Member]
Segment Reporting Information [Line Items]
Total revenues	101,899	102,307	203,457	204,562
Energy and Commodities [Member]
Segment Reporting Information [Line Items]
Total revenues	73,430	56,277	143,295	116,426
Equities, Insurance, and Other Asset Classes [Member]
Segment Reporting Information [Line Items]
Total revenues	104,947	87,965	205,936	185,719
Brokerage [Member]
Segment Reporting Information [Line Items]
Total revenues	511,401	463,475	1,025,151	963,250
All Other Revenue [Member]
Segment Reporting Information [Line Items]
Total revenues	$ 39,786	$ 27,568	$ 70,787	$ 52,567