Fair Value of Financial Assets and Liabilities - Fair Value Hierarchy of Financial Assets and Liabilities under U.S. GAAP Guidance (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Net amounts of recognized assets
Marketable securities, Assets	$ 303	$ 14,228
Government debt, Assets	57,805	56,761
Securities owned - Equities, Assets	79	36
FX/commodities options, Assets	271	26
Forwards, Assets	62	564
FX swaps, Assets	198	1,278
Corporate bonds, Assets	663	728
Total, Assets	59,381	73,621
Net amounts of recognized Liabilities
FX swaps, Liabilities	309	2,244
Forwards, Liabilities	892	1,285
Futures, Liabilities	642	624
Contingent consideration, Liabilities	36,515	42,159
Total, Liabilities	38,358	46,312
Netting and Collateral [Member]
Net amounts of recognized assets
FX/commodities options, Assets	(2)
Forwards, Assets	(9)	(142)
FX swaps, Assets	(24)	(394)
Futures, Assets	(27,092)	(2,044)
Total, Assets	(27,127)	(2,580)
Net amounts of recognized Liabilities
FX/commodities options, Liabilities	(2)
FX swaps, Liabilities	(24)	(394)
Forwards, Liabilities	(9)	(142)
Futures, Liabilities	(27,092)	(2,044)
Total, Liabilities	(27,127)	(2,580)
Level 1 [Member]
Net amounts of recognized assets
Marketable securities, Assets	303	14,228
Government debt, Assets	57,805	56,761
Securities owned - Equities, Assets	79	36
FX/commodities options, Assets	273	26
Total, Assets	58,460	71,051
Net amounts of recognized Liabilities
FX/commodities options, Liabilities	2
Total, Liabilities	2
Level 2 [Member]
Net amounts of recognized assets
Forwards, Assets	71	706
FX swaps, Assets	222	1,672
Futures, Assets	27,092	2,044
Corporate bonds, Assets	663	728
Total, Assets	28,048	5,150
Net amounts of recognized Liabilities
FX swaps, Liabilities	333	2,638
Forwards, Liabilities	901	1,427
Futures, Liabilities	27,734	2,668
Total, Liabilities	28,968	6,733
Level 3 [Member]
Net amounts of recognized Liabilities
Contingent consideration, Liabilities	36,515	42,159
Total, Liabilities	$ 36,515	$ 42,159