Fair Value of Financial Assets and Liabilities - Fair Value Hierarchy of Financial Assets and Liabilities under U.S. GAAP Guidance (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Net amounts of recognized assets
Marketable securities, Assets	$ 338	$ 349
Government debt, Assets	58,806	57,918
Securities owned - Equities, Assets	88	75
FX/commodities options, Assets		74
Forwards, Assets	581	295
FX swaps, Assets	2,756	562
Corporate bonds, Assets	633	579
Total, Assets	63,202	59,852
Net amounts of recognized Liabilities
FX/commodities options, Liabilities	151
FX swaps, Liabilities	1,527	319
Forwards, Liabilities	531	215
Futures, Liabilities	481	926
Contingent consideration, Liabilities	40,056	39,791
Total, Liabilities	42,746	41,251
Netting and Collateral [Member]
Net amounts of recognized assets
Forwards, Assets	(261)	(43)
FX swaps, Assets	(580)	(21)
Futures, Assets	(28,563)	(41,257)
Total, Assets	(29,404)	(41,321)
Net amounts of recognized Liabilities
FX swaps, Liabilities	(580)	(21)
Forwards, Liabilities	(261)	(43)
Futures, Liabilities	(28,563)	(41,257)
Total, Liabilities	(29,404)	(41,321)
Level 1 [Member]
Net amounts of recognized assets
Marketable securities, Assets	338	349
Government debt, Assets	58,806	57,918
Securities owned - Equities, Assets	88	75
FX/commodities options, Assets		74
Total, Assets	59,232	58,416
Net amounts of recognized Liabilities
FX/commodities options, Liabilities	151
Total, Liabilities	151
Level 2 [Member]
Net amounts of recognized assets
Forwards, Assets	842	338
FX swaps, Assets	3,336	583
Futures, Assets	28,563	41,257
Corporate bonds, Assets	633	579
Total, Assets	33,374	42,757
Net amounts of recognized Liabilities
FX swaps, Liabilities	2,107	340
Forwards, Liabilities	792	258
Futures, Liabilities	29,044	42,183
Total, Liabilities	31,943	42,781
Level 3 [Member]
Net amounts of recognized Liabilities
Contingent consideration, Liabilities	40,056	39,791
Total, Liabilities	$ 40,056	$ 39,791