Fair Value of Financial Assets and Liabilities - Fair Value Hierarchy of Financial Assets and Liabilities under GAAP Guidance (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Net amounts of recognized assets
Marketable securities, Assets	$ 12,772	$ 32,064
Government debt, Assets	59,929	57,788
Corporate bonds, Assets	1,300
Securities owned - Equities, Assets	856	620
Foreign exchange/commodities options, Assets	30	39
Forwards, Assets	93	236
Foreign exchange swaps, Assets	1,156	1,611
Futures, Assets	180	93
Total, Assets	76,316	92,451
Net amounts of recognized Liabilities
Foreign exchange/commodities options, Liabilities	5	88,016
Foreign exchange swaps, Liabilities	1,831	2,072
Forwards, Liabilities	3,757	639
Futures, Liabilities		402
Contingent consideration, Liabilities	42,152	45,984
Total, Liabilities	47,745	137,113
Netting and Collateral [Member]
Net amounts of recognized assets
Foreign exchange/commodities options, Assets	(4,713)	(482,712)
Forwards, Assets	(7)	(17)
Foreign exchange swaps, Assets	(1,058)	(327)
Futures, Assets	(1,633)	(57,386)
Total, Assets	(7,411)	(540,442)
Net amounts of recognized Liabilities
Foreign exchange/commodities options, Liabilities	(4,713)	(482,712)
Foreign exchange swaps, Liabilities	(1,058)	(327)
Forwards, Liabilities	(7)	(17)
Futures, Liabilities	(1,633)	(57,386)
Total, Liabilities	(7,411)	(540,442)
Level 1 [Member]
Net amounts of recognized assets
Marketable securities, Assets	12,772	32,064
Government debt, Assets	59,929	57,788
Securities owned - Equities, Assets	856	620
Foreign exchange/commodities options, Assets	4,743	482,751
Total, Assets	78,300	573,223
Net amounts of recognized Liabilities
Foreign exchange/commodities options, Liabilities	4,718	570,728
Total, Liabilities	4,718	570,728
Level 2 [Member]
Net amounts of recognized assets
Corporate bonds, Assets	1,300
Forwards, Assets	100	253
Foreign exchange swaps, Assets	2,214	1,938
Futures, Assets	1,813	57,479
Total, Assets	5,427	59,670
Net amounts of recognized Liabilities
Foreign exchange swaps, Liabilities	2,889	2,399
Forwards, Liabilities	3,764	656
Futures, Liabilities	1,633	57,788
Total, Liabilities	8,286	60,843
Level 3 [Member]
Net amounts of recognized Liabilities
Contingent consideration, Liabilities	42,152	45,984
Total, Liabilities	$ 42,152	$ 45,984