Fair Value of Financial Assets and Liabilities - Fair Value Hierarchy of Financial Assets and Liabilities under U.S. GAAP Guidance (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Net amounts of recognized assets
Loans held for sale, at fair value, Assets	$ 547,968	$ 362,635
Marketable securities, Assets	78,109	208,176
Government debt, Assets	56,776	32,744
Securities owned - Equities, Assets	638	263
Forwards, Assets	35,917	4,262
Rate lock commitments, Assets	14,448	2,923
Foreign exchange swaps, Assets	896	801
Futures, Assets	159	25
Foreign exchange/commodities options, Assets	137	4,416
Interest rate swaps, Assets		242
Total, Assets	735,048	616,487
Net amounts of recognized Liabilities
Foreign exchange/commodities options, Liabilities	33,849	130
Rate lock commitments, Liabilities	13,945	2,390
Foreign exchange swaps, Liabilities	2,075	2,846
Forwards, Liabilities	1,647	1,483
Futures, Liabilities	632	1,066
Equity options, Liabilities		18
Contingent consideration, Liabilities	53,164	67,172
Total, Liabilities	105,312	75,105
Netting and Collateral [Member]
Net amounts of recognized assets
Forwards, Assets	(105)	(101)
Foreign exchange swaps, Assets	(503)	(334)
Futures, Assets	(14,151)	(78,944)
Foreign exchange/commodities options, Assets	(192,174)	(27,418)
Total, Assets	(206,933)	(106,797)
Net amounts of recognized Liabilities
Foreign exchange/commodities options, Liabilities	(192,174)	(27,418)
Foreign exchange swaps, Liabilities	(503)	(334)
Forwards, Liabilities	(105)	(101)
Futures, Liabilities	(14,151)	(78,944)
Total, Liabilities	(206,933)	(106,797)
Level 1 [Member]
Net amounts of recognized assets
Marketable securities, Assets	78,109	208,176
Government debt, Assets	56,776	32,744
Securities owned - Equities, Assets	638	263
Foreign exchange/commodities options, Assets	192,311	31,834
Total, Assets	327,834	273,017
Net amounts of recognized Liabilities
Foreign exchange/commodities options, Liabilities	226,023	27,548
Equity options, Liabilities		18
Total, Liabilities	226,023	27,566
Level 2 [Member]
Net amounts of recognized assets
Loans held for sale, at fair value, Assets	547,968	362,635
Forwards, Assets	19,580	610
Foreign exchange swaps, Assets	1,399	1,135
Futures, Assets	14,310	78,969
Interest rate swaps, Assets		242
Total, Assets	583,257	443,591
Net amounts of recognized Liabilities
Foreign exchange swaps, Liabilities	2,578	3,180
Forwards, Liabilities	1,111	927
Futures, Liabilities	14,783	80,010
Total, Liabilities	18,472	84,117
Level 3 [Member]
Net amounts of recognized assets
Forwards, Assets	16,442	3,753
Rate lock commitments, Assets	14,448	2,923
Total, Assets	30,890	6,676
Net amounts of recognized Liabilities
Rate lock commitments, Liabilities	13,945	2,390
Forwards, Liabilities	641	657
Contingent consideration, Liabilities	53,164	67,172
Total, Liabilities	$ 67,750	$ 70,219