Derivative Financial Instruments Summary of Derivative Activity and Related Impact on Consolidated Statements of Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity warrant assets
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives	$ 11,862	$ 9,251	$ 54,678
Derivatives designated as hedging instruments | Interest rate risks
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives	1,046	2,306	2,506
Derivatives designated as hedging instruments | Interest rate risks | Interest expense-borrowings
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives	1,053	2,341	2,526
Derivatives designated as hedging instruments | Interest rate risks | Net gains on derivative instruments
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives	(7)	(35)	(20)
Derivatives not designated as hedging instruments | Foreign Exchange Contract [Member] | Net gains on derivative instruments
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives	(32,286)	16,136	12,377
Derivatives not designated as hedging instruments | Foreign Exchange Contract [Member] | Other noninterest income
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives	(9,969)
Derivatives not designated as hedging instruments | Other derivative instruments | Net gains on derivative instruments
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives	(9,969)	(5,674)	694
Derivatives not designated as hedging instruments | Other derivative instruments | Other derivatives | Net gains on derivative instruments
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives	(564)	262	(209)
Derivatives not designated as hedging instruments | Equity warrant assets | gains and losses on equity warrant assets [Domain]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives	54,555	37,892	70,963
Internal Instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Foreign Currency Transaction Gain (Loss), before Tax	33,161	(16,676)	(12,735)
Internal Instruments [Member] | Derivatives not designated as hedging instruments | Foreign Exchange Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives	875	(540)	(358)
Internal Instruments [Member] | Derivatives not designated as hedging instruments | Foreign Exchange Contract [Member] | Other noninterest income
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives	(32,286)	16,136	12,377
Foreign Currency Transaction Gain (Loss), before Tax	33,161	(16,676)	(12,735)
Client Instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Foreign Currency Transaction Gain (Loss), before Tax	10,882	4,215	115
Client Instruments [Member] | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives	913	(1,459)	809
Client Instruments [Member] | Derivatives not designated as hedging instruments | Foreign Exchange Contract [Member] | Other noninterest income
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized Gain (Loss) on Derivatives		(5,674)	694
Foreign Currency Transaction Gain (Loss), before Tax	$ 10,882	$ 4,215	$ 115