Fair Value Measurements - Carrying Amounts and Estimated Fair Value for Financial Instrument Assets (Liabilities) (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014
Financial assets:
Cash and cash equivalents	$ 1,332,202		$ 1,368,040	$ 1,249,704		$ 1,373,357
Interest-bearing deposits at banks	10,777,636		7,594,350
Trading account assets	488,588		273,783
Investment securities	14,733,574		15,656,439
Loans and leases:
Allowance for credit losses	(976,121)	$ (970,496)	(955,992)	$ (933,798)	$ (929,987)	$ (919,562)
Loans and leases, net	88,670,371		86,533,507
Financial liabilities:
Noninterest-bearing deposits	(33,127,627)		(29,110,635)
Time deposits	(11,721,005)		(13,110,392)
Deposits at Cayman Islands office	(223,183)		(170,170)
Short-term borrowings	(213,846)		(150,546)
Long-term borrowings	(10,211,160)		(10,653,858)
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Financial assets:
Cash and cash equivalents	1,258,882		1,276,678
Trading account assets	50,699		56,763
Investment securities	62,546		65,178
Significant Observable Inputs (Level 2) [Member]
Financial assets:
Cash and cash equivalents	73,320		91,362
Interest-bearing deposits at banks	10,777,636		7,594,350
Trading account assets	437,889		217,020
Investment securities	14,591,141		15,406,404
Loans and leases:
Commercial real estate loans	290,347		38,774
Residential real estate loans	4,607,044		4,727,816
Loans and leases, net	4,897,391		4,766,590
Accrued interest receivable	311,414		306,496
Financial liabilities:
Noninterest-bearing deposits	(33,127,627)		(29,110,635)
Savings and interest-checking deposits	(53,065,030)		(49,566,644)
Time deposits	(11,829,119)		(13,135,042)
Deposits at Cayman Islands office	(223,183)		(170,170)
Short-term borrowings	(213,846)		(2,132,182)
Long-term borrowings	(10,277,155)		(10,639,556)
Accrued interest payable	(76,230)		(85,145)
Trading account liabilities	(292,713)		(160,745)
Other financial instruments:
Commitments to sell real estate loans	(11,375)		875
Interest rate swap agreements used for interest rate risk management	21,060		43,892
Significant Unobservable Inputs (Level 3) [Member]
Financial assets:
Investment securities	126,034		189,295
Loans and leases:
Commercial loans and leases	21,563,190		20,146,201
Commercial real estate loans	31,653,994		29,005,470
Residential real estate loans	19,173,949		21,539,955
Consumer loans	12,009,873		11,550,270
Loans and leases, net	84,401,006		82,241,896
Other financial instruments:
Commitments to originate real estate loans for sale	29,800		9,879
Other credit-related commitments	(124,499)		(122,334)
Carrying Amount [Member]
Financial assets:
Cash and cash equivalents	1,332,202		1,368,040
Interest-bearing deposits at banks	10,777,636		7,594,350
Trading account assets	488,588		273,783
Investment securities	14,733,574		15,656,439
Loans and leases:
Commercial loans and leases	21,917,163		20,422,338
Commercial real estate loans	32,078,762		29,197,311
Residential real estate loans	23,584,420		26,270,103
Consumer loans	12,066,147		11,599,747
Allowance for credit losses	(976,121)		(955,992)
Loans and leases, net	88,670,371		86,533,507
Accrued interest receivable	311,414		306,496
Financial liabilities:
Noninterest-bearing deposits	(33,127,627)		(29,110,635)
Savings and interest-checking deposits	(53,065,030)		(49,566,644)
Time deposits	(11,721,005)		(13,110,392)
Deposits at Cayman Islands office	(223,183)		(170,170)
Short-term borrowings	(213,846)		(2,132,182)
Long-term borrowings	(10,211,160)		(10,653,858)
Accrued interest payable	(76,230)		(85,145)
Trading account liabilities	(292,713)		(160,745)
Other financial instruments:
Commitments to originate real estate loans for sale	29,800		9,879
Commitments to sell real estate loans	(11,375)		875
Other credit-related commitments	(124,499)		(122,334)
Interest rate swap agreements used for interest rate risk management	21,060		43,892
Estimate Fair Value [Member]
Financial assets:
Cash and cash equivalents	1,332,202		1,368,040
Interest-bearing deposits at banks	10,777,636		7,594,350
Trading account assets	488,588		273,783
Investment securities	14,779,721		15,660,877
Loans and leases:
Commercial loans and leases	21,563,190		20,146,201
Commercial real estate loans	31,944,341		29,044,244
Residential real estate loans	23,780,993		26,267,771
Consumer loans	12,009,873		11,550,270
Loans and leases, net	89,298,397		87,008,486
Accrued interest receivable	311,414		306,496
Financial liabilities:
Noninterest-bearing deposits	(33,127,627)		(29,110,635)
Savings and interest-checking deposits	(53,065,030)		(49,566,644)
Time deposits	(11,829,119)		(13,135,042)
Deposits at Cayman Islands office	(223,183)		(170,170)
Short-term borrowings	(213,846)		(2,132,182)
Long-term borrowings	(10,277,155)		(10,639,556)
Accrued interest payable	(76,230)		(85,145)
Trading account liabilities	(292,713)		(160,745)
Other financial instruments:
Commitments to originate real estate loans for sale	29,800		9,879
Commitments to sell real estate loans	(11,375)		875
Other credit-related commitments	(124,499)		(122,334)
Interest rate swap agreements used for interest rate risk management	$ 21,060		$ 43,892