Fair Value Measurements - Carrying Amounts and Estimated Fair Value for Financial Instrument Assets (Liabilities) (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Financial assets:
Cash and cash equivalents	$ 1,366,853	$ 1,373,357	$ 1,763,118	$ 1,672,934
Interest-bearing deposits at banks	6,291,491	6,470,867
Trading account assets	363,085	308,175
Loans and leases:
Allowance for credit losses	(921,373)	(919,562)	(916,768)	(916,676)
Loans and leases, net	66,177,669	65,749,394
Financial liabilities:
Noninterest-bearing deposits	(27,181,120)	(26,947,880)
Time deposits	(2,946,126)	(3,063,973)
Deposits at Cayman Islands office	(178,545)	(176,582)
Short-term borrowings	(193,495)	(192,676)
Long-term borrowings	(10,509,143)	(9,006,959)
Trading account liabilities	(240,168)	(203,464)
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Financial assets:
Cash and cash equivalents	1,311,917	1,296,923
Trading account assets	48,978	51,416
Investment securities	71,804	64,841
Significant Observable Inputs (Level 2) [Member]
Financial assets:
Cash and cash equivalents	54,936	76,434
Interest-bearing deposits at banks	6,291,491	6,470,867
Trading account assets	314,107	256,759
Investment securities	14,162,805	12,750,396
Loans and leases:
Commercial real estate loans	117,366	307,667
Residential real estate loans	5,119,739	5,189,086
Loans and leases, net	5,237,105	5,496,753
Accrued interest receivable	244,079	227,348
Financial liabilities:
Noninterest-bearing deposits	(27,181,120)	(26,947,880)
Savings deposits and NOW accounts	(43,288,329)	(43,393,618)
Time deposits	(2,967,329)	(3,086,126)
Deposits at Cayman Islands office	(178,545)	(176,582)
Short-term borrowings	(193,495)	(192,676)
Long-term borrowings	(10,641,367)	(9,139,789)
Accrued interest payable	(77,903)	(63,372)
Trading account liabilities	(240,168)	(203,464)
Other financial instruments:
Commitments to sell real estate loans	(9,848)	(7,065)
Interest rate swap agreements used for interest rate risk management	72,855	73,251
Significant Unobservable Inputs (Level 3) [Member]
Financial assets:
Investment securities	209,683	208,719
Loans and leases:
Commercial loans and leases	19,484,920	19,188,574
Commercial real estate loans	27,628,800	27,180,151
Residential real estate loans	3,489,509	3,539,970
Consumer loans	10,880,895	10,909,623
Loans and leases, net	61,484,124	60,818,318
Other financial instruments:
Commitments to originate real estate loans for sale	26,230	17,347
Other credit-related commitments	(112,511)	(119,079)
Carrying Amount [Member]
Financial assets:
Cash and cash equivalents	1,366,853	1,373,357
Interest-bearing deposits at banks	6,291,491	6,470,867
Trading account assets	363,085	308,175
Investment securities	14,393,270	12,993,542
Loans and leases:
Commercial loans and leases	19,775,494	19,461,292
Commercial real estate loans	27,845,710	27,567,569
Residential real estate loans	8,504,119	8,657,301
Consumer loans	10,973,719	10,982,794
Allowance for credit losses	(921,373)	(919,562)
Loans and leases, net	66,177,669	65,749,394
Accrued interest receivable	244,079	227,348
Financial liabilities:
Noninterest-bearing deposits	(27,181,120)	(26,947,880)
Savings deposits and NOW accounts	(43,288,329)	(43,393,618)
Time deposits	(2,946,126)	(3,063,973)
Deposits at Cayman Islands office	(178,545)	(176,582)
Short-term borrowings	(193,495)	(192,676)
Long-term borrowings	(10,509,143)	(9,006,959)
Accrued interest payable	(77,903)	(63,372)
Trading account liabilities	(240,168)	(203,464)
Other financial instruments:
Commitments to originate real estate loans for sale	26,230	17,347
Commitments to sell real estate loans	(9,848)	(7,065)
Other credit-related commitments	(112,511)	(119,079)
Interest rate swap agreements used for interest rate risk management	72,855	73,251
Estimate Fair Value [Member]
Financial assets:
Cash and cash equivalents	1,366,853	1,373,357
Interest-bearing deposits at banks	6,291,491	6,470,867
Trading account assets	363,085	308,175
Investment securities	14,444,292	13,023,956
Loans and leases:
Commercial loans and leases	19,484,920	19,188,574
Commercial real estate loans	27,746,166	27,487,818
Residential real estate loans	8,609,248	8,729,056
Consumer loans	10,880,895	10,909,623
Loans and leases, net	66,721,229	66,315,071
Accrued interest receivable	244,079	227,348
Financial liabilities:
Noninterest-bearing deposits	(27,181,120)	(26,947,880)
Savings deposits and NOW accounts	(43,288,329)	(43,393,618)
Time deposits	(2,967,329)	(3,086,126)
Deposits at Cayman Islands office	(178,545)	(176,582)
Short-term borrowings	(193,495)	(192,676)
Long-term borrowings	(10,641,367)	(9,139,789)
Accrued interest payable	(77,903)	(63,372)
Trading account liabilities	(240,168)	(203,464)
Other financial instruments:
Commitments to originate real estate loans for sale	26,230	17,347
Commitments to sell real estate loans	(9,848)	(7,065)
Other credit-related commitments	(112,511)	(119,079)
Interest rate swap agreements used for interest rate risk management	$ 72,855	$ 73,251