Fair Value Measurements - Carrying Amounts and Estimated Fair Value for Financial Instrument Assets (Liabilities) (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Financial assets:
Cash and cash equivalents	$ 1,373,357	$ 1,672,934	$ 1,986,615	$ 1,452,397
Interest-bearing deposits at banks	6,470,867	1,651,138
Trading account assets	308,175	376,131
Investment securities	12,993,542	8,796,497
Loans and leases:
Residential real estate loans	8,636,794	8,911,554
Consumer loans	10,969,879	10,280,527
Allowance for credit losses	(919,562)	(916,676)	(925,860)	(908,290)
Loans and leases, net	65,749,394	63,156,483
Financial liabilities:
Noninterest-bearing deposits	(26,947,880)	(24,661,007)
Time deposits	(3,063,973)	(3,523,838)
Deposits at Cayman Islands office	(176,582)	(322,746)
Long-term borrowings	(9,006,959)	(5,108,870)
Trading account liabilities	(203,464)	(249,797)
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Financial assets:
Cash and cash equivalents	1,296,923	1,596,877
Trading account assets	51,416	51,386
Investment securities	64,841	82,450
Significant Observable Inputs (Level 2) [Member]
Financial assets:
Cash and cash equivalents	76,434	76,057
Interest-bearing deposits at banks	6,470,867	1,651,138
Trading account assets	256,759	324,745
Investment securities	12,750,396	8,384,106
Loans and leases:
Commercial real estate loans	307,667	67,505
Residential real estate loans	5,189,086	5,432,207
Loans and leases, net	5,496,753	5,499,712
Accrued interest receivable	227,348	222,558
Financial liabilities:
Noninterest-bearing deposits	(26,947,880)	(24,661,007)
Savings deposits and NOW accounts	(43,393,618)	(38,611,021)
Time deposits	(3,086,126)	(3,542,789)
Deposits at Cayman Islands office	(176,582)	(322,746)
Short-term borrowings	(192,676)	(260,455)
Long-term borrowings	(9,139,789)	(5,244,902)
Accrued interest payable	(63,372)	(43,419)
Trading account liabilities	(203,464)	(249,797)
Other financial instruments:
Commitments to sell real estate loans	(7,065)	12,360
Interest rate swap agreements used for interest rate risk management	73,251	102,875
Significant Unobservable Inputs (Level 3) [Member]
Financial assets:
Investment securities	208,719	223,938
Loans and leases:
Commercial loans and leases	19,188,574	18,457,288
Commercial real estate loans	27,180,151	25,950,690
Residential real estate loans	3,539,970	3,435,665
Consumer loans	10,909,623	10,201,087
Loans and leases, net	60,818,318	58,044,730
Other financial instruments:
Commitments to originate real estate loans for sale	17,347	3,941
Other credit-related commitments	(119,079)	(118,886)
Carrying Amount [Member]
Financial assets:
Cash and cash equivalents	1,373,357	1,672,934
Interest-bearing deposits at banks	6,470,867	1,651,138
Trading account assets	308,175	376,131
Investment securities	12,993,542	8,796,497
Loans and leases:
Commercial loans and leases	19,461,292	18,705,216
Commercial real estate loans	27,567,569	26,148,208
Residential real estate loans	8,657,301	8,928,221
Consumer loans	10,982,794	10,291,514
Allowance for credit losses	(919,562)	(916,676)
Loans and leases, net	65,749,394	63,156,483
Accrued interest receivable	227,348	222,558
Financial liabilities:
Noninterest-bearing deposits	(26,947,880)	(24,661,007)
Savings deposits and NOW accounts	(43,393,618)	(38,611,021)
Time deposits	(3,063,973)	(3,523,838)
Deposits at Cayman Islands office	(176,582)	(322,746)
Short-term borrowings	(192,676)	(260,455)
Long-term borrowings	(9,006,959)	(5,108,870)
Accrued interest payable	(63,372)	(43,419)
Trading account liabilities	(203,464)	(249,797)
Other financial instruments:
Commitments to originate real estate loans for sale	17,347	3,941
Commitments to sell real estate loans	(7,065)	12,360
Other credit-related commitments	(119,079)	(118,886)
Interest rate swap agreements used for interest rate risk management	73,251	102,875
Estimate Fair Value [Member]
Financial assets:
Cash and cash equivalents	1,373,357	1,672,934
Interest-bearing deposits at banks	6,470,867	1,651,138
Trading account assets	308,175	376,131
Investment securities	13,023,956	8,690,494
Loans and leases:
Commercial loans and leases	19,188,574	18,457,288
Commercial real estate loans	27,487,818	26,018,195
Residential real estate loans	8,729,056	8,867,872
Consumer loans	10,909,623	10,201,087
Loans and leases, net	66,315,071	63,544,442
Accrued interest receivable	227,348	222,558
Financial liabilities:
Noninterest-bearing deposits	(26,947,880)	(24,661,007)
Savings deposits and NOW accounts	(43,393,618)	(38,611,021)
Time deposits	(3,086,126)	(3,542,789)
Deposits at Cayman Islands office	(176,582)	(322,746)
Short-term borrowings	(192,676)	(260,455)
Long-term borrowings	(9,139,789)	(5,244,902)
Accrued interest payable	(63,372)	(43,419)
Trading account liabilities	(203,464)	(249,797)
Other financial instruments:
Commitments to originate real estate loans for sale	17,347	3,941
Commitments to sell real estate loans	(7,065)	12,360
Other credit-related commitments	(119,079)	(118,886)
Interest rate swap agreements used for interest rate risk management	$ 73,251	$ 102,875