Fair Value Measurements - Carrying Amounts and Estimated Fair Value for Financial Instrument Assets (Liabilities) (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
Financial assets:
Cash and cash equivalents	$ 1,474,502	$ 1,986,615	$ 1,422,831	$ 1,452,397
Interest-bearing deposits at banks	2,555,354	129,945
Trading account assets	378,235	488,966
Investment securities	5,210,526	6,074,361
Loans and leases:
Allowance for credit losses	(927,065)	(925,860)
Loans and leases, net	65,044,801	65,645,097
Financial liabilities:
Noninterest-bearing deposits	(24,074,815)	(24,240,802)
Time deposits	(4,054,524)	(4,562,366)
Deposits at Cayman Islands office	(284,443)	(1,044,519)
Long-term borrowings	(5,122,398)	(4,607,758)
Carrying Amount [Member]
Financial assets:
Cash and cash equivalents	1,474,502	1,986,615
Interest-bearing deposits at banks	2,555,354	129,945
Trading account assets	378,235	488,966
Investment securities	5,210,526	6,074,361
Loans and leases:
Commercial loans and leases	18,021,812	17,776,953
Commercial real estate loans	26,116,394	25,993,790
Residential real estate loans	10,399,749	11,240,837
Consumer loans	11,433,911	11,559,377
Allowance for credit losses	(927,065)	(925,860)
Loans and leases, net	65,044,801	65,645,097
Accrued interest receivable	222,705	222,897
Financial liabilities:
Noninterest-bearing deposits	(24,074,815)	(24,240,802)
Savings deposits and NOW accounts	(37,247,688)	(35,763,566)
Time deposits	(4,054,524)	(4,562,366)
Deposits at Cayman Islands office	(284,443)	(1,044,519)
Short-term borrowings	(307,740)	(1,074,482)
Long-term borrowings	(5,122,398)	(4,607,758)
Accrued interest payable	(52,958)	(54,281)
Trading account liabilities	(277,904)	(374,274)
Other financial instruments:
Commitments to originate real estate loans for sale	7,408	47,859
Commitments to sell real estate loans	76,139	(7,299)
Other credit-related commitments	(118,338)	(119,464)
Interest rate swap agreements used for interest rate risk management	114,168	143,179
Estimate Fair Value [Member]
Financial assets:
Cash and cash equivalents	1,474,502	1,986,615
Interest-bearing deposits at banks	2,555,354	129,945
Trading account assets	378,235	488,966
Investment securities	5,128,413	6,018,968
Loans and leases:
Commercial loans and leases	17,763,265	17,554,562
Commercial real estate loans	26,107,812	25,858,482
Residential real estate loans	10,328,995	11,381,319
Consumer loans	11,373,978	11,504,799
Loans and leases, net	65,574,050	66,299,162
Accrued interest receivable	222,705	222,897
Financial liabilities:
Noninterest-bearing deposits	(24,074,815)	(24,240,802)
Savings deposits and NOW accounts	(37,247,688)	(35,763,566)
Time deposits	(4,071,757)	(4,584,384)
Deposits at Cayman Islands office	(284,443)	(1,044,519)
Short-term borrowings	(307,740)	(1,074,482)
Long-term borrowings	(5,260,847)	(4,768,408)
Accrued interest payable	(52,958)	(54,281)
Trading account liabilities	(277,904)	(374,274)
Other financial instruments:
Commitments to originate real estate loans for sale	7,408	47,859
Commitments to sell real estate loans	76,139	(7,299)
Other credit-related commitments	(118,338)	(119,464)
Interest rate swap agreements used for interest rate risk management	114,168	143,179
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Financial assets:
Cash and cash equivalents	1,412,924	1,895,423
Trading account assets	48,001	56,106
Investment securities	121,254	98,364
Significant Observable Inputs (Level 2) [Member]
Financial assets:
Cash and cash equivalents	61,578	91,192
Interest-bearing deposits at banks	2,555,354	129,945
Trading account assets	330,234	432,860
Investment securities	4,779,417	4,687,211
Loans and leases:
Commercial real estate loans	132,729	199,997
Residential real estate loans	6,980,989	8,100,915
Loans and leases, net	7,113,718	8,300,912
Accrued interest receivable	222,705	222,897
Financial liabilities:
Noninterest-bearing deposits	(24,074,815)	(24,240,802)
Savings deposits and NOW accounts	(37,247,688)	(35,763,566)
Time deposits	(4,071,757)	(4,584,384)
Deposits at Cayman Islands office	(284,443)	(1,044,519)
Short-term borrowings	(307,740)	(1,074,482)
Long-term borrowings	(5,260,847)	(4,768,408)
Accrued interest payable	(52,958)	(54,281)
Trading account liabilities	(277,904)	(374,274)
Other financial instruments:
Commitments to sell real estate loans	76,139	(7,299)
Interest rate swap agreements used for interest rate risk management	114,168	143,179
Significant Unobservable Inputs (Level 3) [Member]
Financial assets:
Investment securities	227,742	1,233,393
Loans and leases:
Commercial loans and leases	17,763,265	17,554,562
Commercial real estate loans	25,975,083	25,658,485
Residential real estate loans	3,348,006	3,280,404
Consumer loans	11,373,978	11,504,799
Loans and leases, net	58,460,332	57,998,250
Other financial instruments:
Commitments to originate real estate loans for sale	7,408	47,859
Other credit-related commitments	$ (118,338)	$ (119,464)