Fair value measurements - Carrying Amounts and Estimated Fair Value for Financial Instrument Assets (Liabilities) (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Financial assets:
Cash and cash equivalents	$ 1,986,615	$ 1,452,397	$ 933,755	$ 1,246,342
Interest-bearing deposits at banks	129,945	154,960
Trading account assets	488,966	561,834
Investment securities	6,074,361	7,673,154
Loans and leases:
Residential real estate loans	11,161,323	7,889,569
Consumer loans	11,550,274	12,020,229
Allowance for credit losses	(925,860)	(908,290)
Loans and leases, net	65,645,097	59,187,715
Financial liabilities:
Noninterest-bearing deposits	(24,240,802)	(20,017,883)
Time deposits	(4,562,366)	(6,107,530)
Deposits at Cayman Islands office	(1,044,519)	(355,927)
Long-term borrowings	(4,607,758)	(6,686,226)
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Financial assets:
Cash and cash equivalents	1,895,423
Trading account assets	56,106
Investment securities	98,364
Significant Observable Inputs (Level 2) [Member]
Financial assets:
Cash and cash equivalents	91,192
Interest-bearing deposits at banks	129,945
Trading account assets	432,860
Investment securities	4,687,211
Loans and leases:
Commercial real estate loans	199,997
Residential real estate loans	8,100,915
Loans and leases, net	8,300,912
Accrued interest receivable	222,897
Financial liabilities:
Noninterest-bearing deposits	(24,240,802)
Savings deposits and NOW accounts	(35,763,566)
Time deposits	(4,584,384)
Deposits at Cayman Islands office	(1,044,519)
Short-term borrowings	(1,074,482)
Long-term borrowings	(4,768,408)
Accrued interest payable	(54,281)
Trading account liabilities	(374,274)
Other financial instruments:
Commitments to sell real estate loans	(7,299)
Interest rate swap agreements used for interest rate risk management	143,179
Significant Unobservable Inputs (Level 3) [Member]
Financial assets:
Investment securities	1,233,393
Loans and leases:
Commercial loans and leases	17,554,562
Commercial real estate loans	25,658,485
Residential real estate loans	3,280,404
Consumer loans	11,504,799
Loans and leases, net	57,998,250
Other financial instruments:
Commitments to originate real estate loans for sale	47,859
Other credit-related commitments	(119,464)
Carrying Amount [Member]
Financial assets:
Cash and cash equivalents	1,986,615	1,452,397
Interest-bearing deposits at banks	129,945	154,960
Trading account assets	488,966	561,834
Investment securities	6,074,361	7,673,154
Loans and leases:
Commercial loans and leases	17,776,953	15,734,436
Commercial real estate loans	25,993,790	24,411,114
Residential real estate loans	11,240,837	7,923,165
Consumer loans	11,559,377	12,027,290
Allowance for credit losses	(925,860)	(908,290)
Loans and leases, net	65,645,097	59,187,715
Accrued interest receivable	222,897	222,618
Financial liabilities:
Noninterest-bearing deposits	(24,240,802)	(20,017,883)
Savings deposits and NOW accounts	(35,763,566)	(32,913,309)
Time deposits	(4,562,366)	(6,107,530)
Deposits at Cayman Islands office	(1,044,519)	(355,927)
Short-term borrowings	(1,074,482)	(782,082)
Long-term borrowings	(4,607,758)	(6,686,226)
Accrued interest payable	(54,281)	(67,900)
Trading account liabilities	(374,274)	(434,559)
Other financial instruments:
Commitments to originate real estate loans for sale	47,859	6,923
Commitments to sell real estate loans	(7,299)	(3,498)
Other credit-related commitments	(119,464)	(109,828)
Interest rate swap agreements used for interest rate risk management	143,179	147,302
Calculated Estimate [Member]
Financial assets:
Cash and cash equivalents	1,986,615	1,452,397
Interest-bearing deposits at banks	129,945	154,960
Trading account assets	488,966	561,834
Investment securities	6,018,968	7,608,008
Loans and leases:
Commercial loans and leases	17,554,562	15,507,342
Commercial real estate loans	25,858,482	24,024,585
Residential real estate loans	11,381,319	7,782,935
Consumer loans	11,504,799	11,869,813
Loans and leases, net	66,299,162	59,184,675
Accrued interest receivable	222,897	222,618
Financial liabilities:
Noninterest-bearing deposits	(24,240,802)	(20,017,883)
Savings deposits and NOW accounts	(35,763,566)	(32,913,309)
Time deposits	(4,584,384)	(6,133,806)
Deposits at Cayman Islands office	(1,044,519)	(355,927)
Short-term borrowings	(1,074,482)	(782,082)
Long-term borrowings	(4,768,408)	(6,720,174)
Accrued interest payable	(54,281)	(67,900)
Trading account liabilities	(374,274)	(434,559)
Other financial instruments:
Commitments to originate real estate loans for sale	47,859	6,923
Commitments to sell real estate loans	(7,299)	(3,498)
Other credit-related commitments	(119,464)	(109,828)
Interest rate swap agreements used for interest rate risk management	$ 143,179	$ 147,302