Fair value measurements - Carrying Amounts and Estimated Fair Value for Financial Instrument Assets (Liabilities) (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Financial assets:
Cash and cash equivalents	$ 1,826,067	$ 1,986,615	$ 1,344,092	$ 1,452,397
Interest-bearing deposits at banks	1,304,770	129,945
Trading account assets	420,144	488,966
Investment securities	5,660,831	6,074,361
Loans and leases:
Allowance for credit losses	(927,117)	(925,860)
Loans and leases, net	64,997,150	65,645,097
Financial liabilities:
Noninterest-bearing deposits	(23,603,971)	(24,240,802)
Time deposits	(4,304,033)	(4,562,366)
Deposits at Cayman Islands office	(266,076)	(1,044,519)
Long-term borrowings	(5,394,563)	(4,607,758)
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Financial assets:
Cash and cash equivalents	1,764,954	1,895,423
Trading account assets	46,567	56,106
Investment securities	112,606	98,364
Significant Observable Inputs (Level 2) [Member]
Financial assets:
Cash and cash equivalents	61,113	91,192
Interest-bearing deposits at banks	1,304,770	129,945
Trading account assets	373,577	432,860
Investment securities	4,289,603	4,687,211
Loans and leases:
Commercial real estate loans	79,400	199,997
Residential real estate loans	7,861,298	8,100,915
Loans and leases, net	7,940,698	8,300,912
Accrued interest receivable	243,547	222,897
Financial liabilities:
Noninterest-bearing deposits	(23,603,971)	(24,240,802)
Savings deposits and NOW accounts	(36,915,646)	(35,763,566)
Time deposits	(4,326,064)	(4,584,384)
Deposits at Cayman Islands office	(266,076)	(1,044,519)
Short-term borrowings	(374,593)	(1,074,482)
Long-term borrowings	(5,571,132)	(4,768,408)
Accrued interest payable	(72,059)	(54,281)
Trading account liabilities	(327,608)	(374,274)
Other financial instruments:
Commitments to sell real estate loans	(4,568)	(7,299)
Interest rate swap agreements used for interest rate risk management	134,306	143,179
Significant Unobservable Inputs (Level 3) [Member]
Financial assets:
Investment securities	1,210,206	1,233,393
Loans and leases:
Commercial loans and leases	17,165,948	17,554,562
Commercial real estate loans	25,709,104	25,658,485
Residential real estate loans	3,372,086	3,280,404
Consumer loans	11,321,522	11,504,799
Loans and leases, net	57,568,660	57,998,250
Other financial instruments:
Commitments to originate real estate loans for sale	36,119	47,859
Other credit-related commitments	(115,024)	(119,464)
Carrying Amount [Member]
Financial assets:
Cash and cash equivalents	1,826,067	1,986,615
Interest-bearing deposits at banks	1,304,770	129,945
Trading account assets	420,144	488,966
Investment securities	5,660,831	6,074,361
Loans and leases:
Commercial loans and leases	17,469,138	17,776,953
Commercial real estate loans	25,944,819	25,993,790
Residential real estate loans	11,094,577	11,240,837
Consumer loans	11,415,733	11,559,377
Allowance for credit losses	(927,117)	(925,860)
Loans and leases, net	64,997,150	65,645,097
Accrued interest receivable	243,547	222,897
Financial liabilities:
Noninterest-bearing deposits	(23,603,971)	(24,240,802)
Savings deposits and NOW accounts	(36,915,646)	(35,763,566)
Time deposits	(4,304,033)	(4,562,366)
Deposits at Cayman Islands office	(266,076)	(1,044,519)
Short-term borrowings	(374,593)	(1,074,482)
Long-term borrowings	(5,394,563)	(4,607,758)
Accrued interest payable	(72,059)	(54,281)
Trading account liabilities	(327,608)	(374,274)
Other financial instruments:
Commitments to originate real estate loans for sale	36,119	47,859
Commitments to sell real estate loans	(4,568)	(7,299)
Other credit-related commitments	(115,024)	(119,464)
Interest rate swap agreements used for interest rate risk management	134,306	143,179
Estimate Fair Value [Member]
Financial assets:
Cash and cash equivalents	1,826,067	1,986,615
Interest-bearing deposits at banks	1,304,770	129,945
Trading account assets	420,144	488,966
Investment securities	5,612,415	6,018,968
Loans and leases:
Commercial loans and leases	17,165,948	17,554,562
Commercial real estate loans	25,788,504	25,858,482
Residential real estate loans	11,233,384	11,381,319
Consumer loans	11,321,522	11,504,799
Loans and leases, net	65,509,358	66,299,162
Accrued interest receivable	243,547	222,897
Financial liabilities:
Noninterest-bearing deposits	(23,603,971)	(24,240,802)
Savings deposits and NOW accounts	(36,915,646)	(35,763,566)
Time deposits	(4,326,064)	(4,584,384)
Deposits at Cayman Islands office	(266,076)	(1,044,519)
Short-term borrowings	(374,593)	(1,074,482)
Long-term borrowings	(5,571,132)	(4,768,408)
Accrued interest payable	(72,059)	(54,281)
Trading account liabilities	(327,608)	(374,274)
Other financial instruments:
Commitments to originate real estate loans for sale	36,119	47,859
Commitments to sell real estate loans	(4,568)	(7,299)
Other credit-related commitments	(115,024)	(119,464)
Interest rate swap agreements used for interest rate risk management	$ 134,306	$ 143,179