Fair value measurements - Carrying Amounts and Estimated Fair Value for Financial Instrument Assets (Liabilities) (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2011	Dec. 31, 2010
Financial assets:
Cash and cash equivalents	$ 1,622,928	$ 1,452,397	$ 1,354,057	$ 933,755
Interest-bearing deposits at banks	411,994	154,960
Trading account assets	526,844	561,834
Investment securities	6,624,004	7,673,154
Loans and leases:
Allowance for credit losses	(921,223)	(908,290)
Loans and leases, net	63,190,732	59,187,715
Financial liabilities:
Noninterest-bearing deposits	(22,968,401)	(20,017,883)
Time deposits	(4,972,409)	(6,107,530)
Deposits at Cayman Islands office	(1,402,753)	(355,927)
Long-term borrowings	(4,969,536)	(6,686,226)
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Financial assets:
Cash and cash equivalents	1,548,290
Trading account assets	55,391
Investment securities	106,789
Significant Observable Inputs (Level 2) [Member]
Financial assets:
Cash and cash equivalents	74,638
Interest-bearing deposits at banks	411,994
Trading account assets	471,453
Investment securities	5,192,015
Loans and leases:
Commercial real estate loans	175,518
Residential real estate loans	7,727,137
Loans and leases, net	7,902,655
Accrued interest receivable	244,021
Financial liabilities:
Noninterest-bearing deposits	(22,968,401)
Savings deposits and NOW accounts	(34,663,695)
Time deposits	(4,999,049)
Deposits at Cayman Islands office	(1,402,753)
Short-term borrowings	(592,154)
Long-term borrowings	(5,090,619)
Accrued interest payable	(78,591)
Trading account liabilities	(424,312)
Other financial instruments:
Commitments to sell real estate loans	(36,809)
Interest rate swap agreements used for interest rate risk management	153,220
Significant Unobservable Inputs (Level 3) [Member]
Financial assets:
Investment securities	1,278,646
Loans and leases:
Commercial loans and leases	16,482,866
Commercial real estate loans	24,527,925
Residential real estate loans	3,265,181
Consumer loans	11,512,448
Loans and leases, net	55,788,420
Other financial instruments:
Commitments to originate real estate loans for sale	70,345
Other credit-related commitments	(112,713)
Carrying amount [Member]
Financial assets:
Cash and cash equivalents	1,622,928	1,452,397
Interest-bearing deposits at banks	411,994	154,960
Trading account assets	526,844	561,834
Investment securities	6,624,004	7,673,154
Loans and leases:
Commercial loans and leases	16,704,575	15,734,436
Commercial real estate loans	24,970,416	24,411,114
Residential real estate loans	10,808,220	7,923,165
Consumer loans	11,628,744	12,027,290
Allowance for credit losses	(921,223)	(908,290)
Loans and leases, net	63,190,732	59,187,715
Accrued interest receivable	244,021	222,618
Financial liabilities:
Noninterest-bearing deposits	(22,968,401)	(20,017,883)
Savings deposits and NOW accounts	(34,663,695)	(32,913,309)
Time deposits	(4,972,409)	(6,107,530)
Deposits at Cayman Islands office	(1,402,753)	(355,927)
Short-term borrowings	(592,154)	(782,082)
Long-term borrowings	(4,969,536)	(6,686,226)
Accrued interest payable	(78,591)	(67,900)
Trading account liabilities	(424,312)	(434,559)
Other financial instruments:
Commitments to originate real estate loans for sale	70,345	6,923
Commitments to sell real estate loans	(36,809)	(3,498)
Other credit-related commitments	(112,713)	(109,828)
Interest rate swap agreements used for interest rate risk management	153,220	147,302
Estimate fair value [Member]
Financial assets:
Cash and cash equivalents	1,622,928	1,452,397
Interest-bearing deposits at banks	411,994	154,960
Trading account assets	526,844	561,834
Investment securities	6,577,450	7,608,008
Loans and leases:
Commercial loans and leases	16,482,866	15,507,342
Commercial real estate loans	24,703,443	24,024,585
Residential real estate loans	10,992,318	7,782,935
Consumer loans	11,512,448	11,869,813
Loans and leases, net	63,691,075	59,184,675
Accrued interest receivable	244,021	222,618
Financial liabilities:
Noninterest-bearing deposits	(22,968,401)	(20,017,883)
Savings deposits and NOW accounts	(34,663,695)	(32,913,309)
Time deposits	(4,999,049)	(6,133,806)
Deposits at Cayman Islands office	(1,402,753)	(355,927)
Short-term borrowings	(592,154)	(782,082)
Long-term borrowings	(5,090,619)	(6,720,174)
Accrued interest payable	(78,591)	(67,900)
Trading account liabilities	(424,312)	(434,559)
Other financial instruments:
Commitments to originate real estate loans for sale	70,345	6,923
Commitments to sell real estate loans	(36,809)	(3,498)
Other credit-related commitments	(112,713)	(109,828)
Interest rate swap agreements used for interest rate risk management	$ 153,220	$ 147,302