Fair Value Measurements - Carrying Amounts and Estimated Fair Value for Financial Instrument Assets (Liabilities) (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
Financial assets:
Cash and cash equivalents	$ 2,059,753	$ 1,986,615	$ 1,622,928	$ 1,452,397
Interest-bearing deposits at banks	1,925,811	129,945
Trading account assets	371,370	488,966
Loans and leases:
Allowance for credit losses	(916,370)	(925,860)
Loans and leases, net	62,742,672	65,645,097
Financial liabilities:
Noninterest-bearing deposits	(24,150,771)	(24,240,802)
Time deposits	(3,831,443)	(4,562,366)
Deposits at Cayman Islands office	(316,510)	(1,044,519)
Long-term borrowings	(5,121,326)	(4,607,758)
Carrying Amount [Member]
Financial assets:
Cash and cash equivalents	2,059,753	1,986,615
Interest-bearing deposits at banks	1,925,811	129,945
Trading account assets	371,370	488,966
Investment securities	8,309,773	6,074,361
Loans and leases:
Commercial loans and leases	17,911,149	17,776,953
Commercial real estate loans	26,345,267	25,993,790
Residential real estate loans	9,228,003	11,240,837
Consumer loans	10,174,623	11,559,377
Allowance for credit losses	(916,370)	(925,860)
Loans and leases, net	62,742,672	65,645,097
Accrued interest receivable	234,208	222,897
Financial liabilities:
Noninterest-bearing deposits	(24,150,771)	(24,240,802)
Savings deposits and NOW accounts	(38,253,417)	(35,763,566)
Time deposits	(3,831,443)	(4,562,366)
Deposits at Cayman Islands office	(316,510)	(1,044,519)
Short-term borrowings	(246,019)	(1,074,482)
Long-term borrowings	(5,121,326)	(4,607,758)
Accrued interest payable	(62,773)	(54,281)
Trading account liabilities	(261,449)	(374,274)
Other financial instruments:
Commitments to originate real estate loans for sale	13,075	47,859
Commitments to sell real estate loans	(9,743)	(7,299)
Other credit-related commitments	(114,048)	(119,464)
Interest rate swap agreements used for interest rate risk management	114,083	143,179
Estimate Fair Value [Member]
Financial assets:
Cash and cash equivalents	2,059,753	1,986,615
Interest-bearing deposits at banks	1,925,811	129,945
Trading account assets	371,370	488,966
Investment securities	8,268,045	6,018,968
Loans and leases:
Commercial loans and leases	17,685,646	17,554,562
Commercial real estate loans	26,256,932	25,858,482
Residential real estate loans	9,198,681	11,381,319
Consumer loans	10,151,004	11,504,799
Allowance for credit losses
Loans and leases, net	63,292,263	66,299,162
Accrued interest receivable	234,208	222,897
Financial liabilities:
Noninterest-bearing deposits	(24,150,771)	(24,240,802)
Savings deposits and NOW accounts	(38,253,417)	(35,763,566)
Time deposits	(3,853,552)	(4,584,384)
Deposits at Cayman Islands office	(316,510)	(1,044,519)
Short-term borrowings	(246,019)	(1,074,482)
Long-term borrowings	(5,257,947)	(4,768,408)
Accrued interest payable	(62,773)	(54,281)
Trading account liabilities	(261,449)	(374,274)
Other financial instruments:
Commitments to originate real estate loans for sale	13,075	47,859
Commitments to sell real estate loans	(9,743)	(7,299)
Other credit-related commitments	(114,048)	(119,464)
Interest rate swap agreements used for interest rate risk management	114,083	143,179
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Financial assets:
Cash and cash equivalents	1,992,949	1,895,423
Interest-bearing deposits at banks
Trading account assets	48,344	56,106
Investment securities	91,075	98,364
Loans and leases:
Commercial loans and leases
Commercial real estate loans
Residential real estate loans
Consumer loans
Allowance for credit losses
Loans and leases, net
Accrued interest receivable
Financial liabilities:
Noninterest-bearing deposits
Savings deposits and NOW accounts
Time deposits
Deposits at Cayman Islands office
Short-term borrowings
Long-term borrowings
Accrued interest payable
Trading account liabilities
Other financial instruments:
Commitments to originate real estate loans for sale
Commitments to sell real estate loans
Other credit-related commitments
Interest rate swap agreements used for interest rate risk management
Significant Observable Inputs (Level 2) [Member]
Financial assets:
Cash and cash equivalents	66,804	91,192
Interest-bearing deposits at banks	1,925,811	129,945
Trading account assets	323,026	432,860
Investment securities	7,945,975	4,687,211
Loans and leases:
Commercial loans and leases
Commercial real estate loans	151,690	199,997
Residential real estate loans	5,757,491	8,100,915
Consumer loans
Allowance for credit losses
Loans and leases, net	5,909,181	8,300,912
Accrued interest receivable	234,208	222,897
Financial liabilities:
Noninterest-bearing deposits	(24,150,771)	(24,240,802)
Savings deposits and NOW accounts	(38,253,417)	(35,763,566)
Time deposits	(3,853,552)	(4,584,384)
Deposits at Cayman Islands office	(316,510)	(1,044,519)
Short-term borrowings	(246,019)	(1,074,482)
Long-term borrowings	(5,257,947)	(4,768,408)
Accrued interest payable	(62,773)	(54,281)
Trading account liabilities	(261,449)	(374,274)
Other financial instruments:
Commitments to originate real estate loans for sale
Commitments to sell real estate loans	(9,743)	(7,299)
Other credit-related commitments
Interest rate swap agreements used for interest rate risk management	114,083	143,179
Significant Unobservable Inputs (Level 3) [Member]
Financial assets:
Cash and cash equivalents
Interest-bearing deposits at banks
Trading account assets
Investment securities	230,995	1,233,393
Loans and leases:
Commercial loans and leases	17,685,646	17,554,562
Commercial real estate loans	26,105,242	25,658,485
Residential real estate loans	3,441,190	3,280,404
Consumer loans	10,151,004	11,504,799
Allowance for credit losses
Loans and leases, net	57,383,082	57,998,250
Accrued interest receivable
Financial liabilities:
Noninterest-bearing deposits
Savings deposits and NOW accounts
Time deposits
Deposits at Cayman Islands office
Short-term borrowings
Long-term borrowings
Accrued interest payable
Trading account liabilities
Other financial instruments:
Commitments to originate real estate loans for sale	13,075	47,859
Commitments to sell real estate loans
Other credit-related commitments	(114,048)	(119,464)
Interest rate swap agreements used for interest rate risk management