Fair Value Measurements - Carrying Amounts and Estimated Fair Value for Financial Instrument Assets (Liabilities) (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Financial assets:
Cash and cash equivalents	$ 1,249,704		$ 1,373,357	$ 1,523,643		$ 1,672,934
Interest-bearing deposits at banks	4,713,266		6,470,867
Trading account assets	340,710		308,175
Investment securities	14,494,539		12,993,542
Loans and leases:
Allowance for credit losses	(933,798)	$ (929,987)	(919,562)	$ (918,633)	$ (917,666)	$ (916,676)
Loans and leases, net	67,606,450		65,749,394
Financial liabilities:
Noninterest-bearing deposits	(28,189,330)		(26,947,880)
Time deposits	(2,791,367)		(3,063,973)
Deposits at Cayman Islands office	(206,185)		(176,582)
Short-term borrowings	(173,783)		(192,676)
Long-term borrowings	(10,174,289)		(9,006,959)
Trading account liabilities	(233,006)		(203,464)
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Financial assets:
Cash and cash equivalents	1,193,831		1,296,923
Trading account assets	48,006		51,416
Investment securities	40,370		64,841
Significant Observable Inputs (Level 2) [Member]
Financial assets:
Cash and cash equivalents	55,873		76,434
Interest-bearing deposits at banks	4,713,266		6,470,867
Trading account assets	292,704		256,759
Investment securities	14,283,075		12,750,396
Loans and leases:
Commercial real estate loans	71,357		307,667
Residential real estate loans	4,918,613		5,189,086
Loans and leases, net	4,989,970		5,496,753
Accrued interest receivable	242,935		227,348
Financial liabilities:
Noninterest-bearing deposits	(28,189,330)		(26,947,880)
Savings deposits and NOW accounts	(41,757,661)		(43,393,618)
Time deposits	(2,810,224)		(3,086,126)
Deposits at Cayman Islands office	(206,185)		(176,582)
Short-term borrowings	(173,783)		(192,676)
Long-term borrowings	(10,219,180)		(9,139,789)
Accrued interest payable	(73,475)		(63,372)
Trading account liabilities	(233,006)		(203,464)
Other financial instruments:
Commitments to sell real estate loans	(8,379)		(7,065)
Interest rate swap agreements used for interest rate risk management	60,782		73,251
Significant Unobservable Inputs (Level 3) [Member]
Financial assets:
Investment securities	198,295		208,719
Loans and leases:
Commercial loans and leases	19,920,031		19,188,574
Commercial real estate loans	28,562,616		27,180,151
Residential real estate loans	3,384,017		3,539,970
Consumer loans	11,280,973		10,909,623
Loans and leases, net	63,147,637		60,818,318
Other financial instruments:
Commitments to originate real estate loans for sale	17,647		17,347
Other credit-related commitments	(118,656)		(119,079)
Carrying Amount [Member]
Financial assets:
Cash and cash equivalents	1,249,704		1,373,357
Interest-bearing deposits at banks	4,713,266		6,470,867
Trading account assets	340,710		308,175
Investment securities	14,494,539		12,993,542
Loans and leases:
Commercial loans and leases	20,233,177		19,461,292
Commercial real estate loans	28,720,537		27,567,569
Residential real estate loans	8,211,062		8,657,301
Consumer loans	11,375,472		10,982,794
Allowance for credit losses	(933,798)		(919,562)
Loans and leases, net	67,606,450		65,749,394
Accrued interest receivable	242,935		227,348
Financial liabilities:
Noninterest-bearing deposits	(28,189,330)		(26,947,880)
Savings deposits and NOW accounts	(41,757,661)		(43,393,618)
Time deposits	(2,791,367)		(3,063,973)
Deposits at Cayman Islands office	(206,185)		(176,582)
Short-term borrowings	(173,783)		(192,676)
Long-term borrowings	(10,174,289)		(9,006,959)
Accrued interest payable	(73,475)		(63,372)
Trading account liabilities	(233,006)		(203,464)
Other financial instruments:
Commitments to originate real estate loans for sale	17,647		17,347
Commitments to sell real estate loans	(8,379)		(7,065)
Other credit-related commitments	(118,656)		(119,079)
Interest rate swap agreements used for interest rate risk management	60,782		73,251
Estimate Fair Value [Member]
Financial assets:
Cash and cash equivalents	1,249,704		1,373,357
Interest-bearing deposits at banks	4,713,266		6,470,867
Trading account assets	340,710		308,175
Investment securities	14,521,740		13,023,956
Loans and leases:
Commercial loans and leases	19,920,031		19,188,574
Commercial real estate loans	28,633,973		27,487,818
Residential real estate loans	8,302,630		8,729,056
Consumer loans	11,280,973		10,909,623
Loans and leases, net	68,137,607		66,315,071
Accrued interest receivable	242,935		227,348
Financial liabilities:
Noninterest-bearing deposits	(28,189,330)		(26,947,880)
Savings deposits and NOW accounts	(41,757,661)		(43,393,618)
Time deposits	(2,810,224)		(3,086,126)
Deposits at Cayman Islands office	(206,185)		(176,582)
Short-term borrowings	(173,783)		(192,676)
Long-term borrowings	(10,219,180)		(9,139,789)
Accrued interest payable	(73,475)		(63,372)
Trading account liabilities	(233,006)		(203,464)
Other financial instruments:
Commitments to originate real estate loans for sale	17,647		17,347
Commitments to sell real estate loans	(8,379)		(7,065)
Other credit-related commitments	(118,656)		(119,079)
Interest rate swap agreements used for interest rate risk management	$ 60,782		$ 73,251