Fair Value Measurements - Carrying Amounts and Estimated Fair Value for Financial Instrument Assets (Liabilities) (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Financial assets:
Cash and cash equivalents	$ 1,286,962	$ 1,320,549	$ 1,178,175	$ 1,368,040
Interest-bearing deposits at banks	6,945,149	5,000,638
Trading account	174,854	323,867
Investment securities	15,968,415	16,250,468
Loans and leases:
Allowance for credit losses	(1,001,430)	(988,997)	$ (962,752)	$ (955,992)
Loans and leases, net	88,311,572	89,864,419
Financial liabilities:
Noninterest-bearing deposits	(34,279,591)	(32,813,896)
Savings and interest-checking deposits	(53,542,149)	(52,346,207)
Time deposits	(9,028,018)	(10,131,846)
Deposits at Cayman Islands office	(192,763)	(201,927)
Short-term borrowings	(185,102)	(163,442)
Long-term borrowings	(8,087,619)	(9,493,835)
Carrying Amount [Member]
Financial assets:
Cash and cash equivalents	1,286,962	1,320,549
Interest-bearing deposits at banks	6,945,149	5,000,638
Trading account	174,854	323,867
Investment securities	15,968,415	16,250,468
Loans and leases:
Commercial loans and leases	22,295,376	22,610,047
Commercial real estate loans	33,071,654	33,506,394
Residential real estate loans	21,724,491	22,590,912
Consumer loans	12,221,481	12,146,063
Allowance for credit losses	(1,001,430)	(988,997)
Loans and leases, net	88,311,572	89,864,419
Accrued interest receivable	318,152	308,805
Financial liabilities:
Noninterest-bearing deposits	(34,279,591)	(32,813,896)
Savings and interest-checking deposits	(53,542,149)	(52,346,207)
Time deposits	(9,028,018)	(10,131,846)
Deposits at Cayman Islands office	(192,763)	(201,927)
Short-term borrowings	(185,102)	(163,442)
Long-term borrowings	(8,087,619)	(9,493,835)
Accrued interest payable	(60,737)	(75,172)
Trading account	(138,695)	(174,376)
Other financial instruments:
Commitments to originate real estate loans for sale	16,202	7,325
Commitments to sell real estate loans	1,048	36,653
Other credit-related commitments	(129,968)	(136,295)
Interest rate swap agreements used for interest rate risk management	7,773	11,892
Estimate Fair Value [Member]
Financial assets:
Cash and cash equivalents	1,286,962	1,320,549
Interest-bearing deposits at banks	6,945,149	5,000,638
Trading account	174,854	323,867
Investment securities	15,958,181	16,244,412
Loans and leases:
Commercial loans and leases	21,882,456	22,239,428
Commercial real estate loans	32,607,794	33,129,428
Residential real estate loans	21,776,908	22,638,167
Consumer loans	12,083,513	12,061,590
Loans and leases, net	88,350,671	90,068,613
Accrued interest receivable	318,152	308,805
Financial liabilities:
Noninterest-bearing deposits	(34,279,591)	(32,813,896)
Savings and interest-checking deposits	(53,542,149)	(52,346,207)
Time deposits	(9,106,654)	(10,222,585)
Deposits at Cayman Islands office	(192,763)	(201,927)
Short-term borrowings	(185,102)	(163,442)
Long-term borrowings	(8,104,055)	(9,473,844)
Accrued interest payable	(60,737)	(75,172)
Trading account	(138,695)	(174,376)
Other financial instruments:
Commitments to originate real estate loans for sale	16,202	7,325
Commitments to sell real estate loans	1,048	36,653
Other credit-related commitments	(129,968)	(136,295)
Interest rate swap agreements used for interest rate risk management	7,773	11,892
Estimate Fair Value [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Financial assets:
Cash and cash equivalents	1,213,120	1,249,654
Trading account	46,137	46,135
Investment securities	63,698	301,711
Estimate Fair Value [Member] | Significant Observable Inputs (Level 2) [Member]
Financial assets:
Cash and cash equivalents	73,842	70,895
Interest-bearing deposits at banks	6,945,149	5,000,638
Trading account	128,717	277,732
Investment securities	15,775,917	15,821,176
Loans and leases:
Commercial real estate loans	74,563	642,590
Residential real estate loans	4,686,339	4,912,488
Loans and leases, net	4,760,902	5,555,078
Accrued interest receivable	318,152	308,805
Financial liabilities:
Noninterest-bearing deposits	(34,279,591)	(32,813,896)
Savings and interest-checking deposits	(53,542,149)	(52,346,207)
Time deposits	(9,106,654)	(10,222,585)
Deposits at Cayman Islands office	(192,763)	(201,927)
Short-term borrowings	(185,102)	(163,442)
Long-term borrowings	(8,104,055)	(9,473,844)
Accrued interest payable	(60,737)	(75,172)
Trading account	(138,695)	(174,376)
Other financial instruments:
Commitments to sell real estate loans	1,048	36,653
Interest rate swap agreements used for interest rate risk management	7,773	11,892
Estimate Fair Value [Member] | Significant Unobservable Inputs (Level 3) [Member]
Financial assets:
Investment securities	118,566	121,525
Loans and leases:
Commercial loans and leases	21,882,456	22,239,428
Commercial real estate loans	32,533,231	32,486,838
Residential real estate loans	17,090,569	17,725,679
Consumer loans	12,083,513	12,061,590
Loans and leases, net	83,589,769	84,513,535
Other financial instruments:
Commitments to originate real estate loans for sale	16,202	7,325
Other credit-related commitments	$ (129,968)	$ (136,295)