Fair Value Measurements - Carrying Amounts and Estimated Fair Value for Financial Instrument Assets (Liabilities) (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Financial assets:
Cash and cash equivalents	$ 1,320,549	$ 1,368,040	$ 1,373,357	$ 1,672,934
Interest-bearing deposits at banks	5,000,638	7,594,350
Trading account assets	323,867	273,783
Investment securities	16,250,468	15,656,439
Loans and leases:
Residential real estate loans	22,584,141	26,249,059
Consumer loans	12,130,094	11,584,347
Allowance for credit losses	(988,997)	(955,992)	$ (919,562)	$ (916,676)
Loans and leases, net	89,864,419	86,533,507
Financial liabilities:
Noninterest-bearing deposits	(32,813,896)	(29,110,635)
Savings and interest-checking deposits	(52,346,207)	(49,566,644)
Time deposits	(10,131,846)	(13,110,392)
Deposits at Cayman Islands office	(201,927)	(170,170)
Short-term borrowings	(163,442)	(150,546)
Long-term borrowings	(9,493,835)	(10,653,858)
Carrying Amount [Member]
Financial assets:
Cash and cash equivalents	1,320,549	1,368,040
Interest-bearing deposits at banks	5,000,638	7,594,350
Trading account assets	323,867	273,783
Investment securities	16,250,468	15,656,439
Loans and leases:
Commercial loans and leases	22,610,047	20,422,338
Commercial real estate loans	33,506,394	29,197,311
Residential real estate loans	22,590,912	26,270,103
Consumer loans	12,146,063	11,599,747
Allowance for credit losses	(988,997)	(955,992)
Loans and leases, net	89,864,419	86,533,507
Accrued interest receivable	308,805	306,496
Financial liabilities:
Noninterest-bearing deposits	(32,813,896)	(29,110,635)
Savings and interest-checking deposits	(52,346,207)	(49,566,644)
Time deposits	(10,131,846)	(13,110,392)
Deposits at Cayman Islands office	(201,927)	(170,170)
Short-term borrowings	(163,442)	(2,132,182)
Long-term borrowings	(9,493,835)	(10,653,858)
Accrued interest payable	(75,172)	(85,145)
Trading account liabilities	(174,376)	(160,745)
Other financial instruments:
Commitments to originate real estate loans for sale	7,325	9,879
Commitments to sell real estate loans	36,653	875
Other credit-related commitments	(136,295)	(122,334)
Interest rate swap agreements used for interest rate risk management	11,892	43,892
Estimate Fair Value [Member]
Financial assets:
Cash and cash equivalents	1,320,549	1,368,040
Interest-bearing deposits at banks	5,000,638	7,594,350
Trading account assets	323,867	273,783
Investment securities	16,244,412	15,660,877
Loans and leases:
Commercial loans and leases	22,239,428	20,146,201
Commercial real estate loans	33,129,428	29,044,244
Residential real estate loans	22,638,167	26,267,771
Consumer loans	12,061,590	11,550,270
Loans and leases, net	90,068,613	87,008,486
Accrued interest receivable	308,805	306,496
Financial liabilities:
Noninterest-bearing deposits	(32,813,896)	(29,110,635)
Savings and interest-checking deposits	(52,346,207)	(49,566,644)
Time deposits	(10,222,585)	(13,135,042)
Deposits at Cayman Islands office	(201,927)	(170,170)
Short-term borrowings	(163,442)	(2,132,182)
Long-term borrowings	(9,473,844)	(10,639,556)
Accrued interest payable	(75,172)	(85,145)
Trading account liabilities	(174,376)	(160,745)
Other financial instruments:
Commitments to originate real estate loans for sale	7,325	9,879
Commitments to sell real estate loans	36,653	875
Other credit-related commitments	(136,295)	(122,334)
Interest rate swap agreements used for interest rate risk management	11,892	43,892
Estimate Fair Value [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Financial assets:
Cash and cash equivalents	1,249,654	1,276,678
Trading account assets	46,135	56,763
Investment securities	301,711	65,178
Estimate Fair Value [Member] | Significant Observable Inputs (Level 2) [Member]
Financial assets:
Cash and cash equivalents	70,895	91,362
Interest-bearing deposits at banks	5,000,638	7,594,350
Trading account assets	277,732	217,020
Investment securities	15,821,176	15,406,404
Loans and leases:
Commercial real estate loans	642,590	38,774
Residential real estate loans	4,912,488	4,727,816
Loans and leases, net	5,555,078	4,766,590
Accrued interest receivable	308,805	306,496
Financial liabilities:
Noninterest-bearing deposits	(32,813,896)	(29,110,635)
Savings and interest-checking deposits	(52,346,207)	(49,566,644)
Time deposits	(10,222,585)	(13,135,042)
Deposits at Cayman Islands office	(201,927)	(170,170)
Short-term borrowings	(163,442)	(2,132,182)
Long-term borrowings	(9,473,844)	(10,639,556)
Accrued interest payable	(75,172)	(85,145)
Trading account liabilities	(174,376)	(160,745)
Other financial instruments:
Commitments to sell real estate loans	36,653	875
Interest rate swap agreements used for interest rate risk management	11,892	43,892
Estimate Fair Value [Member] | Significant Unobservable Inputs (Level 3) [Member]
Financial assets:
Investment securities	121,525	189,295
Loans and leases:
Commercial loans and leases	22,239,428	20,146,201
Commercial real estate loans	32,486,838	29,005,470
Residential real estate loans	17,725,679	21,539,955
Consumer loans	12,061,590	11,550,270
Loans and leases, net	84,513,535	82,241,896
Other financial instruments:
Commitments to originate real estate loans for sale	7,325	9,879
Other credit-related commitments	$ (136,295)	$ (122,334)