Fair Value Measurements - Carrying Amounts and Estimated Fair Value for Financial Instrument Assets (Liabilities) (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015
Financial assets:
Cash and cash equivalents	$ 1,345,478		$ 1,320,549	$ 1,284,442		$ 1,368,040
Interest-bearing deposits at banks	5,023,829		5,000,638
Trading Account	174,646		323,867
Investment securities	15,816,060		16,250,468
Loans and leases:
Allowance for credit losses	(1,008,225)	$ (1,001,430)	(988,997)	$ (970,496)	$ (962,752)	$ (955,992)
Loans and leases, net	88,072,330		89,864,419
Financial liabilities:
Noninterest-bearing deposits	(32,366,426)		(32,813,896)
Savings and interest-checking deposits	(52,871,146)		(52,346,207)
Time deposits	(8,107,749)		(10,131,846)
Deposits at Cayman Islands office	(195,617)		(201,927)
Short-term borrowings	(195,453)		(163,442)
Long-term borrowings	(7,649,580)		(9,493,835)
Carrying Amount [Member]
Financial assets:
Cash and cash equivalents	1,345,478		1,320,549
Interest-bearing deposits at banks	5,023,829		5,000,638
Trading Account	174,646		323,867
Investment securities	15,816,060		16,250,468
Loans and leases:
Commercial loans and leases	22,191,051		22,610,047
Commercial real estate loans	33,348,991		33,506,394
Residential real estate loans	20,960,171		22,590,912
Consumer loans	12,580,342		12,146,063
Allowance for credit losses	(1,008,225)		(988,997)
Loans and leases, net	88,072,330		89,864,419
Accrued interest receivable	310,187		308,805
Financial liabilities:
Noninterest-bearing deposits	(32,366,426)		(32,813,896)
Savings and interest-checking deposits	(52,871,146)		(52,346,207)
Time deposits	(8,107,749)		(10,131,846)
Deposits at Cayman Islands office	(195,617)		(201,927)
Short-term borrowings	(1,695,453)		(163,442)
Long-term borrowings	(7,649,580)		(9,493,835)
Accrued interest payable	(70,204)		(75,172)
Trading account	(117,209)		(174,376)
Other financial instruments:
Commitments to originate real estate loans for sale	12,425		7,325
Commitments to sell real estate loans	3,200		36,653
Other credit-related commitments	(121,960)		(136,295)
Interest rate swap agreements used for interest rate risk management	4,882		11,892
Estimate Fair Value [Member]
Financial assets:
Cash and cash equivalents	1,345,478		1,320,549
Interest-bearing deposits at banks	5,023,829		5,000,638
Trading Account	174,646		323,867
Investment securities	15,808,324		16,244,412
Loans and leases:
Commercial loans and leases	21,790,807		22,239,428
Commercial real estate loans	32,941,849		33,129,428
Residential real estate loans	21,007,049		22,638,167
Consumer loans	12,485,100		12,061,590
Loans and leases, net	88,224,805		90,068,613
Accrued interest receivable	310,187		308,805
Financial liabilities:
Noninterest-bearing deposits	(32,366,426)		(32,813,896)
Savings and interest-checking deposits	(52,871,146)		(52,346,207)
Time deposits	(8,176,159)		(10,222,585)
Deposits at Cayman Islands office	(195,617)		(201,927)
Short-term borrowings	(1,695,453)		(163,442)
Long-term borrowings	(7,676,747)		(9,473,844)
Accrued interest payable	(70,204)		(75,172)
Trading account	(117,209)		(174,376)
Other financial instruments:
Commitments to originate real estate loans for sale	12,425		7,325
Commitments to sell real estate loans	3,200		36,653
Other credit-related commitments	(121,960)		(136,295)
Interest rate swap agreements used for interest rate risk management	4,882		11,892
Estimate Fair Value [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Financial assets:
Cash and cash equivalents	1,251,750		1,249,654
Trading Account	47,276		46,135
Investment securities	47,892		301,711
Estimate Fair Value [Member] | Significant Observable Inputs (Level 2) [Member]
Financial assets:
Cash and cash equivalents	93,728		70,895
Interest-bearing deposits at banks	5,023,829		5,000,638
Trading Account	127,370		277,732
Investment securities	15,644,774		15,821,176
Loans and leases:
Commercial real estate loans	207,971		642,590
Residential real estate loans	4,643,237		4,912,488
Loans and leases, net	4,851,208		5,555,078
Accrued interest receivable	310,187		308,805
Financial liabilities:
Noninterest-bearing deposits	(32,366,426)		(32,813,896)
Savings and interest-checking deposits	(52,871,146)		(52,346,207)
Time deposits	(8,176,159)		(10,222,585)
Deposits at Cayman Islands office	(195,617)		(201,927)
Short-term borrowings	(1,695,453)		(163,442)
Long-term borrowings	(7,676,747)		(9,473,844)
Accrued interest payable	(70,204)		(75,172)
Trading account	(117,209)		(174,376)
Other financial instruments:
Commitments to sell real estate loans	3,200		36,653
Interest rate swap agreements used for interest rate risk management	4,882		11,892
Estimate Fair Value [Member] | Significant Unobservable Inputs (Level 3) [Member]
Financial assets:
Investment securities	115,658		121,525
Loans and leases:
Commercial loans and leases	21,790,807		22,239,428
Commercial real estate loans	32,733,878		32,486,838
Residential real estate loans	16,363,812		17,725,679
Consumer loans	12,485,100		12,061,590
Loans and leases, net	83,373,597		84,513,535
Other financial instruments:
Commitments to originate real estate loans for sale	12,425		7,325
Other credit-related commitments	$ (121,960)		$ (136,295)