Fair Value Measurements - Estimated Fair Value of Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and short-term investments	$ 3,155,647	$ 3,276,794
Available for sale securities	8,719,246	7,447,362
Securities held to maturity (exclusive of allowance for credit losses)	1,070,307	1,116,102
Trading securities	49,154	45,618
Other securities	159,994	108,420
Loans (exclusive of allowance for credit/loan losses)	15,961,155	13,439,525
Derivatives	116,765	55,276
Demand and savings deposits	24,051,444	20,376,880
Time deposits	686,463	1,226,364
Other borrowings	1,944,004	1,896,508
Long-term debt	269,044	70,372
Derivatives	10,194	5,997
Estimate of Fair Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and short-term investments	3,155,647	3,276,794
Available for sale securities	8,719,246	7,447,362
Securities held to maturity (exclusive of allowance for credit losses)	1,099,289	1,082,345
Trading securities	49,154	45,618
Other securities	159,994	108,420
Loans (exclusive of allowance for credit/loan losses)	16,323,448	13,601,595
Derivatives	116,765	55,276
Demand and savings deposits	24,051,444	20,376,880
Time deposits	692,365	1,226,646
Other borrowings	1,944,004	1,896,508
Long-term debt	300,428	70,713
Derivatives	10,194	5,997
Commitments to extend credit for loans	4,563	5,908
Commercial letters of credit	84	113
Standby letters of credit	2,394	2,966
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and short-term investments	2,054,334	1,701,449
Available for sale securities	93,024	252,630
Trading securities	19,472	22,216
Other securities	6,804
Demand and savings deposits	24,051,444	20,376,880
Other borrowings	28,783	31,873
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and short-term investments	1,101,313	1,575,345
Available for sale securities	8,626,222	7,194,732
Securities held to maturity (exclusive of allowance for credit losses)	1,099,289	1,082,345
Trading securities	29,682	23,402
Other securities	153,190	108,420
Loans (exclusive of allowance for credit/loan losses)	16,323,448	13,601,595
Derivatives	116,765	55,276
Time deposits	692,365	1,226,646
Other borrowings	1,915,221	1,864,635
Long-term debt	300,428	70,713
Derivatives	$ 10,194	$ 5,997

Amount of commitments to extend credit for loans to customers.

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Loans and leases receivable gross carrying amount included loans held for sale.

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Fair value of other investments not classified as trading, available-for-sale or held-to-maturity.

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Savings and demand deposits.

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Amount of the contingent obligation under standby letters of credit.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Cash includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the customer may deposit additional funds at any time and effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid Investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. Short-term investments, exclusive of cash equivalents, generally consist of marketable securities intended to be sold within one year (or the normal operating cycle if longer) and may include trading securities, available-for-sale securities, or held-to-maturity securities (if maturing within one year), as applicable.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled within one year or normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled within one year or normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount of investment in debt security measured at amortized cost (held-to-maturity).

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The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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The carrying amount as of the balance sheet date for the aggregate of other miscellaneous borrowings owed by the reporting entity.

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Amount of time deposit liabilities, including certificates of deposit.

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Amount of investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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