Estimated Fair Value of Financial Instruments (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and short-term investments	$ 1,224,102	$ 1,936,084
Securities available for sale	5,932,741	6,258,577
Securities held to maturity	1,199,114	1,261,014
Trading securities	81,159	54,055
Other securities	65,252	65,897
Loans (exclusive of allowance for loan loss)	11,966,946	11,281,973
Derivatives	2,499	10,149
Demand and savings deposits	16,696,192	16,742,736
Time deposits	1,040,512	1,280,264
Other borrowings	1,192,985	1,260,704
Long-term debt	78,523	79,281
Derivatives	11,465	8,906
Estimate of Fair Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and short-term investments	1,224,102	1,936,084
Securities available for sale	5,932,741	6,258,577
Securities held to maturity	1,091,640	1,207,447
Trading securities	81,159	54,055
Other securities	65,252	65,897
Loans (exclusive of allowance for loan loss)	11,938,756	11,318,764
Derivatives	2,499	10,149
Demand and savings deposits	16,696,192	16,742,736
Time deposits	1,040,512	1,280,264
Other borrowings	1,192,985	1,260,704
Long-term debt	78,775	79,496
Derivatives	11,465	8,906
Commitments to extend credit for loans	3,970	6,654
Commercial letters of credit	79	136
Standby letters of credit	1,935	2,514
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	1,024,423	1,749,618
Securities available for sale	37,938	51,909
Trading securities	22,070	14,337
Demand and savings deposits	16,696,192	16,742,736
Other borrowings		11,334
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	199,679	186,466
Securities available for sale	5,894,803	6,206,668
Securities held to maturity	1,091,640	1,207,447
Trading securities	59,089	39,718
Other securities	65,252	65,897
Loans (exclusive of allowance for loan loss)	11,938,756	11,318,764
Derivatives	2,499	10,149
Time deposits	1,040,512	1,280,264
Other borrowings	1,192,985	1,249,370
Long-term debt	78,775	79,496
Derivatives	$ 11,465	$ 8,906

Amount of commitments to extend credit for loans to customers.

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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 and equity securities categorized neither as held-to-maturity nor trading.

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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 allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes loans held for sale. Excludes loans and leases covered under loss sharing agreements.

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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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