Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	$ 61,177	$ 45,618
Available for sale securities	7,639,451	7,447,362
Derivative Assets, Fair Value	145,347	55,276
Derivative Liabilities, Fair Value	10,437	5,997
Securities sold not yet purchased	17,600	14,600
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	22,436	22,216
Available for sale securities	210,494	252,630
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	38,741	23,402
Available for sale securities	7,428,957	7,194,732
U.S. Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities	100,616	93,021
Mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities	4,234,649	4,071,794
State and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities	3,093,692	3,029,917
Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities	178,889	188,552
U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities	31,605	64,078
Fair Value Measurement, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	61,177	45,618
Available for sale securities	7,639,451	7,447,362
Company-owned life insurance	47,726	63,900
Bank-owned life insurance	282,562	280,709
Derivative Assets, Fair Value	145,347	55,276
Total	8,176,263	7,892,865
Derivative Liabilities, Fair Value	10,437	5,997
Securities sold not yet purchased	17,631	14,599
Total	28,068	20,596
Fair Value Measurement, Recurring [Member] \| Trading - other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	17,193	16,567
Fair Value Measurement, Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	22,436	22,216
Available for sale securities	210,494	252,630
Total	232,930	274,846
Fair Value Measurement, Recurring [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Trading - other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	17,193	16,567
Fair Value Measurement, Recurring [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	38,741	23,402
Available for sale securities	7,428,957	7,194,732
Company-owned life insurance	47,726	63,900
Bank-owned life insurance	282,562	280,709
Derivative Assets, Fair Value	145,347	55,276
Total	7,943,333	7,618,019
Derivative Liabilities, Fair Value	10,437	5,997
Securities sold not yet purchased	17,631	14,599
Total	28,068	20,596
Fair Value Measurement, Recurring [Member] \| U.S. Agencies [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	3,477	1,246
Available for sale securities	100,616	93,021
Fair Value Measurement, Recurring [Member] \| U.S. Agencies [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	3,477	1,246
Available for sale securities	100,616	93,021
Fair Value Measurement, Recurring [Member] \| Mortgage-backed [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	5	392
Available for sale securities	4,234,649	4,071,794
Fair Value Measurement, Recurring [Member] \| Mortgage-backed [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	5	392
Available for sale securities	4,234,649	4,071,794
Fair Value Measurement, Recurring [Member] \| State and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	35,259	21,764
Available for sale securities	3,093,692	3,029,917
Fair Value Measurement, Recurring [Member] \| State and political subdivisions [Member] \| Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	35,259	21,764
Available for sale securities	3,093,692	3,029,917
Fair Value Measurement, Recurring [Member] \| Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	5,243	5,649
Available for sale securities	178,889	188,552
Fair Value Measurement, Recurring [Member] \| Corporates [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities	5,243	5,649
Available for sale securities	178,889	188,552
Fair Value Measurement, Recurring [Member] \| U.S. Treasury [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities	31,605	64,078
Fair Value Measurement, Recurring [Member] \| U.S. Treasury [Member] \| Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale securities	$ 31,605	$ 64,078

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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The carrying amount of a life insurance policy on an officer, executive or employee for which the reporting entity (a bank) is entitled to proceeds from the policy upon death of the insured or surrender of the insurance policy.

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Fair value portion of arrangements with third parties, including, but not limited to, operating lease arrangement and arrangement in which the entity has agreed to expend funds to procure goods or services, or has agreed to commit resources to supply goods or services.

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Fair value, after offset of derivative liability, of 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, before offset against an obligation to return collateral under a master netting arrangement. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Fair value, after offset of derivative asset, of 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, elected not to be and before offset against a right to receive collateral under a master netting arrangement. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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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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No definition available.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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This item represents the amount that could be realized under a life insurance contract or contracts owned by the Entity as of the date of the statement of financial position. Such Entity-owned life insurance policies are commonly known as corporate-owned life insurance (COLI) or bank-owned life insurance (BOLI).

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Fair value of financial instruments sold short by a broker-dealer for their own account (proprietary securities) for trading or investment purposes determined by management based upon quoted prices of a financial instrument with similar characteristics or on a valuation technique or model.

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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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