Fair Value Disclosures - Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)	Sep. 30, 2024	Dec. 31, 2023	Sep. 30, 2023
Available-for-sale investment securities:
Estimated Fair Value	$ 4,612,299,000	$ 4,732,762,000	$ 4,652,537,000
Loans held for sale	20,114,000	14,253,000	12,229,000
U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	293,014,000	482,234,000	485,447,000
Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Estimated Fair Value	1,471,483,000	1,497,157,000	1,463,754,000
Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	2,423,930,000	2,364,092,000	2,297,235,000
Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	316,836,000	284,324,000	305,348,000
IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	499,000	427,000	409,000
Forward mortgage-backed securities trades asset (liability)	0	0	0
Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	0	0	237,000
Forward mortgage-backed securities trades asset (liability)	98,000	288,000	0
Fair Value, Measurements, Recurring [Member]
Available-for-sale investment securities:
Estimated Fair Value	4,612,299,000	4,732,762,000	4,652,537,000
Loans held for sale	16,513,000	11,077,000	11,908,000
Fair Value, Measurements, Recurring [Member] \| U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	293,014,000	482,234,000	485,447,000
Fair Value, Measurements, Recurring [Member] \| Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Estimated Fair Value	1,471,483,000	1,497,157,000	1,463,754,000
Fair Value, Measurements, Recurring [Member] \| Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	2,423,930,000	2,364,092,000	2,297,235,000
Fair Value, Measurements, Recurring [Member] \| Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	316,836,000	284,324,000	305,348,000
Fair Value, Measurements, Recurring [Member] \| IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	499,000	427,000	409,000
Fair Value, Measurements, Recurring [Member] \| Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Forward mortgage-backed securities trades asset (liability)	(98,000)	(288,000)	237,000
Fair Value, Measurements, Recurring [Member] \| Corporate Bonds [Member]
Available-for-sale investment securities:
Estimated Fair Value	102,471,000	100,471,000	96,961,000
Fair Value, Measurements, Recurring [Member] \| Collateralized Securities, Other [Member]
Available-for-sale investment securities:
Estimated Fair Value	4,565,000	4,484,000	3,792,000
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member]
Available-for-sale investment securities:
Estimated Fair Value	297,579,000	486,718,000	489,239,000
Loans held for sale	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member] \| U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	293,014,000	482,234,000	485,447,000
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member] \| Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member] \| Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member] \| Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member] \| IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member] \| Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Forward mortgage-backed securities trades asset (liability)	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member] \| Corporate Bonds [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member] \| Collateralized Securities, Other [Member]
Available-for-sale investment securities:
Estimated Fair Value	4,565,000	4,484,000	3,792,000
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member]
Available-for-sale investment securities:
Estimated Fair Value	4,314,720,000	4,246,044,000	4,163,298,000
Loans held for sale	16,513,000	11,077,000	11,908,000
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Estimated Fair Value	1,471,483,000	1,497,157,000	1,463,754,000
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	2,423,930,000	2,364,092,000	2,297,235,000
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	316,836,000	284,324,000	305,348,000
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Forward mortgage-backed securities trades asset (liability)	(98,000)	(288,000)	237,000
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Corporate Bonds [Member]
Available-for-sale investment securities:
Estimated Fair Value	102,471,000	100,471,000	96,961,000
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Collateralized Securities, Other [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 Inputs [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Loans held for sale	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 Inputs [Member] \| U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 Inputs [Member] \| Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 Inputs [Member] \| Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 Inputs [Member] \| Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 Inputs [Member] \| IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	499,000	427,000	409,000
Fair Value, Measurements, Recurring [Member] \| Level 3 Inputs [Member] \| Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Forward mortgage-backed securities trades asset (liability)	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 Inputs [Member] \| Corporate Bonds [Member]
Available-for-sale investment securities:
Estimated Fair Value	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 3 Inputs [Member] \| Collateralized Securities, Other [Member]
Available-for-sale investment securities:
Estimated Fair Value	$ 0	$ 0	$ 0

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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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- Definition

Fair value, before 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. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Fair value, before 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. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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- Definition

Amount, after valuation allowance, of financing receivable held for sale and not part of disposal group. Excludes loan covered under loss sharing agreement and loan classified as investment in debt security.

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