Fair Value Disclosures - Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)	Sep. 30, 2025	Dec. 31, 2024	Sep. 30, 2024
Available-for-sale investment securities:
Estimated Fair Value	$ 5,260,813,000	$ 4,617,759,000	$ 4,612,299,000
Loans held for sale	26,015,000	8,235,000	20,114,000
U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	101,970,000	273,562,000	293,014,000
Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Estimated Fair Value	1,733,911,000	1,440,245,000	1,471,483,000
Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	2,470,798,000	2,350,534,000	2,423,930,000
Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	854,188,000	447,752,000	316,836,000
IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	726,000	238,000	499,000
Forward mortgage-backed securities trades	0	0	0
Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	0	84,000	0
Forward mortgage-backed securities trades	(214,000)	0	(98,000)
Fair Value, Measurements, Recurring [Member]
Available-for-sale investment securities:
Estimated Fair Value	5,260,813,000	4,617,759,000	4,612,299,000
Loans held for sale	25,180,000	7,793,000	16,513,000
Fair Value, Measurements, Recurring [Member] \| U.S. Treasury Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	101,970,000	273,562,000	293,014,000
Fair Value, Measurements, Recurring [Member] \| Obligations of State and Political Subdivisions [Member]
Available-for-sale investment securities:
Estimated Fair Value	1,733,911,000	1,440,245,000	1,471,483,000
Fair Value, Measurements, Recurring [Member] \| Corporate Bonds [Member]
Available-for-sale investment securities:
Estimated Fair Value	83,396,000	101,238,000	102,471,000
Fair Value, Measurements, Recurring [Member] \| Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	2,470,798,000	2,350,534,000	2,423,930,000
Fair Value, Measurements, Recurring [Member] \| Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	854,188,000	447,752,000	316,836,000
Fair Value, Measurements, Recurring [Member] \| IRLCs [Member]
Available-for-sale investment securities:
Asset Derivative Fair Value	726,000	238,000	499,000
Fair Value, Measurements, Recurring [Member] \| Forward Mortgage-Backed Securities Trades [Member]
Available-for-sale investment securities:
Forward mortgage-backed securities trades	214,000	84,000	(98,000)
Fair Value, Measurements, Recurring [Member] \| Collateralized Securities, Other [Member]
Available-for-sale investment securities:
Estimated Fair Value	16,550,000	4,428,000	4,565,000
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member]
Available-for-sale investment securities:
Estimated Fair Value	118,520,000	277,990,000	297,579,000
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	101,970,000	273,562,000	293,014,000
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] \| Corporate Bonds [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	0	0	0
Fair Value, Measurements, Recurring [Member] \| Level 1 Inputs [Member] \| Collateralized Securities, Other [Member]
Available-for-sale investment securities:
Estimated Fair Value	16,550,000	4,428,000	4,565,000
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member]
Available-for-sale investment securities:
Estimated Fair Value	5,142,293,000	4,339,769,000	4,314,720,000
Loans held for sale	25,180,000	7,793,000	16,513,000
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	1,733,911,000	1,440,245,000	1,471,483,000
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Corporate Bonds [Member]
Available-for-sale investment securities:
Estimated Fair Value	83,396,000	101,238,000	102,471,000
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Residential Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	2,470,798,000	2,350,534,000	2,423,930,000
Fair Value, Measurements, Recurring [Member] \| Level 2 Inputs [Member] \| Commercial Mortgage-Backed Securities [Member]
Available-for-sale investment securities:
Estimated Fair Value	854,188,000	447,752,000	316,836,000
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	214,000	84,000	(98,000)
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] \| Corporate Bonds [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	726,000	238,000	499,000
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	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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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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