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Investment Securities - Amortized Cost and Estimated Fair Values of Investments Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Debt Securities Available for Sale
Amortized Cost	$ 1,906,961	$ 2,138,533
Gross Unrealized Gains	1,950	1,007
Gross Unrealized Losses	(168,155)	(234,655)
Estimated Fair Value	1,740,756	1,904,885
Debt Securities Held to Maturity
Totals	95,446	111,866
Gross Unrealized Gains	3	5
Gross Unrealized Losses	(4,150)	(7,522)
Estimated Fair Value	91,299	104,349
Obligations of U.S. government agencies
Debt Securities Available for Sale
Amortized Cost	1,183,001	1,268,654
Gross Unrealized Gains	1,323	16
Gross Unrealized Losses	(122,177)	(174,485)
Estimated Fair Value	1,062,147	1,094,185
Debt Securities Held to Maturity
Totals	93,882	109,155
Gross Unrealized Gains	3	3
Gross Unrealized Losses	(4,138)	(7,443)
Estimated Fair Value	89,747	101,715
Obligations of states and political subdivisions
Debt Securities Available for Sale
Amortized Cost	240,306	249,627
Gross Unrealized Gains	64	66
Gross Unrealized Losses	(22,771)	(28,949)
Estimated Fair Value	217,599	220,744
Debt Securities Held to Maturity
Totals	1,564	2,711
Gross Unrealized Gains	0	2
Gross Unrealized Losses	(12)	(79)
Estimated Fair Value	1,552	2,634
Corporate bonds
Debt Securities Available for Sale
Amortized Cost	4,944	6,182
Gross Unrealized Gains	2	0
Gross Unrealized Losses	(66)	(345)
Estimated Fair Value	4,880	5,837
Asset backed securities
Debt Securities Available for Sale
Amortized Cost	280,761	314,814
Gross Unrealized Gains	321	687
Gross Unrealized Losses	(1,718)	(1,238)
Estimated Fair Value	279,364	314,263
Non-agency collateralized mortgage obligations
Debt Securities Available for Sale
Amortized Cost	197,949	299,256
Gross Unrealized Gains	240	238
Gross Unrealized Losses	(21,423)	(29,638)
Estimated Fair Value	$ 176,766	$ 269,856

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