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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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- Definition Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-2 + Details
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