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Securities | 9 Months Ended
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Sep. 30, 2025
Cash and Cash Equivalents [Abstract] |
Securities | Note 2 - Securities Debt securities have been classified in the consolidated balance sheets according to management’s intent. The amortized cost, related gross unrealized gains and losses, allowance for credit losses and the fair value of available-for-sale securities are as follows (dollars in thousands):
|  | September 30, 2025 |
|  |  |  |  | Gross |  |  | Gross |  |  |  |
|  | Amortized |  |  | Unrealized |  |  | Unrealized |  |  | Estimated |
|  | Cost Basis |  |  | Holding Gains |  |  | Holding Losses |  |  | Fair Value |
Securities available-for-sale: |  |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury securities |  | $ | 102,865 |  |  | $ | 2 |  |  | $ | (897 | ) |  | $ | 101,970 |
Obligations of states and political subdivisions |  |  | 1,859,132 |  |  |  | 8,751 |  |  |  | (133,972 | ) |  |  | 1,733,911 |
Residential mortgage-backed securities |  |  | 2,737,201 |  |  |  | 5,622 |  |  |  | (272,025 | ) |  |  | 2,470,798 |
Commercial mortgage-backed securities |  |  | 849,026 |  |  |  | 8,580 |  |  |  | (3,418 | ) |  |  | 854,188 |
Corporate bonds and other |  |  | 103,544 |  |  |  | 20 |  |  |  | (3,618 | ) |  |  | 99,946 |
Total securities available-for-sale |  | $ | 5,651,768 |  |  | $ | 22,975 |  |  | $ | (413,930 | ) |  | $ | 5,260,813 |
|  | September 30, 2024 |
|  |  |  |  | Gross |  |  | Gross |  |  |  |
|  | Amortized |  |  | Unrealized |  |  | Unrealized |  |  | Estimated |
|  | Cost Basis |  |  | Holding Gains |  |  | Holding Losses |  |  | Fair Value |
Securities available-for-sale: |  |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury securities |  | $ | 298,671 |  |  | $ | 14 |  |  | $ | (5,671 | ) |  | $ | 293,014 |
Obligations of states and political subdivisions |  |  | 1,586,300 |  |  |  | 699 |  |  |  | (115,516 | ) |  |  | 1,471,483 |
Residential mortgage-backed securities |  |  | 2,712,305 |  |  |  | 1,655 |  |  |  | (290,030 | ) |  |  | 2,423,930 |
Commercial mortgage-backed securities |  |  | 320,685 |  |  |  | 1,946 |  |  |  | (5,795 | ) |  |  | 316,836 |
Corporate bonds and other |  |  | 112,163 |  |  |  | 59 |  |  |  | (5,186 | ) |  |  | 107,036 |
Total securities available-for-sale |  | $ | 5,030,124 |  |  | $ | 4,373 |  |  | $ | (422,198 | ) |  | $ | 4,612,299 |
|  | December 31, 2024 |
|  |  |  |  | Gross |  |  | Gross |  |  |  |
|  | Amortized |  |  | Unrealized |  |  | Unrealized |  |  | Estimated |
|  | Cost Basis |  |  | Holding Gains |  |  | Holding Losses |  |  | Fair Value |
Securities available-for-sale: |  |  |  |  |  |  |  |  |  |  |  |
U.S. Treasury securities |  | $ | 278,155 |  |  | $ | 2 |  |  | $ | (4,595 | ) |  | $ | 273,562 |
Obligations of states and political subdivisions |  |  | 1,578,749 |  |  |  | 338 |  |  |  | (138,842 | ) |  |  | 1,440,245 |
Residential mortgage-backed securities |  |  | 2,729,096 |  |  |  | 184 |  |  |  | (378,746 | ) |  |  | 2,350,534 |
Commercial mortgage-backed securities |  |  | 457,311 |  |  |  | 99 |  |  |  | (9,658 | ) |  |  | 447,752 |
Corporate bonds and other |  |  | 111,994 |  |  |  | 50 |  |  |  | (6,378 | ) |  |  | 105,666 |
Total securities available-for-sale |  | $ | 5,155,305 |  |  | $ | 673 |  |  | $ | (538,219 | ) |  | $ | 4,617,759 |
The Company invests in mortgage-backed securities that have expected maturities that differ from their contractual maturities. These differences arise because borrowers may have the right to call or prepay obligations with or without a prepayment penalty. These securities include collateralized mortgage obligations (CMOs) and other asset backed securities. The expected maturities of these securities at September 30, 2025 and 2024, and December 31, 2024, were computed by using scheduled amortization of balances and historical prepayment rates. The carrying value and estimated fair value of available-for-sale securities at September 30, 2025, by contractual and expected maturity, are shown below (dollars in thousands):
|  | Carrying |  |  | Estimated |
|  | Value |  |  | Fair Value |
Due within one year |  | $ | 225,746 |  |  | $ | 223,945 |
Due after one year through five years |  |  | 2,493,891 |  |  |  | 2,383,120 |
Due after five years through ten years |  |  | 2,145,923 |  |  |  | 1,966,228 |
Due after ten years |  |  | 786,208 |  |  |  | 687,520 |
Total |  | $ | 5,651,768 |  |  | $ | 5,260,813 |
The following tables disclose as of September 30, 2025 and 2024, and December 31, 2024, the Company’s investment securities that have been in a continuous unrealized-loss position for less than 12 months and for 12 or more months (dollars in thousands):
|  | Less than 12 Months |  |  | 12 Months or Longer |  |  | Total |
September 30, 2025 |  | Fair Value |  |  | Unrealized Loss |  |  | Fair Value |  |  | Unrealized Loss |  |  | Fair Value |  |  | Unrealized Loss |
U.S. Treasury securities |  | $ | 1,000 |  |  | $ | — |  |  | $ | 99,476 |  |  | $ | (897 | ) |  | $ | 100,476 |  |  | $ | (897 | )
Obligations of states and political subdivisions |  |  | 21,278 |  |  |  | (854 | ) |  |  | 1,344,598 |  |  |  | (133,118 | ) |  |  | 1,365,876 |  |  |  | (133,972 | )
Residential mortgage-backed securities |  |  | 125,497 |  |  |  | (450 | ) |  |  | 2,042,826 |  |  |  | (271,575 | ) |  |  | 2,168,323 |  |  |  | (272,025 | )
Commercial mortgage-backed securities |  |  | 66,851 |  |  |  | (74 | ) |  |  | 166,330 |  |  |  | (3,344 | ) |  |  | 233,181 |  |  |  | (3,418 | )
Corporate bonds and other |  |  | — |  |  |  | — |  |  |  | 82,982 |  |  |  | (3,618 | ) |  |  | 82,982 |  |  |  | (3,618 | )
Total |  | $ | 214,626 |  |  | $ | (1,378 | ) |  | $ | 3,736,212 |  |  | $ | (412,552 | ) |  | $ | 3,950,838 |  |  | $ | (413,930 | )
|  | Less than 12 Months |  |  | 12 Months or Longer |  |  | Total |
September 30, 2024 |  | Fair Value |  |  | Unrealized Loss |  |  | Fair Value |  |  | Unrealized Loss |  |  | Fair Value |  |  | Unrealized Loss |
U.S. Treasury securities |  | $ | — |  |  | $ | — |  |  | $ | 290,535 |  |  | $ | (5,671 | ) |  | $ | 290,535 |  |  | $ | (5,671 | )
Obligations of states and political subdivisions |  |  | 6,899 |  |  |  | (43 | ) |  |  | 1,395,597 |  |  |  | (115,473 | ) |  |  | 1,402,496 |  |  |  | (115,516 | )
Residential mortgage-backed securities |  |  | 124,439 |  |  |  | (335 | ) |  |  | 2,205,425 |  |  |  | (289,695 | ) |  |  | 2,329,864 |  |  |  | (290,030 | )
Commercial mortgage-backed securities |  |  | 7,118 |  |  |  | (32 | ) |  |  | 209,632 |  |  |  | (5,763 | ) |  |  | 216,750 |  |  |  | (5,795 | )
Corporate bonds and other |  |  | — |  |  |  | — |  |  |  | 97,183 |  |  |  | (5,186 | ) |  |  | 97,183 |  |  |  | (5,186 | )
Total |  | $ | 138,456 |  |  | $ | (410 | ) |  | $ | 4,198,372 |  |  | $ | (421,788 | ) |  | $ | 4,336,828 |  |  | $ | (422,198 | )
|  | Less than 12 Months |  |  | 12 Months or Longer |  |  | Total |
December 31, 2024 |  | Fair Value |  |  | Unrealized Loss |  |  | Fair Value |  |  | Unrealized Loss |  |  | Fair Value |  |  | Unrealized Loss |
U.S. Treasury securities |  | $ | — |  |  | $ | — |  |  | $ | 271,088 |  |  | $ | (4,595 | ) |  | $ | 271,088 |  |  | $ | (4,595 | )
Obligations of states and political subdivisions |  |  | 56,777 |  |  |  | (1,224 | ) |  |  | 1,361,628 |  |  |  | (137,618 | ) |  |  | 1,418,405 |  |  |  | (138,842 | )
Residential mortgage-backed securities |  |  | 198,385 |  |  |  | (4,990 | ) |  |  | 2,048,727 |  |  |  | (373,756 | ) |  |  | 2,247,112 |  |  |  | (378,746 | )
Commercial mortgage-backed securities |  |  | 196,640 |  |  |  | (2,249 | ) |  |  | 190,189 |  |  |  | (7,409 | ) |  |  | 386,829 |  |  |  | (9,658 | )
Corporate bonds and other |  |  | 4,871 |  |  |  | (44 | ) |  |  | 95,840 |  |  |  | (6,334 | ) |  |  | 100,711 |  |  |  | (6,378 | )
Total |  | $ | 456,673 |  |  | $ | (8,507 | ) |  | $ | 3,967,472 |  |  | $ | (529,712 | ) |  | $ | 4,424,145 |  |  | $ | (538,219 | )
The number of investments in an unrealized loss position totaled 682 at September 30, 2025. We believe any unrealized losses in the U.S. treasury securities, obligations of state and political subdivisions, residential and commercial mortgage-backed and asset-backed investment securities, and corporate bonds and other at September 30, 2025 and 2024, and December 31, 2024, are due to changes in interest rates and not credit-related events. As such, no allowance for credit losses is required on these securities at September 30, 2025 and 2024, and December 31, 2024. Unrealized losses on investment securities are expected to recover over time as these securities approach maturity. Based on evaluations of impaired securities as of September 30, 2025, the Company does not intend to sell any impaired available-for-sale securities before fair value recovers to the current amortized cost, and it is more-likely-than-not that the Company will not be required to sell impaired securities before fair value recovers, which may be maturity. Our mortgage related securities are backed by GNMA, FNMA and FHLMC or are collateralized by securities backed by these agencies. At September 30, 2025, 72.57% of our available-for-sale securities that are obligations of states and political subdivisions were issued within the State of Texas, of which 55.25% are guaranteed by the Texas Permanent School Fund. Securities, carried at approximately $2,335,928,000 on September 30, 2025, were pledged as collateral for public or trust fund deposits, repurchase agreements, borrowings and for other purposes required or permitted by law. During the three and nine-months ended September 30, 2025 and 2024, respectively, there were no sales of investment securities that were classified as available-for-sale. There were no gross realized security gains or losses from sales during the three and nine-months ended September 30, 2025 and 2024. The specific identification method was used to determ ine cost in order to compute the realized gains and losses.

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- Definition The entire disclosure for cash, cash equivalent, investment in debt and equity securities, equity method investment, investment in joint venture, and investment classified as other. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 65 -Paragraph 13 -Subparagraph (d)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482477/820-10-65-13 + Details
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