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Investment Securities, Equity Securities with Readily Determinable Fair Values and Other Investments (Tables) | 12 Months Ended
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Dec. 31, 2025
Investment Securities, Equity Securities with Readily Determinable Fair Values and Other Investments |
Amortized cost and estimated fair value by type of investment security | The amortized cost and estimated fair value by type of investment security at December 31, 2025 are as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Held to Maturity | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | unrealized | ​ | unrealized | ​ | Estimated | ​ | Carrying | ​
​ | ​ | cost | ​ | gains | ​ | losses | ​ | fair value | ​ | value | ​
​ | ​ | (Dollars in Thousands) | ​
Other securities | ​ ​ ​ | $ | 4,400 | ​ ​ ​ | $ | — | ​ ​ ​ | $ | — | ​ ​ ​ | $ | 4,400 | ​ ​ ​ | $ | 4,400 | ​
Total investment securities | ​ | $ | 4,400 | ​ | $ | — | ​ | $ | — | ​ | $ | 4,400 | ​ | $ | 4,400 | ​
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Available for Sale Debt Securities | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | unrealized | ​ | unrealized | ​ | Estimated | ​ | Carrying | ​
​ | ​ | cost | ​ | gains | ​ | losses | ​ | fair value | ​ | value(1) | ​
​ | ​ | (Dollars in Thousands) | ​
Residential mortgage-backed securities | ​ | $ | 5,140,013 | ​ | $ | 22,759 | ​ | $ | (332,184) | ​ | $ | 4,830,588 | ​ | $ | 4,830,588 | ​
Obligations of states and political subdivisions | ​ |  | 141,077 | ​ |  | 86 | ​ |  | (5,483) | ​ |  | 135,680 | ​ |  | 135,680 | ​
Total investment securities | ​ | $ | 5,281,090 | ​ | $ | 22,845 | ​ | $ | (337,667) | ​ | $ | 4,966,268 | ​ | $ | 4,966,268 | ​
(1) | Included in the carrying value of residential mortgage- backed securities are $854,726 of mortgage-backed securities issued by Ginnie Mae and $3,975,862 of mortgage-backed securities issued by Fannie Mae and Freddie Mac
The amortized cost and estimated fair value by type of investment security at December 31, 2024 are as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Held to Maturity | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | unrealized | ​ | unrealized | ​ | Estimated | ​ | Carrying | ​
​ | ​ | cost | ​ | gains | ​ | losses | ​ | fair value | ​ | value | ​
​ | ​ | (Dollars in Thousands) | ​
Other securities | ​ ​ ​ | $ | 4,400 | ​ ​ ​ | $ | — | ​ ​ ​ | $ | — | ​ ​ ​ | $ | 4,400 | ​ ​ ​ | $ | 4,400 | ​
Total investment securities | ​ | $ | 4,400 | ​ | $ | — | ​ | $ | — | ​ | $ | 4,400 | ​ | $ | 4,400 | ​
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Available for Sale | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | Estimated | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | unrealized | ​ | unrealized | ​ | fair | ​ | Carrying | ​
​ | ​ | cost | ​ | gains | ​ | losses | ​ | value | ​ | value(1) | ​
​ | ​ | (Dollars in Thousands) | ​
Residential mortgage-backed securities | ​ ​ ​ | $ | 5,315,488 | ​ | $ | 8,858 | ​ | $ | (489,170) | ​ ​ ​ | ​ | 4,835,176 | ​ ​ ​ | ​ | 4,835,176 | ​
Obligations of states and political subdivisions | ​ |  | 156,822 | ​ |  | 331 | ​ |  | (4,413) | ​ |  | 152,740 | ​ |  | 152,740 | ​
Total investment securities | ​ | $ | 5,472,310 | ​ | $ | 9,189 | ​ | $ | (493,583) | ​ | $ | 4,987,916 | ​ | $ | 4,987,916 | ​
(1) | Included in the carrying value of residential mortgage- backed securities are $1,001,184 of mortgage-backed securities issued by Ginnie Mae, $3,833,992 of mortgage-backed securities issued by Fannie Mae and Freddie Mac
Amortized cost and fair value of investment securities, by contractual maturity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Held to Maturity | ​ | Available for Sale | ​
​ | ​ | Amortized | ​ | Estimated | ​ | Amortized | ​ | Estimated | ​
​ | ​ | Cost | ​ | fair value | ​ | Cost | ​ | fair value | ​
​ | ​ | (Dollars in Thousands) | ​
Due in one year or less | ​ ​ ​ | $ | 3,075 | ​ ​ ​ | $ | 3,075 | ​ ​ ​ | $ | — | ​ ​ ​ | $ | — | ​
Due after one year through five years | ​ |  | 1,325 | ​ |  | 1,325 | ​ |  | — | ​ | ​ | — | ​
Due after five years through ten years | ​ |  | — | ​ |  | — | ​ |  | 2,240 | ​ |  | 2,240 | ​
Due after ten years | ​ |  | — | ​ |  | — | ​ |  | 138,837 | ​ |  | 133,440 | ​
Residential mortgage-backed securities | ​ |  | — | ​ |  | — | ​ |  | 5,140,013 | ​ |  | 4,830,588 | ​
Total investment securities | ​ | $ | 4,400 | ​ | $ | 4,400 | ​ | $ | 5,281,090 | ​ | $ | 4,966,268 | ​
Gross unrealized losses on debt investment securities and the related fair value | Gross unrealized losses on investment securities and the fair value of the related securities, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position, at December 31, 2025 were as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Less than 12 months | ​ | 12 months or more | ​ | Total | ​
​ | ​ | ​ | ​ | ​ | Unrealized | ​ | ​ | ​ | ​ | Unrealized | ​ | ​ | ​ | ​ | Unrealized | ​
​ | ​ | Fair Value | ​ | Losses | ​ | Fair Value | ​ | Losses | ​ | Fair Value | ​ | Losses | ​
​ | ​ | (Dollars in Thousands) | ​
Available for sale: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Residential mortgage-backed securities | ​ ​ ​ | $ | 130,561 | ​ ​ ​ | $ | (81) | ​ ​ ​ | $ | 3,002,781 | ​ ​ ​ | $ | (332,103) | ​ ​ ​ | $ | 3,133,342 | ​ ​ ​ | $ | (332,184) | ​
Obligations of states and political subdivisions | ​ |  | 4,361 | ​ |  | (59) | ​ |  | 105,263 | ​ |  | (5,424) | ​ |  | 109,624 | ​ |  | (5,483) | ​
​ | ​ | $ | 134,922 | ​ | $ | (140) | ​ | $ | 3,108,044 | ​ | $ | (337,527) | ​ | $ | 3,242,966 | ​ | $ | (337,667) | ​
​ Gross unrealized losses on investment securities and the fair value of the related securities, aggregated by investment category and length of time that individual securities have been in a continuous loss position, at December 31, 2024 were as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Less than 12 months | ​ | 12 months or more | ​ | Total
​ | ​ | ​ | ​ | ​ | Unrealized | ​ | ​ | ​ | ​ | Unrealized | ​ | ​ | ​ | ​ | Unrealized
​ | ​ | Fair Value | ​ | Losses | ​ | Fair Value | ​ | Losses | ​ | Fair Value | ​ | Losses
​ | ​ | (Dollars in Thousands)
Available for sale: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Residential mortgage-backed securities | ​ ​ ​ | $ | 544,375 | ​ ​ ​ | $ | (4,126) | ​ ​ ​ | $ | 3,358,586 | ​ ​ ​ | $ | (485,044) | ​ ​ ​ | $ | 3,902,961 | ​ ​ ​ | $ | (489,170)
Obligations of states and political subdivisions | ​ |  | 66,450 | ​ |  | (2,389) | ​ |  | 57,551 | ​ |  | (2,024) | ​ |  | 124,001 | ​ |  | (4,413)
​ | ​ | $ | 610,825 | ​ | $ | (6,515) | ​ | $ | 3,416,137 | ​ | $ | (487,068) | ​ | $ | 4,026,962 | ​ | $ | (493,583)
​
Summary of unrealized and realized gains and losses recognized in net income on equity securities | ​
​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended
​ | ​ | December 31, 2025
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
Net gains recognized during the period on equity securities | ​ ​ ​ | $ | ​ | 180
Less: Net gains and (losses) recognized during the period on equity securities sold during the period | ​ |  | ​ | —
​ | ​ | ​ | ​ | ​
Unrealized gains recognized during the reporting period on equity securities still held at the reporting date | ​ | $ | ​ | 180
​
​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended
​ | ​ | December 31, 2024
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
Net losses recognized during the period on equity securities | ​ ​ ​ | $ | ​ | (24)
Less: Net gains and (losses) recognized during the period on equity securities sold during the period | ​ |  | ​ | —
​ | ​ | ​ | ​ | ​
Unrealized losses recognized during the reporting period on equity securities still held at the reporting date | ​ | $ | ​ | (24)
​ ​ ​
​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended
​ | ​ | December 31, 2023
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
Net gains recognized during the period on equity securities | ​ ​ ​ | $ | ​ | 59
Less: Net gains and (losses) recognized during the period on equity securities sold during the period | ​ |  | ​ | —
​ | ​ | ​ | ​ | ​
Unrealized gains recognized during the reporting period on equity securities still held at the reporting date | ​ | $ | ​ | 59

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