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Investment Securities, Equity Securities with Readily Determinable Fair Values and Other Investments (Tables) | 9 Months Ended
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Sep. 30, 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 September 30, 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 | ​ | $ | 5,175,459 | ​ | $ | 23,676 | ​ | $ | (353,611) | ​ | $ | 4,845,524 | ​ | $ | 4,845,524 | ​
Obligations of states and political subdivisions | ​ |  | 141,121 | ​ |  | 1 | ​ |  | (6,304) | ​ |  | 134,818 | ​ |  | 134,818 | ​
Total investment securities | ​ | $ | 5,316,580 | ​ | $ | 23,677 | ​ | $ | (359,915) | ​ | $ | 4,980,342 | ​ | $ | 4,980,342 | ​
(1) | Included in the carrying value of residential mortgage-backed securities are $880,747 of mortgage-backed securities issued by Ginnie Mae and $3,964,777 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 and $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 | ​ |  | — | ​ |  | — | ​ |  | 138,881 | ​ |  | 132,578 | ​
Residential mortgage-backed securities | ​ |  | — | ​ |  | — | ​ |  | 5,175,459 | ​ |  | 4,845,524 | ​
Total investment securities | ​ | $ | 4,400 | ​ | $ | 4,400 | ​ | $ | 5,316,580 | ​ | $ | 4,980,342 | ​
Gross unrealized losses on debt investment securities and the related fair value | Gross unrealized losses on debt investment securities and the fair value of those related securities, aggregated by investment category and length of time that individual debt securities have been in a continuous unrealized loss position at September 30, 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 |  | $ | 5,651 |  | $ | (11) |  | $ | 3,096,713 |  | $ | (353,600) |  | $ | 3,102,364 |  | $ | (353,611) | ​
Obligations of states and political subdivisions | ​ |  | 41,480 | ​ |  | (1,243) | ​ |  | 82,702 | ​ |  | (5,061) | ​ |  | 124,182 | ​ |  | (6,304) | ​
​ | ​ | $ | 47,131 | ​ | $ | (1,254) | ​ | $ | 3,179,415 | ​ | $ | (358,661) | ​ | $ | 3,226,546 | ​ | $ | (359,915) | ​
​ Gross unrealized losses on debt investment securities and the fair value of those related securities, aggregated by investment category and length of time that individual debt securities have been in a continuous unrealized 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 | ​
​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended
​ | ​ | September 30, 2025
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
​ | ​ | ​ | ​ | ​
Net gains recognized during the period on equity securities |  | $ | ​ | 63
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 | ​ | $ | ​ | 63
​
​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended
​ | ​ | September 30, 2024
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
​ | ​ | ​ | ​ | ​
Net gains recognized during the period on equity securities |  | $ | ​ | 201
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 | ​ | $ | ​ | 201
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​
​ | ​ | ​ | ​ | ​
​ | ​ | Nine Months Ended
​ | ​ | September 30, 2025
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
Net gains recognized during the period on equity securities |  | $ | ​ | 152
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 | ​ | $ | ​ | 152
​
​ | ​ | ​ | ​ | ​
​ | ​ | Nine Months Ended
​ | ​ | September 30, 2024
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
Net gains recognized during the period on equity securities |  | $ | ​ | 121
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 | ​ | $ | ​ | 121

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