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Investment Securities, Equity Securities with Readily Determinable Fair Values and Other Investments (Tables) | 3 Months Ended
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Mar. 31, 2024
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 March 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 Debt Securities | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | unrealized | ​ | unrealized | ​ | Estimated | ​ | Carrying | ​
​ | ​ | cost | ​ | gains | ​ | losses | ​ | fair value | ​ | value(1) | ​
​ | ​ | (Dollars in Thousands) | ​
Residential mortgage-backed securities | ​ | $ | 5,147,065 | ​ | $ | 3,720 | ​ | $ | (538,479) | ​ | $ | 4,612,306 | ​ | $ | 4,612,306 | ​
Obligations of states and political subdivisions | ​ |  | 159,169 | ​ |  | 717 | ​ |  | (2,562) | ​ |  | 157,324 | ​ |  | 157,324 | ​
Total investment securities | ​ | $ | 5,306,234 | ​ | $ | 4,437 | ​ | $ | (541,041) | ​ | $ | 4,769,630 | ​ | $ | 4,769,630 | ​
(1) | Included in the carrying value of residential mortgage-backed securities are $945,465 of mortgage-backed securities issued by Ginnie Mae and $3,666,841 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, 2023 are as follows:
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Held to Maturity | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | unrealized | ​ | unrealized | ​ | Estimated | ​ | Carrying | ​
​ | ​ | cost | ​ | gains | ​ | losses | ​ | fair value | ​ | value | ​
​ | ​ | (Dollars in Thousands) | ​
Other securities |  | $ | 3,400 |  | $ | — |  | $ | — |  | $ | 3,400 |  | $ | 3,400 | ​
Total investment securities | ​ | $ | 3,400 | ​ | $ | — | ​ | $ | — | ​ | $ | 3,400 | ​ | $ | 3,400 | ​
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Available for Sale | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | Estimated | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | unrealized | ​ | unrealized | ​ | fair | ​ | Carrying | ​
​ | ​ | cost | ​ | gains | ​ | losses | ​ | value | ​ | value(1) | ​
​ | ​ | (Dollars in Thousands) | ​
Residential mortgage-backed securities |  | $ | 5,169,813 | ​ | $ | 9,541 | ​ | $ | (519,255) |  | ​ | 4,660,099 |  | ​ | 4,660,099 | ​
Obligations of states and political subdivisions | ​ |  | 161,001 | ​ |  | 1,602 | ​ |  | (361) | ​ |  | 162,242 | ​ |  | 162,242 | ​
Total investment securities | ​ | $ | 5,330,814 | ​ | $ | 11,143 | ​ | $ | (519,616) | ​ | $ | 4,822,341 | ​ | $ | 4,822,341 | ​
(1) | Included in the carrying value of residential mortgage-backed securities are $959,421 of mortgage-backed securities issued by Ginnie Mae and $3,700,678 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 |  | $ | 2,200 |  | $ | 2,200 |  | $ | — |  | $ | — | ​
Due after one year through five years | ​ |  | 2,200 | ​ |  | 2,200 | ​ |  | — | ​ | ​ | — | ​
Due after five years through ten years | ​ |  | — | ​ |  | — | ​ |  | 1,455 | ​ |  | 1,444 | ​
Due after ten years | ​ |  | — | ​ |  | — | ​ |  | 157,714 | ​ |  | 155,880 | ​
Residential mortgage-backed securities | ​ |  | — | ​ |  | — | ​ |  | 5,147,065 | ​ |  | 4,612,306 | ​
Total investment securities | ​ | $ | 4,400 | ​ | $ | 4,400 | ​ | $ | 5,306,234 | ​ | $ | 4,769,630 | ​
Summary of unrealized and realized gains and losses recognized in net income on equity securities | ​
​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended
​ | ​ | March 31, 2024
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
​ | ​ | ​ | ​ | ​
Net losses recognized during the period on equity securities |  | $ | ​ | ​
Less: Net gains and (losses) recognized during the period on equity securities sold during the period | ​ |  | ​ | (57)
​ | ​ | ​ | ​ | ​
Unrealized losses recognized during the reporting period on equity securities still held at the reporting date | ​ | $ | ​ | (57)
​
​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended
​ | ​ | March 31, 2023
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
​ | ​ | ​ | ​ | ​
Net gains recognized during the period on equity securities |  | $ | ​ | 34
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 | ​ | $ | ​ | 34
​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

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