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Investment Securities (Tables) | 12 Months Ended
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Dec. 31, 2020
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, 2020 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 Debt Securities | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | unrealized | ​ | unrealized | ​ | Estimated | ​ | Carrying | ​
​ | ​ | cost | ​ | gains | ​ | losses | ​ | fair value | ​ | value(1) | ​
​ | ​ | (Dollars in Thousands) | ​
Residential mortgage-backed securities |  | $ | 3,006,592 |  | $ | 32,701 |  | $ | (9,339) |  | $ | 3,029,954 |  | $ | 3,029,954 | ​
Obligations of states and political subdivisions | ​ |  | 47,697 | ​ |  | 3,131 | ​ |  | (14) | ​ |  | 50,814 | ​ |  | 50,814 | ​
Total investment securities | ​ | $ | 3,054,289 | ​ | $ | 35,832 | ​ | $ | (9,353) | ​ | $ | 3,080,768 | ​ | $ | 3,080,768 | ​
(1) | Included in the carrying value of residential mortgage- backed securities are $371,407 of mortgage-backed securities issued by Ginnie Mae and $2,658,547 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, 2019 are as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Held to Maturity | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | unrealized | ​ | unrealized | ​ | Estimated | ​ | Carrying | ​
​ | ​ | cost | ​ | gains | ​ | losses | ​ | fair value | ​ | value | ​
​ | ​ | (Dollars in Thousands) | ​
Other securities |  | $ | 2,400 |  | $ | — |  | $ | — |  | $ | 2,400 |  | $ | 2,400 | ​
Total investment securities | ​ | $ | 2,400 | ​ | $ | — | ​ | $ | — | ​ | $ | 2,400 | ​ | $ | 2,400 | ​
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Available for Sale | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | Estimated | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | unrealized | ​ | unrealized | ​ | fair | ​ | Carrying | ​
​ | ​ | cost | ​ | gains | ​ | losses | ​ | value | ​ | value(1) | ​
​ | ​ | (Dollars in Thousands) | ​
Residential mortgage-backed securities |  | $ | 3,285,623 |  | $ | 16,534 |  | $ | (16,609) |  | $ | 3,285,548 |  | $ | 3,285,548 | ​
Obligations of states and political subdivisions | ​ |  | 90,447 | ​ |  | 2,933 | ​ |  | (5) | ​ |  | 93,375 | ​ |  | 93,375 | ​
Total investment securities | ​ | $ | 3,376,070 | ​ | $ | 19,467 | ​ | $ | (16,614) | ​ | $ | 3,378,923 | ​ | $ | 3,378,923 | ​
(1) | Included in the carrying value of residential mortgage- backed securities are $571,247 of mortgage-backed securities issued by Ginnie Mae, $2,714,301 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 |  | $ | 1,200 |  | $ | 1,200 |  | $ | — |  | $ | — | ​
Due after one year through five years | ​ |  | 2,200 | ​ |  | 2,200 | ​ |  | — | ​ |  | — | ​
Due after five years through ten years | ​ |  | — | ​ |  | — | ​ |  | 355 | ​ |  | 356 | ​
Due after ten years | ​ |  | — | ​ |  | — | ​ |  | 47,342 | ​ |  | 50,458 | ​
Residential mortgage-backed securities | ​ |  | — | ​ |  | — | ​ |  | 3,006,592 | ​ |  | 3,029,954 | ​
Total investment securities | ​ | $ | 3,400 | ​ | $ | 3,400 | ​ | $ | 3,054,289 | ​ | $ | 3,080,768 | ​
Gross unrealized losses on 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, 2020 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 |  | $ | 1,462,232 |  | $ | (9,339) |  | $ | — |  | $ | — |  | $ | 1,462,232 |  | $ | (9,339) | ​
Obligations of states and political subdivisions | ​ |  | — | ​ |  | — | ​ |  | 757 | ​ |  | (14) | ​ |  | 757 | ​ |  | (14) | ​
​ | ​ | $ | 1,462,232 | ​ | $ | (9,339) | ​ | $ | 757 | ​ | $ | (14) | ​ | $ | 1,462,989 | ​ | $ | (9,353) | ​
​ 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, 2019 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 |  | $ | 523,031 |  | $ | (2,269) |  | $ | 1,448,109 |  | $ | (14,340) |  | $ | 1,971,140 |  | $ | (16,609) | ​
Obligations of states and political subdivisions | ​ |  | 766 | ​ |  | (5) | ​ |  | — | ​ |  | — | ​ |  | 766 | ​ |  | (5) | ​
​ | ​ | $ | 523,797 | ​ | $ | (2,274) | ​ | $ | 1,448,109 | ​ | $ | (14,340) | ​ | $ | 1,971,906 | ​ | $ | (16,614) | ​
​
Summary of unrealized and realized gains and losses recognized in net income on equity securities | ​
​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended
​ | ​ | December 31, 2020
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
Net gains recognized during the period on equity securities |  | $ | ​ | 107
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 | ​ | $ | ​ | 107
​
​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended
​ | ​ | December 31, 2019
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
Net gains recognized during the period on equity securities |  | $ | ​ | 158
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 | ​ | $ | ​ | 158
​ | ​ | ​ | ​ | ​
​ ​ ​
​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended
​ | ​ | December 31, 2018
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
Net losses recognized during the period on equity securities |  | $ | ​ | (388)
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 | ​ | $ | ​ | (388)

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