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Investment Securities and Equity Securities with Readily Determinable Fair Values (Tables) | 3 Months Ended
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Mar. 31, 2021
Investment Securities and Equity Securities with Readily Determinable Fair Values |
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, 2021 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) | ​
U.S. Treasury securities |  | $ | 945,682 |  | $ | — |  | $ | — |  | $ | 945,682 |  | $ | 945,682 | ​
Residential mortgage-backed securities | ​ | ​ | 3,043,953 | ​ | ​ | 28,203 | ​ | ​ | (9,120) | ​ | ​ | 3,063,036 | ​ | ​ | 3,063,036 | ​
Obligations of states and political subdivisions | ​ |  | 45,111 | ​ |  | 3,538 | ​ |  | (10) | ​ |  | 48,639 | ​ |  | 48,639 | ​
Total investment securities | ​ | $ | 4,034,746 | ​ | $ | 31,741 | ​ | $ | (9,130) | ​ | $ | 4,057,357 | ​ | $ | 4,057,357 | ​
(1) | Included in the carrying value of residential mortgage-backed securities are $326,756 of mortgage-backed securities issued by Ginnie Mae and $2,736,280 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, 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 | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | Estimated | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | unrealized | ​ | unrealized | ​ | fair | ​ | Carrying | ​
​ | ​ | cost | ​ | gains | ​ | losses | ​ | 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.
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 |  | $ | 1,075 |  | $ | 1,075 |  | $ | 945,682 |  | $ | 945,682 | ​
Due after one year through five years | ​ |  | 2,325 | ​ |  | 2,325 | ​ |  | — | ​ |  | — | ​
Due after five years through ten years | ​ |  | — | ​ |  | — | ​ |  | 355 | ​ |  | 356 | ​
Due after ten years | ​ |  | — | ​ |  | — | ​ |  | 44,756 | ​ |  | 48,283 | ​
Residential mortgage-backed securities | ​ |  | — | ​ |  | — | ​ |  | 3,043,953 | ​ |  | 3,063,036 | ​
Total investment securities | ​ | $ | 3,400 | ​ | $ | 3,400 | ​ | $ | 4,034,746 | ​ | $ | 4,057,357 | ​
Gross unrealized losses on investment securities and the related fair value | ​ Gross unrealized losses on debt investment securities and the fair value of the related securities, aggregated by investment category and length of time that individual debt securities have been in a continuous unrealized loss position at March 31, 2021, 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 |  | $ | 1,518,962 |  | $ | (9,120) |  | $ | — |  | $ | — |  | $ | 1,518,962 |  | $ | (9,120) | ​
Obligations of states and political subdivisions | ​ |  | — | ​ |  | — | ​ |  | 536 | ​ |  | (10) | ​ |  | 536 | ​ |  | (10) | ​
​ | ​ | $ | 1,518,962 | ​ | $ | (9,120) | ​ | $ | 536 | ​ | $ | (10) | ​ | $ | 1,519,498 | ​ | $ | (9,130) | ​
​ 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) | ​
​
Summary of unrealized and realized gains and losses recognized in net income on equity securities | ​
​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended
​ | ​ | March 31, 2021
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
​ | ​ | ​ | ​ | ​
Net losses recognized during the period on equity securities |  | $ | ​ | (77)
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 | ​ | $ | ​ | (77)
​ ​
​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended
​ | ​ | March 31, 2020
​ | ​ | (Dollars in Thousands)
​ | ​ | ​
​ | ​ | ​ | ​ | ​
Net gains recognized during the period on equity securities |  | $ | ​ | 30
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 | ​ | $ | ​ | 30
​ ​ ​ ​

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