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Securities (Tables) | 3 Months Ended
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Mar. 31, 2021
Securities |
Schedule of amortized cost, unrealized gains and losses and fair values of securities classified available for sale and held to maturity | The table below provides the amortized cost, unrealized gains and losses, and fair values of debt securities summarized by major category (dollars in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ |  | ​ | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | ​ | ​ | ​ | Fair
March 31, 2021: |  | Cost |  | Gains |  | Losses |  | ​ | ACL |  | Value
U.S. Treasury securities | ​ | $ | 23,514 | ​ | $ | 243 | ​ | $ | — | ​ | $ | — | ​ | $ | 23,757
Obligations of U.S. government corporations and agencies | ​ |  | 64,903 | ​ |  | 1,720 | ​ |  | (45) | ​ | ​ | — | ​ |  | 66,578
Obligations of states and political subdivisions | ​ |  | 280,150 | ​ |  | 8,854 | ​ |  | (1,542) | ​ | ​ | — | ​ |  | 287,462
Commercial mortgage-backed securities | ​ | ​ | 499,996 | ​ | ​ | 5,308 | ​ | ​ | (6,786) | ​ | ​ | — | ​ | ​ | 498,518
Residential mortgage-backed securities | ​ |  | 1,786,062 | ​ |  | 18,587 | ​ |  | (18,329) | ​ | ​ | — | ​ |  | 1,786,320
Corporate debt securities | ​ |  | 134,783 | ​ |  | 964 | ​ |  | (1,427) | ​ | ​ | — | ​ |  | 134,320
Debt securities available for sale | ​ | $ | 2,789,408 | ​ | $ | 35,676 | ​ | $ | (28,129) | ​ | $ | — | ​ | $ | 2,796,955
​ ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ |  | ​ | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | ​ | ​ | ​ | Fair
December 31, 2020: |  | Cost |  | Gains |  | Losses |  | ​ | ACL |  | Value
U.S. Treasury securities | ​ | $ | 27,481 | ​ | $ | 356 | ​ | $ | — | ​ | $ | — | ​ | $ | 27,837
Obligations of U.S. government corporations and agencies | ​ |  | 67,406 | ​ |  | 2,162 | ​ |  | (49) | ​ | ​ | — | ​ |  | 69,519
Obligations of states and political subdivisions | ​ |  | 292,940 | ​ |  | 11,779 | ​ |  | (8) | ​ | ​ | — | ​ |  | 304,711
Commercial mortgage-backed securities | ​ | ​ | 408,716 | ​ | ​ | 10,212 | ​ | ​ | (312) | ​ | ​ | — | ​ | ​ | 418,616
Residential mortgage-backed securities | ​ |  | 1,344,047 | ​ |  | 24,571 | ​ |  | (303) | ​ | ​ | — | ​ |  | 1,368,315
Corporate debt securities | ​ |  | 70,953 | ​ |  | 1,237 | ​ |  | (1) | ​ | ​ | — | ​ |  | 72,189
Debt securities available for sale | ​ | $ | 2,211,543 | ​ | $ | 50,317 | ​ | $ | (673) | ​ | ​ | — | ​ | $ | 2,261,187
Schedule of amortized cost and fair value of debt securities available for sale and held to maturity by contractual maturity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ |  | ​ | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | ​ | ​ | ​ | Fair
December 31, 2020: |  | Cost |  | Gains |  | Losses |  | ​ | ACL |  | Value
U.S. Treasury securities | ​ | $ | 27,481 | ​ | $ | 356 | ​ | $ | — | ​ | $ | — | ​ | $ | 27,837
Obligations of U.S. government corporations and agencies | ​ |  | 67,406 | ​ |  | 2,162 | ​ |  | (49) | ​ | ​ | — | ​ |  | 69,519
Obligations of states and political subdivisions | ​ |  | 292,940 | ​ |  | 11,779 | ​ |  | (8) | ​ | ​ | — | ​ |  | 304,711
Commercial mortgage-backed securities | ​ | ​ | 408,716 | ​ | ​ | 10,212 | ​ | ​ | (312) | ​ | ​ | — | ​ | ​ | 418,616
Residential mortgage-backed securities | ​ |  | 1,344,047 | ​ |  | 24,571 | ​ |  | (303) | ​ | ​ | — | ​ |  | 1,368,315
Corporate debt securities | ​ |  | 70,953 | ​ |  | 1,237 | ​ |  | (1) | ​ | ​ | — | ​ |  | 72,189
Debt securities available for sale | ​ | $ | 2,211,543 | ​ | $ | 50,317 | ​ | $ | (673) | ​ | ​ | — | ​ | $ | 2,261,187
Schedule of realized gains and losses related to sales of securities | Realized gains and losses related to sales and calls of debt securities available for sale are summarized as follows (dollars in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended March 31,
​ |  | 2021 |  | 2020
Gross security gains | ​ | $ | 25 | ​ | $ | 1,561
Gross security (losses) | ​ | ​ | — | ​ | ​ | (5)
Net gains (losses) on sales of debt securities (1) | ​ | $ | 25 | ​ | $ | 1,556
| (1) | Net gains (losses) on sales of securities reported on the unaudited Consolidated Statements of Income includes sales of equity securities, excluded in this table.
​
Schedule of securities with gross unrealized losses aggregated by investment category and length of time that individual securities have been in a continuous loss position | The following information pertains to debt securities with gross unrealized losses, aggregated by investment category and the length of time that individual securities have been in a continuous loss position (dollars in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Less than 12 months | ​ | 12 months or more | ​ | Total
​ | ​ | Fair |  | Unrealized |  | Fair |  | Unrealized |  | Fair |  | Unrealized
March 31, 2021: |  | Value |  | Losses |  | Value |  | Losses |  | Value |  | Losses
Debt securities available for sale | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Obligations of U.S. government corporations and agencies | ​ | $ | — | ​ | $ | — | ​ | $ | 4,695 | ​ | $ | (45) | ​ | $ | 4,695 | ​ | $ | (45)
Obligations of states and political subdivisions | ​ | ​ | 58,555 | ​ | ​ | (1,542) | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 58,555 | ​ | ​ | (1,542)
Commercial mortgage-backed securities | ​ | ​ | 314,109 | ​ | ​ | (6,786) | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 314,109 | ​ | ​ | (6,786)
Residential mortgage-backed securities | ​ |  | 1,019,462 | ​ |  | (18,325) | ​ |  | 352 | ​ |  | (4) | ​ |  | 1,019,814 | ​ |  | (18,329)
Corporate debt securities | ​ |  | 95,164 | ​ |  | (1,427) | ​ |  | — | ​ |  | — | ​ |  | 95,164 | ​ |  | (1,427)
Total temporarily impaired securities | ​ | $ | 1,487,290 | ​ | $ | (28,080) | ​ | $ | 5,047 | ​ | $ | (49) | ​ | $ | 1,492,337 | ​ | $ | (28,129)
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Less than 12 months | ​ | 12 months or more | ​ | Total
​ | ​ | Fair |  | Unrealized |  | Fair |  | Unrealized |  | Fair |  | Unrealized
December 31, 2020: |  | Value |  | Losses |  | Value |  | Losses |  | Value |  | Losses
Debt securities available for sale | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Obligations of U.S. government corporations and agencies | ​ | $ | — | ​ | $ | — | ​ | $ | 4,957 | ​ | $ | (49) | ​ | $ | 4,957 | ​ | $ | (49)
Obligations of states and political subdivisions | ​ | ​ | 762 | ​ | ​ | (8) | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 762 | ​ | ​ | (8)
Commercial mortgage-backed securities | ​ | ​ | 129,655 | ​ | ​ | (312) | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 129,655 | ​ | ​ | (312)
Residential mortgage-backed securities | ​ |  | 89,997 | ​ |  | (300) | ​ |  | 139 | ​ |  | (3) | ​ |  | 90,136 | ​ |  | (303)
Corporate debt securities | ​ |  | 1,499 | ​ |  | (1) | ​ |  | — | ​ |  | — | ​ |  | 1,499 | ​ |  | (1)
Total temporarily impaired securities | ​ | $ | 221,913 | ​ | $ | (621) | ​ | $ | 5,096 | ​ | $ | (52) | ​ | $ | 227,009 | ​ | $ | (673)

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