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Securities (Tables) | 9 Months Ended
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Sep. 30, 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): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | As of September 30, 2021
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | ​ | ​ | ​ | Fair
​ |  | Cost |  | Gains |  | Losses |  | ​ | ACL |  | Value
Debt securities available for sale | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. Treasury securities | ​ | $ | 185,636 | ​ | $ | 72 | ​ | $ | (255) | ​ | $ | — | ​ | $ | 185,453
Obligations of U.S. government corporations and agencies | ​ |  | 37,784 | ​ |  | 1,213 | ​ |  | — | ​ | ​ | — | ​ |  | 38,997
Obligations of states and political subdivisions | ​ |  | 288,023 | ​ |  | 8,743 | ​ |  | (754) | ​ | ​ | — | ​ |  | 296,012
Commercial mortgage-backed securities | ​ | ​ | 607,097 | ​ | ​ | 5,278 | ​ | ​ | (9,078) | ​ | ​ | — | ​ | ​ | 603,297
Residential mortgage-backed securities | ​ |  | 2,108,388 | ​ |  | 13,973 | ​ |  | (19,753) | ​ | ​ | — | ​ |  | 2,102,608
Asset-backed securities | ​ | ​ | 468,618 | ​ | ​ | 323 | ​ | ​ | (134) | ​ | ​ | — | ​ | ​ | 468,807
Corporate debt securities | ​ |  | 301,878 | ​ |  | 1,374 | ​ |  | (1,182) | ​ | ​ | — | ​ |  | 302,070
Total debt securities available for sale | ​ | $ | 3,997,424 | ​ | $ | 30,976 | ​ | $ | (31,156) | ​ | $ | — | ​ | $ | 3,997,244
​ ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | As of December 31, 2020
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | ​ | ​ | ​ | Fair
​ |  | Cost |  | Gains |  | Losses |  | ​ | ACL |  | Value
Debt securities available for sale | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
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
Total 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 | All mortgage-backed securities were issued by U.S. government corporations and agencies (dollars in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | As of September 30, 2021
​ |  | Amortized |  | Fair
​ |  | Cost |  | Value
Debt securities available for sale | ​ | ​ | ​ | ​ | ​ | ​
Due in one year or less | ​ | $ | 103,158 | ​ | $ | 103,787
Due after one year through five years | ​ |  | 603,868 | ​ |  | 608,074
Due after five years through ten years | ​ |  | 385,479 | ​ |  | 393,540
Due after ten years | ​ |  | 2,904,919 | ​ |  | 2,891,843
Total debt securities available for sale | ​ | $ | 3,997,424 | ​ | $ | 3,997,244
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 September 30, | ​ | Nine Months Ended September 30,
​ |  | 2021 |  | 2020 |  | 2021 |  | 2020
Realized gains and losses on sales of debt securities | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Gross security gains | ​ | $ | — | ​ | $ | 11 | ​ | $ | 524 | ​ | $ | 1,718
Gross security (losses) | ​ | ​ | (5) | ​ | ​ | — | ​ | ​ | (410) | ​ | ​ | (8)
Net gains (losses) on sales of debt securities | ​ | $ | (5) | ​ | $ | 11 | ​ | $ | 114 | ​ | $ | 1,710
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): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | As of September 30, 2021
​ | ​ | Less than 12 months | ​ | 12 months or more | ​ | Total
​ | ​ | Fair |  | Unrealized |  | Fair |  | Unrealized |  | Fair |  | Unrealized
​ |  | Value |  | Losses |  | Value |  | Losses |  | Value |  | Losses
Debt securities available for sale | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. Treasury securities | ​ | $ | 180,317 | ​ | $ | (255) | ​ | $ | — | ​ | $ | — | ​ | $ | 180,317 | ​ | $ | (255)
Obligations of states and political subdivisions | ​ | ​ | 55,434 | ​ | ​ | (754) | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 55,434 | ​ | ​ | (754)
Commercial mortgage-backed securities | ​ | ​ | 434,220 | ​ | ​ | (9,036) | ​ | ​ | 3,063 | ​ | ​ | (42) | ​ | ​ | 437,283 | ​ | ​ | (9,078)
Residential mortgage-backed securities | ​ |  | 1,392,160 | ​ |  | (19,707) | ​ |  | 5,568 | ​ |  | (46) | ​ |  | 1,397,728 | ​ |  | (19,753)
Asset-backed securities | ​ | ​ | 51,183 | ​ | ​ | (134) | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 51,183 | ​ | ​ | (134)
Corporate debt securities | ​ |  | 210,951 | ​ |  | (1,182) | ​ |  | — | ​ |  | — | ​ |  | 210,951 | ​ |  | (1,182)
Total temporarily impaired securities | ​ | $ | 2,324,265 | ​ | $ | (31,068) | ​ | $ | 8,631 | ​ | $ | (88) | ​ | $ | 2,332,896 | ​ | $ | (31,156)
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | As of December 31, 2020
​ | ​ | Less than 12 months | ​ | 12 months or more | ​ | Total
​ | ​ | Fair |  | Unrealized |  | Fair |  | Unrealized |  | Fair |  | Unrealized
​ |  | 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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