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Debt securities | 12 Months Ended
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Dec. 31, 2020
Debt securities |
Debt securities | Note 3. Debt Securities ​ The table below provides information on debt securities (dollars in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | Allowance |  | ​ | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | ​ | for Credit | ​ | Fair
December 31, 2020: |  | Cost |  | Gains |  | Losses |  | ​ | Losses |  | 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
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | Gross | ​ | Gross | ​ | ​ | ​
​ | ​ |  | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | Fair
December 31, 2019: | ​ | ​ |  | Cost |  | Gains |  | Losses |  | Value
U.S. Treasury securities | ​ | ​ | ​ | ​ | $ | 51,472 | ​ | $ | 265 | ​ | $ | — | ​ | $ | 51,737
Obligations of U.S. government corporations and agencies | ​ |  | 160,364 | ​ |  | 2,684 | ​ |  | (48) | ​ |  | 163,000
Obligations of states and political subdivisions | ​ | ​ | ​ | ​ |  | 262,492 | ​ |  | 5,810 | ​ |  | (11) | ​ |  | 268,291
Commercial mortgage-backed securities | ​ | ​ | ​ | ​ | ​ | 137,733 | ​ | ​ | 1,700 | ​ | ​ | (146) | ​ | ​ | 139,287
Residential mortgage-backed securities | ​ | ​ | ​ | ​ |  | 912,308 | ​ |  | 10,282 | ​ |  | (624) | ​ |  | 921,966
Corporate debt securities | ​ | ​ | ​ | ​ |  | 102,696 | ​ |  | 1,280 | ​ |  | — | ​ |  | 103,976
Debt securities available for sale | ​ | ​ | ​ | ​ | $ | 1,627,065 | ​ | $ | 22,021 | ​ | $ | (829) | ​ | $ | 1,648,257
​ The amortized cost and fair value of debt securities, by contractual maturity or pre-refunded date, are shown below. Mortgages underlying mortgage-backed securities may be called or prepaid; therefore, actual maturities could differ from the contractual maturities. All mortgage-backed securities were issued by U.S. government corporations and agencies (dollars in thousands). ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | As of December 31, 2020
​ |  | Amortized |  | Fair
​ |  | Cost |  | Value
Due in one year or less | ​ | $ | 121,208 | ​ | $ | 122,213
Due after one year through five years | ​ |  | 239,021 | ​ |  | 247,833
Due after five years through ten years | ​ |  | 256,485 | ​ |  | 267,859
Due after ten years | ​ |  | 1,594,829 | ​ |  | 1,623,282
Debt securities available for sale | ​ | $ | 2,211,543 | ​ | $ | 2,261,187
​ Realized gains and losses related to sales and calls of debt securities available for sale are summarized as follows (dollars in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Years Ended December 31,
​ |  | 2020 |  | 2019 |  | 2018
Gross security gains | ​ | $ | 1,732 | ​ | $ | 1,318 | ​ | $ | —
Gross security (losses) | ​ | ​ | (8) |  | ​ | (585) |  | ​ | (1,780)
Net gains (losses) on sales of securities(1) | ​ | $ | 1,724 | ​ | $ | 733 | ​ | $ | (1,780)
(1) | Net gains (losses) on sales of securities reported on the Consolidated Statements of Income includes sale of equity securities, excluded in this table.
​ Debt securities with carrying amounts of $628.0 million and $704.7 million on December 31, 2020 and 2019, respectively, were pledged as collateral for public deposits, securities sold under agreements to repurchase, and for other purposes as required or permitted by law. ​ 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, gross | ​ | 12 months or more, gross | ​ | Total, gross
​ | ​ | Fair |  | Unrealized |  | Fair |  | Unrealized |  | Fair |  | Unrealized
December 31, 2020: |  | Value |  | Losses |  | Value |  | Losses |  | Value |  | Losses
Debt securities available for sale | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. Treasury securities | ​ | $ | — | ​ | $ | — | ​ | $ | — | ​ | $ | — | ​ | $ | — | ​ | $ | —
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)
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Less than 12 months, gross | ​ | 12 months or more, gross | ​ | Total, gross
​ | ​ | Fair |  | Unrealized |  | Fair |  | Unrealized |  | Fair |  | Unrealized
December 31, 2019: |  | Value |  | Losses |  | Value |  | Losses |  | Value |  | Losses
Debt securities available for sale | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. Treasury securities | ​ | $ | — | ​ | $ | — | ​ | $ | — | ​ | $ | — | ​ | $ | — | ​ | $ | —
Obligations of U.S. government corporations and agencies | ​ | ​ | 6,362 | ​ | ​ | (48) | ​ | ​ | — | ​ | ​ | — | ​ | ​ | 6,362 | ​ | ​ | (48)
Obligations of states and political subdivisions(1) | ​ | ​ | 4,981 | ​ | ​ | (11) | ​ | ​ | 1,548 | ​ | ​ | — | ​ | ​ | 6,529 | ​ | ​ | (11)
Commercial mortgage-backed securities | ​ | ​ | 33,322 | ​ | ​ | (144) | ​ | ​ | 2,044 | ​ | ​ | (2) | ​ | ​ | 35,366 | ​ | ​ | (146)
Residential mortgage-backed securities | ​ |  | 78,326 | ​ |  | (245) | ​ |  | 50,259 | ​ |  | (379) | ​ |  | 128,585 | ​ |  | (624)
Corporate debt securities | ​ |  | — | ​ |  | — | ​ |  | — | ​ |  | — | ​ |  | — | ​ |  | —
Total temporarily impaired securities | ​ | $ | 122,991 | ​ | $ | (448) | ​ | $ | 53,851 | ​ | $ | (381) | ​ | $ | 176,842 | ​ | $ | (829)
(1) | Unrealized losses for greater than 12 months, gross, was less than one thousand dollars.
​ Debt securities available for sale are not within the scope of CECL, however, the accounting for credit losses on these securities is affected by ASC 326-30. As of December 31, 2020, the Company’s debt security portfolio consisted of 1,114 securities, compared to 1,178 securities at December 31, 2019. The total number of debt securities in the investment portfolio in an unrealized loss position as of December 31, 2020, was 23 and represented an unrealized loss of 0.30% of the aggregate fair value. The total number of debt securities in the investment portfolio in an unrealized loss position as of December 31, 2019, was 55 and represented an unrealized loss of 0.47% of the aggregate fair value. Unrealized losses relate to changes in market interest rates and market conditions that do not represent credit-related impairments. Furthermore, the Company does not intend to sell such securities and it is more likely than not that the Company will recover the amortized cost prior to being required to sell the debt securities. Full collection of the amounts due according to the contractual terms of the debt securities is expected; therefore, the impairment related to noncredit factors is recognized in accumulated other comprehensive income (loss), net of applicable taxes, at December 31, 2020. As of December 31, 2020, the Company did not hold general obligation bonds of any single issuer, the aggregate of which exceeded 10% of the Company’s stockholders’ equity.

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