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Debt securities (Tables) | 12 Months Ended
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Dec. 31, 2020
Debt 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 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
Schedule of amortized cost and fair value of debt securities available for sale and held to maturity by contractual maturity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | 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
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): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | 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.
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, 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.

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