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Debt Securities (Tables) | 12 Months Ended
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Dec. 31, 2019
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 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
December 31, 2019: |  | Cost |  | Gains |  | Losses |  | Value
Debt securities available for sale | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
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
Total | ​ | $ | 1,627,065 | ​ | $ | 22,021 | ​ | $ | (829) | ​ | $ | 1,648,257
​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  |  | ​ |  | Gross |  | Gross |  |  | ​
​ | ​ | Amortized | ​ | Unrealized | ​ | Unrealized | ​ | Fair
December 31, 2018: |  | Cost |  | Gains |  | Losses |  | Value
Debt securities available for sale | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. Treasury securities | ​ | $ | 25,824 | ​ | $ | 1 | ​ | $ | (414) | ​ | $ | 25,411
Obligations of U.S. government corporations and agencies | ​ |  | 53,096 | ​ |  | 7 | ​ |  | (761) | ​ |  | 52,342
Obligations of states and political subdivisions | ​ |  | 171,131 | ​ |  | 484 | ​ |  | (1,571) | ​ |  | 170,044
Commercial mortgage-backed securities | ​ | ​ | 2,003 | ​ | ​ | — | ​ | ​ | (61) | ​ | ​ | 1,942
Residential mortgage-backed securities | ​ |  | 322,646 | ​ |  | 245 | ​ |  | (7,143) | ​ |  | 315,748
Corporate debt securities | ​ |  | 132,513 | ​ |  | 61 | ​ |  | (376) | ​ |  | 132,198
Total | ​ | $ | 707,213 | ​ | $ | 798 | ​ | $ | (10,326) | ​ | $ | 697,685
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Debt securities held to maturity | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Obligations of states and political subdivisions | ​ | $ | 33,947 | ​ | $ | 68 | ​ | $ | (87) | ​ | $ | 33,928
Commercial mortgage-backed securities | ​ | ​ | 59,054 | ​ | ​ | 11 | ​ | ​ | (1,003) | ​ | ​ | 58,062
Residential mortgage-backed securities | ​ | ​ | 515,659 | ​ | ​ | 1,748 | ​ | ​ | (6,037) | ​ | ​ | 511,370
Total | ​ | $ | 608,660 | ​ | $ | 1,827 | ​ | $ | (7,127) | ​ | $ | 603,360
Schedule of amortized cost and fair value of debt securities available for sale and held to maturity by contractual maturity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Debt securities available for sale
​ |  | Amortized |  | Fair
December 31, 2019: |  | Cost |  | Value
Due in one year or less | ​ | $ | 149,566 | ​ | $ | 150,109
Due after one year through five years | ​ |  | 355,497 | ​ |  | 361,500
Due after five years through ten years | ​ |  | 228,518 | ​ |  | 232,950
Due after ten years | ​ |  | 893,484 | ​ |  | 903,698
Total | ​ | $ | 1,627,065 | ​ | $ | 1,648,257
Schedule of realized gains and losses related to sales of securities | Realized gains and losses related to sales and calls of securities available for sale are summarized as follows (dollars in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | For the Years Ended December 31,
​ |  | 2019 |  | 2018 |  | 2017
Gross security gains | ​ | $ | 1,318 | ​ | $ | — | ​ | $ | 1,259
Gross security (losses) | ​ | ​ | (585) |  | ​ | (1,780) |  | ​ | (116)
Net gains (losses) on sales of securities(1) | ​ | $ | 733 | ​ | $ | (1,780) | ​ | $ | 1,143
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, 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(2) | ​ | 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)
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
| (2) | Unrealized losses for greater than 12 months, gross, was less than one thousand dollars.
​ ​ ​ ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | Less than 12 months, gross | ​ | 12 months or more, gross | ​ | Total, gross
​ | Fair |  | Unrealized |  | Fair |  | Unrealized |  | Fair |  | Unrealized
December 31, 2018: | Value |  | Losses |  | Value |  | Losses |  | Value |  | Losses
Debt securities available for sale | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. Treasury securities | $ | 995 | ​ | $ | (4) | ​ | $ | 24,343 | ​ | $ | (410) | ​ | $ | 25,338 | ​ | $ | (414)
Obligations of U.S. government corporations and agencies | ​ | 749 | ​ | ​ | (3) | ​ | ​ | 50,744 | ​ | ​ | (758) | ​ | ​ | 51,493 | ​ | ​ | (761)
Obligations of states and political subdivisions | ​ | 49,893 | ​ | ​ | (460) | ​ | ​ | 77,651 | ​ | ​ | (1,111) | ​ | ​ | 127,544 | ​ | ​ | (1,571)
Commercial mortgage-backed securities | ​ | — | ​ | ​ | — | ​ | ​ | 1,942 | ​ | ​ | (61) | ​ | ​ | 1,942 | ​ | ​ | (61)
Residential mortgage-backed securities |  | 48,387 | ​ |  | (496) | ​ |  | 247,573 | ​ |  | (6,647) | ​ |  | 295,960 | ​ |  | (7,143)
Corporate debt securities |  | 90,713 | ​ |  | (268) | ​ |  | 15,083 | ​ |  | (108) | ​ |  | 105,796 | ​ |  | (376)
Total temporarily impaired securities | $ | 190,737 | ​ | $ | (1,231) | ​ | $ | 417,336 | ​ | $ | (9,095) | ​ | $ | 608,073 | ​ | $ | (10,326)
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Debt securities held to maturity | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Obligations of states and political subdivisions | $ | 9,531 | ​ | $ | (33) | ​ | $ | 9,538 | ​ | $ | (54) | ​ | $ | 19,069 | ​ | $ | (87)
Commercial mortgage-backed securities | ​ | 12,067 | ​ | ​ | (212) | ​ | ​ | 45,041 | ​ | ​ | (791) | ​ | ​ | 57,108 | ​ | ​ | (1,003)
Residential mortgage-backed securities | ​ | 77,071 | ​ | ​ | (974) | ​ | ​ | 245,128 | ​ | ​ | (5,063) | ​ | ​ | 322,199 | ​ | ​ | (6,037)
Total temporarily impaired securities | $ | 98,669 | ​ | $ | (1,219) | ​ | $ | 299,707 | ​ | $ | (5,908) | ​ | $ | 398,376 | ​ | $ | (7,127)

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