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Employee benefit plans (Tables) | 12 Months Ended
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Dec. 31, 2023
Employee benefit plans |
Summary of pension and postretirement life insurance plans | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ |
​ | ​ | Pension Benefits |
(Dollars in thousands) | ​ | 2023 | ​ | 2022 |
Change in benefit obligation: |  | ​ | ​ |  | ​ | ​ | ​
Benefit obligation, beginning | ​ | $ | 13,792 | ​ | $ | 18,066 | ​
Interest cost | ​ |  | 655 | ​ |  | 456 | ​
Change in experience gain | ​ |  | 2 | ​ |  | 26 | ​
Change in actuarial assumptions | ​ |  | 306 | ​ |  | (3,899) | ​
Benefits paid | ​ |  | (910) | ​ |  | (857) | ​
Benefit obligation, ending | ​ |  | 13,845 | ​ |  | 13,792 | ​
Change in plan assets: | ​ | ​ | ​ | ​ | ​ | ​ | ​
Fair value of plan assets, beginning | ​ |  | 16,106 | ​ |  | 19,257 | ​
Actual return on plan assets | ​ |  | 2,413 | ​ |  | (2,294) | ​
Employer contributions | ​ |  | ​ | ​ |  | ​ | ​
Benefits paid | ​ |  | (910) | ​ |  | (857) | ​
Fair value of plan assets, ending | ​ |  | 17,609 | ​ |  | 16,106 | ​
Funded status at end of year | ​ | $ | 3,764 | ​ | $ | 2,314 | ​
Schedule of amounts recognized in balance sheet | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ |
​ | ​ | Pension Benefits | ​ |
(Dollars in thousands) | ​ | 2023 | ​ | 2022 | ​ |
(Other Assets)/Other Liabilities |  | $ | (3,764) |  | $ | (2,314) |  | ​
Amounts recognized in the accumulated other comprehensive loss consist of: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Net actuarial gain | ​ |  | (4,370) | ​ |  | (5,499) | ​ | ​
Deferred taxes | ​ |  | 956 | ​ |  | 1,184 | ​ | ​
Net amount recognized | ​ | $ | (3,414) | ​ | $ | (4,315) | ​ | ​
Schedule of components of net periodic benefit cost | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
(Dollars in thousands) | ​ | Pension Benefits | ​
Twelve Months Ended December 31, | ​ | 2023 | ​ | 2022 | ​ | 2021 | ​
Net periodic pension income: |  | ​ | ​ |  | ​ | ​ | ​ | ​ | ​ | ​
Interest cost | ​ | $ | 655 | ​ | $ | 456 | ​ | $ | 419 | ​
Expected return on plan assets | ​ |  | (1,170) | ​ |  | (1,407) | ​ |  | (1,288) | ​
Amortization of unrecognized net loss | ​ |  | 194 | ​ |  | 198 | ​ |  | 301 | ​
Net periodic pension income: | ​ | ​ | (321) | ​ | ​ | (753) | ​ | ​ | (568) | ​
Other changes in plan assets and benefit obligations recognized in other comprehensive income (loss): | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Net loss | ​ |  | 655 | ​ |  | 456 | ​ |  | 419 | ​
Deferred tax | ​ |  | (141) | ​ |  | (96) | ​ |  | (88) | ​
Total recognized in other comprehensive income (loss) | ​ |  | 514 | ​ |  | 360 | ​ |  | 331 | ​
Total recognized in net period pension cost and other comprehensive income (loss) | ​ | $ | 193 | ​ | $ | (393) | ​ | $ | (237) | ​
Schedule of weighted-average assumptions used to determine benefit obligations and related expenses | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ |
​ | ​ | Pension Benefits | ​ |
​ | ​ | 2023 | ​ | 2022 | ​ | 2021 | ​ |
Discount rate: |  | ​ | ​ | ​ |  | ​ |  | ​
Obligation |  | 4.73 | % | 4.93 | % | 2.59 | % | ​
Expense |  | 4.93 | ​ | 2.59 | ​ | 2.25 | ​ | ​
Expected long-term return on plan assets |  | 7.50 | % | 7.50 | % | 7.50 | % | ​
Schedule of pension plan weighted-average asset allocations | ​
​ | ​ | ​ | ​ | ​ | ​
​ |  | 2023 |  | 2022 |
Asset Category: | ​ | ​ | ​ | ​ | ​
Cash and cash equivalents |  | 4.4 | % | 4.7 | %
Equity securities |  | 63.3 | ​ | 60.3 | ​
Corporate bonds |  | 18.1 | ​ | 19.5 | ​
U.S. government securities |  | 14.2 | ​ | 15.5 | ​
​ |  | 100.0 | % | 100.0 | %
Schedule of fair value measurement of pension plan assets | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | December 31, 2023 |
​ |  | ​ | ​ |  | Quoted Prices in |  | ​ | ​ |  | ​ | ​ |
​ | ​ | ​ | ​ | Active Markets | ​ | Significant | ​ | Significant |
​ | ​ | ​ | ​ | for Identical | ​ | Observable | ​ | Observable |
​ | ​ | ​ | ​ | Assets | ​ | Inputs | ​ | Inputs |
(Dollars in thousands) | ​ | Total | ​ | (Level 1) | ​ | (Level 2) | ​ | (Level 3) |
Cash and cash equivalents | ​ | $ | 783 | ​ | $ | 783 | ​ | $ | ​ |  | $ | ​ | ​
Equity securities: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. large cap | ​ |  | 9,482 | ​ | ​ | 9,482 | ​ | ​ | ​ | ​ | ​ | ​ | ​
International | ​ |  | 1,671 | ​ | ​ | 1,671 | ​ | ​ | ​ | ​ | ​ | ​ | ​
Fixed income securities: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. Treasuries | ​ |  | 343 | ​ | ​ | ​ | ​ | ​ | 343 | ​ | ​ | ​ | ​
U.S. government agencies | ​ |  | 2,149 | ​ | ​ | ​ | ​ | ​ | 2,149 | ​ | ​ | ​ | ​
Corporate bonds | ​ |  | 3,181 | ​ | ​ | ​ | ​ | ​ | 3,181 | ​ | ​ | ​ | ​
Total | ​ | $ | 17,609 | ​ | $ | 11,936 | ​ | $ | 5,673 | ​ | $ | ​ | ​
​ ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
| ​ | December 31, 2022 | ​
​ |  | ​ | ​ |  | Quoted Prices in |  | ​ | ​ |  | ​ | ​ |
​ | ​ | ​ | ​ | ​ | Active Markets | ​ | Significant | ​ | Significant |
​ | ​ | ​ | ​ | ​ | for Identical | ​ | Observable | ​ | Observable |
​ | ​ | ​ | ​ | ​ | Assets | ​ | Inputs | ​ | Inputs |
(Dollars in thousands) | ​ | Total | ​ | (Level 1) | ​ | (Level 2) | ​ | (Level 3) |
Cash and cash equivalents | ​ | $ | 759 | ​ | $ | 759 | ​ | $ | ​ | ​ | $ | ​ | ​
Equity securities: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. large cap | ​ |  | 7,365 | ​ |  | 7,365 | ​ | ​ | ​ | ​ | ​ | ​ | ​
International | ​ |  | 2,338 | ​ |  | 2,338 | ​ | ​ | ​ | ​ | ​ | ​ | ​
Fixed income securities: | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
U.S. Treasuries | ​ |  | 511 | ​ | ​ | ​ | ​ | ​ | 511 | ​ | ​ | ​ | ​
U.S. government agencies | ​ |  | 1,991 | ​ | ​ | ​ | ​ |  | 1,991 | ​ | ​ | ​ | ​
Corporate bonds | ​ |  | 3,142 | ​ | ​ | ​ | ​ |  | 3,142 | ​ | ​ | ​ | ​
Total | ​ | $ | 16,106 | ​ | $ | 10,462 | ​ | $ | 5,644 | ​ | $ | ​ | ​
Schedule of expected benefit payments | ​
​ | ​ | ​ | ​ | ​
(Dollars in thousands) |  | Pension Benefits |
2024 | ​ | $ | 966 | ​
2025 | ​ |  | 999 | ​
2026 | ​ |  | 1,022 | ​
2027 | ​ |  | 1,030 | ​
2028 | ​ |  | 1,024 | ​
Thereafter | ​ | ​ | 4,946 | ​
​

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- Definition Tabular disclosure of the major categories of plan assets of pension plans and/or other employee benefit plans. This information may include, but is not limited to, the target allocation of plan assets, the fair value of each major category of plan assets, and the level within the fair value hierarchy in which the fair value measurements fall. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 715 -SubTopic 20 -Subparagraph (d)(5) -Name Accounting Standards Codification -Paragraph 1 -Section 50 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Tabular disclosure of the amounts that are recognized in the balance sheet (or statement of financial position) for pension plans and/or other employee benefit plans, showing separately the assets and current and noncurrent liabilities (if applicable) recognized. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Tabular disclosure of assumption used to determine benefit obligation and net periodic benefit cost of defined benefit plan. Includes, but is not limited to, discount rate, rate of compensation increase, expected long-term rate of return on plan assets and interest crediting rate. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (k) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Tabular disclosure of the effect of fair value measurements using significant unobservable inputs (Level 3) on changes in plan assets of pension plans and/or other employee benefit plans for the period. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(02) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Tabular disclosure of benefits expected to be paid by pension plans and/or other employee benefit plans in each of the next five fiscal years and in the aggregate for the five fiscal years thereafter. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 715 -SubTopic 20 -Subparagraph (f) -Name Accounting Standards Codification -Paragraph 1 -Section 50 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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- Definition Tabular disclosure of the components of net benefit costs for pension plans and/or other employee benefit plans including service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) recognized due to settlements or curtailments. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 715 -SubTopic 20 -Subparagraph (h) -Name Accounting Standards Codification -Paragraph 1 -Section 50 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480506/715-20-50-1 + Details
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