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Employee Benefit Plans 8 (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019
Post-Retirement Medical Benefits |  |
Employee benefit plans |  |
Employer contributions | $ 269 | $ 170
Pension Benefits |  |
Employee benefit plans |  |
Employer contributions | 3,718 | $ 3,913
Expected future employer's contribution | $ 3,600 |
Minimum | Fixed income fund | Post-Retirement Medical Benefits |  |
Employee benefit plans |  |
Maturity period of investments | 1 year |
Maximum | Fixed income fund | Post-Retirement Medical Benefits |  |
Employee benefit plans |  |
Maturity period of investments | 20 years |

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