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Retirement Plans and Other Postretirement Benefits - Contributions and Plan Assets (Details) - Pension Plan - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Contributions to employees' accounts | $ 0 | $ 0 | $ 0 |
Total assets at the end of the year | 21,945 | 23,164 | 22,090 | $ 25,843
Qualified Plan | 401(k) plan |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Contributions to employees' accounts | 8,187 | 8,233 | 7,655 |
Total assets at the end of the year | 228,449 | 206,247 | 167,767 |
Nonqualified Plan |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Contributions to employees' accounts | 68 | 58 | 70 |
Total assets at the end of the year | $ 0 | $ 0 | $ 0 |

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