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Pension Plans and Other Postretirement Benefits - Contributions and Plan Assets (Details) - Pension Plan - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Contributions to employees' accounts | $ 0 | $ 0 | $ 0 |
Total assets at the end of the year | 22,090 | 25,843 | 25,446 | $ 27,667
Qualified plan | 401(k) plan |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Contributions to employees' accounts | 7,655 | 7,637 | 6,918 |
Total assets at the end of the year | 167,767 | 180,514 | 177,352 |
Nonqualified plan |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Contributions to employees' accounts | 70 | 84 | 72 |
Total assets at the end of the year | $ 0 | $ 0 | $ 0 |

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