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Employee Benefit Plans Employee Benefit Plans 8 (Details) - USD ($) $ in Thousands | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Post-Retirement Medical Benefits |  |  |
Employee benefit plans |  |  |
Fair value of post-retirement plan assets | $ 11,053 | $ 10,538 | $ 10,614
Post-Retirement Medical Benefits | Level 1 |  |  |
Employee benefit plans |  |  |
Fair value of post-retirement plan assets | 11,060 | 10,538 |
Post-Retirement Medical Benefits | Level 2 |  |  |
Employee benefit plans |  |  |
Fair value of post-retirement plan assets | 0 | 0 |
Post-Retirement Medical Benefits | Level 3 |  |  |
Employee benefit plans |  |  |
Fair value of post-retirement plan assets | 0 | 0 |
Post-Retirement Medical Benefits | Cash equivalents | Level 1 |  |  |
Employee benefit plans |  |  |
Fair value of post-retirement plan assets | 189 | 360 |
Post-Retirement Medical Benefits | Cash equivalents | Level 2 |  |  |
Employee benefit plans |  |  |
Fair value of post-retirement plan assets | 0 | 0 |
Post-Retirement Medical Benefits | Cash equivalents | Level 3 |  |  |
Employee benefit plans |  |  |
Fair value of post-retirement plan assets | 0 | 0 |
Post-Retirement Medical Benefits | Fixed income fund | Level 1 |  |  |
Employee benefit plans |  |  |
Fair value of post-retirement plan assets | 4,364 | 4,072 |
Post-Retirement Medical Benefits | Fixed income fund | Level 2 |  |  |
Employee benefit plans |  |  |
Fair value of post-retirement plan assets | 0 | 0 |
Post-Retirement Medical Benefits | Fixed income fund | Level 3 |  |  |
Employee benefit plans |  |  |
Fair value of post-retirement plan assets | 0 | 0 |
Post-Retirement Medical Benefits | U.S. large cap funds | Level 1 |  |  |
Employee benefit plans |  |  |
Fair value of post-retirement plan assets | 6,507 | 6,106 |
Post-Retirement Medical Benefits | U.S. large cap funds | Level 2 |  |  |
Employee benefit plans |  |  |
Fair value of post-retirement plan assets | 0 | 0 |
Post-Retirement Medical Benefits | U.S. large cap funds | Level 3 |  |  |
Employee benefit plans |  |  |
Fair value of post-retirement plan assets | 0 | 0 |
Fair Value | Post-Retirement Medical Benefits |  |  |
Employee benefit plans |  |  |
Fair value of post-retirement plan assets | 11,060 | 10,538 |
Fair Value | Post-Retirement Medical Benefits | Cash equivalents |  |  |
Employee benefit plans |  |  |
Fair value of post-retirement plan assets | 189 | 360 |
Fair Value | Post-Retirement Medical Benefits | Fixed income fund |  |  |
Employee benefit plans |  |  |
Fair value of post-retirement plan assets | 4,364 | 4,072 |
Fair Value | Post-Retirement Medical Benefits | U.S. large cap funds |  |  |
Employee benefit plans |  |  |
Fair value of post-retirement plan assets | $ 6,507 | $ 6,106 |
Minimum | Fixed income fund |  |  |
Employee benefit plans |  |  |
Maturity period of investments | 1 year |  |
Maximum | Fixed income fund |  |  |
Employee benefit plans |  |  |
Maturity period of investments | 20 years |  |

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