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Components of Net Periodic Benefit Cost (Details Textual) - USD ($) $ in Thousands | 6 Months Ended |
--- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefits Plan Estimated Future Employer Contributions In Next Fiscal Year |  |  | $ 2,300
Defined Benefit Plan, Expected Future Benefit Payment, Remainder of Fiscal Year |  |  | 813
Defined Contribution Plan Employer Accrued Discretionary Contribution Amount | $ 837 | $ 867 |
Post Retirement Health Benefits [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefits Plan Estimated Future Employer Contributions In Next Fiscal Year |  |  | $ 264
Defined Benefit Plan, Plan Assets, Contributions by Employer | 122 |  |
Pension Plan [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Defined Benefit Plan, Plan Assets, Contributions by Employer | $ 792 |  |

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