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Employee Retirement Plans (Schedule of Total Expense for All Retirement Plans) (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Defined Benefit Plan [Abstract] |  |  |
Total retirement plan expense | $ 3,044 | $ 2,887 | $ 2,934

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- References No definition available. + Details
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- Definition Period expense related to postemployment benefits. + References No definition available. + Details
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