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Fair Value Measurements (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017
Assets and liabilities measured at fair value on a recurring basis |  |
Marketable securities in deferred compensation plan | $ 3,868 | $ 4,616
Deferred Compensation, Excluding Share-based Payments and Retirement Benefits [Member] | Trustees and Management [Member] |  |
Assets and liabilities measured at fair value on a recurring basis |  |
Maximum percentage of participants' compensation which is deferrable (as a percent) | 100.00% |
Marketable securities in deferred compensation plan | $ 3,900 | $ 4,600

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