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Fair Value Measurements (Details) - USD ($) $ in Thousands | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2019 | Sep. 30, 2019 | Sep. 30, 2018 | Dec. 31, 2018
Assets and liabilities measured at fair value on a recurring basis |  |  |  |
Marketable securities in deferred compensation plan | $ 4,257 | $ 4,257 |  | $ 3,868
Impairment losses |  | $ 327 | $ 0 |
Fair value measurement on a recurring basis | Deferred compensation plan | Trustees and Management |  |  |  |
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 | 4,300 | $ 4,300 |  |
Fair value measurement on a nonrecurring basis | Frederick, Maryland |  |  |  |
Assets and liabilities measured at fair value on a recurring basis |  |  |  |
Impairment losses | $ 327 |  |  |

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