Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements (Details)

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Fair Value Measurements (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Assets and liabilities measured at fair value    
Marketable securities in deferred compensation plan $ 4,194 $ 4,616
Deferred compensation plan | Trustees and Management    
Assets and liabilities measured at fair value    
Maximum percentage of participants' compensation which is deferrable (as a percent) 100.00%  
Marketable securities in deferred compensation plan $ 4,200