Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements (Narrative) (Details)

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Fair Value Measurements (Narrative) (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Sep. 30, 2019
Dec. 31, 2017
Jun. 30, 2017
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Assets and liabilities measured at fair value on a recurring basis            
Marketable securities in deferred compensation plan       $ 3,060 $ 3,868  
Impairment losses       $ 329 2,367 $ 15,116
Fair Value, Recurring | Deferred Compensation, Excluding Share-based Payments and Retirement Benefits | 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       $ 3,100 $ 3,900  
Fair Value, Nonrecurring | Frederick, Maryland            
Assets and liabilities measured at fair value on a recurring basis            
Impairment losses $ 327          
Other | Fair Value, Nonrecurring | Aberdeen, Maryland            
Assets and liabilities measured at fair value on a recurring basis            
Impairment losses   $ 9,000        
Land in Development or held-for-future Development | Fair Value, Nonrecurring | Aberdeen, Maryland            
Assets and liabilities measured at fair value on a recurring basis            
Impairment losses   $ 4,700        
Held-for-sale | Fair Value, Nonrecurring | White Marsh, Maryland            
Assets and liabilities measured at fair value on a recurring basis            
Impairment losses     $ 1,600