Quarterly report pursuant to Section 13 or 15(d)

Real Estate Acquisitions and Discontinued Operations (Operating Income (Loss) By Property Classified as Discontinued Operations) (Details)

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Real Estate Acquisitions and Discontinued Operations (Operating Income (Loss) By Property Classified as Discontinued Operations) (Details) (USD $)
In Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2011
Jun. 30, 2010
Jun. 30, 2011
Jun. 30, 2010
Income from operations of properties held for sale $ (10) $ 985 $ (468) $ 1,884
Parklawn Plaza [Member]
       
Real Estate and Accumulated Depreciation, Description of Property     Office  
Income from operations of properties held for sale   61   132
Lexington Building [Member]
       
Real Estate and Accumulated Depreciation, Description of Property     Office  
Income from operations of properties held for sale   46   65
Saratoga Building [Member]
       
Real Estate and Accumulated Depreciation, Description of Property     Office  
Income from operations of properties held for sale   121   225
Charleston Business Center [Member]
       
Real Estate and Accumulated Depreciation, Description of Property     Industrial  
Income from operations of properties held for sale   176   370
The Ridges [Member]
       
Real Estate and Accumulated Depreciation, Description of Property     Office  
Income from operations of properties held for sale   84   191
Ammendale I&II [Member]
       
Real Estate and Accumulated Depreciation, Description of Property     Industrial  
Income from operations of properties held for sale   265   516
Amvax [Member]
       
Real Estate and Accumulated Depreciation, Description of Property     Industrial  
Income from operations of properties held for sale   88   178
Dulles Station, Phase I [Member]
       
Real Estate and Accumulated Depreciation, Description of Property     Office  
Income from operations of properties held for sale $ (10) $ 144 $ (468) $ 207