Quarterly report pursuant to Section 13 or 15(d)

Real Estate Acquisitions And Discontinued Operations (Narrative) (Details)

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Real Estate Acquisitions And Discontinued Operations (Narrative) (Details) (USD $)
0 Months Ended 3 Months Ended 9 Months Ended 9 Months Ended 0 Months Ended 9 Months Ended
Sep. 02, 2011
Sep. 30, 2011
Sep. 30, 2011
Sep. 30, 2010
Aug. 05, 2011
Sep. 30, 2011
Tenant Origination Costs [Member]
months
Sep. 30, 2011
Leasing Commissions/Absorption Costs [Member]
months
Sep. 30, 2011
Net Lease Intangible Assets [Member]
months
Sep. 30, 2011
Net Lease Intangible Liabilities [Member]
months
Sep. 30, 2011
Dulles Station, Phase I [Member]
Jun. 15, 2011
650 North Glebe Road [Member]
Sep. 30, 2011
John Marshall II and Olney Village Center [Member]
Real Estate Properties [Line Items]                        
Weighted remaining average life, in months           65 54 68 65      
Payments to Acquire Businesses, Net of Cash Acquired                     $ 11,800,000  
Contract purchase price   371,600,000 371,600,000                  
Acquisition cost     281,701,000 68,000,000                
Notes Assumed     (78,500,000)                 76,700,000
Credits received at settlement     1,300,000                  
Number of purchase and sale agreements entered   5 5   5              
Number of office assets         2              
Number of purchase and sale agreements closed on September 2, 2011 3                      
Number of remaining purchase and sale agreements closed at end of period   2 2                  
Aggregate purchase price     350,000,000                  
Proceeds from sale of real estate 235,800,000                      
Gain on sale of real estate   56,600,000                    
Impairment charge                   $ 600,000