Annual report pursuant to Section 13 and 15(d)

Real Estate Investments Continuing Operations (Details)

v2.4.0.6
Real Estate Investments Continuing Operations (Details) (USD $)
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Real Estate Properties [Line Items]      
Real estate investment property, at cost $ 2,406,783,000 $ 2,051,936,000  
Held for development 43,089,000 26,240,000  
Real estate impairment 14,526,000 0 0
Land Available for Development 9,400,000    
Office [Member]
     
Real Estate Properties [Line Items]      
Real estate investment property, at cost 1,268,136,000 980,263,000  
Medical Office [Member]
     
Real Estate Properties [Line Items]      
Real estate investment property, at cost 404,793,000 398,559,000  
Retail [Member]
     
Real Estate Properties [Line Items]      
Real estate investment property, at cost 408,897,000 351,395,000  
Multifamily [Member]
     
Real Estate Properties [Line Items]      
Real estate investment property, at cost 324,957,000 321,719,000  
Office [Member]
     
Real Estate Properties [Line Items]      
Held for development 8,953,000 20,172,000  
Medical Office [Member]
     
Real Estate Properties [Line Items]      
Held for development 5,758,000 5,463,000  
Retail [Member]
     
Real Estate Properties [Line Items]      
Held for development 576,000 546,000  
Multifamily [Member]
     
Real Estate Properties [Line Items]      
Held for development 27,802,000 59,000  
Dulles Station II [Member]
     
Real Estate Properties [Line Items]      
Real estate impairment 14,526,000    
Land Available for Development $ 12,100,000