Quarterly report pursuant to Section 13 or 15(d)

Mortgage Notes Payable (Details)

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Mortgage Notes Payable (Details) (USD $)
9 Months Ended
Sep. 30, 2011
Dec. 31, 2010
Sep. 30, 2011
Olney Village Center [Member]
Mortgages [Member]
Sep. 30, 2011
John Marshall II [Member]
Mortgages [Member]
Sep. 30, 2011
Shady Grove Medical Village II [Member]
Sep. 30, 2011
Columbia Park Road And Dulles Business Park [Member]
Mortgage Loans on Real Estate [Line Items]            
Mortgage notes payable $ 428,909,000 $ 361,860,000 $ 22,600,000 $ 54,100,000    
Mortgage note interest rate, percentage 3.875%   6.37% 5.79%    
Mortgage note, fair value interest rate     4.94%      
Mortgage note maturity date Nov. 01, 2023 May 06, 2016
Fair value of mortgage     24,400,000      
Repayment of secured mortgage notes         9,100,000 17,900,000
Increase in sales price for partially offsetting the penalties incurred due to prepayment of mortgage notes           875,000
Prepayment penalties incurred on mortgage notes           $ 1,000,000