Real Estate Transactions (Narrative) (Details) (USD $)
 3 Months Ended 6 Months Ended 0 Months Ended 6 Months Ended 1 Months Ended
Jun. 30, 2013
Jun. 30, 2013
Jul. 15, 2013
Jan. 18, 2013
Alterra At Overlook Ridge IA [Member]
item
Apr. 04, 2013
Alterra At Overlook Ridge IB [Member]
item
Jul. 10, 2013
106 Allen Road [Member]
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Jun. 30, 2013
19 Skyline Drive [Member]
Jul. 31, 2013
Subsequent Event [Member]
Scenario, Forecast [Member]
Keystone Affiliates [Member]
Jul. 31, 2013
Subsequent Event [Member]
Scenario, Forecast [Member]
150 Monument Road [Member]
Keystone Affiliates [Member]
Jul. 31, 2013
Subsequent Event [Member]
Scenario, Forecast [Member]
Pennsylvania Office Portfolio [Member]
sqft
Jul. 31, 2013
Subsequent Event [Member]
Scenario, Forecast [Member]
Pennsylvania Office Portfolio [Member]
Keystone Affiliates [Member]
Jun. 30, 2013
Blue Bell, Lower Providence And Plymouth Meeting, Pennsylvania [Member]
sqft
property
Real Estate Transactions [Line Items]                        
Number of units       310 412              
Business acquisition, cash paid       $ 61,300,000 $ 88,000,000              
Transaction costs 214,000 214,000                    
Area of property (in square feet)           132,010       1,660,000   462,378
Proceeds from the sale of property   161,727,000       18,000,000       201,000,000    
Sale price of property                   233,000,000    
New mortgage loan                 16,500,000 10,000,000    
Capital balance                   22,000,000    
Mortgage loan, maturity period             10 years   2 years 2 years    
Loan extension period                 1 year 1 year    
Spread over LIBOR     1.25%           6.00% 6.00%    
Initial internal rate of return                     15.00%  
Internal rate of return                   10.00%    
Amount of debt guaranteed by company               50,000,000        
Number of impaired properties                       5
Impairments $ 23,851,000 $ 23,851,000