Recent Transactions (Schedule Of Disposed Properties) (Details)
$ in Thousands
 6 Months Ended 12 Months Ended
Jun. 30, 2019
USD ($)
Dec. 31, 2020
USD ($)
ft²
item
Dec. 31, 2019
USD ($)
ft²
item
property
shares
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Rentable Square Feet/Units | ft²     4,490,578    
Valuation allowance   $ 16,200     $ 5,300
Number of units | item   326 208    
721 Route 202/206 South [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Rentable Square Feet/Units | ft²     192,741    
Park Square Apartments [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Rentable Square Feet/Units | ft²     159    
2115 Linwood Avenue [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Rentable Square Feet/Units | ft²     68,000    
201 Littleton Road [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Rentable Square Feet/Units | ft²     88,369    
320 & 321 University Avenue [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Rentable Square Feet/Units | ft²     147,406    
Flex Portfolio [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Rentable Square Feet/Units | ft²     3,148,512    
650 From Road [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Rentable Square Feet/Units | ft²     348,510    
3600 Route 66 [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Rentable Square Feet/Units | ft²     180,000    
Chase and Alterra Portfolio [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Rentable Square Feet/Units | ft²     1,386    
5 Wood Hollow Road [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Rentable Square Feet/Units | ft²     317,040    
Disposal Group, Not Discontinued Operations [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of buildings | item   20 68    
Net Sales Proceeds   $ 370,754 $ 1,052,427    
Net Carrying Value   338,350 668,574    
Realized Gains (losses)/Unrealized Losses, net   $ 7,163 382,151    
Area of real estate property held for sale (in square feet) | ft²   2,725,639      
Unrealized losses on rental properties held for sale   $ (1,682) (39,049)    
Totals   $ 5,481 $ 343,102    
Disposal Group, Not Discontinued Operations [Member] | 721 Route 202/206 South [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of buildings | item     1    
Net Sales Proceeds     $ 5,651    
Net Carrying Value     5,410    
Realized Gains (losses)/Unrealized Losses, net     $ 241    
Valuation allowance       $ 9,300  
Disposal Group, Not Discontinued Operations [Member] | Park Square Apartments [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of buildings | item     1    
Net Sales Proceeds     $ 34,045    
Net Carrying Value     34,032    
Realized Gains (losses)/Unrealized Losses, net     $ 13    
Valuation allowance       6,300  
Disposal Group, Not Discontinued Operations [Member] | 2115 Linwood Avenue [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of buildings | item     1    
Net Sales Proceeds     $ 15,197    
Net Carrying Value     7,433    
Realized Gains (losses)/Unrealized Losses, net     $ 7,764    
Disposal Group, Not Discontinued Operations [Member] | 201 Littleton Road [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of buildings | item     1    
Net Sales Proceeds     $ 4,842    
Net Carrying Value     4,937    
Realized Gains (losses)/Unrealized Losses, net     $ (95)    
Valuation allowance       3,600  
Disposal Group, Not Discontinued Operations [Member] | 320 & 321 University Avenue [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of buildings | item     2    
Net Sales Proceeds     $ 25,552    
Net Carrying Value     18,456    
Realized Gains (losses)/Unrealized Losses, net     $ 7,096    
Disposal Group, Not Discontinued Operations [Member] | Flex Portfolio [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of buildings | item     56    
Net Sales Proceeds     $ 470,348    
Net Carrying Value     214,758    
Realized Gains (losses)/Unrealized Losses, net     $ 255,590    
Number of properties disposed | property     2    
Repayment of mortgage loans     $ 90,000    
Funds from qualified intermediary     217,400    
Mortgage loan     $ 350,000    
Number of units | item     377    
Redemption of common units, shares | shares     301,638    
Value of units redeemed     $ 6,600    
Disposal Group, Not Discontinued Operations [Member] | 650 From Road [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of buildings | item     1    
Net Sales Proceeds     $ 37,801    
Net Carrying Value     40,046    
Realized Gains (losses)/Unrealized Losses, net     $ (2,245)    
Valuation allowance       $ 900  
Disposal Group, Not Discontinued Operations [Member] | 3600 Route 66 [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of buildings | item     1    
Net Sales Proceeds     $ 25,237    
Net Carrying Value     $ 17,246    
Disposal Group, Not Discontinued Operations [Member] | Chase and Alterra Portfolio [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of buildings | item     3    
Net Sales Proceeds     $ 406,817    
Net Carrying Value     293,030    
Realized Gains (losses)/Unrealized Losses, net     113,787    
Mortgage loan     $ 235,800    
Disposal Group, Not Discontinued Operations [Member] | 5 Wood Hollow Road [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of buildings | item     1    
Net Sales Proceeds     $ 26,937    
Net Carrying Value     33,226    
Impairment charge $ 5,800        
Disposal Group, Not Discontinued Operations [Member] | One Bridge Plaza [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of buildings | item   1      
Rentable Square Feet/Units | ft²   200,000      
Net Sales Proceeds   $ 35,065      
Net Carrying Value   $ 17,743      
Disposal Group, Not Discontinued Operations [Member] | 3 Giralda Farms [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of buildings | item   1      
Rentable Square Feet/Units | ft²   141,000      
Net Sales Proceeds   $ 7,510      
Net Carrying Value   9,534      
Valuation allowance   $ 2,000 16,700    
Disposal Group, Not Discontinued Operations [Member] | Morris Portfolio [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of buildings | item   10      
Rentable Square Feet/Units | ft²   1,448,420      
Net Sales Proceeds   $ 155,116      
Net Carrying Value   175,772      
Valuation allowance   $ 21,600 32,500    
Disposal Group, Not Discontinued Operations [Member] | 325 Columbia Turnpike [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of buildings | item   1      
Rentable Square Feet/Units | ft²   168,144      
Net Sales Proceeds   $ 24,276      
Net Carrying Value   $ 8,020      
Disposal Group, Not Discontinued Operations [Member] | 9 Campus Drive [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of buildings | item   1      
Rentable Square Feet/Units | ft²   156,945      
Net Sales Proceeds   $ 20,678      
Net Carrying Value   $ 22,162      
Valuation allowance     3,500    
Disposal Group, Not Discontinued Operations [Member] | 385 Vaughn Drive [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of buildings | item   1      
Rentable Square Feet/Units | ft²   98,500      
Net Sales Proceeds   $ 7,282      
Net Carrying Value   $ 5,754      
Disposal Group, Not Discontinued Operations [Member] | 7 Campus Drive [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of buildings | item   1      
Rentable Square Feet/Units | ft²   154,395      
Net Sales Proceeds   $ 12,278      
Net Carrying Value   $ 11,804      
Valuation allowance     6,000    
Disposal Group, Not Discontinued Operations [Member] | 581 Main Street [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of buildings | item   1      
Rentable Square Feet/Units | ft²   200,000      
Net Sales Proceeds   $ 58,400      
Net Carrying Value   $ 43,113      
Disposal Group, Not Discontinued Operations [Member] | 500 College Road East [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of buildings | item   1      
Rentable Square Feet/Units | ft²   158,235      
Net Sales Proceeds   $ 4,582      
Net Carrying Value   6,044      
Valuation allowance   $ 1,900      
Disposal Group, Not Discontinued Operations [Member] | 5/10 Dennis St And 100 Hiram Sq [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of buildings | item   2      
Rentalable Units | item   200      
Net Sales Proceeds   $ 45,567      
Net Carrying Value   38,404      
Realized Gains (losses)/Unrealized Losses, net   7,163      
Disposal Group, Not Discontinued Operations [Member] | Unsecured Revolving Credit Facility [Member] | Flex Portfolio [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Mortgage loan     119,900    
Discontinued Operations [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Realized Gains (losses)/Unrealized Losses, net   25,241 1,702    
Unrealized losses on rental properties held for sale   (14,040) (135,052)    
Totals   11,201 (133,350)    
Discontinued Operations [Member] | 3600 Route 66 [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Realized Gains (losses)/Unrealized Losses, net     7,991    
Discontinued Operations [Member] | 5 Wood Hollow Road [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Realized Gains (losses)/Unrealized Losses, net     $ (6,289)    
Discontinued Operations [Member] | One Bridge Plaza [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Realized Gains (losses)/Unrealized Losses, net   17,322      
Discontinued Operations [Member] | 3 Giralda Farms [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Realized Gains (losses)/Unrealized Losses, net   (2,024)      
Discontinued Operations [Member] | Morris Portfolio [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Realized Gains (losses)/Unrealized Losses, net   (20,656)      
Discontinued Operations [Member] | 325 Columbia Turnpike [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Realized Gains (losses)/Unrealized Losses, net   16,256      
Discontinued Operations [Member] | 9 Campus Drive [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Realized Gains (losses)/Unrealized Losses, net   (1,484)      
Discontinued Operations [Member] | 385 Vaughn Drive [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Realized Gains (losses)/Unrealized Losses, net   1,528      
Discontinued Operations [Member] | 7 Campus Drive [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Realized Gains (losses)/Unrealized Losses, net   474      
Discontinued Operations [Member] | 581 Main Street [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Realized Gains (losses)/Unrealized Losses, net   15,287      
Discontinued Operations [Member] | 500 College Road East [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Realized Gains (losses)/Unrealized Losses, net   $ (1,462)