Recent Transactions (Schedule Of Disposed Properties) (Details)
 3 Months Ended 6 Months Ended
Apr. 30, 2019
USD ($)
Apr. 01, 2019
USD ($)
item
Mar. 31, 2019
USD ($)
shares
Jun. 30, 2019
USD ($)
ft²
item
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Valuation allowance       $ 11,100,000   $ 5,300,000
Jersey City, New Jersey [Member]            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Number Of Units | item   377        
Funds from qualified intermediary   $ 217,400,000        
Flex Portfolio [Member]            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Proceeds from sale of properties     $ 6,600,000      
Disposal Group, Not Discontinued Operations [Member]            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Number of Buildings, Disposed | item       63    
Rentable Square Feet, Disposed | ft²       3,993,538    
Net Sales Proceeds       $ 593,436,000    
Net Carrying Value       325,072,000    
Totals       $ 268,364,000    
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, Disposed | item       1    
Rentable Square Feet, Disposed | ft²       192,741    
Net Sales Proceeds       $ 5,651,000    
Net Carrying Value       5,410,000    
Realized Gains (losses)/Unrealized Losses, net       $ 241,000    
Valuation allowance         $ 9,300,000  
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, Disposed | item       1    
Rentable Square Feet, Disposed | ft²       159    
Net Sales Proceeds       $ 34,045,000    
Net Carrying Value       34,032,000    
Realized Gains (losses)/Unrealized Losses, net       $ 13,000    
Valuation allowance         6,300,000  
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, Disposed | item       1    
Rentable Square Feet, Disposed | ft²       68,000    
Net Sales Proceeds       $ 15,197,000    
Net Carrying Value       7,433,000    
Realized Gains (losses)/Unrealized Losses, net       $ 7,764,000    
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, Disposed | item       1    
Rentable Square Feet, Disposed | ft²       88,369    
Net Sales Proceeds       $ 4,842,000    
Net Carrying Value       4,937,000    
Realized Gains (losses)/Unrealized Losses, net       $ (95,000)    
Valuation allowance         3,600,000  
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, Disposed | item       2    
Rentable Square Feet, Disposed | ft²       147,406    
Net Sales Proceeds       $ 25,552,000    
Net Carrying Value       18,456,000    
Realized Gains (losses)/Unrealized Losses, net       $ 7,096,000    
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, Disposed | item       1    
Rentable Square Feet, Disposed | ft²       348,510    
Net Sales Proceeds       $ 37,801,000    
Net Carrying Value       40,046,000    
Realized Gains (losses)/Unrealized Losses, net       $ (2,245,000)    
Valuation allowance         $ 900,000  
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, Disposed | item       56    
Number of properties disposed | item       2    
Rentable Square Feet, Disposed | ft²       3,148,512    
Net Sales Proceeds       $ 470,348,000    
Net Carrying Value       214,758,000    
Realized Gains (losses)/Unrealized Losses, net       255,590,000    
Carrying Amount of Mortgages       350,000,000    
Redemption of common units, shares | shares     301,638      
Proceeds from sale of properties       6,600,000    
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]            
Carrying Amount of Mortgages       119,900,000    
Repayment of mortgage loans       $ 90,000,000    
Disposal Group, Not Discontinued Operations [Member] | Malden, MA [Member]            
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]            
Net Sales Proceeds $ 685,000          
Realized Gains (losses)/Unrealized Losses, net $ 270,000