Recent Transactions (Schedule Of Dispositions/Rental Property Held For Sale) (Details)
$ in Thousands
 6 Months Ended
Jun. 30, 2018
USD ($)
ft²
property
item
Mar. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
Number of rental units | item 858    
Net sales proceeds held by qualified intermediary $ 26,900    
Valuation allowance $ 10,100    
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 20    
Rentable Square Feet, Disposed | ft² 1,737,954    
Net Sales Proceeds $ 224,559    
Net Carrying Value 164,863    
Realized Gains (losses)/Unrealized Losses, net 59,696    
Unrealized losses on rental properties held for sale (500)    
Total 59,196    
Disposal Group, Not Discontinued Operations [Member] | Notes Receivable 3.0 Interest Rate [Member] | Buyer [Member]      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
Notes receivable $ 4,000    
Disposal Group, Not Discontinued Operations [Member] | 35 Waterview Boulevard [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 172,498    
Net Sales Proceeds $ 25,994    
Net Carrying Value 25,739    
Realized Gains (losses)/Unrealized Losses, net $ 255    
Valuation allowance     $ 700
Disposal Group, Not Discontinued Operations [Member] | Hamilton Portfolio [Member]      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
Number of Buildings, Disposed | item 6    
Rentable Square Feet, Disposed | ft² 239,262    
Net Sales Proceeds $ 17,546    
Net Carrying Value 17,501    
Realized Gains (losses)/Unrealized Losses, net $ 45    
Valuation allowance     600
Disposal Group, Not Discontinued Operations [Member] | Wall Portfolio First Closing [Member]      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
Number of Buildings, Disposed | item 5    
Rentable Square Feet, Disposed | ft² 179,601    
Net Sales Proceeds $ 14,053    
Net Carrying Value 10,526    
Realized Gains (losses)/Unrealized Losses, net $ 3,527    
Disposal Group, Not Discontinued Operations [Member] | 700 Horizon Drive [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 120,000    
Net Sales Proceeds $ 33,020    
Net Carrying Value 16,053    
Realized Gains (losses)/Unrealized Losses, net $ 16,967    
Disposal Group, Not Discontinued Operations [Member] | Wall Portfolio Second Closing [Member]      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
Number of Buildings, Disposed | item 3    
Rentable Square Feet, Disposed | ft² 217,822    
Net Sales Proceeds $ 30,209    
Net Carrying Value 12,961    
Realized Gains (losses)/Unrealized Losses, net $ 17,248    
Disposal Group, Not Discontinued Operations [Member] | 75 Livingston 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 94,221    
Net Sales Proceeds $ 7,983    
Net Carrying Value 5,609    
Realized Gains (losses)/Unrealized Losses, net $ 2,374    
Disposal Group, Not Discontinued Operations [Member] | 20 Waterview Boulevard [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 225,550    
Net Sales Proceeds $ 12,475    
Net Carrying Value 11,795    
Realized Gains (losses)/Unrealized Losses, net $ 680    
Valuation allowance     $ 11,000
Disposal Group, Not Discontinued Operations [Member] | 20 Waterview Boulevard [Member] | Notes Receivable 6.0 Interest Rate [Member] | Buyer [Member]      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
Noncash net sales proceeds   $ 2,800  
Disposal Group, Not Discontinued Operations [Member] | Westchester Financial Center [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 489,000    
Net Sales Proceeds $ 81,769    
Net Carrying Value 64,679    
Realized Gains (losses)/Unrealized Losses, net 17,090    
Disposal Group, Not Discontinued Operations [Member] | Westchester Financial Center [Member] | Notes Receivable 3.0 Interest Rate [Member] | Buyer [Member]      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
Noncash net sales proceeds 4,000    
Disposal Group, Not Discontinued Operations [Member] | 65 Jackson Drive [Member]      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
Net Sales Proceeds 1,510    
Realized Gains (losses)/Unrealized Losses, net $ 1,510    
Land [Member] | Disposal Group, Not Discontinued Operations [Member] | Hamilton Portfolio [Member]      
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]      
Number of properties disposed | property 2