Recent Transactions (Schedule Of Dispositions/Rental Property Held For Sale) (Details)
$ in Thousands
 3 Months Ended
Mar. 31, 2018
USD ($)
ft²
property
item
Dec. 31, 2017
USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
Number of rental units | item 365  
Net sales proceeds held by qualified intermediary $ 26,900  
Valuation allowance $ 9,700  
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 $ 223,049  
Net Carrying Value 164,863  
Realized Gains (losses)/Unrealized Losses, net 58,186  
Disposal Group, Not Discontinued Operations [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]    
Notes receivable 2,800  
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] 1  
Rentable Square Feet, Disposed | ft² [1] 172,498  
Net Sales Proceeds [1] $ 25,994  
Net Carrying Value [1] 25,739  
Realized Gains (losses)/Unrealized Losses, net [1] $ 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 [2] 6  
Rentable Square Feet, Disposed | ft² [2] 239,262  
Net Sales Proceeds [2] $ 17,546  
Net Carrying Value [2] 17,501  
Realized Gains (losses)/Unrealized Losses, net [2] $ 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 [3] 1  
Rentable Square Feet, Disposed | ft² [3] 225,550  
Net Sales Proceeds [3] $ 12,475  
Net Carrying Value [3] 11,795  
Realized Gains (losses)/Unrealized Losses, net [3] 680  
Valuation allowance   $ 11,000
Disposal Group, Not Discontinued Operations [Member] | 20 Waterview Boulevard [Member] | Notes Receivable 6.0 Interest Rate [Member]    
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]    
Notes receivable $ 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 [4] 2  
Rentable Square Feet, Disposed | ft² [4] 489,000  
Net Sales Proceeds [4] $ 81,769  
Net Carrying Value [4] 64,679  
Realized Gains (losses)/Unrealized Losses, net [4] 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  
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  
[1] The Company recorded a valuation allowance of $0.7 million on this property during the year ended December 31, 2017.
[2] The Company recorded a valuation allowance of $0.6 million on these properties during the year ended December 31, 2017. The disposition additionally included two land properties.
[3] The Company recorded a valuation allowance of $11 million on this property during the year ended December 31, 2017. Prior to closing, the Company provided short term financing through a note receivable to an affiliate of the buyers of $2.8 million, which is a noncash component of the net sales proceeds. See Note 5: Deferred charges, goodwill and other assets, net.
[4] Prior to closing, the Company provided financing through a note receivable to an affiliate of the buyers of $4.0 million, which is a noncash component of the net sales proceeds. See Note 5: Deferred Charges, Goodwill and Other Assets, Net.