Recent Transactions (Schedule Of Dispositions/Rental Property Held For Sale) (Details)
$ in Thousands
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2015
USD ($)
Sep. 30, 2016
USD ($)
item
Sep. 30, 2015
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2012
USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of Buildings, Disposed | item   13      
Net Sales Proceeds   $ 452,183      
Net Book Value   359,126      
Realized Gain (loss)/Unrealized Loss   93,057      
Unrealized losses on properties held for sale   (24,393)      
Totals   $ 68,664      
Impairments $ 164,176   $ 164,176    
2 Independence Way [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of Buildings, Disposed | item [1]   1      
Net Sales Proceeds [1]   $ 4,119      
Net Book Value [1]   4,283      
Realized Gain (loss)/Unrealized Loss [1]   $ (164)      
Impairments       $ 3,200  
1201 Connecticut Avenue NW [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of Buildings, Disposed | item   1      
Net Sales Proceeds   $ 90,591      
Net Book Value   31,827      
Realized Gain (loss)/Unrealized Loss   $ 58,764      
125 Broad Street [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of Buildings, Disposed | item [2]   1      
Net Sales Proceeds [2]   $ 192,323      
Net Book Value [2]   200,183      
Realized Gain (loss)/Unrealized Loss [2]   $ (7,860)      
Impairments       83,200  
9200 Edmonston Road [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of Buildings, Disposed | item   1      
Net Sales Proceeds [3]   $ 4,083      
Net Book Value   3,837      
Realized Gain (loss)/Unrealized Loss   $ 246      
Impairments         $ 3,000
1400 L Street [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of Buildings, Disposed | item   1      
Net Sales Proceeds [4]   $ 68,399      
Net Book Value   30,053      
Realized Gain (loss)/Unrealized Loss   38,346      
Net sales proceeds held by qualified intermediary   $ 28,500      
600 Parsippany Road [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of Buildings, Disposed | item   1      
Net Sales Proceeds [5]   $ 10,465      
Net Book Value   5,875      
Realized Gain (loss)/Unrealized Loss   4,590      
Net sales proceeds held by qualified intermediary   $ 10,500      
4, 5, 6 Century Drive [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of Buildings, Disposed | item [6]   3      
Net Sales Proceeds [6]   $ 14,533      
Net Book Value [6]   17,308      
Realized Gain (loss)/Unrealized Loss [6]   $ (2,775)      
Impairments       9,800  
Andover Place [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of Buildings, Disposed | item   1      
Net Sales Proceeds   $ 39,863      
Net Book Value   37,150      
Realized Gain (loss)/Unrealized Loss   $ 2,713      
222, 233 Mount Airy Road [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of Buildings, Disposed | item [7]   2      
Net Sales Proceeds [7]   $ 8,817      
Net Book Value [7]   9,039      
Realized Gain (loss)/Unrealized Loss [7]   $ (222)      
Impairments       $ 1,000  
10 Mountainview Road [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of Buildings, Disposed | item   1      
Net Sales Proceeds   $ 18,990      
Net Book Value   19,571      
Realized Gain (loss)/Unrealized Loss   $ (581)      
Multi-Family Properties [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of rental units | item   5,214      
[1] The Company recorded an impairment charge of $3.2 million on this property during the year ended December 31, 2015.
[2] The Company recorded impairment charges of $83.2 million on this property during the year ended December 31, 2015.
[3] The Company transferred the deed for this property to the lender in satisfaction of its obligations. The Company recorded an impairment charge of $3.0 million on this property during the year ended December 31, 2012.
[4] $28.5 million of the net sales proceeds from this sale were held by a qualified intermediary until such funds are used in acquisitions.
[5] $10.5 million of the net sales proceeds from this sale were held by a qualified intermediary until such funds are used in acquisitions.
[6] The Company recorded impairment charges of $9.8 million on these properties during the year ended December 31, 2015.
[7] The Company recorded impairment charges of $1 million on these properties during the year ended December 31, 2015.