Recent Transactions (Schedule Of Property Sold And Disposed) (Details) $ in Thousands				3 Months Ended	12 Months Ended
		Mar. 24, 2016 USD ($) ft² item	Mar. 11, 2016 USD ($) ft² item	Mar. 31, 2016 USD ($) ft² item	Dec. 31, 2015 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings | item				2
Rentable Square Feet | ft²				236,950
Net Sales Proceeds				$ 94,710
Net Book Value				36,110
Realized Gain				58,600
Mortgage loan				$ 1,823,311
2 Independence Way [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Number of Buildings | item	[1]		1
Rentable Square Feet | ft²	[1]		67,401
Net Sales Proceeds	[1]		$ 4,119
Net Book Value	[1]		4,283
Realized Gain	[1]		$ (164)
Impairment charge					$ 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 | item		1
Rentable Square Feet | ft²		169,549
Net Sales Proceeds		$ 90,591
Net Book Value		31,827
Realized Gain		$ 58,764

[1] The Company recorded an impairment charge of $3.2 million during the year ended December 31, 2015 as it estimated that the carrying value of the property may not be recoverable over its anticipated holding period.