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Investments in Unconsolidated Joint Ventures (Investments in Unconsolidated Joint Ventures) (Details) (USD $)
In Thousands, unless otherwise specified
 9 Months Ended 9 Months Ended
Sep. 30, 2014
Dec. 31, 2013
Sep. 30, 2014
Square 407 Limited Partnership [Member]
Sep. 30, 2014
The Metropolitan Square Associates LLC [Member]
Sep. 30, 2014
BP/CRF 901 New York Avenue LLC [Member]
Sep. 30, 2014
WP Project Developer LLC [Member]
Sep. 30, 2014
Annapolis Junction NFM, LLC [Member]
Land_Parcels
Buildings
Sep. 30, 2014
2 GCT Venture LLC [Member]
Sep. 30, 2014
540 Madison Venture LLC [Member]
Sep. 30, 2014
500 North Capitol LLC [Member]
Sep. 30, 2014
Entity Owning Land And Infrastructure Of Project [Member]
Sep. 30, 2014
501 K Street [Member]
payments
sqft
Schedule of Equity Method Investments [Line Items]                        
Properties     Market Square North Metropolitan Square 901 New York Avenue Wisconsin Place Land and Infrastructure Annapolis Junction N/A 540 Madison Avenue 500 North Capitol Street, NW   501 K Street
Ownership Percentage     50.00% 51.00% 25.00% [1] 33.30% [2] 50.00% [3] 60.00% [4] 60.00% 30.00% 33.30% 50.00% [5]
Carrying value of the Company's investments in unconsolidated joint ventures $ 180,149 $ 126,084 $ (8,540) $ 8,343 $ (1,436) $ 46,011 $ 24,484 $ 424 $ 70,418 $ (1,467)   $ 41,912
Number of real estate properties             2          
Number of buildings under construction             2          
Parcels of undeveloped land             2          
Potential additonal payments to joint venture partner                       2
Minimum square footage to make a potential additional payment to joint venture partner (in sqft)                       520,000
[1] The Company’s economic ownership has increased based on the achievement of certain return thresholds.
[2] The Company’s wholly-owned entity that owns the office component of the project also owns a 33.3% interest in the entity owning the land, parking garage and infrastructure of the project.
[3] The joint venture owns two in-service buildings, two buildings under construction and two undeveloped land parcels.
[4] Two Grand Central Tower was sold on October 25, 2011.
[5] Under the joint venture agreement, the partner will be entitled to up to two additional payments from the venture based on increases in total square footage of the project above 520,000 square feet and achieving certain project returns at stabilization.