Investments in Unconsolidated Joint Ventures (Investments in Unconsolidated Joint Ventures) (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2015 USD ($) ft² Land_Parcels Buildings payments	Dec. 31, 2014 USD ($)
Schedule of Equity Method Investments [Line Items]
Investments in unconsolidated joint ventures		$ 235,224	$ 193,394
Investments In Affiliates Subsidiaries Associates And Joint Ventures net	[1]	210,023	182,042
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Investments in unconsolidated joint ventures		$ (25,200)	(11,400)
Square 407 Limited Partnership [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Market Square North
Ownership Percentage		50.00%
Investments in unconsolidated joint ventures	[1]	$ (9,951)	(8,022)
The Metropolitan Square Associates LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Metropolitan Square
Ownership Percentage		51.00%
Investments in unconsolidated joint ventures	[1]	$ 9,179	8,539
BP/CRF 901 New York Avenue LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		901 New York Avenue
Ownership Percentage	[2]	25.00%
Investments in unconsolidated joint ventures	[1]	$ (11,958)	(1,080)
WP Project Developer LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Wisconsin Place Land and Infrastructure
Ownership Percentage	[3]	33.30%
Investments in unconsolidated joint ventures	[1]	$ 43,524	45,514
Annapolis Junction NFM, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Annapolis Junction
Ownership Percentage	[4]	50.00%
Number of real estate properties | Buildings		4
Parcels of undeveloped land | Land_Parcels		2
Investments in unconsolidated joint ventures	[1]	$ 29,009	25,246
540 Madison Venture LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		540 Madison Avenue
Ownership Percentage		60.00%
Investments in unconsolidated joint ventures	[1]	$ 68,983	68,128
500 North Capitol LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		500 North Capitol Street, NW
Ownership Percentage		30.00%
Investments in unconsolidated joint ventures	[1]	$ (3,292)	(2,250)
501 K Street LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		1001 6th Street
Ownership Percentage	[5]	50.00%
Investments in unconsolidated joint ventures	[1]	$ 42,584	41,736
Potential additonal payments to joint venture partner | payments		2
Minimum square footage to make a potential additional payment to joint venture partner (in sqft) | ft²		520,000
Podium Developer LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		The Hub on Causeway
Ownership Percentage		50.00%
Investments in unconsolidated joint ventures	[1]	$ 18,508	$ 4,231
Entity Owning Land And Infrastructure Of Project [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage		33.30%
1265 Main Office JV LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		1265 Main Street
Ownership Percentage		50.00%
Investments in unconsolidated joint ventures	[1]	$ 11,916
BNY Tower Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Dock72 at the Brooklyn Navy Yard
Ownership Percentage		50.00%
Investments in unconsolidated joint ventures	[1]	$ 11,521

[1] Investments with deficit balances aggregating approximately $25.2 million and $11.4 million at December 31, 2015 and 2014, respectively, have been reflected within Other Liabilities on the Company's Consolidated Balance Sheets.

[2] The Company’s economic ownership has increased based on the achievement of certain return thresholds.

[3] 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.

[4] The joint venture owns four in-service buildings and two undeveloped land parcels

[5] Under the joint venture agreement, the partner may 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.