Investments in Unconsolidated Joint Ventures (Investments in Unconsolidated Joint Ventures) (Details) $ in Thousands		6 Months Ended
		Jun. 30, 2015 USD ($) ft² Land_Parcels Buildings payments	Dec. 31, 2014 USD ($)
Schedule of Equity Method Investments [Line Items]
Carrying value of the Company's investments in unconsolidated joint ventures		$ 209,974	$ 193,394
Square 407 Limited Partnership [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Market Square North
Ownership Percentage		50.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]	$ (10,161)	(8,022)
The Metropolitan Square Associates LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Metropolitan Square
Ownership Percentage		51.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]	$ 9,188	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%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]	$ (12,411)	(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%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]	$ 44,597	45,514
Entity Owning Land And Infrastructure Of Project [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage		33.30%
Annapolis Junction NFM, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Annapolis Junction
Ownership Percentage	[4]	50.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]	$ 26,812	25,246
Number of real estate properties | Buildings		2
Number of buildings under construction | Buildings		2
Parcels of undeveloped land | Land_Parcels		2
540 Madison Venture LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		540 Madison Avenue
Ownership Percentage		60.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]	$ 68,627	68,128
500 North Capitol LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		500 North Capitol Street, NW
Ownership Percentage		30.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]	$ (2,755)	(2,250)
501 K Street LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		1001 6th Street
Ownership Percentage	[5]	50.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]	$ 42,704	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		North Station (Phase I - Air Rights)
Ownership Percentage		50.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]	$ 6,609	4,231
1265 Main Office JV LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		1265 Main Street
Ownership Percentage		50.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]	$ 2,209
BNY Tower Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Dock72 at the Brooklyn Navy Yard
Ownership Percentage		50.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]	$ 9,228
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value of the Company's investments in unconsolidated joint ventures		(25,300)	(11,400)
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value of the Company's investments in unconsolidated joint ventures	[1],[6]	$ 184,647	$ 182,042

[1] (1)Investments with deficit balances aggregating approximately $25.3 million and $11.4 million at June 30, 2015 and December 31, 2014, respectively, have been reflected within Other Liabilities on the Company's Consolidated Balance Sheets.

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

[3] (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] 4)The joint venture owns two in-service buildings, two buildings under construction and two undeveloped land parcels.

[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.

[6] (2)Investments with deficit balances aggregating approximately $25.3 million and $11.4 million at June 30, 2015 and December 31, 2014, respectively, have been reflected within Other Liabilities on the Company's Consolidated Balance Sheets.