Investments in Unconsolidated Joint Ventures (Investments in Unconsolidated Joint Ventures) (Details) $ in Thousands		9 Months Ended
		Sep. 30, 2017 USD ($) ft² Land_Parcels Buildings payments		Sep. 06, 2017	Aug. 07, 2017	Dec. 31, 2016 USD ($)
Schedule of Equity Method Investments [Line Items]
Carrying Value of investment		$ (419,215)				$ (450,821)
Carrying value of the Company's investments in unconsolidated joint ventures		$ 611,800				775,198
Square 407 Limited Partnership [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Market Square North
Ownership Percentage		50.00%
Carrying Value of investment	[1]	$ (8,474)				(8,134)
The Metropolitan Square Associates LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Metropolitan Square
Ownership Percentage		20.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]	$ 2,537				2,004
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 investment	[1]	$ (10,747)				(10,564)
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]	$ 40,158				41,605
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]	$ 18,771				20,539
Number of real estate properties | Buildings		4
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]	$ 67,046				67,816
500 North Capitol Venture LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		500 North Capitol Street, NW
Ownership Percentage		30.00%
Carrying Value of investment	[1]	$ (3,642)				(3,389)
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]	$ 42,442				42,528
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%		50.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]	$ 55,917				29,869
Residential Tower Developer LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		The Hub on Causeway - Residential
Ownership Percentage		50.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]	$ 25,811				20,803
Hotel Tower Developer LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		The Hub on Causeway - Hotel
Ownership Percentage		50.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]	$ 1,596				933
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]	$ 4,686				4,779
BNY Tower Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Dock 72 at the Brooklyn Navy Yard
Ownership Percentage	[6]	50.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]	$ 67,901				33,699
CA-Colorado Center Limited Partnership [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Colorado Center
Ownership Percentage		50.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]	$ 263,834				510,623
7750 Wisconsin Avenue [Member]
Schedule of Equity Method Investments [Line Items]
Properties		7750 Wisconsin Avenue
Ownership Percentage		50.00%	[6]		50.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]	$ 21,101
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value of the Company's investments in unconsolidated joint ventures		(22,900)				(22,100)
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Carrying Value of investment		(86,971)				(67,167)
Carrying value of the Company's investments in unconsolidated joint ventures	[1],[7]	$ 588,937				$ 753,111

[1] Investments with deficit balances aggregating approximately $22.9 million and $22.1 million at September 30, 2017 and December 31, 2016, respectively, have been reflected within Other Liabilities in 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 for this land parcel, the partner will be entitled to up to two additional payments from the venture based on increases in total entitled square footage of the project above 520,000 square feet and achieving certain project returns at stabilization.

[6] The entity is a VIE (See Note 2).

[7] Investments with deficit balances aggregating approximately $22.9 million and $22.1 million at September 30, 2017 and December 31, 2016, respectively, have been reflected within Other Liabilities in the Company’s Consolidated Balance Sheets.