Investments in Unconsolidated Joint Ventures (Investments in Unconsolidated Joint Ventures) (Details) $ in Thousands		3 Months Ended
		Mar. 31, 2018 USD ($) ft² Land_Parcels Buildings payments	Dec. 31, 2017 USD ($)
Schedule of Equity Method Investments [Line Items]
Carrying Value of investment		$ (470,140)	$ (443,980)
Carrying value of the Company's investments in unconsolidated joint ventures		$ 666,718	619,925
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]	$ (7,811)	(8,258)
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]	$ 3,372	3,339
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]	$ (13,262)	(13,811)
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]	$ 39,340	39,710
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]	$ 17,974	18,381
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]	$ 66,259	66,179
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]	$ (4,129)	(3,876)
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]	$ 42,636	42,657
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%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]	$ 67,883	67,120
Residential Tower Developer LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		The Hub on Causeway - Residential
Ownership Percentage	[6]	50.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]	$ 29,752	28,212
Hotel Tower Developer LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		The Hub on Causeway - Hotel Air Rights
Ownership Percentage		50.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]	$ 1,751	1,690
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]	$ 4,539	4,641
BNY Tower Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Dock 72 at the Brooklyn Navy Yard
Ownership Percentage		50.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]	$ 71,582	72,104
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]	$ 254,226	254,440
7750 Wisconsin Avenue LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		7750 Wisconsin Avenue
Ownership Percentage	[6]	50.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]	$ 67,404	21,452
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Carrying Value of investment		25,200	25,900
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Carrying Value of investment		(95,597)	(99,215)
Carrying value of the Company's investments in unconsolidated joint ventures	[1],[7]	$ 641,516	$ 593,980

[1] Investments with deficit balances aggregating approximately $25.2 million and $25.9 million at March 31, 2018 and December 31, 2017, 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 subsidiary that owns Wisconsin Place Office also owns a 33.3% interest in the joint venture entity that owns 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] This entity is a VIE (See Note 2).

[7] Investments with deficit balances aggregating approximately $25.2 million and $25.9 million at March 31, 2018 and December 31, 2017, respectively, have been reflected within Other Liabilities in the Company’s Consolidated Balance Sheets.