Investments in Unconsolidated Joint Ventures (Investments in Unconsolidated Joint Ventures) (Details) $ in Thousands		6 Months Ended
		Jun. 30, 2020 USD ($) ft² payments Buildings		Jun. 25, 2020 Land_Parcels	Jan. 28, 2020	Dec. 31, 2019 USD ($)
Schedule of Equity Method Investments [Line Items]
Other Liabilities		$ 373,281				$ 387,994
Investments in unconsolidated joint ventures		1,339,724				955,647
Investments In Affiliates Subsidiaries Associates And Joint Ventures net	[1]	1,310,578				933,223
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Other Liabilities		$ 29,100				22,400
Square 407 Limited Partnership [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Market Square North
Ownership Percentage		50.00%
Other Liabilities	[1]	$ 4,123				4,872
BP/CRF Metropolitan Square LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Metropolitan Square
Ownership Percentage		20.00%
Investments in unconsolidated joint ventures	[1]	$ (7,319)				9,134
901 New York Avenue LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		901 New York Avenue
Ownership Percentage	[2]	25.00%
Other Liabilities	[1]	$ 12,174				12,113
901 New York Avenue LLC (economic ownership) [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage		50.00%
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]	$ 36,102				36,789
Annapolis Junction NFM, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Annapolis Junction
Ownership Percentage		50.00%	[4]	50.00%
Investments in unconsolidated joint ventures	[1]	$ 12,967				25,391
Number of real estate properties | 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	[5]	60.00%
Investments in unconsolidated joint ventures	[1]	$ 201				2,953
500 North Capitol Venture LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		500 North Capitol Street, NW
Ownership Percentage		30.00%
Other Liabilities	[1]	$ 5,530				5,439
501 K Street LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		1001 6th Street
Ownership Percentage	[6]	50.00%
Investments in unconsolidated joint ventures	[1]	$ 42,673				42,496
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 - Podium
Ownership Percentage		50.00%
Investments in unconsolidated joint ventures	[1]	$ 49,160				49,466
Residential Tower Developer LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Hub50House
Ownership Percentage		50.00%
Investments in unconsolidated joint ventures	[1]	$ 53,340				55,092
Hotel Tower Developer LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		The Hub on Causeway - Hotel Air Rights
Ownership Percentage		50.00%
Investments in unconsolidated joint ventures	[1]	$ 10,228				9,883
Office Tower Developer LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		100 Causeway Street
Ownership Percentage		50.00%
Investments in unconsolidated joint ventures	[1]	$ 56,544				56,606
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]	$ 4,116				3,780
BNY Tower Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Dock 72
Ownership Percentage		50.00%
Investments in unconsolidated joint ventures	[1]	$ 96,013				94,804
BNYA Amenity Operator LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Dock 72
Ownership Percentage		50.00%
Investments in unconsolidated joint ventures	[1]	$ 0				0
CA-Colorado Center Limited Partnership [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Colorado Center
Ownership Percentage		50.00%
Investments in unconsolidated joint ventures	[1]	$ 236,287				252,069
7750 Wisconsin Avenue LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		7750 Wisconsin Avenue
Ownership Percentage		50.00%
Investments in unconsolidated joint ventures	[1]	$ 57,130				56,247
BP-M 3HB Venture LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		3 Hudson Boulevard
Ownership Percentage		25.00%
Investments in unconsolidated joint ventures	[1]	$ 91,422				67,499
SMBP Venture LP [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Santa Monica Business Park
Ownership Percentage		55.00%
Investments in unconsolidated joint ventures	[1]	$ 148,665				163,937
Platform 16 Holdings LP [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Platform 16
Ownership Percentage		55.00%	[7]		55.00%
Investments in unconsolidated joint ventures	[1]	$ 99,734				$ 29,501
Gateway Portfolio Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties		Gateway Commons
Ownership Percentage		50.00%	[8]		50.00%
Investments in unconsolidated joint ventures	[1]	$ 345,142
Entity Owning Land And Infrastructure Of Project [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage		33.30%
Gateway Commons Complex prior to deferred contribution [Member] [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage					55.00%
Gateway Commons Complex Future Development[Member] [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage					49.00%
Joint Venture Partner [Member] | Gateway Portfolio Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage					50.00%
Joint Venture Partner [Member] | Gateway Commons Complex Future Development[Member] [Member]
Schedule of Equity Method Investments [Line Items]
Ownership Percentage					51.00%

[1] Investments with deficit balances aggregating approximately $29.1 million and $22.4 million at June 30, 2020 and December 31, 2019, respectively, are included 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. At June 30, 2020 and December 31, 2019, the Company’s economic ownership was approximately 50%.

[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 two in-service buildings.

[5] The property was sold on June 27, 2019. As of June 30, 2020 and December 31, 2019, the investment is comprised of undistributed cash.

[6] 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.

[7] This entity is a VIE (See Note 2).

[8] As a result of the partner’s deferred contribution, the Company owns an approximately 55% interest in the joint venture at June 30, 2020. Future development projects will be owned 49% by the Company and 51% by its partner.