Investments in Unconsolidated Joint Ventures (Investments in Unconsolidated Joint Ventures) (Details) $ in Thousands					9 Months Ended
		Sep. 05, 2019 USD ($) ft² yr	Jun. 27, 2019 ft²	Apr. 26, 2019 USD ($) ft² yr	Sep. 30, 2019 USD ($) ft² Land_Parcels Buildings payments		Sep. 20, 2019 ft²	Dec. 31, 2018 USD ($)
Schedule of Equity Method Investments [Line Items]
Carrying Value of investment					$ (356,338)			$ (503,726)
Carrying value of the Company's investments in unconsolidated joint ventures					$ 933,255			956,309
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]				$ (5,114)			(6,424)
BP/CRF Metropolitan Square 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]				$ 5,726			2,644
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]				$ (12,385)			(13,640)
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]				$ 37,179			38,214
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]				$ 25,314			25,268
Number of real estate properties | Buildings					3
Parcels of undeveloped land | Land_Parcels					2
540 Madison Venture LLC [Member]
Schedule of Equity Method Investments [Line Items]
Debt Instrument, Basis Spread on Variable Rate			1.10%
Properties					540 Madison Avenue
Ownership Percentage			60.00%		60.00%	[5]
Carrying value of the Company's investments in unconsolidated joint ventures	[1]				$ 3,075			66,391
Net Rentable Area (in sf) | ft²			284,000
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]				$ (5,661)			(5,026)
501 K Street LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties					1001 6th Street
Ownership Percentage	[6]				50.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]				$ 42,555			42,557
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%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]				$ 41,434			69,302
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]				$ 48,491			47,505
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]				$ 3,917			3,022
Office Tower Developer LLC [Member]
Schedule of Equity Method Investments [Line Items]
Construction Loan Facility Borrowing Capacity		$ 400,000
Number of extensions		2
Extension Option (in years) | yr		1
Debt Instrument, Basis Spread on Variable Rate		1.50%
Properties					100 Causeway Street
Ownership Percentage		50.00%			50.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]				$ 74,083			23,804
Net Rentable Area (in sf) | ft²		632,000
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]				$ 3,932			3,918
BNY Tower Holdings LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties					Dock 72
Ownership Percentage					50.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]				$ 94,700			82,520
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]				$ 250,213			253,495
7750 Wisconsin Avenue LLC [Member]
Schedule of Equity Method Investments [Line Items]
Construction Loan Facility Borrowing Capacity				$ 255,000
Number of extensions				2
Extension Option (in years) | yr				1
Debt Instrument, Basis Spread on Variable Rate				1.25%
Properties					7750 Wisconsin Avenue
Ownership Percentage				50.00%	50.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]				$ 55,662			69,724
Net Rentable Area (in sf) | ft²				734,000
BP-M 3HB Venture LLC [Member]
Schedule of Equity Method Investments [Line Items]
Properties					3 Hudson Boulevard
Ownership Percentage					25.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]				$ 54,037			46,993
SMBP Venture LP [Member]
Schedule of Equity Method Investments [Line Items]
Properties					Santa Monica Business Park
Ownership Percentage					55.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]				$ 163,809			180,952
Platform 16 Holdings LP [Member]
Schedule of Equity Method Investments [Line Items]
Properties					Platform 16
Ownership Percentage					55.00%	[7]	55.00%
Carrying value of the Company's investments in unconsolidated joint ventures	[1]				$ 29,128			0
Net Rentable Area (in sf) | ft²							1,100,000
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Carrying Value of investment					23,200			25,100
Unconsolidated Joint Ventures [Member]
Schedule of Equity Method Investments [Line Items]
Carrying Value of investment					(773,033)	[8]		(582,006)
Carrying value of the Company's investments in unconsolidated joint ventures	[1],[9]				$ 910,095			$ 931,219

[1] Investments with deficit balances aggregating approximately $23.2 million and $25.1 million at September 30, 2019 and December 31, 2018, 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 three in-service buildings and two undeveloped land parcels.

[5] The property was sold on June 27, 2019. As of September 30, 2019, the investment is comprised of undistributed cash. See note below for additional details.

[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] At September 30, 2019, this amount includes lease liabilities - finance leases and lease liabilities - operating leases totaling approximately $511.7 million and $17.3 million, respectively.

[9] Investments with deficit balances aggregating approximately $23.2 million and $25.1 million at September 30, 2019 and December 31, 2018, respectively, have been reflected within Other Liabilities in the Company’s Consolidated Balance Sheet