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Investments in Unconsolidated Joint Ventures (Investments in Unconsolidated Joint Ventures) (Details) $ in Thousands |  |  |  | 9 Months Ended |  |
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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

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