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Investment in Unconsolidated Joint Ventures (Narrative) (Details) $ in Thousands |  |  |  |  |  | 3 Months Ended | 9 Months Ended |  |  |
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Sep. 01, 2021 USD ($) ft² | Aug. 31, 2021 USD ($) ft² years loanExtension | Jun. 15, 2021 | Apr. 01, 2021 USD ($) | Mar. 30, 2021 USD ($) ft² | Sep. 30, 2021 USD ($) | Sep. 30, 2020 USD ($) | Sep. 30, 2021 USD ($) | Sep. 30, 2020 USD ($) | Jun. 11, 2021 ft² | Feb. 25, 2021 ft² | Dec. 31, 2020 USD ($)
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mortgage notes payable, net |  |  |  |  |  |  | $ 2,898,699 |  |  | $ 2,898,699 |  |  |  |  | $ 2,909,081 |
Debt Instrument, Basis Spread on Variable Rate |  |  |  | 0.775% |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from Sale of Equity Method Investments |  |  |  |  |  |  |  |  |  | 17,789 |  | $ 0 |  |  |  |
Payments to Acquire Equity Method Investments |  |  |  |  |  |  |  |  |  | 95,462 |  | 158,374 |  |  |  |
Income (Loss) from Equity Method Investments |  |  |  |  |  |  | $ (5,597) |  | $ (6,873) | (1,745) |  | (5,410) |  |  |  |
Capital distributions from unconsolidated joint ventures |  |  |  |  |  |  |  |  |  | 122 |  | 55,123 |  |  |  |
Proceeds from sales of real estate |  |  |  |  |  |  |  |  |  | 0 |  | 505,679 |  |  |  |
Repayments of Secured Debt |  |  |  |  |  |  |  |  |  | $ 13,261 |  | 12,795 |  |  |  |
Gateway Commons Complex [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  |  | 50.00% | [1] |  | 50.00% | [1] |  |  | 54.00% | 55.00% |
Gateway Commons Complex [Member] | Scenario, Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  |  | 50.00% |  |  | 50.00% |  |  |  |  |  |
Gateway Commons Complex [Member] | Scenario, Plan | 751 Gateway |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  |  | 49.00% |  |  | 49.00% |  |  |  |  |  |
Gateway Commons Complex prior to deferred contribution [Member] [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  |  | 52.00% |  |  | 52.00% |  |  |  |  | 55.00% |
Platform 16 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage | [2] |  |  |  |  |  | 55.00% |  |  | 55.00% |  |  |  |  |  |
Annapolis Junction NFM, LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  |  | 50.00% | [3]
Contractual Sales Price |  |  |  |  |  | $ 65,900 |  |  |  |  |  |  |  |  |  |
Proceeds from Sale of Equity Method Investments |  |  |  |  |  | 17,800 |  |  |  |  |  |  |  |  |  |
Capital distributions from unconsolidated joint ventures |  |  |  |  | $ 5,800 |  |  |  |  |  |  |  |  |  |  |
Gain on Sale of Investments |  |  |  |  |  | 10,300 |  |  |  | $ 10,300 |  |  |  |  |  |
Repayments of Secured Debt |  |  |  |  |  | $ 15,100 |  |  |  |  |  |  |  |  |  |
Annapolis Junction NFM, LLC [Member] | Annapolis Junction Building Six and Seven | Unconsolidated Properties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  | 247,000 |  |  |  |  |  |  |  |  |  |
One Hundred Causeway Street [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  |  | 50.00% |  |  | 50.00% |  |  |  |  |  |
One Hundred Causeway Street [Member] | Unconsolidated Properties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  |  |  |  |  |  |  |  | 50.00% |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  |  | 632,000 |  |  |
Safeco Plaza REIT LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  | 33.67% |  |  |  |  | 33.67% | [4] |  | 33.67% | [4] |  |  |  |  |
Mortgage notes payable, net |  | $ 250,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  | 765,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Equity Method Investments |  | $ 72,600 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Description of Variable Rate Basis |  | variable rate equal to the greater of (x) 2.35% or (y) LIBOR plus 2.20% per annum |  |  |  |  |  |  |  |  |  |  |  |  |  |
Aggregate purchase price |  | $ 465,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of floors |  | 50 |  |  |  |  |  |  |  |  |  |  |  |  |  |
number of investors |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Podium Developer LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  | 50.00% |  |  |  | 50.00% |  |  | 50.00% |  |  |  |  |  |
Mortgage notes payable, net |  |  | $ 174,300 |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  |  | 2.25% |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  | 382,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Number of extensions | loanExtension |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |
Extension Option (in years) | years |  |  | 1 |  |  |  |  |  |  |  |  |  |  |  |  |
Unconsolidated Joint Ventures [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mortgage notes payable, net |  |  |  |  |  |  | $ 2,995,161 |  |  | $ 2,995,161 |  |  |  |  | $ 2,637,911 |
Income (Loss) from Equity Method Investments |  |  |  |  |  |  | (5,597) |  | (6,873) | (1,745) |  | (5,410) |  |  |  |
Gain on Sale of Investments |  |  |  |  |  |  | $ 0 |  | $ 0 | $ 10,257 | [5] | $ 0 |  |  |  |
Construction in Progress | Gateway Commons Complex [Member] | 751 Gateway | Unconsolidated Properties |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  |  |  | 229,000 |  |
Company's Share [Member] | Safeco Plaza REIT LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  | 33.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Company's Share [Member] | Safeco Partner Entity One [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  | 1.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Company's Share [Member] | Safeco Partner Entity Two |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  | 1.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Institutional investor number one | Safeco Plaza REIT LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  | 33.165% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Equity Method Investments |  | $ 71,900 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Institutional investor number two | Safeco Plaza REIT LLC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  | 33.165% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Equity Method Investments |  | $ 71,900 |  |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | As a result of the partner’s deferred contribution, the Company owned an approximately 52% and 55% interest in the joint venture at September 30, 2021 and December 31, 2020, respectively. (9)The Company’s ownership includes (1) a 33.0% direct interest in the joint venture, and (2) an additional 1% interest in each of the two entities (each, a “Safeco Partner Entity”) through which each partner owns its interest in the joint venture.
[2] | This entity is a VIE (See Note 2).
[3] | On March 30, 2021, the Company sold its interest in the joint venture to the partner. See below for additional details.
[4] | The Company’s ownership includes (1) a 33.0% direct interest in the joint venture, and (2) an additional 1% interest in each of the two entities (each, a “Safeco Partner Entity”) through which each partner owns its interest in the joint venture.
[5] | During the nine months ended September 30, 2021, the Company completed the sale of its 50% ownership interest in Annapolis Junction NFM LLC. The Company recognized a gain on sale of investment of approximately $10.3 million.

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