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Investments in Unconsolidated Joint Ventures Narrative (Details) $ in Thousands |  |  |  |  |  |  | 3 Months Ended | 9 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 20, 2019 USD ($) ft² a | Sep. 16, 2019 USD ($) | Sep. 05, 2019 USD ($) ft² yr | Jun. 27, 2019 USD ($) ft² | Apr. 26, 2019 USD ($) ft² yr | Jan. 24, 2019 USD ($) ft² | Sep. 30, 2019 USD ($) | Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Jun. 15, 2019 ft² | May 28, 2019 USD ($) | Dec. 31, 2018 USD ($)
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Mortgage notes payable, net |  |  |  |  |  |  | $ 2,952,006 |  | $ 2,952,006 |  |  |  |  | $ 2,964,572
Payments to Acquire Equity Method Investments |  |  |  |  |  |  |  |  | $ 65,499 |  | $ 314,075 |  |  |
Annapolis Junction Building Six [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  | 50.00% |  |  |  |  |  |  |  |
Mortgage notes payable, net |  |  |  |  |  | $ 13,000 |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  |  |  |  |  | 2.00% |  |  |  |  |  |  |  |
Extension Option (in years) |  |  |  |  |  | 1 |  |  |  |  |  |  |  |
Debt instrument, borrowing capacity |  |  |  |  |  | $ 14,300 |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  | 119,000 |  |  |  |  |  |  |  |
7750 Wisconsin Avenue LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  | 50.00% |  | 50.00% |  | 50.00% |  |  |  |  |
Construction Loan Facility Borrowing Capacity |  |  |  |  | $ 255,000 |  |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  |  |  |  | 1.25% |  |  |  |  |  |  |  |  |
Number of extensions |  |  |  |  | 2 |  |  |  |  |  |  |  |  |
Extension Option (in years) | yr |  |  |  |  | 1 |  |  |  |  |  |  |  |  |
Proceeds from Construction Loans Payable |  |  |  |  |  |  | $ 39,600 |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  | 734,000 |  |  |  |  |  |  |  |  |
The Hub on Causeway - Podium and 100 Causeway [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  |  |  |  |  |  |  |  |  |  | 50.00% |
Maximum proceeds from a state infrastructure development program |  |  |  |  |  |  |  |  |  |  |  |  | $ 30,000 |
Net proceeds from a state infrastructure development program |  | $ 28,800 |  |  |  |  |  |  |  |  |  |  |  |
Hub on Causeway - Podium [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Repayments of Construction Loans Payable |  | $ 28,800 |  |  |  |  |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  |  |  |  |  |  | 375,000 |  |
540 Madison Venture LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  |  | 60.00% |  |  | 60.00% | [1] | 60.00% | [1] |  |  |  |
Contractual Sales Price |  |  |  | $ 310,300 |  |  |  |  |  |  |  |  |  |
Loans Assumed |  |  |  | $ 120,000 |  |  |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  |  |  | 1.10% |  |  |  |  |  |  |  |  |  |
Proceeds from Sale of Equity Method Investments |  |  |  | $ 178,700 |  |  |  |  |  |  |  |  |  |
Equity Method Investment, Realized Gain (Loss) on Disposal |  |  |  | $ 47,300 |  |  | $ (500) |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  | 284,000 |  |  |  |  |  |  |  |  |  |
100 Causeway Street [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage |  |  | 50.00% |  |  |  | 50.00% |  | 50.00% |  |  |  |  |
Construction Loan Facility Borrowing Capacity |  |  | $ 400,000 |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  |  | 1.50% |  |  |  |  |  |  |  |  |  |  |
Number of extensions |  |  | 2 |  |  |  |  |  |  |  |  |  |  |
Extension Option (in years) | yr |  |  | 1 |  |  |  |  |  |  |  |  |  |  |
Proceeds from Construction Loans Payable |  |  |  |  |  |  | $ 47,700 |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  | 632,000 |  |  |  |  |  |  |  |  |  |  |
Platform 16 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage | 55.00% |  |  |  |  |  | 55.00% | [2] | 55.00% | [2] |  |  |  |
Net Rentable Area (in sf) | ft² | 1,100,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Equity Method Investments | $ 28,200 |  |  |  |  |  |  |  |  |  |  |  |  |
Lessee, Finance Lease, Term of Contract | 65 years |  |  |  |  |  |  |  |  |  |  |  |  |
Area of Land | a | 5.6 |  |  |  |  |  |  |  |  |  |  |  |  |
Company's Share [Member] | 540 Madison Venture LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from Sale of Equity Method Investments |  |  |  | $ 107,100 |  |  |  |  |  |  |  |  |  |
Joint Venture Partner [Member] | Platform 16 [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ownership Percentage | 45.00% |  |  |  |  |  |  |  |  |  |  |  |  |
Payments to Acquire Equity Method Investments | $ 23,100 |  |  |  |  |  |  |  |  |  |  |  |  |
Stabilization as defined in the agreement [Member] | 100 Causeway Street [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument, Basis Spread on Variable Rate |  |  | 1.375% |  |  |  |  |  |  |  |  |  |  |
[1] | 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.
[2] | This entity is a VIE (See Note 2).

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