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Investments in Unconsolidated Joint Ventures Narrative (Details) $ in Thousands | Jun. 27, 2019 USD ($) ft² | May 28, 2019 USD ($) | Apr. 26, 2019 USD ($) ft² yr | Jan. 24, 2019 USD ($) ft² | Jun. 30, 2019 USD ($) | Jun. 15, 2019 ft² | Dec. 31, 2018 USD ($)
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Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Mortgage notes payable, net |  |  |  |  | $ 2,956,833 |  |  | $ 2,964,572
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% |  |  |  |
Number of extensions |  |  |  | 1 |  |  |  |
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% | [1] |  |
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 |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  | 734,000 |  |  |  |  |
The Hub on Causeway - Podium and One hundred Causeway [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Ownership Percentage |  | 50.00% |  |  |  |  |  |
Repayments of Construction Loans Payable |  | $ 30,000 |  |  |  |  |  |
Hub on Causeway - Podium [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² |  |  |  |  |  |  | 385,000 |
540 Madison Venture LLC [Member] |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Ownership Percentage | 60.00% |  |  |  | 60.00% | [2] |  |
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 | $ 47,800 |  |  |  |  |  |  |
Net Rentable Area (in sf) | ft² | 284,000 |  |  |  |  |  |  |
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 |  |  |  |  |  |  |
[1] | This entity is a VIE (See Note 2).
[2] | The property was sold on June 27, 2019. As of June 30, 2019, the investment is comprised of undistributed cash. See note below for additional details.

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