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INVESTMENT IN UNCONSOLIDATED REAL ESTATE VENTURES - Hernadon Innovation Center Metro Portfolio Venture, LLC (Details) |  |  |  |  | 3 Months Ended | 12 Months Ended
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Apr. 12, 2019 USD ($) | Apr. 11, 2019 USD ($) | Mar. 29, 2019 USD ($) property | Dec. 20, 2018 USD ($) ft² property | Sep. 30, 2018 USD ($) | Dec. 31, 2020 USD ($) ft² | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Area of real estate property | ft² |  |  |  |  |  | 13,852,464 |  |
Proceeds from the sale of properties |  |  |  |  |  | $ 278,114,000 | $ 41,795,000 | $ 324,090,000
Net gain on disposition of real estate |  |  |  |  |  | $ 289,461,000 | $ 356,000 | $ 2,932,000
Herndon Innovation Center Metro Portfolio Venture, LLC |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Ownership percentage |  |  |  |  |  | 15.00% |  |
Metropolitan Washington, D.C. - Office | Herndon Innovation Center Metro Portfolio Venture, LLC |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Number of properties contributed to a joint venture | property |  |  |  | 8 |  |  |  |
Area of real estate property | ft² |  |  |  | 1,293,197 |  |  |  |
Sales price |  |  |  | $ 312,000,000.0 |  |  |  |
Ownership percentage |  |  |  | 15.00% |  |  |  |
Cash proceeds received from sale of properties |  |  |  | $ 265,200,000 |  |  |  |
Closing costs |  |  |  | 2,200,000 |  |  |  |
Working capital contribution |  |  |  | 600,000 |  |  |  |
Proceeds from the sale of properties |  |  |  | 262,400,000 |  |  |  |
Capital credit |  |  |  | $ 47,700,000 |  |  |  |
Provision for impairment on assets held for sale |  |  |  |  | $ 56,900,000 |  |  |
Net gain on disposition of real estate |  |  |  |  | $ 400,000 |  |  |
Rockpoint | Metropolitan Washington, D.C. - Office | Herndon Innovation Center Metro Portfolio Venture, LLC |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Ownership percentage |  |  |  | 85.00% |  |  |  |
Secured Debt | Herndon Innovation Center Metro Portfolio Venture, LLC |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Debt instrument, face amount |  |  | $ 134,100,000 |  |  |  |  |
Number of properties used to secure debt | property |  |  | 4 |  |  |  |  |
Proceeds from issuance of debt |  | $ 94,200,000 | $ 113,100,000 |  |  |  |  |
Distributions From Loan One | Secured Debt | Herndon Innovation Center Metro Portfolio Venture, LLC |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Proceeds from issuance of debt | $ 16,700,000 |  |  |  |  |  |  |
Variable interest rate |  |  |  |  |  | 1.95% |  |
Distributions From Loan Two | Secured Debt | Herndon Innovation Center Metro Portfolio Venture, LLC |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Debt instrument, face amount |  | $ 115,300,000 |  |  |  |  |  |
Proceeds from issuance of debt | $ 13,800,000 |  |  |  |  |  |  |
Variable interest rate |  |  |  |  |  | 1.80% |  |
Minimum | Distributions From Loan One | Secured Debt | Herndon Innovation Center Metro Portfolio Venture, LLC |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Effective interest rate |  |  |  |  |  | 5.45% |  |
Maximum | Distributions From Loan One | Secured Debt | Herndon Innovation Center Metro Portfolio Venture, LLC |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Effective interest rate |  |  |  |  |  | 6.45% |  |
Maximum | Distributions From Loan Two | Secured Debt | Herndon Innovation Center Metro Portfolio Venture, LLC |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |
Debt instrument, interest rate, stated percentage |  |  |  |  |  | 6.30% |  |

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