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INVESTMENT IN UNCONSOLIDATED REAL ESTATE VENTURES - Hernadon Innovation Center Metro Portfolio Venture, LLC (Details)					3 Months Ended	12 Months Ended
	Apr. 12, 2019 USD ($)	Apr. 11, 2019 USD ($)	Mar. 29, 2019 USD ($) property	Dec. 20, 2018 USD ($) ft² property	Sep. 30, 2018 USD ($)	Dec. 31, 2019 USD ($) ft²	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Equity Method Investments [Line Items]
Area of real estate property \| ft²						16,711,387
Proceeds from the sale of properties						$ 41,795,000	$ 324,090,000	$ 171,860,000
Net gain on disposition of real estate						$ 356,000	$ 2,932,000	$ 32,017,000
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%