REAL ESTATE INVESTMENTS - Summary of Acquired Properties (Details) $ in Thousands				12 Months Ended
	Nov. 30, 2020 USD ($) ft²	Nov. 05, 2020 USD ($) ft²	Feb. 27, 2020 USD ($) a	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Business Acquisition [Line Items]
Purchase Price (a)				$ 41,950	$ 0	$ 196,625
Land | 145 King of Prussia Road
Business Acquisition [Line Items]
Acres | a			7.75
Office Properties | 145 King of Prussia Road
Business Acquisition [Line Items]
Purchase Price (a)			$ 11,250
Office Properties | 1505-11 Race Street
Business Acquisition [Line Items]
Rentable square feet | ft²		119,763
Purchase Price (a)		$ 9,700
Office Properties | 250 King of Prussia Road (b)
Business Acquisition [Line Items]
Rentable square feet | ft²	169,843
Purchase Price (a)	$ 20,250