INVESTMENT IN UNCONSOLIDATED REAL ESTATE VENTURES - Commerce Square Venture (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 21, 2020	Jul. 21, 2020	Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]
Sale of ownership interest percentage		60.00%
Outstanding indebtedness	$ 1,830,595			$ 2,144,418
Secured Debt
Schedule of Equity Method Investments [Line Items]
Outstanding indebtedness	$ 0			$ 313,812
Philadelphia Office | Secured Debt
Schedule of Equity Method Investments [Line Items]
Outstanding indebtedness			$ 222,100
Commerce Square
Schedule of Equity Method Investments [Line Items]
Sale of ownership interest percentage			30.00%
Commerce Square
Schedule of Equity Method Investments [Line Items]
Ownership percentage	70.00%
Commerce Square | Preferred Equity Securities
Schedule of Equity Method Investments [Line Items]
Ownership percentage	32.00%
Commerce Square | Common Equity Securities
Schedule of Equity Method Investments [Line Items]
Ownership percentage	38.00%