SEGMENT INFORMATION - Unconsolidated Real Estate Ventures of Company's Reportable Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, equity method	$ 389,811	$ 120,294
Equity in loss of Real Estate Ventures	(18,584)	(9,922)	$ (15,231)
Philadelphia CBD
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, equity method	268,562	17,524
Equity in loss of Real Estate Ventures	(9,090)	328	(105)
Metropolitan DC
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, equity method	99,769	102,840
Equity in loss of Real Estate Ventures	(3,019)	(4,234)	(15,065)
Mid-Atlantic Office JV
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, equity method	32,996	0
Equity in loss of Real Estate Ventures	96	0	0
MAP Venture
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, equity method	(11,516)	(70)
Equity in loss of Real Estate Ventures	(6,571)	(6,102)	(2,155)
Other
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, equity method	0	0
Equity in loss of Real Estate Ventures	0	86	407
Austin, Texas
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, equity method	0	0
Equity in loss of Real Estate Ventures	$ 0	$ 0	$ 1,687