SEGMENT INFORMATION - Unconsolidated Real Estate Ventures of Company's Reportable Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Investment in unconsolidated real estate ventures	$ 411,110	$ 389,811
Equity in income (loss) of real estate venture	(26,697)	(18,584)	$ (9,922)
Philadelphia CBD
Segment Reporting Information [Line Items]
Investment in unconsolidated real estate ventures	317,959	268,562
Equity in income (loss) of real estate venture	(15,191)	(9,090)	328
Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Investment in unconsolidated real estate ventures	85,867	99,769
Equity in income (loss) of real estate venture	(3,755)	(3,019)	(4,234)
Mid-Atlantic Office Venture
Segment Reporting Information [Line Items]
Investment in unconsolidated real estate ventures	31,680	32,996
Equity in income (loss) of real estate venture	932	96	0
MAP Venture
Segment Reporting Information [Line Items]
Investment in unconsolidated real estate ventures	(24,396)	(11,516)
Equity in income (loss) of real estate venture	(8,683)	(6,571)	(6,102)
Other
Segment Reporting Information [Line Items]
Investment in unconsolidated real estate ventures	0	0
Equity in income (loss) of real estate venture	$ 0	$ 0	$ 86