SEGMENT INFORMATION - Unconsolidated Real Estate Ventures of Company's Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Segment Reporting Information [Line Items]
Investment in real estate ventures	$ 434,555		$ 411,110
Equity in income (loss) of real estate venture	(4,563)	$ (6,924)
Philadelphia CBD
Segment Reporting Information [Line Items]
Investment in real estate ventures	343,502		317,959
Equity in income (loss) of real estate venture	(2,772)	(4,279)
Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Investment in real estate ventures	85,976		85,867
Equity in income (loss) of real estate venture	(186)	(417)
Mid-Atlantic Office JV
Segment Reporting Information [Line Items]
Investment in real estate ventures	31,911		31,680
Equity in income (loss) of real estate venture	418	207
MAP Venture
Segment Reporting Information [Line Items]
Investment in real estate ventures	(26,834)		$ (24,396)
Equity in income (loss) of real estate venture	$ (2,023)	$ (2,435)