SEGMENT INFORMATION - Unconsolidated Real Estate Ventures of Company's Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021
Segment Reporting Information [Line Items]
Investment in real estate ventures	$ 546,935		$ 546,935		$ 411,110
Equity in income (loss) of real estate venture	(6,260)	$ (6,634)	(15,804)	$ (20,798)
Philadelphia CBD
Segment Reporting Information [Line Items]
Investment in real estate ventures	397,973		397,973		317,959
Equity in income (loss) of real estate venture	(3,452)	(3,602)	(8,798)	(12,542)
Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Investment in real estate ventures	85,206		85,206		85,867
Equity in income (loss) of real estate venture	(706)	(1,173)	(1,273)	(2,368)
Mid-Atlantic Office JV
Segment Reporting Information [Line Items]
Investment in real estate ventures	31,229		31,229		31,680
Equity in income (loss) of real estate venture	(83)	310	234	634
MAP Venture
Segment Reporting Information [Line Items]
Investment in real estate ventures	(32,522)		(32,522)		(24,396)
Equity in income (loss) of real estate venture	(2,019)	(2,169)	(5,967)	(6,522)
Austin, Texas
Segment Reporting Information [Line Items]
Investment in real estate ventures	65,049		65,049		$ 0
Equity in income (loss) of real estate venture	$ 0	$ 0	$ 0	$ 0