SEGMENT INFORMATION - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information [Line Items]
Total revenue	$ 127,505	$ 120,769
Operating expenses	(47,918)	(46,674)
Net operating income	79,587	74,095
Operating Segments | Philadelphia CBD
Segment Reporting Information [Line Items]
Total revenue	53,471	51,227
Operating expenses	(19,743)	(17,411)
Net operating income	33,728	33,816
Operating Segments | Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Total revenue	31,807	31,740
Operating expenses	(10,158)	(10,694)
Net operating income	21,649	21,046
Operating Segments | Austin, Texas
Segment Reporting Information [Line Items]
Total revenue	24,915	26,175
Operating expenses	(10,293)	(9,720)
Net operating income	14,622	16,455
Operating Segments | Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Total revenue	5,195	4,675
Operating expenses	(3,428)	(4,199)
Net operating income	1,767	476
Operating Segments | Other
Segment Reporting Information [Line Items]
Total revenue	3,604	3,213
Operating expenses	(2,010)	(2,282)
Net operating income	1,594	931
Corporate
Segment Reporting Information [Line Items]
Total revenue	8,513	3,739
Operating expenses	(2,286)	(2,368)
Net operating income	$ 6,227	$ 1,371