SEGMENT INFORMATION - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Total revenue	$ 129,372	$ 125,569	$ 384,481	$ 377,116
Operating expenses	(46,399)	(47,486)	(143,253)	(145,053)
Net operating income	82,973	78,083	241,228	232,063
Operating Segments | Philadelphia CBD
Segment Reporting Information [Line Items]
Total revenue	56,367	56,105	169,265	165,002
Operating expenses	(18,609)	(20,080)	(59,452)	(60,361)
Net operating income	37,758	36,025	109,813	104,641
Operating Segments | Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Total revenue	32,611	32,709	97,484	95,849
Operating expenses	(9,770)	(11,022)	(29,633)	(31,438)
Net operating income	22,841	21,687	67,851	64,411
Operating Segments | Austin, Texas
Segment Reporting Information [Line Items]
Total revenue	24,457	22,509	73,475	71,345
Operating expenses	(10,044)	(8,597)	(30,021)	(29,328)
Net operating income	14,413	13,912	43,454	42,017
Operating Segments | Other
Segment Reporting Information [Line Items]
Total revenue	10,295	8,874	28,572	27,078
Operating expenses	(5,526)	(5,089)	(15,788)	(15,985)
Net operating income	4,769	3,785	12,784	11,093
Corporate
Segment Reporting Information [Line Items]
Total revenue	5,642	5,372	15,685	17,842
Operating expenses	(2,450)	(2,698)	(8,359)	(7,941)
Net operating income	$ 3,192	$ 2,674	$ 7,326	$ 9,901