SEGMENT INFORMATION - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Total revenue	$ 514,651	$ 506,100	$ 486,819
Operating expenses	(189,947)	(194,401)	(188,311)
Net operating income	324,704	311,699	298,508
Philadelphia CBD
Segment Reporting Information [Line Items]
Total revenue		220,875
Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Total revenue		128,940
Austin, Texas
Segment Reporting Information [Line Items]
Total revenue		96,329
Operating Segments | Philadelphia CBD
Segment Reporting Information [Line Items]
Total revenue	230,933	220,876	207,858
Operating expenses	(79,579)	(79,827)	(73,695)
Net operating income	151,354	141,049	134,163
Operating Segments | Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Total revenue	129,300	128,940	124,566
Operating expenses	(39,584)	(41,814)	(40,011)
Net operating income	89,716	87,126	84,555
Operating Segments | Austin, Texas
Segment Reporting Information [Line Items]
Total revenue	95,505	96,328	101,680
Operating expenses	(38,453)	(41,141)	(39,374)
Net operating income	57,052	55,187	62,306
Operating Segments | Other
Segment Reporting Information [Line Items]
Total revenue	39,306	36,610	33,880
Operating expenses	(21,134)	(21,165)	(25,226)
Net operating income	18,172	15,445	8,654
Corporate
Segment Reporting Information [Line Items]
Total revenue	19,607	23,346	18,835
Operating expenses	(11,197)	(10,454)	(10,005)
Net operating income	$ 8,410	$ 12,892	$ 8,830