SEGMENT INFORMATION - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information [Line Items]
Total revenue	$ 125,882	$ 124,042	$ 255,109	$ 251,547
Operating expenses	(46,019)	(49,649)	(96,854)	(97,567)
Net operating income	79,863	74,393	158,255	153,980
Operating Segments | Philadelphia CBD
Segment Reporting Information [Line Items]
Total revenue	56,671	55,426	112,898	108,897
Operating expenses	(20,257)	(20,538)	(40,843)	(40,281)
Net operating income	36,414	34,888	72,055	68,616
Operating Segments | Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Total revenue	32,102	31,333	64,873	63,140
Operating expenses	(10,084)	(10,258)	(19,863)	(20,416)
Net operating income	22,018	21,075	45,010	42,724
Operating Segments | Austin, Texas
Segment Reporting Information [Line Items]
Total revenue	23,781	23,921	49,018	48,836
Operating expenses	(8,683)	(10,438)	(19,977)	(20,731)
Net operating income	15,098	13,483	29,041	28,105
Operating Segments | Other
Segment Reporting Information [Line Items]
Total revenue	8,888	9,405	18,277	18,204
Operating expenses	(4,850)	(5,458)	(10,262)	(10,896)
Net operating income	4,038	3,947	8,015	7,308
Corporate
Segment Reporting Information [Line Items]
Total revenue	4,440	3,957	10,043	12,470
Operating expenses	(2,145)	(2,957)	(5,909)	(5,243)
Net operating income	$ 2,295	$ 1,000	$ 4,134	$ 7,227