SEGMENT INFORMATION - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Total revenue	$ 506,100	$ 486,819	$ 534,852
Operating expenses	(194,401)	(188,311)	(205,456)
Net operating income	311,699	298,508	329,396
Philadelphia CBD
Segment Reporting Information [Line Items]
Total revenue	220,875	207,858	232,028
Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Total revenue	128,940	124,566	141,613
Austin, Texas
Segment Reporting Information [Line Items]
Total revenue	96,329	101,680	102,982
Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Total revenue	21,899	19,865	40,223
Operating Segments | Philadelphia CBD
Segment Reporting Information [Line Items]
Total revenue	220,876	207,858	232,028
Operating expenses	(79,827)	(73,695)	(82,505)
Net operating income	141,049	134,163	149,523
Operating Segments | Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Total revenue	128,940	124,566	141,613
Operating expenses	(41,814)	(40,011)	(46,281)
Net operating income	87,126	84,555	95,332
Operating Segments | Austin, Texas
Segment Reporting Information [Line Items]
Total revenue	96,328	101,680	102,982
Operating expenses	(41,141)	(39,374)	(39,759)
Net operating income	55,187	62,306	63,223
Operating Segments | Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Total revenue	21,900	19,865	40,223
Operating expenses	(12,539)	(15,386)	(20,791)
Net operating income	9,361	4,479	19,432
Operating Segments | Other
Segment Reporting Information [Line Items]
Total revenue	14,710	14,015	13,469
Operating expenses	(8,626)	(9,840)	(9,815)
Net operating income	6,084	4,175	3,654
Corporate
Segment Reporting Information [Line Items]
Total revenue	23,346	18,835	4,537
Operating expenses	(10,454)	(10,005)	(6,305)
Net operating income	$ 12,892	$ 8,830	$ (1,768)