SEGMENT INFORMATION - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Total revenue	$ 486,819	$ 534,852	$ 580,417
Operating expenses	(188,311)	(205,456)	(225,846)
Net operating income (loss)	298,508	329,396	354,571
Philadelphia CBD
Segment Reporting Information [Line Items]
Total revenue	207,858	232,028	263,769
Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Total revenue	124,566	141,613	141,084
Austin, Texas
Segment Reporting Information [Line Items]
Total revenue	101,680	102,982	104,157
Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Total revenue	19,865	40,223	51,498
Operating Segments | Philadelphia CBD
Segment Reporting Information [Line Items]
Total revenue	207,858	232,028	263,769
Operating expenses	(73,695)	(82,505)	(100,219)
Net operating income (loss)	134,163	149,523	163,550
Operating Segments | Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Total revenue	124,566	141,613	141,084
Operating expenses	(40,011)	(46,281)	(47,418)
Net operating income (loss)	84,555	95,332	93,666
Operating Segments | Austin, Texas
Segment Reporting Information [Line Items]
Total revenue	101,680	102,982	104,157
Operating expenses	(39,374)	(39,759)	(38,285)
Net operating income (loss)	62,306	63,223	65,872
Operating Segments | Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Total revenue	19,865	40,223	51,498
Operating expenses	(15,386)	(20,791)	(23,455)
Net operating income (loss)	4,479	19,432	28,043
Operating Segments | Other
Segment Reporting Information [Line Items]
Total revenue	14,015	13,469	14,558
Operating expenses	(9,840)	(9,815)	(9,328)
Net operating income (loss)	4,175	3,654	5,230
Corporate
Segment Reporting Information [Line Items]
Total revenue	18,835	4,537	5,351
Operating expenses	(10,005)	(6,305)	(7,141)
Net operating income (loss)	$ 8,830	$ (1,768)	$ (1,790)