SEGMENT INFORMATION - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Total revenue	$ 534,852	$ 580,417	$ 544,345
Operating expenses	(205,456)	(225,846)	(218,099)
Net operating income (loss)	329,396	354,571	326,246
Philadelphia CBD
Segment Reporting Information [Line Items]
Total revenue	232,028	263,769
Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Total revenue	141,613	141,084
Austin, Texas
Segment Reporting Information [Line Items]
Total revenue	102,982	104,157
Metropolitan DC
Segment Reporting Information [Line Items]
Total revenue	40,223	51,498
Operating Segments | Philadelphia CBD
Segment Reporting Information [Line Items]
Total revenue	232,028	263,769	256,717
Operating expenses	(82,505)	(100,219)	(99,449)
Net operating income (loss)	149,523	163,550	157,268
Operating Segments | Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Total revenue	141,613	141,084	138,279
Operating expenses	(46,281)	(47,418)	(49,433)
Net operating income (loss)	95,332	93,666	88,846
Operating Segments | Austin, Texas
Segment Reporting Information [Line Items]
Total revenue	102,982	104,157	38,665
Operating expenses	(39,759)	(38,285)	(16,739)
Net operating income (loss)	63,223	65,872	21,926
Operating Segments | Metropolitan DC
Segment Reporting Information [Line Items]
Total revenue	40,223	51,498	90,308
Operating expenses	(20,791)	(23,455)	(34,072)
Net operating income (loss)	19,432	28,043	56,236
Operating Segments | Other
Segment Reporting Information [Line Items]
Total revenue	13,469	14,558	16,757
Operating expenses	(9,815)	(9,328)	(11,888)
Net operating income (loss)	3,654	5,230	4,869
Corporate
Segment Reporting Information [Line Items]
Total revenue	4,537	5,351	3,619
Operating expenses	(6,305)	(7,141)	(6,518)
Net operating income (loss)	$ (1,768)	$ (1,790)	$ (2,899)