SEGMENT INFORMATION - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total revenue	$ 147,039	$ 145,331	$ 144,151	$ 143,896	$ 139,203	$ 134,998	$ 133,786	$ 136,358	$ 580,417	$ 544,345	$ 520,493
Operating expenses									(225,846)	(218,099)	(206,075)
Net operating income (loss)									354,571	326,246	314,418
Philadelphia CBD
Segment Reporting Information [Line Items]
Total revenue									263,769
Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Total revenue									141,084
Austin, Texas
Segment Reporting Information [Line Items]
Total revenue									104,157
Metropolitan DC
Segment Reporting Information [Line Items]
Total revenue									51,498
Operating Segments | Philadelphia CBD
Segment Reporting Information [Line Items]
Total revenue									263,769	256,717	226,673
Operating expenses									(100,219)	(99,449)	(88,818)
Net operating income (loss)									163,550	157,268	137,855
Operating Segments | Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Total revenue									141,084	138,279	139,785
Operating expenses									(47,418)	(49,433)	(47,845)
Net operating income (loss)									93,666	88,846	91,940
Operating Segments | Austin, Texas
Segment Reporting Information [Line Items]
Total revenue									104,157	38,665	34,301
Operating expenses									(38,285)	(16,739)	(15,456)
Net operating income (loss)									65,872	21,926	18,845
Operating Segments | Metropolitan DC
Segment Reporting Information [Line Items]
Total revenue									51,498	90,308	92,024
Operating expenses									(23,455)	(34,072)	(35,014)
Net operating income (loss)									28,043	56,236	57,010
Operating Segments | Other
Segment Reporting Information [Line Items]
Total revenue									14,558	16,757	18,347
Operating expenses									(9,328)	(11,888)	(11,749)
Net operating income (loss)									5,230	4,869	6,598
Corporate
Segment Reporting Information [Line Items]
Total revenue									5,351	3,619	9,363
Operating expenses									(7,141)	(6,518)	(7,193)
Net operating income (loss)									$ (1,790)	$ (2,899)	$ 2,170