SEGMENT INFORMATION - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Segment Reporting Information [Line Items]
Total revenue	$ 124,042	$ 120,102	$ 251,547	$ 240,871
Operating expenses	(49,649)	(47,427)	(97,567)	(94,101)
Net operating income	74,393	72,675	153,980	146,770
Operating Segments | Philadelphia CBD
Segment Reporting Information [Line Items]
Total revenue	55,426	51,445	108,897	102,672
Operating expenses	(20,538)	(18,259)	(40,281)	(35,670)
Net operating income	34,888	33,186	68,616	67,002
Operating Segments | Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Total revenue	31,333	30,294	63,140	62,034
Operating expenses	(10,258)	(9,777)	(20,416)	(20,471)
Net operating income	21,075	20,517	42,724	41,563
Operating Segments | Austin, Texas
Segment Reporting Information [Line Items]
Total revenue	23,921	25,513	48,836	51,688
Operating expenses	(10,438)	(9,962)	(20,731)	(19,682)
Net operating income	13,483	15,551	28,105	32,006
Operating Segments | Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Total revenue	5,486	4,812	10,681	9,487
Operating expenses	(3,116)	(3,902)	(6,544)	(8,101)
Net operating income	2,370	910	4,137	1,386
Operating Segments | Other
Segment Reporting Information [Line Items]
Total revenue	3,919	3,662	7,523	6,875
Operating expenses	(2,342)	(2,528)	(4,352)	(4,810)
Net operating income	1,577	1,134	3,171	2,065
Corporate
Segment Reporting Information [Line Items]
Total revenue	3,957	4,376	12,470	8,115
Operating expenses	(2,957)	(2,999)	(5,243)	(5,367)
Net operating income	$ 1,000	$ 1,377	$ 7,227	$ 2,748