SEGMENT INFORMATION - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information [Line Items]
Total revenue	$ 125,569	$ 120,418	$ 377,116	$ 361,289
Operating expenses	(47,486)	(47,052)	(145,053)	(141,153)
Net operating income	78,083	73,366	232,063	220,136
Operating Segments | Philadelphia CBD
Segment Reporting Information [Line Items]
Total revenue	56,105	51,587	165,002	154,259
Operating expenses	(20,080)	(18,823)	(60,361)	(54,493)
Net operating income	36,025	32,764	104,641	99,766
Operating Segments | Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Total revenue	32,709	31,327	95,849	93,361
Operating expenses	(11,022)	(10,154)	(31,438)	(30,625)
Net operating income	21,687	21,173	64,411	62,736
Operating Segments | Austin, Texas
Segment Reporting Information [Line Items]
Total revenue	22,509	24,813	71,345	76,501
Operating expenses	(8,597)	(9,580)	(29,328)	(29,262)
Net operating income	13,912	15,233	42,017	47,239
Operating Segments | Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Total revenue	5,360	5,224	16,041	14,711
Operating expenses	(3,030)	(3,806)	(9,574)	(11,907)
Net operating income	2,330	1,418	6,467	2,804
Operating Segments | Other
Segment Reporting Information [Line Items]
Total revenue	3,514	3,638	11,037	10,513
Operating expenses	(2,059)	(2,534)	(6,411)	(7,344)
Net operating income	1,455	1,104	4,626	3,169
Corporate
Segment Reporting Information [Line Items]
Total revenue	5,372	3,829	17,842	11,944
Operating expenses	(2,698)	(2,155)	(7,941)	(7,522)
Net operating income	$ 2,674	$ 1,674	$ 9,901	$ 4,422