SEGMENT INFORMATION - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Operating properties	$ 3,632,495	$ 3,617,240
Right of use asset - operating leases, net	19,505	19,664
Construction-in-progress	236,040	218,869
Land held for development	67,923	76,499
Prepaid leasehold interests in land held for development, net	27,762	35,576
Total Core Segments
Segment Reporting Information [Line Items]
Operating properties	3,259,535	3,248,182
Philadelphia CBD
Segment Reporting Information [Line Items]
Operating properties	1,519,693	1,517,801
Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Operating properties	886,165	878,546
Austin, Texas
Segment Reporting Information [Line Items]
Operating properties	853,677	851,835
Other
Segment Reporting Information [Line Items]
Operating properties	$ 372,960	$ 369,058