SEGMENT INFORMATION - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Operating properties	$ 3,542,232	$ 3,617,240	$ 3,472,602
Right of use asset - operating leases, net	19,031	19,664
Construction-in-progress	135,529	218,869
Land held for development	82,510	76,499
Prepaid leasehold interests in land held for development, net	27,762	35,576
Total Core Segments | Continuing Operations
Segment Reporting Information [Line Items]
Operating properties	3,237,096	3,248,182
Philadelphia CBD | Continuing Operations
Segment Reporting Information [Line Items]
Operating properties	1,534,893	1,517,801
Pennsylvania Suburbs | Continuing Operations
Segment Reporting Information [Line Items]
Operating properties	900,230	878,546
Austin, Texas | Continuing Operations
Segment Reporting Information [Line Items]
Operating properties	801,973	851,835
Other
Segment Reporting Information [Line Items]
Operating properties	$ 305,136	$ 369,058