SEGMENT INFORMATION - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Operating properties	$ 3,694,633	$ 3,617,240
Right of use asset - operating leases, net	19,188	19,664
Construction-in-progress	143,929	218,869
Land held for development	76,058	76,499
Prepaid leasehold interests in land held for development, net	27,762	35,576
Continuing Operations
Segment Reporting Information [Line Items]
Operating properties	3,696,066	3,617,240
Total Core Segments | Continuing Operations
Segment Reporting Information [Line Items]
Operating properties	3,235,523	3,248,182
Philadelphia CBD | Continuing Operations
Segment Reporting Information [Line Items]
Operating properties	1,521,723	1,517,801
Pennsylvania Suburbs | Continuing Operations
Segment Reporting Information [Line Items]
Operating properties	915,659	878,546
Austin, Texas | Continuing Operations
Segment Reporting Information [Line Items]
Operating properties	798,141	851,835
Other | Continuing Operations
Segment Reporting Information [Line Items]
Operating properties	$ 460,543	$ 369,058