SEGMENT INFORMATION - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Operating properties	$ 3,587,083	$ 3,472,602
Right of use asset - operating leases, net	19,826	20,313
Construction-in-progress	227,231	277,237
Land held for development	74,272	114,604
Prepaid leasehold interests in land held for development, net	35,576	27,762
Continuing Operations
Segment Reporting Information [Line Items]
Operating properties	3,608,133	3,472,602
Discontinued Operations
Segment Reporting Information [Line Items]
Operating properties	21,050	0
Philadelphia CBD
Segment Reporting Information [Line Items]
Operating properties	1,515,324	1,460,510
Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Operating properties	852,658	866,223
Austin, Texas
Segment Reporting Information [Line Items]
Operating properties	843,213	778,145
Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Operating properties	289,904	280,921
Other
Segment Reporting Information [Line Items]
Operating properties	$ 85,984	$ 86,803