SEGMENT INFORMATION - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Operating properties	$ 3,617,240	$ 3,472,602	$ 3,474,109
Right of use asset - operating leases, net	19,664	20,313
Construction-in-progress	218,869	277,237
Land held for development	76,499	114,604
Prepaid leasehold interests in land held for development, net	35,576	27,762
Philadelphia CBD
Segment Reporting Information [Line Items]
Operating properties	1,517,801	1,460,510
Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Operating properties	878,546	866,223
Austin, Texas
Segment Reporting Information [Line Items]
Operating properties	851,835	778,145
Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Operating properties	282,458	280,921
Other
Segment Reporting Information [Line Items]
Operating properties	$ 86,600	$ 86,803