SEGMENT INFORMATION - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands	Jun. 30, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Operating properties	$ 3,545,102	$ 3,472,602
Right of use asset - operating leases, net	19,988	20,313
Construction-in-progress	366,823	277,237
Land held for development	93,887	114,604
Prepaid leasehold interests in land held for development, net	35,576	27,762
Philadelphia CBD
Segment Reporting Information [Line Items]
Operating properties	1,506,350	1,460,510
Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Operating properties	870,510	866,223
Austin, Texas
Segment Reporting Information [Line Items]
Operating properties	794,734	778,145
Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Operating properties	287,215	280,921
Other
Segment Reporting Information [Line Items]
Operating properties	$ 86,293	$ 86,803