SEGMENT INFORMATION - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Segment Reporting Information [Line Items]
Operating properties	$ 3,517,995	$ 3,472,602
Right of use asset - operating leases, net	20,150	20,313
Construction-in-progress	283,323	277,237
Land held for development	94,411	114,604
Prepaid leasehold interests in land held for development, net	27,762	27,762
Philadelphia CBD
Segment Reporting Information [Line Items]
Operating properties	1,494,819	1,460,510
Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Operating properties	859,470	866,223
Austin, Texas
Segment Reporting Information [Line Items]
Operating properties	791,600	778,145
Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Operating properties	285,386	280,921
Other
Segment Reporting Information [Line Items]
Operating properties	$ 86,720	$ 86,803