SEGMENT INFORMATION - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Operating properties	$ 3,472,602	$ 3,474,109	$ 4,006,459
Right of use asset - operating leases, net	20,313	20,977
Construction-in-progress	277,237	210,311
Land held for development	114,604	117,984
Prepaid leasehold interests in land held for development, net	27,762	39,185
Philadelphia CBD
Segment Reporting Information [Line Items]
Operating properties	1,460,510	1,433,927
Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Operating properties	866,223	871,530
Austin, Texas
Segment Reporting Information [Line Items]
Operating properties	778,145	728,741
Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Operating properties	280,921	352,794
Other
Segment Reporting Information [Line Items]
Operating properties	$ 86,803	$ 87,117