SEGMENT INFORMATION - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Operating properties	$ 3,474,109	$ 4,006,459	$ 3,951,719
Right of use asset - operating leases, net	20,977	21,656
Construction-in-progress	210,311	180,718
Land held for development	117,984	96,124
Prepaid leasehold interests in land held for development, net	39,185	39,592
Philadelphia CBD
Segment Reporting Information [Line Items]
Operating properties	1,433,927	1,726,299
Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Operating properties	871,530	1,003,890
Austin, Texas
Segment Reporting Information [Line Items]
Operating properties	728,741	721,255
Metropolitan DC
Segment Reporting Information [Line Items]
Operating properties	352,794	468,035
Other
Segment Reporting Information [Line Items]
Operating properties	$ 87,117	$ 86,980