SEGMENT INFORMATION - Real Estate Investments, at Cost of Company's Reportable Segments (Details) $ in Thousands		12 Months Ended
	Jun. 29, 2018 USD ($)	Dec. 31, 2019 USD ($) a	Dec. 31, 2018 USD ($) a	Dec. 31, 2017 USD ($) a
Segment Reporting Information [Line Items]
Operating properties		$ 4,006,459	$ 3,951,719	$ 3,830,824
Right of use asset - operating leases, net		21,656	0	0
Construction-in-progress		180,718	150,263	121,188
Land held for development		96,124	86,401	98,242
Prepaid leasehold interests in land held for development, net		$ 39,592	$ 39,999	$ 0
Held for Sale Properties Included in Continuing Operations | Land Held For Development
Segment Reporting Information [Line Items]
Acres | a		35.2	37.9	13.1
3025 JFK Boulevard
Segment Reporting Information [Line Items]
Prepaid leasehold interests in land held for development, net	$ 15,000
Lease agreement term	99 years	99 years
Philadelphia CBD
Segment Reporting Information [Line Items]
Operating properties		$ 1,726,299	$ 1,670,388	$ 1,643,296
Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Operating properties		1,003,890	1,002,937	957,272
Austin, Texas
Segment Reporting Information [Line Items]
Operating properties		721,255	667,698	163,653
Metropolitan DC
Segment Reporting Information [Line Items]
Operating properties		468,035	524,190	978,257
Other
Segment Reporting Information [Line Items]
Operating properties		$ 86,980	$ 86,506	$ 88,346