Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Operating properties	$ 3,392,734	$ 3,474,109
Right of use asset - operating leases, net	20,643	20,977
Construction-in-progress	286,208	210,311
Land held for development	118,181	117,984
Prepaid leasehold interests in land held for development, net	27,762	39,185
Philadelphia CBD
Segment Reporting Information [Line Items]
Operating properties	1,428,769	1,433,927
Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Operating properties	875,475	871,530
Austin, Texas
Segment Reporting Information [Line Items]
Operating properties	729,829	728,741
Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Operating properties	271,661	352,794
Other
Segment Reporting Information [Line Items]
Operating properties	$ 87,000	$ 87,117