Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Operating properties	$ 4,004,118	$ 4,006,459
Right of use asset - operating leases, net	21,485	21,656
Construction-in-progress	193,160	180,718
Land held for development	108,213	96,124
Prepaid leasehold interests in land held for development, net	39,490	39,592
Philadelphia CBD
Segment Reporting Information [Line Items]
Operating properties	1,733,987	1,726,299
Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Operating properties	991,788	1,003,890
Austin, Texas
Segment Reporting Information [Line Items]
Operating properties	722,873	721,255
Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Operating properties	468,416	468,035
Other
Segment Reporting Information [Line Items]
Operating properties	$ 87,054	$ 86,980