Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Operating properties	$ 3,715,765	$ 4,006,459
Operating Properties	3,715,765	4,006,459
Right of use asset - operating leases, net	21,145	21,656
Construction-in-progress	186,853	180,718
Land held for development	115,474	96,124
Prepaid leasehold interests in land held for development, net	39,287	39,592
Philadelphia CBD
Segment Reporting Information [Line Items]
Operating properties	1,414,041	1,726,299
Pennsylvania Suburbs
Segment Reporting Information [Line Items]
Operating properties	1,000,214	1,003,890
Austin, Texas
Segment Reporting Information [Line Items]
Operating properties	726,295	721,255
Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Operating properties	488,139	468,035
Other
Segment Reporting Information [Line Items]
Operating properties	$ 87,076	$ 86,980