Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Operating properties		$ 3,864,629	$ 3,832,348
Construction-in-progress		120,593	121,188
Land held for development	[1]	95,658	98,242
Prepaid leasehold interests in land held for development	[2]	40,177	0
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Operating properties		1,647,843	1,643,296
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Operating properties		979,720	958,796
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Operating properties		979,573	978,257
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Operating properties		170,912	163,653
Other [Member]
Segment Reporting Information [Line Items]
Operating properties		$ 86,581	$ 88,346

[1] As of June 30, 2018, the Company categorized 2.7 acres of land held for development, located in the Pennsylvania Suburbs segment, as held for sale in accordance with applicable accounting standards for long lived assets. As of December 31, 2017, the Company categorized 13.1 acres of land held for development, located in the Other segment, as held for sale in accordance with applicable accounting standards for long lived assets. As of June 30, 2018, this caption comprised leasehold interests in prepaid 99-year ground leases at 3025 and 3001-3003 JFK Boulevard, in Philadelphia, Pennsylvania. See Note 3, “Real Estate Investments,” for further information.

[2] As of June 30, 2018, this caption comprised leasehold interests in prepaid 99-year ground leases at 3025 and 3001-3003 JFK Boulevard, in Philadelphia, Pennsylvania. See Note 3, “Real Estate Investments,” for further information