Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Operating properties		$ 3,429,048	$ 3,832,348
Assets held for sale - real estate investments	[1]	396,072	0
Operating properties		3,825,120	3,832,348
Construction-in-progress		157,075	121,188
Land held for development	[2]	77,578	98,242
Prepaid leasehold interests in land held for development, net	[3]	40,100	0
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Operating properties		1,652,819	1,643,296
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Operating properties		979,068	958,796
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Operating properties	[1]	539,105	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		$ 87,144	$ 88,346

[1] As of September 30, 2018, eight office properties in the Metropolitan Washington, D.C. segment were classified as held for sale. See Note 3, “Real Estate Investments,” for further information.

[2] As of September 30, 2018, the Company categorized 37.9 acres of land held for development, comprised of 2.7 acres and 35.2 acres, located in the Pennsylvania Suburbs segment and Other segment, respectively, 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.

[3] As of September 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.