Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Operating properties		$ 3,864,706	$ 3,832,348
Construction-in-progress		129,413	121,188
Land held for development	[1]	99,436	98,242
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Operating properties		1,653,373	1,643,296
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Operating properties		978,838	958,796
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Operating properties		979,041	978,257
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Operating properties		163,788	163,653
Other [Member]
Segment Reporting Information [Line Items]
Operating properties		$ 89,666	$ 88,346

[1] 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.