Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands		Sep. 30, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Operating properties		$ 3,769,067	$ 3,586,295
Assets held for sale - real estate investments	[1],[2]	47,291	73,591
Operating properties		3,816,358	3,659,886
Construction-in-progress		127,141	297,462
Land held for development		120,696	150,970
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Operating properties		1,591,828	1,320,974
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Operating properties	[3]	966,128	1,005,446
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Operating properties		976,286	975,987
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Operating properties		146,859	146,794
Other [Member]
Segment Reporting Information [Line Items]
Operating properties		$ 87,966	$ 137,094

[1] Real estate investments related to assets held for sale above represents gross real estate assets and does not include accumulated depreciation, land held for development or other assets on the balance sheet of the property held for sale. See Note 3, “Real Estate Investments,” for further information.

[2] Real estate investments related to assets held for sale above represents gross real estate assets and does not include accumulated depreciation, land held for development or other assets on the balance sheet of the property held for sale. See “Held for Sale” below in this Note 3.

[3] As of September 30, 2017, the Company categorized six office properties in its Pennsylvania Suburbs segment as held for sale in accordance with applicable accounting standards for long lived assets. See Note 3, “Real Estate Investments,” for further information.