Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands		Jun. 30, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Operating properties		$ 3,769,678	$ 3,586,295
Assets held for sale	[1]	0	73,591
Operating properties		3,769,678	3,659,886
Construction-in-progress		119,690	297,462
Land held for development	[2]	125,157	150,970
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Operating properties		1,536,488	1,320,974
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Operating properties		1,024,252	1,005,446
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Operating properties		974,152	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		$ 87,927	$ 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 “Held for Sale” below in this Note 3.

[2] As of June 30, 2017, the Company categorized 50 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. See Note 3, “Real Estate Investments,” for further information.