Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Details) - USD ($) $ in Thousands		Mar. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Operating properties		$ 3,702,204	$ 3,586,295
Assets held for sale	[1],[2]	5,576	73,591
Operating Properties		3,707,780	3,659,886
Construction-in-progress		166,372	297,462
Land held for development		153,268	150,970
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Operating properties		1,471,258	1,320,974
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Operating properties		1,006,780	1,005,446
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Operating properties		974,838	975,987
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Operating properties		146,858	146,794
Other [Member]
Segment Reporting Information [Line Items]
Operating properties		$ 102,470	$ 137,094

[1] As of March 31, 2017, the Company categorized a retail property 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.

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