Segment Information - Unconsolidated Real Estate Ventures of Company's Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		$ 264,941		$ 281,331
Equity in loss of Real Estate Ventures		(748)	$ (403)
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		44,618		48,691
Equity in loss of Real Estate Ventures		(67)	455
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		3,147		15,421
Equity in loss of Real Estate Ventures		276	265
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		143,591		141,786
Equity in loss of Real Estate Ventures		467	(448)
MAP Venture [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[1]	17,776		20,893
Equity in loss of Real Estate Ventures	[1]	(1,117)	(556)
Other [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		1,713		1,654
Equity in loss of Real Estate Ventures		58	161
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		54,096		$ 52,886
Equity in loss of Real Estate Ventures		$ (365)	$ (280)

[1] The MAP Venture represents a joint venture formed between the Company and MAP Ground Lease Holdings LLC, an affiliate of Och-Ziff Capital Management Group, LLC, on February 4, 2016. The MAP Venture’s business operations, including properties in Richmond, Virginia; Metropolitan Washington, D.C.; New Jersey/Delaware and Pennsylvania Suburbs, are centrally managed with the results reported to management of the Company on a consolidated basis. As a result, the investment in the MAP Venture is separately presented. All other unconsolidated real estate ventures are managed consistently with the Company’s regional segments.