Segment Information - Unconsolidated Real Estate Ventures of Company's Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		$ 262,107		$ 262,107		$ 281,331
Equity in income (loss) of Real Estate Ventures		1,084	$ (1,666)	336	$ (2,069)
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		43,164		43,164		48,691
Equity in income (loss) of Real Estate Ventures		45	(475)	(21)	(20)
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		3,296		3,296		15,421
Equity in income (loss) of Real Estate Ventures		148	315	424	580
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		143,088		143,088		141,786
Equity in income (loss) of Real Estate Ventures		(75)	(332)	392	(781)
MAP Venture [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[1]	16,989		16,989		20,893
Equity in income (loss) of Real Estate Ventures	[1]	(787)	(1,042)	(1,904)	(1,598)
Other [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		1,818		1,818		1,654
Equity in income (loss) of Real Estate Ventures		107	327	164	488
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		53,752		53,752		$ 52,886
Equity in income (loss) of Real Estate Ventures		$ 1,646	$ (459)	$ 1,281	$ (738)

[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.