Segment Information - Unconsolidated Real Estate Ventures of Company's Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		$ 272,148		$ 272,148		$ 241,004
Equity in loss of Real Estate Ventures		(1,666)	$ (873)	(2,069)	$ (742)
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[1]	50,542		50,542		44,089
Equity in loss of Real Estate Ventures	[1]	(475)	(150)	(20)	(450)
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		20,199		20,199		16,408
Equity in loss of Real Estate Ventures		315	(17)	580	(22)
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		120,656		120,656		118,422
Equity in loss of Real Estate Ventures		(332)	(179)	(781)	(229)
MAP Venture [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[2]	23,513		23,513		0
Equity in loss of Real Estate Ventures	[2]	(1,042)	0	(1,598)	0
Other [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[3]	1,570		1,570		1,657
Equity in loss of Real Estate Ventures	[3]	327	362	488	495
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[4]	55,668		55,668		$ 60,428
Equity in loss of Real Estate Ventures	[4]	$ (459)	$ (889)	$ (738)	$ (536)

[1] Net increase of investment of $6.5 million primary relates to the evo at Cira real estate venture. See Note 4, “Investment in Unconsolidated Real Estate Ventures,” for further information.

[2] 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. See Note 4 “Investment in Unconsolidated Real Estate Ventures,” to our consolidated financial statements for further information. 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.

[3] See footnote (e) to the “Real estate investments, at cost” table above for further information regarding this segment.

[4] Investment in real estate ventures does not include the $1.1 million negative investment balance in one real estate venture as of December 31, 2015, which is included in other liabilities. The Company disposed of its interest in this venture during the three-month period ended March 31, 2016. See Note 4, "Investment in Unconsolidated Real Estate Ventures," for further information. The decrease primarily relates to distributions from the DRA-Austin real estate venture.