Segment Information - Unconsolidated Real Estate Ventures of Company's Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		$ 278,659		$ 241,004
Equity in income (loss) of Real Estate Ventures		(403)	$ 131
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[1]	57,477		44,089
Equity in income (loss) of Real Estate Ventures	[1]	455	(300)
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		16,737		16,408
Equity in income (loss) of Real Estate Ventures		265	(6)
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		119,713		118,422
Equity in income (loss) of Real Estate Ventures		(448)	(50)
MAP Venture [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[2]	24,554		0
Equity in income (loss) of Real Estate Ventures	[2]	(556)	0
Other [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[3]	1,551		1,657
Equity in income (loss) of Real Estate Ventures	[3]	161	134
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[4]	58,627		$ 60,428
Equity in income (loss) of Real Estate Ventures	[4]	$ (280)	$ 353

[1] Investment in real estate ventures increased by $12.8 million as a result of the Company’s cash payment to evo at Cira to purchase Campus Crest’s entire 30% interest in evo at Cira and, as a result, each of the Company and HSRE owns a 50% interest in evo at Cira. 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; 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 (c) at 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 first quarter of 2016. See Note 4, "Investment in Unconsolidated Real Estate Ventures," for further information.