Segment Information - Unconsolidated Real Estate Ventures of Company's Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		$ 282,162		$ 282,162		$ 241,004
Equity in loss of Real Estate Ventures		(7,254)	$ (1,093)	(9,323)	$ (1,835)
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[1]	48,904		48,904		44,089
Equity in loss of Real Estate Ventures	[1]	(453)	(186)	(473)	(636)
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		15,025		15,025		16,408
Equity in loss of Real Estate Ventures		(170)	(120)	410	(142)
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[2]	140,219		140,219		118,422
Equity in loss of Real Estate Ventures	[2]	(5,287)	(343)	(6,068)	(572)
MAP Venture [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[3]	22,503		22,503		0
Equity in loss of Real Estate Ventures	[3]	(1,010)	0	(2,608)	0
Other [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[4]	1,554		1,554		1,657
Equity in loss of Real Estate Ventures	[4]	227	173	715	669
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[5]	53,957		53,957		$ 60,428
Equity in loss of Real Estate Ventures	[5]	$ (561)	$ (617)	$ (1,299)	$ (1,154)

[1] Net increase of investment of $4.8 million primary relates to the evo at Cira real estate venture. See Note 4, “Investment in Unconsolidated Real Estate Ventures,” for further information.

[2] On August 31, 2016, the Company terminated its lease for the regional management and leasing office at 3141 Fairview Park Drive, located in Falls Church, Virginia. Accordingly, the Company no longer has any continuing involvement with 3141 Fairview Park Drive and recorded the partial sale under the full accrual method of accounting. As a result, the Company deconsolidated net assets of $45.6 million, a mortgage loan of $20.6 million and a financing liability of $12.4 million related to the property from its consolidated balance sheet and recorded a $12.6 million equity method investment to the Company’s Brandywine - AI Venture LLC, in which the Company owns a 50% interest. See Note 4, “Investment in Unconsolidated Real Estate Ventures,” for details.

[3] 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.

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

[5] 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 to our investment balance primarily relates to distributions from the G&I VII Austin Office LLC real estate venture.