Segment Information - Unconsolidated Real Estate Ventures of Company's Reportable Segments (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		$ 281,331	$ 241,004	$ 225,004
Equity in loss of Real Estate Ventures		(11,503)	(811)	(790)
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		48,691	44,089	27,137
Equity in loss of Real Estate Ventures		(686)	(188)	46
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		15,421	16,408	17,385
Equity in loss of Real Estate Ventures		748	310	(777)
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[1]	141,786	118,422	73,127
Equity in loss of Real Estate Ventures	[1]	(6,293)	(336)	(317)
MAP Venture [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[2]	20,893	0	0
Equity in loss of Real Estate Ventures	[2]	(4,218)	0	0
Other [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[3]	1,654	1,657	1,574
Equity in loss of Real Estate Ventures	[3]	814	930	1,338
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[4]	52,886	60,428	105,781
Equity in loss of Real Estate Ventures	[4]	$ (1,868)	$ (1,527)	$ (1,080)

[1] 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. 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,” 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 (a) 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 to the Company’s investment balance primarily relates to distributions from the G&I VII Austin Office LLC real estate venture.