Segment Information - Unconsolidated Real Estate Ventures of Company's Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		$ 161,568		$ 169,100
Equity in income (loss) of real estate venture		(1,358)	$ (825)
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		19,325		19,897
Equity in income (loss) of real estate venture		78	(236)
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity	[1]	135,149		136,142
Equity in income (loss) of real estate venture	[1]	(175)	(37)
MAP Venture [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity	[2]	5,101		11,173
Equity in income (loss) of real estate venture	[2]	(1,367)	(736)
Other [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		1,993		1,888
Equity in income (loss) of real estate venture		106	104
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity	[3]	0		$ 0
Equity in income (loss) of real estate venture	[3]	$ 0	$ 80

[1] On December 20, 2018, the Company formed the Herndon Innovation Center Venture. See footnote (b) to the “Net operating income” table above for further information regarding this transaction.

[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. 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] On December 11, 2018, the Company acquired from DRA Advisors its 50% ownership interest in the G&I Austin Office LLC real estate venture. The DRA Austin Venture owned twelve office properties containing an aggregate 1,570,123 square feet, located in Austin, Texas. As a result of the acquisition, the Company acquired complete ownership of the Austin properties.