Segment Information - Unconsolidated Real Estate Ventures of Company's Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		$ 167,782		$ 167,782		$ 194,621
Equity in income (loss) of real estate venture		1	$ (5,723)	(1,182)	$ (5,387)
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		20,544		20,544		39,939
Equity in income (loss) of real estate venture		(36)	134	(183)	113
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		0		0		3,503
Equity in income (loss) of real estate venture		0	24	0	448
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		119,032		119,032		119,817
Equity in income (loss) of real estate venture		(31)	(5,264)	(431)	(4,872)
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		14,411		14,411		13,973
Equity in income (loss) of real estate venture		423	(15)	1,156	1,266
MAP Venture [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity	[1]	11,939		11,939		15,450
Equity in income (loss) of real estate venture	[1]	(444)	(706)	(2,011)	(2,610)
Other [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		1,856		1,856		$ 1,939
Equity in income (loss) of real estate venture		$ 89	$ 104	$ 287	$ 268

[1] The MAP Venture represents a joint venture, formed on February 4, 2016 between the Company and MAP Ground Lease Holdings LLC, an affiliate of Och-Ziff Capital Management Group, LLC. 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.