Segment Information - Unconsolidated Real Estate Ventures of Company's Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		$ 170,361		$ 170,361		$ 194,621
Equity in income (loss) of real estate ventures		(358)	$ 1,084	(1,183)	$ 336
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		21,230		21,230		39,939
Equity in income (loss) of real estate ventures		89	45	(147)	(21)
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 ventures		0	148	0	424
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		119,678		119,678		119,817
Equity in income (loss) of real estate ventures		(363)	(75)	(400)	392
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		14,684		14,684		13,973
Equity in income (loss) of real estate ventures		653	1,646	734	1,281
MAP Venture [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity	[1]	12,883		12,883		15,450
Equity in income (loss) of real estate ventures	[1]	(830)	(787)	(1,567)	(1,904)
Other [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		1,886		1,886		$ 1,939
Equity in income (loss) of real estate ventures		$ 93	$ 107	$ 197	$ 164

[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.