Segment Information - Unconsolidated Real Estate Ventures of Company's Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		$ 171,383		$ 194,621
Equity in income (loss) of real estate ventures		(825)	$ (748)
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		21,641		39,939
Equity in income (loss) of real estate ventures		(236)	(67)
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		0		3,503
Equity in income (loss) of real estate ventures		0	276
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		120,041		119,817
Equity in income (loss) of real estate ventures		(37)	467
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		14,069		15,450
Equity in income (loss) of real estate ventures		80	(1,117)
MAP Venture [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity	[1]	13,713		1,939
Equity in income (loss) of real estate ventures	[1]	(736)	58
Other [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		1,919		$ 13,973
Equity in income (loss) of real estate ventures		$ 104	$ (365)

[1] 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.