Segment Information - Unconsolidated Real Estate Ventures of Company's Reportable Segments (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		$ 169,100	$ 194,621	$ 281,331
Equity in loss of Real Estate Ventures		(15,231)	(8,306)	(11,503)
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		19,897	39,939	48,691
Equity in loss of Real Estate Ventures		(105)	255	(686)
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		0	3,503	15,421
Equity in loss of Real Estate Ventures		0	631	748
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity	[1]	136,142	119,817	141,786
Equity in loss of Real Estate Ventures	[1]	(15,065)	(5,044)	(6,293)
MAP Venture [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity	[2]	11,173	15,450	20,893
Equity in loss of Real Estate Ventures	[2]	(2,155)	(3,443)	(4,218)
Other [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		1,888	1,939	1,654
Equity in loss of Real Estate Ventures		407	285	814
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity	[3]	0	13,973	52,886
Equity in loss of Real Estate Ventures	[3]	$ 1,687	$ (990)	$ (1,868)

[1] On December 20, 2018, the Company formed a joint venture with Rockpoint. See footnote (a) to the “Real estate investments, at cost” 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] See footnote (b) to the “Real estate investments, at cost” table above for further information regarding this transaction.