Segment Information - Unconsolidated Real Estate Ventures of Company's Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		$ 236,313		$ 236,313		$ 281,331
Equity in income (loss) of real estate ventures		(5,723)	$ (7,254)	(5,387)	$ (9,323)
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		42,298		42,298		48,691
Equity in income (loss) of real estate ventures		134	(453)	113	(473)
Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		3,320		3,320		15,421
Equity in income (loss) of real estate ventures		24	(170)	448	410
Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity	[1]	119,550		119,550		141,786
Equity in income (loss) of real estate ventures	[1]	(5,264)	(5,287)	(4,872)	(6,068)
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		52,940		52,940		52,886
Equity in income (loss) of real estate ventures		(15)	(561)	1,266	(1,299)
MAP Venture [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity	[2]	16,283		16,283		20,893
Equity in income (loss) of real estate ventures	[2]	(706)	(1,010)	(2,610)	(2,608)
Other [Member]
Segment Reporting Information [Line Items]
Investment in real estate ventures, at equity		1,922		1,922		$ 1,654
Equity in income (loss) of real estate ventures		$ 104	$ 227	$ 268	$ 715

[1] The three- and nine-month periods ended September 30, 2017 include a $4.8 million other than temporary impairment charge related to the BDN - AI Venture that was recorded during the three months ended September 30, 2017.

[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.