Segment Information Summary of Unconsolidated Real Estate Ventures (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		$ 211,771		$ 211,771		$ 225,004
Equity in loss of Real Estate Ventures		(1,093)	$ (486)	(1,835)	$ (733)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		211,771		211,771		225,004
Equity in loss of Real Estate Ventures		(1,093)	(486)	(1,835)	(733)
Operating Segments [Member] | Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[1]	43,236		43,236		27,137
Equity in loss of Real Estate Ventures	[1]	(186)	32	(636)	13
Operating Segments [Member] | Pennsylvania [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		16,796		16,796		17,385
Equity in loss of Real Estate Ventures		(120)	(578)	(142)	(937)
Operating Segments [Member] | Metropolitan Dc [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[2]	113,222		113,222		73,127
Equity in loss of Real Estate Ventures	[2]	(343)	(142)	(572)	(117)
Operating Segments [Member] | New Jersey/ Delaware [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		0		0		0
Equity in loss of Real Estate Ventures		67	82	231	204
Operating Segments [Member] | Richmond, Virginia [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		1,512		1,512		1,574
Equity in loss of Real Estate Ventures		106	184	438	253
Operating Segments [Member] | Austin, Texas [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[3],[4]	37,005		37,005		$ 105,781
Equity in loss of Real Estate Ventures	[3],[4]	$ (617)	$ (64)	$ (1,154)	$ (149)

[1] evo at Cira was placed into service during the third quarter ended September 30, 2014.

[2] Investment in real estate ventures as of September 30, 2015 includes the JBG Ventures, which were formed on May 29, 2015

[3] Investment in real estate ventures does not include the $1.1 million and $1.2 million negative investment balance in one real estate venture as of September 30, 2015 and December 31, 2014, respectively, which is included in other liabilities.

[4] On June 22, 2015 the Company acquired the remaining 50.0% of the common interest in Broadmoor Austin Associates. As such, equity method investment at June 30, 2015 related to the Austin Venture only. See Note 3, "Real Estate Investments," for further information regarding the purchase of Broadmoor Austin Associates.