Segment Information - Unconsolidated Real Estate Ventures (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		$ 241,004	$ 225,004	$ 180,512
Equity in income (loss) of Real Estate Ventures		(811)	(790)	3,664
Operating Segments [Member] | Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[1]	44,089	27,137	19,975
Equity in income (loss) of Real Estate Ventures	[1]	(188)	46	1,547
Operating Segments [Member] | Pennsylvania [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		16,408	17,385	17,272
Equity in income (loss) of Real Estate Ventures		310	(777)	925
Operating Segments [Member] | Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[2]	118,422	73,127	59,905
Equity in income (loss) of Real Estate Ventures	[2]	(336)	(317)	130
Operating Segments [Member] | New Jersey/ Delaware [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		0	0	0
Equity in income (loss) of Real Estate Ventures		348	989	1,245
Operating Segments [Member] | Richmond, Virginia [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity		1,657	1,574	1,400
Equity in income (loss) of Real Estate Ventures		582	349	381
Operating Segments [Member] | Austin, Texas [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	[3],[4]	60,428	105,781	81,960
Equity in income (loss) of Real Estate Ventures	[3],[4]	$ (1,527)	$ (1,080)	$ (564)

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

[2] Investment in real estate ventures as of December 31, 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 December 31, 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, the equity method investment at December 31, 2015 related to the Austin Venture only. See Note 3, "Real Estate Investments," for further information regarding the purchase of Broadmoor Austin Associates.