Segment Information - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Segment Reporting Information [Line Items]
Total revenue		$ 136,502	$ 150,406
Operating expenses	[1]	(54,775)	(60,698)
Net operating income		81,727	89,708
Operating Segments [Member] | Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Total revenue		49,670	52,973
Operating expenses	[1]	(19,256)	(18,390)
Net operating income		30,414	34,583
Operating Segments [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Total revenue		37,106	39,890
Operating expenses	[1]	(13,155)	(14,585)
Net operating income		23,951	25,305
Operating Segments [Member] | Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Total revenue		27,339	27,406
Operating expenses	[1]	(10,899)	(11,860)
Net operating income		16,440	15,546
Operating Segments [Member] | Austin, Texas [Member]
Segment Reporting Information [Line Items]
Total revenue	[2]	8,547	934
Operating expenses	[1],[2]	(3,255)	(870)
Net operating income	[2]	5,292	64
Operating Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Total revenue	[3]	12,740	27,781
Operating expenses	[1],[3]	(7,602)	(14,578)
Net operating income	[3]	5,138	13,203
Corporate [Member]
Segment Reporting Information [Line Items]
Total revenue		1,100	1,422
Operating expenses	[1]	(608)	(415)
Net operating income		$ 492	$ 1,007

[1] Includes property operating expense, real estate taxes and third party management expense.

[2] On June 22, 2015 the Company acquired the remaining 50.0% interest in Broadmoor Austin Associates. As such, the Company has seven wholly owned properties in its Austin, Texas business segment at March 31, 2016. In addition, net operating income for the three months ended March 31, 2016 and 2015 includes management fees and related expenses for services provided by the Company to the Austin Venture.

[3] See footnote (c) at the “Real estate investments, at cost” table above for further information regarding this segment.