Segment Information - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information [Line Items]
Total revenue		$ 129,694	$ 152,585	$ 393,377	$ 448,639
Operating expenses	[1]	(51,317)	(58,618)	(156,313)	(175,665)
Net operating income		78,377	93,967	237,064	272,974
Operating Segments [Member] | Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Total revenue		50,744	52,203	148,496	157,595
Operating expenses	[1]	(19,071)	(18,750)	(58,102)	(56,587)
Net operating income		31,673	33,453	90,394	101,008
Operating Segments [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Total revenue		35,763	39,507	107,971	118,407
Operating expenses	[1]	(11,247)	(14,004)	(36,982)	(41,286)
Net operating income		24,516	25,503	70,989	77,121
Operating Segments [Member] | Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Total revenue		24,251	27,587	76,881	81,947
Operating expenses	[1]	(9,900)	(10,792)	(29,567)	(33,197)
Net operating income		14,351	16,795	47,314	48,750
Operating Segments [Member] | Austin, Texas [Member]
Segment Reporting Information [Line Items]
Total revenue	[2]	8,726	8,533	25,123	11,999
Operating expenses	[1],[2]	(3,523)	(2,625)	(9,689)	(4,993)
Net operating income	[2]	5,203	5,908	15,434	7,006
Operating Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Total revenue	[3]	8,526	24,275	30,328	76,129
Operating expenses	[1],[3]	(4,837)	(12,061)	(17,513)	(38,238)
Net operating income	[3]	3,689	12,214	12,815	37,891
Corporate [Member]
Segment Reporting Information [Line Items]
Total revenue	[4]	1,684	480	4,578	2,562
Operating expenses	[1],[4]	(2,739)	(386)	(4,460)	(1,364)
Net operating income	[4]	$ (1,055)	$ 94	$ 118	$ 1,198

[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 June 30, 2016. In addition, net operating income for the three and nine months ended September 30, 2016 and 2015 includes management fees and related expenses for services provided by the Company to the Austin Venture.

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

[4] Increase in revenue and operating expenses primarily relates to the third party management operations of the Subaru Headquarters Development and third party management of the MAP Venture.