Segment Information - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Total revenue		$ 132,086	$ 129,694	$ 127,181	$ 136,502	$ 153,992	$ 152,585	$ 145,648	$ 150,406	$ 525,463	$ 602,631	$ 596,982
Operating expenses	[1]									(209,448)	(238,087)	(235,965)
Net operating income										316,015	364,544	361,017
Operating Segments [Member] | Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Total revenue										200,245	209,298	201,809
Operating expenses	[1]									(78,708)	(77,352)	(75,262)
Net operating income										121,537	131,946	126,547
Operating Segments [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Total revenue										144,338	158,398	160,630
Operating expenses	[1]									(49,208)	(57,319)	(55,062)
Net operating income										95,130	101,079	105,568
Operating Segments [Member] | Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Total revenue										99,781	110,657	113,834
Operating expenses	[1]									(39,036)	(44,294)	(43,399)
Net operating income										60,745	66,363	70,435
Operating Segments [Member] | Austin, Texas [Member]
Segment Reporting Information [Line Items]
Total revenue	[2]									34,585	20,910	5,610
Operating expenses	[1],[2]									(13,222)	(8,010)	(3,223)
Net operating income	[2]									21,363	12,900	2,387
Operating Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Total revenue										39,359	98,799	113,971
Operating expenses	[1]									(23,204)	(49,604)	(57,214)
Net operating income										16,155	49,195	56,757
Corporate [Member]
Segment Reporting Information [Line Items]
Total revenue										7,155	4,569	1,128
Operating expenses	[1]									(6,070)	(1,508)	(1,805)
Net operating income										$ 1,085	$ 3,061	$ (677)

[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% of the common interest in Broadmoor Austin Associates. As such, the Company has seven wholly owned properties in its Austin, Texas business segment at December 31, 2016. In addition, net operating income for the years ended December 31, 2016 and 2015 includes management fees and related expenses for services provided by the Company to the Austin Venture. See Note 3, "Real Estate Investments," for further information regarding these transactions.