Segment Information - Net Operating Income (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Total revenue		$ 153,992	$ 152,585	$ 145,648	$ 150,406	$ 147,810	$ 146,558	$ 150,500	$ 152,114	$ 602,631	$ 596,982	$ 562,210
Operating expenses	[1]									(238,087)	(235,965)	(221,769)
Net operating income										364,544	361,017	340,441
Operating Segments [Member] | Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Total revenue										209,298	201,809	146,081
Operating expenses	[1]									(77,352)	(75,262)	(55,702)
Net operating income										131,946	126,547	90,379
Operating Segments [Member] | Pennsylvania [Member]
Segment Reporting Information [Line Items]
Total revenue										158,398	160,630	153,426
Operating expenses	[1]									(57,319)	(55,062)	(54,506)
Net operating income										101,079	105,568	98,920
Operating Segments [Member] | Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Total revenue										110,657	113,834	116,048
Operating expenses	[1]									(44,294)	(43,399)	(42,641)
Net operating income										66,363	70,435	73,407
Operating Segments [Member] | New Jersey/ Delaware [Member]
Segment Reporting Information [Line Items]
Total revenue										47,726	60,403	60,262
Operating expenses	[1]									(26,854)	(31,650)	(29,981)
Net operating income										20,872	28,753	30,281
Operating Segments [Member] | Richmond, Virginia [Member]
Segment Reporting Information [Line Items]
Total revenue										36,486	34,180	35,058
Operating expenses	[1]									(15,073)	(15,399)	(14,916)
Net operating income										21,413	18,781	20,142
Operating Segments [Member] | California [Member]
Segment Reporting Information [Line Items]
Total revenue										14,587	19,388	18,369
Operating expenses	[1]									(7,677)	(10,165)	(9,411)
Net operating income										6,910	9,223	8,958
Operating Segments [Member] | Austin, Texas [Member]
Segment Reporting Information [Line Items]
Total revenue	[2]									20,910	5,610	31,451
Operating expenses	[1],[2]									(8,010)	(3,223)	(13,298)
Net operating income	[2]									12,900	2,387	18,153
Corporate [Member]
Segment Reporting Information [Line Items]
Total revenue										4,569	1,128	1,515
Operating expenses	[1]									(1,508)	(1,805)	(1,314)
Net operating income										$ 3,061	$ (677)	$ 201

[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, 2015. See Note 3, "Real Estate Investments," for further information regarding these transactions.