Segment Information - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Reporting Information [Line Items]
Total revenue		$ 127,181	$ 145,648	$ 263,683	$ 296,054
Operating expenses	[1]	(50,221)	(56,349)	(104,996)	(117,047)
Net operating income		76,960	89,299	158,687	179,007
Operating Segments [Member] | Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Total revenue		48,082	52,420	97,752	105,393
Operating expenses	[1]	(19,775)	(19,448)	(39,031)	(37,838)
Net operating income		28,307	32,972	58,721	67,555
Operating Segments [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Total revenue		35,102	39,010	72,208	78,900
Operating expenses	[1]	(12,580)	(12,856)	(25,735)	(27,441)
Net operating income		22,522	26,154	46,473	51,459
Operating Segments [Member] | Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Total revenue		25,291	26,953	52,630	54,359
Operating expenses	[1]	(8,768)	(10,544)	(19,667)	(22,404)
Net operating income		16,523	16,409	32,963	31,955
Operating Segments [Member] | Austin, Texas [Member]
Segment Reporting Information [Line Items]
Total revenue	[2]	7,850	2,533	16,397	3,467
Operating expenses	[1],[2]	(2,911)	(1,498)	(6,166)	(2,369)
Net operating income	[2]	4,939	1,035	10,231	1,098
Operating Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Total revenue	[3]	9,062	24,074	21,802	51,853
Operating expenses	[1],[3]	(5,074)	(11,407)	(12,676)	(25,985)
Net operating income	[3]	3,988	12,667	9,126	25,868
Corporate [Member]
Segment Reporting Information [Line Items]
Total revenue		1,794	658	2,894	2,082
Operating expenses	[1]	(1,113)	(596)	(1,721)	(1,010)
Net operating income		$ 681	$ 62	$ 1,173	$ 1,072

[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 six months ended June 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.