Segment Information Summary of Net Operating Income (Details) - USD ($) $ in Thousands		3 Months Ended				6 Months Ended
		Jun. 30, 2015		Jun. 30, 2014		Jun. 30, 2015		Jun. 30, 2014
Segment Reporting Information [Line Items]
Total revenue		$ 145,648		$ 150,500		$ 296,054		$ 302,614
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenue		145,648		150,500		296,054		302,614
Operating expenses	[1]	(56,349)		(57,707)		(117,047)		(119,681)
Net operating income (loss)		89,299		92,793		179,007		182,933
Operating Segments [Member] | Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Total revenue		52,420		51,420		105,393		100,834
Operating expenses	[1]	(19,448)		(18,789)		(37,838)		(37,231)
Net operating income (loss)		32,972		32,631		67,555		63,603
Operating Segments [Member] | Pennsylvania [Member]
Segment Reporting Information [Line Items]
Total revenue		39,010		40,743		78,900		81,317
Operating expenses	[1]	(12,856)		(13,742)		(27,441)		(28,431)
Net operating income (loss)		26,154		27,001		51,459		52,886
Operating Segments [Member] | Metropolitan Dc [Member]
Segment Reporting Information [Line Items]
Total revenue		26,953		28,949		54,359		59,639
Operating expenses	[1]	(10,544)		(10,505)		(22,404)		(22,462)
Net operating income (loss)		16,409		18,444		31,955		37,177
Operating Segments [Member] | New Jersey/ Delaware [Member]
Segment Reporting Information [Line Items]
Total revenue		11,727		14,841		25,226		30,466
Operating expenses	[1]	(6,203)		(7,531)		(14,071)		(16,223)
Net operating income (loss)		5,524		7,310		11,155		14,243
Operating Segments [Member] | Richmond, Virginia [Member]
Segment Reporting Information [Line Items]
Total revenue		8,956		8,322		18,251		17,189
Operating expenses	[1]	(3,632)		(3,566)		(7,904)		(7,906)
Net operating income (loss)		5,324		4,756		10,347		9,283
Operating Segments [Member] | California
Segment Reporting Information [Line Items]
Total revenue		3,391		4,895		8,376		9,497
Operating expenses	[1]	(1,572)		(2,515)		(4,010)		(4,923)
Net operating income (loss)		1,819		2,380		4,366		4,574
Operating Segments [Member] | Austin, Texas [Member]
Segment Reporting Information [Line Items]
Total revenue		2,533	[2]	976	[2]	3,467	[1]	3,057	[1]
Operating expenses	[1]	(1,498)	[2]	(617)	[2]	(2,369)		(1,793)
Net operating income (loss)		1,035	[2]	359	[2]	1,098	[1]	1,264	[1]
Operating Segments [Member] | Corporate Segment [Member]
Segment Reporting Information [Line Items]
Total revenue		658		354		2,082		615
Operating expenses	[1]	(596)		(442)		(1,010)		(712)
Net operating income (loss)		$ 62		$ (88)		$ 1,072		$ (97)

[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. See Note 3, "Real Estate Investments," for further information regarding this transaction. On April 3, 2014, the Company contributed Four Points Centre to an unconsolidated real estate venture. See Note 3, "Real Estate Investments," in notes to the audited financial statements included in the Company's Annual Report on Form 10-K for the fiscal year ended December 31, 2014.