Segment Information Summary of Net Operating Income (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Information [Line Items]
Total revenue		$ 152,585	$ 146,558	$ 448,639	$ 449,172
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total revenue		152,585	146,558	448,639	449,172
Operating expenses	[1]	(58,618)	(57,231)	(175,665)	(176,912)
Net operating income		93,967	89,327	272,974	272,260
Operating Segments [Member] | Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Total revenue		52,203	49,469	157,595	150,303
Operating expenses	[1]	(18,750)	(18,168)	(56,587)	(55,399)
Net operating income		33,453	31,301	101,008	94,904
Operating Segments [Member] | Pennsylvania [Member]
Segment Reporting Information [Line Items]
Total revenue		39,507	39,996	118,407	121,313
Operating expenses	[1]	(14,004)	(13,409)	(41,286)	(41,542)
Net operating income		25,503	26,587	77,121	79,771
Operating Segments [Member] | Metropolitan Dc [Member]
Segment Reporting Information [Line Items]
Total revenue		27,587	27,764	81,947	87,403
Operating expenses	[1]	(10,792)	(10,672)	(33,197)	(33,134)
Net operating income		16,795	17,092	48,750	54,269
Operating Segments [Member] | New Jersey/ Delaware [Member]
Segment Reporting Information [Line Items]
Total revenue		11,922	14,975	37,149	45,440
Operating expenses	[1]	(6,594)	(7,506)	(20,857)	(23,728)
Net operating income		5,328	7,469	16,292	21,712
Operating Segments [Member] | Richmond, Virginia [Member]
Segment Reporting Information [Line Items]
Total revenue		9,130	8,354	27,381	25,543
Operating expenses	[1]	(3,700)	(3,724)	(11,603)	(11,629)
Net operating income		5,430	4,630	15,778	13,914
Operating Segments [Member] | Austin, Texas [Member]
Segment Reporting Information [Line Items]
Total revenue	[2]	8,533	843	11,999	3,900
Operating expenses	[1],[2]	(2,625)	(569)	(4,993)	(2,362)
Net operating income	[2]	5,908	274	7,006	1,538
Operating Segments [Member] | California
Segment Reporting Information [Line Items]
Total revenue		3,223	4,902	11,599	14,399
Operating expenses	[1]	(1,767)	(2,711)	(5,778)	(7,634)
Net operating income		1,456	2,191	5,821	6,765
Operating Segments [Member] | Corporate Segment [Member]
Segment Reporting Information [Line Items]
Total revenue		480	255	2,562	871
Operating expenses	[1]	(386)	(472)	(1,364)	(1,484)
Net operating income		$ 94	$ (217)	$ 1,198	$ (613)

[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.