Segment Information Summary of Net Operating Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014
Segment Reporting Information [Line Items]
Total revenue	$ 150,406		$ 152,114
Operating expenses	(60,698)		(61,974)	[1]
Net operating income	89,708		90,140
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Total revenue	52,973		49,414
Operating expenses	(18,390)		(18,443)	[1]
Net operating income	34,583		30,971
Pennsylvania [Member]
Segment Reporting Information [Line Items]
Total revenue	39,890		40,574
Operating expenses	(14,585)		(14,687)	[1]
Net operating income	25,305		25,887
Metropolitan Dc [Member]
Segment Reporting Information [Line Items]
Total revenue	27,406		30,690
Operating expenses	(11,860)		(11,957)	[1]
Net operating income	15,546		18,733
New Jersey/ Delaware [Member]
Segment Reporting Information [Line Items]
Total revenue	13,500		15,625
Operating expenses	(7,868)		(8,692)	[1]
Net operating income	5,632		6,933
Richmond, Virginia [Member]
Segment Reporting Information [Line Items]
Total revenue	9,296		8,867
Operating expenses	(4,272)		(4,340)	[1]
Net operating income	5,024		4,527
CALIFORNIA
Segment Reporting Information [Line Items]
Total revenue	4,985		4,602
Operating expenses	(2,438)		(2,409)	[1]
Net operating income	2,547		2,193
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Total revenue	934	[2]	2,081	[2]
Operating expenses	(870)	[2]	(1,176)	[1],[2]
Net operating income	64	[2]	905	[2]
Corporate Segment [Member]
Segment Reporting Information [Line Items]
Total revenue	1,422		261
Operating expenses	(415)		(270)	[1]
Net operating income	$ 1,007		$ (9)

[1] Includes property operating expense, real estate taxes and third party management expense.

[2] On April 3, 2014, the Company contributed Four Points Centre to an unconsolidated real estate venture. Following the contribution of this property, there are no wholly owned properties in the Austin, Texas segment. Activity subsequent to the contribution is related to management fees and related expenses provided by the Company to the Austin Venture.