Segment Information Summary of Unconsolidated Real Estate Ventures (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Dec. 31, 2014
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	$ 233,478				$ 225,004
Equity in income of Real Estate Ventures	131		242
Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	29,164				27,137
Equity in income of Real Estate Ventures	(300)		(21)
Pennsylvania [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	17,273				17,385
Equity in income of Real Estate Ventures	(6)		32
Metropolitan Dc [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	78,069				73,127
Equity in income of Real Estate Ventures	(50)		67
New Jersey/ Delaware [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	0				0
Equity in income of Real Estate Ventures	48		18
Richmond, Virginia [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	1,410				1,574
Equity in income of Real Estate Ventures	86		(45)
Austin, Texas [Member]
Segment Reporting Information [Line Items]
Investment in Real Estate Ventures, at equity	107,562	[1]			105,781	[1]
Equity in income of Real Estate Ventures	$ 353	[1]	$ 191	[1]

[1] Investment in real estate ventures does not include the $1.2 million negative investment balance in one real estate venture as of March 31, 2015 and December 31, 2014, which is included in other liabilities.