Commitments and Contingencies (Details Textuals) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2011 Property Tenants Year sqft ac Room Buildings	Aug. 05, 2010
Commitments and Contingencies (Textuals) [Abstract]
Minimum remaining lease period	18
Maximum remaining lease period	92
Annual rental payment under ground lease	The annual rental payment after 2022 will be adjusted at the lower of $3.0 million or the prevailing market rent at that time until 2030. Subsequent to 2030, the annual rental payment will be adjusted at the lower of $4.0 million or the prevailing market rent at the time until 2042 and at fair market value until 2091.
Minimum amount of annual rental payment	$ 3.0
Minimum amount of annual rental payments	4.0
Aggregate fair market rent future payment as of the date of acquisition		27.4
Area of Two Logan Square in Philadelphia	708,856
Amount to be paid if the Company takes fee title to Two Logan Square upon foreclosure of related mortgage	2.9
Liability related to acquisition of TRC	0.7
Accretion Of Liability Related To Acquisition	2.9
Balance portion of acquisition related liability	$ 1.3
Agreed holding period (not to sell) for properties acquired as part of the TRC acquisition	15 years