Commitments and Contingencies (Details) (USD $)	12 Months Ended
	Dec. 31, 2011 sqft Years	Dec. 31, 2010	Aug. 05, 2010
Commitments and Contingencies (Textuals) [Abstract]
Mortgage lenders with letters-of-credit	$ 1,900,000	$ 13,200,000
Minimum remaining lease period	18
Maximum remaining lease period	92
Annual rental payment under ground lease	The annual rental payment under this ground lease is ten dollars through August 2022 which is when the initial term of the ground lease will end. After the initial term, the Company has the option to renew the lease until 2091.
Minimum amount of annual rental payment	3,000,000
Minimum amount of annual rental payments	4,000,000
Aggregate fair market rent future payment as of the date of acquisition			27,400,000
Area of Two Logan Square in Philadelphia	708,844
Amount to be paid if the Company takes fee title to Two Logan Square upon foreclosure of related mortgage	2,900,000
Liability related to acquisition of TRC	700,000
Accretion Of Liability Related To Acquisition	2,900,000
Balance portion of acquisition related liability	1,300,000
Agreed holding period (not to sell) for properties acquired as part of the TRC acquisition	15 years
Three Logan Square [Member]
Commitments and Contingencies (Textuals) [Abstract]
Liability related to acquisition of TRC			900,000
Three Logan Square [Member] | Ground Lease [Member]
Commitments and Contingencies (Textuals) [Abstract]
Annual rental payment during initial term of ground lease	$ 10