Commitments and Contingencies (Details) (USD $)	12 Months Ended		12 Months Ended
	Dec. 31, 2012 sqft Y	Dec. 31, 2011	Dec. 31, 2012 Three Logan Square [Member] Ground Lease [Member]	Aug. 05, 2010 Three Logan Square [Member] Ground Lease [Member]
Long-term Purchase Commitment [Line Items]
Letters-of-credit, funded required leasing and capital reserve accounts for benefit of mortgage lenders		$ 1,900,000
Commitments and Contingencies (Textuals) [Abstract]
Minimum remaining lease period	17
Maximum remaining lease period	91
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.
Annual rental payment during initial term of ground lease			10
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
Leased Area Of Building	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	$ 1,500,000
Agreed holding period (not to sell) for properties acquired as part of the TRC acquisition	15 years