Real Estate Investments (Acquistions) (Details) (USD $)	12 Months Ended											0 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Aug. 12, 2011 sqft	Dec. 31, 2011 Overlook I And II [Member]	Mar. 28, 2011 Overlook I And II [Member] sqft	Jan. 20, 2011 Parcel of land in PA [Member] A	Jan. 20, 2011 Parcel of land in PA [Member] Partnership Interest [Member]	Dec. 31, 2011 Three Logan Square [Member] sqft	Aug. 05, 2010 Three Logan Square [Member] Storey	Aug. 05, 2010 Three Logan Square [Member] Partnership Interest [Member] Units_of_limited_partnership_interest	Dec. 29, 2010 Gibbsboro, New Jersey [Member] A
Real Estate Investments (Textuals) [Abstract]
Acquired real estate, cost of property				$ 18,800,000		$ 12,600,000	$ 9,300,000			$ 129,000,000
Date which note receivable obtained from third party in connection with acquisition				Aug. 02, 2011
Net rentable space of property acquired				192,707		126,496			1,000,000
Lease percentage of acquired property	57.20%				100.00%				67.20%
Acquisitions through cash										51,200,000
Business Acquisition, Equity Interest Issued or Issuable, Number of Units											7,111,112
Acquisition related costs				300,000			500,000			400,000
Business acquisition, purchase price allocated to building				21,400,000
Business acquisition, purchase price allocated to intangible assets				12,000,000
Business acquisition, purchase price allocated to market lease liabilities				14,400,000
Business acquisition, purchase price allocated to other assets				200,000
Business acquisition, purchase price allocated to assumed liabilities				400,000
Area of land acquired							1					12
Business Acquisition, Equity Interest Issued or Issuable, Description								50% general partner interest in the venture
Number of storey of office tower acquired										53
Assumed security deposits obligations	700,000									900,000
Net amount of cash paid after assumed liabilities										50,300,000
Closing market price of Parent Company's common shares on acquisition date										11.54
Annual dividend rate per share										$ 0.60
Recognized tenant and other receivables from acquisition										1,100,000
Prepaid real estate taxes from acquisition										1,500,000
Settlement of note receivable through foreclosure of a parcel of land	0	2,795,000	0									2,800,000
Land available for development	109,008,000	110,055,000	97,368,000									300,000
Business Acquisition, consideration received							$ 4,900,000