Annual report pursuant to Section 13 and 15(d)

Discontinued Operations (Narrative) (Details)

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Discontinued Operations (Narrative) (Details) (USD $)
 12 Months Ended 0 Months Ended 12 Months Ended 0 Months Ended 3 Months Ended 0 Months Ended 3 Months Ended 12 Months Ended 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Mar. 28, 2012
2200 Renaissance Boulevard [Member]
sqft
Dec. 31, 2012
2200 Renaissance Boulevard [Member]
Dec. 31, 2010
2200 Renaissance Boulevard [Member]
Jul. 25, 2012
95 Chestnut Ridge Road [Member]
sqft
Mar. 31, 2012
95 Chestnut Ridge Road [Member]
sqft
Nov. 07, 2012
Moorestown, New Jersey [Member]
Jun. 30, 2012
Moorestown, New Jersey [Member]
Mar. 31, 2012
Moorestown, New Jersey [Member]
property
sqft
Dec. 31, 2012
19 Skyline Drive [Member]
sqft
Dec. 31, 2012
55 Corporate Drive [Member]
sqft
Dec. 31, 2012
19 Skyline Drive And 55 Corporate Drive [Member]
property
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]                          
Area of property (in square feet)     174,124     47,700 47,700     222,258 248,400 204,057  
Mortgage loan face amount     $ 16,200,000                    
Mortgage loan, maturity date       December 1, 2012                  
Impairment charge on rental property         9,500,000                
Gain recorded on disposal of office property     4,500,000         100,000          
Valuation allowance recognized             500,000   1,600,000   7,100,000   7,100,000
Proceeds from the sale of property 23,429,000         4,000,000   19,400,000          
Number of office buildings                   3      
Number of properties held for sale                         2
Value of property held for sale                         60,900,000
Accumulated depreciation $ 1,478,214,000 $ 1,409,163,000                     $ 16,800,000