Quarterly report pursuant to Section 13 or 15(d)

Investments In Unconsolidated Joint Ventures (Boston-Downtown Crossing) (Details)

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Investments In Unconsolidated Joint Ventures (Boston-Downtown Crossing) (Details) (Boston-Downtown Crossing [Member], USD $)
In Millions, unless otherwise specified
 0 Months Ended 1 Months Ended 12 Months Ended
May 15, 2012
Jan. 29, 2007
Oct. 31, 2006
sqft
Dec. 31, 2008
Schedule of Equity Method Investments [Line Items]        
Area of office property, square feet     1,200,000  
Percentage of interest in venture     15.00%  
Venture property acquisition cost   $ 100    
Impairment of property       69.5
Optional purchase amount $ 45      
Holding and distribution pattern under operating agreement     Distributions will generally be in proportion to its members' respective ownership interests and, depending upon the development unit, promotes will be available to specified partners after the achievement of certain internal rates of return ranging from 10 to 15 percent.  
Maximum [Member]
        
Schedule of Equity Method Investments [Line Items]        
Internal rate of return     15.00%  
Minimum [Member]
        
Schedule of Equity Method Investments [Line Items]        
Internal rate of return     10.00%