Annual report pursuant to Section 13 and 15(d)

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

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Investments In Unconsolidated Joint Ventures (Boston-Downtown Crossing) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended 0 Months Ended 1 Months Ended 12 Months Ended 1 Months Ended
Dec. 31, 2011
Jan. 29, 2007
Boston-Downtown Crossing [Member]
Oct. 31, 2006
Boston-Downtown Crossing [Member]
sqft
Dec. 31, 2008
Boston-Downtown Crossing [Member]
Oct. 31, 2006
Maximum [Member]
Boston-Downtown Crossing [Member]
Oct. 31, 2006
Minimum [Member]
Boston-Downtown Crossing [Member]
Schedule of Equity Method Investments [Line Items]            
Percentage of interest in venture     15.00%      
Internal rate of return 10.00%       15.00% 10.00%
Area of office property     1,200,000      
Venture property acquisition cost   $ 100        
Impairment of property       $ 69.5    
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.