Annual report pursuant to Section 13 and 15(d)

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

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Investments In Unconsolidated Joint Ventures (Boston-Downtown Crossing) (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2012
sqft
May 15, 2012
Boston Downtown Crossing [Member]
   
Schedule of Equity Method Investments [Line Items]    
Area of property (in square feet) 1,200,000  
Percentage of interest in venture 15.00%  
Optional purchase amount   $ 45
Minimum [Member]
   
Schedule of Equity Method Investments [Line Items]    
Percentage of interest in venture 7.50%  
Maximum [Member]
   
Schedule of Equity Method Investments [Line Items]    
Percentage of interest in venture 80.00%