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Real Estate and Accumulated Depreciation (Details Textual) (USD $)
In Billions, unless otherwise specified
12 Months Ended
Dec. 31, 2011
Real Estate and Accumulated Depreciation (Textual) [Abstract]  
Tenant Improvements, Leasehold Improvements Life of lease
Aggregate cost of land and buildings and equipment for federal income tax purpose $ 3.1
Building And Building Improvements And Land [Member]
 
Depreciation Expense on the Useful Lives  
Property, Plant and Equipment, Useful Life, Minimum 3
Property, Plant and Equipment, Useful Life, Maximum 50