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Real Estate and Accumulated Depreciation - Additional Information (Detail) (USD $)
In Billions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
SEC Schedule III, Real Estate and Accumulated Depreciation Disclosure [Abstract]    
Real estate investments for federal income tax $ 1.2us-gaap_RealEstateFederalIncomeTaxBasis $ 1.1us-gaap_RealEstateFederalIncomeTaxBasis