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Nature of Business and Financial Statement Presentation (Useful Lives) (Details)
9 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Building and building and land improvements | Minimum    
Property, Plant and Equipment [Line Items]    
Useful life 20 years  
Building and building and land improvements | Maximum    
Property, Plant and Equipment [Line Items]    
Useful life 40 years  
Furniture, fixtures, and equipment | Minimum    
Property, Plant and Equipment [Line Items]    
Useful life 5 years  
Furniture, fixtures, and equipment | Maximum    
Property, Plant and Equipment [Line Items]    
Useful life 10 years  
Tenant improvements    
Property, Plant and Equipment [Line Items]    
Estimated useful lives   The shorter of the term of the related lease or useful life