Annual report pursuant to Section 13 and 15(d)

SELF STORAGE PROPERTY ACQUISITIONS AND DISPOSITIONS - Narrative (Details)

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SELF STORAGE PROPERTY ACQUISITIONS AND DISPOSITIONS - Narrative (Details)
 1 Months Ended 12 Months Ended
Dec. 31, 2016
USD ($)
property
Dec. 31, 2016
USD ($)
property
Dec. 31, 2015
USD ($)
property
Dec. 31, 2014
USD ($)
property
Business Acquisition [Line Items]        
Number of self storage properties acquired | property   107 58 83
Estimated fair value of acquired self storage properties | $ $ 721,400,000 $ 721,400,000 $ 313,000,000  
Self storage properties, net | $ $ 703,700,000 703,700,000 304,500,000  
Pro forma revenue | $   35,600,000 19,300,000  
Pro forma operating income | $   3,700,000 2,400,000  
Acquisition costs | $   6,546,000 4,765,000 $ 9,558,000
Number self storage properties sold | property 1      
Gross selling price of self storage property sold | $ $ 4,900,000 4,900,000    
Gain on sale of self storage property | $ 0 0 0 $ 1,427,000
Leases, Acquired-in-Place [Member]        
Business Acquisition [Line Items]        
Recognized fair value of identifiable intangible assets | $ 17,700,000 $ 17,700,000 $ 8,500,000  
Member Of Board Of Trustees [Member]        
Business Acquisition [Line Items]        
Number of self storage properties acquired | property     13  
Estimated fair value of acquired self storage properties | $     $ 65,300,000  
Participating Regional Operator [Member] | Affiliated Entity [Member]        
Business Acquisition [Line Items]        
Number of self storage properties acquired | property   23 25  
Estimated fair value of acquired self storage properties | $ $ 176,300,000 $ 176,300,000 $ 134,400,000  
2016 Acquisitions [Member]        
Business Acquisition [Line Items]        
Number of self storage properties acquired | property   107    
Number of businesses acquired for which pro forma financial information is available | property   100    
Number of businesses acquired for which pro forma financial information is not available | property   7    
2015 Acquisitions [Member]        
Business Acquisition [Line Items]        
Number of self storage properties acquired | property     58  
Number of businesses acquired for which pro forma financial information is available | property     42  
Number of businesses acquired for which pro forma financial information is not available | property     16