Quarterly report pursuant to Section 13 or 15(d)

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue Recognition (Details)

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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue Recognition (Details)
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
USD ($)
property
Sep. 30, 2017
USD ($)
Sep. 30, 2018
USD ($)
property
Sep. 30, 2017
USD ($)
Dec. 31, 2016
USD ($)
property
Dec. 31, 2017
USD ($)
Disaggregation of Revenue [Line Items]            
Number of self storage properties | property 668   668      
Initial term of Joint Venture agreement     4 years      
Performance obligation period of recognition     4 years      
Joint Venture | 2016 Joint Venture            
Disaggregation of Revenue [Line Items]            
Number of self storage properties | property         66  
Joint Venture | 2018 Joint Venture            
Disaggregation of Revenue [Line Items]            
Number of self storage properties | property 103   103      
Tenant Insurance and Protection Plan Fees and Commissions            
Disaggregation of Revenue [Line Items]            
Revenue $ 2,000,000 $ 1,600,000 $ 5,500,000 $ 4,400,000    
Retail Products and Supplies            
Disaggregation of Revenue [Line Items]            
Revenue 300,000 400,000 1,100,000 1,000,000    
Property Management, Call Center, and Platform Fees            
Disaggregation of Revenue [Line Items]            
Revenue 1,900,000 1,300,000 $ 4,600,000 3,600,000    
Property management fees as percent of monthly gross revenues and net sales revenue of Joint Venture assets     6.00%      
Platform fees per Joint Venture property per month     $ 1,250      
Call center fees as percent of monthly gross revenues and net sales revenue of Joint Venture assets     1.00%      
Acquisition Fees            
Disaggregation of Revenue [Line Items]            
Revenue 500,000 200,000 $ 1,100,000 1,100,000    
Acquisition fee for the Initial JV Portfolio 4,000,000       $ 4,100,000  
Acquisition fees received as a percent of gross capitalization of Initial JV Portfolio         0.65%  
Deferred revenue 5,000,000   5,000,000     $ 2,800,000
Tenant Warranty Protection or Tenant Insurance            
Disaggregation of Revenue [Line Items]            
Revenue $ 700,000 $ 400,000 $ 1,700,000 $ 1,400,000    
Percent of total warranty protection plan proceeds received per Joint Venture property (percent)     50.00%