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 12 Months Ended
Mar. 31, 2018
USD ($)
property
Mar. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
property
Dec. 31, 2017
USD ($)
Disaggregation of Revenue [Line Items]        
Other property-related revenue $ 2,321,000 $ 1,881,000    
Property management fees, call center fees and platform fees $ 2,161,000 1,838,000    
Number of self storage properties | property 468      
Deferred revenue related to the Initial JV Portfolio $ 13,542,000     $ 12,687,000
Joint Venture | Joint Venture        
Disaggregation of Revenue [Line Items]        
Number of self storage properties | property 72   66  
Initial term of Joint Venture (in years)     4 years  
Tenant Insurance and Protection Plan Fees and Commissions        
Disaggregation of Revenue [Line Items]        
Other property-related revenue $ 1,700,000 1,400,000    
Retail Products and Supplies        
Disaggregation of Revenue [Line Items]        
Other property-related revenue $ 400,000 300,000    
Property Management, Call Center, and Platform Fees        
Disaggregation of Revenue [Line Items]        
Property management fees as percent of monthly gross revenues and net sales revenue of Joint Venture assets 6.00%      
Call center fees as percent of monthly gross revenues and net sales revenue of Joint Venture assets 1.00%      
Platform fees per Joint Venture property per month $ 1,250      
Property management fees, call center fees and platform fees 1,300,000 1,100,000    
Acquisition Fees        
Disaggregation of Revenue [Line Items]        
Property management fees, call center fees and platform fees 300,000 300,000    
Acquisition fee for the Initial JV Portfolio     $ 4,100,000  
Acquisition fees received as a percent of gross capitalization of Initial JV Portfolio     0.65%  
Deferred revenue related to the Initial JV Portfolio 2,600,000     $ 2,800,000
Tenant Warranty Protection        
Disaggregation of Revenue [Line Items]        
Property management fees, call center fees and platform fees $ 500,000 $ 500,000    
Percent of total warranty protection plan proceeds received per Joint Venture property (percent) 50.00%