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 6 Months Ended 12 Months Ended
Jun. 30, 2018
USD ($)
property
Jun. 30, 2017
USD ($)
Jun. 30, 2018
USD ($)
property
Jun. 30, 2017
USD ($)
Dec. 31, 2016
USD ($)
property
Dec. 31, 2017
USD ($)
Disaggregation of Revenue [Line Items]            
Other property-related revenue $ 2,549,000 $ 2,045,000 $ 4,870,000 $ 3,926,000    
Property management fees, call center fees and platform fees $ 2,155,000 2,142,000 $ 4,316,000 3,980,000    
Number of self storage properties | property 551   551      
Initial term of Joint Venture agreement         4 years  
Deferred revenue related to the Initial JV Portfolio $ 13,546,000   $ 13,546,000     $ 12,687,000
Joint Venture | Unconsolidated real estate venture            
Disaggregation of Revenue [Line Items]            
Number of self storage properties | property         66  
Tenant Insurance and Protection Plan Fees and Commissions            
Disaggregation of Revenue [Line Items]            
Other property-related revenue 1,800,000 1,400,000 3,500,000 2,800,000    
Retail Products and Supplies            
Disaggregation of Revenue [Line Items]            
Other property-related revenue 400,000 300,000 $ 800,000 600,000    
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 1,400,000 1,200,000 2,700,000 2,300,000    
Acquisition Fees            
Disaggregation of Revenue [Line Items]            
Property management fees, call center fees and platform fees 300,000 600,000 600,000 900,000    
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 2,300,000   2,300,000     $ 2,800,000
Tenant Warranty Protection            
Disaggregation of Revenue [Line Items]            
Property management fees, call center fees and platform fees $ 500,000 $ 500,000 $ 1,000,000 $ 1,000,000    
Percent of total warranty protection plan proceeds received per Joint Venture property (percent)     50.00%