Annual report pursuant to Section 13 and 15(d)

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue Recognition (Details)

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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue Recognition (Details)
12 Months Ended
Dec. 31, 2020
USD ($)
property
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2016
USD ($)
property
Disaggregation of Revenue [Line Items]        
Number of self storage properties | property 821      
Initial term of unconsolidated real estate venture agreements 4 years      
Performance obligation period of recognition 4 years      
Joint Venture | Unconsolidated properties        
Disaggregation of Revenue [Line Items]        
Number of self storage properties | property 177      
Joint Venture | 2016 Joint Venture | Unconsolidated properties        
Disaggregation of Revenue [Line Items]        
Number of self storage properties | property 74     66
Tenant insurance and tenant warranty protection plan        
Disaggregation of Revenue [Line Items]        
Revenue $ 11,100,000 $ 9,100,000 $ 7,500,000  
Retail products and supplies        
Disaggregation of Revenue [Line Items]        
Revenue 1,800,000 1,700,000 1,500,000  
Property management fees, call center fees, and platform fees        
Disaggregation of Revenue [Line Items]        
Revenue $ 13,100,000 12,800,000 7,800,000  
Property management fees as percent of monthly gross revenues and net sales revenues from the assets of the unconsolidated real estate ventures 6.00%      
Platform fees per month per unconsolidated real estate venture property $ 1,250      
Call center fees as percent of of monthly gross revenues and net sales revenues from the 2016 Joint Venture properties 1.00%      
Acquisition fees        
Disaggregation of Revenue [Line Items]        
Revenue $ 1,700,000 1,800,000 1,600,000  
Acquisition fees received     4,000,000.0 $ 4,100,000
Acquisition fees received as a percent of gross capitalization of Initial 2016 JV Portfolio       0.65%
Deferred revenue related to acquisition fees 1,300,000 2,800,000    
Tenant warranty protection or tenant insurance        
Disaggregation of Revenue [Line Items]        
Revenue $ 6,300,000 $ 4,700,000 $ 2,400,000  
Percent of total warranty protection plan proceeds received per unconsolidated real estate venture property (percent) 50.00%