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, 2021
USD ($)
property
Jun. 30, 2020
USD ($)
Jun. 30, 2021
USD ($)
property
Jun. 30, 2020
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2016
USD ($)
property
Dec. 31, 2020
USD ($)
Disaggregation of Revenue [Line Items]              
Number of self storage properties | property 864   864        
Initial term of unconsolidated real estate venture agreements     4 years        
Performance obligation period of recognition     4 years        
Joint venture | 2016 Joint Venture | Unconsolidated properties              
Disaggregation of Revenue [Line Items]              
Number of self storage properties | property 74   74     66  
Tenant Insurance and Tenant Warranty Protection Plan Revenues              
Disaggregation of Revenue [Line Items]              
Revenue $ 3,600,000 $ 2,700,000 $ 6,700,000 $ 5,300,000      
Retail Products and Supplies              
Disaggregation of Revenue [Line Items]              
Revenue 700,000 400,000 1,200,000 800,000      
Property Management, Call Center, and Platform Fees              
Disaggregation of Revenue [Line Items]              
Revenue 3,600,000 3,200,000 $ 7,100,000 6,400,000      
Property management fees as percent of monthly gross revenues and net sales revenue of unconsolidated real estate venture assets     6.00%        
Platform fees per unconsolidated real estate venture property per month     $ 1,250        
Call center fees as percent of monthly gross revenues and net sales revenue of unconsolidated real estate venture assets     1.00%        
Acquisition Fees              
Disaggregation of Revenue [Line Items]              
Revenue 200,000 400,000 $ 400,000 900,000      
Acquisition fee         $ 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 900,000   900,000       $ 1,300,000
Tenant Warranty Protection or Tenant Insurance              
Disaggregation of Revenue [Line Items]              
Revenue $ 2,200,000 $ 1,900,000 $ 4,100,000 $ 3,500,000      
Percent of total warranty protection plan proceeds received per unconsolidated real estate venture property     50.00%