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
Mar. 31, 2020
USD ($)
property
Mar. 31, 2019
USD ($)
Jun. 30, 2018
USD ($)
Dec. 31, 2016
USD ($)
property
Dec. 31, 2019
USD ($)
Disaggregation of Revenue [Line Items]          
Number of self storage properties | property 780        
Initial term of unconsolidated real estate venture agreements 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  
Tenant Insurance and Tenant Warranty Protection Plan Revenues          
Disaggregation of Revenue [Line Items]          
Revenue $ 2,600,000 $ 2,100,000      
Retail Products and Supplies          
Disaggregation of Revenue [Line Items]          
Revenue 400,000 400,000      
Property Management, Call Center, and Platform Fees          
Disaggregation of Revenue [Line Items]          
Revenue $ 3,200,000 3,200,000      
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 $ 500,000 500,000      
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 2,400,000       $ 2,800,000
Tenant Warranty Protection or Tenant Insurance          
Disaggregation of Revenue [Line Items]          
Revenue $ 1,600,000 $ 1,200,000      
Percent of total warranty protection plan proceeds received per unconsolidated real estate venture property (percent) 50.00%