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 4 Months Ended 6 Months Ended
Jun. 30, 2019
USD ($)
property
Jun. 30, 2018
USD ($)
Dec. 31, 2016
USD ($)
property
Dec. 31, 2018
USD ($)
Jun. 30, 2019
USD ($)
property
Jun. 30, 2018
USD ($)
Disaggregation of Revenue [Line Items]            
Number of self storage properties | property 729       729  
Initial term of unconsolidated real estate venture agreements         4 years  
Performance obligation period of recognition         4 years  
Corporate 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 $ 2,200,000 $ 1,800,000     $ 4,300,000 $ 3,500,000
Retail Products and Supplies            
Disaggregation of Revenue [Line Items]            
Revenue 500,000 400,000     900,000 800,000
Property Management, Call Center, and Platform Fees            
Disaggregation of Revenue [Line Items]            
Revenue 3,200,000 1,400,000     $ 6,400,000 2,700,000
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 500,000 300,000     $ 1,000,000.0 600,000
Acquisition fee     $ 4,100,000 $ 4,000,000.0    
Acquisition fees received as a percent of gross capitalization of Initial 2016 JV Portfolio     0.65%      
Deferred revenue 3,700,000     $ 4,600,000 3,700,000  
Tenant Warranty Protection or Tenant Insurance            
Disaggregation of Revenue [Line Items]            
Revenue $ 1,200,000 $ 500,000     $ 2,400,000 $ 1,000,000.0
Percent of total warranty protection plan proceeds received per unconsolidated real estate venture property (percent)         50.00%