Annual report pursuant to Section 13 and 15(d)

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)

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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)
 12 Months Ended
Dec. 31, 2022
USD ($)
property
partnership
segment
$ / shares
Dec. 31, 2021
USD ($)
$ / shares
Dec. 31, 2020
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2016
USD ($)
Property, Plant and Equipment [Line Items]          
Number of self storage properties | property 1,101        
Net book value of real estate owned $ 5,618,911,000 $ 5,219,471,000      
Carrying value 3,560,570,000 2,948,944,000      
Real estate impairment charges $ 0 0 $ 0    
Initial term of unconsolidated real estate venture agreements 4 years        
Performance obligation period of recognition 4 years        
Advertising expenses $ 10,000,000 6,600,000 5,800,000    
Deferred tax asset 0 0      
Deferred tax liabilities 0 0      
Unrecognized tax benefits $ 0 $ 0      
Common shares of beneficial interest, par value (in dollars per share) | $ / shares $ 0.01 $ 0.01      
Number of reportable segments | segment 1        
Goodwill impairment $ 0 $ 0      
Unconsolidated properties | Joint Venture          
Property, Plant and Equipment [Line Items]          
Number of self storage properties | property 185        
Tenant insurance and tenant warranty protection plan          
Property, Plant and Equipment [Line Items]          
Revenue $ 19,800,000 15,000,000 11,100,000    
Retail products and supplies          
Property, Plant and Equipment [Line Items]          
Revenue 2,600,000 2,300,000 1,800,000    
Property management fees, call center fees, and platform fees          
Property, Plant and Equipment [Line Items]          
Revenue $ 16,500,000 14,800,000 13,100,000    
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          
Property, Plant and Equipment [Line Items]          
Revenue $ 1,200,000 800,000 1,700,000    
Acquisition fees received as a percent of gross capitalization of Initial 2016 JV Portfolio         0.65%
Acquisition fees received       $ 4,000,000 $ 4,100,000
Deferred revenue related to acquisition fees $ 0 500,000      
Tenant warranty protection or tenant insurance          
Property, Plant and Equipment [Line Items]          
Number of self storage properties | property 1        
Revenue $ 9,500,000 7,300,000 $ 6,300,000    
Percent of total warranty protection plan proceeds received per unconsolidated real estate venture property (percent) 50.00%        
Customer in-place leases          
Property, Plant and Equipment [Line Items]          
Estimated useful life 12 months        
Building and improvements | Minimum          
Property, Plant and Equipment [Line Items]          
Estimated useful life 7 years        
Building and improvements | Maximum          
Property, Plant and Equipment [Line Items]          
Estimated useful life 40 years        
Furniture and equipment | Minimum          
Property, Plant and Equipment [Line Items]          
Estimated useful life 3 years        
Furniture and equipment | Maximum          
Property, Plant and Equipment [Line Items]          
Estimated useful life 10 years        
Mortgages | Fixed rate mortgages payable          
Property, Plant and Equipment [Line Items]          
Carrying value $ 299,570,000 303,944,000      
VIE, Primary Beneficiary          
Property, Plant and Equipment [Line Items]          
Number of partnerships considered to be VIEs | partnership 22        
Number of self storage properties | property 48        
Net book value of real estate owned $ 412,900,000 425,700,000      
VIE, Primary Beneficiary | Mortgages | Fixed rate mortgages payable          
Property, Plant and Equipment [Line Items]          
Carrying value $ 188,700,000 $ 188,700,000