Annual report pursuant to Section 13 and 15(d)

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)

v3.22.0.1
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)
 12 Months Ended
Dec. 31, 2021
USD ($)
segment
partnership
property
$ / shares
Dec. 31, 2020
USD ($)
$ / shares
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2016
USD ($)
property
Property, Plant and Equipment [Line Items]          
Number of self storage properties | property 1,050        
Net book value of real estate owned $ 5,219,471,000 $ 3,195,569,000      
Carrying value 2,948,944,000 1,922,614,000      
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 $ 6,600,000 5,800,000 5,200,000    
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 177        
2016 Joint Venture | Unconsolidated properties | Joint Venture          
Property, Plant and Equipment [Line Items]          
Number of self storage properties | property 74       66
Tenant insurance and tenant warranty protection plan          
Property, Plant and Equipment [Line Items]          
Revenue $ 15,000,000 11,100,000 9,100,000    
Retail products and supplies          
Property, Plant and Equipment [Line Items]          
Revenue 2,300,000 1,800,000 1,700,000    
Property management fees, call center fees, and platform fees          
Property, Plant and Equipment [Line Items]          
Revenue $ 14,800,000 13,100,000 12,800,000    
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          
Property, Plant and Equipment [Line Items]          
Revenue $ 800,000 1,700,000 1,800,000    
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 500,000 1,300,000      
Tenant warranty protection or tenant insurance          
Property, Plant and Equipment [Line Items]          
Revenue $ 7,300,000 6,300,000 $ 4,700,000    
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        
Fixed rate mortgages payable | Fixed rate mortgages payable          
Property, Plant and Equipment [Line Items]          
Carrying value $ 303,944,000 223,614,000      
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 $ 425,700,000 225,100,000      
VIE, Primary Beneficiary | Fixed rate mortgages payable | Fixed rate mortgages payable          
Property, Plant and Equipment [Line Items]          
Carrying value $ 188,700,000 $ 100,700,000