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, 2019
USD ($)
partnership
property
segment
$ / shares
Dec. 31, 2018
USD ($)
$ / shares
Dec. 31, 2017
USD ($)
Property, Plant and Equipment [Line Items]      
Number of self storage properties | property 742    
Net book value of real estate owned $ 2,753,897,000 $ 2,391,462,000  
Carrying value 1,539,260,000 1,277,638,000  
Real estate impairment charges 0 0 $ 0
Advertising expenses 5,200,000 4,100,000 $ 3,700,000
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  
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 $ 264,260,000 268,138,000  
VIE, Primary Beneficiary      
Property, Plant and Equipment [Line Items]      
Number of partnerships considered to be VIEs | partnership 21    
Number of self storage properties | property 34    
Net book value of real estate owned $ 233,100,000 240,400,000  
VIE, Primary Beneficiary | Mortgages | Fixed rate mortgages payable      
Property, Plant and Equipment [Line Items]      
Carrying value $ 136,400,000 $ 138,400,000