Quarterly report pursuant to Section 13 or 15(d)

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)

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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)
$ in Thousands
Mar. 31, 2022
USD ($)
property
partnership
Dec. 31, 2021
USD ($)
partnership
property
Variable Interest Entity [Line Items]    
Number of self storage properties | property 1,061  
Net book value of real estate owned $ 5,268,038 $ 5,219,471
Carrying value of fixed rate mortgages 3,007,860 2,948,944
Fixed rate mortgages payable | Fixed rate mortgages payable    
Variable Interest Entity [Line Items]    
Carrying value of fixed rate mortgages $ 302,860 $ 303,944
VIE, Primary Beneficiary    
Variable Interest Entity [Line Items]    
Number of partnerships considered to be VIEs | partnership 22 22
Number of self storage properties | property 48 48
Net book value of real estate owned $ 421,400 $ 425,700
VIE, Primary Beneficiary | Fixed rate mortgages payable | Fixed rate mortgages payable    
Variable Interest Entity [Line Items]    
Carrying value of fixed rate mortgages $ 188,700 $ 188,700