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, 2020
USD ($)
property
partnership
Dec. 31, 2019
USD ($)
partnership
property
Variable Interest Entity [Line Items]    
Number of self storage properties | property 780  
Net book value of real estate owned $ 2,951,322 $ 2,753,897
Carrying value of fixed rate mortgages 1,736,680 1,539,260
Mortgages | Fixed rate mortgages payable    
Variable Interest Entity [Line Items]    
Carrying value of fixed rate mortgages $ 261,680 $ 264,260
VIE, Primary Beneficiary    
Variable Interest Entity [Line Items]    
Number of partnerships considered to be VIEs | partnership 21 21
Number of self storage properties | property 34 34
Net book value of real estate owned $ 231,100 $ 233,100
VIE, Primary Beneficiary | Mortgages | Fixed rate mortgages payable    
Variable Interest Entity [Line Items]    
Carrying value of fixed rate mortgages $ 135,900 $ 136,400