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
Sep. 30, 2018
USD ($)
property
partnership
Dec. 31, 2017
USD ($)
property
partnership
Variable Interest Entity [Line Items]    
Number of self storage properties | property 668  
Net book value of real estate owned $ 2,357,980 $ 2,104,875
Carrying value of fixed rate mortgages 1,227,882 $ 954,991
Number of properties held for sale | property   1
Mortgages | Fixed Rate Mortgages    
Variable Interest Entity [Line Items]    
Carrying value of fixed rate mortgages $ 269,382 $ 271,491
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 $ 242,200 $ 248,000
VIE, Primary Beneficiary | Mortgages | Fixed Rate Mortgages    
Variable Interest Entity [Line Items]    
Carrying value of fixed rate mortgages $ 138,900 $ 140,300