Annual report pursuant to Section 13 and 15(d)

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)

v3.10.0.1
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details)
 12 Months Ended
Dec. 31, 2018
USD ($)
partnership
segment
property
$ / shares
Dec. 31, 2017
USD ($)
property
$ / shares
Dec. 31, 2016
USD ($)
Property, Plant and Equipment [Line Items]      
Number of self storage properties | property 675    
Net book value of real estate owned $ 2,391,462,000 $ 2,104,875,000  
Carrying value of fixed rate mortgages 1,277,638,000 954,991,000  
Real estate impairment charges 0 0 $ 0
Advertising expenses 4,100,000 3,700,000 $ 3,100,000
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    
Number of properties held for sale | property   1  
Goodwill impairment $ 0 $ 0  
Customer In-Place Leases [Member]      
Property, Plant and Equipment [Line Items]      
Estimated useful life of in-place leases 12 months    
Building and Improvements [Member] | Minimum [Member]      
Property, Plant and Equipment [Line Items]      
Estimated useful life 7 years    
Building and Improvements [Member] | Maximum [Member]      
Property, Plant and Equipment [Line Items]      
Estimated useful life 40 years    
Furniture and Equipment [Member] | Minimum [Member]      
Property, Plant and Equipment [Line Items]      
Estimated useful life 3 years    
Furniture and Equipment [Member] | Maximum [Member]      
Property, Plant and Equipment [Line Items]      
Estimated useful life 10 years    
Mortgages [Member] | Fixed Rate Mortgages [Member]      
Property, Plant and Equipment [Line Items]      
Carrying value of fixed rate mortgages $ 268,138,000 271,491,000  
Variable Interest Entity, Primary Beneficiary [Member]      
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 $ 240,400,000 248,000,000  
Variable Interest Entity, Primary Beneficiary [Member] | Mortgages [Member] | Fixed Rate Mortgages [Member]      
Property, Plant and Equipment [Line Items]      
Carrying value of fixed rate mortgages $ 138,400,000 $ 140,300,000