Annual report pursuant to Section 13 and 15(d)

RELATED PARTY TRANSACTIONS - Narrative (Details)

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RELATED PARTY TRANSACTIONS - Narrative (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
USD ($)
property
shares
Dec. 31, 2017
USD ($)
property
shares
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
property
Related Party Transaction [Line Items]        
Property operating expenses $ 103,875 $ 84,455 $ 64,798  
Affiliated Entity [Member] | Issuance of units to affiliates of Northwest | Subordinated performance units [Member]        
Related Party Transaction [Line Items]        
Consideration, equity interest issued (in shares) | shares 5,824      
Affiliated Entity [Member] | Issuance of units to affiliates of Northwest | OP units [Member]        
Related Party Transaction [Line Items]        
Consideration, equity interest issued (in shares) | shares 19,047      
Participating Regional Operator [Member] | Management [Member]        
Related Party Transaction [Line Items]        
Number of self storage properties acquired | property       16
Receivable balance netted against equity interest issued in business acquisition       $ 5,200
Business Combination, Receivable Netted Against Equity Interests Issued And Issuable, Reimbursements Received for Interest Payments $ 1,200 1,300    
Reimbursements received on assumed mortgages $ 1,200 $ 1,300    
Units issued upon reimbursement of assumed mortgages (in shares) | shares 44,502 47,339    
Participating Regional Operator [Member] | Management [Member] | Supervisory and Administrative Fee Agreement [Member]        
Related Party Transaction [Line Items]        
Supervisory and administrative fees for PROs $ 16,900 $ 14,400 11,000  
Participating Regional Operator [Member] | Management [Member] | Supervisory and Administrative Fee Agreement [Member] | Minimum [Member]        
Related Party Transaction [Line Items]        
Supervisory and administrative fee arrangement of gross revenue (percent) 5.00%      
Participating Regional Operator [Member] | Management [Member] | Supervisory and Administrative Fee Agreement [Member] | Maximum [Member]        
Related Party Transaction [Line Items]        
Supervisory and administrative fee arrangement of gross revenue (percent) 6.00%      
Participating Regional Operator [Member] | Management [Member] | Affiliate Payroll Services [Member]        
Related Party Transaction [Line Items]        
Property operating expenses $ 29,500 24,600 19,400  
Participating Regional Operator [Member] | Management [Member] | Due Diligence Costs [Member]        
Related Party Transaction [Line Items]        
Due diligence expenses payable to the PROs $ 400 $ 700 $ 1,100  
Participating Regional Operator [Member] | Affiliated Entity [Member]        
Related Party Transaction [Line Items]        
Number of self storage properties acquired | property 4 10    
NSA OP, LP [Member] | OP units [Member]        
Related Party Transaction [Line Items]        
Issuance of shares/units (in shares) | shares 2,024,170      
NSA OP, LP [Member] | Affiliated Entity [Member] | Issuance of units to affiliate of Personal Mini        
Related Party Transaction [Line Items]        
Value Of Co-Investment In Acquisition Received Upon Issuance Of Shares $ 300      
NSA OP, LP [Member] | Affiliated Entity [Member] | Issuance of units to affiliate of Personal Mini | Subordinated performance units [Member]        
Related Party Transaction [Line Items]        
Issuance of shares/units (in shares) | shares 11,490