Quarterly report pursuant to Section 13 or 15(d)

RELATED PARTY TRANSACTIONS (Details)

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RELATED PARTY TRANSACTIONS (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Related Party Transaction [Line Items]        
Payroll and related costs reimbursed by the Company $ 21,618 $ 17,330 $ 61,170 $ 46,064
Supervisory and Administrative Fee Agreement [Member] | Management [Member] | Participating Regional Operator [Member]        
Related Party Transaction [Line Items]        
Supervisory and administrative fees for PROs 3,700 3,000 $ 10,500 7,800
Supervisory and Administrative Fee Agreement [Member] | Management [Member] | Participating Regional Operator [Member] | Minimum [Member]        
Related Party Transaction [Line Items]        
Supervisory and administrative fee agreement of gross revenue, percent     5.00%  
Supervisory and Administrative Fee Agreement [Member] | Management [Member] | Participating Regional Operator [Member] | Maximum [Member]        
Related Party Transaction [Line Items]        
Supervisory and administrative fee agreement of gross revenue, percent     6.00%  
Payroll Services [Member] | Management [Member] | Participating Regional Operator [Member]        
Related Party Transaction [Line Items]        
Payroll and related costs reimbursed by the Company 6,100 5,100 $ 17,800 13,800
Due Diligence Costs [Member] | Management [Member] | Participating Regional Operator [Member]        
Related Party Transaction [Line Items]        
Due diligence expenses payable to the PROs 200 $ 400 500 $ 700
Customary Fees [Member] | Equity Method Investee [Member] | Management Fees And Other Revenue [Member]        
Related Party Transaction [Line Items]        
Management fees and other revenue from Joint Venture $ 2,000   $ 6,000  
Acquisition of Self Storage Property SA-SCMI LLC [Member] | Affiliated Entity [Member] | Subordinated performance units [Member]        
Related Party Transaction [Line Items]        
Consideration, equity interest issued (in shares)     44,917  
Acquisition of Self Storage Property Howard Family Limited Partnership I [Member] | Affiliated Entity [Member] | Subordinated performance units [Member]        
Related Party Transaction [Line Items]        
Consideration, equity interest issued (in shares)     23,666  
Acquisition of Self Storage Property Howard Family Limited Partnership I [Member] | Affiliated Entity [Member] | OP units [Member]        
Related Party Transaction [Line Items]        
Consideration, equity interest issued (in shares)     75,147  
Acquisition of Self Storage Property Van Mourick Diversified LP [Member] | Affiliated Entity [Member] | OP units [Member]        
Related Party Transaction [Line Items]        
Consideration, equity interest issued (in shares)     10,766