Annual report pursuant to Section 13 and 15(d)

RELATED PARTY TRANSACTIONS - Narrative (Details)

v3.8.0.1
RELATED PARTY TRANSACTIONS - Narrative (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
property
shares
Dec. 31, 2016
USD ($)
property
shares
Dec. 31, 2015
USD ($)
Dec. 31, 2014
USD ($)
property
Related Party Transaction [Line Items]        
Payroll and related costs reimbursed by the company | $ $ 84,455 $ 64,798 $ 45,412  
Affiliated Entity [Member] | Acquisition of Self Storage Property from SA-SCMI LLC [Member] | Subordinated performance units [Member]        
Related Party Transaction [Line Items]        
Consideration, equity interest issued (in shares) | shares 44,917      
Affiliated Entity [Member] | Acquisition of Self Storage Property From Nordhagen LLLP [Member] | OP units [Member]        
Related Party Transaction [Line Items]        
Consideration, equity interest issued (in shares) | shares 26,049      
Affiliated Entity [Member] | Acquisition of Self Storage Property From SecurCare SelfStorage Inc [Member] | Subordinated performance units [Member]        
Related Party Transaction [Line Items]        
Consideration, equity interest issued (in shares) | shares 22,214      
Affiliated Entity [Member] | Acquisition of Self Storage Property From Howard Family Limited Partnership I [Member] | Subordinated performance units [Member]        
Related Party Transaction [Line Items]        
Consideration, equity interest issued (in shares) | shares 88,435      
Affiliated Entity [Member] | Acquisition of Self Storage Property From Howard Family Limited Partnership I [Member] | OP units [Member]        
Related Party Transaction [Line Items]        
Consideration, equity interest issued (in shares) | shares 101,270      
Affiliated Entity [Member] | Acquisition of Self Storage Property From Van Mourick Diversified LP [Member] | OP units [Member]        
Related Party Transaction [Line Items]        
Consideration, equity interest issued (in shares) | shares 10,766      
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 | $ $ 1,300 1,400    
Reimbursements received on assumed mortgages | $ $ 1,300 $ 1,400    
Units issued upon reimbursement of assumed mortgages (in shares) | shares 47,339 67,832    
Participating Regional Operator [Member] | Management [Member] | Supervisory and Administrative Fee Agreement [Member]        
Related Party Transaction [Line Items]        
Supervisory and administrative fees for PROs | $ $ 14,400 $ 11,000 7,600  
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]        
Payroll and related costs reimbursed by the company | $ $ 24,600 19,400 13,400  
Participating Regional Operator [Member] | Management [Member] | Due Diligence Costs [Member]        
Related Party Transaction [Line Items]        
Due diligence expenses payable to the PROs | $ $ 700 $ 1,100 $ 600  
Participating Regional Operator [Member] | Affiliated Entity [Member]        
Related Party Transaction [Line Items]        
Number of self storage properties acquired | property 10 23