Annual report pursuant to Section 13 and 15(d)

INVESTMENT IN UNCONSOLIDATED REAL ESTATE VENTURE - Narrative (Details)

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INVESTMENT IN UNCONSOLIDATED REAL ESTATE VENTURE - Narrative (Details)
storage_unit in Thousands, $ in Thousands, ft² in Millions
 12 Months Ended
Oct. 04, 2016
USD ($)
Dec. 31, 2017
USD ($)
ft²
storage_unit
state
property
$ / property
Dec. 31, 2016
USD ($)
property
Dec. 31, 2015
USD ($)
Schedule of Equity Method Investments [Line Items]        
Contributions to unconsolidated real estate venture   $ 15,289 $ 82,950 $ 0
Number of self storage properties | property   444    
Total rentable square feet in self storage properties | ft²   27.2    
Number of storage units owned | storage_unit   215    
Number of states that self storage properties are owned in | state   26    
Investment in unconsolidated real estate venture   $ 89,093 81,486  
Outside basis capitalized in investment in unconsolidated real estate venture   3,700    
Equity in losses of unconsolidated real estate venture   $ (2,339) (1,484) $ 0
State Pension Fund [Member] | iStorage Facilities JV Portfolio [Member]        
Schedule of Equity Method Investments [Line Items]        
Equity method investment ownership percentage (percent) 75.00%      
Joint Venture [Member]        
Schedule of Equity Method Investments [Line Items]        
Equity method investment, monthly property management fee (percent)   6.00%    
Equity method investment, annual call center fee (percent)   1.00%    
Equity method investment, monthly platform fee (in dollars per property) | $ / property   1,250    
Equity method investment, acquisition fee (percent)   0.65%    
Equity method investment, acquisition fee term (years)   4 years    
Equity method investment, development management fee (percent)   3.00%    
Equity method investment, warranty protection exchange, proceeds given in exchange (percent)   50.00%    
Joint Venture [Member] | iStorage Facilities JV Portfolio [Member]        
Schedule of Equity Method Investments [Line Items]        
Aggregate purchase price of JV portfolio     $ 630,000  
Number of self storage properties acquired | property     66  
Total equity contributed by JV Partners to Joint Venture $ 320,000      
Equity contributed by JV Partner to Joint Venture 80,000      
Joint Venture [Member] | Unconsolidated Real Estate Venture [Member]        
Schedule of Equity Method Investments [Line Items]        
Number of self storage properties acquired | property   5    
Estimated fair value of self storage properties acquired   $ 59,300    
Contributions to unconsolidated real estate venture   $ 14,900    
Joint Venture [Member] | State Pension Fund [Member] | iStorage Facilities JV Portfolio [Member]        
Schedule of Equity Method Investments [Line Items]        
Equity contributed by JV Partner to Joint Venture $ 240,000      
Equity Method Investee [Member] | Unconsolidated Real Estate Venture [Member]        
Schedule of Equity Method Investments [Line Items]        
Equity method investment ownership percentage (percent) 25.00%      
Number of self storage properties | property   71    
Total rentable square feet in self storage properties | ft²   4.9    
Number of storage units owned | storage_unit   39    
Number of states that self storage properties are owned in | state   13    
Equity Method Investee [Member] | Customary Fees [Member]        
Schedule of Equity Method Investments [Line Items]        
Management and other fees   $ 8,100 $ 1,800