Annual report pursuant to Section 13 and 15(d)

INVESTMENT IN UNCONSOLIDATED REAL ESTATE VENTURES - Narrative (Details)

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INVESTMENT IN UNCONSOLIDATED REAL ESTATE VENTURES - Narrative (Details)
storage_unit in Thousands, $ in Thousands, ft² in Millions
12 Months Ended
Dec. 31, 2020
USD ($)
ft²
property
state
storage_unit
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2016
property
Schedule of Equity Method Investments [Line Items]        
Number of self storage properties 821      
Total rentable square feet in self storage properties | ft² 52.0      
Number of storage units 413,000      
Number of states in which self storage properties are located | state 36      
Contributions to unconsolidated real estate venture | $ $ 4,382 $ 0 $ 165,642  
Joint Venture | 2018 Joint Venture        
Schedule of Equity Method Investments [Line Items]        
Company's equity interest in unconsolidated real estate ventures (percent) 25.00%      
Debt financing | $ $ 4,700      
Capital contributions | $ 5,000      
Contributions to unconsolidated real estate venture | $ 1,300      
Joint Venture | 2016 Joint Venture        
Schedule of Equity Method Investments [Line Items]        
Contributions to unconsolidated real estate venture | $ $ 3,100      
Unconsolidated properties | Joint Venture        
Schedule of Equity Method Investments [Line Items]        
Number of self storage properties 177      
Total rentable square feet in self storage properties | ft² 12.7      
Number of storage units 104,000      
Number of states in which self storage properties are located | state 21      
Unconsolidated properties | Joint Venture | 2018 Joint Venture        
Schedule of Equity Method Investments [Line Items]        
Company's equity interest in unconsolidated real estate ventures (percent) 25.00%      
Number of self storage properties 103      
Total rentable square feet in self storage properties | ft² 7.8      
Number of storage units | storage_unit 64      
Number of states in which self storage properties are located | state 17      
Number of properties acquired 1      
Payments to acquire property | $ $ 9,700      
Unconsolidated properties | Joint Venture | 2016 Joint Venture        
Schedule of Equity Method Investments [Line Items]        
Company's equity interest in unconsolidated real estate ventures (percent) 25.00%      
Number of self storage properties 74     66
Total rentable square feet in self storage properties | ft² 4.9      
Number of storage units 40,000      
Number of states in which self storage properties are located | state 13      
Number of properties acquired 2      
Payments to acquire property | $ $ 12,100