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
3 Months Ended 12 Months Ended
Dec. 31, 2019
USD ($)
ft²
property
state
storage_unit
Sep. 30, 2019
USD ($)
property
Jun. 30, 2019
USD ($)
property
Mar. 31, 2019
USD ($)
property
Dec. 31, 2018
USD ($)
property
Sep. 30, 2018
USD ($)
property
Jun. 30, 2018
USD ($)
property
Mar. 31, 2018
USD ($)
property
Dec. 31, 2019
USD ($)
ft²
property
state
storage_unit
Dec. 31, 2018
USD ($)
property
Dec. 31, 2016
property
Schedule of Equity Method Investments [Line Items]                      
Number of self storage properties 742               742    
Total rentable square feet in self storage properties | ft² 47.1               47.1    
Number of storage units 378,000               378,000    
Number of states in which self storage properties are located | state 35               35    
Number of properties acquired 7 6 24 32 7 13 12 25 69 57  
Gross sales price of acquisition | $ $ 32,163 $ 35,771 $ 185,335 $ 194,561 $ 51,395 $ 106,528 $ 62,937 $ 135,789 $ 447,830 $ 356,649  
Joint Venture | Asset acquired from 2016 Joint Venture                      
Schedule of Equity Method Investments [Line Items]                      
Number of properties acquired                 1    
Gross sales price of acquisition | $                 $ 4,100    
Joint Venture | 2016 Joint Venture                      
Schedule of Equity Method Investments [Line Items]                      
Number of self storage properties                     66
Unconsolidated properties | Joint Venture                      
Schedule of Equity Method Investments [Line Items]                      
Company's equity interest in unconsolidated real estate ventures (percent) 25.00%               25.00%    
Number of self storage properties 175               175    
Total rentable square feet in self storage properties | ft² 12.6               12.6    
Number of storage units 103,000               103,000    
Number of states in which self storage properties are located | state 21               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%               25.00%    
Number of self storage properties 103               103    
Total rentable square feet in self storage properties | ft² 7.7               7.7    
Number of storage units | storage_unit 63               63    
Number of states in which self storage properties are located | state 17               17    
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%               25.00%    
Number of self storage properties 72               72    
Total rentable square feet in self storage properties | ft² 4.9               4.9    
Number of storage units 40,000               40,000    
Number of states in which self storage properties are located | state 13               13