Annual report pursuant to Section 13 and 15(d)

SELF STORAGE PROPERTY ACQUISITIONS AND DISPOSITIONS - Narrative (Details)

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SELF STORAGE PROPERTY ACQUISITIONS AND DISPOSITIONS - Narrative (Details)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2019
USD ($)
property
Sep. 30, 2019
property
Jun. 30, 2019
property
Mar. 31, 2019
property
Dec. 31, 2018
USD ($)
property
Sep. 30, 2018
property
Jun. 30, 2018
property
Mar. 31, 2018
property
Dec. 31, 2019
USD ($)
property
Dec. 31, 2018
USD ($)
property
Schedule of Asset Acquisition [Line Items]                    
Number of properties acquired | property 7 6 24 32 7 13 12 25 69 57
Estimated fair value of net assets acquired                 $ 447.8 $ 356.6
Acquisition-related costs capitalized                 3.6 1.9
Aggregate revenue related to properties acquired during the year                 30.4 21.9
Operating income related to properties acquired during the year                 $ 2.5 $ 2.5
Number of self storage properties sold | property                 1 2
Gross selling price of self storage property sold $ 6.5       $ 5.5       $ 6.5 $ 5.5
Gain on sale of self storage properties                 $ 2.8 $ 0.4
Asset acquired from PROs | Affiliated entity                    
Schedule of Asset Acquisition [Line Items]                    
Number of properties acquired | property                 19 4
Estimated fair value of net assets acquired                 $ 131.3 $ 23.1
Asset acquired from 2016 Joint Venture | Joint Venture                    
Schedule of Asset Acquisition [Line Items]                    
Number of properties acquired | property                 1  
Estimated fair value of net assets acquired                 $ 4.1  
Customer in-place leases                    
Schedule of Asset Acquisition [Line Items]                    
Recognized fair value allocated to intangible assets                 10.9 9.1
Real Estate                    
Schedule of Asset Acquisition [Line Items]                    
Recognized fair value allocated to real estate                 $ 436.9 $ 347.5