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)
 1 Months Ended 3 Months Ended 12 Months Ended
Dec. 31, 2016
USD ($)
property
Dec. 31, 2017
USD ($)
property
Sep. 30, 2017
property
Jun. 30, 2017
property
Mar. 31, 2017
property
Dec. 31, 2017
USD ($)
property
Dec. 31, 2016
USD ($)
property
Business Acquisition [Line Items]              
Recognized fair value allocated to real estate $ 703,700,000 $ 416,300,000       $ 416,300,000 $ 703,700,000
Acquisition-related costs capitalized           3,600,000  
Pro forma revenue           15,500,000 35,600,000
Pro forma operating income           $ 500,000 3,700,000
Number of self storage properties sold | property 1         3  
Gross selling price of self storage property sold $ 4,900,000 17,800,000       $ 17,800,000 4,900,000
Gain on sale of self storage properties           5,700,000 0
Customer In-Place Leases [Member]              
Business Acquisition [Line Items]              
Recognized fair value allocated to in-place leases 17,700,000 10,500,000       $ 10,500,000 $ 17,700,000
Participating Regional Operator [Member] | Affiliated Entity [Member]              
Business Acquisition [Line Items]              
Number of self storage properties acquired | property           10 23
Estimated fair value of acquired self storage properties 176,300,000 $ 73,200,000       $ 73,200,000 $ 176,300,000
2017 Acquisitions [Member]              
Business Acquisition [Line Items]              
Number of self storage properties acquired | property   31 19 10 5 65  
Estimated fair value of acquired self storage properties   $ 426,800,000       $ 426,800,000  
2016 Acquisitions [Member]              
Business Acquisition [Line Items]              
Number of self storage properties acquired | property             107
Estimated fair value of acquired self storage properties $ 721,400,000           $ 721,400,000