Annual report pursuant to Section 13 and 15(d)

COMMITMENTS AND CONTINGENCIES - Operating Leases (Details)

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COMMITMENTS AND CONTINGENCIES - Operating Leases (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
USD ($)
property
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Other Commitments [Line Items]      
Number of self storage properties that are subject to non-cancelable leasehold interest agreements | property 6    
Operating lease rent expense $ 1,600 $ 1,200 $ 1,100
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]      
2019 1,679    
2020 1,777    
2021 1,791    
2022 1,800    
2023 1,770    
2024 through 2092 37,147    
Total operating leases 45,964    
Leasehold Interests [Member]      
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]      
2019 1,334    
2020 1,379    
2021 1,404    
2022 1,419    
2023 1,424    
2024 through 2092 36,074    
Total operating leases 43,034    
Office Building [Member]      
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]      
2019 345    
2020 398    
2021 387    
2022 381    
2023 346    
2024 through 2092 1,073    
Total operating leases 2,930    
Office Building [Member]      
Other Commitments [Line Items]      
Operating lease rent expense $ 200 $ 200 $ 100