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
 1 Months Ended 12 Months Ended
Mar. 31, 2014
Dec. 31, 2017
USD ($)
property
Dec. 31, 2016
USD ($)
Dec. 31, 2015
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   $ 1,200 $ 1,100 $ 1,000
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]        
2018   1,513    
2019   1,522    
2020   1,491    
2021   1,404    
2022   1,419    
2023 through 2092   37,498    
Total operating leases   44,847    
Leasehold Interests [Member]        
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]        
2018   1,329    
2019   1,334    
2020   1,379    
2021   1,404    
2022   1,419    
2023 through 2092   37,498    
Total operating leases   44,363    
Office Building [Member]        
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]        
2018   184    
2019   188    
2020   112    
2021   0    
2022   0    
2023 through 2092   0    
Total operating leases   484    
Office Building [Member] | Greenwood Village, Colorado        
Other Commitments [Line Items]        
Operating lease rent expense   $ 200 $ 100 $ 100
Office Lease renewal additional term 5 years