Annual report pursuant to Section 13 and 15(d)

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue Recognition (Details)

v3.10.0.1
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue Recognition (Details)
 12 Months Ended
Dec. 31, 2018
USD ($)
property
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
property
Disaggregation of Revenue [Line Items]      
Number of self storage properties | property 675    
Initial Term Of Joint Venture Agreement 4 years    
Revenue, Performance Obligation, Recognition Period 4 years    
Joint Venture [Member] | 2016 Joint Venture [Member]      
Disaggregation of Revenue [Line Items]      
Number of self storage properties | property     66
Tenant Insurance And Protection Plan Fees And Commissions [Member]      
Disaggregation of Revenue [Line Items]      
Revenue $ 7,500,000 $ 6,000,000 $ 4,200,000
Retail Products And Supplies [Member]      
Disaggregation of Revenue [Line Items]      
Revenue 1,500,000 1,300,000 1,000,000
Property Management, Call Center, And Platform Fees [Member]      
Disaggregation of Revenue [Line Items]      
Revenue $ 7,800,000 4,800,000 1,100,000
Property Management Fees, Percent Of Monthly Gross Revenues And Net Sales Revenue Of Joint Venture Assets 6.00%    
Platform Fees, Per Joint Venture Property, Per Month $ 1,250    
Call Center Fees, Percent Of Monthly Gross Revenues And Net Sales Revenue Of Joint Venture Assets 1.00%    
Acquisition Fees [Member]      
Disaggregation of Revenue [Line Items]      
Revenue $ 1,600,000 1,500,000 300,000
Proceeds from Fees Received 4,000,000   $ 4,100,000
Fees Received, Percent Of Gross Capitalization Of Joint Venture     0.65%
Contract with Customer, Liability 4,600,000 2,800,000  
Tenant Warranty Protection [Member]      
Disaggregation of Revenue [Line Items]      
Revenue $ 2,400,000 $ 1,900,000 $ 500,000
Tenant Warranty Plan, Proceeds Per Property, Percent 50.00%