INVESTMENT ACTIVITY (Details)
$ / shares in Units, $ in Thousands
 3 Months Ended 9 Months Ended 12 Months Ended 18 Months Ended 21 Months Ended
Jun. 02, 2017
USD ($)
Apr. 12, 2017
USD ($)
shares
Aug. 12, 2016
USD ($)
Apr. 18, 2016
USD ($)
shares
Apr. 05, 2016
USD ($)
May 14, 2015
USD ($)
shares
Sep. 30, 2017
USD ($)
$ / shares
Dec. 31, 2016
USD ($)
Dec. 31, 2015
Sep. 30, 2017
USD ($)
property
facility
$ / shares
shares
Dec. 31, 2016
USD ($)
property
item
facility
Jun. 30, 2017
USD ($)
Sep. 30, 2017
USD ($)
facility
$ / shares
Self-storage facilities                          
Storage facilities             $ 4,086,960 $ 3,998,180   $ 4,086,960 $ 3,998,180   $ 4,086,960
Restricted cash             4,189 7,893   4,189 $ 7,893   $ 4,189
OP units issued (in shares) | shares       61,224   20,408              
Issuance of OP Shares       $ 1,500   $ 500              
Acquisition of noncontrolling interest $ 8,600                 9,033      
Self storage in operation                          
Self-storage facilities                          
Number of self-storage facilities | facility                         5
Total Construction Costs                       $ 182,700  
Self storage under construction | PSI                          
Self-storage facilities                          
Development costs             145,600     145,600     $ 145,600
Expected construction cost                   $ 282,400      
Number of facilities under contract | facility                   7      
Self-storage facilities located in New York                          
Self-storage facilities                          
Ownership interest held by the entity (as a percent)                 90.00%   51.00%    
Number of joint ventures | item                     2    
Minority ownership interest (as a percent) 0.00%                        
Acquisition of remaining interest in real estate ventures (as a percent) 10.00%                        
Total consideration $ 9,000                        
Repayments of debt $ 9,800                        
Self-storage Facility in Washington DC                          
Self-storage facilities                          
Number of self-storage facilities | facility                         1
Ownership interest held by the entity (as a percent)                         100.00%
Total Construction Costs                       27,800  
Self-storage Facility In North Palm Beach, FL                          
Self-storage facilities                          
Number of self-storage facilities | facility                         1
Ownership interest held by the entity (as a percent)                         100.00%
Total Construction Costs                       9,700  
Self-storage Facility in Queens, NY                          
Self-storage facilities                          
Number of self-storage facilities | facility                         1
Ownership interest held by the entity (as a percent)                         100.00%
Total Construction Costs                       31,800  
Minority ownership interest (as a percent)         49.00%                
Amount of net proceeds applied to acquisitions closed during the year         $ 12,500                
Self-storage Facility in Brooklyn, NY                          
Self-storage facilities                          
Number of self-storage facilities | facility                         1
Ownership interest held by the entity (as a percent)                         90.00%
Total Construction Costs                       81,200  
Self-storage Facility in Bronx, NY                          
Self-storage facilities                          
Number of self-storage facilities | facility                         1
Ownership interest held by the entity (as a percent)                         100.00%
Total Construction Costs                       $ 32,200  
Minority ownership interest (as a percent)     49.00%                    
Amount of net proceeds applied to acquisitions closed during the year     $ 17,000                    
2017 Acquisitions                          
Self-storage facilities                          
Number of self-storage facilities acquired | facility                   3      
Aggregate purchase price                   $ 33,050      
Summary of the amounts of revenue and earnings of the 2016 and 2015 acquisitions since the acquisition dates                          
Total revenue             510     785      
Net loss             (362)     (701)      
2017 Acquisitions | Self-storage facilities located in US                          
Self-storage facilities                          
Intangible value of the in-place leases             2,000     $ 2,000     $ 2,000
Estimated life                   12 months      
Amortization expense                   $ 800      
Number of properties, assumed mortgage | facility                   1      
Units issued                   $ 12,300      
Assumed mortgage debt, at fair value             6,200     6,200     6,200
Outstanding principal balance of mortgage debt assumed on acquisitions             5,900     5,900     5,900
Premium on debt assumed on acquisitions             300     300     300
2016 Acquisitions                          
Self-storage facilities                          
Number of self-storage facilities acquired | facility                     28    
Aggregate purchase price                     $ 403,550    
2016 Acquisitions | Self storage under construction                          
Self-storage facilities                          
Number of facilities under contract | facility                     3    
2016 Acquisitions | Self-storage facilities located in US                          
Self-storage facilities                          
Number of self-storage facilities | facility                     28    
Aggregate purchase price                     $ 403,600    
Intangible value of the in-place leases               18,800     $ 18,800    
Estimated life                     12 months    
Amortization expense                   7,900      
Number of properties, assumed mortgage | facility                     1    
Assumed mortgage debt, at fair value               6,500     $ 6,500    
Outstanding principal balance of mortgage debt assumed on acquisitions               6,300     6,300    
Premium on debt assumed on acquisitions               200     $ 200    
Real Estate Acquisition Expected in 2018                          
Self-storage facilities                          
Deposit             1,800     $ 1,800     1,800
Number of facilities under contract | facility                   3      
Expected aggregate purchase or sales price             49,900     $ 49,900     49,900
Real Estate Acquisition Expected in 2018 | Self storage under construction                          
Self-storage facilities                          
Number of self-storage facilities | facility                   3      
Self-storage facilities                          
Self-storage facilities                          
Number of self-storage facilities acquired | property                   3 28    
Aggregate purchase price               $ 403,600   $ 33,100      
Self-storage facilities | Self-storage facilities located in US                          
Self-storage facilities                          
Number of self-storage facilities acquired | facility                   2      
Aggregate purchase price                   $ 21,900      
Self-storage facilities | 2017 Acquisitions                          
Self-storage facilities                          
Maximum number of months to finalize purchase price allocation                   12 months      
Business Combination, Measurement Period Adjustments Recorded                   $ 0      
Measurement period adjustments                   $ 0      
New York | Self storage under construction | PSI                          
Self-storage facilities                          
Number of facilities under contract | facility                   5      
Massachusetts | Self storage under construction | PSI                          
Self-storage facilities                          
Number of facilities under contract | facility                   1      
New Jersey | Self storage under construction | PSI                          
Self-storage facilities                          
Number of facilities under contract | facility                   1      
Illinois                          
Self-storage facilities                          
Cash paid for acquisition   $ 9,700               $ 9,700      
Contract amount to purchase real estate             $ 11,200     11,200     $ 11,200
Units issued (in shares) | shares   58,000                      
Total consideration   $ 11,200                      
Cash paid for acquisition of remaining interest in real estate ventures   $ 9,700               $ 9,700      
Class C units | Illinois                          
Self-storage facilities                          
Units issued (in shares) | shares                   58,400      
Stated value (in dollars per share) | $ / shares             $ 25     $ 25     $ 25
Distribution rate (as a percent)             3.00%     3.00%     3.00%
Redemption period, holder                   6 months      
Redemption period, issuer                   12 months      
Redemption price (in dollars per share) | $ / shares             $ 25     $ 25     $ 25