Quarterly report pursuant to Section 13 or 15(d)

SHAREHOLDERS' EQUITY AND NONCONTROLLING INTERESTS - Narrative (Details)

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SHAREHOLDERS' EQUITY AND NONCONTROLLING INTERESTS - Narrative (Details) - USD ($)
1 Months Ended 3 Months Ended 9 Months Ended
Oct. 06, 2020
Sep. 30, 2020
Sep. 22, 2020
Feb. 27, 2019
Oct. 06, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
Class of Stock [Line Items]                            
Common shares of beneficial interest, par value (in dollars per share)   $ 0.01   $ 0.01   $ 0.01           $ 0.01   $ 0.01
Value of common and preferred shares authorized under ATM program       $ 250,000,000.0                    
Preferred share/unit dividend rate (percent)       6.00%                    
Proceeds from issuance of common shares                       $ 22,033,000 $ 70,637,000  
Preferred units stated value (in dollars per share)   $ 0.01       $ 0.01           $ 0.01   0.01
Common Shares                            
Class of Stock [Line Items]                            
Issuance of shares/units (in shares)                   2,375,000        
Redemptions/conversions of units (in shares)             292,291 118,961 32,951 224,618 29,910      
Contribution of units arising from the mergers (in shares)               8,105,192            
Arlen Nordhagen | SecurCare                            
Class of Stock [Line Items]                            
Related party ownership interest   53.00%       53.00%           53.00%    
David Cramer | SecurCare                            
Class of Stock [Line Items]                            
Related party ownership interest   24.00%       24.00%           24.00%    
SecurCare and DLAN                            
Class of Stock [Line Items]                            
Number common shares issued upon conversion (in shares)   8,105,192       8,105,192           8,105,192    
Discount rate on conversion   1.00%       1.00%           1.00%    
SecurCare and DLAN | Arlen Nordhagen                            
Class of Stock [Line Items]                            
Number common shares issued upon conversion (in shares)   4,063,571       4,063,571           4,063,571    
SecurCare and DLAN | David Cramer                            
Class of Stock [Line Items]                            
Number common shares issued upon conversion (in shares)   1,858,737       1,858,737           1,858,737    
Series A-1 Preferred Units | NSA OP, LP                            
Class of Stock [Line Items]                            
Preferred share/unit dividend rate (percent)                       6.00%    
Preferred units stated value (in dollars per share)   $ 25.00       $ 25.00           $ 25.00    
Unit conversion ratio                       1    
Redemptions/conversions of units (in shares)           5,600                
OP units | SecurCare and DLAN                            
Class of Stock [Line Items]                            
Number of units to own/receive (in shares)   8,187,052       8,187,052           8,187,052    
OP units | NSA OP, LP                            
Class of Stock [Line Items]                            
Issuance of shares/units (in shares)                       445,701    
Unit conversion ratio                       1    
Minimum conversion period (in years)                       1 year    
Conversion of units (in shares)                       368,500    
Units retired during the period (in shares)                       710,462    
OP units | NSA OP, LP | Common Shares                            
Class of Stock [Line Items]                            
Redemptions/conversions of units (in shares)                       704,335    
OP units | NSA OP, LP | Self Storage Properties                            
Class of Stock [Line Items]                            
Issuance of shares/units (in shares)                       378,239    
OP units | DownREIT Partnership                            
Class of Stock [Line Items]                            
Issuance of shares/units (in shares)                       115,888    
Unit conversion ratio                       1    
Minimum conversion period (in years)                       5 years    
Conversion of units (in shares)                       368,500    
Contribution of units arising from the mergers (in shares)                       407,731    
Subordinated performance units                            
Class of Stock [Line Items]                            
Time frame prior to conversion for conversion metric (in years)                       1 year    
Percentage of cash available for distribution used in conversion ratio calculation                       110.00%    
Subordinated performance units | NSA OP, LP                            
Class of Stock [Line Items]                            
Redemptions/conversions of units (in shares)                       332,738    
Unit conversion, lock out period (in years)                       2 years    
Units retired during the period (in shares)                       2,001,441    
Subordinated performance units | NSA OP, LP | Self Storage Properties                            
Class of Stock [Line Items]                            
Issuance of shares/units (in shares)                       90,728    
Subordinated performance units | DownREIT Partnership                            
Class of Stock [Line Items]                            
Unit conversion ratio                       1    
Minimum conversion period (in years)                       5 years    
Redemptions/conversions of units (in shares)                       34,511    
LTIP units | NSA OP, LP                            
Class of Stock [Line Items]                            
Issuance of shares/units (in shares)                       236,786    
Unit conversion ratio                       1    
Conversion of units (in shares)                       214,512    
Common shares | Common Shares                            
Class of Stock [Line Items]                            
Issuance of shares/units (in shares)           181,915 387,000 125,000            
Redemptions/conversions of units (in shares)           293,083                
Common shares | At the Market Program                            
Class of Stock [Line Items]                            
Issuance of shares/units (in shares)                       693,915    
Average offering price (in dollars per share)   $ 32.92       $ 32.92           $ 32.92    
Proceeds from issuance of common shares                       $ 22,000,000.0    
Common shares | Public Stock Offering                            
Class of Stock [Line Items]                            
Sale of stock, number of shares issued in transaction (in shares)     4,500,000                      
Sale of stock, price per share (in dollars per share)     $ 33.15                      
Common shares | Public Stock Offering | Subsequent Event                            
Class of Stock [Line Items]                            
Sale of stock, additional number of shares issued in transaction (in shares) 400,000                          
Common shares | Public Stock Offering - From Third Parties                            
Class of Stock [Line Items]                            
Sale of stock, price per share (in dollars per share)   $ 32.832       $ 32.832           $ 32.832    
Sale of stock, consideration received on transaction   $ 160,000,000                        
Common shares | Public Stock Offering - From Third Parties | Subsequent Event                            
Class of Stock [Line Items]                            
Sale of stock, number of shares issued in transaction (in shares)         4,900,000                  
Sale of stock, price per share (in dollars per share) $ 33.15       $ 33.15                  
Sale of stock, consideration received on transaction         $ 162,400,000                  
Series A preferred shares | NSA OP, LP                            
Class of Stock [Line Items]                            
Preferred units cash redemption price (in dollars per share)                           $ 25.00
Over-Allotment Option | Public Stock Offering                            
Class of Stock [Line Items]                            
Sale of stock, number of shares issued in transaction (in shares)     675,000