Annual report pursuant to Section 13 and 15(d)

SHAREHOLDERS' EQUITY AND NONCONTROLLING INTERESTS - Narrative (Details)

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SHAREHOLDERS' EQUITY AND NONCONTROLLING INTERESTS - Narrative (Details) - USD ($)
1 Months Ended 12 Months Ended
Oct. 06, 2020
Sep. 22, 2020
Feb. 27, 2019
Oct. 11, 2017
Oct. 06, 2020
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Mar. 31, 2020
Dec. 31, 2017
Class of Stock [Line Items]                    
Common shares of beneficial interest, outstanding (in shares)           71,293,117 59,659,108      
Preferred shares dividend rate (percent)     6.00%              
Value of common and preferred shares authorized under ATM program     $ 250,000,000.0              
Net proceeds from issuance of common shares           $ 82,917,000 $ 70,637,000 $ 175,616,000    
Net proceeds from issuance of preferred shares           $ 0 $ 43,624,000 $ 0    
Preferred units stated value (in dollars per share)           $ 0.01 $ 0.01      
Common Shares                    
Class of Stock [Line Items]                    
Common shares of beneficial interest, outstanding (in shares)           71,293,117 59,659,108 56,654,009   50,284,934
Issuance of shares/units (in shares)             2,412,770 5,900,000    
Redemptions/conversions of units (in shares)           892,070 581,001 462,778    
Merger and internalization of PRO, net of issuance costs (in shares)           8,105,192        
Chief Operating Officer | SecurCare                    
Class of Stock [Line Items]                    
Related party ownership interest (percent)                 24.00%  
Executive chairman and former chief executive officer | SecurCare                    
Class of Stock [Line Items]                    
Related party ownership interest (percent)                 53.00%  
SecurCare and DLAN                    
Class of Stock [Line Items]                    
Common shares expected to be issued upon conversion of units (in shares)                 8,105,192  
Discount on unit to common share conversion (percent)                 1.00%  
SecurCare and DLAN | Chief Operating Officer                    
Class of Stock [Line Items]                    
Number of common share issued upon merger and internalization                 1,858,737  
SecurCare and DLAN | Executive chairman and former chief executive officer                    
Class of Stock [Line Items]                    
Common shares expected to be issued upon conversion of units (in shares)                 4,063,571  
OP units | SecurCare and DLAN                    
Class of Stock [Line Items]                    
Units expected to be owned or have right to receive (in units)                 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        
Number of units converted (in units)           368,500        
Shares retired during period           710,462        
OP units | NSA OP, LP | Common Shares                    
Class of Stock [Line Items]                    
Redemptions/conversions of units (in shares)           892,070        
OP units | NSA OP, LP | Self Storage Properties                    
Class of Stock [Line Items]                    
Issuance of shares/units (in shares)           755,007        
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        
Number of units converted (in units)           368,500        
Merger and internalization of PRO, net of issuance costs (in shares)           407,731        
Series A-1 preferred units | NSA OP, LP                    
Class of Stock [Line Items]                    
Preferred shares dividend rate (percent)           6.00%        
Preferred units stated value (in dollars per share)           $ 25.00        
Unit conversion ratio           1        
Minimum conversion period (in years)           10 years        
Redemptions/conversions of units (in shares)           5,600        
Subordinated performance units                    
Class of Stock [Line Items]                    
Percentage of CAD per unit used in conversion rate calculation           110.00%        
Subordinated performance units | NSA OP, LP                    
Class of Stock [Line Items]                    
Minimum conversion period (in years)           2 years        
Redemptions/conversions of units (in shares)           332,738        
Number of units converted (in units)           47,328        
Shares retired during period           2,001,441        
Unit conversion, lock out period (in years)           2 years        
Subordinated performance units | NSA OP, LP | Self Storage Properties                    
Class of Stock [Line Items]                    
Issuance of shares/units (in shares)           303,528        
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        
Number of units converted (in units)           246,156        
Common Shares | Common Shares                    
Class of Stock [Line Items]                    
Issuance of shares/units (in shares)           2,622,892        
Common Shares | Public Stock Offering                    
Class of Stock [Line Items]                    
Number of shares issued in transaction   4,500,000                
Sale of stock, price per share (USD per share)   $ 33.15                
Additional number of shares issued in transaction 400,000                  
Common Shares | Public Stock Offering - From Third Parties                    
Class of Stock [Line Items]                    
Number of shares issued in transaction         4,900,000 1,850,510        
Sale of stock, price per share (USD per share) $ 33.15       $ 33.15          
Consideration received on transaction         $ 162,400,000 $ 60,000,000.0        
Common shares of beneficial interest, outstanding (in shares)           3,049,490        
Common Shares | At the Market Program                    
Class of Stock [Line Items]                    
Issuance of shares/units (in shares)           743,915 2,375,000      
Average offering price (in dollars per share)           $ 33.01 $ 30.06      
Net proceeds from issuance of common shares           $ 22,900,000 $ 70,600,000      
Underwriter option to purchase | Public Stock Offering                    
Class of Stock [Line Items]                    
Number of shares issued in transaction   675,000                
Series A preferred shares                    
Class of Stock [Line Items]                    
Preferred shares dividend rate (percent)       6.00%            
Preferred shares per share amount of dividends (in dollars per share)       $ 1.50            
Series A preferred shares | NSA OP, LP                    
Class of Stock [Line Items]                    
Preferred shares cash redemption price (in dollars per share)       $ 25.00   $ 25.00        
Series A preferred shares | At the Market Program                    
Class of Stock [Line Items]                    
Issuance of shares/units (in shares)             1,785,680      
Average offering price (in dollars per share)             $ 24.84      
Net proceeds from issuance of preferred shares             $ 43,600,000