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 ($)
3 Months Ended 6 Months Ended
Feb. 27, 2019
Jun. 30, 2020
Mar. 31, 2020
Jun. 30, 2019
Mar. 31, 2019
Jun. 30, 2020
Jun. 30, 2019
Dec. 31, 2019
Oct. 11, 2017
Class of Stock [Line Items]                  
Value of common and preferred shares authorized under ATM program $ 250,000,000.0                
Common shares of beneficial interest, par value (in dollars per share) $ 0.01 $ 0.01       $ 0.01   $ 0.01  
Preferred share/unit dividend rate (percent) 6.00%                
Proceeds from issuance of common shares           $ 16,165,000 $ 70,637,000    
Preferred units stated value (in dollars per share)   $ 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)   292,291 118,961 224,618 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%      
David Cramer | SecurCare                  
Class of Stock [Line Items]                  
Related party ownership interest   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      
Discount rate on conversion   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      
SecurCare and DLAN | David Cramer                  
Class of Stock [Line Items]                  
Number common shares issued upon conversion (in shares)   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      
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      
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)           411,252      
OP units | NSA OP, LP | Self Storage Properties                  
Class of Stock [Line Items]                  
Issuance of shares/units (in shares)           356,392      
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)           35,735      
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)   387,000 125,000            
Common shares | At the Market Program                  
Class of Stock [Line Items]                  
Issuance of shares/units (in shares)           512,000      
Average offering price (in dollars per share)   $ 32.41       $ 32.41      
Proceeds from issuance of common shares           $ 16,200,000      
Series A preferred shares | NSA OP, LP                  
Class of Stock [Line Items]                  
Preferred units cash redemption price (in dollars per share)                 $ 25.00