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
Feb. 27, 2019
Mar. 31, 2020
Mar. 31, 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  
Preferred share/unit dividend rate (percent) 6.00%        
Proceeds from issuance of common shares   $ 4,249,000 $ 0    
Preferred units stated value (in dollars per share)   $ 0.01   $ 0.01  
Common Shares          
Class of Stock [Line Items]          
Issuance of shares/units (in shares)   125,000      
Redemptions/conversions of units (in shares)   118,961 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%      
David Cramer | SecurCare          
Class of Stock [Line Items]          
Related party ownership interest   24.00%      
SecurCare and DLAN          
Class of Stock [Line Items]          
Number common shares issued upon conversion (in shares)   8,105,192      
Discount rate on conversion   1.00%      
SecurCare and DLAN | Arlen Nordhagen          
Class of Stock [Line Items]          
Number common shares issued upon conversion (in shares)   4,063,571      
SecurCare and DLAN | David Cramer          
Class of Stock [Line Items]          
Number common shares issued upon conversion (in shares)   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      
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      
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)   118,961      
OP units | NSA OP, LP | Self Storage Properties          
Class of Stock [Line Items]          
Issuance of shares/units (in shares)   181,981      
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]          
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)   13,105      
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)   154,806      
Unit conversion ratio   1      
Conversion of units (in shares)   133,637      
Common shares | At the Market Program          
Class of Stock [Line Items]          
Issuance of shares/units (in shares)   125,000      
Average offering price (in dollars per share)   $ 36.18      
Proceeds from issuance of common shares   $ 4,200,000      
Series A preferred shares | NSA OP, LP          
Class of Stock [Line Items]          
Preferred units cash redemption price (in dollars per share)         $ 25.00