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 ($)
$ / shares in Units, $ in Thousands
3 Months Ended 9 Months Ended
Feb. 27, 2019
Sep. 30, 2019
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Oct. 11, 2017
Class of Stock [Line Items]            
Value of common and preferred shares authorized under ATM program $ 250,000          
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     $ 70,637 $ 175,691    
Proceeds from issuance of preferred shares     $ 43,624 $ 0    
Preferred units stated value (in dollars per share)   0.01 $ 0.01   $ 0.01  
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)   41,439        
OP units | NSA OP, LP            
Class of Stock [Line Items]            
Issuance of shares/units (in shares)     863,148      
Unit conversion ratio     1      
Minimum conversion period (in years)     1 year      
Redemptions/conversions of units (in shares)     287,479      
OP units | NSA OP, LP | Self Storage Properties            
Class of Stock [Line Items]            
Issuance of shares/units (in shares)     343,657      
OP units | DownREIT Partnership            
Class of Stock [Line Items]            
Issuance of shares/units (in shares)     13,475      
Unit conversion ratio     1      
Minimum conversion period (in years)     5 years      
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)     913,680      
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)     1,125,351      
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)     15,377      
LTIP units | NSA OP, LP            
Class of Stock [Line Items]            
Issuance of shares/units (in shares)     208,119      
Unit conversion ratio     1      
Conversion of LTIP units into OP units (in shares)     379,617      
Common shares | At the Market Program            
Class of Stock [Line Items]            
Issuance of shares/units (in shares)     2,375,000      
Average offering price (in dollars per share)   $ 30.06 $ 30.06      
Proceeds from issuance of common shares     $ 70,600      
Series A preferred shares | NSA OP, LP            
Class of Stock [Line Items]            
Preferred units cash redemption price (in dollars per share)           $ 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 $ 24.84      
Proceeds from issuance of preferred shares     $ 43,600      
Acquisition of Interest in Reinsurance Company | OP units | NSA OP, LP            
Class of Stock [Line Items]            
Issuance of shares/units (in shares)     285,512