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
 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Class of Stock [Line Items]      
Common and preferred shares, shares authorized under ATM program $ 250,000    
Common shares of beneficial interest, par value (in dollars per share) $ 0.01   $ 0.01
Preferred share/unit dividend rate (percent) 6.00%    
Proceeds from issuance of common shares $ 70,637 $ 0  
Proceeds from issuance of preferred shares $ 43,624 $ 0  
Series A-1 preferred units stated value (in dollars per share) $ 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%    
Series A-1 preferred units stated value (in dollars per share) $ 25.00    
Unit conversion ratio 1    
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) 254,528    
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) 1,096,840    
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) 204,213    
Unit conversion ratio 1    
Conversion of LTIP units into OP units (in shares) 379,617    
Par value of common shares of beneficial interest (in dollars per share) | At the Market Program [Member]      
Class of Stock [Line Items]      
Issuance of shares/units (in shares) 2,375,000    
Average offering price (in dollars per share) $ 30.06    
Proceeds from issuance of common shares $ 70,600    
Series A preferred shares | At the Market Program [Member]      
Class of Stock [Line Items]      
Issuance of shares/units (in shares) 1,785,680    
Average offering price (in dollars per share) $ 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