Quarterly report pursuant to Section 13 or 15(d)

SHAREHOLDERS' EQUITY AND NONCONTROLLING INTERESTS - Narrative (Details)

v3.10.0.1
SHAREHOLDERS' EQUITY AND NONCONTROLLING INTERESTS - Narrative (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Class of Stock [Line Items]        
Proceeds from issuance of common shares $ 175,700 $ 175,691 $ 0  
Series A-1 preferred units stated value (in dollars per share) $ 0.01 $ 0.01   $ 0.01
Common shares of beneficial interest, par value (in dollars per share) 0.01 $ 0.01   $ 0.01
Series A-1 Preferred Units        
Class of Stock [Line Items]        
Series A-1 preferred units dividend rate (percent)   6.00%    
Series A-1 preferred units stated value (in dollars per share) $ 25 $ 25    
Series A-1 Preferred Units | NSA OP, LP        
Class of Stock [Line Items]        
Unit conversion ratio   1    
OP units | NSA OP, LP        
Class of Stock [Line Items]        
Issuance of common shares/units (in shares)   2,024,170    
Unit conversion ratio   1    
Minimum conversion period (in years)   1 year    
Redemptions/conversions of units (in shares)   364,348    
OP units | NSA OP, LP | Self Storage Properties        
Class of Stock [Line Items]        
Issuance of common shares/units (in shares)   520,455    
OP units | DownREIT Partnership        
Class of Stock [Line Items]        
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)   997,074    
Unit conversion, lock out period   2 years    
Subordinated performance units | NSA OP, LP | Self Storage Properties        
Class of Stock [Line Items]        
Issuance of common shares/units (in shares)   102,110    
Subordinated performance units | DownREIT Partnership        
Class of Stock [Line Items]        
Unit conversion ratio   1    
Minimum conversion period (in years)   5 years    
LTIP units | NSA OP, LP        
Class of Stock [Line Items]        
Unit conversion ratio   1    
Conversion of LTIP units into OP units (in shares)   9,100    
Common Shares [Member] | Follow On Offering [Member]        
Class of Stock [Line Items]        
Issuance of common shares/units (in shares) 5,900,000      
Share price (in dollars per share) $ 29.86 $ 29.86