Annual report pursuant to Section 13 and 15(d)

SHAREHOLDERS' EQUITY AND NONCONTROLLING INTERESTS - Narrative (Details)

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SHAREHOLDERS' EQUITY AND NONCONTROLLING INTERESTS - Narrative (Details) - USD ($)
 12 Months Ended
Feb. 27, 2019
Jul. 13, 2018
Dec. 11, 2017
Oct. 11, 2017
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Class of Stock [Line Items]              
Value of common and preferred shares authorized under ATM program $ 250,000,000.0            
Preferred shares dividend rate (percent) 6.00%            
Net proceeds from issuance of common shares   $ 175,600,000 $ 140,300,000   $ 70,637,000 $ 175,616,000 $ 140,261,000
Net proceeds from issuance of preferred shares         $ 43,624,000 $ 0 $ 166,566,000
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 shares dividend rate (percent)         6.00%    
Preferred units stated value (in dollars per share)         $ 25.00    
Unit conversion ratio         1    
Minimum conversion period (in years)         10 years    
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)         581,001    
OP units | NSA OP, LP | Acquisition of interest in reinsurance company              
Class of Stock [Line Items]              
Issuance of shares/units (in shares)         285,512    
OP units | NSA OP, LP | Self Storage Properties              
Class of Stock [Line Items]              
Issuance of shares/units (in shares)         350,319    
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 CAD per unit used in conversion rate calculation         110.00%    
Subordinated performance units | NSA OP, LP              
Class of Stock [Line Items]              
Minimum conversion period (in years)         2 years    
Redemptions/conversions of units (in shares)         913,680    
Number of units converted (in units)         332,738    
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)         1,178,400    
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    
Number of units converted (in units)         396,224    
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    
Net proceeds from issuance of common shares         $ 70,600,000    
Common Shares | Underwriter option to purchase              
Class of Stock [Line Items]              
Issuance of shares/units (in shares)     750,000        
Common Shares | Follow-on offering              
Class of Stock [Line Items]              
Issuance of shares/units (in shares)   5,900,000 5,750,000        
Average offering price (in dollars per share)   $ 29.86 $ 25.50        
Series A preferred shares              
Class of Stock [Line Items]              
Preferred shares dividend rate (percent)       6.00%      
Issuance of shares/units (in shares)       6,900,000      
Net proceeds from issuance of preferred shares       $ 166,600,000      
Preferred shares per share amount of dividends (in dollars per share)       $ 1.50      
Series A preferred shares | NSA OP, LP              
Class of Stock [Line Items]              
Preferred shares cash redemption price (in dollars per share)       $ 25.00 $ 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    
Net proceeds from issuance of preferred shares         $ 43,600,000    
Series A preferred shares | Underwriter option to purchase              
Class of Stock [Line Items]              
Issuance of shares/units (in shares)       900,000