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
Feb. 27, 2019
Mar. 31, 2022
Mar. 31, 2021
Dec. 31, 2021
May 19, 2021
Class of Stock [Line Items]          
Common shares of beneficial interest, par value (in dollars per share)   $ 0.01   $ 0.01  
Preferred units stated value (in dollars per share)   $ 0.01   $ 0.01  
Value of OP equity   $ 16,576      
Common Shares          
Class of Stock [Line Items]          
Issuance of series A-1 preferred units (in shares)     0    
Redemptions/conversions of units (in shares)   258,477 190,248    
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      
Issuance of series A-1 preferred units (in shares)   353,030      
Redemptions/conversions of units (in shares)   8,216      
OP units | NSA OP, LP          
Class of Stock [Line Items]          
Unit conversion ratio   1      
Minimum conversion period (in years)   1 year      
Issuance of shares/units (in shares)   3,911,260      
OP units | NSA OP, LP | Common Shares          
Class of Stock [Line Items]          
Redemptions/conversions of units (in shares)   258,477      
OP units | NSA OP, LP | Self Storage Properties          
Class of Stock [Line Items]          
Issuance of shares/units (in shares)   92,567      
OP units | NSA OP, LP | Northwest's Right to Property Management Contracts, Brand, Intellectual Property, Certain Tangible Assets          
Class of Stock [Line Items]          
Number of shares issued   46,540      
Conversion of units (in shares)   651,734      
OP units | NSA OP, LP | Northwest Retirement          
Class of Stock [Line Items]          
Issuance of shares/units (in shares)   235,241      
OP units | NSA OP, LP | Retirement of Northwest          
Class of Stock [Line Items]          
Issuance of shares/units (in shares)   3,911,260      
Conversion of units (in shares)   651,234      
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]          
Time frame prior to conversion for conversion metric (in years)   1 year      
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)   2,078,357      
Issuance of shares/units, non-voluntary (in shares)   2,078,357      
Unit conversion, lock out period (in years)   2 years      
Subordinated performance units | NSA OP, LP | Northwest's Right to Property Management Contracts, Brand, Intellectual Property, Certain Tangible Assets          
Class of Stock [Line Items]          
Issuance of shares/units (in shares)   244,792      
Subordinated performance units | NSA OP, LP | Northwest Retirement          
Class of Stock [Line Items]          
Redemptions/conversions of units (in shares)   82,611      
Subordinated performance units | NSA OP, LP | Retirement of Northwest          
Class of Stock [Line Items]          
Redemptions/conversions of units (in shares)   244,792      
Issuance of shares/units (in shares)   15,061      
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      
Issuance of shares/units (in shares)   111,199      
Conversion of units (in shares)   117,216      
At The Market Program          
Class of Stock [Line Items]          
Value of common and preferred shares authorized under ATM program         $ 400,000
Value of remaining available offered shares   $ 169,100     $ 31,000
Prior Sales Agreement          
Class of Stock [Line Items]          
Value of common and preferred shares authorized under ATM program $ 250,000        
Preferred share/unit dividend rate (percent) 6.00%        
Series A preferred shares | NSA OP, LP          
Class of Stock [Line Items]          
Preferred units cash redemption price (in dollars per share)   $ 25.00