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 Millions
3 Months Ended 6 Months Ended
Feb. 27, 2019
Jun. 30, 2022
Mar. 31, 2022
Jun. 30, 2021
Mar. 31, 2021
Jun. 30, 2022
Dec. 31, 2021
May 19, 2021
Class of Stock [Line Items]                
Value of common and preferred shares authorized under ATM program               $ 400.0
Common shares of beneficial interest, par value (in dollars per share)   $ 0.01       $ 0.01 $ 0.01  
Value of remaining available offered shares               $ 31.0
Preferred units stated value (in dollars per share)   $ 0.01       $ 0.01 $ 0.01  
Common Shares                
Class of Stock [Line Items]                
Redemptions/conversions of units (in shares)   294,573 258,477 121,923 190,248      
Issuance of shares/units (in shares)       2,390,000 3,692,216      
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    
Issuance of series A-1 preferred units (in shares)           353,030    
Redemptions/conversions of units (in shares)           280,869    
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)           553,050    
OP units | NSA OP, LP | Self Storage Properties                
Class of Stock [Line Items]                
Issuance of shares/units (in shares)           387,638    
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)           141,918    
Conversion of units (in shares)           192,296    
At The Market Program                
Class of Stock [Line Items]                
Value of remaining available offered shares   $ 169.1       $ 169.1    
Prior Sales Agreement                
Class of Stock [Line Items]                
Value of common and preferred shares authorized under ATM program $ 250.0              
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       $ 25.00