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 9 Months Ended
Feb. 27, 2019
Sep. 30, 2022
Jun. 30, 2022
Mar. 31, 2022
Sep. 30, 2021
Jun. 30, 2021
Mar. 31, 2021
Sep. 30, 2022
Jul. 11, 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   $ 0.01  
Value of remaining available offered shares                     $ 31.0
Value of shares authorized to be repurchased                 $ 400.0    
Repurchase of common shares (in shares)   (953,924)                  
Repurchase of common shares   $ 50.0                  
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)     294,573 258,477 340,349 121,923 190,248        
Issuance of shares/units (in shares)           2,390,000 3,692,216        
NSA OP, LP | Series A preferred shares                      
Class of Stock [Line Items]                      
Preferred units cash redemption price (in dollars per share)   25.00           $ 25.00      
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)               614,712      
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 | Self Storage Properties                      
Class of Stock [Line Items]                      
Issuance of shares/units (in shares)               458,958      
OP units | NSA OP, LP | Northwest's Right to Property Management Contracts, Brand, Intellectual Property, Certain Tangible Assets                      
Class of Stock [Line Items]                      
Conversion of units (in shares)               800,556      
Number of shares issued (in shares)               46,540      
OP units | NSA OP, LP | Retirement of Northwest                      
Class of Stock [Line Items]                      
Issuance of shares/units (in shares)               3,911,260      
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 Retirement                      
Class of Stock [Line Items]                      
Redemptions/conversions of units (in shares)               82,611      
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)               393,614      
Subordinated performance units | NSA OP, LP | Retirement of Northwest                      
Class of Stock [Line Items]                      
Issuance of shares/units (in shares)               45,555      
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)               145,739      
Conversion of units (in shares)               192,296      
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%                    
At The Market Program                      
Class of Stock [Line Items]                      
Value of remaining available offered shares   $ 169.1           $ 169.1