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 ($)
1 Months Ended 3 Months Ended 6 Months Ended 27 Months Ended
Jul. 23, 2021
Mar. 22, 2021
Dec. 30, 2020
Oct. 06, 2020
Sep. 22, 2020
Feb. 27, 2019
Aug. 04, 2021
Oct. 06, 2020
Jun. 30, 2021
Mar. 31, 2021
Jun. 30, 2020
Mar. 31, 2020
Jun. 30, 2021
May 19, 2021
Dec. 31, 2020
Class of Stock [Line Items]                              
Common shares of beneficial interest, par value (in dollars per share)         $ 0.01       $ 0.01       $ 0.01   $ 0.01
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)                 121,923 190,248 292,291 118,961      
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    
Redemptions/conversions of units (in shares)                         4,000    
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)                         63,033    
OP units | NSA OP, LP | Common Shares                              
Class of Stock [Line Items]                              
Redemptions/conversions of units (in shares)                         312,171    
OP units | NSA OP, LP | Self Storage Properties                              
Class of Stock [Line Items]                              
Issuance of shares/units (in shares)                         559,463    
OP 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)                         186,571    
Conversion of units (in shares)                         142,405    
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)                         32,741    
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)                         512,763    
Subordinated performance units | DownREIT Partnership                              
Class of Stock [Line Items]                              
Unit conversion ratio                         1    
Minimum conversion period (in years)                         5 years    
Prior Sales Agreement                              
Class of Stock [Line Items]                              
Value of common and preferred shares authorized under ATM program           $ 250,000,000.0                  
Preferred share/unit dividend rate (percent)           6.00%                  
Aggregate gross sales price of shares                           $ 219,000,000.0  
Value of remaining available offered shares                           31,000,000.0  
Amended At The Market Program                              
Class of Stock [Line Items]                              
Value of common and preferred shares authorized under ATM program                           $ 400,000,000.0  
Common shares | Common Shares                              
Class of Stock [Line Items]                              
Issuance of shares/units (in shares)                 2,390,000 3,692,216 387,000 125,000      
Common shares | Public Stock Offering                              
Class of Stock [Line Items]                              
Sale of stock, number of shares issued in transaction (in shares)         4,500,000                    
Sale of stock, price per share (in dollars per share)         $ 33.15                    
Sale of stock, additional number of shares issued in transaction (in shares)       400,000                      
Common shares | Public Stock Offering - From Third Parties                              
Class of Stock [Line Items]                              
Sale of stock, number of shares issued in transaction (in shares)   3,049,490 1,850,510         4,900,000              
Sale of stock, price per share (in dollars per share)       $ 33.15       $ 33.15              
Proceeds from sale of stock   $ 97,300,000 $ 60,000,000.0         $ 162,400,000              
Common shares | Follow-On Public Offering | Subsequent Event                              
Class of Stock [Line Items]                              
Sale of stock, number of shares issued in transaction (in shares) 10,120,000                            
Sale of stock, price per share (in dollars per share) $ 51.25                            
Proceeds from sale of stock $ 497,400,000                            
Common shares | At the Market Program                              
Class of Stock [Line Items]                              
Sale of stock, number of shares issued in transaction (in shares)                 2,390,000       3,032,726    
Proceeds from sale of stock                 $ 103,700,000       $ 129,100,000    
Average price per share (in dollars per share)                 $ 43.86       $ 42.92    
Common shares | At the Market Program | Subsequent Event                              
Class of Stock [Line Items]                              
Sale of stock, number of shares issued in transaction (in shares)             782,000                
Proceeds from sale of stock             $ 40,000,000.0                
Average price per share (in dollars per share)             $ 51.82                
Underwriter option to purchase | Public Stock Offering                              
Class of Stock [Line Items]                              
Sale of stock, number of shares issued in transaction (in shares)         675,000                    
Underwriter option to purchase | Follow-On Public Offering | Subsequent Event                              
Class of Stock [Line Items]                              
Sale of stock, number of shares issued in transaction (in shares) 1,320,000                            
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