Annual report pursuant to Section 13 and 15(d)

SHAREHOLDERS' EQUITY AND NONCONTROLLING INTERESTS - Equity Interests (Details)

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SHAREHOLDERS' EQUITY AND NONCONTROLLING INTERESTS - Equity Interests (Details) - shares
Dec. 31, 2020
Dec. 31, 2019
Partnership Subsidiaries    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 46,281,288 48,808,931
Series A-1 preferred units | NSA OP, LP    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 637,382 642,982
OP units | NSA OP, LP    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 29,616,809 30,188,305
OP units | DownREIT Partnership    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 1,924,918 1,848,261
Subordinated performance units | NSA OP, LP    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 9,030,872 11,014,195
Subordinated performance units | DownREIT Partnership    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 4,337,111 4,371,622
LTIP units | NSA OP, LP    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 734,196 743,566