Quarterly report pursuant to Section 13 or 15(d)

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

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SHAREHOLDERS' EQUITY AND NONCONTROLLING INTERESTS - Equity Interests (Details) - shares
Sep. 30, 2020
Dec. 31, 2019
Partnership Subsidiaries    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 45,879,455 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) 29,443,460 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) 8,770,744 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) 765,840 743,566