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, 2019
Dec. 31, 2018
Partnership Subsidiaries [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 49,042,742 47,120,753
Series A-1 preferred units | NSA OP, LP    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 642,982 343,719
OP units | NSA OP, LP    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 30,458,558 28,874,103
OP units | DownREIT Partnership    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 1,848,261 1,834,786
Subordinated performance units | NSA OP, LP    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 10,961,146 10,749,475
Subordinated performance units | DownREIT Partnership    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 4,371,622 4,386,999
LTIP units | NSA OP, LP    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 760,173 931,671