Annual report pursuant to Section 13 and 15(d)

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

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SHAREHOLDERS' EQUITY AND NONCONTROLLING INTERESTS - Schedule of Outstanding Equity Interests (Details) - shares
Dec. 31, 2018
Dec. 31, 2017
Partnership Subsidiaries [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 47,120,753 45,317,526
NSA OP, LP [Member] | Series A-1 Preferred Units [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 343,719 0
NSA OP, LP [Member] | OP units [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 28,874,103 26,719,607
NSA OP, LP [Member] | Subordinated performance units [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 10,749,475 11,604,738
NSA OP, LP [Member] | LTIP units [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 931,671 771,396
DownREIT Partnership [Member] | OP units [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 1,834,786 1,834,786
DownREIT Partnership [Member] | Subordinated performance units [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 4,386,999 4,386,999