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, 2017
Dec. 31, 2016
Partnership Subsidiaries [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 45,317,526 44,913,512
NSA OP, LP [Member] | OP units [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 26,719,607 26,125,444
NSA OP, LP [Member] | Subordinated performance units [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 11,604,738 11,022,378
NSA OP, LP [Member] | LTIP units [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 771,396 1,543,905
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