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, 2016
Dec. 31, 2015
Partnership Subsidiaries [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 44,913,512 39,865,541
OP units [Member] | NSA OP, LP [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 26,125,444 21,556,006
OP units [Member] | DownREIT Partnership [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 1,834,786 1,834,786
Subordinated performance units [Member] | NSA OP, LP [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 11,022,378 9,302,989
Subordinated performance units [Member] | DownREIT Partnership [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 4,386,999 4,386,999
LTIP units [Member] | NSA OP, LP [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 1,543,905 2,784,761