Annual report pursuant to Section 13 and 15(d)

NONCONTROLLING INTERESTS - Equity Interests (Details)

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NONCONTROLLING INTERESTS - Equity Interests (Details)
 12 Months Ended
Dec. 31, 2015
unit / shares
shares
Dec. 31, 2014
shares
Partnership Subsidiaries [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 39,865,541 34,240,410
OP units [Member] | NSA OP, LP And DownREIT Partnership [Member]    
Noncontrolling Interest [Line Items]    
Minimum conversion period 1 year  
OP units [Member] | NSA OP, LP [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 21,556,006 18,817,088
Unit conversion ratio | unit / shares 1  
OP units [Member] | DownREIT Partnership [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 1,834,786 1,275,979
Unit conversion ratio | unit / shares 1  
Subordinated performance units [Member] | NSA OP, LP And DownREIT Partnership [Member]    
Noncontrolling Interest [Line Items]    
Minimum conversion period 2 years  
Unit conversion ratio | unit / shares 1  
Subordinated performance units [Member] | NSA OP, LP [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 9,302,989 8,447,679
Subordinated performance units [Member] | DownREIT Partnership [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 4,386,999 3,009,884
LTIP units [Member] | NSA OP, LP [Member]    
Noncontrolling Interest [Line Items]    
Outstanding equity interest (in shares) 2,784,761 2,689,780