Annual report pursuant to Section 13 and 15(d)

EARNINGS PER SHARE - Narrative (Details)

v3.19.3.a.u2
EARNINGS PER SHARE - Narrative (Details)
 12 Months Ended
Dec. 31, 2019
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Dec. 31, 2018
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Dec. 31, 2017
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NSA OP, LP | OP units      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Unit conversion ratio 1    
Minimum conversion period (in years) 1 year    
NSA OP, LP | Subordinated performance units      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Minimum conversion period (in years) 2 years    
NSA OP, LP | LTIP units      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Unit conversion ratio 1    
DownREIT Partnership | OP units      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Unit conversion ratio 1    
Minimum conversion period (in years) 5 years    
DownREIT Partnership | Subordinated performance units      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Unit conversion ratio 1    
Minimum conversion period (in years) 5 years    
NSA OP, LP And DownREIT Partnership | Subordinated performance units      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Unit conversion ratio 1    
Minimum conversion period (in years) 2 years    
LTIP units with vesting based on service or market condition      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Equity interests excluded from computation of diluted earnings per share (in units) 448,089    
LTIP units with vesting based on future acquisitions      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Equity interests excluded from computation of diluted earnings per share (in units) 224,000    
OP units, DownREIT OP units, Subordinated performance units, DownREIT subordinated performance units, Vested LTIP units      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Equity interests excluded from computation of diluted earnings per share (in units) 54,200,000 50,600,000 50,600,000