Annual report pursuant to Section 13 and 15(d)

EARNINGS PER SHARE - Narrative (Details)

v3.10.0.1
EARNINGS PER SHARE - Narrative (Details)
 12 Months Ended
Dec. 31, 2018
shares
Dec. 31, 2017
shares
NSA OP, LP [Member] | LTIP units [Member]    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Unit conversion ratio 1  
NSA OP, LP [Member] | OP units [Member]    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Unit conversion ratio 1  
Minimum conversion period 1 year  
NSA OP, LP [Member] | Subordinated performance units [Member]    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Minimum conversion period 2 years  
DownREIT Partnership [Member] | OP units [Member]    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Unit conversion ratio 1  
Minimum conversion period 5 years  
DownREIT Partnership [Member] | Subordinated performance units [Member]    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Unit conversion ratio 1  
Minimum conversion period 5 years  
NSA OP, LP And DownREIT Partnership [Member] | Subordinated performance units [Member]    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Unit conversion ratio 1  
Minimum conversion period 2 years  
Long-Term Incentive Plan Unit Based On Service [Member]    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Outstanding equity interests excluded from computation of earnings per share (in shares/units) 383,712  
Long-Term Incentive Plan Unit Based On Future Acquisitions [Member]    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Outstanding equity interests excluded from computation of earnings per share (in shares/units) 224,000  
OP units, DownREIT OP units, Subordinated performance units, DownREIT subordinated performance units [Member]    
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]    
Outstanding equity interests excluded from computation of earnings per share (in shares/units) 50,600,000 50,600,000