Annual report pursuant to Section 13 and 15(d)

EARNINGS PER SHARE - Narrative (Details)

v3.8.0.1
EARNINGS PER SHARE - Narrative (Details)
 12 Months Ended
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]  
Weighted average outstanding equity interests excluded from computation of earnings per share (in units) 318,640
Long-Term Incentive Plan Unit Based On Future Acquisitions [Member]  
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]  
Weighted average outstanding equity interests excluded from computation of earnings per share (in 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]  
Weighted average outstanding equity interests excluded from computation of earnings per share (in units) 50,600,000