Annual report pursuant to Section 13 and 15(d)

SHAREHOLDERS' EQUITY AND NONCONTROLLING INTERESTS - Narrative (Details)

v3.8.0.1
SHAREHOLDERS' EQUITY AND NONCONTROLLING INTERESTS - Narrative (Details)
12 Months Ended
Dec. 11, 2017
USD ($)
$ / shares
shares
Oct. 11, 2017
USD ($)
$ / shares
shares
Dec. 16, 2016
USD ($)
$ / shares
shares
Jul. 06, 2016
USD ($)
$ / shares
shares
Apr. 28, 2015
USD ($)
$ / shares
shares
Dec. 31, 2017
USD ($)
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Dec. 31, 2015
USD ($)
Oct. 11, 2016
USD ($)
agent
Class of Stock [Line Items]                  
Net proceeds from issuance of common shares | $ $ 140,300,000   $ 105,500,000 $ 237,500,000   $ 140,261,000 $ 378,281,000 $ 278,070,000  
Proceeds from issuance of preferred shares | $           $ 166,566,000 0 $ 0  
ATM [Member]                  
Class of Stock [Line Items]                  
Net proceeds from issuance of common shares | $             $ 33,600,000    
Number of agents used in ATM program | agent                 4
Total value of common shares authorized to be sold through ATM program | $                 $ 200,000,000
Common Shares [Member]                  
Class of Stock [Line Items]                  
Net proceeds from issuance of common shares in IPO | $         $ 278,100,000        
Common Shares [Member] | IPO [Member]                  
Class of Stock [Line Items]                  
Issuance of shares/units (in shares)         23,000,000        
Share price (in dollars per share) | $ / shares         $ 13.00        
Common Shares [Member] | Over-Allotment Option [Member]                  
Class of Stock [Line Items]                  
Issuance of shares/units (in shares) 750,000   675,000 1,571,250 3,000,000        
Common Shares [Member] | Follow On Offering [Member]                  
Class of Stock [Line Items]                  
Issuance of shares/units (in shares) 5,750,000   5,175,000 12,046,250          
Share price (in dollars per share) | $ / shares $ 25.50   $ 20.48 $ 20.75          
Common Shares [Member] | ATM [Member]                  
Class of Stock [Line Items]                  
Issuance of shares/units (in shares)             1,740,959    
Share price (in dollars per share) | $ / shares             $ 19.54    
Series A Preferred Shares [Member]                  
Class of Stock [Line Items]                  
Issuance of shares/units (in shares)   6,900,000              
Preferred shares dividend rate (percent)   6.00%              
Proceeds from issuance of preferred shares | $   $ 166,600,000              
Preferred shares per share amount of dividends (in dollars per share) | $ / shares   $ 1.50              
Preferred shares redemption price (in dollars per share) | $ / shares   $ 25.00              
Series A Preferred Shares [Member] | Over-Allotment Option [Member]                  
Class of Stock [Line Items]                  
Issuance of shares/units (in shares)   900,000              
Subordinated performance units [Member]                  
Class of Stock [Line Items]                  
Number of units converted (in units)           997,074      
LTIP units [Member]                  
Class of Stock [Line Items]                  
Issuance of shares/units (in shares)           219,701      
NSA OP, LP [Member] | OP units [Member]                  
Class of Stock [Line Items]                  
Issuance of shares/units (in shares)           992,210      
Number of units converted (in units)           11,050      
Unit conversion ratio           1      
Minimum conversion period           1 year      
Redemptions of units (in shares)           1,409,715      
NSA OP, LP [Member] | OP units [Member] | Acquisition of Self Storage Properties [Member]                  
Class of Stock [Line Items]                  
Issuance of shares/units (in shares)           1,022,718      
NSA OP, LP [Member] | Subordinated performance units [Member]                  
Class of Stock [Line Items]                  
Minimum conversion period           2 years      
Redemptions of units (in shares)           11,050      
Unit conversion, lock out period           2 years      
NSA OP, LP [Member] | Subordinated performance units [Member] | Acquisition of Self Storage Properties [Member]                  
Class of Stock [Line Items]                  
Issuance of shares/units (in shares)           271,267      
NSA OP, LP [Member] | Subordinated performance units [Member] | Affiliated Entity [Member]                  
Class of Stock [Line Items]                  
Issuance of shares/units (in shares)           300,043      
NSA OP, LP [Member] | LTIP units [Member]                  
Class of Stock [Line Items]                  
Number of units converted (in units)           992,210      
Unit conversion ratio           1      
DownREIT Partnership [Member] | OP units [Member]                  
Class of Stock [Line Items]                  
Unit conversion ratio           1      
Minimum conversion period           5 years      
DownREIT Partnership [Member] | Subordinated performance units [Member]                  
Class of Stock [Line Items]                  
Unit conversion ratio           1      
Minimum conversion period           5 years