Annual report pursuant to Section 13 and 15(d)

DEBT FINANCING - Narrative (Details)

v3.8.0.1
DEBT FINANCING - Narrative (Details)
1 Months Ended 12 Months Ended
Jan. 29, 2018
USD ($)
Jun. 30, 2016
USD ($)
Aug. 31, 2017
USD ($)
property
Dec. 31, 2017
USD ($)
component
agency
Dec. 31, 2016
USD ($)
property
Dec. 31, 2015
USD ($)
Debt Instrument [Line Items]            
Repayment of long term debt       $ 679,104,000 $ 558,597,000 $ 357,273,000
Credit Facility [Member]            
Debt Instrument [Line Items]            
Covenant compliance, maximum leverage ratio (percent)       60.00%    
Covenant compliance, minimum interest coverage ratio       1.5    
Covenant compliance, required minimum net worth       $ 682,600,000    
Covenant compliance, required minimum net worth, addition to base, percent of equity issuances (percent)       75.00%    
Covenant compliance, maximum unsecured debt to unencumbered asset ratio (percent)       60.00%    
Covenant compliance, unencumbered net operating income to unsecured interest expense ratio       2.0    
Line of Credit [Member] | Credit Facility [Member] | Revolving line of credit [Member]            
Debt Instrument [Line Items]            
Maximum borrowing capacity       $ 400,000,000    
Extension fee (percent)       0.15%    
Remaining borrowing capacity       $ 306,800,000    
Effective interest rate (percent)       2.96% 2.17%  
Line of Credit [Member] | Credit Facility [Member] | Revolving line of credit [Member] | Minimum [Member]            
Debt Instrument [Line Items]            
Elective unused borrowing capacity fee (percent)       0.15%    
Unused borrowing capacity fee (percent)       0.125%    
Line of Credit [Member] | Credit Facility [Member] | Revolving line of credit [Member] | Maximum [Member]            
Debt Instrument [Line Items]            
Elective unused borrowing capacity fee (percent)       0.25%    
Unused borrowing capacity fee (percent)       0.30%    
Line of Credit [Member] | Credit Facility [Member] | Letter of Credit [Member]            
Debt Instrument [Line Items]            
Outstanding letters of credit       $ 4,700,000    
Line of Credit [Member] | Amended Credit Facility [Member] | Revolving line of credit [Member] | Subsequent Event [Member]            
Debt Instrument [Line Items]            
Maximum borrowing capacity $ 400,000,000          
Unsecured Debt [Member] | Credit Facility [Member]            
Debt Instrument [Line Items]            
Maximum borrowing capacity       $ 895,000,000    
Number of lender agreement components | component       4    
Unsecured Debt [Member] | Credit Facility [Member] | Federal Funds Effective Swap Rate [Member]            
Debt Instrument [Line Items]            
Borrowing rate spread (percent)       0.50%    
Unsecured Debt [Member] | Credit Facility [Member] | LIBOR [Member]            
Debt Instrument [Line Items]            
Borrowing rate spread (percent)       1.00%    
Unsecured Debt [Member] | Credit Facility [Member] | Base Rate [Member]            
Debt Instrument [Line Items]            
Minimum number of rating agencies required | agency       2    
Unsecured Debt [Member] | Credit Facility [Member] | Minimum [Member] | LIBOR [Member]            
Debt Instrument [Line Items]            
Leverage based margin threshold (percent)       1.35%    
Elective leverage based margin threshold (percent)       0.85%    
Unsecured Debt [Member] | Credit Facility [Member] | Minimum [Member] | Base Rate [Member]            
Debt Instrument [Line Items]            
Leverage based margin threshold (percent)       0.35%    
Elective leverage based margin threshold (percent)       0.00%    
Unsecured Debt [Member] | Credit Facility [Member] | Maximum [Member] | LIBOR [Member]            
Debt Instrument [Line Items]            
Leverage based margin threshold (percent)       2.25%    
Elective leverage based margin threshold (percent)       2.45%    
Unsecured Debt [Member] | Credit Facility [Member] | Maximum [Member] | Base Rate [Member]            
Debt Instrument [Line Items]            
Leverage based margin threshold (percent)       1.25%    
Elective leverage based margin threshold (percent)       1.45%    
Unsecured Debt [Member] | Credit Facility [Member] | Tranche A Term Loan [Member]            
Debt Instrument [Line Items]            
Maximum borrowing capacity       $ 235,000,000    
Effective interest rate (percent)       2.63%    
Unsecured Debt [Member] | Credit Facility [Member] | Tranche B Term Loan [Member]            
Debt Instrument [Line Items]            
Maximum borrowing capacity       $ 155,000,000    
Effective interest rate (percent)       3.24%    
Unsecured Debt [Member] | Credit Facility [Member] | Tranche C Term Loan [Member]            
Debt Instrument [Line Items]            
Maximum borrowing capacity       $ 105,000,000    
Effective interest rate (percent)       3.71%    
Unsecured Debt [Member] | Term Loan Facility [Member]            
Debt Instrument [Line Items]            
Debt instrument, face amount   $ 100,000,000        
Effective interest rate (percent)       3.08%    
Unsecured Debt [Member] | Term Loan Facility [Member] | Federal Funds Effective Swap Rate [Member]            
Debt Instrument [Line Items]            
Borrowing rate spread (percent)   0.50%        
Unsecured Debt [Member] | Term Loan Facility [Member] | LIBOR [Member]            
Debt Instrument [Line Items]            
Borrowing rate spread (percent)   1.00%        
Unsecured Debt [Member] | Term Loan Facility [Member] | Minimum [Member] | LIBOR [Member]            
Debt Instrument [Line Items]            
Leverage based margin threshold (percent)   1.75%        
Elective leverage based margin threshold (percent)   1.50%        
Unsecured Debt [Member] | Term Loan Facility [Member] | Minimum [Member] | Base Rate [Member]            
Debt Instrument [Line Items]            
Leverage based margin threshold (percent)   0.75%        
Elective leverage based margin threshold (percent)   0.50%        
Unsecured Debt [Member] | Term Loan Facility [Member] | Maximum [Member]            
Debt Instrument [Line Items]            
Debt instrument, face amount   $ 200,000,000        
Unsecured Debt [Member] | Term Loan Facility [Member] | Maximum [Member] | LIBOR [Member]            
Debt Instrument [Line Items]            
Leverage based margin threshold (percent)   2.35%        
Elective leverage based margin threshold (percent)   2.45%        
Unsecured Debt [Member] | Term Loan Facility [Member] | Maximum [Member] | Base Rate [Member]            
Debt Instrument [Line Items]            
Leverage based margin threshold (percent)   1.35%        
Elective leverage based margin threshold (percent)   1.45%        
Unsecured Debt [Member] | Amended Credit Facility [Member] | Subsequent Event [Member]            
Debt Instrument [Line Items]            
Maximum borrowing capacity 1,300,000,000          
Current borrowing capacity 1,000,000,000          
Unsecured Debt [Member] | Amended Credit Facility [Member] | Tranche A Term Loan [Member] | Subsequent Event [Member]            
Debt Instrument [Line Items]            
Maximum borrowing capacity 235,000,000          
Unsecured Debt [Member] | Amended Credit Facility [Member] | Tranche B Term Loan [Member] | Subsequent Event [Member]            
Debt Instrument [Line Items]            
Maximum borrowing capacity 155,000,000          
Unsecured Debt [Member] | Amended Credit Facility [Member] | Tranche C Term Loan [Member] | Subsequent Event [Member]            
Debt Instrument [Line Items]            
Maximum borrowing capacity $ 105,000,000          
Unsecured Debt [Member] | Amended Credit Facility [Member] | Tranche D Term Loan [Member] | Subsequent Event [Member]            
Debt Instrument [Line Items]            
Debt instrument, term 5 years          
Debt instrument, face amount $ 125,000,000.00          
Unsecured Debt [Member] | Amended Credit Facility [Member] | Tranche D Term Loan [Member] | Federal Funds Effective Swap Rate [Member] | Subsequent Event [Member]            
Debt Instrument [Line Items]            
Borrowing rate spread (percent) 0.50%          
Unsecured Debt [Member] | Amended Credit Facility [Member] | Tranche D Term Loan [Member] | LIBOR [Member] | Subsequent Event [Member]            
Debt Instrument [Line Items]            
Borrowing rate spread (percent) 1.00%          
Unsecured Debt [Member] | Amended Credit Facility [Member] | Tranche D Term Loan [Member] | Minimum [Member] | LIBOR [Member] | Subsequent Event [Member]            
Debt Instrument [Line Items]            
Leverage based margin threshold (percent) 1.30%          
Elective leverage based margin threshold (percent) 0.90%          
Unsecured Debt [Member] | Amended Credit Facility [Member] | Tranche D Term Loan [Member] | Minimum [Member] | Base Rate [Member] | Subsequent Event [Member]            
Debt Instrument [Line Items]            
Leverage based margin threshold (percent) 0.30%          
Elective leverage based margin threshold (percent) 0.00%          
Unsecured Debt [Member] | Amended Credit Facility [Member] | Tranche D Term Loan [Member] | Maximum [Member] | LIBOR [Member] | Subsequent Event [Member]            
Debt Instrument [Line Items]            
Leverage based margin threshold (percent) 1.85%          
Elective leverage based margin threshold (percent) 1.75%          
Unsecured Debt [Member] | Amended Credit Facility [Member] | Tranche D Term Loan [Member] | Maximum [Member] | Base Rate [Member] | Subsequent Event [Member]            
Debt Instrument [Line Items]            
Leverage based margin threshold (percent) 0.85%          
Elective leverage based margin threshold (percent) 0.75%          
Mortgages [Member] | Fixed Rate Mortgages [Member]            
Debt Instrument [Line Items]            
Effective interest rate (percent)       4.15%    
Liabilities assumed, mortgages         $ 61,600,000  
Number of self storage properties acquired with assumed mortgages | property         17  
Repayment of long term debt         $ 12,200,000  
Mortgages [Member] | Fixed Rate Mortgages [Member] | Minimum [Member]            
Debt Instrument [Line Items]            
Effective interest rate (percent)       2.55%    
Mortgages [Member] | Fixed Rate Mortgages [Member] | Maximum [Member]            
Debt Instrument [Line Items]            
Effective interest rate (percent)       5.00%    
Secured Debt [Member] | Fixed Rate Mortgages [Member]            
Debt Instrument [Line Items]            
Debt instrument, face amount     $ 84,900,000      
Number of properties used to secure debt | property     22      
Debt instrument, fixed interest rate (percent)     4.14%