Annual report pursuant to Section 13 and 15(d)

DEBT FINANCING - Narrative (Details)

v3.6.0.2
DEBT FINANCING - Narrative (Details)
12 Months Ended
Feb. 08, 2017
USD ($)
Jun. 30, 2016
USD ($)
Dec. 31, 2016
USD ($)
property
component
agency
Dec. 31, 2015
USD ($)
property
Dec. 31, 2014
USD ($)
property
Debt Instrument [Line Items]          
Repayment of long term debt     $ 558,597,000 $ 357,273,000 $ 143,970,000
Number of self storage properties acquired with assumed mortgages | property     107 58 83
Line of Credit [Member] | Credit Facility [Member]          
Debt Instrument [Line Items]          
Total borrowing capacity     $ 725,000,000.0    
Number of lender agreement components | component     3    
Covenant compliance, maximum leverage ratio     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     75.00%    
Covenant compliance, maximum unsecured debt to unencumbered asset ratio     60.00%    
Covenant compliance, unencumbered net operating income to unsecured interest expense ratio     2.0    
Line of Credit [Member] | Credit Facility [Member] | Federal Funds Effective Swap Rate [Member]          
Debt Instrument [Line Items]          
Borrowing rate spread     0.50%    
Line of Credit [Member] | Credit Facility [Member] | London Interbank Offered Rate (LIBOR) [Member]          
Debt Instrument [Line Items]          
Borrowing rate spread     1.00%    
Line of Credit [Member] | Credit Facility [Member] | Base Rate [Member]          
Debt Instrument [Line Items]          
Minimum rating agencies required | agency     2    
Line of Credit [Member] | Credit Facility [Member] | Minimum [Member] | London Interbank Offered Rate (LIBOR) [Member]          
Debt Instrument [Line Items]          
Leverage based margin threshold     1.35%    
Elective leverage based margin threshold     0.85%    
Line of Credit [Member] | Credit Facility [Member] | Minimum [Member] | Base Rate [Member]          
Debt Instrument [Line Items]          
Leverage based margin threshold     0.35%    
Elective leverage based margin threshold     0.00%    
Line of Credit [Member] | Credit Facility [Member] | Maximum [Member] | London Interbank Offered Rate (LIBOR) [Member]          
Debt Instrument [Line Items]          
Leverage based margin threshold     2.15%    
Elective leverage based margin threshold     2.30%    
Line of Credit [Member] | Credit Facility [Member] | Maximum [Member] | Base Rate [Member]          
Debt Instrument [Line Items]          
Leverage based margin threshold     1.15%    
Elective leverage based margin threshold     1.30%    
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     $ 143,600,000    
Line of Credit [Member] | Credit Facility [Member] | Revolving line of credit [Member] | Minimum [Member]          
Debt Instrument [Line Items]          
Elective unused borrowing capacity, fee     0.15%    
Unused borrowing capacity, fee     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     0.25%    
Unused borrowing capacity, fee     0.30%    
Line of Credit [Member] | Credit Facility [Member] | Letter of Credit [Member]          
Debt Instrument [Line Items]          
Line of credit outstanding     $ 9,900,000    
Unsecured Debt [Member] | Credit Facility [Member] | Tranche A Term Loan [Member]          
Debt Instrument [Line Items]          
Maximum borrowing capacity     225,000,000    
Unsecured Debt [Member] | Credit Facility [Member] | Tranche B Term Loan [Member]          
Debt Instrument [Line Items]          
Maximum borrowing capacity     $ 100,000,000    
Unsecured Debt [Member] | Term Loan Facility [Member]          
Debt Instrument [Line Items]          
Debt instrument, face amount   $ 100,000,000      
Unsecured Debt [Member] | Term Loan Facility [Member] | Federal Funds Effective Swap Rate [Member]          
Debt Instrument [Line Items]          
Borrowing rate spread   0.50%      
Unsecured Debt [Member] | Term Loan Facility [Member] | London Interbank Offered Rate (LIBOR) [Member]          
Debt Instrument [Line Items]          
Borrowing rate spread   1.00%      
Unsecured Debt [Member] | Term Loan Facility [Member] | Minimum [Member] | London Interbank Offered Rate (LIBOR) [Member]          
Debt Instrument [Line Items]          
Leverage based margin threshold   1.75%      
Elective leverage based margin threshold   1.50%      
Unsecured Debt [Member] | Term Loan Facility [Member] | Minimum [Member] | Base Rate [Member]          
Debt Instrument [Line Items]          
Leverage based margin threshold   0.75%      
Elective leverage based margin threshold   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] | London Interbank Offered Rate (LIBOR) [Member]          
Debt Instrument [Line Items]          
Leverage based margin threshold   2.35%      
Elective leverage based margin threshold   2.45%      
Unsecured Debt [Member] | Term Loan Facility [Member] | Maximum [Member] | Base Rate [Member]          
Debt Instrument [Line Items]          
Leverage based margin threshold   1.35%      
Elective leverage based margin threshold   1.45%      
Mortgages [Member] | Fixed Rate Mortgages [Member]          
Debt Instrument [Line Items]          
Minimum effective interest rate     2.55%    
Maximum effective interest rate     5.00%    
Repayment of long term debt     $ 12,200,000 $ 34,700,000  
Liabilities assumed, mortgages     $ 61,600,000 $ 73,500,000  
Number of self storage properties acquired with assumed mortgages | property     17 31  
Subsequent Event [Member] | Line of Credit [Member] | Amended Credit Facility [Member] | Tranche C Term Loan [Member]          
Debt Instrument [Line Items]          
Maximum borrowing capacity $ 105,000,000.0        
Subsequent Event [Member] | Line of Credit [Member] | Amended Credit Facility [Member] | Revolving line of credit [Member]          
Debt Instrument [Line Items]          
Maximum borrowing capacity 400,000,000        
Subsequent Event [Member] | Line of Credit [Member] | Expansion Option Of Amended Credit Facility [Member]          
Debt Instrument [Line Items]          
Maximum borrowing capacity 1,000,000,000.0        
Subsequent Event [Member] | Unsecured Debt [Member] | Amended Credit Facility [Member]          
Debt Instrument [Line Items]          
Maximum borrowing capacity 895,000,000        
Increase in total borrowing capacity 170,000,000.0        
Subsequent Event [Member] | Unsecured Debt [Member] | Amended Credit Facility [Member] | Tranche C Term Loan [Member]          
Debt Instrument [Line Items]          
Maximum borrowing capacity $ 105,000,000        
Subsequent Event [Member] | Unsecured Debt [Member] | Amended Credit Facility [Member] | Tranche C Term Loan [Member] | Federal Funds Effective Swap Rate [Member]          
Debt Instrument [Line Items]          
Borrowing rate spread 0.50%        
Subsequent Event [Member] | Unsecured Debt [Member] | Amended Credit Facility [Member] | Tranche C Term Loan [Member] | London Interbank Offered Rate (LIBOR) [Member]          
Debt Instrument [Line Items]          
Borrowing rate spread 1.00%        
Subsequent Event [Member] | Unsecured Debt [Member] | Amended Credit Facility [Member] | Tranche C Term Loan [Member] | Minimum [Member] | London Interbank Offered Rate (LIBOR) [Member]          
Debt Instrument [Line Items]          
Leverage based margin threshold 1.70%        
Elective leverage based margin threshold 1.50%        
Subsequent Event [Member] | Unsecured Debt [Member] | Amended Credit Facility [Member] | Tranche C Term Loan [Member] | Minimum [Member] | Base Rate [Member]          
Debt Instrument [Line Items]          
Leverage based margin threshold 0.70%        
Elective leverage based margin threshold 0.50%        
Subsequent Event [Member] | Unsecured Debt [Member] | Amended Credit Facility [Member] | Tranche C Term Loan [Member] | Maximum [Member] | London Interbank Offered Rate (LIBOR) [Member]          
Debt Instrument [Line Items]          
Leverage based margin threshold 2.25%        
Elective leverage based margin threshold 2.45%        
Subsequent Event [Member] | Unsecured Debt [Member] | Amended Credit Facility [Member] | Tranche C Term Loan [Member] | Maximum [Member] | Base Rate [Member]          
Debt Instrument [Line Items]          
Leverage based margin threshold 1.25%        
Elective leverage based margin threshold 1.45%        
Subsequent Event [Member] | Unsecured Debt [Member] | Amended Credit Facility [Member] | Tranche A Term Loan [Member]          
Debt Instrument [Line Items]          
Maximum borrowing capacity $ 235,000,000        
Subsequent Event [Member] | Unsecured Debt [Member] | Amended Credit Facility [Member] | Tranche B Term Loan [Member]          
Debt Instrument [Line Items]          
Maximum borrowing capacity $ 155,000,000