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CREDIT FACILITY (Details)
12 Months Ended
Feb. 02, 2016
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Line of Credit Facility [Line Items]        
Maximum borrowing capacity   $ 100,000,000    
Debt covenant, maximum debt to total capitalization ratio 0.35      
Interest expense incurred   0 $ 0 $ 0
Credit Facility        
Line of Credit Facility [Line Items]        
Maximum borrowing capacity $ 50,000,000 50,000,000    
Debt instrument, term 4 years      
Outstanding balance   $ 0 $ 0  
Letter of Credit Subfacility        
Line of Credit Facility [Line Items]        
Maximum borrowing capacity $ 20,000,000      
Swingline Subfacility        
Line of Credit Facility [Line Items]        
Maximum borrowing capacity $ 5,000,000