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DEBT AND CREDIT FACILITY - Narrative (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Jul. 06, 2018
Dec. 31, 2017
Dec. 31, 2016
Aug. 04, 2015
Investment [Line Items]          
Debt $ 250,729,000   $ 236,358,000 $ 223,211,000  
Equity component of convertible notes 68,347,000   68,347,000    
Senior Convertible Unsecured Notes          
Investment [Line Items]          
Debt $ 265,000,000        
Debt instrument, interest rate, stated percentage 2.875%        
Debt conversion, converted instrument, shares issued 50        
Debt instrument, convertible, conversion price (in usd per share) $ 20.00        
Number of trading days sale price exceeds conversion price 20 days        
Number of consecutive trading days 30 days        
Debt instrument, convertible, threshold percentage of stock price trigger 130.00%        
Debt instrument, redemption price, percentage 100.00%        
Debt instrument, fair value disclosure $ 178,358,000        
Debt instrument, term 7 years        
Fair value inputs, discount rate 8.50%        
Equity component of convertible notes, gross $ 77,723,000        
Equity component of convertible notes 68,347,000        
Revolving Credit Facility          
Investment [Line Items]          
Debt   $ 10,000,000      
Notional amount         $ 75,000,000
Borrowing capacity, Accordion feature $ 25,000,000        
Line of credit facility, financial letters of credit, percentage 66.00%        
Line of credit facility, undrawn fees, percentage 25.00%        
Revolving Credit Facility | Minimum | London Interbank Offered Rate (LIBOR)          
Investment [Line Items]          
Borrowings, adjustment to interest rate basis 2.25%        
Revolving Credit Facility | Maximum | London Interbank Offered Rate (LIBOR)          
Investment [Line Items]          
Borrowings, adjustment to interest rate basis 3.75%        
Letter of Credit          
Investment [Line Items]          
Debt $ 8,000,000   $ 7,700,000