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SHAREHOLDERS’ EQUITY AND SHARE-BASED COMPENSATION Black-Scholes and Binomial model Assumptions and Summary of Activity related to our stock (Details) - USD ($)
$ / shares in Units, $ in Thousands
9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Number of options      
Outstanding at the beginning of the period (in shares) 1,745,906    
Granted (in shares) 271,876 216,000  
Exercised (in shares) (326,737)    
Forfeited (in shares) (232,247)    
Outstanding at the end of the period (in shares) 1,458,798   1,745,906
Exercisable at the end of the period (in shares) 886,720    
Weighted average exercise price      
Outstanding at the beginning of the period (in usd per share) $ 28.20    
Weighted average exercise price of stock options granted (in usd per share) 25.06 $ 34.07  
Exercised (in usd per share) 11.24    
Forfeited (in usd per share) 32.47    
Outstanding at the end of the period (in usd per share) 30.73   $ 28.20
Exercisable at the end of the period (in usd per share) $ 27.18    
Weighted average contractual term      
Weighted average contractual term 5 years 4 months 24 days   4 years 11 months 16 days
Exercisable at the end of the period 3 years 8 months 1 day    
Aggregate intrinsic value      
Aggregate intrinsic value, beginning balance (in dollars) $ 10,202    
Aggregate intrinsic value, ending balance (in dollars) 10,053   $ 10,202
Exercisable at the end of the period (in dollars) $ 7,364    
Expected dividend yield 0.00% 0.00%  
Black Scholes | Minimum      
Aggregate intrinsic value      
Risk-free interest rate (%) 2.66% 1.89%  
Expected stock price volatility (%) 70.31% 61.49%  
Expected dividend yield 0.00% 0.00%  
Expected option life (in years) 6 years 6 years  
Fair value (in usd per share) $ 16.17 $ 13.57  
Black Scholes | Maximum      
Aggregate intrinsic value      
Risk-free interest rate (%) 2.98% 2.29%  
Expected stock price volatility (%) 71.86% 71.31%  
Expected dividend yield 0.00% 0.00%  
Expected option life (in years) 6 years 3 months 7 years 6 months  
Fair value (in usd per share) $ 19.06 $ 24.80  
Binomial | Minimum      
Aggregate intrinsic value      
Risk-free interest rate (%) 1.64% 0.77%  
Expected stock price volatility (%) 71.81% 66.68%  
Expected dividend yield 0.00% 0.00%  
Expected option life (in years) 2 years 6 months 22 days 2 years 6 months 18 days  
Fair value (in usd per share) $ 14.67 $ 11.94  
Binomial | Maximum      
Aggregate intrinsic value      
Risk-free interest rate (%) 2.83% 2.38%  
Expected stock price volatility (%) 71.86% 71.31%  
Expected dividend yield 0.00% 0.00%  
Expected option life (in years) 4 years 3 months 26 days 4 years 3 months 26 days  
Fair value (in usd per share) $ 18.28 $ 24.30