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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
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Number of options          
Outstanding at the beginning of the period (in shares)     1,745,906    
Granted (in shares)     271,876 129,000  
Exercised (in shares)     (295,752)    
Forfeited (in shares)     (180,578)    
Outstanding at the end of the period (in shares) 1,541,452   1,541,452   1,745,906
Exercisable at the end of the period (in shares) 921,627   921,627    
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 $ 39.13  
Exercised (in usd per share)     9.48    
Forfeited (in usd per share)     33.91    
Outstanding at the end of the period (in usd per share) $ 30.56   30.56   $ 28.20
Exercisable at the end of the period (in usd per share) $ 27.03   $ 27.03    
Weighted average contractual term          
Weighted average contractual term     5 years 9 months 4 days   4 years 11 months 16 days
Exercisable at the end of the period     3 years 11 months 16 days    
Aggregate intrinsic value          
Aggregate intrinsic value, beginning balance (in dollars)     $ 10,202    
Aggregate intrinsic value, ending balance (in dollars) $ 5,235   5,235   $ 10,202
Exercisable at the end of the period (in dollars) $ 5,132   $ 5,132    
Expected dividend yield 0.00% 0.00%   0.00%  
Black Scholes | Minimum          
Aggregate intrinsic value          
Risk-free interest rate (%)     2.66% 2.06%  
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 $ 23.91 $ 16.17 $ 23.91  
Black Scholes | Maximum          
Aggregate intrinsic value          
Risk-free interest rate (%)     2.98% 2.29%  
Expected stock price volatility (%)     71.86% 66.68%  
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 $ 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 3 years 6 months 11 days  
Fair value (in usd per share) 14.67 23.54 $ 14.67 $ 23.54  
Binomial | Maximum          
Aggregate intrinsic value          
Risk-free interest rate (%)     2.83% 2.38%  
Expected stock price volatility (%)     71.86% 66.68%  
Expected dividend yield     0.00% 0.00%  
Expected option life (in years)     4 years 3 months 26 days 3 years 10 months 6 days  
Fair value (in usd per share) $ 18.28 $ 24.30 $ 18.28 $ 24.30