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Black-Scholes and Binomial model Assumptions and Summary of Activity related to our stock (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Option      
Summary of activity related to our stock      
Outstanding at the beginning of the period (in shares) 3,163,125    
Exercised (in shares) (40,000)    
Forfeited (in shares) (24,968)    
Outstanding at the end of the period (in shares) 3,164,157   3,163,125
Exercisable at the end of the period (in shares) 2,181,083    
Weighted average exercise price      
Outstanding at the beginning of the period (in usd per share) $ 20.13    
Exercised (in usd per share) 9.14    
Forfeited (in usd per share) 24.25    
Outstanding at the end of the period (in usd per share) 20.40   $ 20.13
Exercisable at the end of the period (in usd per share) $ 14.70    
Weighted average contractual term      
Weighted average contractual term 4 years 6 months 26 days   4 years 11 months 9 days
Exercisable at the end of the period 2 years 8 months 12 days    
Aggregate intrinsic value      
Aggregate intrinsic value, beginning balance (in dollars) $ 35,842    
Aggregate intrinsic value, ending balance (in dollars) 26,028   $ 35,842
Exercisable at the end of the period (in dollars) $ 24,063    
Binomial      
Equity And Share-Based Compensation      
Expected stock price volatility (%) 62.05% 55.06%  
Binomial | Minimum      
Equity And Share-Based Compensation      
Risk-free interest rate (%) 0.27% 0.02%  
Expected stock price volatility (%) 59.76%    
Expected option life (in years) 4 years 6 months 18 days 4 years 5 months 12 days  
Fair value 11.06 10.68  
Binomial | Maximum      
Equity And Share-Based Compensation      
Risk-free interest rate (%) 1.83% 2.01%  
Expected option life (in years) 4 years 10 months 17 days 4 years 11 months 1 day  
Fair value 15.18 11.93  
Black Scholes      
Equity And Share-Based Compensation      
Risk-free interest rate (%)   1.67%  
Expected stock price volatility (%)   55.06%  
Expected option life (in years)   6 years 3 months  
Fair value   11.69  
Black Scholes | Minimum      
Equity And Share-Based Compensation      
Risk-free interest rate (%) 1.25%    
Expected stock price volatility (%) 59.75%    
Fair value 11.15    
Black Scholes | Maximum      
Equity And Share-Based Compensation      
Risk-free interest rate (%) 1.89%    
Expected stock price volatility (%) 62.05%    
Expected option life (in years) 6 years 3 months    
Fair value 16.30