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Note 9 - Stock-based Compensation - Assumptions Used in the Black-Scholes Option Pricing Model (Details) - Employee Stock Option [Member]
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Risk-free interest rate   2.24%
Expected life (years) (Year) 6 years 91 days 6 years 182 days
Expected volatility   44.68%
Expected dividend yield 0.00% 0.00%
Minimum [Member]    
Risk-free interest rate 2.52%  
Expected life (years) (Year) 6 years 91 days  
Expected volatility 41.95%  
Expected dividend yield 0.00%  
Maximum [Member]    
Risk-free interest rate 3.07%  
Expected life (years) (Year) 6 years 91 days  
Expected volatility 42.22%  
Expected dividend yield 0.00%