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Note 10 - Common Stock and Stock-based Compensation Plans - Valuation Assumptions (Details) - Employee Stock Purchase Plan [Member]
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Expected dividend yield 0.00% 0.00%
Expected volatility 43.00%  
Risk-free interest rate 2.50%  
Contractual term of up to (Month) 2 years 2 years
Minimum [Member]    
Expected volatility   35.00%
Risk-free interest rate   0.70%
Maximum [Member]    
Expected volatility   42.00%
Risk-free interest rate   1.60%