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Note 6 - Common Stock Options (Details Textual) - The Plan [Member] - USD ($)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Employee Stock Option [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Expiration Period 10 years    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Not yet Recognized, Stock Options $ 5,000,000    
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition 2 years 7 days    
Any One Person [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Percentage of Outstanding Stock Maximum 5.00%    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value $ 17.95 $ 8.23 $ 11.37
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested in Period, Intrinsic Value, Amount Per Share $ 2.86 $ 2.90 $ 2.51
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value $ 18,697,000    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value 19,044,000    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested in Period, Fair Value $ 5,463,000 $ 6,999,000 $ 632,500