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Note 13 - Stock-based Compensation - Information about Stock Options Outstanding (Details) - $ / shares
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Options outstanding, vested and expected to vest, number outstanding (in shares) 252,616 399,055 569,639 766,042
Options outstanding, vested and expected to vest, weighted average exercise price (in dollars per share) $ 64.71 $ 51.13 $ 45 $ 36.52
Exercise Price Range One [Member]        
Range of exercise prices, lower limit (in dollars per share) 3.67      
Range of exercise prices, upper limit (in dollars per share) $ 31.43      
Options outstanding, vested and expected to vest, number outstanding (in shares) 30,972      
Options outstanding, vested and expected to vest, weighted average remaining contractual life (Year) 2 years 346 days      
Options outstanding, vested and expected to vest, weighted average exercise price (in dollars per share) $ 24.59      
Options exercisable, number exercisable (in shares) 30,972      
Options exercisable, weighted average exercise price (in dollars per share) $ 24.59      
Exercise Price Range Two [Member]        
Range of exercise prices, lower limit (in dollars per share) 31.44      
Range of exercise prices, upper limit (in dollars per share) $ 66.87      
Options outstanding, vested and expected to vest, number outstanding (in shares) 133,492      
Options outstanding, vested and expected to vest, weighted average remaining contractual life (Year) 5 years 131 days      
Options outstanding, vested and expected to vest, weighted average exercise price (in dollars per share) $ 58.52      
Options exercisable, number exercisable (in shares) 69,210      
Options exercisable, weighted average exercise price (in dollars per share) $ 58.78      
Exercise Price Range Three [Member]        
Range of exercise prices, lower limit (in dollars per share) 66.88      
Range of exercise prices, upper limit (in dollars per share) $ 96.20      
Options outstanding, vested and expected to vest, number outstanding (in shares) 51,894      
Options outstanding, vested and expected to vest, weighted average remaining contractual life (Year) 5 years 200 days      
Options outstanding, vested and expected to vest, weighted average exercise price (in dollars per share) $ 72.49      
Options exercisable, number exercisable (in shares) 25,743      
Options exercisable, weighted average exercise price (in dollars per share) $ 71.73      
Exercise Price Range Four [Member]        
Range of exercise prices, lower limit (in dollars per share) 96.21      
Range of exercise prices, upper limit (in dollars per share) $ 123.10      
Options outstanding, vested and expected to vest, number outstanding (in shares) 36,258      
Options outstanding, vested and expected to vest, weighted average remaining contractual life (Year) 9 years 83 days      
Options outstanding, vested and expected to vest, weighted average exercise price (in dollars per share) $ 110.59      
Options exercisable, number exercisable (in shares) 0      
Options exercisable, weighted average exercise price (in dollars per share) $ 0