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Note 12 - Stock-based Compensation - Information about Stock Options Outstanding (Details) - $ / shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Options outstanding, vested and expected to vest, number outstanding (in shares) 399,055 569,639 766,042 998,987
Options outstanding, vested and expected to vest, weighted average exercise price (in dollars per share) $ 51.13 $ 45 $ 36.52 $ 26.49
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) $ 7.85      
Options outstanding, vested and expected to vest, number outstanding (in shares) 6,000      
Options outstanding, vested and expected to vest, weighted average remaining contractual life (Year) 1 year 120 days      
Options outstanding, vested and expected to vest, weighted average exercise price (in dollars per share) $ 5.56      
Options exercisable, number exercisable (in shares) 6,000      
Options exercisable, weighted average exercise price (in dollars per share) $ 5.56      
Exercise Price Range Two [Member]        
Options outstanding, vested and expected to vest, number outstanding (in shares) 53,100      
Options outstanding, vested and expected to vest, weighted average remaining contractual life (Year) 1 year 160 days      
Options outstanding, vested and expected to vest, weighted average exercise price (in dollars per share) $ 7.86      
Options exercisable, number exercisable (in shares) 53,100      
Options exercisable, weighted average exercise price (in dollars per share) $ 7.86      
Exercise Price Range Three [Member]        
Range of exercise prices, lower limit (in dollars per share) 7.87      
Range of exercise prices, upper limit (in dollars per share) $ 31.43      
Options outstanding, vested and expected to vest, number outstanding (in shares) 47,788      
Options outstanding, vested and expected to vest, weighted average remaining contractual life (Year) 4 years 10 days      
Options outstanding, vested and expected to vest, weighted average exercise price (in dollars per share) $ 26.68      
Options exercisable, number exercisable (in shares) 47,788      
Options exercisable, weighted average exercise price (in dollars per share) $ 26.68      
Exercise Price Range Four [Member]        
Range of exercise prices, lower limit (in dollars per share) 31.44      
Range of exercise prices, upper limit (in dollars per share) $ 47.15      
Options outstanding, vested and expected to vest, number outstanding (in shares) 7,816      
Options outstanding, vested and expected to vest, weighted average remaining contractual life (Year) 1 year 277 days      
Options outstanding, vested and expected to vest, weighted average exercise price (in dollars per share) $ 47.08      
Options exercisable, number exercisable (in shares) 5,390      
Options exercisable, weighted average exercise price (in dollars per share) $ 47.08      
Exercise Price Range Five [Member]        
Range of exercise prices, lower limit (in dollars per share) 47.16      
Range of exercise prices, upper limit (in dollars per share) $ 66.87      
Options outstanding, vested and expected to vest, number outstanding (in shares) 179,032      
Options outstanding, vested and expected to vest, weighted average remaining contractual life (Year) 6 years 83 days      
Options outstanding, vested and expected to vest, weighted average exercise price (in dollars per share) $ 58.35      
Options exercisable, number exercisable (in shares) 78,874      
Options exercisable, weighted average exercise price (in dollars per share) $ 58.83      
Exercise Price Range Six [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) 105,319      
Options outstanding, vested and expected to vest, weighted average remaining contractual life (Year) 6 years 105 days      
Options outstanding, vested and expected to vest, weighted average exercise price (in dollars per share) $ 74.69      
Options exercisable, number exercisable (in shares) 48,330      
Options exercisable, weighted average exercise price (in dollars per share) $ 71.57