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Note 2 - Stock-based Compensation - Stock Option Activity (Details) - $ / shares
6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Outstanding stock options weighted-average exercise price, January 1 (in dollars per share) $ 23.67 $ 25.03
Outstanding stock options, January 1 (in shares) 29,324 31,949
Exercised stock options (in shares) (7,949)
Exercised stock options weighted-average exercise price (in dollars per share) $ 29.87
Outstanding stock options, March 31 (in shares) 21,375 31,949
Outstanding stock options weighted-average exercise price, March 31 (in dollars per share) $ 26.91 $ 25.03
Exercisable stock options weighted-average exercise price, March 31 (in dollars per share) $ 26.91 $ 25.03
Exercisable stock options, March 31 (in shares) 21,375 31,949