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Note 11 - Share Option Plans - Summary of Exercise Prices, Weighted Average Remaining Contractual Life and Intrinsic Values of Outstanding Options (Details)
 12 Months Ended
Dec. 31, 2017
USD ($)
$ / shares
shares
Number outstanding (in shares) | shares 653,571
Weighted average exercise price per share, options outstanding (in dollars per share) $ 36.69
Weighted average remaining contractual life, options outstanding (Year) 4 years 328 days
Aggregate intrinsic value, options outstanding | $ $ 21,801,765
Number exercisable (in shares) | shares 243,771
Weighted average exercise price per share, options exercisable (in dollars per share) $ 14.79
Weighted average remaining contractual life, options exercisable (Year) 2 years 292 days
Aggregate intrinsic value, options exercisable | $ $ 13,471,695
Exercise Price Range 01 [Member]  
Exercise price - lower (in dollars per share) $ 2.92
Exercise price - higher (in dollars per share) $ 8.92
Number outstanding (in shares) | shares 91,133
Weighted average exercise price per share, options outstanding (in dollars per share) $ 6.63
Weighted average remaining contractual life, options outstanding (Year) 1 year 219 days
Aggregate intrinsic value, options outstanding | $ $ 5,779,404
Number exercisable (in shares) | shares 91,133
Weighted average exercise price per share, options exercisable (in dollars per share) $ 6.63
Weighted average remaining contractual life, options exercisable (Year) 1 year 219 days
Aggregate intrinsic value, options exercisable | $ $ 5,779,404
Exercise Price Range 02 [Member]  
Exercise price - lower (in dollars per share) $ 10.16
Exercise price - higher (in dollars per share) $ 19.95
Number outstanding (in shares) | shares 120,913
Weighted average exercise price per share, options outstanding (in dollars per share) $ 16.30
Weighted average remaining contractual life, options outstanding (Year) 3 years 182 days
Aggregate intrinsic value, options outstanding | $ $ 6,499,346
Number exercisable (in shares) | shares 92,638
Weighted average exercise price per share, options exercisable (in dollars per share) $ 15.87
Weighted average remaining contractual life, options exercisable (Year) 3 years 109 days
Aggregate intrinsic value, options exercisable | $ $ 5,018,978
Exercise Price Range 03 [Member]  
Exercise price - lower (in dollars per share) $ 21.10
Exercise price - higher (in dollars per share) $ 27.53
Number outstanding (in shares) | shares 81,000
Weighted average exercise price per share, options outstanding (in dollars per share) $ 23.65
Weighted average remaining contractual life, options outstanding (Year) 4 years 73 days
Aggregate intrinsic value, options outstanding | $ $ 3,758,393
Number exercisable (in shares) | shares 55,000
Weighted average exercise price per share, options exercisable (in dollars per share) $ 24.61
Weighted average remaining contractual life, options exercisable (Year) 3 years 292 days
Aggregate intrinsic value, options exercisable | $ $ 2,499,313
Exercise Price Range 04 [Member]  
Exercise price - lower (in dollars per share) $ 35.25
Exercise price - higher (in dollars per share) $ 37.35
Number outstanding (in shares) | shares 14,375
Weighted average exercise price per share, options outstanding (in dollars per share) $ 35.89
Weighted average remaining contractual life, options outstanding (Year) 5 years 146 days
Aggregate intrinsic value, options outstanding | $ $ 491,062
Number exercisable (in shares) | shares 5,000
Weighted average exercise price per share, options exercisable (in dollars per share) $ 35.25
Weighted average remaining contractual life, options exercisable (Year) 6 years
Aggregate intrinsic value, options exercisable | $ $ 174,000
Exercise Price Range 05 [Member]  
Exercise price - lower (in dollars per share) $ 43.15
Exercise price - higher (in dollars per share) $ 47
Number outstanding (in shares) | shares 18,500
Weighted average exercise price per share, options outstanding (in dollars per share) $ 44.19
Weighted average remaining contractual life, options outstanding (Year) 6 years 36 days
Aggregate intrinsic value, options outstanding | $ $ 478,400
Number exercisable (in shares) | shares
Weighted average exercise price per share, options exercisable (in dollars per share)
Weighted average remaining contractual life, options exercisable (Year)
Aggregate intrinsic value, options exercisable | $
Exercise Price Range 06 [Member]  
Exercise price - lower (in dollars per share) $ 53.20
Exercise price - higher (in dollars per share) $ 55.65
Number outstanding (in shares) | shares 327,650
Weighted average exercise price per share, options outstanding (in dollars per share) $ 55.41
Weighted average remaining contractual life, options outstanding (Year) 6 years 146 days
Aggregate intrinsic value, options outstanding | $ $ 4,795,160
Number exercisable (in shares) | shares
Weighted average exercise price per share, options exercisable (in dollars per share)
Weighted average remaining contractual life, options exercisable (Year)
Aggregate intrinsic value, options exercisable | $