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Note 9 - Share-based Payments - Black-scholes Assumptions (Details)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Risk-free interest rate, average 2.21% 1.52% 2.00%
Expected option life, average (in years) (Year) 7 years 328 days 6 years 36 days 6 years 36 days
Expected price volatility, average 40.71% 43.75% 44.30%
Expected dividend yield 2.83% 1.71% 1.10%