Note 13 - Employee Benefits - Stock Options Valuation Assumptions (Details) - $ / shares		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Expected option life in years (Year)	[1]	6 years	6 years	6 years
Expected stock price volatility percentage	[2]	28.00%	28.00%	26.00%
Risk-free interest rate percentage	[3]	0.60%	2.20%	2.80%
Expected dividend yield	[4]	0.80%	0.58%	0.48%
Fair value as of the date of grant (in dollars per share)		$ 3.85	$ 3.93	$ 4.13

[1] Expected option life – We use historical employee exercise and option expiration data to estimate the expected life assumption for the Black-Scholes grant-date valuation. We believe that this historical data is currently the best estimate of the expected term of a new option. We use a weighted-average expected life for all awards.

[2] Expected stock price volatility – We use our stock’s historical volatility for the same period of time as the expected life. We have no reason to believe that its future volatility will differ from the past.

[3] Risk-free interest rate – The rate is based on the U.S. Treasury yield curve in effect at the time of the grant for the same period of time as the expected life.

[4] Expected dividend yield – The calculation is based on the total expected annual dividend payout divided by the average stock price.