Note 1 - Nature of Business and Significant Accounting Policies - Option Pricing Model Valuation Assumptions (Details) - $ / shares	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Expected life of option grants (Year)	10 years
Weighted-average grant date fair value (in dollars per share)	$ 10.85	$ 10.07	$ 11.29
Employee Stock Option
Dividend yield	0.55%		0.67%
Expected volatility	23.57%		28.28%
Risk-free interest rate	1.62%		2.90%
Expected life of option grants (Year)	6 years 3 months	6 years 3 months	6 years 3 months
Weighted-average grant date fair value (in dollars per share)	$ 10.85	$ 10.07	$ 11.29
Employee Stock Option | Minimum
Dividend yield	0.55%	0.60%	0.67%
Expected volatility	23.57%	25.49%	28.28%
Risk-free interest rate	1.62%	0.79%	2.90%
Employee Stock Option | Maximum
Dividend yield		0.81%
Expected volatility		25.70%
Risk-free interest rate		1.31%
Stock Purchase Grants [Member]
Weighted-average grant date fair value (in dollars per share)	$ 7.67	$ 6.03	$ 4.81
Stock Purchase Grants [Member] | Minimum
Dividend yield	0.49%	0.55%	0.69%
Expected volatility	37.51%	24.59%	20.15%
Risk-free interest rate	0.00%	0.15%	2.02%
Expected life of option grants (Year)	3 months	3 months	3 months
Stock Purchase Grants [Member] | Maximum
Dividend yield	0.61%	0.81%	0.75%
Expected volatility	38.65%	36.38%	21.06%
Risk-free interest rate	0.06%	1.65%	2.46%
Expected life of option grants (Year)	6 months	6 months	6 months