Note 1 - Nature of Business and Significant Accounting Policies - Option Pricing Model Valuation Assumptions (Details) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Expected life of option grants (Year)	10 years
Weighted-average grant date fair value (in dollars per share)	$ 14.68	$ 14.75	$ 7.31
Employee Stock Option
Expected life of option grants (Year)	6 years	6 years	6 years
Weighted-average grant date fair value (in dollars per share)	$ 14.68	$ 14.75	$ 7.31
Employee Stock Option | Minimum
Dividend yield	0.45%	0.36%	0.35%
Expected volatility	29.51%	29.64%	29.32%
Risk-free interest rate	2.60%	2.50%	1.73%
Employee Stock Option | Maximum
Dividend yield	0.48%	0.47%	0.51%
Expected volatility	29.59%	29.95%	29.37%
Risk-free interest rate	2.94%	2.81%	2.18%
Stock Purchase Grants [Member]
Weighted-average grant date fair value (in dollars per share)	$ 6.63	$ 6.42	$ 3.28
Stock Purchase Grants [Member] | Minimum
Dividend yield	0.37%	0.37%	0.33%
Expected volatility	20.90%	19.80%	12.70%
Risk-free interest rate	1.59%	0.67%	0.39%
Expected life of option grants (Year)	3 months	3 months	3 months
Stock Purchase Grants [Member] | Maximum
Dividend yield	0.51%	0.42%	0.59%
Expected volatility	21.40%	19.86%	15.60%
Risk-free interest rate	2.22%	1.18%	0.57%
Expected life of option grants (Year)	6 years	6 months	6 months