Note 1 - Nature of Business and Significant Accounting Policies - Option Pricing Model Valuation Assumptions (Details) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Expected life of option grants (Year)	10 years
Weighted-average grant date fair value (in dollars per share)	$ 14.75	$ 7.31	$ 5.11
Employee Stock Option [Member]
Expected life of option grants (Year)	6 years	6 years	6 years
Weighted-average grant date fair value (in dollars per share)	$ 14.75	$ 7.31	$ 5.11
Employee Stock Option [Member] | Minimum [Member]
Dividend yield	0.36%	0.35%	0.37%
Expected volatility	29.64%	29.32%	28.92%
Risk-free interest rate	2.50%	1.73%	1.89%
Employee Stock Option [Member] | Maximum [Member]
Dividend yield	0.47%	0.51%	0.46%
Expected volatility	29.95%	29.37%	29.32%
Risk-free interest rate	2.81%	2.18%	2.37%
Stock Purchase Grants [Member]
Weighted-average grant date fair value (in dollars per share)	$ 6.42	$ 3.28	$ 2.39
Stock Purchase Grants [Member] | Minimum [Member]
Dividend yield	0.37%	0.33%	0.37%
Expected volatility	19.80%	12.70%	8.81%
Risk-free interest rate	0.67%	0.39%	0.09%
Expected life of option grants (Year)	90 days	90 days	90 days
Stock Purchase Grants [Member] | Maximum [Member]
Dividend yield	0.42%	0.59%	0.45%
Expected volatility	19.86%	15.60%	13.10%
Risk-free interest rate	1.18%	0.57%	0.016%
Expected life of option grants (Year)	180 days	180 days	180 days