Note 1 - Nature of Business and Significant Accounting Policies - Option Pricing Model Valuation Assumptions (Details) - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Expected life of option grants (years) (Year)	10 years
Weighted-average grant date fair value (in dollars per share)	$ 7.31	$ 5.11	$ 5.68
Employee Stock Option [Member]
Dividend yield			0.47%
Expected life of option grants (years) (Year)	6 years	6 years	6 years
Weighted-average grant date fair value (in dollars per share)	$ 7.31	$ 5.11	$ 5.68
Employee Stock Option [Member] | Minimum [Member]
Dividend yield	0.35%	0.37%
Expected volatility	29.32%	28.92%	29.07%
Risk-free interest rate	1.73%	1.89%	2.69%
Employee Stock Option [Member] | Maximum [Member]
Dividend yield	0.51%	0.46%
Expected volatility	29.37%	29.32%	29.18%
Risk-free interest rate	2.18%	2.37%	2.82%
Stock Purchase Grants [Member]
Weighted-average grant date fair value (in dollars per share)	$ 3.28	$ 2.39	$ 2.37
Stock Purchase Grants [Member] | Minimum [Member]
Dividend yield	0.33%	0.37%	0.46%
Expected volatility	12.70%	8.81%	16.96%
Risk-free interest rate	0.39%	0.09%	0.04%
Expected life of option grants (years) (Year)	3 years	3 years	3 years
Stock Purchase Grants [Member] | Maximum [Member]
Dividend yield	0.59%	0.45%	0.47%
Expected volatility	15.60%	13.10%	19.35%
Risk-free interest rate	0.57%	0.16%	0.12%
Expected life of option grants (years) (Year)	6 years	6 years	6 years