Note 1 - Nature of Business and Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2017

Notes Tables

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]

	2017				2016				201 5
Dividend yield		0.36%	to	0.47%		0.35%	to	0.51%		0.37%	to	0.46%
Expected volatilit y		29.64%	to	29.95%		29.32%	to	29.37%		28.92%	to	29.32%
Risk-free interest rate		2.50%	to	2.81%		1.73%	to	2.18%		1.89%	to	2.37%
Expected life of option grants (in years)			6				6				6
Weighted-average grant date fair value			$14.75				$7.31				$5.11

	2017				2016				20 15
Dividend yield		0.37%	to	0.42%		0.33%	to	0.59%		0.37%	to	0.45%
Expect ed volatility		19.80%	to	19.86%		12.70%	to	15.60%		8.81%	to	13.10%
Risk-free interest rate		0.67%	to	1.18%		0.39%	to	0.57%		0.09%	to	.016%
Expected life of purchase grants (in months)		3	to	6		3	to	6		3	to	6
Weighted-average grant date fair value			$6.42				$3.28				$2.39