Note 1 - Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

Notes Tables

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

The Company uses the Black-Scholes option pricing model to estimate the fair value of stock option grants with the following assumptions for the indicated periods:

	2018		2017		2016
Dividend yield		0.45% to 0.48%		0.36% to 0.47%		0.35% to 0.51%
Expected volatility		29.51% to 29.59%		29.64% to 29.95%		29.32% to 29.37%
Risk-free interest rate		2.60% to 2.94%		2.50% to 2.81%		1.73% to 2.18%
Expected life of option grants (in years)		6 years		6 years		6 years
Weighted-average grant date fair value	$	14.68	$	14.75	$	7.31

The Company also uses the Black-Scholes option pricing model to estimate the fair value of stock purchase grants with the following assumptions for the indicated periods:

	2018		2017		2016
Dividend yield		0.37% to 0.51%		0.37% to 0.42%		0.33% to 0.59%
Expected volatility		20.90% to 21.40%		19.80% to 19.86%		12.70% to 15.60%
Risk-free interest rate		1.59% to 2.22%		0.67% to 1.18%		0.39% to 0.57%
Expected life of purchase grants (in months)		3 to 6 months		3 to 6 months		3 to 6 months
Weighted-average grant date fair value	$	6.63	$	6.42	$	3.28