Note 7 - Stock-based Compensation - Valuation Assumptions (Details) - 2004 Employee Stock Plan [Member] - Employee Stock Purchase Plan [Member]	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Expected term (years) (Year)	182 days	182 days	182 days
Expected volatility	23.50%	28.60%	30.30%
Risk-free interest rate	0.90%	0.40%	0.20%
Dividend yield	0.90%	1.20%	1.40%