Note 9 - Share-based Compensation - Stock Options Valuation Assumptions (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Common Class A [Member]
Expected dividend yield at date of grant	2.62%	2.99%	2.22%
Expected stock price volatility	32.45%	32.74%	31.97%
Risk-free interest rate	2.18%	1.69%	1.58%
Expected life of options (in years) (Year)	6 years 292 days	6 years 313 days	5 years 178 days
Common Class B [Member]
Expected dividend yield at date of grant	8.06%	7.29%	5.48%
Expected stock price volatility	26.75%	29.41%	31.17%
Risk-free interest rate	2.18%	1.69%	1.60%
Expected life of options (in years) (Year)	6 years 292 days	6 years 313 days	5 years 226 days