Note 11 - Share-based Compensation - Stock Options Valuation Assumptions (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Common Class B [Member]
Expected dividend yield at date of grant		8.06%	7.29%
Expected stock price volatility		26.75%	29.41%
Risk-free interest rate		2.18%	1.69%
Expected life of options (in years) (Year)		6 years 292 days	6 years 313 days
Common Stock [Member]
Expected dividend yield at date of grant	2.59%	2.62%	2.99%
Expected stock price volatility	32.47%	32.45%	32.74%
Risk-free interest rate	2.51%	2.18%	1.69%
Expected life of options (in years) (Year)	7 years 102 days	6 years 292 days	6 years 313 days