Note 9 - Share-based Compensation - Stock Options Valuation Assumptions (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Common Class A [Member]
Expected dividend yield at date of grant			2.62%
Expected stock price volatility			32.45%
Risk-free interest rate			2.18%
Expected life of options (in years) (Year)			6 years 292 days
Common Class B [Member]
Expected dividend yield at date of grant			8.06%
Expected stock price volatility			26.75%
Risk-free interest rate			2.18%
Expected life of options (in years) (Year)			6 years 292 days
Common Stock [Member]
Expected dividend yield at date of grant	2.60%	2.59%
Expected stock price volatility	34.01%	32.47%
Risk-free interest rate	2.38%	2.51%
Expected life of options (in years) (Year)	7 years 167 days	7 years 102 days