Note 9 - Share-based Compensation - Stock Options Valuation Assumptions (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Common Class A [Member] | Minimum [Member]
Expected dividend yield at date of grant	2.96%	2.00%	1.47%
Expected stock price volatility	31.33%	30.86%	27.52%
Risk-free interest rate	1.36%	1.41%	1.63%
Expected life of options (in years) (Year)	6 years	5 years	5 years
Common Class A [Member] | Maximum [Member]
Expected dividend yield at date of grant	3.02%	2.57%	1.97%
Expected stock price volatility	34.61%	34.87%	32.03%
Risk-free interest rate	2.12%	1.78%	2.37%
Expected life of options (in years) (Year)	8 years	7 years	7 years
Common Class B [Member] | Minimum [Member]
Expected dividend yield at date of grant	6.67%	5.29%	4.03%
Expected stock price volatility	27.64%	29.72%	30.13%
Risk-free interest rate	1.36%	1.41%	1.63%
Expected life of options (in years) (Year)	6 years	5 years	5 years
Common Class B [Member] | Maximum [Member]
Expected dividend yield at date of grant	8.12%	5.72%	4.87%
Expected stock price volatility	31.77%	33.94%	32.65%
Risk-free interest rate	2.12%	1.78%	2.37%
Expected life of options (in years) (Year)	8 years	7 years	7 years