Note 6 - Share-based Compensation - Stock Options Valuation Assumptions (Details)	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Common Class A [Member] | Minimum [Member]
Expected dividend yield at date of grant	2.14%	1.47%
Expected stock price volatility	30.86%	27.52%
Risk-free interest rate	1.55%	1.63%
Expected life of options (in years)	5 years	5 years
Common Class A [Member] | Maximum [Member]
Expected dividend yield at date of grant	2.57%	1.97%
Expected stock price volatility	34.87%	32.03%
Risk-free interest rate	1.78%	2.37%
Expected life of options (in years)	7 years	7 years
Common Class B [Member] | Minimum [Member]
Expected dividend yield at date of grant	5.29%	4.03%
Expected stock price volatility	29.96%	30.13%
Risk-free interest rate	1.55%	1.63%
Expected life of options (in years)	5 years	5 years
Common Class B [Member] | Maximum [Member]
Expected dividend yield at date of grant	5.72%	4.87%
Expected stock price volatility	33.94%	32.65%
Risk-free interest rate	1.78%	2.37%
Expected life of options (in years)	7 years	7 years