Note 5 - Share-based Compensation - Stock Options Valuation Assumptions (Details)	9 Months Ended
Sep. 30, 2016	Sep. 30, 2015
Common Class A [Member] \| Minimum [Member]
Expected dividend yield at date of grant	2.96%	2.14%
Expected stock price volatility	31.33%	30.86%
Risk-free interest rate	1.36%	1.55%
Expected life of options (in years)	6 years	5 years
Common Class A [Member] \| Maximum [Member]
Expected dividend yield at date of grant	3.02%	2.57%
Expected stock price volatility	34.61%	34.87%
Risk-free interest rate	2.12%	1.78%
Expected life of options (in years)	8 years	7 years
Common Class B [Member] \| Minimum [Member]
Expected dividend yield at date of grant	6.67%	5.29%
Expected stock price volatility	27.64%	29.96%
Risk-free interest rate	1.36%	1.55%
Expected life of options (in years)	6 years	5 years
Common Class B [Member] \| Maximum [Member]
Expected dividend yield at date of grant	8.12%	5.72%
Expected stock price volatility	31.77%	33.94%
Risk-free interest rate	2.12%	1.78%
Expected life of options (in years)	8 years	7 years

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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- Definition

The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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- Definition

The risk-free interest rate assumption that is used in valuing an option on its own shares.

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- Definition

Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (f)(2)(i)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

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