Note 4 - Share-based Compensation - Stock Options Valuation Assumptions (Details)	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016
Common Class A [Member] \| Minimum [Member]
Expected dividend yield at date of grant	2.46%	2.96%
Expected stock price volatility	32.20%	31.33%
Risk-free interest rate	2.08%	1.36%
Expected life of options (Year)	6 years	6 years
Common Class A [Member] \| Maximum [Member]
Expected dividend yield at date of grant	2.87%	3.02%
Expected stock price volatility	32.62%	34.61%
Risk-free interest rate	2.33%	2.12%
Expected life of options (Year)	8 years	8 years
Common Class B [Member] \| Minimum [Member]
Expected dividend yield at date of grant	7.99%	6.67%
Expected stock price volatility	26.47%	27.64%
Risk-free interest rate	2.08%	1.36%
Expected life of options (Year)	6 years	6 years
Common Class B [Member] \| Maximum [Member]
Expected dividend yield at date of grant	8.10%	8.12%
Expected stock price volatility	27.18%	31.77%
Risk-free interest rate	2.33%	2.12%
Expected life of options (Year)	8 years	8 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.

+ References

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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=96867065&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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