Note 6 - Share-based Compensation - Stock Options Valuation Assumptions (Details)	6 Months Ended
Jun. 30, 2018	Jun. 30, 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	2.59%
Expected stock price volatility	32.47%
Risk-free interest rate	2.51%
Expected life of options (in years) (Year)	7 years 109 days

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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
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-Subparagraph (SAB Topic 14.D.2)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

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