Note 3 - Share-based Compensation (Details Textual) $ in Millions	3 Months Ended
Mar. 31, 2018 USD ($) shares
Restricted Stock Units (RSUs) [Member]
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Total	$ 77.2
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition	3 years 219 days
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period \| shares	3,432
Performance Shares [Member]
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Total	$ 24.8
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition	2 years 219 days

Unrecognized cost of unvested share-based compensation awards.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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- Definition

Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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- Definition

Number of share instruments newly issued under a share-based compensation plan.

+ References

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-Name Accounting Standards Codification
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