Note 3 - Share-based Compensation (Details Textual) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018 USD ($) shares	Sep. 30, 2018 USD ($) shares
Restricted Stock Units (RSUs) [Member]
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Total \| $	$ 63.2	$ 63.2
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	291,045	295,186
Performance Shares [Member]
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Total \| $	$ 17.1	$ 17.1
Employee Service Share-based Compensation, Nonvested Awards, Compensation Cost Not yet Recognized, Period for Recognition		2 years 182 days
Share-based Compensation Arrangement by Share-based Payment Award, Shares Issued in Period \| shares	118,438	118,438

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.

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