Note 10 - Common Stock and Stock-based Compensation Plans - Summary of Restricted Stock Units Activity (Details) - Restricted Stock Units (RSUs) [Member] - $ / shares	3 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Unvested, number (in shares)	564,390
Unvested, weighted average fair value (in dollars per share)	$ 31.43	$ 32.28
Granted, number (in shares)	340,050
Granted, weighted average fair value (in dollars per share)	$ 28.57
Vested, number (in shares)	(220,908)
Vested, weighted average fair value (in dollars per share)	$ 29.23
Forfeited or expired, number (in shares)	(6,265)
Forfeited or expired, weighted average fair value (in dollars per share)	$ 29.63
Unvested, number (in shares)	677,267

The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iv)(3)
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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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-Topic 718
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-Section 50
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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

+ References

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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