Note 6 - Stock-based Compensation - Restricted Stock Units (Details)	9 Months Ended
Sep. 30, 2018 $ / shares shares
Time-based Restricted Stock Units [Member]
Balance (in shares) \| shares	221,738
Balance (in dollars per share) \| $ / shares	$ 32.72
Granted (in shares) \| shares	141,810
Granted (in dollars per share) \| $ / shares	$ 43.30
Vested (in shares) \| shares	(65,721)
Vested (in dollars per share) \| $ / shares	$ 41.59
Forfeited (in shares) \| shares	(12,535)
Forfeited (in dollars per share) \| $ / shares	$ 35.46
Balance (in shares) \| shares	285,292
Balance (in dollars per share) \| $ / shares	$ 35.82
Performance-based Restricted Stock Units [Member]
Balance (in shares) \| shares	339,872
Balance (in dollars per share) \| $ / shares	$ 33.52
Granted (in shares) \| shares	55,455
Granted (in dollars per share) \| $ / shares	$ 39.46
Vested (in shares) \| shares	(68,459)
Vested (in dollars per share) \| $ / shares	$ 41.22
Forfeited (in shares) \| shares
Forfeited (in dollars per share) \| $ / shares
Balance (in shares) \| shares	326,868
Balance (in dollars per share) \| $ / shares	$ 32.91

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

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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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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).

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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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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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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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