Note 6 - Share-based Compensation - Stock Appreciation Rights Outstanding (Details) - $ / shares	6 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Lower range of exercise prices on options (in dollars per share)	$ 3.82
Upper range of exercise prices on options (in dollars per share)	24.28
Stock Appreciation Rights (SARs) [Member]
Lower range of exercise prices on options (in dollars per share)	16.35
Upper range of exercise prices on options (in dollars per share)	$ 23.59
Shares (in shares)	182,894	146,504
Weighted average remaining contratual life (Year)	3 years 40 days
Weighted average exercise price (in dollars per share)	$ 18.99	$ 17.38
Exercise Price Range 1 [Member]
Lower range of exercise prices on options (in dollars per share)	3.82
Upper range of exercise prices on options (in dollars per share)	5.88
Exercise Price Range 1 [Member] \| Stock Appreciation Rights (SARs) [Member]
Lower range of exercise prices on options (in dollars per share)	16.35
Upper range of exercise prices on options (in dollars per share)	$ 18.66
Shares (in shares)	134,379
Weighted average remaining contratual life (Year)	2 years 211 days
Weighted average exercise price (in dollars per share)	$ 17.33
Exercise Price Range 2 [Member]
Lower range of exercise prices on options (in dollars per share)	7.36
Upper range of exercise prices on options (in dollars per share)	10.38
Exercise Price Range 2 [Member] \| Stock Appreciation Rights (SARs) [Member]
Lower range of exercise prices on options (in dollars per share)	23.59
Upper range of exercise prices on options (in dollars per share)	$ 23.59
Shares (in shares)	48,515
Weighted average remaining contratual life (Year)	4 years 211 days
Weighted average exercise price (in dollars per share)	$ 23.59

The weighted average exercise price for non-option equity instruments.

No definition available.

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- Definition

Weighted average remaining contractual term for equity-based awards excluding options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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- Definition

Number of equity instruments other than options outstanding, including both vested and non-vested instruments.

+ 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
-Subparagraph (c)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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

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- Definition

The floor of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

+ References

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- Definition

The ceiling of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

+ References

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