Note 13 - Stock-based Compensation - Performance-based Restricted Stock Units (Details) - $ / shares	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Expected shares to be awarded (in shares)	696,711
Performance Shares [Member]
Vesting period (Year)	3 years
Performance Shares [Member] \| Executive Officer [Member]
Vesting period (Year)	3 years	3 years	3 years
Shares awarded, grant date weighted average cost (in dollars per share)	$ 31.54	$ 35.66	$ 22.61
Expected shares to be awarded (in shares)	50,352	61,893	69,161

Period which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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