Note 14 - Stock-based Compensation - Performance-based Restricted Stock Units (Details) - $ / shares	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Expected shares to be awarded (in shares)	506,000
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)	$ 32.03	$ 31.54	$ 35.66
Expected shares to be awarded (in shares)	43,602	50,352	61,893

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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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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