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Note 18 - Weighted-average Assumption Used to Determine Net Post-retirement Benefit Cost (Details) - Other Postretirement Benefit Plans [Member]
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Weighted Average Assumption Used To Determine Net Postretirement Benefit Cost [Line Items]      
Effective rate for interest cost 2.87% 2.85% 3.49%
Salary increase rate 2.35% 1.80% 2.60%