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Note 12 - Pension and Other Postretirement Benefit Plans - Assumptions Used in Determining the Actuarial Present Value of the Accumulated Postretirement Benefit Obligations (Details)
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Rate of increase in health care costs      
Initial 8.00% 7.00% 8.00%
Ultimate (year 2023) 5.00% 5.00% 5.00%
Other Postretirement Benefits Plan [Member]      
Weighted average discount rate 3.42% 3.88%  
Rate of increase in health care costs      
Initial 7.00% 8.00%  
Ultimate (year 2023) 5.00% 5.00%  
Rate of increase in future compensation levels 0.00% 0.00%