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Note 18 - Pension and Postretirement Benefits - Summary of Weighted-average Assumptions Used in Calculating Net Periodic Benefit (Credit) Cost (Details)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Pension Plan and SERP Plan [Member]      
Discount rate 2.49% 3.24%  
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligations, Expected Return On Plan Assets 6.50% 6.75%  
Cash balance interest credit rate 2.57% 2.57%  
Discount rate 2.83% 3.82% 3.50%
Expected return on plan assets 6.75% 7.00% 7.00%
Cash balance interest credit rate 2.57% 2.57% 2.57%
Other Postretirement Benefits Plan [Member]      
Discount rate 1.50% 2.50%  
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligations, Expected Return On Plan Assets 2.00% 2.00%  
Discount rate 2.50% 3.75% 3.00%
Expected return on plan assets 2.00% 2.00% 2.00%