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Note 11 - Pension Plan and Other Postretirement Benefits (Details Textual) - USD ($)
 1 Months Ended 12 Months Ended
Jan. 31, 2020
Jul. 30, 2015
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year     $ 3,300,000    
Defined Benefit Plan, Expected Future Prescription Drug Subsidy Receipt, Next Twelve Months     100,000    
Defined Contribution Plan, Cost     5,265,000 $ 4,532,000 $ 4,211,000
Employee Stock Ownership Plan (ESOP), Cash Contributions to ESOP     $ 374,000 398,000 $ 612,000
Pension Plan [Member]          
Defined Benefit Plan, Percentage Vesting After the Specified Employment Period     100.00%    
Defined Benefit Plan, Vesting Period     5 years    
Defined Benefit Plan, Age for Compensation     65    
Defined Benefit Plan, Age Below Which Compensation is Reduced     62    
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets for Next Fiscal Year     6.88%    
Defined Benefit Plan, Plan Assets at Fair Value, Valuation Inputs, Period Over Which Investment Gains or Losses are Recognized     5 years    
Defined Benefit Plan, Plan Assets, Contributions by Employer     $ 22,500,000 20,000,000  
Pension Plan [Member] | SEI Energy Debt Fund [Member]          
Amount Invested in Pension Fund Assets   $ 10,000,000.0      
Pension Plan [Member] | Subsequent Event [Member]          
Defined Benefit Plan, Plan Assets, Contributions by Employer $ 11,200,000        
Other Postretirement Benefits Plan [Member]          
Defined Benefit Plan, Plan Assets, Contributions by Employer     $ 2,757,000 3,183,000  
Health Insurance Benefits, Requisite Age     55 years    
Health Insurance Benefits, Requisite Service Period     10 years    
Health Insurance Benefits, Reduction in Prescription Reimbursement, Percent     27.00%    
Defined Benefit Plan, Benefit Obligation, Increase (Decrease) for Plan Amendment     $ (20,864,000) $ 0  
Health Insurance Benefits, Reduction in Benefits, Period for Recognition     4 years 3 months 18 days