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Savings Plans, Pension Plans and Other Postretirement Employee Benefits (Narrative) (Details) - USD ($)
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2009
Defined Benefit Plan Disclosure [Line Items]        
Contributions by employer $ 3,900,000 $ 3,700,000 $ 2,400,000  
Accumulated benefit obligation 458,100,000 417,400,000    
Funding of pension and other postretirement benefit plans $ 5,678,000 57,580,000 $ 10,638,000  
Expected return on plan assets 5.75%      
Expected increase in net periodic cost due to 25 basis point decrease in pension discount rate $ 1,100,000      
Expected increase in projected benefit obligation due to 25 basis point decrease in pension discount rate 13,600,000      
Expected increase in net periodic cost due to 25 basis point decrease in the assumption for expected return on plan assets 900,000      
Level 3 [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Investments in Level 3 0 0    
Qualified Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Pension benefit contributions, voluntary   $ 44,000,000.0    
Non-qualified pension plan and other postretirement employee benefit, estimated payments in next fiscal year $ 4,400,000      
Accumulated Other Comprehensive Loss [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Decrease in accumulated postretirement benefit obligations       $ 76,700,000
Management Deferred Compensation Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Annual incentive plan, employee permitted minimum defer percentage 50.00%      
Annual incentive plan, employee permitted maximum defer percentage 100.00%      
Management Deferred Compensation Plan [Member] | Minimum [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Basic salary defer percentage 50.00%      
Pension Plans [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Estimated net loss to be amortized from Accumulated Other Comprehensive Loss into Net Periodic Benefit Cost $ 17,000,000.0      
Estimated prior service cost to be amortized from Accumulated Other Comprehensive Loss into Net Periodic Benefit Cost 100,000      
Non-qualified pension plan and other postretirement employee benefit, estimated payments in next fiscal year 30,769,000      
Pension Plans [Member] | Nonqualified Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Non-qualified pension plan and other postretirement employee benefit, estimated payments in next fiscal year 2,200,000      
Other Postretirement Benefit Plans [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Estimated net loss to be amortized from Accumulated Other Comprehensive Loss into Net Periodic Benefit Cost 1,700,000      
Estimated prior service cost to be amortized from Accumulated Other Comprehensive Loss into Net Periodic Benefit Cost 1,300,000      
Non-qualified pension plan and other postretirement employee benefit, estimated payments in next fiscal year 4,548,000      
Other Postretirement Benefit Plans [Member] | Nonqualified Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Non-qualified pension plan and other postretirement employee benefit, estimated payments in next fiscal year $ 4,500,000      
Non Deltic Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Health care cost trend rate assumed for next fiscal year 7.51%      
Defined Benefit Plan, Assumption Health Care Cost Trend Rate for Next Fiscal Year 4.50%      
Deltic Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Health care cost trend rate assumed for next fiscal year 7.18%      
Defined Benefit Plan, Assumption Health Care Cost Trend Rate for Next Fiscal Year 4.50%