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Pension and Other Postretirement Benefits - Assumptions Used to Determine Benefit Obligations (Details)
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Postretirement Plans [Member]      
Discount rate, net periodic benefit 4.06% 3.42% 3.88%
Initial 7.00% 7.00% 8.00%
Ultimate (year 2023) 5.00% 5.00% 5.00%
Retirement Plan [Member]      
Discount rate, benefit obligation 3.00% 4.06%  
Discount rate, net periodic benefit 4.06% 3.42% 3.88%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets 5.25% 7.00% 7.00%
Rate of return on plan assets 5.25% 7.00% 7.00%
Other Postretirement Benefits Plan [Member]      
Discount rate, benefit obligation 3.00% 4.06%  
Initial 7.50% 7.00%  
Ultimate (year 2023) 5.00% 5.00%  
Directors' Plan [Member]      
Discount rate, benefit obligation 3.00% 4.06% 3.42%
Discount rate, net periodic benefit 4.06% 3.42% 3.88%