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Retirement Plans and Other Benefits - Weighted-Average Assumptions for Pensions and Other Benefits (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Weighted-average assumptions used to determine benefit obligations      
Rate of compensation increase (as a percent) 4.00% 4.00%  
Initial pre-65 healthcare cost trend rate (as a percent) 7.00% 7.00%  
Initial post-65 healthcare cost trend rate (as a percent) 4.75% 4.75%  
Ultimate health care cost trend rate (as a percent) 4.75% 4.75%  
Number of years to ultimate trend rate (pre-65 participants) 7 years 8 years  
Weighted-average assumptions used to determine net periodic benefit costs      
Initial pre-65 health care cost trend rate (as a percent) 7.00% 7.00% 7.00%
Initial post-65 health care cost trend rate (as a percent) 4.75% 5.00% 5.00%
Ultimate healthcare cost trend rate (as a percent) 4.75% 5.00% 5.00%
Number of years to ultimate trend rate (pre-65 participants) 8 years 4 years 4 years
Pension Benefits      
Weighted-average assumptions used to determine benefit obligations      
Discount rate (as a percent) 4.34% 3.65%  
Weighted-average assumptions used to determine net periodic benefit costs      
Discount rate (as a percent) 3.65% 4.08% 4.37%
Rate of compensation increase (as a percent) 4.00% 4.00% 4.00%
Expected long-term return on plan assets (as a percent) 6.05% 6.55% 6.90%
Other Benefits      
Weighted-average assumptions used to determine benefit obligations      
Discount rate (as a percent) 4.39% 3.71%  
Weighted-average assumptions used to determine net periodic benefit costs      
Discount rate (as a percent) 3.71% 4.17% 4.52%
Expected long-term return on plan assets (as a percent) 5.40% 6.05% 4.45%
Effects of one percentage point change in the assumed initial and ultimate health care cost trend rates      
Effect of 1% increase on other postretirement benefits expense, after consideration of amounts capitalized or billed to electric plant participants $ 10,235    
Effect of 1% decrease on other postretirement benefits expense, after consideration of amounts capitalized or billed to electric plant participants (4,322)    
Effect of 1% increase on service and interest cost components of net periodic other postretirement benefit costs 11,223    
Effect of 1% decrease on service and interest cost components of net periodic other postretirement benefit costs (8,479)    
Effect of 1% increase on the accumulated other postretirement benefit obligation 101,224    
Effect of 1% decrease on the accumulated other postretirement benefit obligation $ (81,144)