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Benefit Plans - Actuarial Assumptions and Year-End Asset Allocations (Details)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Pension and Supplemental Executive Retirement Plans    
Defined Benefit Plan Disclosure [Line Items]    
Discount Rate (in hundredths) 4.40% 3.75%
Rate of Compensation Increase (in hundredths) 3.00% 3.00%
Discount Rate (in hundredths) 3.75% 4.30%
Expected Long-term Return on Plan Assets (in hundredths) 5.75% 5.75%
Rate of Compensation Increase (in hundredths) 3.00% 3.00%
Other Postretirement Benefit Plan    
Defined Benefit Plan Disclosure [Line Items]    
Discount Rate (in hundredths) 4.25% 3.55%
Discount Rate (in hundredths) 3.55% 3.95%
Expected Long-term Return on Plan Assets (in hundredths) 7.50% 7.50%
Health Care Cost Trend Rate Assumed for Next Year (in hundredths) 6.25% 6.50%
Rate to Which the Cost Trend Rate is Assumed to Decline (Ultimate Trend Rate) (in hundredths) 5.00% 5.00%
Year That the Rate Reaches the Ultimate Trend Rate 2024 2024
Weighted-average asset allocations of plans (in hundredths) 100.00% 100.00%
Other Postretirement Benefit Plan | Equity securities    
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocations of plans (in hundredths) 100.00% 100.00%
Other Postretirement Benefit Plan | Total fixed income securities    
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocations of plans (in hundredths) 0.00% 0.00%
Pension Plan    
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocations of plans (in hundredths) 100.00% 100.00%
Pension Plan | Equity securities    
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocations of plans (in hundredths) 23.00% 21.00%
Pension Plan | Total fixed income securities    
Defined Benefit Plan Disclosure [Line Items]    
Weighted-average asset allocations of plans (in hundredths) 77.00% 79.00%