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Pension, Savings, and Other Employee Benefits - Pension Plan and Savings Plan, Additional Information (Details) - USD ($)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2021
Defined Benefit Plan Disclosure [Line Items]        
Minimum required outstanding par value to each issue of bonds $ 300,000,000      
Pension Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Defined benefit plan, benefit obligation 893,000,000 $ 836,000,000 $ 765,000,000  
Fair value of plan assets 896,000,000 826,000,000 731,000,000  
Employer contributions $ 4,000,000 3,000,000    
Qualified pension | Pension Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Estimated social security benefits age 65 years      
Nonqualified pension | Pension Benefits        
Defined Benefit Plan Disclosure [Line Items]        
Expected pension contribution $ 5,000,000      
Nonqualified pension | Pension Benefits | Forecast        
Defined Benefit Plan Disclosure [Line Items]        
Expected pension contribution       $ 5,000,000
Flexible Benefits Contribution        
Defined Benefit Plan Disclosure [Line Items]        
Employer contributions $ 37,000,000 $ 28,000,000 $ 29,000,000  
Qualified pension/ postretirement benefits        
Defined Benefit Plan Disclosure [Line Items]        
Expected time horizon 30 years      
Defined benefit plan assumptions used calculating benefit obligation expected long term return on assets 3.45%      
Qualified pension/ postretirement benefits | Maximum        
Defined Benefit Plan Disclosure [Line Items]        
Term used for assumption calculation 30 years      
Postretirement benefit (retirees prior to January 1, 1993)        
Defined Benefit Plan Disclosure [Line Items]        
Defined benefit plan assumptions used calculating benefit obligation expected long term return on assets 0.90%      
Postretirement benefit (retirees post January 1, 1993)        
Defined Benefit Plan Disclosure [Line Items]        
Defined benefit plan assumptions used calculating benefit obligation expected long term return on assets 6.40%      
Savings Plan        
Defined Benefit Plan Disclosure [Line Items]        
Employer investment in qualified defined contribution plan 100.00%      
Maximum percent of employee pre-tax contributions that may be matched by the Company (percent) 6.00%