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Benefit Plans (Narrative) (Details) - USD ($)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]      
Amounts charged to expense for defined contribution plans $ 88,000,000 $ 78,000,000 $ 85,000,000
Pension [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Accumulated benefit obligation 9,100,000,000 8,400,000,000  
Accumulated benefit obligation 393,000,000 357,000,000  
Fair value of plan assets 149,000,000 134,000,000  
Projected benefit obligation 9,324,000,000 8,522,000,000  
Defined benefit plan, pension plan with projected benefit obligation in excess of plan assets, plan assets 8,750,000,000 8,023,000,000  
Defined benefit plan, plan assets, amount $ 8,911,000,000 $ 8,167,000,000 $ 7,138,000,000
Defined benefit plan, assumptions used calculating net periodic benefit cost, expected long-term rate of return on plan assets 6.60% 7.80% 7.80%
Pension [Member] | Maximum [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Expected future employer contributions $ 30,000,000    
Pension [Member] | Emerging Markets [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Composition of plan assets 1.00%    
Pension [Member] | Below grade investments [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Composition of plan assets 8.00%    
Pension [Member] | Equity Securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets, target allocation percentage 70.00%    
Pension [Member] | Equity Securities [Member] | Emerging Markets [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Composition of plan assets 1.00%    
Pension [Member] | Fixed Income Securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets, target allocation percentage 30.00%    
Pension [Member] | Fixed Income Securities [Member] | Below grade investments [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Composition of plan assets 11.00%    
Postretirement Benefit Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan, plan assets, amount $ 201,000,000 $ 213,000,000 $ 211,000,000
Defined benefit plan, assumptions used calculating net periodic benefit cost, expected long-term rate of return on plan assets 7.70% 7.80% 7.80%
Postretirement Benefit Plan [Member] | Maximum [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Expected future employer contributions $ 60,000,000    
Postretirement Benefit Plan [Member] | Emerging Markets [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan, plan assets, amount $ 0    
Postretirement Benefit Plan [Member] | Below grade investments [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Composition of plan assets 3.00%    
Postretirement Benefit Plan [Member] | Equity Securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets, target allocation percentage 55.00%    
Postretirement Benefit Plan [Member] | Fixed Income Securities [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Plan assets, target allocation percentage 45.00%    
Postretirement Benefit Plan [Member] | Fixed Income Securities [Member] | Below grade investments [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Composition of plan assets 8.00%    
Level 3 [Member] | Postretirement Benefit Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan, plan assets, amount $ 0 $ 0