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Pension and Other Postretirement Plans - Reconciliation of the Change in Benefit Obligation and Plan Asset, Funded Status and Amounts Recognized in the Financial Statements (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Amounts recognized in the consolidated balance sheets consist of:      
Total liability $ (2,456) $ (1,434)  
Pension Benefits      
Defined Benefit Plan Disclosure [Line Items]      
Accumulated benefit obligation 5,387 5,333  
Change in projected benefit obligation:      
Projected benefit obligation at beginning of year 6,398 5,396  
Service cost 216 184 $ 228
Interest cost 209 226 217
Benefits paid (1,445) (200)  
Actuarial loss 1,181 784  
Special termination benefit 54 0  
Benefits paid (1,445) (200)  
Curtailment (105) 0  
Other 17 8  
Projected benefit obligation at end of year 6,525 6,398 5,396
Change in plan assets:      
Fair value of plan assets at beginning of year 4,964 3,827  
Actual return on plan assets 521 684  
Employer contributions 16 649  
Benefits paid (1,445) (200)  
Other 13 4  
Fair value of plan assets at end of year 4,069 4,964 3,827
Funded status—Net amount recognized (2,456) (1,434)  
Amounts recognized in the consolidated balance sheets consist of:      
Noncurrent asset 8 14  
Current liability (4) (2)  
Noncurrent liability (2,460) (1,446)  
Amounts recognized in accumulated other comprehensive loss consist of:      
Net actuarial loss (1,924) (1,652)  
Prior service cost (3) (4)  
Total accumulated other comprehensive loss (1,927) (1,656)  
Other Postretirement Benefits      
Change in projected benefit obligation:      
Projected benefit obligation at beginning of year 842 1,391  
Service cost 10 10 12
Interest cost 28 47 61
Plan participants' contributions 58 67  
Benefits paid (164) (180)  
Actuarial loss 107 99  
Plan amendments 0 (597)  
Special termination benefit 201 0  
Benefits paid (164) (180)  
Other 0 5  
Projected benefit obligation at end of year 1,082 842 1,391
Change in plan assets:      
Fair value of plan assets at beginning of year 52 53  
Actual return on plan assets 1 1  
Employer contributions 104 111  
Plan participants' contributions 58 67  
Benefits paid (164) (180)  
Fair value of plan assets at end of year 51 52 $ 53
Funded status—Net amount recognized (1,031) (790)  
Amounts recognized in the consolidated balance sheets consist of:      
Current liability (37) (1)  
Noncurrent liability (994) (789)  
Total liability (1,031) (790)  
Amounts recognized in accumulated other comprehensive loss consist of:      
Net actuarial loss (255) (403)  
Prior service cost (570) (693)  
Total accumulated other comprehensive loss $ (825) $ (1,096)