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Employee Benefit Plans - Change in Benefit Obligation (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Pension Benefits      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation — beginning of year $ 4,498 $ 4,000  
Service cost 4 4 $ 4
Interest cost 127 159 142
Plan participants’ contributions 0 0  
Actuarial loss (gain) 12 48  
Amendments 0 0  
Changes in assumptions 437 488  
Benefits and expenses paid (203) (201)  
Benefit obligation — end of year 4,875 4,498 4,000
Other Postretirement Benefits      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation — beginning of year 223 220  
Service cost 0 0 0
Interest cost 6 8 7
Plan participants’ contributions 11 13  
Actuarial loss (gain) (2) 6  
Amendments 0 (2)  
Changes in assumptions 16 19  
Benefits and expenses paid (34) (41)  
Benefit obligation — end of year $ 220 $ 223 $ 220