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Pension and Other Postretirement Benefits - Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Pension    
Components of net periodic benefit cost:    
Service cost $ 13 $ 14
Interest cost 10 15
Expected return on plan assets (25) (28)
Amortization of actuarial loss (gain) 13 12
Total net periodic benefit cost 11 13
Other Postretirement Benefits    
Components of net periodic benefit cost:    
Service cost 2 2
Interest cost 3 4
Expected return on plan assets (7) (6)
Amortization of actuarial loss (gain) 0 0
Total net periodic benefit cost $ (2) $ 0