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Pensions and Other Postretirement Benefits - Benefit Cost Components (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 40 $ 35 $ 39
Interest cost 74 93 83
Expected return on plan assets (190) (179) (177)
Amortization of net losses 51 43 57
Amortization of prior service cost 1 1 0
Net cost (24) (7) 2
OPEB      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 6 6 7
Interest cost 12 17 15
Expected return on plan assets (14) (14) (15)
Amortization of prior service cost (25) (24) (24)
Net cost $ (21) $ (15) $ (17)