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Pension and Other Postretirement Plans - Accumulated Benefit Obligation and Projected Benefit Obligation in Excess of Plan Assets (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Retirement Benefits [Abstract]    
Projected benefit obligation $ 6,250 $ 6,161
Accumulated benefit obligation 5,163 5,137
Fair value of plan assets $ 3,786 $ 4,714