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Retirement Plans and Other Retiree Benefits - Benefit Obligation Exceeds Fair Value of Plan Assets (Details) - Pension Plans - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]    
Projected benefit obligation $ 1,092 $ 2,862
Fair value of plan assets 299 2,094
Accumulated benefit obligation 882 875
Fair value of plan assets $ 134 $ 166