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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, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure - Plans with Projected Obligations in Excess of Plan Assets and Plans with Accumulated Benefit Obligations in Excess of Plan Assets [Line Items]    
Projected benefit obligation $ 2,834 $ 2,973
Fair value of plan assets 1,992 2,024
Accumulated benefit obligation 2,641 2,840
Fair value of plan assets $ 1,905 $ 2,003