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Pension and Other Postretirement Benefits - Schedule of Obligations and Funded Status (Parenthetical) (Detail) - USD ($)
$ in Millions
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]    
Fair value of plan assets $ 4,646 $ 10,926
Domestic Pension Plans, Defined Benefit [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Benefit obligation 5,707 10,983
Fair value of plan assets 3,495 8,077
Funded status $ (2,212) $ (2,906)