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Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]        
Net periodic benefit cost     $ 163 $ 246
Net periodic benefit costs continuing and discontinued        
Net periodic benefit cost     163 246
Pension Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 22 $ 43 67 124
Interest cost 59 114 175 358
Expected return on plan assets (82) (187) (248) (558)
Recognized net actuarial loss 55 104 165 308
Amortization of prior service cost (benefits) 1 4 4 12
Settlements   13   15
Special termination benefits       1
Net periodic benefit cost 55 91 163 260
Net periodic benefit costs continuing and discontinued        
Discontinued operations   41   114
Net amount recognized in Statement of Consolidated Operations 55 50 163 146
Net periodic benefit cost 55 91 163 260
Other Postretirement Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 2 3 6 10
Interest cost 7 16 22 53
Recognized net actuarial loss 2 8 4 19
Amortization of prior service cost (benefits) (2) (6) (6) (19)
Net periodic benefit cost 9 21 26 63
Net periodic benefit costs continuing and discontinued        
Discontinued operations   12   37
Net amount recognized in Statement of Consolidated Operations 9 9 26 26
Net periodic benefit cost $ 9 $ 21 $ 26 $ 63