Pension Plans, Other Postretirement Benefits, and Defined-Contribution Plans - Components of Net Periodic Benefit Cost Table (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015
Pension Plans, Defined Benefit [Member]
Components of net periodic benefit cost
Service cost	$ 87	$ 99		$ 118
Interest cost	84	95		101
Expected (return) loss on plan assets	(84)	(97)		(109)
Amortization of net actuarial (gain) loss	25	42		52
Amortization of net prior service (credit) cost	(1)	0		0
Settlement expense	91	146	[1]	11
Termination benefits expense	4	44	[1]	0
Curtailment expense	0	8	[1]	0
Net periodic benefit cost	206	337		173
Other Postretirement Benefits Plan [Member]
Components of net periodic benefit cost
Service cost	2	3		9
Interest cost	12	12		15
Expected (return) loss on plan assets	0	0		0
Amortization of net actuarial (gain) loss	0	0		0
Amortization of net prior service (credit) cost	(24)	(25)		(4)
Settlement expense	0	0	[1]	0
Termination benefits expense	0	0	[1]	0
Curtailment expense	0	0	[1]	0
Net periodic benefit cost	$ (10)	$ (10)		$ 20

[1] Settlement expense, termination benefits expense, and curtailment expense for 2016 relate to the workforce reduction program initiated in the first quarter of 2016. See Note 18—Restructuring Charges.