Pension Plans and Other Postretirement Benefits - Components of Net Periodic Benefit Cost Table (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2018		Sep. 30, 2017	Sep. 30, 2018		Sep. 30, 2017
Pension Benefits [Member]
Components of net periodic benefit cost
Service cost	$ 23		$ 22	$ 68		$ 64
Interest cost	19		21	57		63
Expected (return) loss on plan assets	(20)		(21)	(61)		(63)
Amortization of net actuarial loss (gain)	6		7	19		20
Amortization of net prior service cost (credit)	0		(1)	0		(1)
Settlement expense	9	[1]	22	9		80
Termination benefits expense	7		0	7		4
Net periodic benefit cost	44		50	99	[1]	167
Other Benefits [Member]
Components of net periodic benefit cost
Service cost	0		0	1		1
Interest cost	3		3	8		9
Expected (return) loss on plan assets	0		0	0		0
Amortization of net actuarial loss (gain)	0		0	0		0
Amortization of net prior service cost (credit)	(6)		(6)	(18)		(18)
Settlement expense	0	[1]	0	0		0
Termination benefits expense	0		0	0		0
Net periodic benefit cost	$ (3)		$ (3)	$ (9)	[1]	$ (8)

[1] The service cost component of net periodic benefit cost is included in G&A; oil and gas operating expense; gathering, processing, and marketing expense; and exploration expense, and all other components of net periodic benefit cost are included in other (income) expense on the Company’s Consolidated Statements of Income.