Pension Plans and Other Postretirement Benefits - Components of Net Periodic Benefit Cost Table (Detail) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Pension Benefits [Member]
Components of net periodic benefit cost
Service cost		$ 23	$ 21
Interest cost		19	21
Expected (return) loss on plan assets		(21)	(21)
Amortization of net actuarial loss (gain)		7	6
Amortization of net prior service cost (credit)		0	0
Settlement expense (1)		0	3
Termination benefits expense		0	4
Net periodic benefit cost	[1]	28	34
Other Benefits [Member]
Components of net periodic benefit cost
Service cost		0	0
Interest cost		3	3
Expected (return) loss on plan assets		0	0
Amortization of net actuarial loss (gain)		0	0
Amortization of net prior service cost (credit)		(6)	(6)
Settlement expense (1)		0	0
Termination benefits expense		0	0
Net periodic benefit cost	[1]	$ (3)	$ (3)

[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.