Retirement Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 49	$ 60	$ 67
Interest cost	102	107	102
Expected return on plan assets	(168)	(166)	(168)
Amortization of unrecognized net actuarial losses	58	60	75
Settlement loss	19	0	17
Special termination benefit recognized	0	1	1
Net Periodic Benefit Cost	60	62	94
Postretirement Medical Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	4	4	4
Interest cost	3	3	3
Expected return on plan assets	0	0	0
Amortization of unrecognized net actuarial losses	0	0	0
Settlement loss	0	0	0
Special termination benefit recognized	0	0	0
Net Periodic Benefit Cost	$ 7	$ 7	$ 7