Retirement Plans - Components of Net Periodic Pension Cost (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Compensation And Retirement Disclosure [Abstract]
Service cost	$ 10		$ 13	$ 34	$ 41
Interest cost	24		25	71	77
Expected return on plan assets	(49)		(42)	(147)	(125)
Amortization of unrecognized net actuarial losses	8		13	29	46
Curtailment gains	0	$ (2)	0	(2)	0
Settlement loss	0		4	0	11
Pension (income) expense	$ (7)		$ 13	$ (15)	$ 50