Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 52	$ 47	$ 156	$ 140
Interest cost	56	55	168	165
Expected return on plan assets	(95)	(71)	(284)	(213)
Prior service cost (credit) amortization				(1)
Actuarial loss (gain) amortization	36	31	109	95
Net periodic benefit cost	49	62	149	186
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost			1	2
Expected return on plan assets		(1)	(2)	(2)
Prior service cost (credit) amortization	(1)	(1)	(2)	(3)
Actuarial loss (gain) amortization	(1)	(1)	(4)	(4)
Net periodic benefit cost	$ (2)	$ (3)	$ (7)	$ (7)