Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 59	$ 48	$ 118	$ 96
Interest cost	59	62	117	124
Expected return on plan assets	(100)	(96)	(201)	(191)
Actuarial loss (gain) amortization	33	25	67	49
Net periodic benefit cost	51	39	101	78
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost			1	1
Expected return on plan assets	(1)		(2)	(1)
Prior service cost (credit) amortization	(1)	(1)	(2)	(2)
Actuarial loss (gain) amortization	(1)	(2)	(3)	(3)
Net periodic benefit cost	$ (3)	$ (3)	$ (6)	$ (5)