Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 58	$ 48	$ 176	$ 144
Interest cost	59	63	176	187
Expected return on plan assets	(101)	(96)	(302)	(287)
Actuarial loss (gain) amortization	34	24	101	73
Net periodic benefit cost	50	39	151	117
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost		1	1	2
Expected return on plan assets		(1)	(2)	(2)
Prior service cost (credit) amortization		(1)	(2)	(3)
Actuarial loss (gain) amortization	(2)	(1)	(5)	(4)
Net periodic benefit cost	$ (2)	$ (2)	$ (8)	$ (7)