Employee Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 48	$ 52	$ 96	$ 104
Interest cost	62	56	124	112
Expected return on plan assets	(96)	(94)	(191)	(189)
Actuarial loss (gain) amortization	25	36	49	73
Net periodic benefit cost	39	50	78	100
Postretirement Welfare Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost			1	1
Expected return on plan assets		(1)	(1)	(2)
Prior service cost (credit) amortization	(1)		(2)	(1)
Actuarial loss (gain) amortization	(2)	(1)	(3)	(3)
Net periodic benefit cost	$ (3)	$ (2)	$ (5)	$ (5)