Employee Benefit Plans (Net Periodic Benefit Cost (Credit)) (Detail) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 45	$ 41	$ 131	$ 121
Interest cost		82	97	263	296
Expected return on plan assets		(197)	(177)	(582)	(530)
Amortization of prior service cost (credit)		(1)	0	0	1
Amortization of net actuarial loss		58	42	155	124
Settlements and curtailments	[1]	3	2	5	75
Net periodic benefit cost (credit)		(10)	5	(28)	87
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost		8	7	22	20
Interest cost		15	17	45	51
Expected return on plan assets		(39)	(37)	(117)	(105)
Amortization of prior service cost (credit)		(12)	(13)	(37)	(39)
Amortization of net actuarial loss		1	2	4	9
Settlements and curtailments	[1]	0	0	0	42
Net periodic benefit cost (credit)		$ (27)	$ (24)	$ (83)	$ (22)

[1] 2019 amounts relate primarily to a voluntary retirement program.