Employee Benefit Plans (Net Periodic Benefit Cost (Credit)) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 42	$ 43
Interest cost	79	91
Expected return on plan assets	(208)	(193)
Amortization of prior service (credit) cost	0	0
Amortization of net actuarial loss	48	49
Settlements	(2)	0
Net periodic benefit (credit) cost	(41)	(10)
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	6	7
Interest cost	12	15
Expected return on plan assets	(44)	(39)
Amortization of prior service (credit) cost	(10)	(12)
Amortization of net actuarial loss	1	1
Settlements	0	0
Net periodic benefit (credit) cost	$ (35)	$ (28)