Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6	$ 14	$ 16	$ 41
Interest cost	30	41	88	124
Expected return on plan assets	(71)	(73)	(217)	(220)
Recognized net actuarial loss	48	54	149	158
Settlements	8	5	47	5
Curtailments				4
Net periodic benefit cost	21	41	83	112
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		1	3	3
Interest cost	4	5	12	15
Recognized net actuarial loss	5	5	16	14
Amortization of prior service cost (benefit)	(3)	(4)	(11)	(11)
Settlements			26
Curtailments			(17)	(2)
Net periodic benefit cost	$ 6	$ 7	$ 29	$ 19