Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5	$ 13	$ 10	$ 27
Interest cost	29	41	58	83
Expected return on plan assets	(73)	(73)	(146)	(147)
Recognized net actuarial loss	50	53	101	104
Settlements	39		39
Curtailments		1		4
Net periodic benefit cost	50	35	62	71
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1	3	2
Interest cost	4	5	8	10
Recognized net actuarial loss	5	5	11	9
Amortization of prior service cost (benefit)	(4)	(4)	(8)	(7)
Settlements			26
Curtailments		(2)	(17)	(2)
Net periodic benefit cost	$ 6	$ 5	$ 23	$ 12