Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Curtailments		$ (3)
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5	14
Interest cost	29	42
Expected return on plan assets	(73)	(74)
Recognized net actuarial loss	51	51
Curtailments		3
Net periodic benefit cost	12	36
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	1
Interest cost	4	5
Recognized net actuarial loss	6	4
Amortization of prior service cost (benefit)	(4)	(3)
Settlements	26
Curtailments	(17)
Net periodic benefit cost	$ 17	$ 7