Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Curtailments	$ 3
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	14	$ 12
Interest cost	42	56
Expected return on plan assets	(74)	(81)
Recognized net actuarial loss	51	42
Amortization of prior service cost		1
Curtailments	3
Net periodic benefit cost	36	30
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1
Interest cost	5	8
Recognized net actuarial loss	4	3
Amortization of prior service cost	(3)
Net periodic benefit cost	$ 7	$ 12