Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Curtailments(2)		$ (23)
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 12	14
Interest cost(1)	56	59
Expected return on plan assets(1)	(81)	(90)
Recognized net actuarial loss(1)	42	55
Amortization of prior service cost(1)	1	2
Curtailments(2)		5
Net periodic benefit cost	30	45
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1
Interest cost(1)	8	9
Recognized net actuarial loss(1)	3	3
Curtailments(2)		(28)
Net periodic benefit cost	$ 12	$ (15)