Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Curtailments		$ 38	$ 2	$ 38
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 13	12	27	24
Interest cost	41	56	83	112
Expected return on plan assets	(73)	(82)	(147)	(163)
Recognized net actuarial loss	53	42	104	84
Amortization of prior service cost (benefit)		2		3
Curtailments	1	38	4	38
Net periodic benefit cost	35	68	71	98
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	1	2	2
Interest cost	5	10	10	18
Recognized net actuarial loss	5	2	9	5
Amortization of prior service cost (benefit)	(4)		(7)
Curtailments	(2)		(2)
Net periodic benefit cost	$ 5	$ 13	$ 12	$ 25