Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Settlements(2)	$ (5)		$ (5)
Curtailments(2)			(38)
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	12	$ 13	36	$ 41
Interest cost(1)	55	56	167	170
Expected return on plan assets(1)	(81)	(84)	(244)	(256)
Recognized net actuarial loss(1)	43	47	127	154
Amortization of prior service cost(1)	1	2	4	6
Settlements(2)	5	232	5	399
Curtailments(2)			38	5
Net periodic benefit cost	35	266	133	519
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	2	3	4
Interest cost(1)	10	8	28	26
Recognized net actuarial loss(1)	2	3	7	10
Amortization of prior service cost(1)				(1)
Settlements(2)		(56)		(56)
Curtailments(2)				(28)
Net periodic benefit cost	$ 13	$ (43)	$ 38	$ (45)