Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Settlements	$ (341)
Curtailments	331	$ 8	$ 17
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	54	71	61
Interest cost	227	244	138
Expected return on plan assets(3)	(341)	(398)	(242)
Recognized net actuarial loss(3)	198	185	102
Amortization of prior service cost (benefit)(3)	8	9	7
Settlements	410	5	16
Curtailments	5
Special termination benefits		3	1
Net periodic benefit cost	561	119	83
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	5	2
Interest cost	34	38	16
Recognized net actuarial loss(3)	13	13	8
Amortization of prior service cost (benefit)(3)		(6)	(5)
Settlements	(56)
Curtailments	(28)
Net periodic benefit cost	$ (32)	$ 50	$ 21