Employee Benefits - Costs by Plans (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 62		$ 70	$ 185	$ 215
Amortization of prior service cost (credit)	15		13	46	33
Expected return on plan assets	(282)		(321)	(846)	(979)
Interest cost	173		176	527	508
Remeasurement loss (gain), net	291	$ (96)	(454)	195	(454)
Other components	197		(586)	(78)	(892)
Total	259		(516)	107	(677)
Pension | Cost of services
Defined Benefit Plan Disclosure [Line Items]
Service cost	51		57	151	174
Pension | Selling, general and administrative expense
Defined Benefit Plan Disclosure [Line Items]
Service cost	11		13	34	41
Health Care and Life
Defined Benefit Plan Disclosure [Line Items]
Service cost	24		32	72	96
Amortization of prior service cost (credit)	(242)		(244)	(728)	(732)
Expected return on plan assets	(9)		(11)	(28)	(33)
Interest cost	157		153	472	460
Remeasurement loss (gain), net	0		0	0	0
Other components	(94)		(102)	(284)	(305)
Total	(70)		(70)	(212)	(209)
Health Care and Life | Cost of services
Defined Benefit Plan Disclosure [Line Items]
Service cost	20		26	59	78
Health Care and Life | Selling, general and administrative expense
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4		$ 6	$ 13	$ 18