Employee Benefits - Costs by Plans (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 70	$ 70	$ 215	$ 210
Amortization of prior service cost (credit)	13	10	33	29
Expected return on plan assets	(321)	(315)	(979)	(947)
Interest cost	176	171	508	513
Remeasurement gain, net	(454)	0	(454)	0
Other components	(586)	(134)	(892)	(405)
Total	(516)	(64)	(677)	(195)
Pension | Cost of services
Defined Benefit Plan Disclosure [Line Items]
Service cost	57	55	174	165
Pension | Selling, general and administrative expense
Defined Benefit Plan Disclosure [Line Items]
Service cost	13	15	41	45
Health Care and Life
Defined Benefit Plan Disclosure [Line Items]
Service cost	32	37	96	111
Amortization of prior service cost (credit)	(244)	(235)	(732)	(705)
Expected return on plan assets	(11)	(13)	(33)	(39)
Interest cost	153	164	460	494
Remeasurement gain, net	0	0	0	0
Other components	(102)	(84)	(305)	(250)
Total	(70)	(47)	(209)	(139)
Health Care and Life | Cost of services
Defined Benefit Plan Disclosure [Line Items]
Service cost	26	29	78	87
Health Care and Life | Selling, general and administrative expense
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6	$ 8	$ 18	$ 24