Employee Benefits - Costs by Plans (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 73	$ 70	$ 145	$ 140
Amortization of prior service cost (credit)	10	9	20	19
Expected return on plan assets	(329)	(316)	(658)	(632)
Interest cost	166	171	332	342
Other components	(153)	(136)	(306)	(271)
Total	(80)	(66)	(161)	(131)
Pension | Cost of services
Defined Benefit Plan Disclosure [Line Items]
Service cost	59	55	117	110
Pension | Selling, general and administrative expense
Defined Benefit Plan Disclosure [Line Items]
Service cost	14	15	28	30
Health Care and Life
Defined Benefit Plan Disclosure [Line Items]
Service cost	32	37	64	74
Amortization of prior service cost (credit)	(244)	(235)	(488)	(470)
Expected return on plan assets	(11)	(13)	(22)	(26)
Interest cost	154	165	307	330
Other components	(101)	(83)	(203)	(166)
Total	(69)	(46)	(139)	(92)
Health Care and Life | Cost of services
Defined Benefit Plan Disclosure [Line Items]
Service cost	26	29	52	58
Health Care and Life | Selling, general and administrative expense
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6	$ 8	$ 12	$ 16