Employee Benefits - Costs by Plans (Detail) - USD ($) $ in Millions	3 Months Ended					9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 76			$ 62		$ 222	$ 185
Amortization of prior service cost (credit)	16			15		46	46
Expected return on plan assets	(295)			(282)		(888)	(846)
Interest cost	124			173		401	527
Remeasurement loss, net	1,092	$ 153	$ 182	291	$ (96)	1,427	195
Other components	937			197		986	(78)
Total	1,013			259		1,208	107
Pension | Cost of services
Defined Benefit Plan Disclosure [Line Items]
Service cost	61			51		179	151
Pension | Selling, general and administrative expense
Defined Benefit Plan Disclosure [Line Items]
Service cost	15			11		43	34
Health Care and Life
Defined Benefit Plan Disclosure [Line Items]
Service cost	27			24		82	72
Amortization of prior service cost (credit)	(242)			(242)		(725)	(728)
Expected return on plan assets	(7)			(9)		(20)	(28)
Interest cost	108			157		322	472
Remeasurement loss, net	0			0		0	0
Other components	(141)			(94)		(423)	(284)
Total	(114)			(70)		(341)	(212)
Health Care and Life | Cost of services
Defined Benefit Plan Disclosure [Line Items]
Service cost	22			20		67	59
Health Care and Life | Selling, general and administrative expense
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5			$ 4		$ 15	$ 13