Employee Benefits - Costs by Plans (Detail) - USD ($) $ in Millions	3 Months Ended					9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 67		$ 76			$ 212	$ 222
Amortization of prior service cost (credit)	16		16			46	46
Expected return on plan assets	(309)		(295)			(927)	(888)
Interest cost	102		124			296	401
Remeasurement loss, net	144	$ (1,300)	1,092	$ 153	$ 182	(1,170)	1,427
Other components	(47)		937			(1,755)	986
Total	20		1,013			(1,543)	1,208
Pension | Cost of services
Defined Benefit Plan Disclosure [Line Items]
Service cost	59		61			186	179
Pension | Selling, general and administrative expense
Defined Benefit Plan Disclosure [Line Items]
Service cost	8		15			26	43
Health Care and Life
Defined Benefit Plan Disclosure [Line Items]
Service cost	28		27			85	82
Amortization of prior service cost (credit)	(223)		(242)			(670)	(725)
Expected return on plan assets	(6)		(7)			(17)	(20)
Interest cost	72		108			217	322
Remeasurement loss, net	0		0			0	0
Other components	(157)		(141)			(470)	(423)
Total	(129)		(114)			(385)	(341)
Health Care and Life | Cost of services
Defined Benefit Plan Disclosure [Line Items]
Service cost	24		22			71	67
Health Care and Life | Selling, general and administrative expense
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4		$ 5			$ 14	$ 15