Retirement Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Settlement loss		$ 0	$ 93	$ 4
Net non-service pension (income) costs included in Other, net		(59)	55	(61)
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost		50	44	42
Interest cost		73	89	91
Expected return on plan assets		(180)	(180)	(194)
Amortization of unrecognized net actuarial losses (gains)		48	52	39
Settlement loss	$ (88)	0	93	4
Curtailment gain		0	0	0
Net Periodic Benefit Cost		(9)	98	(18)
Postretirement Medical Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost		3	2	2
Interest cost		1	2	3
Expected return on plan assets		0	0	0
Amortization of unrecognized net actuarial losses (gains)		(1)	(1)	(2)
Settlement loss		0	0	0
Curtailment gain		0	0	(2)
Net Periodic Benefit Cost		$ 3	$ 3	$ 1