Retirement Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Settlement loss		$ 93	$ 4	$ 19
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost		44	42	49
Interest cost		89	91	102
Expected return on plan assets		(180)	(194)	(168)
Amortization of unrecognized net actuarial losses (gains)		52	39	58
Settlement loss	$ 88	93	4	19
Curtailment gain		0	0	0
Net Periodic Benefit Cost		98	(18)	60
Postretirement Medical Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost		2	2	4
Interest cost		2	3	3
Expected return on plan assets		0	0	0
Amortization of unrecognized net actuarial losses (gains)		(1)	(2)	0
Settlement loss		0	0	0
Curtailment gain		0	(2)	0
Net Periodic Benefit Cost		$ 3	$ 1	$ 7