Pension and Other Benefit Plans - Changes in Accumulated Postretirement Benefit Obligation, Plan Assets and Funded Status (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in Plan Assets
Benefits paid	$ (257)
Postretirement medical plan
Change in Projected Benefit Obligations
Projected benefit obligation at beginning of year	1,193	$ 1,106
Service cost	31	29	$ 32
Interest cost	36	45	43
Contribution by plan participants	8	8
Actuarial losses	64	65
Benefits paid	(58)	(60)
Curtailment	(40)
Projected benefit obligation at end of year	1,234	1,193	1,106
Change in Plan Assets
Plan assets at fair value at beginning of year	1,185	997
Actual return on plan assets	221	240
Contributions by plan participants	8	8
Benefits paid	(58)	(60)
Plan assets at fair value at end of year	1,356	1,185	$ 997
Asset (Unfunded Liability)	122	(8)
Amounts Recognized in Accumulated Other Comprehensive Loss
Actuarial losses	(186)	(98)
Prior service credit	(104)	(158)
Total amount recognized in accumulated other comprehensive income	$ (290)	$ (256)