Postretirement Benefit Plans - Reconciliation of Benefit Obligations, Plan Assets, and Unfunded or Funded Status (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in plan assets
Beginning balance at fair value	$ 37,331
Ending balance at fair value	40,566	$ 37,331
Qualified Defined  Benefit Pension Plans
Change in benefit obligation
Beginning balance	48,674	43,305
Service cost	101	516	$ 630
Interest cost	1,538	1,806	1,740
Benefits paid	(2,188)	(2,294)
Settlements	(1,392)	(1,933)
Actuarial losses (gains)	5,036	6,403
Changes in longevity assumptions	(426)	860
Plan amendments and curtailments	9	11
Medicare Part D subsidy	0	0
Participants’ contributions	0	0
Ending balance	51,352	48,674	43,305
Change in plan assets
Beginning balance at fair value	35,442	32,002
Actual return on plan assets	5,594	6,667
Benefits paid	(2,188)	(2,294)
Settlements	(1,392)	(1,933)
Company contributions	1,025	1,000
Medicare Part D subsidy	0	0
Participants’ contributions	0	0
Ending balance at fair value	38,481	35,442	32,002
Unfunded status of the plans	(12,871)	(13,232)
Retiree Medical and Life Insurance Plans
Change in benefit obligation
Beginning balance	2,226	2,348
Service cost	13	14	19
Interest cost	70	97	91
Benefits paid	(220)	(229)
Settlements	0	0
Actuarial losses (gains)	135	(1)
Changes in longevity assumptions	(18)	(70)
Plan amendments and curtailments	(8)	(6)
Medicare Part D subsidy	3	2
Participants’ contributions	70	71
Ending balance	2,271	2,226	2,348
Change in plan assets
Beginning balance at fair value	1,889	1,644
Actual return on plan assets	298	342
Benefits paid	(220)	(229)
Settlements	0	0
Company contributions	45	59
Medicare Part D subsidy	3	2
Participants’ contributions	70	71
Ending balance at fair value	2,085	1,889	$ 1,644
Unfunded status of the plans	$ (186)	$ (337)