Postretirement Benefit Plans - Reconciliation of Benefit Obligations, Plan Assets, and Unfunded or Funded Status (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Change in plan assets
Beginning balance at fair value	$ 33,646
Ending balance at fair value	37,331	$ 33,646
Qualified Defined Benefit Pension Plans
Change in benefit obligation
Beginning balance	43,305	48,686
Service cost	516	630	$ 635
Interest cost	1,806	1,740	1,835
Benefits paid	(2,294)	(2,379)
Settlements	(1,933)	(1,821)
Actuarial losses (gains)	6,403	(3,281)
Changes in longevity assumptions	860	(162)
Plan amendments and curtailments	11	(108)
Medicare Part D subsidy	0	0
Participants’ contributions	0	0
Ending balance	48,674	43,305	48,686
Change in plan assets
Beginning balance at fair value	32,002	33,095
Actual return on plan assets	6,667	(1,893)
Benefits paid	(2,294)	(2,379)
Settlements	(1,933)	(1,821)
Company contributions	1,000	5,000
Medicare Part D subsidy	0	0
Participants’ contributions	0	0
Ending balance at fair value	35,442	32,002	33,095
Unfunded status of the plans	(13,232)	(11,303)
Curtailment gain		119
Retiree Medical and Life Insurance Plans
Change in benefit obligation
Beginning balance	2,348	2,602
Service cost	14	19	19
Interest cost	97	91	103
Benefits paid	(229)	(224)
Settlements	0	0
Actuarial losses (gains)	(1)	(311)
Changes in longevity assumptions	(70)	(8)
Plan amendments and curtailments	(6)	101
Medicare Part D subsidy	2	9
Participants’ contributions	71	69
Ending balance	2,226	2,348	2,602
Change in plan assets
Beginning balance at fair value	1,644	1,883
Actual return on plan assets	342	(94)
Benefits paid	(229)	(224)
Settlements	0	0
Company contributions	59	1
Medicare Part D subsidy	2	9
Participants’ contributions	71	69
Ending balance at fair value	1,889	1,644	$ 1,883
Unfunded status of the plans	$ (337)	$ (704)