Postretirement Benefit Plans - Reconciliation of Benefit Obligations, Plan Assets, and Unfunded or Funded Status (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Change in plan assets
Beginning balance at fair value	$ 34,978
Ending balance at fair value	33,646	$ 34,978
Qualified Defined Benefit Pension Plans
Change in benefit obligation
Beginning balance	48,686	45,064
Service cost	630	635	$ 671
Interest cost	1,740	1,835	1,890
Benefits paid	(2,379)	(2,310)
Settlements	(1,821)	0
Actuarial losses (gains)	(3,281)	3,536
Changes in longevity assumptions	(162)	(352)
Plan amendments and curtailments (b)	(108)	278
Medicare Part D subsidy	0	0
Participants’ contributions	0	0
Ending balance	43,305	48,686	45,064
Change in plan assets
Beginning balance at fair value	33,095	31,417
Actual return on plan assets	(1,893)	3,942
Benefits paid	(2,379)	(2,310)
Settlements	(1,821)	0
Company contributions	5,000	46
Medicare Part D subsidy	0	0
Participants’ contributions	0	0
Ending balance at fair value	32,002	33,095	31,417
Unfunded status of the plans	(11,303)	(15,591)
Curtailment gain	119
Retiree Medical and Life Insurance Plans
Change in benefit obligation
Beginning balance	2,602	2,649
Service cost	19	19	23
Interest cost	91	103	120
Benefits paid	(224)	(232)
Settlements	0	0
Actuarial losses (gains)	(311)	23
Changes in longevity assumptions	(8)	(24)
Plan amendments and curtailments (b)	101	0
Medicare Part D subsidy	9	0
Participants’ contributions	69	64
Ending balance	2,348	2,602	2,649
Change in plan assets
Beginning balance at fair value	1,883	1,787
Actual return on plan assets	(94)	224
Benefits paid	(224)	(232)
Settlements	0	0
Company contributions	1	40
Medicare Part D subsidy	9	0
Participants’ contributions	69	64
Ending balance at fair value	1,644	1,883	$ 1,787
Unfunded status of the plans	$ (704)	$ (719)