Postretirement Benefit Plans - Schedule of Net Periodic Benefit Costs (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Qualified Defined  Benefit Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 101	$ 516	$ 630
Interest cost	1,538	1,806	1,740
Expected return on plan assets	(2,264)	(2,300)	(2,395)
Recognized net actuarial losses (gains)	849	1,404	1,777
Amortization of net prior service (credit) cost	(342)	(333)	(321)
Total net periodic benefit cost (income)	(118)	1,093	1,431
Retiree Medical and Life Insurance Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	13	14	19
Interest cost	70	97	91
Expected return on plan assets	(127)	(110)	(135)
Recognized net actuarial losses (gains)	(4)	2	5
Amortization of net prior service (credit) cost	39	42	15
Total net periodic benefit cost (income)	$ (9)	$ 45	$ (5)