Postretirement Benefit Plans - Qualified Defined Benefit Pension Plans and Retiree Medical and Life Insurance Plans by Asset Category (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	$ 40,566	$ 37,331
Buy-In Contract
Defined Benefit Plan Disclosure [Line Items]
Plan with benefit obligations equal to plan assets	1,700	857
Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	31,500	29,459
Level 1, 2 and 3 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	1,109	1,961
Level 1, 2 and 3 | U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	7,535	7,189
Level 1, 2 and 3 | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	6,844	7,244
Level 1, 2 and 3 | Commingled equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	1,671	1,933
Level 1, 2 and 3 | Corporate debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	5,732	5,208
Level 1, 2 and 3 | U.S. Government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	2,506	2,260
Level 1, 2 and 3 | U.S. Government-sponsored enterprise securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	230	530
Level 1, 2 and 3 | Other fixed income investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	5,873	3,134
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	15,891	16,977
Level 1 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	1,109	1,961
Level 1 | U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	7,467	7,182
Level 1 | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	6,836	7,217
Level 1 | Commingled equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	442	582
Level 1 | Corporate debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	0	0
Level 1 | U.S. Government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	0	0
Level 1 | U.S. Government-sponsored enterprise securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	0	0
Level 1 | Other fixed income investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	37	35
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	13,765	11,505
Level 2 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	0	0
Level 2 | U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	8	0
Level 2 | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	0	23
Level 2 | Commingled equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	1,228	1,351
Level 2 | Corporate debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	5,730	5,206
Level 2 | U.S. Government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	2,506	2,260
Level 2 | U.S. Government-sponsored enterprise securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	230	530
Level 2 | Other fixed income investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	4,063	2,135
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	1,844	977
Level 3 | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	0	0
Level 3 | U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	60	7
Level 3 | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	8	4
Level 3 | Commingled equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	1	0
Level 3 | Corporate debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	2	2
Level 3 | U.S. Government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	0	0
Level 3 | U.S. Government-sponsored enterprise securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	0	0
Level 3 | Other fixed income investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	1,773	964
NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	9,066	7,794
NAV | Other fixed income investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	541	32
NAV | Commingled equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	92	181
NAV | Private equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	4,672	4,019
NAV | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	2,650	2,493
NAV | Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	1,111	1,069
NAV | Receivables, net
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	$ 0	$ 78