Postretirement Benefit Plans - Qualified Defined Benefit Pension Plans and Retiree Medical and Life Insurance Plans by Asset Category (Detail) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	$ 33,646	$ 34,978
Receivables, net
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	72	68
Investments measured at fair value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	26,442	28,080
Investments measured at fair value | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	1,727	1,419
Investments measured at fair value | U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	3,936	4,922
Investments measured at fair value | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	5,406	5,370
Investments measured at fair value | Commingled equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	3,587	4,453
Investments measured at fair value | Corporate debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	4,890	4,910
Investments measured at fair value | U.S. Government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	3,399	3,775
Investments measured at fair value | U.S. Government-sponsored enterprise securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	571	817
Investments measured at fair value | Other fixed income investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	2,926	2,414
Investments measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	7,132	6,830
Investments measured at NAV | Other fixed income investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	29	68
Investments measured at NAV | Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	828	718
Investments measured at NAV | Commingled equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	144	99
Investments measured at NAV | Private equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	4,014	4,334
Investments measured at NAV | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	2,117	1,611
Level 1 | Investments measured at fair value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	12,490	13,172
Level 1 | Investments measured at fair value | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	1,727	1,419
Level 1 | Investments measured at fair value | U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	3,927	4,905
Level 1 | Investments measured at fair value | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	5,400	5,355
Level 1 | Investments measured at fair value | Commingled equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	1,436	1,493
Level 1 | Investments measured at fair value | Corporate debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	0	0
Level 1 | Investments measured at fair value | U.S. Government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	0	0
Level 1 | Investments measured at fair value | 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 | Investments measured at fair value | Other fixed income investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	0	0
Level 2 | Investments measured at fair value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	13,000	14,887
Level 2 | Investments measured at fair value | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	0	0
Level 2 | Investments measured at fair value | U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	3	14
Level 2 | Investments measured at fair value | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	0	13
Level 2 | Investments measured at fair value | Commingled equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	2,151	2,960
Level 2 | Investments measured at fair value | Corporate debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	4,888	4,905
Level 2 | Investments measured at fair value | U.S. Government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	3,399	3,775
Level 2 | Investments measured at fair value | U.S. Government-sponsored enterprise securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	571	817
Level 2 | Investments measured at fair value | Other fixed income investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	1,988	2,403
Level 3 | Investments measured at fair value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	952	21
Level 3 | Investments measured at fair value | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	0	0
Level 3 | Investments measured at fair value | U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	6	3
Level 3 | Investments measured at fair value | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	6	2
Level 3 | Investments measured at fair value | Commingled equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	0	0
Level 3 | Investments measured at fair value | Corporate debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	2	5
Level 3 | Investments measured at fair value | U.S. Government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	0	0
Level 3 | Investments measured at fair value | 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 | Investments measured at fair value | Other fixed income investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	$ 938	$ 11