Postretirement Benefit Plans - Qualified Defined Benefit Pension Plans and Retiree Medical and Life Insurance Plans by Asset Category (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	$ 34,978	$ 33,204
Receivables, net
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	68	53
Investments measured at fair value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	28,087	27,604
Investments measured at fair value | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	1,419	2,301
Investments measured at fair value | U.S. equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	4,922	4,166
Investments measured at fair value | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	5,370	3,971
Investments measured at fair value | Commingled equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	4,453	2,332
Investments measured at fair value | Corporate debt securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	4,910	4,333
Investments measured at fair value | U.S. Government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	3,775	6,811
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	817	919
Investments measured at fair value | Other fixed income investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	2,412	2,215
Investments measured at fair value | Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	7	33
Investments measured at fair value | Commodities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	2	523
Investments measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	6,823	5,547
Investments measured at NAV | Other fixed income investments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	68	0
Investments measured at NAV | Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	711	462
Investments measured at NAV | Commingled equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	99	60
Investments measured at NAV | Private equity funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	4,334	3,614
Investments measured at NAV | Real estate funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	1,611	1,411
Level 1 | Investments measured at fair value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	13,173	11,680
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	1,419	2,301
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	4,905	4,139
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	5,355	3,927
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	1,493	788
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 1 | Investments measured at fair value | Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	0	0
Level 1 | Investments measured at fair value | Commodities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	1	525
Level 2 | Investments measured at fair value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	14,893	15,898
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	14	23
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	13	40
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	2,960	1,544
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	4,905	4,316
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	3,775	6,811
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	817	919
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	2,401	2,214
Level 2 | Investments measured at fair value | Hedge funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	7	33
Level 2 | Investments measured at fair value | Commodities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	1	(2)
Level 3 | Investments measured at fair value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	21	26
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	3	4
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	2	4
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	5	17
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	11	1
Level 3 | Investments measured at fair value | Hedge 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 | Commodities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at end of year	$ 0	$ 0