Postretirement Benefit Plans - Amounts Recognized in Other Comprehensive Income (Loss) Related to Postretirement Benefit Plans, Net of Tax (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Actuarial gains and losses - Incurred but Not Yet Recognized in Net Periodic Benefit Cost	$ (2,178)	$ (416)	$ (1,161)
Net prior service credit and cost - Incurred but Not Yet Recognized in Net Periodic Benefit Cost	(4)	(85)	(219)
Incurred but Not Yet Recognized in Net Periodic Benefit Cost	(2,182)	(501)	(1,380)
Actuarial gains and losses - Recognition of Previously Deferred Amounts	1,148	1,455	1,030
Net prior service credit and cost - Recognition of Previously Deferred Amounts	(240)	(253)	(228)
Recognition of Previously Deferred Amounts	908	1,202	802
Qualified Defined Benefit Pension Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Actuarial gains and losses - Incurred but Not Yet Recognized in Net Periodic Benefit Cost	(2,283)	(570)	(1,172)
Net prior service credit and cost - Incurred but Not Yet Recognized in Net Periodic Benefit Cost	(8)	(6)	(219)
Actuarial gains and losses - Recognition of Previously Deferred Amounts	1,104	1,396	974
Net prior service credit and cost - Recognition of Previously Deferred Amounts	(263)	(255)	(229)
Retiree Medical and Life Insurance Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Actuarial gains and losses - Incurred but Not Yet Recognized in Net Periodic Benefit Cost	238	71	77
Net prior service credit and cost - Incurred but Not Yet Recognized in Net Periodic Benefit Cost	4	(79)	0
Actuarial gains and losses - Recognition of Previously Deferred Amounts	2	4	12
Net prior service credit and cost - Recognition of Previously Deferred Amounts	33	12	10
Other plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Actuarial gains and losses - Incurred but Not Yet Recognized in Net Periodic Benefit Cost	(133)	83	(66)
Net prior service credit and cost - Incurred but Not Yet Recognized in Net Periodic Benefit Cost	0	0	0
Actuarial gains and losses - Recognition of Previously Deferred Amounts	42	55	44
Net prior service credit and cost - Recognition of Previously Deferred Amounts	$ (10)	$ (10)	$ (9)