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, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Actuarial gains and losses - Incurred but Not Yet Recognized in Net Periodic Benefit Cost	$ (1,066)	$ (2,178)	$ (416)
Net prior service credit and cost - Incurred but Not Yet Recognized in Net Periodic Benefit Cost	(1)	(4)	(85)
Incurred but Not Yet Recognized in Net Periodic Benefit Cost	(1,067)	(2,182)	(501)
Actuarial gains and losses - Recognition of Previously Deferred Amounts	689	1,148	1,455
Net prior service credit and cost - Recognition of Previously Deferred Amounts	(249)	(240)	(253)
Recognition of Previously Deferred Amounts	440	908	1,202
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	(1,005)	(2,283)	(570)
Net prior service credit and cost - Incurred but Not Yet Recognized in Net Periodic Benefit Cost	(7)	(8)	(6)
Actuarial gains and losses - Recognition of Previously Deferred Amounts	668	1,104	1,396
Net prior service credit and cost - Recognition of Previously Deferred Amounts	(269)	(263)	(255)
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	43	238	71
Net prior service credit and cost - Incurred but Not Yet Recognized in Net Periodic Benefit Cost	6	4	(79)
Actuarial gains and losses - Recognition of Previously Deferred Amounts	(3)	2	4
Net prior service credit and cost - Recognition of Previously Deferred Amounts	30	33	12
Other plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Actuarial gains and losses - Incurred but Not Yet Recognized in Net Periodic Benefit Cost	(104)	(133)	83
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	24	42	55
Net prior service credit and cost - Recognition of Previously Deferred Amounts	$ (10)	$ (10)	$ (10)