Stockholders' Equity - Changes in Balance of Accumulated Other Comprehensive Loss, Net of Income Taxes (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	$ 3,171	$ 1,449	$ (776)
Other comprehensive loss before reclassifications	(1,011)	(2,164)	(606)
Amounts reclassified from AOCL
Amounts reclassified from AOCL	444	931	1,232
Other comprehensive income (loss), net of tax	(567)	(1,233)	626
Reclassification of income tax effects from tax reform			0
Ending Balance	6,038	3,171	1,449
Postretirement Benefit Plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(15,528)	(14,254)	(12,559)
Other comprehensive loss before reclassifications	(1,067)	(2,182)	(501)
Amounts reclassified from AOCL
Amounts reclassified from AOCL	440	908	1,202
Other comprehensive income (loss), net of tax	(627)	(1,274)	701
Reclassification of income tax effects from tax reform			(2,396)
Ending Balance	(16,155)	(15,528)	(14,254)
AOCI related to postretirement benefit plans, tax benefit	4,400	4,200	3,900
Other, net
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(26)	(67)	20
Other comprehensive loss before reclassifications	56	18	(105)
Amounts reclassified from AOCL
Amounts reclassified from AOCL	4	23	30
Other comprehensive income (loss), net of tax	60	41	(75)
Reclassification of income tax effects from tax reform			(12)
Ending Balance	34	(26)	(67)
AOCL
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning Balance	(15,554)	(14,321)	(12,539)
Amounts reclassified from AOCL
Other comprehensive income (loss), net of tax	(567)	(1,233)	626
Reclassification of income tax effects from tax reform			(2,408)
Ending Balance	(16,121)	(15,554)	(14,321)
Recognition of net actuarial losses
Amounts reclassified from AOCL
Amounts reclassified from AOCL	689	1,148	1,455
Amortization of net prior service credits
Amounts reclassified from AOCL
Amounts reclassified from AOCL	(249)	(240)	(253)
Other
Amounts reclassified from AOCL
Amounts reclassified from AOCL	$ 4	$ 23	$ 30