Income Taxes (Detail) $ in Millions		12 Months Ended
		Dec. 31, 2020 USD ($) aircraft		Dec. 31, 2019 USD ($)		Dec. 31, 2018 USD ($)	Dec. 31, 2015
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent		27.80%		22.20%			35.00%
Deferred Tax Assets, State Taxes		$ 852
Effective Income Tax Rate Reconciliation, Tax Cuts and Jobs Act, Percent						0.21
CARES Act Loss Carryback Period		5 years
Operating Loss Carryforward Expiration Year 2025 | aircraft		2,025
Operating Loss Carryforward Expiration Year 2040 | aircraft		2,040
DEFERRED TAX LIABILITIES:
Accelerated depreciation		$ 2,939		$ 3,096
Prepaid insurance		190		12
Operating lease right-of-use assets		423		293
Other		67		81
Total deferred tax liabilities		3,619		3,482
DEFERRED TAX ASSETS:
Accrued employee benefits		491		346
Rapid rewards loyalty liability		632		305
Operating lease liabilities		443		308
Residual travel funds		117		0
Construction obligation		72		38
Net operating losses and tax credits		60	[1]	8
Other		170		113
Total deferred tax assets		1,985		1,118
Net deferred tax liability		1,634		2,364
CURRENT:
Federal	[2]	(273)		610		$ 338
State		(5)		102		60
Change in federal statutory rate	[3]	(188)		0		0
Total current		(466)		712		398
DEFERRED:
Federal	[2]	(589)		(18)		299
State		(76)		(6)		2
State net operating losses		(51)		0		0
Change in federal statutory tax rate	[4]	0		(31)		0
Total deferred		(716)		(55)		301
Total		(1,182)		657		699
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Tax at statutory U.S. tax rates		(894)		621		664
State income taxes, net of federal benefit		(115)		76		49
Change in federal statutory rate		(188)	[3]	(31)	[4]	0
Other, net		15		(9)		(14)
PROVISION (BENEFIT) FOR INCOME TAXES		$ (1,182)		$ 657		$ 699

[1] At December 31, 2020, the Company had approximately $852 million of state net operating loss carryforwards to reduce future state taxable income. These state net operating loss carryforwards will expire in years 2025 - 2040 if unused.

[2] The CARES Act allows entities to carry back 2020 losses to prior periods of up to five years, and claim refunds of federal taxes paid, and the Company expects to receive a significant cash tax refund once it completes all the necessary requirements to make the appropriate filings with the IRS.

[3] The benefit is representative of the excess refund generated as the result of carrying the 2020 losses back to a period when the federal statutory tax rate was 35 percent as opposed to the current tax rate of 21 percent.

[4] The Tax Cuts and Jobs Act was enacted in December 2017, which reduced the U.S. federal corporate tax rate from the previous rate of 35 percent to 21 percent