Income Taxes (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			21.00%	35.00%
DEFERRED TAX LIABILITIES:
Accelerated depreciation		$ 3,096	$ 3,395
Operating lease right-of-use assets		293
Other		93	92
Deferred Tax Liabilities, Gross		3,482	3,487
DEFERRED TAX ASSETS:
Deferred tax assets construction obligation		38	355
Accrued employee benefits		346	329
Rapid rewards loyalty liability		305	267
Operating lease liabilities		308
Other		121	109
Total deferred tax assets		1,118	1,060
Total deferred tax liabilities		2,364	2,427
CURRENT:
Federal		610	338	$ 904
State		102	60	72
Total current		712	398	976
DEFERRED:
Federal		(18)	299	200
State		(6)	2	2
Change in federal statutory rate	[1]	(31)	0	(1,270)
Total deferred		(55)	301	(1,068)
Total		657	699	(92)
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation [Abstract]
Tax at statutory U.S. tax rates		621	664	1,143
State income taxes, net of federal benefit		76	49	50
Change in federal statutory rate	[1]	(31)	0	(1,270)
Other, net		$ (9)	$ (14)	$ (15)

[1] 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