Significant Accounting Policies - Expected Impact of the Adoption of the New Lease Standard (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended									12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016	[2]
Current assets:
Receivables, less allowance for doubtful accounts (2018—$8; 2017—$7)	$ 1,346				$ 1,340	[1]				$ 1,346	$ 1,340	[1]
Prepaid expenses and other	913				1,071	[1]				913	1,071	[1]
Operating property and equipment:
Other property and equipment	7,919				6,946	[1]				7,919	6,946	[1]
Less-Accumulated depreciation and amortization (owned)	(12,760)				(11,159)	[1]				(12,760)	(11,159)	[1]
Flight equipment	1,029				1,151	[1]				1,029	1,151	[1]
Less-Accumulated amortization	(654)				(777)	[1]				(654)	(777)	[1]
Current liabilities:
Current maturities of capital leases	149				128	[1]				149	128	[1]
Other	619				576	[1]				619	576	[1]
Long-term obligations under finance leases	1,134				996					1,134	996
Other liabilities and deferred credits:
Deferred income taxes	814				204	[1]				814	204	[1]
Other	1,832				1,832	[1]				1,832	1,832	[1]
Stockholders' equity:
Retained earnings	6,668				4,549	[1]				6,668	4,549	[1]
Operating expense:
Regional capacity purchase										2,601	2,232	[2]	$ 2,197
Landing fees and other rent										2,359	2,240	[2]	2,165
Depreciation and amortization										2,240	2,149	[2],[3]	1,977	[3]
Operating expenses										38,011	34,113	[2]	32,214
Operating income	652	$ 1,203	$ 1,161	$ 276	776		$ 1,138	$ 1,437	$ 320	3,292	3,671	[2]	4,344
Nonoperating income (expense):
Interest expense										(729)	(671)	[2]	(674)
Interest capitalized										70	84	[2]	72
Total nonoperating expense, net										(634)	(631)	[2]	(571)
Income before income taxes										2,658	3,040	[2]	3,773
Income tax expense										529	896	[2]	1,539
Net income	$ 462	$ 836	$ 684	$ 147	$ 579		$ 645	$ 821	$ 99	$ 2,129	$ 2,144	[2],[3],[4],[5]	$ 2,234	[3],[4],[5]
Earnings per share, basic (in dollars per share)	$ 1.71	$ 3.07	$ 2.49	$ 0.52	$ 1.99		$ 2.15	$ 2.67	$ 0.32	$ 7.73	$ 7.08	[2]	$ 6.77
Earnings per share, diluted (in dollars per share)	$ 1.70	$ 3.06	$ 2.48	$ 0.52	$ 1.98		$ 2.15	$ 2.67	$ 0.32	$ 7.70	$ 7.06	[2]	$ 6.76
ASU 2016-02
Current assets:
Receivables, less allowance for doubtful accounts (2018—$8; 2017—$7)	$ 1,426				$ 1,466					$ 1,426	$ 1,466
Prepaid expenses and other	733				863					733	863
Operating property and equipment:
Other property and equipment	6,878				6,024					6,878	6,024
Less-Accumulated depreciation and amortization (owned)	(12,620)				(11,067)					(12,620)	(11,067)
Flight equipment (Finance leases)	992				940					992	940
Less-Accumulated amortization	(646)				(608)					(646)	(608)
Current liabilities:
Current maturities of finance leases	123				78					123	78
Current maturities of operating leases	719				949					719	949
Other	553				518					553	518
Long-term obligations under finance leases	224				230					224	230
Long-term obligations under operating leases	5,276				5,789					5,276	5,789
Other liabilities and deferred credits:
Deferred income taxes	828				220					828	220
Other	1,010				1,021					1,010	1,021
Stockholders' equity:
Retained earnings	6,715				4,603					6,715	4,603
Operating expense:
Regional capacity purchase										2,649	2,268
Landing fees and other rent										2,449	2,310
Depreciation and amortization										2,165	2,096
Operating expenses										38,074	34,166
Operating income										3,229	3,618
Nonoperating income (expense):
Interest expense										(670)	(626)
Interest capitalized										65	74
Total nonoperating expense, net										(581)	(595)
Income before income taxes										2,648	3,023
Income tax expense										526	880
Net income										$ 2,122	$ 2,143
Earnings per share, basic (in dollars per share)										$ 7.70	$ 7.08
Earnings per share, diluted (in dollars per share)										$ 7.67	$ 7.06
ASU 2016-02 | Flight Equipment
Operating property and equipment:
Operating lease assets	2,380				3,102					$ 2,380	$ 3,102
ASU 2016-02 | Other Property and Equipment
Operating property and equipment:
Operating lease assets	2,882				2,975					2,882	2,975
ASU 2016-02 | New Lease Standard Adjustments
Current assets:
Receivables, less allowance for doubtful accounts (2018—$8; 2017—$7)	80				126					80	126
Prepaid expenses and other	(180)				(208)					(180)	(208)
Operating property and equipment:
Other property and equipment	(1,041)				(922)					(1,041)	(922)
Less-Accumulated depreciation and amortization (owned)	140				92					140	92
Flight equipment (Finance leases)	(37)				(211)					(37)	(211)
Less-Accumulated amortization	8				169					8	169
Current liabilities:
Current maturities of finance leases	(26)				(50)					(26)	(50)
Current maturities of operating leases	719				949					719	949
Other	(66)				(58)					(66)	(58)
Long-term obligations under finance leases	(910)				(766)					(910)	(766)
Long-term obligations under operating leases	5,276				5,789					5,276	5,789
Other liabilities and deferred credits:
Deferred income taxes	14				16					14	16
Other	(822)				(811)					(822)	(811)
Stockholders' equity:
Retained earnings	47				54					47	54
Operating expense:
Regional capacity purchase										48	36
Landing fees and other rent										90	70
Depreciation and amortization										(75)	(53)
Operating expenses										63	53
Operating income										(63)	(53)
Nonoperating income (expense):
Interest expense										59	45
Interest capitalized										(5)	(10)
Total nonoperating expense, net										53	36
Income before income taxes										(10)	(17)
Income tax expense										(3)	(16)
Net income										$ (7)	$ (1)
Earnings per share, basic (in dollars per share)										$ (0.03)	$ 0.00
Earnings per share, diluted (in dollars per share)										$ (0.03)	$ 0.00
ASU 2016-02 | New Lease Standard Adjustments | Flight Equipment
Operating property and equipment:
Operating lease assets	2,380				3,102					$ 2,380	$ 3,102
ASU 2016-02 | New Lease Standard Adjustments | Other Property and Equipment
Operating property and equipment:
Operating lease assets	$ 2,882				$ 2,975					$ 2,882	$ 2,975

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.

[2] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606) and Accounting Standards Update No. 2017-07, Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.

[3] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.

[4] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.

[5] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.