Significant Accounting Policies - Impact of the Adoption of the New Revenue Standard and the New Retirement 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
Operating revenue:
Revenue	$ 10,491	$ 11,003	$ 10,777	$ 9,032	$ 9,451		$ 9,899	$ 10,008	$ 8,426	$ 41,303	$ 37,784	[1]	$ 36,558	[1]
Operating expenses										38,011	34,113	[1]	32,214	[1]
Operating income	652	1,203	1,161	276	776		1,138	1,437	320	3,292	3,671	[1]	4,344	[1]
Total nonoperating expense, net										(634)	(631)	[1]	(571)	[1]
Income before income taxes										2,658	3,040	[1]	3,773	[1]
Income tax expense										529	896	[1]	1,539	[1]
Net income	$ 462	$ 836	$ 684	$ 147	$ 579		$ 645	$ 821	$ 99	$ 2,129	$ 2,144	[1],[2],[3],[4]	$ 2,234	[1],[2],[3],[4]
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	[1]	$ 6.77	[1]
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	[1]	$ 6.76	[1]
Current assets:
Prepaid expenses and other	$ 913				$ 1,071	[5]				$ 913	$ 1,071	[5]
Current liabilities:
Advance ticket sales	4,381				3,940	[5]				4,381	3,940	[5]
Frequent flyer deferred revenue	2,286				2,192	[5]				2,286	2,192	[5]
Other	619				576	[5]				619	576	[5]
Other liabilities and deferred credits:
Frequent flyer deferred revenue	2,719				2,591	[5]				2,719	2,591	[5]
Deferred income taxes	814				204	[5]				814	204	[5]
Stockholders' equity:
Retained earnings	$ 6,668				4,549	[5]				6,668	4,549	[5]
As Previously Reported
Current assets:
Prepaid expenses and other					1,051						1,051
Current liabilities:
Advance ticket sales					3,876						3,876
Frequent flyer deferred revenue					2,176						2,176
Other					569						569
Other liabilities and deferred credits:
Frequent flyer deferred revenue					2,565						2,565
Deferred income taxes					225						225
Stockholders' equity:
Retained earnings					4,621						4,621
Passenger revenue
Operating revenue:
Revenue										37,706	34,460	[1]	$ 33,429	[1]
Cargo
Operating revenue:
Revenue										1,237	1,114	[1]	934	[1]
Other operating revenue
Operating revenue:
Revenue										$ 2,360	2,210	[1]	2,195	[1]
As Previously Reported
Operating revenue:
Revenue											37,736		36,556
Operating expenses											34,238		32,218
Operating income											3,498		4,338
Total nonoperating expense, net											(499)		(519)
Income before income taxes											2,999		3,819
Income tax expense											868		1,556
Net income											$ 2,131		$ 2,263
Earnings per share, basic (in dollars per share)											$ 7.04		$ 6.86
Earnings per share, diluted (in dollars per share)											$ 7.02		$ 6.85
As Previously Reported | Passenger revenue
Operating revenue:
Revenue											$ 32,404		$ 31,457
As Previously Reported | Cargo
Operating revenue:
Revenue											1,035		876
As Previously Reported | Other operating revenue
Operating revenue:
Revenue											4,297		4,223
ASU 2014-09 | New Revenue Standard Adjustments
Current assets:
Prepaid expenses and other					20						20
Current liabilities:
Advance ticket sales					64						64
Frequent flyer deferred revenue					16						16
Other					7						7
Other liabilities and deferred credits:
Frequent flyer deferred revenue					26						26
Deferred income taxes					(21)						(21)
Stockholders' equity:
Retained earnings					$ (72)						(72)
ASU 2014-09 | Adjustment
Operating revenue:
Revenue											48		2
Operating expenses											(21)		(12)
Operating income											69		14
Total nonoperating expense, net											(28)		(60)
Income before income taxes											41		(46)
Income tax expense											28		(17)
Net income											$ 13		$ (29)
Earnings per share, basic (in dollars per share)											$ 0.04		$ (0.09)
Earnings per share, diluted (in dollars per share)											$ 0.04		$ (0.09)
ASU 2014-09 | Adjustment | Passenger revenue
Operating revenue:
Revenue											$ 2,056		$ 1,972
ASU 2014-09 | Adjustment | Cargo
Operating revenue:
Revenue											79		58
ASU 2014-09 | Adjustment | Other operating revenue
Operating revenue:
Revenue											(2,087)		(2,028)
ASU 2017-07 | Adjustment
Operating revenue:
Revenue											0		0
Operating expenses											(104)		8
Operating income											104		(8)
Total nonoperating expense, net											(104)		8
Income before income taxes											0		0
Income tax expense											0		0
Net income											$ 0		$ 0
Earnings per share, basic (in dollars per share)											$ 0.00		$ 0.00
Earnings per share, diluted (in dollars per share)											$ 0.00		$ 0.00
ASU 2017-07 | Adjustment | Passenger revenue
Operating revenue:
Revenue											$ 0		$ 0
ASU 2017-07 | Adjustment | Cargo
Operating revenue:
Revenue											0		0
ASU 2017-07 | Adjustment | Other operating revenue
Operating revenue:
Revenue											$ 0		$ 0

[1] 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.

[2] 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.

[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.