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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
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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.

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