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Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |
Fuel and oil |  | $ 4,616 | $ 4,076 | $ 3,801
Interest Expense |  | 131 | 114 | 122
Fuel derivatives |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |
Beginning balance |  | 3 | (499) |
Changes in fair value |  | 0 | (50) |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax |  |  | 552 |
Beginning balance |  | (3) | 499 |
Changes in fair value |  | 0 | (50) |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax |  |  | 552 |
Ending balance |  | (56) | 3 | (499)
Interest rate derivatives |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |
Beginning balance |  | (7) | (18) |
Changes in fair value |  | 1 | 0 |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax |  |  | 11 |
Beginning balance |  | 7 | 18 |
Changes in fair value |  | 1 | 0 |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax |  |  | 11 |
Ending balance |  | 0 | (7) | (18)
Defined benefit plan items |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |
Beginning balance |  | (9) | (14) |
Changes in fair value |  | 67 | 5 |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax |  | 0 | 0 |
Beginning balance |  | 9 | 14 |
Changes in fair value |  | 67 | 5 |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax |  | 0 | 0 |
Ending balance |  | 58 | (9) | (14)
Other |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |
Beginning balance |  | 33 | 20 |
Changes in fair value |  | (8) | 13 |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax |  | 0 | 0 |
Beginning balance |  | (33) | (20) |
Changes in fair value |  | (8) | 13 |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax |  | 0 | 0 |
Ending balance |  | 25 | 33 | 20
Deferred tax |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |
Beginning balance |  | (8) | 188 |
Changes in fair value |  | (14) | 11 |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax |  | 7 | (207) |
Beginning balance |  | 8 | (188) |
Changes in fair value |  | (14) | 11 |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax |  | 7 | (207) |
Ending balance |  | (7) | (8) | 188
Accumulated other comprehensive income (loss) |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |
Beginning balance |  | 12 | (323) |
Changes in fair value |  | 46 | (21) |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax |  | (20) | 356 |
Beginning balance |  | (12) | 323 |
Changes in fair value |  | 46 | (21) |
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax |  | (20) | 356 |
Ending balance |  | 20 | $ 12 | $ (323)
Fuel derivatives | Reclassification out of Accumulated Other Comprehensive Income |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |
Fuel and oil |  | (33) |  |
Interest rate derivatives | Reclassification out of Accumulated Other Comprehensive Income |  |  |  |
AOCI Attributable to Parent, Net of Tax [Roll Forward] |  |  |  |
Interest Expense |  | 6 |  |
Accounting Standards Update 2017-12 [Member] | Fuel derivatives |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption | [1] | (26) |  |
Accounting Standards Update 2017-12 [Member] | Interest rate derivatives |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption | [1] | 0 |  |
Accounting Standards Update 2017-12 [Member] | Defined benefit plan items |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption | [1] | 0 |  |
Accounting Standards Update 2017-12 [Member] | Other |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption | [1] | 0 |  |
Accounting Standards Update 2017-12 [Member] | Deferred tax |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption | [1] | 6 |  |
Accounting Standards Update 2017-12 [Member] | Accumulated other comprehensive income (loss) |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption | [1] | (20) |  |
Accounting Standards Update 2018-02 [Domain] | Fuel derivatives |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption | [2] | 0 |  |
Accounting Standards Update 2018-02 [Domain] | Interest rate derivatives |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption | [2] | 0 |  |
Accounting Standards Update 2018-02 [Domain] | Defined benefit plan items |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption | [2] | 0 |  |
Accounting Standards Update 2018-02 [Domain] | Other |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption | [2] | 0 |  |
Accounting Standards Update 2018-02 [Domain] | Deferred tax |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption | [2] | 2 |  |
Accounting Standards Update 2018-02 [Domain] | Accumulated other comprehensive income (loss) |  |  |  |
Accumulated Other Comprehensive Income (Loss) [Line Items] |  |  |  |
Cumulative Effect of New Accounting Principle in Period of Adoption | [2] | $ 2 |  |
[1] | The Company adopted the New Hedging Standard as of January 1, 2018. See Note 2 for further information on this adoption.
[2] | The Company adopted the Reclassification of Certain Tax Effects from AOCI as of January 1, 2018, which allowed the Company to reclassify to Retained earnings any tax effects stranded in AOCI as a result of the Tax Cuts and Jobs Act enacted in December 2017.

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