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Fair Value Measurement - Fair Value Assets and Liabilities Measured on Recurring Basis with Unobservable Inputs (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017
Total losses (realized or unrealized) |  |  |
Included in other comprehensive income |  | $ (1) |
Fuel derivatives |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Beginning Balance |  | (248) | $ 258
Total losses (realized or unrealized) |  |  |
Included in earnings |  |  | (125)
Included in other comprehensive income |  |  | (50)
Purchases | [1] | 66 | 142
Sales | [1] | (4) |
Settlements |  | (171) | (539)
Ending Balance |  | (138) | (248)
The amount of total losses for the period included in earnings attributable to the change in unrealized gains or losses relating to assets still held at December 31, 2017 |  |  | (42)
Quoted prices in active markets for identical assets (Level 1) | Fair Value, Measurements, Recurring |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Assets, Fair Value Disclosure |  | 3,101 | 2,731
Significant other observable inputs (Level 2) | Fair Value, Measurements, Recurring |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Assets, Fair Value Disclosure |  | 715 | 649
Significant unobservable inputs (Level 3) | Fair Value, Measurements, Recurring |  |  |
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Assets, Fair Value Disclosure |  | $ 138 | $ 283
[1] | The purchase and sale of fuel derivatives are recorded gross based on the structure of the derivative instrument and whether a contract with multiple derivatives is purchased as a single instrument or separate instruments.

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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- Definition This item represents the amount of the total realized and unrealized gains or losses for the period which are included in the statement of income (or changes in net assets); the fair value of which assets and liabilities was or is measured on a recurring basis using significant unobservable inputs (Level 3). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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- Definition Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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