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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, 2019 | Dec. 31, 2018
Total losses (realized or unrealized) included in other comprehensive income |  |  |
Gains (Losses) included in Other Comprehensive Income (Loss) |  | $ (112) | $ (1)
Fuel derivatives |  |  |
Total losses (realized or unrealized) included in other comprehensive income |  |  |
Beginning Balance |  | 138 | 248
Purchases | [1] | 133 | 66
Sales | [1] | (2) | (4)
Settlements |  | (47) | (171)
Ending Balance |  | 110 | 138
Quoted prices in active markets for identical assets (Level 1) | Fair Value, Measurements, Recurring |  |  |
Total losses (realized or unrealized) included in other comprehensive income |  |  |
Interest Rate Derivative Assets, at Fair Value |  | 0 |
Assets, Fair Value Disclosure |  | 3,392 | 3,101
Significant other observable inputs (Level 2) | Fair Value, Measurements, Recurring |  |  |
Total losses (realized or unrealized) included in other comprehensive income |  |  |
Interest Rate Derivative Assets, at Fair Value |  | 2 |
Assets, Fair Value Disclosure |  | 879 | 715
Significant unobservable inputs (Level 3) | Fair Value, Measurements, Recurring |  |  |
Total losses (realized or unrealized) included in other comprehensive income |  |  |
Interest Rate Derivative Assets, at Fair Value |  | 0 |
Assets, Fair Value Disclosure |  | 110 | 138
Estimate of Fair Value Measurement | Fair Value, Measurements, Recurring |  |  |
Total losses (realized or unrealized) included in other comprehensive income |  |  |
Interest Rate Derivative Assets, at Fair Value |  | 2 |
Assets, Fair Value Disclosure |  | $ 4,381 | $ 3,954
[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 was purchased as a single instrument or separate instruments.

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- Definition Amount of sale of 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)(2) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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