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Fair Value Measurement - Fair Value Assets and Liabilities Measured on Recurring Basis with Unobservable Inputs (Details) - Fuel derivatives - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2020 | Sep. 30, 2020 | Jun. 30, 2020 | Dec. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Losses for the period included in earnings | [1] | $ (23) | $ (40) |  |
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Asset (Liability), Gain (Loss), OCI |  | 1 | (79) |  |
Purchases | [2] | 24 | 118 |  |
Ending Balance |  | 109 | 109 |  |
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value |  |  |  | $ 107 | $ 110
Commodity Option |  |  |  |  |
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items] |  |  |  |  |
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) | [1] | (21) | (29) |  |
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] |  |  |  |  |
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI |  | $ (1) | $ (73) |  |
[1] | Included in Other (gains) losses, net, within the unaudited Condensed Consolidated Statement of Comprehensive Income (Loss).
[2] | The purchase of fuel derivatives is 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 unrealized gain (loss) recognized in other comprehensive income (OCI) from asset measured at fair value on recurring basis using unobservable input (level 3) and still held. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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- Definition Amount of unrealized gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3) and still held. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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