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Financial Derivative Instruments - Premiums for Fuel Derivative Contracts (Details) - Fuel derivatives - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
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Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020
Designated as Hedging Instrument | Fuel and oil |  |  |  |
Derivative [Line Items] |  |  |  |
Premium expense recognized in income on derivatives | $ 14 | $ 13 | $ 29 | $ 38
Not Designated as Hedging Instrument | Other (gains)/losses, net |  |  |  |
Derivative [Line Items] |  |  |  |
Premium expense recognized in income on derivatives | $ 10 | $ 11 | $ 21 | $ 11

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