Financial Derivative Instruments - Premiums for Fuel Derivative Contracts (Details) - Fuel derivatives - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Designated as Hedging Instrument \| Operating Expense
Derivative [Line Items]
Premiums paid for fuel derivative contracts	$ 64	$ 95	$ 135
Not Designated as Hedging Instrument, Economic Hedge \| Other (gains) losses, net
Derivative [Line Items]
Premiums paid for fuel derivative contracts	$ 34	$ 0	$ 0

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