Financial Derivative Instruments and Risk Management (Details) - Aircraft Fuel Derivatives	12 Months Ended
Dec. 31, 2020 USD ($) aircraft	Dec. 31, 2019 USD ($) aircraft	Dec. 31, 2018 USD ($)
Derivative [Line Items]
Longest remaining term (months)	0 years	6 months
Hedged volume (barrels, in thousands) \| aircraft	0	2,112,000
Estimated amount of existing (gains) losses expected to be reclassified into earnings in the next 12 months	$ 0	$ (2,000,000)
Percentage of actual consumption economically hedged	25.00%	6.00%	4.00%
Offsetting derivative instruments	$ 0	$ 0
Aircraft Fuel Expense
Derivative [Line Items]
Hedge effectiveness (gains) losses recognized in aircraft fuel expense	7,000,000	5,000,000	$ 2,000,000
Other Income
Derivative [Line Items]
Losses on derivatives resulting from the discontinuance of hedge accounting recognized in interest income and other	8,000,000	0	0
Comprehensive Income
Derivative [Line Items]
Hedge (gains) losses on derivatives recognized in comprehensive income	11,000,000	(1,000,000)	$ 6,000,000
Prepaid Expenses and Other Current Assets
Derivative [Line Items]
Asset fair value recorded in prepaid expenses and other	$ 0	$ 8,000,000

Barrels of fuel covered under derivative contracts as of the end of the period.

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Percentage of actual consumption economically hedged.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled within one year or normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Amount of right to receive cash collateral under master netting arrangements that have not been offset against derivative liabilities.

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Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The amount of loss reclassified into earnings in the period when cash flow hedge is discontinued because it is probable that the original forecasted transactions will not occur by the end of the original period or an additional two month time period.

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Maximum period the entity is hedging its exposure to the variability in future cash flows for forecasted transactions, excluding those forecasted transactions related to the payment of variable interest on existing financial instruments, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount before tax of increase (decrease) in accumulated gain (loss) from derivative instruments designated and qualifying as the effective portion of cash flow hedges and an entity's share of an equity investee's increase (decrease) in deferred hedging gain (loss).

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The estimated net amount of unrealized gains or losses on price risk cash flow hedges as of the balance sheet date expected to be reclassified to earnings within the next twelve months.

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