Financial Derivative Instruments - (Gain) Loss in Income by Hedging Relationship (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Interest Expense
Derivative [Line Items]
(Gain) Loss on Derivative Instruments, Net, Pretax	$ 6	$ 29
Fuel
Derivative [Line Items]
(Gain) Loss on Derivative Instruments, Net, Pretax	70	48
Other (gains) losses, net
Derivative [Line Items]
(Gain) Loss on Derivative Instruments, Net, Pretax	39
Cash Flow Hedging \| Interest Expense \| Fuel derivatives
Derivative [Line Items]
Derivative Instruments, (Gain) Loss Reclassified from Accumulated OCI into Income, Effective Portion, Net	0	0
Cash Flow Hedging \| Interest Expense \| Interest rate derivatives
Derivative [Line Items]
Derivative Instruments, (Gain) Loss Reclassified from Accumulated OCI into Income, Effective Portion, Net	1	5
Cash Flow Hedging \| Fuel \| Fuel derivatives
Derivative [Line Items]
Derivative Instruments, (Gain) Loss Reclassified from Accumulated OCI into Income, Effective Portion, Net	70	48
Cash Flow Hedging \| Fuel \| Interest rate derivatives
Derivative [Line Items]
Derivative Instruments, (Gain) Loss Reclassified from Accumulated OCI into Income, Effective Portion, Net	0	0
Cash Flow Hedging \| Other (gains) losses, net \| Fuel derivatives
Derivative [Line Items]
Loss on Discontinuation of Cash Flow Hedge Due to Forecasted Transaction Probable of Not Occurring	39
Cash Flow Hedging \| Other (gains) losses, net \| Interest rate derivatives
Derivative [Line Items]
Derivative Instruments, (Gain) Loss Reclassified from Accumulated OCI into Income, Effective Portion, Net	0
Not Designated as Hedging Instrument \| Fuel derivatives
Derivative [Line Items]
Loss on Discontinuation of Cash Flow Hedge Due to Forecasted Transaction Probable of Not Occurring	39
Not Designated as Hedging Instrument \| Fair Value Hedging \| Interest Expense \| Interest rate derivatives
Derivative [Line Items]
Interest Expense, Debt	11	22
Not Designated as Hedging Instrument \| Fair Value Hedging \| Fuel \| Interest rate derivatives
Derivative [Line Items]
Interest Expense, Debt	0	0
Not Designated as Hedging Instrument \| Fair Value Hedging \| Other (gains) losses, net \| Interest rate derivatives
Derivative [Line Items]
Interest Expense, Debt	0
Designated as Hedging Instrument \| Fair Value Hedging \| Interest Expense \| Interest rate derivatives
Derivative [Line Items]
(Gain) Loss on Derivative Instruments, Net, Pretax	(6)	2
Designated as Hedging Instrument \| Fair Value Hedging \| Fuel \| Interest rate derivatives
Derivative [Line Items]
(Gain) Loss on Derivative Instruments, Net, Pretax	0	$ 0
Designated as Hedging Instrument \| Fair Value Hedging \| Other (gains) losses, net \| Interest rate derivatives
Derivative [Line Items]
(Gain) Loss on Derivative Instruments, Net, Pretax	$ 0

The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
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- Definition

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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No definition available.

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- Definition

Aggregate net gain (loss) on all derivative instruments recognized in earnings during the period, before tax effects.

+ References

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-Publisher FASB
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- Definition

Amount of the cost of borrowed funds accounted for as interest expense for debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
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- Definition

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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