Financial Derivative Instruments - (Gain) Loss in Income by Hedging Relationship (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Fuel
Derivative [Line Items]
Loss on Derivative Instruments, Net, Pretax	$ 11	$ 4
Interest Expense
Derivative [Line Items]
Loss on Derivative Instruments, Net, Pretax	8	10
Cash Flow Hedging \| Fuel \| Fuel derivatives
Derivative [Line Items]
Derivative Instruments, Loss Reclassified from Accumulated OCI into Income, Effective Portion, Net	11	4
Cash Flow Hedging \| Fuel \| Interest rate derivatives
Derivative [Line Items]
Derivative Instruments, Loss Reclassified from Accumulated OCI into Income, Effective Portion, Net	0	0
Cash Flow Hedging \| Interest Expense \| Fuel derivatives
Derivative [Line Items]
Derivative Instruments, 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, Loss Reclassified from Accumulated OCI into Income, Effective Portion, Net	1	2
Designated as Hedging Instrument \| Fair Value Hedging \| Fuel \| Interest rate derivatives
Derivative [Line Items]
Interest Expense, Debt	0	0
Designated as Hedging Instrument \| Fair Value Hedging \| Interest Expense \| Interest rate derivatives
Derivative [Line Items]
Interest Expense, Debt	6	6
Not Designated as Hedging Instrument \| Fair Value Hedging \| Fuel \| Interest rate derivatives
Derivative [Line Items]
Loss on Derivative Instruments, Net, Pretax	0	0
Not Designated as Hedging Instrument \| Fair Value Hedging \| Interest Expense \| Interest rate derivatives
Derivative [Line Items]
Loss on Derivative Instruments, Net, Pretax	$ 1	$ 2

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

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-Publisher FASB
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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.

+ References

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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- Definition

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

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-Publisher FASB
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