Financial Derivative Instruments - Fair Values by Balance Sheet Location (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Derivatives, Fair Value [Line Items]
Gross amounts of recognized assets	$ 503	$ 134
Gross amounts of recognized liabilities	3	6
Fuel derivatives \| Prepaid expenses and other current assets
Derivatives, Fair Value [Line Items]
Gross amounts of recognized assets	224	13
Gross amounts offset in the Balance Sheet	48	3
Gross amounts offset in the Balance Sheet	48	3
Gross amounts of recognized liabilities	48	3
Fuel derivatives \| Other Current Assets
Derivatives, Fair Value [Line Items]
Cash collateral deposits held from counterparties for fuel contracts - current	48	3
Fuel derivatives \| Other assets
Derivatives, Fair Value [Line Items]
Gross amounts of recognized assets	278	121
Gross amounts offset in the Balance Sheet	72	31
Gross amounts offset in the Balance Sheet	72	31
Gross amounts of recognized liabilities	72	31
Cash collateral deposits held from counterparties for fuel contracts - current	72	31
Interest rate derivatives \| Other noncurrent liabilities
Derivatives, Fair Value [Line Items]
Gross amounts offset in the Balance Sheet	0	0
Gross amounts of recognized liabilities	3	6
Interest rate derivatives \| Other assets
Derivatives, Fair Value [Line Items]
Gross amounts of recognized assets	1	0
Gross amounts offset in the Balance Sheet	0	0
Designated as Hedging Instrument
Derivatives, Fair Value [Line Items]
Gross amounts of recognized assets	483	130
Gross amounts of recognized liabilities	3	6
Designated as Hedging Instrument \| Fuel derivatives \| Prepaid expenses and other current assets
Derivatives, Fair Value [Line Items]
Gross amounts of recognized assets	204	9
Designated as Hedging Instrument \| Fuel derivatives \| Other assets
Derivatives, Fair Value [Line Items]
Gross amounts of recognized assets	278	121
Designated as Hedging Instrument \| Interest rate derivatives \| Other noncurrent liabilities
Derivatives, Fair Value [Line Items]
Gross amounts of recognized liabilities	3	6
Designated as Hedging Instrument \| Interest rate derivatives \| Other assets
Derivatives, Fair Value [Line Items]
Gross amounts of recognized assets	1
Not Designated as Hedging Instrument \| Fuel derivatives \| Prepaid expenses and other current assets
Derivatives, Fair Value [Line Items]
Gross amounts of recognized assets	$ 20	$ 4

Amount of obligation to return cash collateral under master netting arrangements offset against derivative assets.

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