Financial Derivative Instruments - Fair Values by Balance Sheet Location (Details) - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019
Derivatives, Fair Value
Derivative Asset, Fair Value, Gross Asset		$ 134	$ 112
Derivative Liability, Fair Value, Gross Liability		6	6
Designated as Hedging Instrument
Derivatives, Fair Value
Derivative Asset, Fair Value, Gross Asset		130	112
Derivative Liability, Fair Value, Gross Liability		6	6
Fuel derivatives | Designated as Hedging Instrument | Prepaid expenses and other current assets
Derivatives, Fair Value
Derivative Asset, Fair Value, Gross Asset	[1]	9	48
Derivative Asset, Fair Value, Gross Liability	[1]	0	0
Fuel derivatives | Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value
Derivative Asset, Fair Value, Gross Asset	[1]	121	62
Derivative Asset, Fair Value, Gross Liability	[1]	0	0
Fuel derivatives | Not Designated as Hedging Instrument | Prepaid expenses and other current assets
Derivatives, Fair Value
Derivative Asset, Fair Value, Gross Asset	[1]	4	0
Derivative Asset, Fair Value, Gross Liability	[1]	0	0
Interest rate derivatives | Other Noncurrent Liabilities
Derivatives, Fair Value
Derivative Liability, Fair Value, Gross Liability		6
Interest rate derivatives | Designated as Hedging Instrument | Other Assets
Derivatives, Fair Value
Derivative Asset, Fair Value, Gross Asset	[1]	0	2
Derivative Asset, Fair Value, Gross Liability	[1]	0	0
Interest rate derivatives | Designated as Hedging Instrument | Accrued Liabilities
Derivatives, Fair Value
Derivative Liability, Fair Value, Gross Liability	[1]	0	5
Derivative Liability, Fair Value, Gross Asset	[1]	0	0
Interest rate derivatives | Designated as Hedging Instrument | Other Noncurrent Liabilities
Derivatives, Fair Value
Derivative Liability, Fair Value, Gross Liability	[1]	6	1
Derivative Liability, Fair Value, Gross Asset	[1]	$ 0	$ 0

[1] Represents the position of each trade before consideration of offsetting positions with each counterparty and does not include the impact of cash collateral deposits provided to or received from counterparties. See discussion of credit risk and collateral following in this Note.