Financial Derivative Instruments - Fair Values by Balance Sheet Location (Details) - Designated as Hedging Instrument - USD ($) $ in Millions		Jun. 30, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	$ 187	$ 138
Derivative Liability, Fair Value, Gross Liability	[1]	37	14
Fuel derivatives | Prepaid expenses and other current assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	67	43
Derivative Asset, Fair Value, Gross Liability	[1]	0	0
Fuel derivatives | Other Assets
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	[1]	120	95
Derivative Asset, Fair Value, Gross Liability	[1]	0	0
Interest rate derivatives | Accrued Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Asset	[1]	0	0
Derivative Liability, Fair Value, Gross Liability	[1]	37	2
Interest rate derivatives | Other Noncurrent Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Asset	[1]	0	0
Derivative Liability, Fair Value, Gross Liability	[1]	$ 0	$ 12

[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.