Fair Value Measurement - Fair Value Assets and Liabilities Measured on Recurring Basis with Unobservable Inputs (Details) - Fuel derivatives - USD ($) $ in Millions		3 Months Ended	6 Months Ended
		Jun. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Losses for the period included in earnings	[1]	$ (15)	$ (17)
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Asset (Liability), Gain (Loss), OCI		26	(80)
Purchases	[2]	11	94
Ending Balance		107	107
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value				$ 85	$ 110
Commodity Option
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	[1]	(8)	(8)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI		$ 19	$ (72)

[1] Included in Other (gains) losses, net, within the unaudited Condensed Consolidated Statement of Comprehensive Income (Loss).

[2] The purchase of fuel derivatives is recorded gross based on the structure of the derivative instrument and whether a contract with multiple derivatives was purchased as a single instrument or separate instruments.