Fair Value Measurement - Fair Value Assets and Liabilities Measured on Recurring Basis with Unobservable Inputs (Details) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Asset (Liability), Gain (Loss), OCI			$ (112)
Fuel derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance			138
Losses for the period included in earnings	[1]	$ (2)
Fair Value, Recurring Basis, Unobservable Input Reconciliation, Asset (Liability), Gain (Loss), OCI		(106)
Purchases	[2]	83	133
Sales	[2]		(2)
Settlements			(47)
Ending Balance		85
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value			$ 110
Commodity Option | Fuel derivatives
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss)	[1]	(2)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss), OCI		$ (100)

[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.