Financial Derivative Instruments - Offsetting of Derivative Assets (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Offsetting Assets [Line Items]
Gross amounts of recognized assets	$ 503	$ 134
Fuel derivatives | Prepaid expenses and other current assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets	224	13
Gross amounts offset in the Balance Sheet	48	3
Net amounts of assets presented in the Balance Sheet	176	10
Fuel derivatives | Other assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets	278	121
Gross amounts offset in the Balance Sheet	72	31
Net amounts of assets presented in the Balance Sheet	206	90
Interest rate derivatives | Other assets
Offsetting Assets [Line Items]
Gross amounts of recognized assets	1	0
Gross amounts offset in the Balance Sheet	0	0
Net amounts of assets presented in the Balance Sheet	$ 1	$ 0