Revenue - Air Traffic Liability Breakout (Details) - USD ($) $ in Millions	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Revenue from Contract with Customer [Abstract]
Air traffic liability - passenger travel and ancillary passenger services	$ 3,464		$ 2,686
Air traffic liability - loyalty program	4,772	$ 4,719	4,447	$ 4,187	$ 3,856	$ 3,385
Total Air traffic liability	$ 8,236		$ 7,133	$ 7,094		$ 5,510