Revenue - Air Traffic Liability Breakout (Details) - USD ($) $ in Millions	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Revenue from Contract with Customer [Abstract]
Air traffic liability - passenger travel and ancillary passenger services	$ 3,960		$ 2,686
Air traffic liability - loyalty program	4,719	$ 4,623	4,447	$ 3,856	$ 3,561	$ 3,385
Total Air traffic liability	$ 8,679		$ 7,133	$ 6,879		$ 5,510