Revenue - Passenger Revenue Breakout (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue [Line Items]
Operating revenue	$ 4,679	$ 1,793	$ 10,739	$ 7,035
Passenger
Disaggregation of Revenue [Line Items]
Operating revenue	4,227	1,454	9,508	6,003
Passenger non-loyalty
Disaggregation of Revenue [Line Items]
Operating revenue	3,443	1,183	7,672	4,966
Passenger loyalty - air transportation
Disaggregation of Revenue [Line Items]
Operating revenue	621	205	1,448	747
Passenger ancillary sold separately
Disaggregation of Revenue [Line Items]
Operating revenue	$ 163	$ 66	$ 388	$ 290