Revenue - Passenger Revenue Breakout (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation of Revenue [Line Items]
Operating revenue	$ 4,008	$ 1,008	$ 6,060	$ 5,242
Passenger
Disaggregation of Revenue [Line Items]
Operating revenue	3,569	704	5,282	4,549
Passenger non-loyalty
Disaggregation of Revenue [Line Items]
Operating revenue	2,875	562	4,230	3,783
Passenger loyalty - air transportation
Disaggregation of Revenue [Line Items]
Operating revenue	549	81	826	542
Passenger ancillary sold separately
Disaggregation of Revenue [Line Items]
Operating revenue	$ 145	$ 61	$ 226	$ 224