Revenue - Air Traffic Liability Rollforward (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Change in Contract With Customer, Liability [Roll Forward]
Beginning balance	$ 7,133	$ 5,510
Current period sales (passenger travel, ancillary services, flight loyalty, and partner loyalty)	6,851	5,946
Revenue from amounts included in contract liability opening balances	(1,600)	(1,936)
Revenue from current period sales	(3,705)	(2,641)
Ending balance	$ 8,679	$ 6,879