Revenue - Roll Forward of Frequent Flyer Deferred Revenue (Details) - Frequent Flyer - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Frequent Flyer Deferred Revenue [Roll Forward]
Total Frequent flyer deferred revenue - beginning balance	$ 6,185	$ 5,670	$ 5,975	$ 5,276
Total miles awarded	439	268	1,071	1,056
Travel miles redeemed (Passenger revenue)	(391)	(87)	(781)	(444)
Non-travel miles redeemed (Other operating revenue)	(16)	(16)	(48)	(53)
Total Frequent flyer deferred revenue - ending balance	$ 6,217	$ 5,835	$ 6,217	$ 5,835