Revenue - Roll Forward of Frequent Flyer Deferred Revenue (Details) - Frequent Flyer - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Frequent Flyer Deferred Revenue [Roll Forward]
Total Frequent flyer deferred revenue - beginning balance	$ 5,975	$ 5,276
Total miles awarded	272	559
Travel miles redeemed (Passenger revenue)	(123)	(322)
Non-travel miles redeemed (Other operating revenue)	(15)	(25)
Total Frequent flyer deferred revenue - ending balance	$ 6,109	$ 5,488