Revenue - Air Traffic Liability Rollforward (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Air Traffic Liability Rollforward [Abstract]
ATL, beginning balance	$ 5,510	$ 5,070
Current Period Sales	9,348	21,296
Revenue amounts in beginning balance	(2,317)	(3,816)
Revenue from Current Period Sales	(5,408)	(17,040)
ATL, ending balance	$ 7,133	$ 5,510