Revenue Recognition - Loyalty Program Liability (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Loyalty Program
Miles earned	$ 1,437	$ 3,156	$ 3,142
Travel miles redeemed	(935)	(2,900)	(2,651)
Non-travel miles redeemed	(48)	(169)	(171)
Loyalty program
Loyalty Program
Deferred revenue (current and noncurrent), beginning	6,728	6,641	6,321
Deferred revenue (current and noncurrent), ending	$ 7,182	$ 6,728	$ 6,641