Revenue Recognition - Loyalty Program Liability (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Frequent Flyer Liability Activity [Roll Forward]
Mileage credits earned	$ 2,352	$ 2,322
Travel mileage credits redeemed	(2,175)	(1,976)
Non-travel mileage credits redeemed	(122)	(125)
Loyalty program
Frequent Flyer Liability Activity [Roll Forward]
Deferred revenue (current and noncurrent), beginning	6,641	6,321
Deferred revenue (current and noncurrent), ending	$ 6,696	$ 6,542