Revenue Recognition - Loyalty Program Liability (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Frequent Flyer Liability Activity [Roll Forward]
Mileage credits earned	$ 1,542	$ 1,550
Travel mileage credits redeemed	(1,443)	(1,298)
Non-travel mileage credits redeemed	(86)	(82)
Loyalty program
Frequent Flyer Liability Activity [Roll Forward]
Deferred revenue (current and noncurrent), beginning	6,641	6,321
Deferred revenue (current and noncurrent), ending	$ 6,654	$ 6,491