Revenue Recognition - Narrative (Details) - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Marketing contracts duration, minimum (in years)	1 year
Marketing contracts duration, maximum (in years)	11 years
Cash sales of miles from marketing agreements	$ 4.2	$ 3.5	$ 3.2
Majority of new miles, redemption period (in years)	2 years
Air traffic
Disaggregation of Revenue [Line Items]
Revenue recognized that was in prior year air traffic liability	$ 3.8	$ 3.5	$ 3.5