Revenue Recognition - Narrative (Details) - USD ($) $ in Billions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Cash sales of mileage credits	$ 2.6	$ 2.3
Majority of new miles redemption period (in years)	2 years
Marketing contracts duration, minimum	1 year
Marketing contracts duration, maximum	8 years
Air Traffic
Disaggregation of Revenue [Line Items]
Revenue recognized that was previously deferred	$ 3.5	$ 3.5