Revenue Recognition - Narrative (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2018	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)		8 years
Cash sales from marketing agreements of mileage credits		$ 3,500	$ 3,200
Majority of new miles, redemption period (in years)		2 years
Proceeds from contribution of assets to newly formed entity	$ 40
New DGS
Disaggregation of Revenue [Line Items]
Equity method investment, ownership percentage	49.00%	49.00%
Air traffic
Disaggregation of Revenue [Line Items]
Revenue recognized that was previously deferred		$ 3,500	$ 3,500