Revenue Recognition - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Revenue from Contract with Customer [Abstract]
Deferred air traffic revenue	$ 2,100	$ 2,100
Air traffic liability	6,233		$ 4,364	$ 4,100
Cash sales of mileage credits	$ 841	$ 757

The value of air transportation services sold but as yet unused by the passenger, including sales for air transportation to be provided by the reporting air carrier and air transportation to be provided by another air carrier for whom sales were made. Used to reflect the current portion of the liability (within one year or within the normal operating cycle if longer).

No definition available.

Name:	us-gaap_AirTrafficLiabilityCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant