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Revenue - Air Traffic Liability - Loyalty Program Rollforward (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Movement in Deferred Revenue    
Air traffic liability - loyalty program - beginning balance $ 2,667 $ 2,485
Amounts deferred associated with points awarded 2,717 2,485
Revenue recognized from points redeemed - Passenger (2,307) (2,263)
Revenue recognized from points redeemed - Other (66) (40)
Air traffic liability - loyalty program - ending balance $ 3,011 $ 2,667