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Significant Accounting Policies - Roll Forward of Frequent Flier Deferred Revenue (Details) - Frequent Flyer - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Movement in Deferred Revenue [Roll Forward]    
Total Frequent flyer deferred revenue - beginning balance $ 5,005 $ 4,783
Total miles awarded 2,621 2,451
Travel miles redeemed (Passenger revenue) (2,213) (2,068)
Non-travel miles redeemed (Other operating revenue) (137) (161)
Total Frequent flyer deferred revenue - ending balance $ 5,276 $ 5,005