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REVENUE FROM CONTRACTS WITH CUSTOMERS - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Deferred Revenue Arrangement [Line Items]          
Receivable $ 536   $ 536   $ 480
Costs and Expenses 1,695 $ 1,272 3,631 $ 3,938  
COVID-19 Credits Issued, Balance 324   324   569
Mileage Plan Revenue [Member]          
Deferred Revenue Arrangement [Line Items]          
Receivable 57   57   $ 48
Mileage Plan Revenue [Member] | Mileage plan other revenue [Member]          
Deferred Revenue Arrangement [Line Items]          
Contract liability revenue recognized     (30) (21)  
Passenger Revenue [Member]          
Deferred Revenue Arrangement [Line Items]          
Contract liability revenue recognized $ 101   $ 276 $ 484