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REVENUE FROM CONTRACTS WITH CUSTOMERS - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Deferred Revenue Arrangement [Line Items]      
Receivable $ 517   $ 480
Costs and Expenses 958 $ 1,957  
COVID-19 Credits Issued, Balance 484    
Mileage Plan Revenue [Member]      
Deferred Revenue Arrangement [Line Items]      
Receivable 52   $ 48
Mileage Plan Revenue [Member] | Mileage plan other revenue [Member]      
Deferred Revenue Arrangement [Line Items]      
Contract liability revenue recognized (4) (22)  
Passenger Revenue [Member]      
Deferred Revenue Arrangement [Line Items]      
Contract liability revenue recognized $ 136 $ 489