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Leases - Narrative (Details)
$ in Millions
 12 Months Ended
Dec. 31, 2020
USD ($)
aircraft
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Lessee, Lease, Description [Line Items]      
Aircraft leases, minimum remaining lease term 9 months    
Aircraft leases, maximum remaining lease term 5 years    
Fleet impairment | $ $ 273 $ 0 $ 0
Facility leases, minimum lease term remaining 2 months    
Facility leases, maximum lease term remaining 14 years    
Minimum      
Lessee, Lease, Description [Line Items]      
Renewal lease term 1 year    
Maximum      
Lessee, Lease, Description [Line Items]      
Renewal lease term 2 years    
Aircraft      
Lessee, Lease, Description [Line Items]      
Number of aircraft operated 267    
Number of aircraft accounted for under operating leases 62    
Finance lease, number of aircraft leases 4    
Number of aircraft variable rate rent 0    
Number of aircraft having purchase options 40