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Leases (Details)
12 Months Ended
Dec. 31, 2020
USD ($)
aircraft
Dec. 31, 2019
USD ($)
aircraft
Dec. 31, 2018
USD ($)
Sublease Income $ 78,000,000 $ 97,000,000  
Sublease income - 2021 41,000,000    
Sublease income - 2022 17,000,000    
Sublease income - 2023 7,000,000    
Sublease income - 2024 1,000,000    
Unrecorded Unconditional Purchase Obligation $ 291,000,000    
Lessee, Operating Lease, Lease Not yet Commenced, Description | aircraft 9    
Proceeds from sale-leaseback transactions $ 815,000,000 0 $ 0
Gain on sale-leaseback transactions $ (222,000,000) $ 0 $ 0
B-717-200      
Property Subject to or Available for Operating Lease, Number of Units | aircraft 45    
Finance Leases, Net Investment in Sales Type Leases, Contingent Payments on Leased Property $ 0    
Finance Leases, Net Investment in Sales Type Leases, Unguaranteed Residual Values of Leased Property $ 0    
Finance Leased Assets, Number of Units | aircraft 2    
Property Subject To Or Available For Lease, Number Of Units, Total | aircraft 88    
B-737-800      
Sale Leaseback Transaction, Description of Asset(s) ten    
Proceeds from sale-leaseback transactions $ 405,000,000    
Sale Leaseback Transaction, Lease Terms ten years    
Gain on sale-leaseback transactions $ 153,000,000    
B-737-8 Max      
Sale Leaseback Transaction, Description of Asset(s) ten    
Proceeds from sale-leaseback transactions $ 410,000,000    
Sale Leaseback Transaction, Lease Terms 13 years    
Gain on sale-leaseback transactions $ 69,000,000    
Minimum | B-717-200      
Sublease Terms 1 month    
Minimum | 737 MAX 8      
Lessee, Renewal Term 8 years    
Minimum | Aircraft      
Lessee, Renewal Term 3 months    
Lessee Operating and Finance Lease Term of Contract 1 month    
Minimum | Airport-Related      
Lessee, Renewal Term 6 months    
Lessee Operating Lease Terms of Contract 1 month    
Minimum | Other Property      
Lessee, Renewal Term 6 months    
Lessee Operating and Finance Lease Term of Contract 3 months    
Maximum | B-717-200      
Sublease Terms 4 years    
Maximum | 737 MAX 8      
Lessee, Operating Lease, Lease Not yet Commenced, Term of Contract 9 years    
Maximum | Aircraft      
Lessee, Renewal Term 6 years    
Lessee Operating and Finance Lease Term of Contract 13 years    
Maximum | Airport-Related      
Lessee, Renewal Term 10 years    
Lessee Operating Lease Terms of Contract 40 years    
Maximum | Other Property      
Lessee, Renewal Term 5 years    
Lessee Operating and Finance Lease Term of Contract 6 years    
Assets Sold to Third Party | B-717-200      
Owned Assets, Number of Units | aircraft   10  
Lease Expired | B-717-200      
Property Subject To Or Available For Lease, Number Of Units, Total | aircraft 20 8