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Fair Value - Fair Value Hierarchy (Details) - USD ($)
$ in Millions
Mar. 31, 2024
Dec. 31, 2023
Assets    
Available-for-sale investment securities $ 256 $ 330
Fair Value, Recurring    
Assets    
Cash equivalents 897 724
Available-for-sale investment securities 256 330
Aircraft fuel derivatives 4 4
Fair Value, Recurring | Level 1    
Assets    
Cash equivalents 897 724
Available-for-sale investment securities 0 0
Aircraft fuel derivatives 0 0
Fair Value, Recurring | Level 2    
Assets    
Cash equivalents 0 0
Available-for-sale investment securities 240 314
Aircraft fuel derivatives 4 4
Fair Value, Recurring | Level 3    
Assets    
Cash equivalents 0 0
Available-for-sale investment securities 16 16
Aircraft fuel derivatives $ 0 $ 0