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Summary of Significant Accounting Policies - Concentrations of Risk (Details) - employeeGroup
12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Total operating expenses | Aircraft fuel expenditure concentration risk | Fuel Costs      
Concentration Risk [Line Items]      
Concentration of risk 31.00% 34.00% 28.00%
Number of employees, total | Unionized employees concentration risk      
Concentration Risk [Line Items]      
Concentration of risk 85.00%    
Union-represented employee groups 6