v3.26.1
Fair Value Measurements and Derivative Instruments - Summary of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Recurring - USD ($)
$ in Millions
Mar. 31, 2026
Sep. 30, 2025
Carrying Value | Other Current Assets | Fuel Derivative Contracts    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total assets $ 1.7  
Carrying Value | Other Assets    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total assets 11.0 $ 7.9
Carrying Value | Other Assets | Interest Rate Derivative Contracts    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total assets 1.3  
Carrying Value | Other Liabilities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total liabilities 8.0 9.5
Carrying Value | Other Liabilities | Interest Rate Derivative Contracts    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total liabilities   1.5
Carrying Value | Accrued Expenses and Other Current Liabilities | Fuel Derivative Contracts    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total liabilities   0.1
Investment Held by Rabbi Trust | Carrying Value | Other Assets    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total assets 8.0 7.9
Investment Held by Rabbi Trust | Carrying Value | Other Liabilities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total liabilities 8.0 7.9
Level 1 | Other Current Assets | Fuel Derivative Contracts    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total assets 0.0  
Level 1 | Other Assets    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total assets 8.0 7.9
Level 1 | Other Liabilities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total liabilities 8.0 7.9
Level 1 | Other Liabilities | Interest Rate Derivative Contracts    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total assets 0.0  
Total liabilities   0.0
Level 1 | Accrued Expenses and Other Current Liabilities | Fuel Derivative Contracts    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total liabilities   0.0
Level 1 | Investment Held by Rabbi Trust | Other Assets    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total assets 8.0 7.9
Level 1 | Investment Held by Rabbi Trust | Other Liabilities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total liabilities 8.0 7.9
Level 2 | Other Current Assets | Fuel Derivative Contracts    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total assets 1.7  
Level 2 | Other Assets    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total assets 3.0 0.0
Level 2 | Other Liabilities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total liabilities 0.0 1.6
Level 2 | Other Liabilities | Interest Rate Derivative Contracts    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total assets 1.3  
Total liabilities   1.5
Level 2 | Accrued Expenses and Other Current Liabilities | Fuel Derivative Contracts    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total liabilities   0.1
Level 2 | Investment Held by Rabbi Trust | Other Assets    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total assets 0.0 0.0
Level 2 | Investment Held by Rabbi Trust | Other Liabilities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total liabilities 0.0 0.0
Level 3 | Other Current Assets | Fuel Derivative Contracts    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total assets 0.0  
Level 3 | Other Assets    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total assets 0.0 0.0
Level 3 | Other Liabilities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total liabilities 0.0 0.0
Level 3 | Other Liabilities | Interest Rate Derivative Contracts    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total assets 0.0  
Total liabilities   0.0
Level 3 | Accrued Expenses and Other Current Liabilities | Fuel Derivative Contracts    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total liabilities   0.0
Level 3 | Investment Held by Rabbi Trust | Other Assets    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total assets 0.0 0.0
Level 3 | Investment Held by Rabbi Trust | Other Liabilities    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Total liabilities $ 0.0 $ 0.0