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Fair Value Measurements and Derivative Instruments - Summary of Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - Recurring - USD ($) $ in Millions | Jun. 30, 2021 | Sep. 30, 2020
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Carrying Value | Other Assets |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total assets | $ 15.9 | $ 12.1
Carrying Value | Other Assets | Fuel Swap Contracts |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total assets | 2.4 | 0.7
Carrying Value | Other Liabilities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total liabilities | 17.7 | 22.2
Carrying Value | Other Liabilities | Interest Rate Swaps |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total liabilities | 4.2 | 5.5
Carrying Value | Accrued Expenses and Other Current Liabilities | Interest Rate Swaps |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total liabilities |  | 4.5
Carrying Value | Accrued Expenses and Other Current Liabilities | Fuel Swap Contracts |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total liabilities |  | 0.8
Investment Held by Rabbi Trust | Carrying Value | Other Assets |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total assets | 13.5 | 11.4
Investment Held by Rabbi Trust | Carrying Value | Other Liabilities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total liabilities | 13.5 | 11.4
Level 1 | Other Assets |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total assets | 13.5 | 11.4
Level 1 | Other Liabilities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total liabilities | 13.5 | 11.4
Level 1 | Investment Held by Rabbi Trust | Other Assets |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total assets | 13.5 | 11.4
Level 1 | Investment Held by Rabbi Trust | Other Liabilities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total liabilities | 13.5 | 11.4
Level 2 | Other Assets |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total assets | 2.4 | 0.7
Level 2 | Other Assets | Fuel Swap Contracts |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total assets | 2.4 | 0.7
Level 2 | Other Liabilities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total liabilities | 4.2 | 10.8
Level 2 | Other Liabilities | Interest Rate Swaps |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total liabilities | $ 4.2 | 5.5
Level 2 | Accrued Expenses and Other Current Liabilities | Interest Rate Swaps |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total liabilities |  | 4.5
Level 2 | Accrued Expenses and Other Current Liabilities | Fuel Swap Contracts |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Total liabilities |  | $ 0.8

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