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, 2026	Sep. 30, 2025
Carrying Value | Location, Statement of Financial Position, Balance [Axis]: us-gaap:AccruedLiabilitiesAndOtherLiabilities | Fuel Derivative Contracts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total liabilities		$ 0.1
Carrying Value | Location, Statement of Financial Position, Balance [Axis]: us-gaap:Assets
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets	$ 13.6	7.9
Carrying Value | Location, Statement of Financial Position, Balance [Axis]: us-gaap:Liabilities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total liabilities	8.6	9.5
Carrying Value | Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherAssetsCurrent | Fuel Derivative Contracts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets	0.9
Carrying Value | Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherAssetsNoncurrent | Interest Rate Derivative Contracts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets	4.1
Carrying Value | Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherLiabilitiesNoncurrent | Interest Rate Derivative Contracts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total liabilities		1.5
Investment Held by Rabbi Trust | Carrying Value | Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherAssetsNoncurrent
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets	8.6	7.9
Investment Held by Rabbi Trust | Carrying Value | Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherLiabilitiesNoncurrent
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total liabilities	8.6	7.9
Level 1 | Location, Statement of Financial Position, Balance [Axis]: us-gaap:AccruedLiabilitiesAndOtherLiabilities | Fuel Derivative Contracts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total liabilities		0.0
Level 1 | Location, Statement of Financial Position, Balance [Axis]: us-gaap:Assets
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets	8.6	7.9
Level 1 | Location, Statement of Financial Position, Balance [Axis]: us-gaap:Liabilities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total liabilities	8.6	7.9
Level 1 | Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherAssetsCurrent | Fuel Derivative Contracts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets	0.0
Level 1 | Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherAssetsNoncurrent | Interest Rate Derivative Contracts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets	0.0
Level 1 | Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherLiabilitiesNoncurrent | Interest Rate 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 | Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherAssetsNoncurrent
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets	8.6	7.9
Level 1 | Investment Held by Rabbi Trust | Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherLiabilitiesNoncurrent
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total liabilities	8.6	7.9
Level 2 | Location, Statement of Financial Position, Balance [Axis]: us-gaap:AccruedLiabilitiesAndOtherLiabilities | Fuel Derivative Contracts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total liabilities		0.1
Level 2 | Location, Statement of Financial Position, Balance [Axis]: us-gaap:Assets
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets	5.0	0.0
Level 2 | Location, Statement of Financial Position, Balance [Axis]: us-gaap:Liabilities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total liabilities	0.0	1.6
Level 2 | Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherAssetsCurrent | Fuel Derivative Contracts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets	0.9
Level 2 | Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherAssetsNoncurrent | Interest Rate Derivative Contracts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets	4.1
Level 2 | Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherLiabilitiesNoncurrent | Interest Rate Derivative Contracts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total liabilities		1.5
Level 2 | Investment Held by Rabbi Trust | Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherAssetsNoncurrent
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 | Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherLiabilitiesNoncurrent
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total liabilities	0.0	0.0
Level 3 | Location, Statement of Financial Position, Balance [Axis]: us-gaap:AccruedLiabilitiesAndOtherLiabilities | Fuel Derivative Contracts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total liabilities		0.0
Level 3 | Location, Statement of Financial Position, Balance [Axis]: us-gaap:Assets
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets	0.0	0.0
Level 3 | Location, Statement of Financial Position, Balance [Axis]: us-gaap:Liabilities
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total liabilities	0.0	0.0
Level 3 | Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherAssetsCurrent | Fuel Derivative Contracts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets	0.0
Level 3 | Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherAssetsNoncurrent | Interest Rate Derivative Contracts
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total assets	0.0
Level 3 | Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherLiabilitiesNoncurrent | Interest Rate 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 | Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherAssetsNoncurrent
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 | Location, Statement of Financial Position, Balance [Axis]: us-gaap:OtherLiabilitiesNoncurrent
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Total liabilities	$ 0.0	$ 0.0