Long-Term Debt (Details) - USD ($) $ in Millions	6 Months Ended
Mar. 31, 2024	Mar. 31, 2023	Sep. 30, 2023
Debt Instrument
Long-term and short-term debt, combined amount	$ 840.0		$ 865.0
Unamortized Debt Issuance Expense	(7.6)		(8.2)
Long-term debt	832.4		856.8
7.00% Senior Notes Maturing in March 2030
Debt Instrument
Long-term debt, gross	840.0		840.0
Revolving Credit Facility
Debt Instrument
Long-term debt, gross	0.0		25.0
Maximum borrowing capacity on line of credit	250.0
Proceeds from borrowing under line of credit	0.0	$ 115.0
Repayments of lines of credit	25.0	$ 75.0
Remaining borrowing capacity on line of credit	250.0		225.0
Letters of credit outstanding, amount	0.0		$ 0.0
Letter of Credit
Debt Instrument
Maximum borrowing capacity on line of credit	$ 20.0

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

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Amount, after deduction of unamortized premium (discount) and debt issuance cost, of long-term debt. Excludes lease obligation.

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Amount of cash inflow from contractual arrangement with the lender, including but not limited to, letter of credit, standby letter of credit and revolving credit arrangements.

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Amount of cash outflow for payment of an obligation from a lender, including but not limited to, letter of credit, standby letter of credit and revolving credit arrangements.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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