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v3.25.3

Long-term Debt Details - Additional Information (Details) - USD ($) $ in Millions |  | 12 Months Ended
--- | --- | ---
Oct. 23, 2025 | Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Long-Term Debt, Gross |  | $ 3,879.1 | $ 3,771.1 |
Interest Paid, Capitalized, Investing Activities |  | 19.5 | 17.0 | $ 8.6
Common stock |  | 59.0 | 57.7 |
Spire and Other Subsidiaries [Member] |  |  |  |
Long-Term Debt, Gross |  | 1,196.1 |  |
Parent Company [Member] | First Mortgage Bond [Member] | Subsequent Event [Member] |  |  |  |
Debt Instrument face amount | $ 150.0 |  |  |
Debt Instrument, Payment Terms | semi-annually on March 15 and September 15 of each year |  |  |
Parent Company [Member] | First Mortgage Bond [Member] | Share-Based Payment Arrangement, Tranche One [Member] | Subsequent Event [Member] |  |  |  |
Debt Instrument face amount | $ 200.0 |  |  |
Debt instrument interest rate | 4.60% |  |  |
Debt instrument maturity date | Sep. 15, 2030 |  |  |
Parent Company [Member] | First Mortgage Bond [Member] | Share-Based Payment Arrangement, Tranche Two [Member] | Subsequent Event [Member] |  |  |  |
Debt Instrument face amount | $ 50.0 |  |  |
Debt instrument interest rate | 4.65% |  |  |
Debt instrument maturity date | Jan. 15, 2031 |  |  |
Spire Missouri [Member] |  |  |  |
Long-Term Debt, Gross |  | 1,968.0 | 1,818.0 |
Interest Paid, Capitalized, Investing Activities |  | 4.8 | 4.6 | 3.1
Regulatory Preferred Stock and Debt Securities Issuance Amount Authorized |  | 850.0 |  |
Common stock |  | 0.1 | 0.1 |
Spire Missouri [Member] | Subsequent Event [Member] |  |  |  |
Common stock | $ 74.4 |  |  |
Debt available for issuance under authorization | 426.0 |  |  |
Spire Missouri [Member] | First Mortgage Bond [Member] | Subsequent Event [Member] |  |  |  |
Debt Instrument face amount | $ 350.0 |  |  |
Spire Alabama Inc [Member] |  |  |  |
Long-Term Debt, Gross |  | 715.0 | 750.0 |
Interest Paid, Capitalized, Investing Activities |  | $ 4.5 | $ 1.8 | $ 2.3

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