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Derivative Instruments and Hedging Activities (Tables) | 12 Months Ended
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Sep. 30, 2025
Derivative Instruments and Hedging Activities Disclosures [Line Items] |
Schedule of Derivative Instruments |
|  | September 30, 2025 |  |  | September 30, 2024 |
Gas Marketing |  | Notional (MMBtu millions) |  |  | Maximum Term (Months) |  |  | Notional (MMBtu millions) |  |  | Maximum Term (Months) |
Natural gas futures purchased |  |  | 48.0 |  |  |  | 48 |  |  |  | 56.0 |  |  |  | 48 |
Natural gas options purchased, net |  |  | 9.9 |  |  |  | 15 |  |  |  | 3.8 |  |  |  | 15 |
Natural gas basis swaps purchased |  |  | 24.7 |  |  |  | 39 |  |  |  | 32.4 |  |  |  | 39 |
Gas Utility |  |  |  |  |  |  |  |  |  |  |  |
Natural gas futures purchased |  |  | 32.3 |  |  |  | 12.0 |  |  |  | 34.6 |  |  |  | 12 |
|  | September 30, 2025 |  |  | September 30, 2024 |
|  | Notional (MMBtu millions) |  |  | Maximum Term (Months) |  |  | Notional (MMBtu millions) |  |  | Maximum Term (Months) |
Natural gas futures purchased |  |  | 32.3 |  |  |  | 12 |  |  |  | 34.6 |  |  |  | 12 |
Derivative Instruments, Gain (Loss) |
Effect of Derivative Instruments on the Consolidated Statements of Income and Comprehensive Income |
|  | Location of Gain (Loss) |  |  |  |  |  |  |  |  |
|  | Recorded in Income |  | 2025 |  |  | 2024 |  |  | 2023 |
Derivatives in Cash Flow Hedging Relationships |  |  |  |  |  |  |  |  |
Portion of gain (loss) recognized in OCI on derivatives: |  |  |  |  |  |  |  |  |
Interest rate swaps |  |  |  | $ | 12.8 |  |  | $ | (15.9 | ) |  | $ | 20.1 |
Portion of gain reclassified from AOCI to income: |  |  |  |  |  |  |  |  |
Interest rate swaps |  | Interest Expense |  | $ | 5.1 |  |  | $ | 11.1 |  |  | $ | 2.4 |
Derivatives Not Designated as Hedging Instruments* |  |  |  |  |  |  |  |  |
(Loss) gain recognized in income on derivatives: |  |  |  |  |  |  |  |  |
Gas / diesel futures |  | Other Income (Expense), Net |  | $ | — |  |  | $ | (0.3 | ) |  | $ | 0.3 |
Natural gas commodity contracts |  | Operating Expenses: Natural Gas |  |  | (6.5 | ) |  |  | 12.5 |  |  |  | 18.0 |
NYMEX / ICE natural gas contracts |  | Operating Expenses: Natural Gas |  |  | (3.1 | ) |  |  | (16.3 | ) |  |  | (35.0 | )
Total |  |  |  | $ | (9.6 | ) |  | $ | (4.1 | ) |  | $ | (16.7 | )
Schedule of Derivative Instruments in Statement of Financial Position, Fair Value |
Fair Value of Derivative Instruments in the Consolidated Balance Sheets |
|  | Derivative Assets* |  |  | Derivative Liabilities* |
September 30, 2025 |  | Balance Sheet Location |  | Fair Value |  |  | Balance Sheet Location |  | Fair Value |
Derivatives designated as hedging instruments |  |  |  |  |  |  |  |
Other: Interest rate swaps |  | Current Assets: Other |  | $ | 10.0 |  |  | Current Liabilities: Other |  | $ | — |
Subtotal |  |  |  |  | 10.0 |  |  |  |  |  | — |
Derivatives not designated as hedging instruments |  |  |  |  |  |  |  |
Gas Utility: |  |  |  |  |  |  |  |  |  |
Natural gas contracts |  | Current Assets: Other |  |  | 2.9 |  |  | Current Liabilities: Other |  |  | 25.9 |
Gas Marketing: |  |  |  |  |  |  |  |  |  |
NYMEX / ICE natural gas contracts |  | Current Assets: Other |  |  | 11.3 |  |  | Current Liabilities: Other |  |  | 17.7 |
|  | Deferred Charges and Other Assets: Other |  |  | 0.1 |  |  | Deferred Credits and Other Liabilities: Other |  |  | 1.3 |
Natural gas commodity |  | Current Assets: Other |  |  | 42.8 |  |  | Current Liabilities: Other |  |  | 15.6 |
|  | Deferred Charges and Other Assets: Other |  |  | 2.0 |  |  | Deferred Credits and Other Liabilities: Other |  |  | 1.1 |
Subtotal |  |  |  |  | 59.1 |  |  |  |  |  | 61.6 |
Total derivatives |  |  |  | $ | 69.1 |  |  |  |  | $ | 61.6 |
September 30, 2024 |  |  |  |  |  |  |  |  |  |
Derivatives designated as hedging instruments |  |  |  |  |  |  |  |
Other: Interest rate swaps |  | Current Assets: Other |  | $ | 0.8 |  |  | Current Liabilities: Other |  | $ | 1.5 |
Subtotal |  |  |  |  | 0.8 |  |  |  |  |  | 1.5 |
Derivatives not designated as hedging instruments |  |  |  |  |  |  |  |
Gas Utility: |  |  |  |  |  |  |  |  |  |
Natural gas contracts |  | Current Assets: Other |  |  | 3.4 |  |  | Current Liabilities: Other |  |  | 13.6 |
Gasoline and heating oil contracts |  | Current Assets: Other |  |  | — |  |  |  |  |  | — |
Gas Marketing: |  |  |  |  |  |  |  |  |  |
NYMEX / ICE natural gas contracts |  | Current Assets: Other |  |  | 6.2 |  |  | Current Liabilities: Other |  |  | 13.7 |
|  | Deferred Charges and Other Assets: Other |  |  | 0.8 |  |  | Deferred Credits and Other Liabilities: Other |  |  | 4.0 |
Natural gas commodity |  | Current Assets: Other |  |  | 42.1 |  |  | Current Liabilities: Other |  |  | 19.8 |
|  | Deferred Charges and Other Assets: Other |  |  | 3.9 |  |  | Deferred Credits and Other Liabilities: Other |  |  | 4.7 |
Subtotal |  |  |  |  | 56.4 |  |  |  |  |  | 55.8 |
Total derivatives |  |  |  | $ | 57.2 |  |  |  |  | $ | 57.3 |
* The fair values of Derivative Assets and Derivative Liabilities exclude the fair value of cash margin receivables or payables with counterparties subject to netting arrangements. Fair value amounts of derivative contracts (including the fair value amounts of cash margin receivables and payables) for which there is a legal right to set off are presented net on the balance sheets. As such, the gross balances presented in the table above are not indicative of the Company’s net economic exposure. Refer to Note 9, Fair Value Measurements, for information on the valuation of derivative instruments.
Fair Value of Derivative Instruments in the Balance Sheets |
|  | Derivative Assets* |  |  | Derivative Liabilities* |
September 30, 2025 |  | Balance Sheet Location |  | Fair Value |  |  | Balance Sheet Location |  | Fair Value |
Derivatives not designated as hedging instruments |  |  |  |  |  |  |  |
Natural gas contracts |  | Current Assets: Other |  | $ | 2.9 |  |  | Current Liabilities: Other |  | $ | 25.9 |
Total derivatives |  |  |  | $ | 2.9 |  |  |  |  | $ | 25.9 |
September 30, 2024 |  |  |  |  |  |  |  |  |  |
Derivatives not designated as hedging instruments |  |  |  |  |  |  |  |
Natural gas contracts |  | Current Assets: Other |  | $ | 3.4 |  |  | Current Liabilities: Other |  | $ | 13.6 |
Gasoline and heating oil contracts |  | Current Assets: Other |  |  | — |  |  |  |  |  | — |
Total derivatives |  |  |  | $ | 3.4 |  |  |  |  | $ | 13.6 |
* The fair values of Derivative Assets and Derivative Liabilities exclude the fair value of cash margin receivables or payables with counterparties subject to netting arrangements. Fair value amounts of derivative contracts (including the fair value amounts of cash margin receivables and payables) for which there is a legal right to set off are presented net on the balance sheets. As such, the gross balances presented in the table above are not indicative of Spire Missouri’s net economic exposure. Refer to Note 9, Fair Value Measurements, for information on the valuation of derivative instruments.
Derivative Instrument Reconciliation |
|  | 2025 |  |  | 2024 |
Fair value of derivative assets presented above |  | $ | 69.1 |  |  | $ | 57.2 |
Fair value of cash margin receivable offset with derivatives |  |  | 6.0 |  |  |  | 11.1 |
Netting of assets and liabilities with the same counterparty |  |  | (23.0 | ) |  |  | (25.0 | )
Total |  | $ | 52.1 |  |  | $ | 43.3 |
Derivative Instrument Assets, per Consolidated Balance Sheets: |  |  |  |  |  |
Current Assets: Othe r |  | $ | 50.1 |  |  | $ | 39.5 |
Deferred Charges and Other Assets: Other |  |  | 2.0 |  |  |  | 3.8 |
Total |  | $ | 52.1 |  |  | $ | 43.3 |
Fair value of derivative liabilities presented above |  | $ | 61.6 |  |  | $ | 57.3 |
Netting of assets and liabilities with the same counterparty |  |  | (23.0 | ) |  |  | (25.0 | )
Total |  | $ | 38.6 |  |  | $ | 32.3 |
Derivative Instrument Liabilities, per Consolidated Balance Sheets: |  |  |  |  |  |
Current Liabilities: Other |  | $ | 37.5 |  |  | $ | 27.6 |
Deferred Credits and Other Liabilities: Other |  |  | 1.1 |  |  |  | 4.7 |
Total |  | $ | 38.6 |  |  | $ | 32.3 |
|  | 2025 |  |  | 2024 |
Fair value of derivative assets presented above |  | $ | 2.9 |  |  | $ | 3.4 |
Fair value of cash margin receivable offset with derivatives |  |  | (1.6 | ) |  |  | 0.4 |
Netting of assets and liabilities with the same counterparty |  |  | (1.3 | ) |  |  | (3.8 | )
Total |  | $ | — |  |  | $ | — |
Fair value of derivative liabilities presented above |  | $ | 25.9 |  |  | $ | 13.6 |
Netting of assets and liabilities with the same counterparty |  |  | (1.3 | ) |  |  | (3.8 | )
Total |  |  | 24.6 |  |  |  | 9.8 |
Derivative Instrument Liabilities, per Balance Sheets: |  |  |  |  |  |
Current Liabilities: Other |  | $ | 24.6 |  |  | $ | 9.8 |
Deferred Credits and Other Liabilities: Other |  |  | - |  |  |  | — |
Total |  | $ | 24.6 |  |  | $ | 9.8 |
Interest Rate Swap [Member] |
Derivative Instruments and Hedging Activities Disclosures [Line Items] |
Schedule of Derivative Instruments |
Period Originated |  | Contract Hedge Term (Years) |  |  | Notional Amount |  |  | Fixed Interest Rate |  |  | Fiscal 2025 Mark-to- Market Gain |
Quarter 3, fiscal 2023 |  |  | 10 |  |  | $ | 25.0 |  |  |  | 3.018 | % |  |  | 0.8 |
Quarter 1, fiscal 2024 |  |  | 10 |  |  |  | 25.0 |  |  |  | 3.400 | % |  |  | 0.8 |
Quarter 1, fiscal 2024 |  |  | 10 |  |  |  | 25.0 |  |  |  | 3.525 | % |  |  | 0.8 |
Quarter 1, fiscal 2024 |  | 10 |  |  |  | 25.0 |  |  |  | 3.535 | % |  |  | 0.8 |
Quarter 1, fiscal 2024 |  |  | 10 |  |  |  | 25.0 |  |  |  | 3.450 | % |  |  | 0.8 |
Quarter 4, fiscal 2024 |  |  | 10 |  |  |  | 25.0 |  |  |  | 3.541 | % |  |  | 0.7 |
Quarter 4, fiscal 2024 |  |  | 10 |  |  |  | 25.0 |  |  |  | 3.552 | % |  |  | 0.8 |
Quarter 4, fiscal 2024 |  |  | 10 |  |  |  | 25.0 |  |  |  | 3.426 | % |  |  | 0.8 |
Quarter 4, fiscal 2024 |  | 10 |  |  |  | 25.0 |  |  |  | 3.577 | % |  |  | 0.8 |
Quarter 4, fiscal 2024 |  | 10 |  |  |  | 25.0 |  |  |  | 3.450 | % |  |  | 0.8 |
Quarter 4, fiscal 2024 |  | 10 |  |  |  | 25.0 |  |  |  | 3.350 | % |  |  | 0.8 |
Quarter 1, fiscal 2025 |  | 1.5 |  |  |  | 125.0 |  |  |  | 3.567 | % |  |  | 0.2 |
Quarter 1, fiscal 2025 |  | 1.5 |  |  |  | 225.0 |  |  |  | 3.567 | % |  |  | 0.4 |
Quarter 3, fiscal 2025 |  | 10 |  |  |  | 25.0 |  |  |  | 3.580 | % |  |  | 0.3 |
Quarter 3, fiscal 2025 |  | 10 |  |  |  | 25.0 |  |  |  | 3.611 | % |  |  | 0.3 |
Quarter 3, fiscal 2025 |  | 10 |  |  |  | 25.0 |  |  |  | 3.657 | % |  |  | 0.3 |
Quarter 3, fiscal 2025 |  | 10 |  |  |  | 25.0 |  |  |  | 3.763 | % |  |  | 0.5 |
|  |  |  |  | $ | 725.0 |  |  |  |  |  | $ | 10.7 |

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