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Fair Value of Financial Instruments (Tables) | 12 Months Ended
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Sep. 30, 2025
Fair Value Disclosures [Abstract] |
Schedule of Estimated Fair Value of Financial Instruments Not Measured at Fair Value on Recurring Basis | The carrying amounts and estimated fair values of financial instruments not measured at fair value on a recurring basis were as follows:
|  |  |  |  |  |  |  | Classification of |
|  |  |  |  |  |  |  | Estimated Fair Value |
|  |  |  |  |  |  |  | Quoted |  |  |  |
|  |  |  |  |  |  |  | Prices in |  |  | Significant |
|  |  |  |  |  |  |  | Active |  |  | Observable |
|  | Carrying |  |  | Fair |  |  | Markets |  |  | Inputs |
|  | Amount |  |  | Value |  |  | (Level 1) |  |  | (Level 2) |
Spire |  |  |  |  |  |  |  |  |  |  |  |
As of September 30, 2025 |  |  |  |  |  |  |  |  |  |  |  |
Cash and cash equivalents |  | $ | 5.7 |  |  | $ | 5.7 |  |  | $ | 5.7 |  |  | $ | — |
Notes payable |  |  | 1,317.0 |  |  |  | 1,317.0 |  |  |  | — |  |  |  | 1,317.0 |
Long-term debt, including current portion |  |  | 3,856.9 |  |  |  | 3,691.5 |  |  |  | — |  |  |  | 3,691.5 |
As of September 30, 2024 |  |  |  |  |  |  |  |  |  |  |  |
Cash and cash equivalents |  | $ | 4.5 |  |  | $ | 4.5 |  |  | $ | 4.5 |  |  | $ | — |
Notes payable |  |  | 947.0 |  |  |  | 947.0 |  |  |  | — |  |  |  | 947.0 |
Long-term debt, including current portion |  |  | 3,746.4 |  |  |  | 3,600.3 |  |  |  | — |  |  |  | 3,600.3 |
Spire Missouri |  |  |  |  |  |  |  |  |  |  |  |
As of September 30, 2025 |  |  |  |  |  |  |  |  |  |  |  |
Notes payable - associated companies |  | $ | 566.3 |  |  | $ | 566.3 |  |  | $ | — |  |  | $ | 566.3 |
Long-term debt |  |  | 1,953.6 |  |  |  | 1,874.0 |  |  |  | — |  |  |  | 1,874.0 |
As of September 30, 2024 |  |  |  |  |  |  |  |  |  |  |  |
Notes payable - associated companies |  | $ | 495.3 |  |  | $ | 495.3 |  |  | $ | — |  |  | $ | 495.3 |
Long-term debt |  |  | 1,803.4 |  |  |  | 1,736.9 |  |  |  | — |  |  |  | 1,736.9 |
Spire Alabama |  |  |  |  |  |  |  |  |  |  |  |
As of September 30, 2025 |  |  |  |  |  |  |  |  |  |  |  |
Cash and cash equivalents |  | $ | 1.9 |  |  | $ | 1.9 |  |  | $ | 1.9 |  |  | $ | — |
Notes payable - associated companies |  |  | 130.1 |  |  |  | 130.1 |  |  |  | — |  |  |  | 130.1 |
Long-term debt |  |  | 711.7 |  |  |  | 675.9 |  |  |  | — |  |  |  | 675.9 |
As of September 30, 2024 |  |  |  |  |  |  |  |  |  |  |  |
Cash and cash equivalents |  | $ | 1.5 |  |  | $ | 1.5 |  |  | $ | 1.5 |  |  | $ | — |
Notes payable - associated companies |  |  | 48.4 |  |  |  | 48.4 |  |  |  | — |  |  |  | 48.4 |
Long-term debt, including current portion |  |  | 746.3 |  |  |  | 711.8 |  |  |  | — |  |  |  | 711.8 |

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- Definition Tabular disclosure of the fair value of financial instruments, including financial assets and financial liabilities, and the measurements of those instruments, assets, and liabilities. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Name Accounting Standards Codification -Section 55 -Paragraph 10 -SubTopic 10 -Topic 825 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482881/825-10-55-10 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 13 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482907/825-10-50-13 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482881/825-10-55-12 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 12 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482907/825-10-50-12 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 825 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 11 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482907/825-10-50-11 + Details
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