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Fair Value Measurements (Tables) | 12 Months Ended
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Sep. 30, 2025
Fair Value Disclosures [Abstract] |
Schedule of Derivative Assets and Liabilities, Including Receivables and Payables |
|  |  |  |  |  |  |  |  |  |  | Effects of |  |  |  |
|  | Quoted |  |  |  |  |  |  |  |  | Netting |  |  |  |
|  | Prices |  |  | Significant |  |  | Significant |  |  | and Cash |  |  |  |
|  | in Active |  |  | Observable |  |  | Unobservable |  |  | Margin |  |  |  |
|  | Markets |  |  | Inputs |  |  | Inputs |  |  | Receivables |  |  |  |
|  | (Level 1) |  |  | (Level 2) |  |  | (Level 3) |  |  | /Payables |  |  | Total |
As of September 30, 2025 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
ASSETS |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gas Utility: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. stock/bond mutual funds |  | $ | 27.5 |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | 27.5 |
NYMEX/ICE natural gas contracts |  |  | 2.9 |  |  |  | — |  |  |  | — |  |  |  | (2.9 | ) |  |  | — |
Gas Marketing: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
NYMEX/ICE natural gas contracts |  |  | 11.4 |  |  |  | — |  |  |  | — |  |  |  | (11.4 | ) |  |  | — |
Natural gas commodity contracts |  |  | — |  |  |  | 44.8 |  |  |  | — |  |  |  | (2.7 | ) |  |  | 42.1 |
Other: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. stock/bond mutual funds |  |  | 27.0 |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | 27.0 |
U.S. bonds |  |  | 23.5 |  |  |  | — |  |  |  | — |  |  |  |  |  |  | 23.5 |
Global Bonds |  |  | 1.5 |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | 1.5 |
Interest rate swaps |  |  | — |  |  |  | 10.0 |  |  |  | — |  |  |  | — |  |  |  | 10.0 |
Total |  | $ | 93.8 |  |  | $ | 54.8 |  |  | $ | — |  |  | $ | (17.0 | ) |  | $ | 131.6 |
LIABILITIES |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gas Utility: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
NYMEX/ICE natural gas contracts |  | $ | 25.9 |  |  | $ | — |  |  | $ | — |  |  | $ | (1.3 | ) |  | $ | 24.6 |
Gas Marketing: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
NYMEX/ICE natural gas contracts |  |  | 19.0 |  |  |  | — |  |  |  | — |  |  |  | (19.0 | ) |  |  | — |
Natural gas commodity contracts |  |  | — |  |  |  | 16.7 |  |  |  | — |  |  |  | (2.7 | ) |  |  | 14.0 |
Total |  | $ | 44.9 |  |  | $ | 16.7 |  |  | $ | — |  |  | $ | (23.0 | ) |  | $ | 38.6 |
As of September 30, 2024 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
ASSETS |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gas Utility: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. stock/bond mutual funds |  | $ | 24.3 |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | 24.3 |
NYMEX/ICE natural gas contracts |  |  | 3.4 |  |  |  | — |  |  |  | — |  |  |  | (3.4 | ) |  |  | — |
Gas Marketing: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
NYMEX/ICE natural gas contracts |  |  | 7.0 |  |  |  | — |  |  |  | — |  |  |  | (7.0 | ) |  |  | — |
Natural gas commodity contracts |  |  | — |  |  |  | 46.0 |  |  |  | — |  |  |  | (3.5 | ) |  |  | 42.5 |
Other: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. stock/bond mutual funds |  |  | 17.9 |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | 17.9 |
U.S. bonds |  |  | 21.9 |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | 21.9 |
Global Bonds |  |  | 5.9 |  |  |  | — |  |  |  | — |  |  |  | — |  |  |  | 5.9 |
Interest rate swaps |  |  | — |  |  |  | 0.8 |  |  |  | — |  |  |  | — |  |  |  | 0.8 |
Total |  | $ | 80.4 |  |  | $ | 46.8 |  |  | $ | — |  |  | $ | (13.9 | ) |  | $ | 113.3 |
LIABILITIES |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Gas Utility: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
NYMEX/ICE natural gas contracts |  | $ | 13.6 |  |  | $ | — |  |  | $ | — |  |  | $ | (3.8 | ) |  | $ | 9.8 |
Gas Marketing: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
NYMEX/ICE natural gas contracts |  |  | 17.7 |  |  |  | — |  |  |  | — |  |  |  | (17.7 | ) |  |  | — |
Natural gas commodity contracts |  |  | — |  |  |  | 24.5 |  |  |  | — |  |  |  | (3.5 | ) |  |  | 21.0 |
Other: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate swaps |  |  | — |  |  |  | 1.5 |  |  |  | — |  |  |  | — |  |  |  | 1.5 |
Total |  | $ | 31.3 |  |  | $ | 26.0 |  |  | $ | — |  |  | $ | (25.0 | ) |  | $ | 32.3 |
Spire Missouri
|  |  |  |  |  |  |  |  |  |  | Effects of |  |  |  |
|  | Quoted |  |  |  |  |  |  |  |  | Netting |  |  |  |
|  | Prices |  |  | Significant |  |  | Significant |  |  | and Cash |  |  |  |
|  | in Active |  |  | Observable |  |  | Unobservable |  |  | Margin |  |  |  |
|  | Markets |  |  | Inputs |  |  | Inputs |  |  | Receivables |  |  |  |
|  | (Level 1) |  |  | (Level 2) |  |  | (Level 3) |  |  | /Payables |  |  | Total |
As of September 30, 2025 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
ASSETS |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. stock/bond mutual funds |  | $ | 27.5 |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | 27.5 |
NYMEX/ICE natural gas contracts |  |  | 2.9 |  |  |  | — |  |  |  | — |  |  |  | (2.9 | ) |  |  | — |
Total |  | $ | 30.4 |  |  | $ | — |  |  | $ | — |  |  | $ | (2.9 | ) |  | $ | 27.5 |
LIABILITIES |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
NYMEX/ICE natural gas contracts |  | $ | 25.9 |  |  | $ | — |  |  | $ | — |  |  | $ | (1.3 | ) |  | $ | 24.6 |
As of September 30, 2024 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
ASSETS |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. stock/bond mutual funds |  | $ | 24.3 |  |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | 24.3 |
NYMEX/ICE natural gas contracts |  |  | 3.4 |  |  |  | — |  |  |  | — |  |  |  | (3.4 | ) |  |  | — |
Total |  | $ | 27.7 |  |  | $ | — |  |  | $ | — |  |  | $ | (3.4 | ) |  | $ | 24.3 |
LIABILITIES |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
NYMEX/ICE natural gas contracts |  | $ | 13.6 |  |  | $ | — |  |  | $ | — |  |  | $ | (3.8 | ) |  | $ | 9.8 |

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- Definition Tabular disclosure of assets and liabilities, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, that are measured at fair value on a recurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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