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Revenue - Schedule of Gross Receipts Taxes Associated Revenue (Details) - USD ($) $ in Millions | 12 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2023
Disaggregation of Revenue [Line Items] |  |  |
Gross receipts tax amounts | $ 115.7 | $ 128.2 | $ 131.8
Spire Missouri [Member] |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Gross receipts tax amounts | 82.9 | 93.1 | 96.7
Spire Alabama Inc [Member] |  |  |
Disaggregation of Revenue [Line Items] |  |  |
Gross receipts tax amounts | $ 27.9 | $ 30.1 | $ 29.9

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- Definition Amount of gross receipts taxes associated with natural gas utility service, which are imposed on the Utility and billed to its customers. + References No definition available. + Details
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