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Income Taxes - Additional Information (Details) - USD ($) $ in Millions | Sep. 30, 2025 | Sep. 30, 2024
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Income Tax Contingency [Line Items] |  |
Unrecognized tax benefits that would impact effective tax rate | $ 2.7 | $ 6.2
Unrecognized tax benefits, interest accrued | 0.0 | 0.0
Spire Missouri [Member] |  |
Income Tax Contingency [Line Items] |  |
Unrecognized tax benefits that would impact effective tax rate | 1.3 | 5.3
Spire Alabama Inc [Member] |  |
Income Tax Contingency [Line Items] |  |
Unrecognized tax benefits that would impact effective tax rate | $ 0.3 | $ 0.3

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- Definition Amount of interest expense accrued for an underpayment of income taxes. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 740 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 15 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482685/740-10-50-15 + Details
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